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Sound Insulation Evaluation Using Transfer Function Measurements by Rodolfo Venegas C., Marco Nabuco and Paulo Massarani

Reprinted from

JOURNAL OF

BUILDING ACOUSTICS Volume 13 · Number 1 · 2006

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BUILDING ACOUSTICS · Volume 13 · Number 1 · 2006 Pages 23 – 31

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Sound Insulation Evaluation Using Transfer Function Measurements Rodolfo Venegas C.1, Marco Nabuco2 and Paulo Massarani3 1Departamento

2, 3Acoustic

de Acústica, Universidad Pérez Rosales, Brown Norte 290, Ñuñoa, Santiago, Chile, [email protected]

Testing Laboratory, Inmetro, Duque de Caxias, 25 250-020, RJ, Brazil, [email protected], [email protected] (Received 13 October 2005 and accepted 11 January 2006)

ABSTRACT The accepted methods for measurement of sound insulation between rooms, audiometric cabins and other adjacent closed spaces deal with average sound pressure measurements and random sound excitation. The acoustic field can be as diffuse as found in reverberation chambers or form well-determined stationary waves in rectangular rooms. The use of random noise excitation requires averages in time to reduce the expected inherent uncertainties. The use of techniques, such as MLS or swept sine excitation, can avoid time consuming averaging processes and reduce measurement. With the advent of new front-end devices and signal analyzers it is now relatively easy to obtain acoustic transfer functions between two points in space, which can be in adjacent enclosures. With these transfer functions, it is possible to obtain the level differences, required by the international standard, as suggested in a new draft ISO document. This paper presents results obtained for the insulation of a cabin, tested in a reverberation chamber, using random noise and swept sine excitation. An analysis is given of the viability of using fewer positions of sound sources when measuring the transfer function. Repeatability tests, for both methods, are also presented.

1. INTRODUCTION The classical way to measure sound level difference involves random excitation and space-time averaging. The use of random signals, for such tests, can lead to large standard deviations. Several measurement positions are required1 and a large number of source/microphone combinations2. During the last ten years, deterministic methods have been developed to obtain impulse responses and/or acoustic transfer functions of rooms3,4. A new international standard is being developed to standardize those methods for different room and building acoustics applications. Depending on the approach, or if it is desired to work in the time or frequency domain, it is better to use MLS or the swept sine technique, to

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get the impulse or the frequency response in a specific position in the room, respectively. In the time domain, ISO/DIS 182335 recommends the Schroeder energy integration method to obtain the sound pressure difference level between rooms. In the frequency domain, the direct transfer function is sufficient to get the same level difference. This paper describes a method to obtain transfer functions from a swept sine technique and shows insulation data for a cabin measured in a reverberation chamber. A comparison is given with results using the classical random noise method. Data repeatability for both methods6 is presented and an analysis of the viability of using fewer positions for the source, for the transfer function method, is also presented. 2. THEORY The basic principle of sound insulation determination is to obtain the sound level difference, which is then corrected for room characteristics, represented by the average reverberation time. As mentioned before, the level difference can be obtained in two ways. The classical method uses random excitation. The transfer function method uses MLS excitation and a decorrelation technique or swept sine excitation and deconvolution technique. For all methods the basic principle involves the sound level difference D between the source room and receiving room (in dB). D = SPLS − SPLR

(1)

Where: —–— SPLS: Space-time average sound pressure level in the source room, in dB —–— SPLR: Space-time average sound pressure level in the receiving room, in dB. The most general and simple definition of a transfer function, H, is the relationship between the input and output of any linear system. For this case, the input is the sound source, which includes the signal generation system, power amplifier, loudspeaker and any other device before the sound source acoustic centre7. The room output includes the microphone, associated pre-amplifier, cable and conditioner amplifier and any other accessory which are necessary for the proper operation of the microphone. The acoustic transfer function can be used to determine the average sound pressure level in a source room as follows,

(

SPL S ( f ) = 10 log H S ( f )

2

)

 X( f ) 2  + 10 log   2  p0 

Where: —– HS ( f ): Spatial average acoustic transfer function in the Source room, in dB. X( f ): Room acoustic input, in N/m2. p0: Reference sound pressure, in N/m2. f : Frequency in Hz.

(2)

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For the receiving room, equation (2) can be rewritten as,

(

SPL R ( f ) = 10 log H R ( f )

2

)

 X( f ) 2  + 10 log   2  p0 

(3)

—– Where HR( f ) is the spatial average acoustic transfer function in the receiving room, in dB. From equations (2) and (3), the general expression for the sound pressure level difference as a function of the acoustic transfer functions, is obtained as,

(

D ( f ) = 10 log HS ( f )

2

)

(

− 10 log H R ( f )

2

)

(4)

The same expression is defined and more rigorously in ISO/DIS 182235, and is:   ∞ 2  ∫ H S ( f ) df   D = Ls − LR = 10 log  −∞  ∞ 2  ∫ H R ( f ) df    −∞

(5)

Where LS is the sound pressure level in the source room, LR is the sound pressure level in the receiving room, HS( f ) is the acoustic transfer function between a microphone and sound source position in the source room and HR( f ) is the acoustic transfer function between a microphone position and sound source position in the receiving room. 3. MEASUREMENT PROCEDURE A typical audiometric cabin was installed inside a 196 [m3] reverberation chamber. Six fixed microphone positions were installed as required in ISO 119571 and ISO 37412 regarding the distances to the sound source, chamber and cabin surfaces (See figure 1). Two microphones were installed inside the cabin, 0.6 m from each other and 1.2 m from the cabin floor. The two methods were used to determine the cabin sound pressure level differences in the frequency range 100 to 10000 Hz. 3.1. Random noise method A reverberant sound field was generated in the chamber by a dodecahedron loudspeaker in two positions. The generated sound level had a continuous spectrum as specified by ISO 119571. The space-time average room sound level was obtained by averaging the sound pressure level for 1 minute and between six fixed microphone positions. The average sound level in the cabin was obtained from two microphones, also for 1 minute each. The complete measuring system was calibrated with a 1000 Hz sound level calibrator at the beginning of each measurement. The total time of the measurement was about twenty minutes. For each trial of three independent measurements, two sound

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Figure 1.

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Sound Insulation Evaluation Using Transfer Function Measurements

Measurement configuration; with source and microphone positions.

source positions were used. The environmental conditions were measured and remained constant during each set of measurement. The sound level generated inside the room was around 80 dB for the frequency range of interest. 3.2. Transfer function method A swept sine input to the dodecahedron and the input signal is analyzed by the software, which also controlled the set-up and recorded each microphone transfer functions directly and simultaneously. The spatial average acoustic transfer function was obtained as a moving average and an energetic sum. The total measurement time was about two minutes and the tests were repeated three times. 4. EXPERIMENTAL RESULTS Figure 2 shows the median sound level difference obtained with both methods. The curves agree, and the level differences between both values (see Figure 3) are of the order of ± 0.4 dB. Figure 4 shows the spatial sound pressure level standard deviation for both methods and the ISO 37412 required values. From the data shown both methods are valid from 160 Hz up to 10000 Hz. For all tests, the measured background noise level remained below the measured signal by more than 40 dB in the frequency range of the tests. The repetition deviation standard was calculated (see figure 5). The swept sine technique showed slightly higher values, although both methods showed very low

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Figure 3.

Difference between the data obtained by the two methods.

values. Of course, it is necessary to test the same cabin in other laboratories and in field conditions to establish a first uncertainty interval. In order to investigate how many positions of the sound source are necessary with the transfer function method, figure 6 shows the measurements obtained in position 1 of the sound source and figure 7 shows the results obtained in position 2 of the source (See figure 1). Figure 8 shows the differences obtained for the six independent measurements between the classical random noise method and the transfer function method. Again, the differences are of the order of 0.4 dB over the frequency range of interest.

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Figure 4.

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Figure 5.

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Three-time repetition deviation for both methods.

The spatial deviation standards, for each measurement with the transfer function method are presented in figure 9. All measurements are valid from 160 Hz to 10,000 Hz according to the tolerance given by the standard ISO 37412, except for the measurement number 1 in the position 1 in the 1 kHz band, where the spatial deviation standard is greater by 0.1 dB. Nevertheless, we can assume that only one sound source position is required, to further reduce the measurement time.

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Sound level differences obtained in three independent measurements for position 1 of the sound source.

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Sound level differences obtained in three independent measurements for position 2 of the sound source.

5. CONCLUSIONS The transfer function method, using a swept sine excitation, proved to be a powerful tool. The time spent, to perform one complete test, was approximately two minutes, for two source positions.

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Spatial standard deviation obtained in six independent measurements with the transfer function method.

It has been demonstrated that one source position gives acceptable results. Tests were not conducted in non specialist rooms, where the background noise levels are normally higher than in reverberation chambers. The ability of this technique to improve the signal to noise ratio, therefore was not tested.

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The calibration using the simplest sound calibrator proved to be enough for this investigation propose. Therefore problems at low frequencies can be responsible for the differences verified at frequencies smaller than 160 Hz. To assure higher precision levels it should be necessary to correct the obtained data for all frequencies from the microphone calibration chart7. ACKNOWLEDGEMENT The authors acknowledge Universidad Pérez Rosales for the use of his set-up at the Acoustic Testing Laboratory of Inmetro. The authors also acknowledge Vibranihil Amortecedores de Vibrações for the use of the audiometric cabin prototype. REFERENCES 1. ISO 11957 – Acoustics – Determination of sound insulation performance of cabins – Laboratory and in situ measurements, 1996, 2004. 2. ISO 3741 – Acoustics - Determination of sound power levels of noise sources Precision methods for broad band sources in reverberation rooms, 2003. 3. Rife D.D., Vanderkooy, J., “Transfer-Function Measurement with MaximumLength Sequences”, J. Audio Eng. Soc., Vol. 37, No. 6, 1989 June. pp. 419–444. 4. Müller, S., Massarani, P., “Transfer Function Measurement with Sweeps”, J. Audio Eng. Soc., Vol. 49, No. 6, 2001. pp. 443. 5. ISO/DIS 18233 – Acoustics – Application of new measurement methods in building acoustics, 2004. 6. Venegas, R., Nabuco, M, Massarani, P., “Sound insulation evaluation using Transfer Function Measurements”, Proc. 34rd Internoise, Rio de Janeiro, Brazil, 2005. 7. Massarani, P., Nabuco, M., Venegas, R., “Level adjustment for multi-channel impulse response measurements in building acoustics”, Proc. 34rd Internoise, Rio de Janeiro, Brazil, 2005.

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the international standard, as suggested in a new draft ISO document. This paper .... 8000. 10000. 1/3 Octave Centre Frequency (Hz). Level difference (dB).

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