GB2192716A

GB2192716A - Measurement of airborne sound in buildings

Info

Publication number: GB2192716A
Application number: GB08716875A
Authority: GB
Inventors: Robin Mackenzie
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1986-07-18
Filing date: 1987-07-17
Publication date: 1988-01-20
Anticipated expiration: 2007-07-17
Also published as: DK371287D0; NO173203B; GB8716875D0; DK169797B1; GB8617566D0; GB2192716B; NO873006L; NO873006D0; DK371287A; NO173203C

Abstract

Apparatus for the measurement of the sound transmission characteristics of the structural elements of a building includes a filter to produce a predetermined nonlinear frequency variation of the relative sensitivity of the receiver and the transmitted sound output level and this variation lies within the limits shown on the graph. In a method of using the apparatus a sound transmitter is placed in a first room and an operator situated in an adjacent room measures purer levels at a number of frequencies whilst making a rotational traverse with the receiver. A similar measurement is also carried out in the transmitter room and the sound level difference between the rooms is calculated. <IMAGE>

Description

SPECIFICATION Measurement of airborne sound in buildings This invention relates to the measurement of airborne sound in buildings, and, in particular, to methods of and apparatus for the assessment of the sound insulation properties of walls and floors.

In this specification, technical terms shall have the definitions and/or meanings ascribed to them in accordance with the following list.

A-Weighted sound pressure, in pascals: The root mean square sound pressure determined by use of frequency-weighting network "A" (see l.E.C. Publication 651).

A-Weighted sound pressure, in decibels: The sound pressure level of A-weighted sound pressure is given by the formula LPA = 10 log(p,,,/P0) 2 Equivalent continuous A-weighted sound pressure level, in decibels: Value of the A-weighted sound pressure level of a continuous, steady sound that, within a specified time interval, T, has the same mean square sound pressure as a sound under consideration whose level varies with time. It is given by the formula

where LAtq7 is the-equivalent continuous A-weighted sound Pressure level, in decibels, determined over a time interval T starting at t and ending at t2; p0 is the reference sound pressure (20,pea); pA(t) is the instantaneous A-weighted sound pressure of the sound signal.

The measurement time interval is that time interval over which the squared A-weighted sound pressure is integrated and averaged.

The average sound pressure level in a room is ten times the common logarithm of the ratio of the space and time average of the sound pressure squared to the square of the reference sound pressure, the space average being taken over the entire room with the exception of those parts where the direct radiation of a sound source or the near field of the boundaries (wall, etc) is of significant influence.

This quantity is denoted by L, where p12 + p22 + , pn2 L = 10 log nip,2 where p1,p2 p are the r.m.s. sound pressures at n different positions in the room; p0 = 20,uPa is the reference sound pressure.

The A-weighted level difference is The difference in space and time average A-weighted sound levels produced in two rooms by one or more sound sources in one of them. This quantity is denoted by DA where DA = LA,1 LA,2 where LA.i is the average sound pressure level in the source room and LA2 is the average sound pressure level in the receiving room. The atandardised level difference is the level difference corresponding to a reference value of the reverberation time in the receiving room. This quantity is denoted by DOT, where DOT = D +pl 10 log(T/TO) dB where D is the level difference, T is the reverberation in the receiving room, To is the reference reverberation time.For dwellings, To is given by To = 0.5s The standardising of the level difference to a reverberation time of 0,5s takes into account that in dwellings with furniture the reverberation time has been found-nearly independently of the volume and of frequency-to be equal to 0.5s. With this standardising, DnT is dependent on the direction of the sound transmission if the two rooms have different volumes.

The standardising of the level difference to the reverberation time in the receiving room of To = 0.5s is equivalent to standardising the level difference with respect to an equivalent absorption area of: Ao = 0.32V where Ap is the equivalent absorption area, in square metres and V is the volume of the receiving room, in cubic metres. The method of measurement of airborne sound insulation in buildings involving field measurements between rooms according to frequency has been standardised in I.S.O. 140/4. 1978 Acoustics-Me-asurement of sound insulation in buildings and of building elements; Part 4: Field measurements of airborne sound insulation between rooms.

These procedures are long and complicated and a need has therefore arisen for a short test method which can provide an estimate of the single number rating.

According to the present invention there is provided a method of measurement of the sound transmission characteristics of structural elements between two rooms of a building comprising the steps of setting up a sound source in the first of said two rooms and measuring the sound level in the second of said two rooms at a plurality of different frequencies wherein the sound power level of the transmitter and the. relative sensitivity of the receiver exhibit a predeterminednonlinear variation with respect to frequency.

The method is designed for use by building control inspectors or persons undertaking a similar role to allow a fast and simple check to be carried out on the airborne sound insulation between rooms.

The invention will now be particularly described by way of example with reference to the accompanying drawings in which Figure 1 is a schematic view showing the layout of apparatus for measurements in a sound source room Figure 2 is a schematic view showing the layout of apparatus for measurements in a receiving room.

Figure 3 is a power level spectrum for combined wall/floor noise source.

Figure 4 is a power level spectrum for wall noise source, and Figure 5 is a power level spectrum for a floor noise source.

The method according to the invention is based upon the determination of the A-weighted sound level difference between adjacent rooms- without regard to the paths of transmission.

The sound level difference for the purpose of this method is that obtained from. the space and time averaged A-weighted sound levels in the source SR and receiving RR rooms respectively due- to the operation of a specific sound source in the source room. In the case of unfurnished rooms, absorption is required to be added to both the source and receiving rooms in order to minimise spectral imbalance caused by variations in room absorption at different frequencies.

In accordance with a preferred embodiment of the invention the sound source to be used for the sound level difference- measurements is a combination of loudspeaker, amplifier and random noise generator. The maximum available octave band sound power level of random noise falls within the shaded limits of Figure 2 for a combined wall/floor source and Figures 3 and 4 for separate wall and floor sources. The loudspeaker enclosure dimensions were restricted to 500mm (Height) x 350mm (Width) x 2500mm (Depth) + 10%. The maximum broadband sound power output is 120dB re. 10-52W. The loudspeaker is fitted with resilient pads on the base of the cabinet two minimise the transmission of loudspeaker vibration into the floor. Ideally, the sound source should be capable of being switched on and off remotely by FM transmitter/receiver circuits.

An absorbent pack each comprising three polyether foam (30kg/m3) panels measuring 1000mm x 500mm x 50mm is used in the source and receiving rooms (i.e. three panels for each room) to compensate fdr operator presence.

During the measurements all doors and windows of the rooms under test are closed.

The sound level meter is calibrated with a cavity-type calibrator, in accordance with the instructions-of the manufacturer, before and after measurements are made for each pair of rooms. After calibrating, the sound level meter is set for A-weighting for the remainder of the test procedure. If the room be unfurnished, an absorbent screen AS is erected similar to the layout shown in Figure 1. This layout applies whilst the operator is present in the source or receiving room. When the -operator 0 leaves the source room the screen is adjusted as shown in Figure 2 in order to compensate for the absorption provided by the operator whilst present in the room. It is important that no person other than the operator is present in the test rooms whilst the measurement is being carried out.The sound source is placed on the floor on the side of the room opposite the partly wall with the front of the loudspeaker facing into the room at an angle of 45". If the sound source equipment is fitted with separate spectra for walls and for floors, then the appropriate spectrum should be selected. When testing floors, the lower room is chosen as the. source mom. The sound output control should be adjusted so as to produce an A-weighted sound level at least 10dB above the background sound level.

A measurement is made initially in the receiving room to allow comparison with background noise level. -[f the sound level in the receiving room is less than 10dB above the background level, then a correction- is applied to the receiving room level in accordance with Table 2. The integrating sound level meter is held out at arms length with the operator standing as near to the centre of the room as possible. The microphone should be rotated FOUR times through 360 moving the arm up and down in a gentle movement during the traverse. The four rotations should be completed in a total time of approximately THIRTY seconds. (It may be necessary to extend the measurement period when using a non-integrating sound level meter).The LAeq(30 seconds) is noted and taken as the value for LA . A measurement is carried out in the source room identical to that undertaken in the receiving room, giving Lea.2.

The sound level difference, DA between the source and receiving rooms is then calculated. The appropriate value for the DnTw should be read directly from Table 1.

Although this invention has been described with reference to transmitting apparatus having a defined transmission spectrum and a receiver with an essentially flat response, it will be clear to one skilled in the art that an equivalent result may be obtained with a transmitter with a flat transmission characteristic and a receiver with a response curve which is the complement of the transmission characteristics shown in Figures 1 and 2 or both transmitter and receiver may have complementary non-linear characteristics to achieve the same overall effect.

Table 1 DA DnTw Receiving Room Condition Furnished Unfurnished+ Wal-ls Floors Added Absorbent 35 32 38 37 36 33 39 38 37 34 40 39 38 35 41 40 39 36 42 41 40 37 42 42 41 38 43 42 42 39 44 43 43 40 45 44 44 41 45 45 45 42 46 46 46 43 47 47 47 44 48 47 48 45 49 48 49 46 50 49 50 47 50 49 51 48 51 50 52 49 52 51 53 50 53 52 54 51 54 53 55 52 54 54 56 53 55 55 57 54 56 56 58 55 57 57 59 56 57 57 60 57 58 58 61 58 59 59 62 59 60 60 63 60 61 61 64 61 62 62 65 62 63 63 TABLE 2 I Difference between sound I Correction to be subtracted pressure level measured I from sound pressure level with sound source measured with sound source operating and background operating to obtain sound level alone pressure level due to sound source alone 3 3 4 to 5 2 6 to 9 1

Claims

1. A method of measurement of the sound transmission characteristics of structural elements between two rooms of a building comprising the steps of setting up a sound source in the first of said two rooms and measuring the sound level in the second of said two rooms at a plurality of different frequencies wherein the sound power level of the transmitter and the relative sensitivity of the receiver exhibit a predetermined nonlinear variation with respect to frequency.

2. A method of measurement of the sound transmission characteristics of structural elements between two rooms of a building as claimed in claim 1 wherein for combined floor and wall measurements the predetermined nonlinear variation falls substantially within the limits depicted in Figure 3 of the accompanying drawings.

3. A method of measurement of the sound transmission characteristics of structural elements between two rooms of a building as claimed in claim 1 wherein for wall measurements the predetermined nonlinear variation falls substantially within the limits depicted in Figure 4 of the accompanying drawings.

4. A method of measurement of the sound transmission characteristics of structural elements between two rooms of a building as claimed in claim 1 wherein for floor measurements the predetermined nonlinear variation falls substantially within the limits depicted in Figure 5 of the accompanying drawings.

5. A method of measurement of the sound transmission characteristics of structural elements between two rooms of a building as claimed in any one of the preceding claims wherein a correction factor is applied to said measurements when the sound level in the receiving room is less than 10dB above background sound level.

6s Apparatus for the measurement of the sound transmission characteristics of structural elements between two rooms of a building having a filter adapted to produced said non-linear variation.

7. Apparatus for the measurement of the sound transmission characteristics of structural elements between two rooms of a building substantially as herein described with reference to and as shown in the accompanying drawings.