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Microphone probe for sound measurement in flames and inaccessible places
MICROPHONE PROBE FOR SOUND MEASUREMENT IN FLAMES A N D INACCESSIBLE PLACES R. A.
KNEZEK,* L. L. FAULKNER and M. J. MORAN
Department of Mechanical Engineering, The Ohio State University, Columbus, Ohio (USA)
S UMMA R Y In this paper details o f the design, construction and performance o f a microphone probe for sound measurement in flames and inaccessible places are presented.
INTRODUCTION
In the course of conducting a research programme aimed at determining the effect of an imposed sound field on natural gas flames, it became necessary to measure sound pressure levels in the vicinity of a flame. ~ The purpose of this paper is to describe the design of the microphone probe developed for this purpose and to present sample frequency response data.
MICROPHONE PROBE
The microphone probe developed is shown in Fig. 1. The probe consists of a sensing tube, a microphone holder and an anechoic termination. The sensing tube is constructed of 1/4 in OD stainless steel tubing approximately 22 in in length. A small port is located near one end of the tube to allow sound pressure waves travelling along the sensing tube to act on the microphone diaphragm. The microphone holder (Fig. 2) is constructed of Lucite and surrounds the port in the sensing tube. It is fixed and sealed in that position with epoxy. The microphone holder serves the purpose of supporting the microphone over the sensing port and sealing against air leaks from the sensing port. Reflection of sound pressure waves passing the microphone diaphragm is prevented by an anechoic termination of the sensing tube. * Present address: General Dynamics Corporation, Fort Worth, Texas (USA). 79 Applied Acoustics (10) 0977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
80
R. A. KNEZEK, L. L. FAULKNER, M. J. MORAN
Fig. 1.
Microphone probe.
The anechoic termination consists of a 50 ft length of 1/4 in OD copper tubing in the form of a coil 8 in in diameter. The end of the coil has been pinched and soldered shut to prevent gas flow past the microphone and external sound waves from being transmitted to the microphone. Sound waves passing the microphone location propagate into the length of tubing and are sufficiently attenuated to render the waves reflected from the sealed end insignificant compared with the incident waves at the microphone location.
PROBE PERFORMANCE
The performance of the microphone probe is defined here as the loss in measured sound pressure level when the probe is used, compared with the sound pressure level determined by means of the microphone without the probe, in the same sound field and location of sensed pressure. The calibration of the probe was performed in an anechoic chamber using a variable-frequency oscillator and a speaker as the sound source for the probe calibration, and a frequency analyser and a level recorder to obtain the frequency response. A sample of the data obtained is shown in Fig. 3.
PROBE FOR SOUND MEASUREMENT IN FLAMES
81
L Cavity for Microphone~
Probe Tube
~\ ....
/
To Anechoic Termination
rm Fig. 2.
Microphone adapter.
In calibrating the probe, the substitution method was used wherein the microphone without the probe was placed perpendicularly to the acoustic wave fronts directly in front of a speaker and approximately 4 ft away. The measurements were then repeated with the same microphone inserted in the microphone probe, the tip of the probe being in the exact location as the microphone diaphragm in the first test. Measurements were made with the probe at angles of 0, 30, 60 and 90 degrees from the speaker axis. All data fall within the curves shown on Fig. 3. The loss of sensitivity at high frequency can be traced to the loss in the microphone free-field response for grazing incidence, which applies for the assembly of Fig. 2. 2
DISCUSSION
A microphone probe similar to that shown in Fig. 1 has been used by Blackshear et al. a to measure combustion screech; however, performance of the probe in terms of absolute sound pressure level versus frequency was not reported. Also, in the Blackshear study, the probe was used for relative amplitude measurements of large amplitude acoustic pressures compared with the present application to low
82
R. A. KNEZEK, L. L. FAULKNER, M. J. MORAN
--4--~ I-~-H +! .... 1 - 4 - ~
20
50
I00
200
...........
500
~i
I000
......
2000
o-
5000
I0000
-
20000
Frequency, Hz
Fig. 3. Performanceof the microphone probe. amplitude acoustic waves in the audible frequency range. Further, the probe system utilised a small diameter hole to connect a probe tube to a microphone cavity, an approach that could possibly result in acoustic resonances at high frequencies. This situation was eliminated in the system being reported upon here because the microphone diaphragm is directly exposed to the sound waves in the probe tube. Snap-on probes for remote sound pressure measurement are available commercially. 4 However, the performance of these probes is limited because of standing waves in the tubes created by reflections from the microphone and the need for manually inserting varying amounts of sound-absorbing material. There is always the danger that the sound-absorbing materials might shift and change the operating characteristics of the probe and it is not uncommon for these types of probe to change acoustic characteristics with time. The noteworthy characteristics of the probe developed in the study reported on here are high sensitivity and the absence of resonances in the audio-frequency range. Another important feature is its rigid construction which is not subject to change in performance with use, as are units employing glass fibre or other soft acoustic materials. Finally, the system shown in Fig. 1 is adaptable to a liquid cooled probe by providing an annulus around the sensing tube as a coolant passage.
PROBE FOR SOUND MEASUREMENT IN FLAMES
83
ACKNOWLEDGEMENT
This research was supported by a grant from the American Gas Association, Arlington, Virginia 22209, USA.
REFERENCES 1. R. A. KrqEZEK, An investigation of the influence of sound on flow stability, flame noise and nitrogen oxide levels in natural gas flames. Doctoral dissertation, The Ohio State University, 1974. 2. DAVID N. KEAST, Measurements in mechanical dynamics, McGraw-Hill Book Company, New York, 1967, p. 86. 3. P. L. BLACKSHEAR,W. D. RAYLE and L. K. TOWER, Study of screeching combustion in a 6-inch simulated afterburner, NACA TN 3567, 1955. 4. Short Form Catalogue, Sound vibration and data analysis instrumentation, B&K Instruments, Inc., Copenhagen, Denmark, 1974.
KNEZEK,* L. L. FAULKNER and M. J. MORAN
Department of Mechanical Engineering, The Ohio State University, Columbus, Ohio (USA)
S UMMA R Y In this paper details o f the design, construction and performance o f a microphone probe for sound measurement in flames and inaccessible places are presented.
INTRODUCTION
In the course of conducting a research programme aimed at determining the effect of an imposed sound field on natural gas flames, it became necessary to measure sound pressure levels in the vicinity of a flame. ~ The purpose of this paper is to describe the design of the microphone probe developed for this purpose and to present sample frequency response data.
MICROPHONE PROBE
The microphone probe developed is shown in Fig. 1. The probe consists of a sensing tube, a microphone holder and an anechoic termination. The sensing tube is constructed of 1/4 in OD stainless steel tubing approximately 22 in in length. A small port is located near one end of the tube to allow sound pressure waves travelling along the sensing tube to act on the microphone diaphragm. The microphone holder (Fig. 2) is constructed of Lucite and surrounds the port in the sensing tube. It is fixed and sealed in that position with epoxy. The microphone holder serves the purpose of supporting the microphone over the sensing port and sealing against air leaks from the sensing port. Reflection of sound pressure waves passing the microphone diaphragm is prevented by an anechoic termination of the sensing tube. * Present address: General Dynamics Corporation, Fort Worth, Texas (USA). 79 Applied Acoustics (10) 0977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
80
R. A. KNEZEK, L. L. FAULKNER, M. J. MORAN
Fig. 1.
Microphone probe.
The anechoic termination consists of a 50 ft length of 1/4 in OD copper tubing in the form of a coil 8 in in diameter. The end of the coil has been pinched and soldered shut to prevent gas flow past the microphone and external sound waves from being transmitted to the microphone. Sound waves passing the microphone location propagate into the length of tubing and are sufficiently attenuated to render the waves reflected from the sealed end insignificant compared with the incident waves at the microphone location.
PROBE PERFORMANCE
The performance of the microphone probe is defined here as the loss in measured sound pressure level when the probe is used, compared with the sound pressure level determined by means of the microphone without the probe, in the same sound field and location of sensed pressure. The calibration of the probe was performed in an anechoic chamber using a variable-frequency oscillator and a speaker as the sound source for the probe calibration, and a frequency analyser and a level recorder to obtain the frequency response. A sample of the data obtained is shown in Fig. 3.
PROBE FOR SOUND MEASUREMENT IN FLAMES
81
L Cavity for Microphone~
Probe Tube
~\ ....
/
To Anechoic Termination
rm Fig. 2.
Microphone adapter.
In calibrating the probe, the substitution method was used wherein the microphone without the probe was placed perpendicularly to the acoustic wave fronts directly in front of a speaker and approximately 4 ft away. The measurements were then repeated with the same microphone inserted in the microphone probe, the tip of the probe being in the exact location as the microphone diaphragm in the first test. Measurements were made with the probe at angles of 0, 30, 60 and 90 degrees from the speaker axis. All data fall within the curves shown on Fig. 3. The loss of sensitivity at high frequency can be traced to the loss in the microphone free-field response for grazing incidence, which applies for the assembly of Fig. 2. 2
DISCUSSION
A microphone probe similar to that shown in Fig. 1 has been used by Blackshear et al. a to measure combustion screech; however, performance of the probe in terms of absolute sound pressure level versus frequency was not reported. Also, in the Blackshear study, the probe was used for relative amplitude measurements of large amplitude acoustic pressures compared with the present application to low
82
R. A. KNEZEK, L. L. FAULKNER, M. J. MORAN
--4--~ I-~-H +! .... 1 - 4 - ~
20
50
I00
200
...........
500
~i
I000
......
2000
o-
5000
I0000
-
20000
Frequency, Hz
Fig. 3. Performanceof the microphone probe. amplitude acoustic waves in the audible frequency range. Further, the probe system utilised a small diameter hole to connect a probe tube to a microphone cavity, an approach that could possibly result in acoustic resonances at high frequencies. This situation was eliminated in the system being reported upon here because the microphone diaphragm is directly exposed to the sound waves in the probe tube. Snap-on probes for remote sound pressure measurement are available commercially. 4 However, the performance of these probes is limited because of standing waves in the tubes created by reflections from the microphone and the need for manually inserting varying amounts of sound-absorbing material. There is always the danger that the sound-absorbing materials might shift and change the operating characteristics of the probe and it is not uncommon for these types of probe to change acoustic characteristics with time. The noteworthy characteristics of the probe developed in the study reported on here are high sensitivity and the absence of resonances in the audio-frequency range. Another important feature is its rigid construction which is not subject to change in performance with use, as are units employing glass fibre or other soft acoustic materials. Finally, the system shown in Fig. 1 is adaptable to a liquid cooled probe by providing an annulus around the sensing tube as a coolant passage.
PROBE FOR SOUND MEASUREMENT IN FLAMES
83
ACKNOWLEDGEMENT
This research was supported by a grant from the American Gas Association, Arlington, Virginia 22209, USA.
REFERENCES 1. R. A. KrqEZEK, An investigation of the influence of sound on flow stability, flame noise and nitrogen oxide levels in natural gas flames. Doctoral dissertation, The Ohio State University, 1974. 2. DAVID N. KEAST, Measurements in mechanical dynamics, McGraw-Hill Book Company, New York, 1967, p. 86. 3. P. L. BLACKSHEAR,W. D. RAYLE and L. K. TOWER, Study of screeching combustion in a 6-inch simulated afterburner, NACA TN 3567, 1955. 4. Short Form Catalogue, Sound vibration and data analysis instrumentation, B&K Instruments, Inc., Copenhagen, Denmark, 1974.