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Elements of voice reinforcement systems in halls
ELEMENTS
OF VOICE REINFORCEMENT IN HALLS
SYSTEMS
R. S. CADDY
Vice-Chancellor’s Unit, University of New South Wales (Australia) (Received: 30 July, 1969)
SUMMARY
The separate elements of voice reinforcement systems in halls-loudspeakers, ampl$ers, pre-amplijers and microphones-are discussed. The use of column loudspeakers, with their advantages and disadvantages, are mentioned, while special attention is paid to the problem of the choice and the installation of microphones in such systems.
INTRODUCTION
There is an ever growing demand for amplification of the human voice in clubs and halls. The reasons are various, including increasing ambient noise and a decrease in the number of people willing to project their voices. The design of voice reinforcement systems, while not an exact science, can be tackled systematically and with reasonable chance of success, if all sections of the microphone-pre-amplifierpower-amplifier-loudspeaker-hall combination are considered. This paper discusses the elements in this chain.
LOUDSPEAKERS--GENERAL
DlSCUSSION
Loudspeakers come in all shapes and sizes, from the two-inch circle of the small transistor radio to the monsters of the large cinema. The large area ‘Quad’ electrostatic loudspeaker represents another design approach. Why the variety? A loudspeaker’s primary use is to cause acoustic wave motions in air. In a transistor radio very small size is all important, in mantel model radio and console type TV Applied Acousrics
(2) (1969)--9
Elsevier Publishing Company Ltd. England-Printed
in Great Britain
260
R . S . CADDY
receivers small size is important--quality of sound reproduction is not. Again, while most loudspeakers are circular in shape, some few are oval. Generally these latter are designed to fit into a confined rectangular space in order to give a larger cone area. All loudspeakers are transducers changing electrical energy into acoustic energy. The 'dynamic' or cone type consists of a moving section which is shaped somewhat like a squat truncated cone. Attached to the vertex is a short cylinder on which is wound a coil of wire, the 'voice coil', the cylinder and wire being set in a magnetic field created by a strong permanent magnet. The wide end of the cone is attached to a frame by a corrugated surround while the narrow end of the cone is held in place by another flexible fitting or 'spider'. The surround and the lower fitting act as springs which restore the cone to its equilibrium position and constrain the cone in its back and forth motion in sympathy with the alternating current fed into the voice coil. The interaction of the current and the magnetic field gives the motional drive to the whole system. A small area of cone can move only a small area of air and so to create a given intensity at a point it must vibrate over a certain amplitude. However, a large cone area will vibrate over a smaller amplitude to create the same intensity. Hence it follows that at a particular frequency, and for a given amplitude of motion of the cone, or 'throw', a larger loudspeaker will be capable of radiating a greater power. The larger cone area also results in a greater efficiency of conversion of electrical energy into sound energy. Not that the efficiency of ordinary cone loudspeakers is great--two to three per cent, or worse, is the usual figure. But loudspeakers must be 'mounted' before being of much use. If a loudspeaker is set up in space and electrical energy fed into it, some sound is heard, low frequencies are missing, the higher frequencies only are apparent. The sound condensations built up in front of the cone as the cone moves out are cancelled by the rarefactions created at the back of the cone. The first thing, then, is to separate these two waves by mounting the loudspeaker in a 'batfle'--a fiat board, the larger the better. This helps, but the low frequencies still suffer. An improvement is to mount the loudspeaker in a'box. The back wave is definitely eliminated but the size and treatment of the interior of the box is important. The box itself must be lined with sound-absorbent material to dissipate the sound energy generated inside the box. But while this procedure eliminates the back wave it also affects a characteristic of a loudspeaker. Since the mechanical system of a loudspeaker consists of a cone with a definite mass attached to a spring, we have the 'mass on the end of a spring" first-year Physics experiment--the problem of resonance. Every loudspeaker has a definite resonant frequency. While a large expensive loudspeaker may have a resonant frequency of about 25 Hz, a smaller low-priced one might have a resonant frequency of 100 Hz. But this is the frequency when the loudspeaker is left to itself. Put into a box the resonant frequency rises--the smaller the box the higher the rise. A loudspeaker becomes an even less efficient sound radiator at frequencies
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below its resonant frequency. This resonance property introduces a further problem. A loudspeaker can be lightly damped at resonance. That means that if shock excited especially with a frequency near to its resonant frequency, it will vibrate at that frequency and not at the exciting frequency! This is the main cause of 'booming bass'. The more expensive a loudspeaker the greater is the strength of the magnetic field, and the better damped the system. This means that the loudspeaker will reproduce the frequencies supplied to it and not go off on its own resonant frequency vibration when suddenly excited. One variant of the loudspeaker box is the vented enclosure. In this. the box is fitted with a hole and possibly a tunnel in the front near the loudspeaker. The springiness of the air volume in the box combined with the mass of air in the hole and tunnel are deliberately designed to give an 'antiresonance' at the same resonant frequency of the loudspeaker. By antiresonance is meant that the box deliberately opposes the relatively easy excursions of the loudspeaker at this critical frequency, and extends the useful frequency range of the loudspeaker down by about one-third of an octave. For example, if a loudspeaker of resonant frequency 50 Hz is placed in a well-designed vented enclosure its useful frequency range would be from about 40 Hz upward. If the 'vent' was blocked the resonant frequency of the system could rise to about 70 Hz. The main drawback of the vented enclosure is that the box must be of reasonable size, for example a 50 Hz resonant, 0.20 m (8 in) diam. loudspeaker requires a box of about 0.09 m 3 (3 ft 3) volume, a 0-30 m (12 in) diam. loudspeaker would need about 0-15 m 3 (5 ft a) box. But loudspeakers have radiation defects at the higher frequencies. Because they radiate waves from a finite area, the waves radiated from one side of the loudspeaker can interfere destructively with waves radiated from the opposite side. This diffraction effect results in 'beaming' of the higher frequencies and is the reason why the isolated unmounted loudspeaker radiates the higher frequencies better than the low frequencies. This beaming is more pronounced at higher frequencies and with larger loudspeakers. Roughly, a 0.30 m (12 in) diam. loudspeaker has an active radiating diameter of about 0.2? m (10.5 in). It starts to beam at frequencies above 1500 Hz. A good rule of thumb is to say that a loudspeaker starts to beam at a wavelength equal to its active cone diameter. So that if wide-angle highfrequency coverage is required of a loudspeaker it should have a small width--on the other hand for good low frequency radiation efficiency large diameter loudspeakers are required!
LOUDSPEAKERS FOR VOICE REINFORCEMENT
Column loudspeakers still offer the simplest and cheapest method of providing voice reinforcement in reverberant spaces. In the author's experience a frequency
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range of 100 to 5000 Hz provides more than adequate range for high quality reproduction. A column loudspeaker is a system where a number of loudspeakers are mounted in a long vertical box generally set high up in the front of the hall and tilted forward. Such a system of sound sources has two very useful properties. In the vertical plane the sound is directional. Normal to the loudspeakers all sources are nearly in phase, each speaker reinforces the output of the other. This reinforcement is roughly proportional to the number of loudspeakers in the column as compared with feeding the same electrical energy into just one of these loudspeakers. A further advantage is that each loudspeaker handles only a fraction of the power a single loudspeaker would be called upon to handle--and the cost of the loudspeaker is not proportional to its power handling capabilities. Loudspeaker distortion is reduced. As one moves from the normal in the vertical plane then the loudspeaker outputs get out of phase because the path difference of the sound from the farthest loudspeaker is increased compared with the nearest speaker. When this path difference is one wavelength complete destructive interference takes place and no sound energy is received there. The energy has been redirected, mostly into the centre beam. Further around from the normal the sound will increase and then decrease again in intensity, but even this increase will be at least 13 dB down on the response directly in front of the loudspeaker. The column beams the sound energy into the audience. People act as good sound absorbers, less energy is directed to the ceiling where it is not wanted anyway-and the result can be interpreted as a reduction in the reverberation time of the hall ! Or if that idea is frowned upon, then the column loudspeaker is a method of directing the sound energy so that less unwanted energy gets back to the microphone, allowing the gain of the amplifying system to be increased before positive audio feedback or howling starts. This beaming effect is a function of wavelength and so a column loudspeaker, from the mathematical viewpoint, cannot work. Of course it does work. Speech is not a series o f continuous sine waves--it is a complex mixture of many frequencies. Directivity below 250 Hz is not important--those frequencies only give body to the voice and anyway a sound amplifying system must have a control to reduce the intensity of these lower frequencies. To anticipate, a normal loss of 3 dB at about 350 Hz is a good practical standard. It is the higher frequencies which give intelligibility to speech and as the frequency rises the wavelength decreases, the column confines the high frequencies into a narrowing beam. One practical inexpensive method to overcome this is to feed the two outside loudspeakers through an inductance, whose impedance at 1000 Hz is half that of each loudspeaker. As the frequency rises the inductance allows less energy into these loudspeakers, effectively reducing the column length of the remaining loudspeakers. For example, suppose the column consists of five loudspeakers each with an impedance of fifteen ohms. Then the two outside loudspeakers are fed through an air-cored coil of 1-3 mH
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inductance. The column reduces to only three loudspeakers at the higher frequencies. A more expensive and mathematically precise method is to use in the same box another set of small loudspeakers, say four inch diameter spread over half the main column length. These are fed from a 'dividing network' which separates the sound spectrum into two bands, one say below 2000 Hz, the other above, the higher frequency band being fed into the smaller column. This is an expensive method but one used by the purists. A further hazard of a column is that each loudspeaker will also start to beam in the horizontal plane as the frequency rises. If the radiation angle to be covered is narrow then this is of not much hazard, but if the angle approaches sixty degrees or greater then large loudspeakers, e.g. 0-30 m diameter, might start to confine the higher frequencies. For wide horizontal coverage a loudspeaker width of 0.15 m should be taken as a maximum. The calculation of the column length is best covered in other papers while the design of the box to hold the loudspeakers is a paper in itself. 1.2
PROBLEM SITUATIONS IN HALLS
The discussion so far holds for a simple auditorium where everyone can 'see" the whole loudspeaker column, that is the radiation from the whole column reaches the listener without obstruction. People under a balcony may be in a 'shadow' area especially for the higher frequencies. This is a difficult problem. If the ear receives the same information by two paths differing in length by more than 17 metres the brain treats the effect as an echo. If a subsidiary loudspeaker is installed to cover this shadow area and the distance between the main speaker and the subsidiary is greater than this seventeen metres then some sort of acoustical time delay must be inserted into the loudspeaker feed system. As far as the author knows there is only one system available to introduce a time delay. This method is to record the sound on an endless magnetic tape. The tape then travels around a circle until a time equal to the required delay time has elapsed. Then the original signal passes over a play head and this second signal is amplified and fed to the 'delay' loudspeaker. The tape then travels past an erase head where the signal is removed and the tape is ready to pass over the record head again. The tape is in continual motion and must be renewed after ten to fifty hours. A rotating magnetic drum with record and play heads set just out of contact with the drum would be a better system, but one firm which did make this equipment has ceased its manufacture. Whether a hall has been designed by an eminent acoustician or not it is an excellent insurance policy to make sure there is the right space in the right place for a voice reinforcement system. The hall should be designed for the best possible unassisted speech and the electronic system should assist when it is wanted.
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R.S. CADDY AMPLIFIERS A N D P R E - A M P L I F I E R S
What electrical power is needed for a given room size? An awkward question. since the answer depends on the efficiency of the loudspeaker, the absorption of the hall and of the audience as well. The author's attitude is to see what amplifiers are available 'off the shelf' and to buy one of suitable size. A loudspeaker in a home might need anything from two to twenty watts of electrical energy input for a robust resulting sound. This is due to the prevalent design of small enclosures and inefficient loudspeakers. In the design of columns highly inefficient loudspeakers are not used and so some rule of thumb can be given for speech. There is no guide at all for ' p o p music' reproduction. These figures are for normal speech where a satisfactory level for easy listening is the desired condition. For example, a hall of seating capacity 1000 and volume about 4000 m 3 using a 1.5 m column equipped with 0.30 m high quality loudspeakers can be serviced with an amplifier with an output of l0 watts! A lecture hall seating 125 with a small 0.9 m column is serviced with a 3 watt amplifier. An 'audience" of 700 in an examination room converted from a 24 alley ten pin bowl uses a 20 watt amplifier-because it was on hand! Remember that to make a voice sound twice as loud ten times as much acoustical energy is required. Buy a standard 'off the shelf' amplifier whose power is more than adequate instead of trying to calculate the power required. Further, an increase of two in acoustical power will just be noticed by the audience. Thus the difference between l0 and 20 watts is not very significant ! So the rule of thumb i s - - u p to 200 seating capacity 8 watts, 1000 people 25 watts. above that call in the expert! But even the term '25 watt amplifier' needs explaining with the present sales talk that is used to rate amplifier outputs. Music power, I H F M and peak power are merely methods of making the power output of an amplifier look better than it really is. So when specifications call for a 20 watt amplifier it means 20 watts continuous output power. Any amplifier is merely a machine that accepts a small input power (the output of the microphone) and generates a much larger replica of that input. No machine is perfectly efficient. An amplifier will produce waste heat power that must be removed. Valve amplifiers generate more heat than transistor amplifiers by virtue of the power needed to heat the filaments, and the volume surrounding such an amplifier is normally hotter than its surroundings. That does not mean that it can be stored in a totally enclosed box--amplifiers must have adequate ventilation. Further, most valve amplifiers generate the same quantity of heat all the time they are switched on. Valve amplifiers are robust, will take reasonable abuse without failure. For example, the output can be short circuited for a few seconds without harm. even over a minute, with only a shortening of life of the output valves and rectifier--and then the valves are merely pulled out to be checked or replaced.
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Transistor amplifiers are much smaller than their valve counterparts for equal continuous power output. Most are designed in the 'class B' mode. This means they dissipate a small amount of heat when quiescent and dissipate much more heat when delivering acoustical power to the loudspeakers. Since speech consists of lots of silences the average heat which must be removed is small. But this type of amplifier is temperature sensitive. Amplifiers are rated at an ambient temperature of 20°C (68°F). On no account must they be kept in confined spaces. Further, transistor amplifiers are not robust against overload. While most amplifiers will have overload protection, few will have protection against high transient overloads. A transistor will be wrecked if a heavy current transient lasts for a few microseconds, so protective circuits must work within the 2 to 3 microsecond range. Be sure if transistor amplifiers are used in voice or any reinforcement system that overload protection of this type is provided. Transistors have to be removed and replaced by the use of soldering irons--a job for the experienced person as indiscreet application of a soldering iron can wreck a perfectly good transistor. Besides specification of power output, a statement of distortion is also necessary. For speech amplification, a distortion figure at the suggested output power of one per cent is more than sufficient, a figure of three per cent is satisfactory. Seldom will that maximum power be needed and distortion only starts to approach the rated figure as the power output rises towards maximum. Do not fall into the trap of buying too much more power output than is needed. Signal-to-noise ratio, that is the ratio of output signal to noise inside the amplifier, is always quoted at maximum power output. To use a 100 watt amplifier when a 10 watt will do merely means a decrease of 10 dB in the signal-to-noise ratio for the particular application. Further, never use an amplifier with a power rating greater than the combined stated power handling capabilities of the loudspeakers. For example, a column loudspeaker of 5 loudspeaker units each rated at 7 watts fed from a 70 watt amplifier could be literally burnt out if accidentally fed from the full power of the amplifier, while, of course, it will have indefinite life if driven from a 25 watt amplifier. However, there is a more subtle trap if a column loudspeaker with a cross-over network and separate high frequency column is used. In normal speech and music this 'tweeter' system will never handle more than one-tenth of the power fed to the lower frequency system. So this 'tweeter' section is never made to handle the power fed to the lower frequency section. Testing the column with a frequency run from low to high frequencies without reducing the power to match the capabilities of the 'tweeter' section can lead to destruction of that section of the loudspeaker system. The power amplifier should always be preceded by a tone control. At its simplest for speech reinforcement it should consist of a simple bass cut device that inserts a 3 dB loss at about 350 Hz. The technical jargon could b e - - a bass cut network with a single time constant of 450 microseconds. However, a better idea is a variable tone control which is able to provide both a gain or a loss of about 12 dB at both
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50 and 10,000 Hz, when referred to 1000 Hz. This will provide adequate bass cut at the low frequency and will be able to provide some control of the overall high frequency response of the microphone-amplifier-loudspeaker-hall combination. It is not a cure-all, but it can control a mild high frequency rise in the response of the microphone or a drop in the high frequency reverberation time of the hall. This use of the tone control will not overcome the reverberation time of the hall--it may ameliorate the condition slightly.
MICROPHONES
Microphones can be divided into three main groups, crystal or ceramic, magnetic and electrostatic. The latter are also known as condensor or capacitor microphones. O f these the crystal is not used in serious work while the electrostatic is too expensive. The electromagnetic microphone is manufactured in three main types, dynamic - - t h a t is equally responsive in all directions, cardioid--responsive mainly from the front with at least a 15 dB difference in response between the direct front position and the direct back position, and the ribbon--a microphone with equal response in the front and back directions but with zero response for signals coming from the two sides. However, this latter microphone is rare and rather expensive and can be damaged by blowing into it. So the safe choice is now limited to two types. Most modern cardioid microphones are of the dynamic cardioid type. This means that it is an omnidirectional microphone modified by the insertion at the rear of the microphone of slots and openings which give reduced sensitivity to sounds impinging on the microphone from the rear. Reasonable quality microphones are available at reasonable prices. Any lower priced microphone from the well advertised brands is satisfactory.
MICROPHONE INSTALLATION AND USE
A microphone must be connected to the amplifier by way of a cable and a preamplifier because the electrical output of the microphone is small. Magnetic microphones have either low or high (some both) output impedances. A high output impedance goes with a higher output voltage than the low impedance but high output impedance microphones cannot be used with long cables because the cable picks up unwanted electrical signals along with the wanted signal from the microphone. Using a cable longer than that supplied with the microphone--no longer than thirty feet as the maximum--is bad practice and only leads to trouble. High frequency response is also affected by the long cable. Do not use the microphone supplied with a tape recorder. They are not high quality types.
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High impedance microphones with their higher output means that less preamplifier gain is needed and therefore the preamplifier is cheaper. I f a hall is to be fitted with a well-designed loudspeaker system, it is stupid to save money on the microphone and amplifier. Wherever the microphone is separated from the amplifier then low impedance microphones equipped with balanced shielded lines connected to microphone balanced input transformers must be specified. Balanced lines means that two wires come from the microphone and the two are enclosed in a metallic braid. Neither conductor is earthed, both go to the input winding of the microphone transformer. The shielded braid is connected to the earthing terminal of the preamplifier. In this way noise-free signals can be carried over long distances and through areas where unwanted signals can be generated. Cable lengths o f 30 metres can be common. The author has run a microphone line of 200 metres without extraneous signal being introduced into the balanced cable. Cardioid microphones emphasise the lower frequencies if used for close talking. While close talking is the standard practice for 'public address systems' a minimum distance of 0.30 m (12 in) should be aimed at with a good voice-reinforcing system. If a person is, say, 0.05 m from a microphone and moves back 0.05 m then the level at the microphone will change by 6 dB. The same change in distance at 0.30 m will cause a change of l dB, an imperceptible change. Thus if the microphone is to be set at a lectern and the position of the speaker is reasonably stationary then a modern dynamic cardioid pointed toward the mouth of the speaker will be satisfactory. If people wish to stand around the microphone and they can be persuaded not to be 'behind' the microphone a cardioid will just be satisfactory. If the people must stand all around the microphone then an omnidirectional microphone must be used, with an attendant loss in gain before howling of about 5 dB. If the microphone can be placed under the loudspeaker (assuming a column type) or just behind the loudspeaker then the front-to-back discrimination of the microphone will allow the gain of the system to be raised at least 6 dB. The use of the column loudspeaker also directs the sound on to the most absorbing section of the hall, cutting down the reflected energy- and thus allowing the gain of the system to be increased--or the level of the person's voice to be decreased (unfortunately generally the latter)--without the system bursting into the familiar howl. If the speaker must walk about, for example a lecturer using a blackboard, or moving from illustration to illustration, then the best microphone is a lavalier type, that is one which hangs about the neck by means of a lanyard. While these are of the omnidirectional type the body of the wearer acts as an acoustic shield and the microphone is always at a constant distance from the m o u t h - - i f the person speaks straight ahead. This type of microphone is designed with a rising high frequency response to counter the drop in the high frequencies reaching it due to the diffraction losses at these frequencies. They also suffer from the defect that if the speaker encloses the microphone in his arms the response drops alarmingly, the high frequencies suffering most, while if the speaker is using a lectern and stands with
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the body hard against the lectern a standing wave pattern can be created which can reduce the gain of the system substantially to counter the onset of howling. When two microphones are to be used and it is demanded that they be on simultaneously the gain of the system before howling will be automatically reduced at least 3 dB, more likely 6 dB or more. I f more than one microphone is used then the obvious way is to have a microphone mixer in circuit and a person in charge of the mixer to switch the microphones on and off as required. This is an added complication.
CONCLUSION
A well-designed conservatively rated speech column system can also be used for incidental music and the playing of, say, national anthems. These latter uses will mean added mixing inputs to the amplifier systems. Remember it will not handle rock and roll and screaming vocalists. A well-designed system will provide voice reinforcement, that is the voice will be reproduced so that the audience will hear it at a natural unobtrusive level and natural personal quality. It will provide effortless long-term listening. It should not make its presence heard. At best the audience can be tricked into wondering if the microphone on the stage is really working. At worst it should provide undistorted intelligible reproduction unlike the distorted gabble associated in the public mind with the 'public address system'.
REFERENCES
1. P. H. PARKINand J. H. TAYLOR,Speech reinforcement in St Paul's cathedral, Wireless World, 58, 54-7, 109-11. 2. R. W. MUNCEYand A. F. B. NICKSON,Improving church acoustics with sound reinforcement, Architecture in Australia, 44(4) (1955) 120.
OF VOICE REINFORCEMENT IN HALLS
SYSTEMS
R. S. CADDY
Vice-Chancellor’s Unit, University of New South Wales (Australia) (Received: 30 July, 1969)
SUMMARY
The separate elements of voice reinforcement systems in halls-loudspeakers, ampl$ers, pre-amplijers and microphones-are discussed. The use of column loudspeakers, with their advantages and disadvantages, are mentioned, while special attention is paid to the problem of the choice and the installation of microphones in such systems.
INTRODUCTION
There is an ever growing demand for amplification of the human voice in clubs and halls. The reasons are various, including increasing ambient noise and a decrease in the number of people willing to project their voices. The design of voice reinforcement systems, while not an exact science, can be tackled systematically and with reasonable chance of success, if all sections of the microphone-pre-amplifierpower-amplifier-loudspeaker-hall combination are considered. This paper discusses the elements in this chain.
LOUDSPEAKERS--GENERAL
DlSCUSSION
Loudspeakers come in all shapes and sizes, from the two-inch circle of the small transistor radio to the monsters of the large cinema. The large area ‘Quad’ electrostatic loudspeaker represents another design approach. Why the variety? A loudspeaker’s primary use is to cause acoustic wave motions in air. In a transistor radio very small size is all important, in mantel model radio and console type TV Applied Acousrics
(2) (1969)--9
Elsevier Publishing Company Ltd. England-Printed
in Great Britain
260
R . S . CADDY
receivers small size is important--quality of sound reproduction is not. Again, while most loudspeakers are circular in shape, some few are oval. Generally these latter are designed to fit into a confined rectangular space in order to give a larger cone area. All loudspeakers are transducers changing electrical energy into acoustic energy. The 'dynamic' or cone type consists of a moving section which is shaped somewhat like a squat truncated cone. Attached to the vertex is a short cylinder on which is wound a coil of wire, the 'voice coil', the cylinder and wire being set in a magnetic field created by a strong permanent magnet. The wide end of the cone is attached to a frame by a corrugated surround while the narrow end of the cone is held in place by another flexible fitting or 'spider'. The surround and the lower fitting act as springs which restore the cone to its equilibrium position and constrain the cone in its back and forth motion in sympathy with the alternating current fed into the voice coil. The interaction of the current and the magnetic field gives the motional drive to the whole system. A small area of cone can move only a small area of air and so to create a given intensity at a point it must vibrate over a certain amplitude. However, a large cone area will vibrate over a smaller amplitude to create the same intensity. Hence it follows that at a particular frequency, and for a given amplitude of motion of the cone, or 'throw', a larger loudspeaker will be capable of radiating a greater power. The larger cone area also results in a greater efficiency of conversion of electrical energy into sound energy. Not that the efficiency of ordinary cone loudspeakers is great--two to three per cent, or worse, is the usual figure. But loudspeakers must be 'mounted' before being of much use. If a loudspeaker is set up in space and electrical energy fed into it, some sound is heard, low frequencies are missing, the higher frequencies only are apparent. The sound condensations built up in front of the cone as the cone moves out are cancelled by the rarefactions created at the back of the cone. The first thing, then, is to separate these two waves by mounting the loudspeaker in a 'batfle'--a fiat board, the larger the better. This helps, but the low frequencies still suffer. An improvement is to mount the loudspeaker in a'box. The back wave is definitely eliminated but the size and treatment of the interior of the box is important. The box itself must be lined with sound-absorbent material to dissipate the sound energy generated inside the box. But while this procedure eliminates the back wave it also affects a characteristic of a loudspeaker. Since the mechanical system of a loudspeaker consists of a cone with a definite mass attached to a spring, we have the 'mass on the end of a spring" first-year Physics experiment--the problem of resonance. Every loudspeaker has a definite resonant frequency. While a large expensive loudspeaker may have a resonant frequency of about 25 Hz, a smaller low-priced one might have a resonant frequency of 100 Hz. But this is the frequency when the loudspeaker is left to itself. Put into a box the resonant frequency rises--the smaller the box the higher the rise. A loudspeaker becomes an even less efficient sound radiator at frequencies
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below its resonant frequency. This resonance property introduces a further problem. A loudspeaker can be lightly damped at resonance. That means that if shock excited especially with a frequency near to its resonant frequency, it will vibrate at that frequency and not at the exciting frequency! This is the main cause of 'booming bass'. The more expensive a loudspeaker the greater is the strength of the magnetic field, and the better damped the system. This means that the loudspeaker will reproduce the frequencies supplied to it and not go off on its own resonant frequency vibration when suddenly excited. One variant of the loudspeaker box is the vented enclosure. In this. the box is fitted with a hole and possibly a tunnel in the front near the loudspeaker. The springiness of the air volume in the box combined with the mass of air in the hole and tunnel are deliberately designed to give an 'antiresonance' at the same resonant frequency of the loudspeaker. By antiresonance is meant that the box deliberately opposes the relatively easy excursions of the loudspeaker at this critical frequency, and extends the useful frequency range of the loudspeaker down by about one-third of an octave. For example, if a loudspeaker of resonant frequency 50 Hz is placed in a well-designed vented enclosure its useful frequency range would be from about 40 Hz upward. If the 'vent' was blocked the resonant frequency of the system could rise to about 70 Hz. The main drawback of the vented enclosure is that the box must be of reasonable size, for example a 50 Hz resonant, 0.20 m (8 in) diam. loudspeaker requires a box of about 0.09 m 3 (3 ft 3) volume, a 0-30 m (12 in) diam. loudspeaker would need about 0-15 m 3 (5 ft a) box. But loudspeakers have radiation defects at the higher frequencies. Because they radiate waves from a finite area, the waves radiated from one side of the loudspeaker can interfere destructively with waves radiated from the opposite side. This diffraction effect results in 'beaming' of the higher frequencies and is the reason why the isolated unmounted loudspeaker radiates the higher frequencies better than the low frequencies. This beaming is more pronounced at higher frequencies and with larger loudspeakers. Roughly, a 0.30 m (12 in) diam. loudspeaker has an active radiating diameter of about 0.2? m (10.5 in). It starts to beam at frequencies above 1500 Hz. A good rule of thumb is to say that a loudspeaker starts to beam at a wavelength equal to its active cone diameter. So that if wide-angle highfrequency coverage is required of a loudspeaker it should have a small width--on the other hand for good low frequency radiation efficiency large diameter loudspeakers are required!
LOUDSPEAKERS FOR VOICE REINFORCEMENT
Column loudspeakers still offer the simplest and cheapest method of providing voice reinforcement in reverberant spaces. In the author's experience a frequency
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range of 100 to 5000 Hz provides more than adequate range for high quality reproduction. A column loudspeaker is a system where a number of loudspeakers are mounted in a long vertical box generally set high up in the front of the hall and tilted forward. Such a system of sound sources has two very useful properties. In the vertical plane the sound is directional. Normal to the loudspeakers all sources are nearly in phase, each speaker reinforces the output of the other. This reinforcement is roughly proportional to the number of loudspeakers in the column as compared with feeding the same electrical energy into just one of these loudspeakers. A further advantage is that each loudspeaker handles only a fraction of the power a single loudspeaker would be called upon to handle--and the cost of the loudspeaker is not proportional to its power handling capabilities. Loudspeaker distortion is reduced. As one moves from the normal in the vertical plane then the loudspeaker outputs get out of phase because the path difference of the sound from the farthest loudspeaker is increased compared with the nearest speaker. When this path difference is one wavelength complete destructive interference takes place and no sound energy is received there. The energy has been redirected, mostly into the centre beam. Further around from the normal the sound will increase and then decrease again in intensity, but even this increase will be at least 13 dB down on the response directly in front of the loudspeaker. The column beams the sound energy into the audience. People act as good sound absorbers, less energy is directed to the ceiling where it is not wanted anyway-and the result can be interpreted as a reduction in the reverberation time of the hall ! Or if that idea is frowned upon, then the column loudspeaker is a method of directing the sound energy so that less unwanted energy gets back to the microphone, allowing the gain of the amplifying system to be increased before positive audio feedback or howling starts. This beaming effect is a function of wavelength and so a column loudspeaker, from the mathematical viewpoint, cannot work. Of course it does work. Speech is not a series o f continuous sine waves--it is a complex mixture of many frequencies. Directivity below 250 Hz is not important--those frequencies only give body to the voice and anyway a sound amplifying system must have a control to reduce the intensity of these lower frequencies. To anticipate, a normal loss of 3 dB at about 350 Hz is a good practical standard. It is the higher frequencies which give intelligibility to speech and as the frequency rises the wavelength decreases, the column confines the high frequencies into a narrowing beam. One practical inexpensive method to overcome this is to feed the two outside loudspeakers through an inductance, whose impedance at 1000 Hz is half that of each loudspeaker. As the frequency rises the inductance allows less energy into these loudspeakers, effectively reducing the column length of the remaining loudspeakers. For example, suppose the column consists of five loudspeakers each with an impedance of fifteen ohms. Then the two outside loudspeakers are fed through an air-cored coil of 1-3 mH
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inductance. The column reduces to only three loudspeakers at the higher frequencies. A more expensive and mathematically precise method is to use in the same box another set of small loudspeakers, say four inch diameter spread over half the main column length. These are fed from a 'dividing network' which separates the sound spectrum into two bands, one say below 2000 Hz, the other above, the higher frequency band being fed into the smaller column. This is an expensive method but one used by the purists. A further hazard of a column is that each loudspeaker will also start to beam in the horizontal plane as the frequency rises. If the radiation angle to be covered is narrow then this is of not much hazard, but if the angle approaches sixty degrees or greater then large loudspeakers, e.g. 0-30 m diameter, might start to confine the higher frequencies. For wide horizontal coverage a loudspeaker width of 0.15 m should be taken as a maximum. The calculation of the column length is best covered in other papers while the design of the box to hold the loudspeakers is a paper in itself. 1.2
PROBLEM SITUATIONS IN HALLS
The discussion so far holds for a simple auditorium where everyone can 'see" the whole loudspeaker column, that is the radiation from the whole column reaches the listener without obstruction. People under a balcony may be in a 'shadow' area especially for the higher frequencies. This is a difficult problem. If the ear receives the same information by two paths differing in length by more than 17 metres the brain treats the effect as an echo. If a subsidiary loudspeaker is installed to cover this shadow area and the distance between the main speaker and the subsidiary is greater than this seventeen metres then some sort of acoustical time delay must be inserted into the loudspeaker feed system. As far as the author knows there is only one system available to introduce a time delay. This method is to record the sound on an endless magnetic tape. The tape then travels around a circle until a time equal to the required delay time has elapsed. Then the original signal passes over a play head and this second signal is amplified and fed to the 'delay' loudspeaker. The tape then travels past an erase head where the signal is removed and the tape is ready to pass over the record head again. The tape is in continual motion and must be renewed after ten to fifty hours. A rotating magnetic drum with record and play heads set just out of contact with the drum would be a better system, but one firm which did make this equipment has ceased its manufacture. Whether a hall has been designed by an eminent acoustician or not it is an excellent insurance policy to make sure there is the right space in the right place for a voice reinforcement system. The hall should be designed for the best possible unassisted speech and the electronic system should assist when it is wanted.
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What electrical power is needed for a given room size? An awkward question. since the answer depends on the efficiency of the loudspeaker, the absorption of the hall and of the audience as well. The author's attitude is to see what amplifiers are available 'off the shelf' and to buy one of suitable size. A loudspeaker in a home might need anything from two to twenty watts of electrical energy input for a robust resulting sound. This is due to the prevalent design of small enclosures and inefficient loudspeakers. In the design of columns highly inefficient loudspeakers are not used and so some rule of thumb can be given for speech. There is no guide at all for ' p o p music' reproduction. These figures are for normal speech where a satisfactory level for easy listening is the desired condition. For example, a hall of seating capacity 1000 and volume about 4000 m 3 using a 1.5 m column equipped with 0.30 m high quality loudspeakers can be serviced with an amplifier with an output of l0 watts! A lecture hall seating 125 with a small 0.9 m column is serviced with a 3 watt amplifier. An 'audience" of 700 in an examination room converted from a 24 alley ten pin bowl uses a 20 watt amplifier-because it was on hand! Remember that to make a voice sound twice as loud ten times as much acoustical energy is required. Buy a standard 'off the shelf' amplifier whose power is more than adequate instead of trying to calculate the power required. Further, an increase of two in acoustical power will just be noticed by the audience. Thus the difference between l0 and 20 watts is not very significant ! So the rule of thumb i s - - u p to 200 seating capacity 8 watts, 1000 people 25 watts. above that call in the expert! But even the term '25 watt amplifier' needs explaining with the present sales talk that is used to rate amplifier outputs. Music power, I H F M and peak power are merely methods of making the power output of an amplifier look better than it really is. So when specifications call for a 20 watt amplifier it means 20 watts continuous output power. Any amplifier is merely a machine that accepts a small input power (the output of the microphone) and generates a much larger replica of that input. No machine is perfectly efficient. An amplifier will produce waste heat power that must be removed. Valve amplifiers generate more heat than transistor amplifiers by virtue of the power needed to heat the filaments, and the volume surrounding such an amplifier is normally hotter than its surroundings. That does not mean that it can be stored in a totally enclosed box--amplifiers must have adequate ventilation. Further, most valve amplifiers generate the same quantity of heat all the time they are switched on. Valve amplifiers are robust, will take reasonable abuse without failure. For example, the output can be short circuited for a few seconds without harm. even over a minute, with only a shortening of life of the output valves and rectifier--and then the valves are merely pulled out to be checked or replaced.
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Transistor amplifiers are much smaller than their valve counterparts for equal continuous power output. Most are designed in the 'class B' mode. This means they dissipate a small amount of heat when quiescent and dissipate much more heat when delivering acoustical power to the loudspeakers. Since speech consists of lots of silences the average heat which must be removed is small. But this type of amplifier is temperature sensitive. Amplifiers are rated at an ambient temperature of 20°C (68°F). On no account must they be kept in confined spaces. Further, transistor amplifiers are not robust against overload. While most amplifiers will have overload protection, few will have protection against high transient overloads. A transistor will be wrecked if a heavy current transient lasts for a few microseconds, so protective circuits must work within the 2 to 3 microsecond range. Be sure if transistor amplifiers are used in voice or any reinforcement system that overload protection of this type is provided. Transistors have to be removed and replaced by the use of soldering irons--a job for the experienced person as indiscreet application of a soldering iron can wreck a perfectly good transistor. Besides specification of power output, a statement of distortion is also necessary. For speech amplification, a distortion figure at the suggested output power of one per cent is more than sufficient, a figure of three per cent is satisfactory. Seldom will that maximum power be needed and distortion only starts to approach the rated figure as the power output rises towards maximum. Do not fall into the trap of buying too much more power output than is needed. Signal-to-noise ratio, that is the ratio of output signal to noise inside the amplifier, is always quoted at maximum power output. To use a 100 watt amplifier when a 10 watt will do merely means a decrease of 10 dB in the signal-to-noise ratio for the particular application. Further, never use an amplifier with a power rating greater than the combined stated power handling capabilities of the loudspeakers. For example, a column loudspeaker of 5 loudspeaker units each rated at 7 watts fed from a 70 watt amplifier could be literally burnt out if accidentally fed from the full power of the amplifier, while, of course, it will have indefinite life if driven from a 25 watt amplifier. However, there is a more subtle trap if a column loudspeaker with a cross-over network and separate high frequency column is used. In normal speech and music this 'tweeter' system will never handle more than one-tenth of the power fed to the lower frequency system. So this 'tweeter' section is never made to handle the power fed to the lower frequency section. Testing the column with a frequency run from low to high frequencies without reducing the power to match the capabilities of the 'tweeter' section can lead to destruction of that section of the loudspeaker system. The power amplifier should always be preceded by a tone control. At its simplest for speech reinforcement it should consist of a simple bass cut device that inserts a 3 dB loss at about 350 Hz. The technical jargon could b e - - a bass cut network with a single time constant of 450 microseconds. However, a better idea is a variable tone control which is able to provide both a gain or a loss of about 12 dB at both
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50 and 10,000 Hz, when referred to 1000 Hz. This will provide adequate bass cut at the low frequency and will be able to provide some control of the overall high frequency response of the microphone-amplifier-loudspeaker-hall combination. It is not a cure-all, but it can control a mild high frequency rise in the response of the microphone or a drop in the high frequency reverberation time of the hall. This use of the tone control will not overcome the reverberation time of the hall--it may ameliorate the condition slightly.
MICROPHONES
Microphones can be divided into three main groups, crystal or ceramic, magnetic and electrostatic. The latter are also known as condensor or capacitor microphones. O f these the crystal is not used in serious work while the electrostatic is too expensive. The electromagnetic microphone is manufactured in three main types, dynamic - - t h a t is equally responsive in all directions, cardioid--responsive mainly from the front with at least a 15 dB difference in response between the direct front position and the direct back position, and the ribbon--a microphone with equal response in the front and back directions but with zero response for signals coming from the two sides. However, this latter microphone is rare and rather expensive and can be damaged by blowing into it. So the safe choice is now limited to two types. Most modern cardioid microphones are of the dynamic cardioid type. This means that it is an omnidirectional microphone modified by the insertion at the rear of the microphone of slots and openings which give reduced sensitivity to sounds impinging on the microphone from the rear. Reasonable quality microphones are available at reasonable prices. Any lower priced microphone from the well advertised brands is satisfactory.
MICROPHONE INSTALLATION AND USE
A microphone must be connected to the amplifier by way of a cable and a preamplifier because the electrical output of the microphone is small. Magnetic microphones have either low or high (some both) output impedances. A high output impedance goes with a higher output voltage than the low impedance but high output impedance microphones cannot be used with long cables because the cable picks up unwanted electrical signals along with the wanted signal from the microphone. Using a cable longer than that supplied with the microphone--no longer than thirty feet as the maximum--is bad practice and only leads to trouble. High frequency response is also affected by the long cable. Do not use the microphone supplied with a tape recorder. They are not high quality types.
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High impedance microphones with their higher output means that less preamplifier gain is needed and therefore the preamplifier is cheaper. I f a hall is to be fitted with a well-designed loudspeaker system, it is stupid to save money on the microphone and amplifier. Wherever the microphone is separated from the amplifier then low impedance microphones equipped with balanced shielded lines connected to microphone balanced input transformers must be specified. Balanced lines means that two wires come from the microphone and the two are enclosed in a metallic braid. Neither conductor is earthed, both go to the input winding of the microphone transformer. The shielded braid is connected to the earthing terminal of the preamplifier. In this way noise-free signals can be carried over long distances and through areas where unwanted signals can be generated. Cable lengths o f 30 metres can be common. The author has run a microphone line of 200 metres without extraneous signal being introduced into the balanced cable. Cardioid microphones emphasise the lower frequencies if used for close talking. While close talking is the standard practice for 'public address systems' a minimum distance of 0.30 m (12 in) should be aimed at with a good voice-reinforcing system. If a person is, say, 0.05 m from a microphone and moves back 0.05 m then the level at the microphone will change by 6 dB. The same change in distance at 0.30 m will cause a change of l dB, an imperceptible change. Thus if the microphone is to be set at a lectern and the position of the speaker is reasonably stationary then a modern dynamic cardioid pointed toward the mouth of the speaker will be satisfactory. If people wish to stand around the microphone and they can be persuaded not to be 'behind' the microphone a cardioid will just be satisfactory. If the people must stand all around the microphone then an omnidirectional microphone must be used, with an attendant loss in gain before howling of about 5 dB. If the microphone can be placed under the loudspeaker (assuming a column type) or just behind the loudspeaker then the front-to-back discrimination of the microphone will allow the gain of the system to be raised at least 6 dB. The use of the column loudspeaker also directs the sound on to the most absorbing section of the hall, cutting down the reflected energy- and thus allowing the gain of the system to be increased--or the level of the person's voice to be decreased (unfortunately generally the latter)--without the system bursting into the familiar howl. If the speaker must walk about, for example a lecturer using a blackboard, or moving from illustration to illustration, then the best microphone is a lavalier type, that is one which hangs about the neck by means of a lanyard. While these are of the omnidirectional type the body of the wearer acts as an acoustic shield and the microphone is always at a constant distance from the m o u t h - - i f the person speaks straight ahead. This type of microphone is designed with a rising high frequency response to counter the drop in the high frequencies reaching it due to the diffraction losses at these frequencies. They also suffer from the defect that if the speaker encloses the microphone in his arms the response drops alarmingly, the high frequencies suffering most, while if the speaker is using a lectern and stands with
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the body hard against the lectern a standing wave pattern can be created which can reduce the gain of the system substantially to counter the onset of howling. When two microphones are to be used and it is demanded that they be on simultaneously the gain of the system before howling will be automatically reduced at least 3 dB, more likely 6 dB or more. I f more than one microphone is used then the obvious way is to have a microphone mixer in circuit and a person in charge of the mixer to switch the microphones on and off as required. This is an added complication.
CONCLUSION
A well-designed conservatively rated speech column system can also be used for incidental music and the playing of, say, national anthems. These latter uses will mean added mixing inputs to the amplifier systems. Remember it will not handle rock and roll and screaming vocalists. A well-designed system will provide voice reinforcement, that is the voice will be reproduced so that the audience will hear it at a natural unobtrusive level and natural personal quality. It will provide effortless long-term listening. It should not make its presence heard. At best the audience can be tricked into wondering if the microphone on the stage is really working. At worst it should provide undistorted intelligible reproduction unlike the distorted gabble associated in the public mind with the 'public address system'.
REFERENCES
1. P. H. PARKINand J. H. TAYLOR,Speech reinforcement in St Paul's cathedral, Wireless World, 58, 54-7, 109-11. 2. R. W. MUNCEYand A. F. B. NICKSON,Improving church acoustics with sound reinforcement, Architecture in Australia, 44(4) (1955) 120.