Noise in enclosed spaces. Phil Joseph

Transcription

1 Noise in enclosed spaces Phil Joseph

2 MODES OF A CLOSED PIPE A 1 A x = 0 x = L Consider a pipe with a rigid termination at x = 0 and x = L. The particle velocity must be zero at both ends. Acoustic resonances occur when an integer number of half wavelengths can fit into the length of the pipe n L L n These values of allow us to determine the natural frequencies, or resonance frequencies of the pipe, f n f n c 0 or nc0 f n L

3 MODES OF A CLOSED OPEN PIPE A 1 A x = 0 x = L Consider a pipe with a rigid termination at x = 0 and open at x = L. The particle velocity must be zero at one end and the pressure is approximately zero at the other. Acoustic resonances occur when an odd number of quarter wavelengths can fit into the length of the pipe. ( n 1) L 4 or, L n 1 for n = 1,, 3, where L' = L + 8a/3, with a the radius of the pipe, includes an end correction. The natural frequencies of a closed-open pipe are f n c n 1) 4L' ( 0

4 PRESSURE MODE SHAPES Closed-closed pipe Closed-open pipe L L

5 ACOUSTIC MODES OF ENCLOSED SPACES One-dimensional closed pipe, length L Acoustic natural frequencies are given by for integer values of n Pressure mode shapes given by Rectangular space L 1 L L 3 Acoustic natural frequencies are given by for integer values of (l, m, n) or Pressure mode shapes given by L nc c f n 0 0 L x n A n cos 3 1 0,, L n L m L l c f n m l ,, n m l m n l f f f f ,, cos cos cos L x n L x m L x l A n m l Limitations: irregular geometry (but the first few modes can be estimated quite well) wall/window flexibility (tends to lower cavity natural frequencies if panel modes are much lower than cavity modes).

6 ROOM MODES The modes of a room can be defined by the number of nodal lines in each direction.

7 ROOM MODES The modes of a room can be defined by the number of nodal lines in each direction.

8 MODES OF A RECTANGULAR CAVITY Example: rectangular space.3 m long x 1.3 m wide x 1.1 m high. First 4 modes: mode (1,0,0) : Hz mode (0,1,0) : Hz / /

9 MODES OF A RECTANGULAR CAVITY Example: rectangular space.3 m long x 1.3 m wide x 1.1 m high. First 4 modes: mode (1,0,0) : Hz mode (0,1,0) : Hz mode (,0,0) : Hz mode (1,1,0) : 150. Hz

10 MODES OF A CAR INTERIOR calculated using acoustic FEM also have modes in width direction (this is a D model) real modes more complicated than simple model but size of vehicle is main parameter

11 NOISE CONTOL APPROACHES IN ROOMS Consider those techniques used to reduce noise levels other than noise control at source and direct protection of the listener's hearing. 1. The use of absorptive treatments (i) (ii) In "Sabine" spaces In "Non-Sabine" spaces. The use of barriers (screens) (i) (ii) (iii) In free field conditions In the presence of ground reflections In enclosed spaces 3. The use of acoustic enclosures

12 DIFFUSE FIELD THEORY Applicable to "large" rooms (dimensions L 1, L,,, L 3 > l) of low aspect ratio (L 1 ~L ~ L 3 ) and low average absorption coefficient. DIFFUSE FIELD MODEL (Morse & Ingard, 1968). Represent sound field as an assemblage of plane waves, each travelling in a direction specified by the angles,,. n q Local time averaged energy density Local time averaged energy intensity w 0 0 q 0 0 p c sin d d I / 0 0 p q c 0 0 sin cos d d

13 DIFFUSE FIELD ASSUMPTIONS 1. Local time averaged energy density uniform throughout room (w independent of position).. Local time averaged acoustic intensity uniform throughout room and I independent of direction. (I independent of position and direction.) Under these conditions I 0 c0 p w 4 4 c 0 3. Absorption properties of a wall surface can be represented by an absorption coefficient,, such that the local rate at which energy is absorbed per unit area is given by I 0 c0w p 4 4 c 0

14 ENERGY BALANCE EQUATION The diffuse field assumptions lead to the energy balance given by W R Ap d V p 40c0 dt 0 c 0 ( A S ) POWER IN = POWER LOST + RATE OF CHANGE OF ROOM ENERGY Steady State Solution Transient Solution Time taken for 4W c 0 A Ac 0 p0 exp 4V p R 0 10log10 p / p0 60dB p t is the reverberation time T 0.161V A - "Sabine's Formula"

15 LIMITATION OF SABINE'S FORMULA Note that with perfectly absorbing surfaces ( A S S) with a finite reverberation time is predicted. Eyring (1930) points out that Sabine's theory assumes a continuous decay of room energy, i.e. energy loss through walls assumed to occur at same instant as energy loss from room volume. Eyring's formula assumes energy loss occurs at each discrete reflection of a sound ray. No. of reflections/s = " Mean c0 free path" p 0 ( c 0 p (1 ) S / 4V ) T 0.161V S ln(1 ) - "Eyring's formula"

16 DIRECT AND REVERBERANT SOUND Sabine's theory does not account for the form of the sound field close to the source. Assume the source power W is partitioned between the "direct" and "reverberant" fields W W D W R rate at which energy is lost on first reflection rate at which energy is fed to "diffuse" reverberant field Assume W D W W R ( 1 )W

17 REDUCTION IN REVERBERANT FIELD SOUND PRESSURE DUE TO THE ADDITION OF ABSORPTION From the steady state solution to the energy balance equation p 4W 0c0 (1 S enables the calculation of the reduction in reverberant field SPL due to increased absorption SPL 10log 10 ) (1 ) 1 (1 ) 1 e.g SPL 9.7 db SPL 3.7 db

18 STEADY STATE SOLUTION REVERBERATION RADIUS Under steady state conditions, there is an exact balance between the rate at which energy is produced by the source, and the rate at which it is dissipated by the enclosure boundaries W c x 4 S p

19 REVERBERATION RADIUS DIRECT SOUND FIELD At positions sufficiently close to the source, the direct (or free field) dominates the reverberant field. An alternative expression for the source sound power may therefore be written as W p r c D 4r Therefore pr D Wc 4r

20 REVERBERATION RADIUS TOTAL MEAN SQUARE PRESSURE Total mean squared pressure in the enclosure is therefore p r p r p r D i.e., p r 1 Wc 4r 4 S

21 REVERBERATION RADIUS The distance from a source at which the direct field and volume averaged reverberant field are equal is called the reverberation radius r r. An expression for r r may be obtained by equating the two terms in square brackets, r r S 16

22 THE USE OF DIFFUSE FIELD THEORY FOR PRACTICAL PREDICTIONS Typical comparison of measured sound field and that predicted from measured 'T' in a large "flat" space.

23 THE IMAGE MODEL Assumes "geometrical acoustics"; enclosures consist of plane boundaries with specular reflections. Example for two parallel plane surfaces: Image sources used to represent specular reflections

24 COMPUTATIONAL COMPLEXITY The computation time mainly is dependent on the desired maximum order of image sources. The number of sources up to the order i of a room with n w walls increases exponentially with i : N N IS IS i 1 k 0 n nw n w w ( n ( n w w 1) 1) i k 1 e.g. for an impulse response duration of 400 ms for a room consisting of 30 walls enclosing a volume of 15000m 3 (average number of reflections per second = 5.5s -1 ) 14 ~ N IS -but the ratio of N IS to the number of visible sources is typically 10 1!

25 THE RAY TRACING MODEL Geometrical acoustics is again assumed. The paths of a number of specularly reflected "particles" are traced. Particles travel at the speed of sound and carry a certain energy, which is reduced on each reflection. The presence of a particle is detected within a finite (usually spherical) detector volume; this enables the construction of the "energy impulse response" of the enclosure.

26 HYBRID RAY TRACING / IMAGE MODELS The ray tracing algorithm can be used to identify efficiently the visible sources in an image model giving considerable savings in computational cost. Example of a highly resolved energy impulse response obtained with the new sound particle-image-source method

27 THE PERFORMANCE OF ACOUSTIC SCREENS Diffraction by a semi-infinite rigid barrier; exact solution given by Sommerfeld (1896). Can also apply Kirchhoff diffraction theory Assume p s, p s / n determined by source in absence of screen Surface, S Assume p s, p s / n zero over surface Source n p S g ps n p s g ds n where g is the free space Green function g the observer from points on the surface S. e jkr s / 4r s and r s is the distance to

28 DIFFRACTION OF A PLANE WAVE BY A RIGID BARRIER

29 Attenuation (db) EXPERIMENTS DUE TO MAEKAWA (1968) Compares results using pulse tone technique with Kirchhoff diffraction theory. Results plotted against Fresnel number N = / where is "path length difference".

30 BARRIER PERFORMANCE ON REFLECTING GROUND SURFACES Maekawa suggests the use of image technique for allowance of ground reflections. SPLs calculated at observer and "image observer" using appropriate "path length difference". Total SPL calculated using energy addition at observer position.

31 BARRIER PERFORMANCE ON REFLECTING GROUND SURFACES Typical experimental results; (1 khz) Without screen With screen 1 h (m) Attenuation (db) In general, large errors (~ 10 db) at low frequencies (< 500 Hz)

32 ATTENUATION DUE TO BARRIERS OF FINITE WIDTH Maekawa suggests energy addition of component waves diffracted by "half infinite" open surface and two "quarter infinite" open surfaces. Ground reflections again accounted for using image observers. Source Observer Experimental results given reasonable agreement ( 5 db) with predictions based on this method

33 THE PEFORMANCE OF BARRIERS IN ENCLOSED SPACES Comprehensive series of measurements undertaken by Kurze (1978) in open plan offices and industrial buildings. Barrier Height (m) Insertion loss of barriers in open plan offices (db) Source/ Receiver distance (m) Mean values and standard deviations in 1 khz octave band; based on 4 measurements.

34 THE PERFORMANCE OF BARRIERS IN ENCLOSED SPACES After Kurze (1978) Insertion loss of barriers in factory halls (db) barrier height room height (source - receiver distance) (room height) Mean values and standard deviations in 1 khz octave band. See also the work of Czarnecki (1978) who advocates the use of the "absorbing belt" in the region above a barrier.

35 ACOUSTIC ENCLOSURE PERFORMANCE: A DIFFUSE FIELD MODEL Assume: Source power output unaffected by the presence of the enclosure. Total sound power radiated Total sound power absorbed W W R D S IdS S IdS W W Total source power R S W s W R W where the overbar indicates an average value. D

36 ENCLOSURE INSERTION LOSS Can be defined in terms of the reduction in sound power radiated IL W If it is assumed that low frequencies 10log , W W S R SWL S SWL R (as is usually the case in practice, e.g. at 0.) WR W Thus the expression for the insertion loss reduces to 1 IL W 10log10 10log 10 S IL W Rfield 10log 10 where R field is the field S.R.I. Thus note that the maximum insertion loss is equal to the (field incidence) sound reduction index of the enclosure material and this is obtained when. 1

37 CLOSE FITTING ENCLOSURES: JACKSON'S THEORY Jackson (196, 1966) considers a model of an infinite plane vibrating surface parallel to an infinite plane enclosure surface assumes only plane wave propagation. Expression given for enclosure insertion loss SPL S - SPL R IL p c M k / 10log 10 1 sin kl sin klcos kl 0c0 0c0

38 SOUND TRANSMISSION THROUGH APERTURES The transmission coefficient has been deduced for plane wave incidence on "thick" or "thin" apertures. For thin apertures a, a, a, SRI 0dB Theories of Lamb (1931), Rayleigh (1877), Gomperts (1964), Wilson and Soroka (1965), Boukamp (1954), all predict SRI 1.5dB Experiments of Mulholland and Parbrook (1967), and theory of Cook (1957), predict SRI 1.5dB For thick apertures SRI is highly frequency-dependent; approximate theory due to Wilson and Soroka (1965) Experimental results for a circular aperture between two reverberant chambers