Generation and control of noise in water supply installations: Part 1: Fundamental aspects

Applied Acoustics 16 (1983) 325-346

Generation and Control of Noise in Water Supply Installations: Part I: Fundamental Aspects H. V. F u c h s Fraunhofer-Institut fiir Bauphysik, 7000 Stuttgart 80 (West Germany) (Received: 1 June, 1981) S UMMA RY This paper, the first in a series of three, is aimed at improving our understanding of, and communication about, problems of noise in water supply installations. Part 1 presents a discussion of the relevant noise measuring standards, their usefulness, applicability and shortcomings. The importance of measuring the waterborne sound emission is stressed and a simplified test procedure is recommended. This is particularly suited for use in noisy environments such as are frequently found in industry. Active control of the noise excitation and propagation processes requires some knowledge of the most relevant hydrodynamic and acoustic parameters. These, together with the discussion of the various sound source mechanisms to follow in Part 2, may help in the design of quiet taps and throttling devices.

INTRODUCTION AND BACKGROUND A long tradition exists at the IBP of measuring and investigating noise generated by appliances and equipment used in water supply installations. The earlier work by G6sele and Bach, 1 and G6sele and Voigtsberger 2-4 concentrated on measuring techniques and standards applicable to the noise of water taps, its dependence on operational parameters such as the flow rate or valve clearance and ways of reducing the transmission of noise along the pipe. 325 Applied Acoustics 0003-682X/83/$03"00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

326

H . V . Fuchs

More recent activities 5'6 were aimed at a better understanding of reactive silencers for reducing the waterborne sound transmission along the water column inside the pipe. Specific types of silencer were also studied as integrated elements of water taps. Various kinds of noise generating (or regenerating) mechanisms became apparent in these studies which have hitherto not been described in any detail. Since a considerable industrial interest exists in reduction schemes for this type of noise, several new activities are now being launched to further our understanding in this field. The present paper is intended as a basis for this work by updating the relevant material. It was found worthy of publication only because the open literature on the subject was found to be very scarce. Part 1 of this review contains the more fundamental aspects of the noise caused by water supply installations. A somewhat novel waterborne sound measuring and testing procedure is described as an alternative to the valid measuring standards. A number of scaling laws are presented for the noise emitted by different types of flow acoustic sources. These may help in comparisons of experimental results obtained for varying flow configurations. They also enable a reduction of the data to a few physically relevant parameters. Part 2 is concerned with a variety of different source mechanisms to be found in valves and taps. A m o n g these, two different types of valve noise are discussed along with that due to cavitation processes. Charts are presented which may help to avoid the latter occurring in practice. The dominant r61e of flow-excited acoustic resonances in pipes and cavities is discussed as the main reason why frequency spectra do not scale according to the conventional hydroacoustic scaling laws. Part 3 deals with silencing devices in fluid bearing pipes with special emphasis on waterborne sound. A transmission loss prediction scheme is based on the 'relative compliance' of silencer elements as the characterizing parameter. Theoretical predictions are compared with experimental results for simple expansion chambers and entrapped air cushions. This paper, in a sense, takes up an earlier discussion by Heymann, 7 published in this Journal, on acoustic performance tests and parameters for fluid piping system components. Many of his practical considerations in the design of acoustic test loops for hydraulic systems are equally valid when dealing with water supply lines in dwellings. It may be worth mentioning in an English Journal that the noise from

Control o f noise in water supply installations: Part 1

327

water supply appliances is considered a much more severe problem in countries like Germany where sanitary installations make use of relatively high water pressures, enabling flow rates of more than 60 litres/min where this is requested.

M E A S U R I N G T E C H N I Q U E S FOR THE NOISE D U E TO WATER SUPPLY INSTALLATIONS Any noisy situation, indoors or outdooors, is generally made up by three sets of characteristic data: (a) The sound power emission of the sources. (b) The transmission characteristics of the sound paths. (c) The noise immission at working or living places. Tackling a noise problem usually starts by setting certain limits for: (a) The sound power, P, emitted. (b) The minimum transmission loss (TL) required. (c) The maximum sound pressure level (SPL) acceptable. Legislative procedures are normally confined to the last mentioned immission data. How to adhere to these limits is left to the technicians or acousticians. If they really want to tackle a problem they need to know both the emission and transmission data. These, however, require very sophisticated measuring techniques which are laid down in a great number of standards for the various types of noise sources and transmission paths.

Regulations and standards For houses and dwellings the German standard DIN41091 sets the limiting airborne SPLs and TLs of all kinds of transmission elements. This may suffice for most of the externally or internally generated noises, but it is totally inadequate with respect to noise due to the water supply and central heating systems, for the following reasons:

(a) The transmission paths for these noise sources are different. (b) This noise is particularly annoying since it carries intimate information and cannot be influenced by the user to make it sound quieter at night as can be done, for example, with a radio or TV set.

328 (c)

H . V . Fuchs

Water taps and the like are rigidly installed noise sources and, as such, are not easily damped or replaced by the occupiers of a house.

It was therefore necessary to pay special attention to these noise emitters and develop a special measuring technique and certification procedure for them. Today, almost any new product, before it can go onto the market, has to pass the following noise examinations: (1) Three specimens of the new product are sent to one of the specially approved testing establishments. (2) The noise emission of these specimens is measured under laboratory conditions. (3) A test report containing the results is sent to a special office (the Institut fiir Bautechnik, IfBt, Berlin). which alone can issue a certificate qualifying the tested element as a 'low noise' device. (4) This certificate enables the producer to label his product correspondingly. The DIN 52 218

The laboratory test arrangement is shown in Fig. 1. The test object (1) is mounted on a test pipe (2) which is rigidly coupled to a solid test wall (3)

,

4 10

5

I ~-~=-

÷

I

/ -8

3

E~7~

Fig. 1. Test arrangement according to DIN 52218, Part 1.9 1--Test object; 2--test pipe; 3--test wall; 4~measuring room; 5--water supply pump; 6--water tank; 7 silencers; 8--manometer; 9--ttow meter; 10~microphone.

Control of noise in water supply installations: Part 1

329

by four metallic clamps. Water is supplied by a pump (5) the noise of which is carefully kept away from the test wall. The noise emission of the test object is then measured as an (averaged) airborne SPL (L) in the measuring room (4), or, even better, its octave band spectrum (L,) between 125 and 4000Hz (a range common in building acoustics). The test rigs thus installed according to the standard, and the noise levels measured, will depend a great deal on: (a) the length of the pipes, (b) the type of clamps used, (c) the weight and size of the wall and (d) the size and absorption characteristics of the measuring room. The whole test arrangement is therefore calibrated by means of a noise standard, the so-called 'Installation Noise Standard' (IGN) or (INS) (Fig. 2). This is a flow noise source which has been exactly specified in Figs

6---~

,g,.I

1

2 Fig. 2. Installation noise standard (INS) as specified in DIN52218, Part 1, and proposed for ISO 3822, Part 1. 1--bushing (1 in); 2--gasket; 3--threaded insert; 4 ~ orifice plate (4 orifices, 2.5 mm in diameter); 5--nozzle body (1 orifice, 5 mm in diameter); 6~flow direction.

4 to 9 of the DIN 52 218, Part 1 (1976). 9 Attributed to this INS is a specific octave-band spectrum, Lso., (Fig. 10 of the same standard) which represents an average of numerous airborne sound measurements in real dwelling-houses at a pressure difference of Ap = 3 bar against the ambient Po. When mounted on the test pipe, the difference, Ks,, between this spectrum and the spectrum levels, Ls., measured in the laboratory:

Ks =Ls -Lso"

(1)

is then used to correct the measured valve noise spectrum, L~., to yield: La~" [dB] = L. - Ks.

(2)

H. V. Fuchs

330

From this normalized spectrum one may easily obtain the A-weighted valve noise level by the standard procedure:

(3)

La~ [dB(A) ] = 10 log~-[ 10~L"_ r,. +kta~,)/lOdB n

This whole evaluation procedure, which is depicted in graphical form in Fig. 3, becomes particularly simple when the spectrum Ls, is merely

50" dB

/.0 .I

.

f

E L-

~ 30-

\

I/)

~ 20"

.J

0

125

250 500 lk Mid f r e q u e n c y f

2k Hz /-,k -,

Fig. 3. Airborne noise spectrum evaluation procedure according to DIN 52 218. 9 C), Lso. standard (reference); x, Ls, standard (laboratory); + , standard simulated (laboratory); - - , Ln test object.

shifted by a constant value, K s, against the reference spectrum K s = L s - Lso = L s , - Leo"

= const

Lso:

(4)

Such a situation may be simulated by employing a variable filter set in order to equalize the two spectra. With the aid of such a filter the valve noise level may then be measured simply as: La6 = L - K s

(5)

in decibels or, preferably, in dB(A). Although the IBP played a decisive r61e in introducing this measuring standard and despite the fact that the DIN 52 218 has recently found its

Control o f noise in water supply installations: Part 1

331

international equivalent in the ISO/DIS 3822 (in preparation), we are presently developing and testing a completely new measuring procedure. The reasons for this are as follows: (1) The installation of the test facilities according to the standards presently valid requires considerable investment. This may be acceptable to the testing establishments as well as for bigger companies. Smaller companies, however, very often cannot afford it. (2) Even if one takes pains to build a large measuring room it often proves very difficult indeed to achieve sufficiently low background noise levels, particularly in the noise environments, which are typical in industry; Lnoise <

20 dB(A).

(3) Since the noise emitted has been transmitted through the water column, the test pipe, the clamps and the test wall into the measuring room, these measurements allow only a more or less indirect approach to the genuine source characteristics. It is certainly true that the method enables reproducible and fully comparable measurements to be made. But it is less suitable when details of the excitation spectrum are to be investigated more thoroughly. Waterborne sound in water supply installations

There was a time when water taps used to be very noisy appliances. A frequent reason for this was vibrations of the metallic parts which were being excited by the throttling mechanism. This structure-borne sound could be directly transmitted through the pipe to the walls. In modern designs of taps such an obvious failure will very rarely be found. Here the sound is primarily generated by turbulent flow pressure fluctuations which excite the water column within the pipe. The corresponding pressure fluctuation levels: L = 20 lOg~o;

/~o = 20#Pa

(6)

are typically around 150dB for a relatively quiet tap. The installation noise standard generates about 170 dB at Ap = 3 bar. These pressure

332

H. V. Fuchs

fluctuations can propagate in an almost undamped manner over large distances. From previous investigations at the IBP 1 we know that it is this waterborne noise which is responsible for airborne sound measured in the laboratory tests described above, as well as for the noise in dwellings. The following tests illustrate how powerful this noise is.

(1) First, an air-filled pipe is excited by hammering its walls. Curve b of Fig. 4 shows that the resulting structureborne noise may very effectively be damped by lagging the pipe with sand over a distance of 2m. (2) If, on the other hand, the water-filled pipe is excited by a tap, the sand lagging has practically no effect on the airborne noise spectrum in the measuring room (Fig. 5). We conclude that the sound emitted by a tap is primarily waterborne. That portion of the acoustic energy transmitted through the pipe walls (which would undoubtedly be damped by external lagging) must be negligibly small.

,% ......

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dB

/

10

l 40 s o

- 10 100

200

500

1000

20OO f Hz

5OOO

Fig. 4. Damping effect of a sand lagging. 1 (a) waterborne noise (tap). (b) Structure borne noise (hammer).

30 tO0

_

200

.__

500

tO00

2000 f

5000 Hz

Fig. 5. Noise generated by a tap. ~ (a) without sand lagging. (b) with sand lagging.

Control of noise in water supply installations: Part 1 .8and

dB

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50 (1¢ 40

i I

,i |

,

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a

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b

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13

Damping of pipe wall vibrations at a frequency of 900 Hz by (a) sand lagging, (b) sand lagging plus waterborne sound silencing.l

(3) This is once again illustrated in Fig. 6, curve a, by now measuring the pipe wall vibration level, LvR, as excited by a tap. The lagging is seen to reduce the vibrational amplitude by approximately 20 dB. But it is equally true that over only a short distance behind the lagging the amplitude is built up again to exactly the same level as before• The only way of lagging the pipe to effectively reduce valve noise is by concentrating it on the area where the pipe is mounted on the wall, as can be seen from Fig. 7. After having demonstrated the prominent r61e waterborne sound plays in installation noise, it is clear that one wants to measure these pressure fluctuations directly by either small transducers mounted flush with the inner pipe wall or by hydrophones coupled to the water column by special adapters. With the sensitivity, E, of such (preferably piezo-electrical) transducers given in V/bar the pressure levels, L, may be deduced from the respective voltage levels, Lu, after: L = L v - 20 log E + 74 dB where: U L U = 10 log Uo ,

U o = 1/~V

(7)

n

,rJ W

,~\\\"k'~\\\\",~

I

I

60 dB ~

t,o

i ~/}

..J

tOO

200

500

1000 2000 f Hz

5000

Fig. 7. Damping of valve noise by lagging the pipe over the whole length where it is mounted on the test wall.1 (a) With sand lagging. (b) Without sand lagging. 3

12

10

Fig. 8. Schematic of the simplified test rig for measuring the waterborne sound directly.S 1--Test object or INS; 2--valve; 3--manometer; 4--base for pressure transducer; 5-base for accelerometer; 6~pressure transducer; 7--preamplifier; 8--measuring amplifier; 9--variable filter set; 10--water tank; 11--pump; 12--pressure reducing valve.

Control of noise in water supply installations." Part 1

335

170 dB 160

150

=

,,--I

m

"W,O

J%

/

\

\

-,

130

/\

120

c .8° I ~, '1'~0

100

90 125

250

500

1000

2000

4000 Hz 8000 f

Fig. 9.

Waterborne sound spectra generated by various noise sources, Ap = 3 bar. Mixing valve; - - O - - , quiet outlet resistance;--+--, background; . . . . . b a c k g r o u n d (electrical); - - 0 - - , installation noise standard; - - O - - , form of reference spectrum. 9

--x--,

Simplified test rig Concentration on the waterborne sound emission of appliances and equipment in water supply systems simplifies the whole test procedure considerably. For convenience, the actual test pipe is chosen to be about 1 m long. For testing mixing valves two of such test pipes are installed in parallel and the signals of two pressure transducers are added before being processed electrically in a standard manner (Fig. 8). Figure 9 shows octave spectra generated by various noise sources. For all of them the signal-to-noise ratio may be seen to be very high. It may still be increased by employing transducers with an even lower electrical noise floor. Also shown is the form of the reference spectrum according to the standards. Of course, one may now go through a similar test

336

H . V . Fuchs

dB (A) 40

/

,~ 3o

o

..-,.,I

20 10

//

0

/ 10

20

30

40 dB(A)

LAG Fig. 10. Comparison of the noise emission of various taps measured as waterborne sound (LA~,.) and as airborne sound according to D I N 52 218 (LAG). 4 O , Ap = 3 bar. O , Ap = 5 bar.

procedure as laid down in the DIN 52218: (a)

Define the octave band level differences relative to the reference spectrum of DIN 52218, Part 1. (b) Correct the spectrum of the test object accordingly. (c) Determine the A-weighted overall noise level, LA~, as usual.

If our hypothesis of waterborne sound dominating the noise emission of the various test objects was valid, the results obtained should not differ from those measured according to the standards. In fact, Fig. 10 shows a one-to-one relationship for a number of such comparisons.* It may be of interest that similar tests have also been performed by CETIM, France. ~o Applicability of the new test procedure

The new test procedure has the following advantages over the former: (1)

It requires a minimum of investment. It is not necessary to build a special soundproof measuring room. The test rig itself costs less than 5000 D M and can be supplied by the IBP. (2) With the test rig mounted on a mobile trolley and the water supplied through flexible hosepipes, it may be employed extremely flexibly. (3) The test procedure is so simple that an acoustically unskilled person can be trained in a few days to do the measuring.

Control o f noise in water supply installations: Part 1

(4)

(5)

337

The m e t h o d is exceptionally safe against contamination by b a c k g r o u n d noises. The measurements m a y therefore be carried out even in extremely noisy environments. This makes it particularly suited to production control applications in industry. Since it enables direct investigations into the genuine source mechanisms, the m e t h o d is equally applicable in research and development problems.

Fig. 11. Simple test rig for measuring waterborne sound in sanitary pipes (a water tap is mounted on the right of the photograph).

Fig. 12. Radiator valve under test.

338

H. V. Fuchs

Figure 11 shows a view of the simplified test rig for measuring waterborne sound generated by sanitary installation equipment. As well as water taps one may test all kinds of fluid throttling device like, for example, radiator valves (Fig. 12). CHARACTERISTICS OF NOISE GENERATED IN WATER SU PPLY INSTALLATIONS We have already pointed out that the noise is primarily fluid-borne. It has also been made clear that this noise is generated hydrodynamically. It is certainly not due to the boundary layer noise as a result of the water flow through straight or (as usual) softly bent pipes. The noise in question is entirely due to the unsteady flow in the various kinds of throttling device (constrictions or obstacles). We may therefore begin our description with the fluid dynamics of such flows. Non-dimensional parameters We may start by listing the variables. The static pressure, Pw, on the pipe certainly drives the flow through the throttling device or outlet resistance with, normally: 0 < Ap = Pw - P o < 5 bar

In this range the mean density of the fluid may be taken as: Pw = const

throughout. We are thus left with the flow rate Q (litres/min). However, with a little experience one knows that, for all important processes in the flow, the truly relevant parameter is the flow velocity: 0 < V = f ( A p ) < 60m/s

at that location where the noise is generated. These variables may be completed by a set of geometric parameters l(mm). In a situation where one is unable to predict details of the flow analytically one is well advised to look out for non-dimensional parameters in order to be able to at least classify the flow a little. We may start with the classical flow parameter, the Reynolds number: Re -

plV

(8)

Control o f noise in water supply installations." Part 1

339

where ~/= viscosity of the fluid. Under atmospheric conditions we thus have: Water

Air

Re w = 1031(mm) V (m/s);

Re L = 691(mm) V(m/s)

Rew ReL

-

-

-

14.5.

We can be sure that for the operating conditions in which we are interested the flow in the water supply system is turbulent throughout; Re w >

10 3

and particularly so at the smallest cross-sections inside a tap. We may, under these circumstances, forget Re w as a relevant parameter (as is usually the case in flow noise problems). The other well-known non-dimensional flow parameter is the Mach number: V Ma = -c

(9)

where c = sound speed in the fluid. Water (Cw = 1400 m/s)

A i r (cL = 350 m/s)

Maw = 7 x 10-4V(m/s);

Ma L = 3 x 10-3V(m/s),

MaL = 4 Maw Needless to say that, with: Ma w =

10 -2 -

10 -3

~ 1

The Mach number, too, has no effect on the structure of the flow inside a water valve. This normally means that the fluid m a y be considered as incompressible. F r o m a flow noise point of view, however, the fluid must necessarily be considered as (if only weakly) compressible, otherwise there would be no sound generation or propagation at all. Small Mach numbers then simply indicate that the efficiency by which sound is generated by this kind of fluid dynamic process must be very low indeed. Ma w ~ 1 may also tell us something about the character of the sources to be expected.

340

H. V. Fuchs

If flow instabilities play a r61e in our problem these are usually assumed to occur at frequencies which scale on the Strouhal number: St =--fl V

(10)

With St typically around 0.2 or 0.3 these might, for I between 1 and 10 m m and Vbetween 1 and 60 m/s, fall well into the relevant range of frequencies between 30 and 10 000 Hz. Nevertheless, we have reason to believe that the spectra of waterborne sound in water supply installations rarely scale on a Strouhal number basis. This will be elaborated in Part 2 when we discuss resonant mechanisms. Water supply appliances are considered as flow noise sources, the frequency characteristics of which are not only affected b y - - b u t almost entirely determined through--all kinds of flow-acoustic interactions. The last parameter that we feel should be considered in our problem is the Helmholtz number: He

jl

I

- MaSt

(11)

This is always very small: He w ~ 1 even when the dimensions, l, of the valve as a whole are considered in relation to the sound wavelength, 2 w, in the water. This means that the sound-producing appliances have to be taken as one compact source even when more than one component takes part in the emission process. Hydrodynamic

scaling laws

After having defined the relevant scaling parameters for a problem, one may, by a semi-empirical approach, find the so-called scaling laws from which one m a y deduce certain features of a flow or, more specifically, of a throttling device. Given, for example, a turbulent incompressible flow around any kind of obstacle or through a nozzle or a grid (Fig. 13), the pressure, or, more specifically, the throttling coefficient, ~: Ap - pV2/2

Pl - P 2 pV2/2 = const (1,...)

(12)

is always found to be a constant depending on the respective geometry of

Control o f noise in water supply installations. Part 1

341

the obstacle, nozzle or grid. (This should be indicated by the symbols in brackets.) In a tap, for example, the pressure drops at typically two locations, at (1) the valve seating and (2) the outlet resistance: AP=Pl--Po

=J(Pl--Pz)+(Pz--Po)=APl +AP2

(13)

with:

~P2 pwV2/2 - ~l(s,...);

py/2

=

Since a number of resistances in series simply add to give the total resistance: AP - pwVZ/2 - ~ + ~2

(14)

one may investigate both components separately. Which one dominates depends entirely on s or, what is the same, on the operating conditions: =f(s .... )

(15)

Figure 13 shows the flow characteristics: Ap = f ( Q ) -,, Q2 ,,~ v 2

(16)

for a valve seating and a standardised outlet resistance: a bundle of five tubes, each 3 m m in diameter and about 30 cm long (Fig. 14). A second hydrodynamic relationship, Bernoulli's law, should also be borne in mind: in a narrow constriction of a given geometry the velocity, Ul, is a function of the pressure drop, Apl,ithere. Consequently, the flow rate has to increase approximately linearly with the area of this constriction: Q _~ n d s u 1 ,~ s ;

Apl = const

(17)

This trend is indicated by curve a in Fig. 15. Curves b and c start deviating from the line of proportionality where Ap~ is no longer constant but drops. This drop in Apl is due to an increasing fraction of the constant total pressure, Ap, being taken over with increasing Q by Ap2 at the outlet resistance.

H . V . Fuchs

342

.i/ bar

/

// /

_//<

#

1

ZI /

/

Q.


~"

./:

,

/; / 6

~1 10

20 I/rain

Q ~

" ~

"

*

Pl

~

APl

',/'212

:

Pl - P2 - -

c~v2/2

40

60

.

P0

P2

;

AP2

=

¢2

o

V2/2

-

P2

- Po

n v2"/2

Fig. 13. Definition of throttling coefficients with respect to a tap model. ©, ~2 flow characteristics of the outlet resistance, ~1 flow characteristics of the valve seating; 0 , s = 0 . 4 m m ; × , s = 1 . 4 m m ; + , s = 10.4ram.

343

Control o f noise in water supply installations." Part 1

A

,

Fig. 14.

IU6L

Quiet outlet resistance (five tubes) according to a DIN52218, Part 3, modification proposal of April, 1981. I/min 50

3O

2O ,4 ...o..-

~0

o,-'"

r'.... S'.

/ 5

O,t

0,2

0,3

q5

2

mm

Fig. 15. Typical throttling characteristics of a tap (Ap =const.). 3 (a), with no outlet resistance; (b) with small outlet resistance; (c) with large outlet resistance.

344

H.V. Fuchs

Type A.

of

Fluctuatinq

source

Acoustic

power

stresses: P ~ 0 12 V 8 / c 5 Pm

O

.

Fluctuating

(Ma)5

forces: p

- ~ 12 V 6 /c 3 Pm

C.

Fluctuating

masses:

(Ma) 3

P -- 0 12 V 4 / c - Pm

(Ma)

I

£p -- i V 2

D.

Fluctuating

losses:

P

~

~,AV 2 Pm

Pm : A p A V £P ~ i< V

Fig. 16. Hydroacoustic scaling laws.11,12

Ma

Re

Control of noise in water supply installations: Part 1

345

Hydroacoustic scaling laws F r o m the hydrodynamic relationships we may already conclude that one should not plot acoustic data as a function of Ap or Q as these were seen not to be the most relevant flow parameters. Instead, we may recall the fundamental aeroacoustic theories which have come up with scaling laws for the acoustic power P radiated by different types of flow noise source (Fig. 16). For details refer to reference 11. Sources of the kind A, B and C are usually treated together. They are all provoked by turbulence, the most prominent representatives being 'jet noise' and 'vortex-shedding noise'. One may imagine fluctuating-mass type sources participating in what is sometimes termed 'air-frame noise' when Helmholtz resonators are built into the fuselages of aircraft. A flow noise source of kind D would represent the limiting case where the pressure drop across the throttling element is a linear function of the velocity, V, or the flow rate, Q. The mechanical power, P,,, would, under such circumstances, be: P,. = ApA V,.~ pA V2

(18)

Pm "~ P 12V3

(19)

as opposed to:

in cases (A) to (C) (Fig. 16). The possibility of such a low Reynolds number flow noise source is discussed in reference 12 for air exhaust silencers of fine-grained porous materials made for use in pneumatic pipe systems. We shall come back to this question when dealing with low-noise outlet resistances used with high-perf0rmance water taps.

REFERENCES 1. K. G6sele and M. R. Bach, Die Schallausbreitung in Installationsleitungen und ihre Verminderung, Gesundheits-Ingenieur, 80 (1959), 1-6. 2. K. G6sele and C. A. Voigtsberger, Armaturenger/iusche und Wege zu ihrer Verminderung, Gesundheits-lngenieur, 89 (1968), 129-35 and 168-72. 3. K. Gfsele and C. A. Voigtsberger, Grundlagen zur Ger/iuschminderung bei Wasserauslaufarmaturen, Gesundheits-Ingenieur 91 (1970), 108-17. 4. K. G6sele and C. A. Voigtsberger, Vereinfachte Anordnung zur Priifung des Ger/iuschverhaltens von Armaturen, Sanitiir- undHeizungstechnik, 3 (1979),

189-94.

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H. V. Fuchs

5. C. A. Voigtsberger and H. V. Fuchs, Messung des von Sanitfir-Armaturen erzeugten Wasserschalls, IBP Mitteilung, 37 (1979). 6. H. V. Fuchs and C. A. Voigtsberger, Wasserschall-D/impfer, IBP Mitteilung, 52 (1980). 7. F. J. Heymann, Acoustic performance tests and parameters for fluid piping system components: A critical evaluation of the state of the art, Applied Acoustics, 4 (1971), 79-101 and 155-73. 8. DIN 4109: 'Schallschutz im Hochbau' Part 5 (1979). 9. DIN 52 218: 'Priifung des Ger/iuschverhaltens yon Armaturen und Ger/iten der Wasserinstallation im Laboratorium', Mel3verfahren und Priifanordnung; Part 1 (1976). 10. G. Bernard, Qualit~ acoustique des robinets: Une nouvelle m&hode de mesure applicable par tousles robinetiers, Centre Technique des Industries M~caniques (CETIM)--lnformations, 52 (1976). 11. H. V. Fuchs and A. Michalke, Introduction to aerodynamic noise theory, Prog. Aerospace Sci., 14 (1973), 227-97. 12. H. V. Fuchs and K. GieBelmann, Ger~iuscherzeugung bei der Durchstr6mung por6ser Materialien. Fortschritte der Akustik, VDEVerlag, Berlin, 1981.