A wall separating two spaces, when hit by sound waves, is forced to vibrate and the resulting vibration field is directly responsible of 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, such as elastic, inertial and dissipative properties.
On site measurements are those performed to verify that a building or components thereof were installed so as to comply with the norms or to resort to remedial steps if a space is hardly usable acoustically. The measurements are carried out as in the laboratory but using easier to handle and less bulky devices.
For on site measurements the apparent soundproofing power R’ is used; the apostrophe means a quantity used in on-site measurements. R’ and R depend on frequency and mass per units of surface.
R' is defined as follows:
W1 = incident sound power on tested wall,
W2 = incident sound power transmitted through tested wall,
W3 = sound power transmitted by lateral elements or other components.
In the hypothesis of sound fields diffusion, the norm UNI EN 140-3 defines the quantity R' itself as:
(1) |
 |
S = separation surface
A2 = receiving room equivalent absorption area
Another commonly used definition is the Standardized difference of DnT levels that refers the receiving room reverberation time to the standard level of 0.5s.
(2) |
 |
T2 = receiving room reverberation time
If the furnished room reverberation time is maintained on around half second values, the DnT corresponds quite well to the actual sound insulation experienced in living or bed rooms.
R’ considers the room dimension, therefore in small spaces (such as bathrooms) it is easier to obtain compliance with the DnT.
For large rooms R’ is the most restrictive between the two.
Having a unique value in the calculation, instead of R', that describes the soundproofing power of a separation wall independent of the frequency, can be useful. As a result of this, the soundproofing power index R'w found in DPCM 5.12.97 was introduced. The calculation rules can be found in the norm UNI 8270. The standardized curve ISO 717-1, the frequencies of which go from 100Hz to 3150Hz in third of octave intervals, is used. At low frequencies the gradient is 3dB per octave, at medium frequencies is 1dB per octave, and is null at high frequencies.
To pass from R' to R'w it's necessary to weigh the experimental levels with the ISO curve by translating it downward, dB by dB, until the sum of positive differences between the ISO curve and the experimental data is less than 2 (with data expressed in thirds of octave) and then the value corresponding to 500Hz is read from the reference curve, that is R'w.
Algebraically:
(3) |
 |
N = number of values,
n = number of points when the standardized curve is above the experimental.
To obtain the index, n must diminish unit by unit, that is to say, translate the standardized curve downward until the inequality is verified.
To perform the measurements in conformity with norm ISO 140-4 a series of conditions are necessary, such as: sound source type, microphones type and position, room dimensions.
The most important are listed here:
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 10dB greater than background noise in the receiving room.
If the signal and background noise difference is between 3dB and 9dB, a correction given by the following table needs to be inserted
Signal and background noise difference |
Correction factor (to be subtracted) |
3dB |
3dB |
between 4 and 5dB |
2 dB |
between 6 and 9dB |
1 dB |
The measurement cannot be performed for differences below 3dB.
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 the integration time should not be less than 6s.
From these measurements a signal and background noise average spectrum is obtained for source and receiving rooms from which L1 and L2 can be obtained and used in (1) and (2).
For the receiving room reverberation time T2 calculation to be used in (2), a series of measurements (minimum 6) in various positions need to be taken in order to obtain the average reverberation time later used in the formula.
For a complete knowledge of the measurement requirements and procedure, please see the norm ISO 140-4.
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.