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The objective of this page is to present the definitions of technical terms relating to the fields of acoustics and vibration.
It is aimed at anyone wishing to deepen their knowledge of acoustics and vibration.
This page is constantly evolving so do not hesitate to contact us if you have any questions or comments!
The definitions are positioned in an educational manner as for a course, therefore normally from the simplest to the most detailed.
We also attach a lot of importance to references because it is thanks to the people who write the books, theses and other publications that we can carry out this educational work. Therefore, we strive to clarify our references as best as possible but if any are missing, do not hesitate to let us know.
Acoustics is the science of sound, including its production, control, transmission, reception, and effects. It is primarily concerned with the study of waves and is therefore considered a branch of mechanics.
A vibration is a mechanical oscillating movement around a stable equilibrium position or average trajectory. The vibration of a system can be free or forced.
This is the variation in pressure around atmospheric pressure.
Since sound pressure is difficult to represent on a linear scale (from approximately 20μPa to 10Pa), the sound level is quantified using a logarithmic scale.
The formula is:
With pref the reference pressure = 20.10-5Pa and p being the effective pressure.
Below is a table representing the scale of sound pressure levels:
30dB + 30dB does not make 60dB! and fortunately for our ears!
Indeed, being a logarithmic scale, the operations must be carried out by returning to the squared effective pressures therefore to sum n sound levels (or intensity or power):
It therefore follows that if we sum two identical levels:
To easily and quickly calculate sums, differences and averages, you can use the Acouvapp module below:
This is the speed at which the sound wave travels. In air, the speed of sound is calculated as follows:
c=(1.4 x P0 /ρ)1/2
with
P0 : Atmospheric pressure in Pa
ρ: air density in kg/m3
The simplified formula is as follows:
c=20.05 (T).
with T in Kelvin
The wavelength (λ) is defined as the distance for a pure sound traveled during a period T:
λ=c/f=c*T
With c, the speed of sound in m/s and f, the frequency in Hz.
The time evolution of a sound can be considered as a sum of sines and cosines evolving with different time periods.
Each of these periods (T) corresponds to a frequency (f=1/T). If we take the example of a "pure" sound, the temporal evolution will present an undulation with a given temporal period. In the frequency domain, this same sound will therefore be represented by a point at a single frequency (1/T).
There are different representations of the acoustic or vibrational spectrum. The most used is the representation in octaves or thirds of octaves.
The width of the octave band is delimited by two frequencies (called center frequencies) whose upper frequency is equal to twice the lower frequency. The frequency 1000 Hz is defined as the "normalized" center frequency from which all other center frequencies are derived.
In order to refine the spectral analysis, the third-octave representation is often used. Three thirds of octaves form an octave. The relationship between two adjacent thirds of an octave is with .
The table below shows the list of octaves and thirds of octaves:
This is a sound whose spectrum amplitude in fine bands decreases by 3dB when the frequency doubles. On the other hand, in thirds of an octave or in octaves, the shape of the spectrum is constant (energy sum between the octave and thirds of octave bands).
It is a sound whose amplitude of the spectrum in fine bands is stable when the frequency doubles. On the other hand, in thirds of an octave or in octaves, the amplitude of the spectrum increases by 3dB per octave (energy sum between the octave and thirds of octave bands).
This definition is too long to write here, so rdv on the acoustic power page!
There are two types of weightings: frequency weightings and time weightings.
The frequency weightings The most used are the following:
Frequency weightings are added to the sound pressure level for each third octave or octave (with units dB or dBLin).
The time weightings the most used are:
Each has its own integration time (exponential average), namely 35ms, 125ms and 1s respectively. As indicated, the choice of one of the weightings will depend on the type of noise to be analyzed (short, long or impulse noise).
If you want to switch from A to C or Lin to A weighting, try the Acouvapp module for calculating acoustic weighting.
Reverberation time is the length of time a sound lasts after the sound source has stopped, due to multiple reflections on the walls of a room.
It is expressed in seconds and represents the time needed for the sound pressure level decreases by 60 dB.
The frequency weightings The most used are the following:
The graph below shows an illustration of how reverberation time is determined
The reverberation time depends mainly on:
An acoustic measurement can be performed with different sources such as a bursting balloon (impulse method) or an omnidirectional source (interrupted noise method). Multiple microphone and source locations are required during the measurement. standards 3382-1 to 3 describe the measurement methods depending on the type of room.
This definition is too long to write here, so rdv on the dynamic stiffness of materials page!
Acoustic insulation is the ability of a material or system to limit the transmission of noise from one room to another.
R insulation is defined in dB and the higher it is, the more efficient the wall is. R is defined by the following formula:
with 
being the transmission coefficient which is the ratio of the acoustic powers of the 2 rooms.
There are several indices to define isolation, and it is true that one can get lost among them (R, Rw, Ra, R'…).
To see the clues more clearly, read our article: Clues and Co.
Generally speaking, a wall is tested in a double reverberation chamber in which (in the laboratory), the sound levels are measured with microphones in several positions then the attenuation index is calculated by making the difference in the sound levels (L2 – L1) by bands of thirds of octaves (or octaves or fine bands) corrected for the equivalent absorption area of the reception room and the surface of the element to be tested.
The attenuation index is generally presented in spectra of thirds of octaves accompanied by the global indices Rw (weighted attenuation index), Rw+C and Rw+Ctr, C and Ctr being terms of adaptation to a given spectrum (C mainly for use in isolation from interior noise and Ctr for isolation from exterior noise). These indices are calculated according to the ISO 717 standard.
The attenuation index is mainly provided over the frequency range from 100Hz to 5000Hz but it can be measured from 50Hz (low frequencies).
Anti-vibration mounts are used to reduce vibration on a structure coming from equipment. Different technologies exist such as elastomer pads, metal pads (springs), etc. Depending on the application and constraints (size, stiffness, damping, temperature resistance, lifespan, etc.), certain technologies will be favored.
The pads act primarily as vibration filters, therefore they will be effective over a certain frequency range.
This frequency will depend mainly on the stiffness of the block (K, ratio of force and displacement) and the suspended mass (M).
These two parameters will allow us to calculate the transmissibility of the plot, that is, the ratio between the downstream vibration and the upstream vibration. This transmissibility is characterized by several distinct frequency zones (see graph below with f the frequency).
As stated in the graph analysis above, when sizing a plot, the objective is to position the excitation frequency of the equipment in the vibration reduction zone (green zone). A vibration study is necessary to correctly size the suspension studs.
There are several indices to define isolation, and it is true that one can get lost among them (R, Rw, Ra, R'…).
To see the clues more clearly, read our article: Clues and Co.
Generally speaking, a wall is tested in a double reverberation chamber in which (in the laboratory), the sound levels are measured with microphones in several positions then the attenuation index is calculated by making the difference in the sound levels (L2 – L1) by bands of thirds of octaves (or octaves or fine bands) corrected for the equivalent absorption area of the reception room and the surface of the element to be tested.
The attenuation index is generally presented in spectra of thirds of octaves accompanied by the global indices Rw (weighted attenuation index), Rw+C and Rw+Ctr, C and Ctr being terms of adaptation to a given spectrum (C mainly for use in isolation from interior noise and Ctr for isolation from exterior noise). These indices are calculated according to the ISO 717 standard.
The attenuation index is mainly provided over the frequency range from 100Hz to 5000Hz but it can be measured from 50Hz (low frequencies).