The objective of this paper is to measure sound isolation according to ISO 140-4:2008 between two classrooms of the Universidad Nacional de Tres de Febrero located in Caseros, Argentina.
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1 UNTREF, Sound Engineering, Acoustic’s Instruments & Measurements July 2013, Argentina SOUND ISOLATION MEASUREMENT APPLYING ISO 140-4:2008 RAMÓN FACUNDO 1 1 Univesidad Nacional de Tres de Febrero, Sound Engineering, Caseros, Argentina. [email protected]1. INTRODUCTION Sound isolation is important for modern life style. Since people moved from rural areas to cities and started to live in residential buildings, their privacy and independence started to depend on sound isolation among other things. Silence is needed for a good quality of life, but the freedom of making noise is too. The way of measuring isolation between two neighboring spaces has been standardized in ISO 140-4 1 . This norm sets a method for isolation measurement of airborne noise between two separated spaces. The objective of this paper is to measure sound isolation according to ISO 140-4:2008 between two classrooms of the Universidad Nacional de Tres de Febrero located in Caseros, Argentina. 2. ISO 140-4 This standard specifies a method to measure in situ the airborne sound isolation between two places in diffuse field condition. It returns a value of isolation dependent of frequency. The method consists on generating a known sound pressure level in one room (emitter) and to measure the transmitted energy in a contiguous room (receiver). Sound sources must be omnidirectional and the noise generated should contain equal energy in all the audible octave bands. It must be at least 10 dB over the background noise at the receiver point. The ISO 140-4 defines a list of parameters described in the following sections. 2.1. Average sound pressure level (L) This value is the logarithmic spatial average of sound pressure level in the room (see eq. 1). ! = 10 !"# ! ! 10 ! ! /!" ! !!! (1) Where ! ! is the sound pressure level measured at one single point of the room. The level must be measured as L eq of minimum 6 second of integration. It is recommended to measure at least 5 points (n>5) separated by at least 0.7 m between them. It is also recommended to locate the microphones farther than 0.5 m from any wall and 1 m of the sound source. This is in order to measure a diffuse field condition. The average level must be obtained in both rooms, emitter and receiver. And it must be obtained in third octave band. 2.2. Difference of levels (D) It is the difference between spatial average level measured in the emitter room and the one measured in the receiver room. ! = ! ! − ! ! (2) Where ! ! is level in the emitter and ! ! in the receiver room. 2.3. Standardized difference of levels (D n ) Because the receiver room can absorb some of the transmitted energy, it was decided to standardize an absorption area A 0 that will compensate the final result. Otherwise, all the loss of energy will be adjudicated to the isolation and values would be grater than they should be. ! ! = ! − 10 log ! ! ! (3) Where A is the absorption area of the receiver room calculated with Sabine’s equation (see eq. 4), and A 0 is the reference absorption area (see eq. 5). ! = 0.16 ! ! (4) ! ! = 0.32 ! (5) Where V is the volume of the receiver room in cubic meters and T is its reverberation time obtained applying ISO 354 2 .
Transcript
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UNTREF, Sound Engineering, Acoustic’s Instruments & Measurements July 2013, Argentina
SOUND ISOLATION MEASUREMENT APPLYING ISO 140-4:2008
RAMÓN FACUNDO1
1 Univesidad Nacional de Tres de Febrero, Sound Engineering, Caseros, Argentina. [email protected]
1. INTRODUCTION
Sound isolation is important for modern life style. Since people moved from rural areas to cities and started to live in residential buildings, their privacy and independence started to depend on sound isolation among other things. Silence is needed for a good quality of life, but the freedom of making noise is too.
The way of measuring isolation between two neighboring spaces has been standardized in ISO 140-41. This norm sets a method for isolation measurement of airborne noise between two separated spaces.
The objective of this paper is to measure sound isolation according to ISO 140-4:2008 between two classrooms of the Universidad Nacional de Tres de Febrero located in Caseros, Argentina.
2. ISO 140-4
This standard specifies a method to measure in situ the airborne sound isolation between two places in diffuse field condition. It returns a value of isolation dependent of frequency.
The method consists on generating a known sound pressure level in one room (emitter) and to measure the transmitted energy in a contiguous room (receiver). Sound sources must be omnidirectional and the noise generated should contain equal energy in all the audible octave bands. It must be at least 10 dB over the background noise at the receiver point.
The ISO 140-4 defines a list of parameters described in the following sections.
2.1. Average sound pressure level (L)
This value is the logarithmic spatial average of sound pressure level in the room (see eq. 1).
! = 10 !"# !
!10!!/!"!
!!! (1)
Where !! is the sound pressure level measured at one single point of the room. The level must be measured as Leq of minimum 6 second of integration.
It is recommended to measure at least 5 points (n>5) separated by at least 0.7 m between them. It is also recommended to locate the microphones farther than 0.5 m from any wall and 1 m of the sound source. This is in order to measure a diffuse field condition.
The average level must be obtained in both rooms, emitter and receiver. And it must be obtained in third octave band.
2.2. Difference of levels (D)
It is the difference between spatial average level measured in the emitter room and the one measured in the receiver room.
! = !! − !! (2)
Where !! is level in the emitter and !! in the
receiver room. 2.3. Standardized difference of levels (Dn)
Because the receiver room can absorb some of the transmitted energy, it was decided to standardize an absorption area A0 that will compensate the final result. Otherwise, all the loss of energy will be adjudicated to the isolation and values would be grater than they should be.
!! = ! − 10 log !
!! (3)
Where A is the absorption area of the receiver
room calculated with Sabine’s equation (see eq. 4), and A0 is the reference absorption area (see eq. 5).
! =0.16 !!
(4)
!! = 0.32 ! (5)
Where V is the volume of the receiver room in cubic meters and T is its reverberation time obtained applying ISO 3542.
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2.4. Standardized difference of levels (DnT)
It is basically equal to the previously described standardized value, but instead of compensate with an absorption area it compensates with the reverberation time. The reference reverberation time (T0) is 0.5 s.
!!! = ! − 10 log!!!
(6)
2.5. Apparent reduction index (R’)
Under the assumption of diffuse sound field in both rooms, this parameter can be calculated as follows:
!! = ! + 10 log!! (7)
where S is the separator element’s surface in square meters and A is the equivalent absorption area of the receiver room calculated with equation (4).
3. METHODOLOGY
Two contiguous classrooms were measured. Namely room 301 and 302 of Universidad Nacional de Tres de Febrero (see fig. 1).
Figure 1: Classrooms scheme.
Two source position at the emitter room and three
microphone positions at each room were measured (see fig. 1).
First, all microphones pre-amplifiers were calibrated with a 1 kHz tone at 94 dB SPL. Then
background noise level was measured and recorded at every microphone position of the receiver room.
A dodecahedral sound source was used to excite the emitter room; its level was set to overcome per 10 dB the background noise level at the receiver positions.
The impulse response of the receiver room was obtained using a balloon as an impulsive source.
All data was digitally recorded as .wav files of 16 bit and 44 kHz and analyzed using Aurora acoustical parameters plug-in and Excel for the calculations.
An L value (see eq. 1) was obtained for each room and at each third octave band. Then D iwas obtained (see eq. 2) also for each room and third octave band.
After processing the impulse response and obtaining the reverberation time T20 of the receiver room the equivalent absorption area was calculated using equation 4. Then the parameter Dn, Dnt and R’ were calculated using the definitions described above (equations 3, 6 and 7). 4. RESULTS
Background noise level at the receiver room is shown in table 1.
Table 1: Background noise at receiver point.
31,7 Leq 1min dB(A)
62,6 Leq 1min dB(Z)
Dimensions of the room and the division wall are
shown in tables 2 and 3.
Table 2: Dimension of the wall.
Height 2.6 m Length 8.0 m Surface 20.8 m2
Table 3: Dimension of the receiver room.
Height 2.6 m Length 8.0 m Width 4.3 m Volume 89.44 m3
Figure 2 shows reverberation time T20 of the
receiver room. From figure 3 to figure 6 the four parameters are
shown, namely: D, Dn, DnT and R’. Finally, figure 7 shows the four parameters in the same graphic to make differences more visible.
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Figure 2: T20 of receiver room.
Figure 3: D per third octave band.
Figure 4: Dn per third octave band.
Figure 5: DnT per third octave band.
Figure 6: R’ of receiver room.
Figure 7: Comparison between the four parameters.
5. CONCLUSIONS
The results are the expected ones. Figure 3 shows a typical isolating curve for a single wall where the coincidence frequency is clearly at 250 Hz.
Reverberation time (figure 2) is not well obtained for frequencies under 400 [Hz], this is adjudicated to the use of a balloon explosion and the existence of low frequency energy in the background noise (see table 1). This can affect the standardization of D at low frequencies because it strongly depends on the reverberation time.
Nevertheless, figure 7 shows that the room is diffuse enough and there is not need of corrections, the three curves remained together. 6. REFERENCES
[1] ISO 140-4:1998. “Field measurements of airborne sound insulation between rooms”. [2] ISO 354:2003. “Measuring sound absorption in a reverberation room”.
