A wall separating two spaces, when hit by sound waves, is forced to vibrate and the resulting vibration field is directly responsible for sound propagation to adjacent rooms. In order to "limit damages", the vibration width must be reduced: here the structure's dynamic properties play their role, for example, elastic, inertial and dissipative properties.
Fig.1
Laboratory measurements (fig.1) are carried out to determine the material's insulation properties or for investigations to establish design or specification data. They can also be used to verify the correspondence of building materials to international or local norms.
Laboratory test rooms are built with due attention to avoid any possible vibration loss so that during the test, all energy arriving in the receiving room comes through the tested wall. Usually the sound source issues a third-of-octave-filtered wide-band noise, that is, pink noise is used for this operation because, using third-of-octave filters, each band of transmitted energy remains constant. The sound pressure measurement is carried out in both the receiving and source room using a sound level meter that records the signals' levels.
The soundproofing power R is defined as 10x the logarithm (base 10) of the ratio between incident sound power W1 on the wall and sound power W2 transmitted by the tested wall in the adjacent room
| (1) |  |
Another definition of the same quantity R is found in UNI EN 140-3, derived from (1) under the diffusion conditions of the sound fields in the two rooms and the sound irradiation in the receiving room coming from the tested room only; this condition is usually obtained through using elastic joints that insulate the wall from the rest of the laboratory. The parameter R is measured from the two rooms, receiving and source, sound level differences, and taking into account the receiving room absorption.
(2) |
 |
L1= source room sound pressure level
L2= receiving room sound pressure level
S= separation wall area
A2 = receiving room absorption
The correction factor for laboratory measurements is obtained from the tested sample surface S and the receiving room absorption A2 in m2.
A2 can be determined by volume V and room reverberation time through the following formula, and is later replaced in the previous formula (2).
(3) |
 |
To perform a measurement according to norm ISO 140-3 a series of conditions are necessary concerning sound source type, microphones type and position, and room dimensions.
The most important are:
The source spectrum must be flat with a maximum difference of 6dB between bands.
The signal power must ensure that each spectrum band signal is at least 6 dB (preferably 15 dB) greater than background noise in the receiving room.
If the difference between signal and background noise is between 6dB and 15dB, a correction given by the following formula needs to be inserted
.
Lsb level with source switched on
Lb level of background noise
Below 6dB a fixed correction of 1.3dB is used, but the measurement report indicates that extreme conditions have been found.
The procedure prescribes a series of measurements of the background noise spectrum and of the spectrum with the source on, in a certain number of positions (minimum 5), in relation to source and receiving room dimension and form.
The minimum distances between microphones and sound sources must be observed:
0.7 m between the various microphone positions.
0.7 m between each microphone and room and diffuser surfaces.
1 m between each microphone and sound source.
1 m between each microphone and tested element.
The measurement must be carried out using 1/3 octave band filters with central frequency
100 - 125 - 160 - 200 - 250 - 315 - 400 - 500 - 630 - 800 - 1000 - 1250 - 1600 - 2000 - 2500 - 3150 (Hz)
or with octave band filters with central frequency from 125Hz to 2000Hz.
For the spectrum measurements, set an integration time on the instrument that generally should not be less than 6s.
With the performed measurements a signal and background noise average spectrum is obtained for source and receiving rooms from which L1 and L2 can be obtained.
For the receiving room reverberation time Tr calculation, a series of measurements (minimum 6) in various positions need to be taken in order to obtain the average reverberation time, later used in formula (3).
For a complete knowledge of the measurement requirements and procedure, please see the norm ISO 140-3.
The Framework Act 447 prescribes the issue of several decrees and assigns the compilation of the necessary technical regulations to the UNI. The UNI got these norms from CEN who got them from ISO: consequently, the rules are mainly the same in all Countries applying them.
Particularly interesting are norms UNI EN 20140, UNI EN ISO 140 and DPCM 5.12.97 that identify the different types of buildings and establish, for each type, the passive acoustic requirements. There are some remarks concerning the decree: the levels for each category are set independently of the area in which the building is located. Consequently, the same specification can be too strict in some cases but insufficient in other ones. Furthermore, the prescribed insulation levels for some buildings are much less severe than those prescribed for plants. Finally, the application scope was not clarified and therefore could also be extended to already existing buildings.
| Category A: residential occupancy or similar buildings; |
| Category B: office occupancy or similar buildings; |
| Category C: hotel, boarding house occupancy and similar buildings; |
| Category D: hospital, clinic, nursing home occupancy and similar buildings; |
| Category E: all schooling levels occupancy and similar buildings; |
| Category F: recreational or worship activities occupancy or similar buildings; |
| Category G: commercial occupancy or similar buildings; |
Table A - Classification of dwelling unit environments
| Categories of Tab. A |
Parameters |
||||
| Rw (*) |
D2m,nT,w |
Ln,w |
LASmax |
LAeq |
|
| 1. D |
55 |
45 |
58 |
35 |
25 |
| 2. A, C |
50 |
40 |
63 |
35 |
35 |
| 3. E |
50 |
48 |
58 |
35 |
25 |
| 4. B, F, G |
50 |
42 |
55 |
35 |
35 |
Table B - Passive acoustic requirements for buildings, their components and technological plants
(*) Values of Rw referred to separation elements between two distinct houses.