Acoustic

Noise represents a series of sound vibrations that correspond to variations of air pressure, which in turn can be heard by the human ear.
In recent decades, noise has become one of the main sources of pollution: man - who does not have the physiological ability to protect himself from noise as he isolates himself from light by closing his eyes - feels the need to protect himself from sound.

The implementations that improve noise inside buildings are mainly related to the following factors:

acoustic characteristics of the building systems and their components
correct construction and compliance with the building structure project

ACOUSTIC INSULATION
Comprises all those measures that are taken to reduce the transmission of sound energy from sources that produce it to places that must be protected.

The comparison between the different regulations is difficult to perform as several countries use different ways of evaluating the user’s demands:

the insulation index used is not the same and can not be directly compared;
the insulation criteria are also different (some use the maximum values of the sound index in the spaces, other use acoustical performance requirements for the partitions, walls and floors);
the specifications used to evaluate the insulation index are not the same in all countries.

DEFINITIONS
Standard insulation indexes (D_n, D_nA, D_nT)
The index D_n gives the difference (for each one-third octave band or for each octave band) between the sound pressure value in the emission room and the sound pressure value in the receiving room. It is used for “in situ” measurements.
If a standard absorption area of 10 m² for the receiving room is used to change the Dn index to a standard value a D_nA value is obtained.
If a standard reverberation time for the receiving room is used to change the D_n index measured for any room a D_nT value is obtained.
The D_nA, D_nT and D_n values are measured or calculated for each band of frequencies normally between 100 Hz and 5000 Hz. It is not a single-value number. It changes with frequency.

Insulation indexes (R_r, R’)
The insulation index R is defined for construction elements which divide two different spaces (walls, partitions, floors). It is evaluated in laboratory and defines the insulation properties of a specific construction product. When the index is evaluated “in situ”, the R’ value is used which is different from R because it considers the flanking transmissions which are insignificant in laboratory measurements. D_n and R can be related by the following formula:

Dn = R + 10 log (0.16 V/TA) where:

V – total volume of the receiving room (m³);
T – reverberation time of the receiving room (seconds);
A – total area of the separating wall

Standard impact sound insulation indexes (Lⁿ, L’ⁿ)
The value Ln is the sound pressure index measured in a room when the above floor is tapped by the standard tapping machine. This index is evaluated in laboratory.
Similarly to the index R, if the measurements are performed “in situ” the index L’_n is used.
If a standard absorption area for the receiving room is used to correct the Ln value a L_nA (or L’_nA) value is obtained. Similarly, if a standard reverberation time is used a L_nT (or L’_nT) is obtained.
In conclusion for impact sound insulation the following indexes may be used: L_n, L_nA, L_nT, L’_n, L’_nA or L’_nT.

The EN-ISO Index
The EN-ISO index is a single value number that is obtained using the rules defined in EN-ISO 717. The index results from the adjustment of the values obtained in the test to a reference curve defined in EN-ISO 717.1, EN-ISO 717.2 and ISO 717.3 (respectively for airborne sound insulation, impact sound and for facades). The EN-ISO indexes use the notation R_w, R_{’w, DnT,w}, D_nA,w, L_’nT,w, L_{’nA, w}, L_{nA, w}, L_{nT, w}, L_{n, w} and L_{’n, w}.
These symbols have the same meanings mentioned in previous paragraphs. The letter “w” means weighted and at the same time indicates single-value number.

The dB(A) index
The dB(A) index is another single-value number. It results from the measured values by one-third octave bands or octave bands using correction values for each band. The index is calculated based on the corrected values for each band using a logarithmic sum. A single value index is obtained.
To represent the dB(A) index the letter A is included in the index defined in previous paragraphs and between the emission and the receiving rooms (m²).
The index DnAT, DnAA, RA, R’A, LnAA, LnAT, L’nAT and L’nAA are obtained.
It is not possible to find an easy correlation between the EN-ISO index and the dB(A)

Exterior walls insulation
The majority of the countries use a ISO index measured “in situ” according to standards ISO 140.5 and ISO 717.3 (R_{θ, w} or R_{tr, w}). Some countries define the requirements indirectly limiting the Lp and/or Leq index values measured inside the buildings. In these cases, the R_{θ, w} or R_{tr, w} are not evaluated directly but must be specified in the projects so that the building is able to respect the regulations for a certain urban area.

SOUND ABSORPTION regards the correction of noise propagation within the same environment. Basically, it intervenes with sound-absorbing materials to reduce the reverberation time (that is, the echo effect) of noise in the place where it is produced.
The quantity that provides a unique value of the sound-absorbing performance of a material is the following:

αw: sound absorption index

Refer to local laws to know the parameters to be respected.

Sound Absorption Coefficient
When a sound wave in a room hits a surface, a certain fraction of it is absorbed, and a certain amount is transmitted into the surface. Both of these amounts are lost from the room, and the fractional loss is characterized by an absorption coefficient that can have values between 0 and 1, 1 being a perfect absorber.

Absorption coefficient = a
Effective absorbing area = aS

The effective absorbing area is a factor in determining the reverberation time of an auditorium. The absorption coefficient of a surface typically changes with frequency, so the reverberation time is likewise frequency dependent. A table of absorption coefficients can be used in calculations of reverberation time with the Sabine formula.