We are still talking here about “Bell telephone laboratories”. Bell engineers needed a reliable metric to develop their new telephone devices. So they created the decibel, or one tenth of a Bel – named in honor of their venerable boss, Alexander Graham Bell.
In 1933, Harvey Fletcher and W. A. Munson from Bell Telephone Laboratories presented loudness level contours (curves of equal perception of a sound). These curves were obtained statistically following listening tests carried out using listeners with hearing considered to be normal.
The audible frequency range varies from 20 Hz to 20 kHz. As for the perception of the volume of a sound, the human ear does not react in a linear way for the perception of the pitch of a sound.
Sounds are defined as an auditory sensation generated by an acoustic wave (Larousse). This definition introduces two notions: acoustic waves and human hearing. When an acoustic wave reaches the head of a listener, it is picked up by the pinna and directed toward the ear canal. These two elements form the outer ear.
Frequency is defined as the number of cycles (or period) per second and is measured in Hz. In high frequency, the number of oscillations per second is high. The relation between the period T and the frequency f is T=1/f.
A sound wave is a pressure disturbance that propagates through the air. This disturbance can be caused by a vibration (for example the vibrations of the vocal cords) or even turbulence in an air flow, such as a whistle.
The absorption measurement in a reverberation chamber or in a small cabin is also called absorption measurement in diffuse fields or under diffuse incidence. It follows the ASTM C423 or ISO 354 standards.
An impedance tube, also called a Kundt tube, is a tube fitted at one end with a loudspeaker and at the other end with a sample holder in which the sample of acoustic material to be tested is installed. The measurement is done using two microphones located between the loudspeaker and the sample holder.
The static thermal permeability is the low frequency limit of the dynamic thermal permeability. The thermal permeability problem is the thermal analogy of the viscous permeability problem. When the frame of a porous medium has a sufficient thermal capacity for the compressibility to reach its isothermal value at low frequencies, the excess acoustical temperature can be considered to vanish at the pore walls (this replaces the no-slip condition for viscous flow) and a static “thermal permeability” exists.
The open porosity (ϕ) is defined as the fraction of volume that is occupied by the fluid in the interconnected porous network. Non-interconnected voids trapped in the solid phase are not part of the open porosity: they are part of the closed porosity.
Viscoelastic materials are added to structures and plates in order to avoid large vibrations, espciallly around resonant frequencies. The properties of these materials are the Young’s modulus (E(ω)), the loss factor (η(ω)) and shear modulus (G(ω)). These parameters are frequency and temperature dependent.
For isotropic poroelastic materials, the elastic properties to be characterized are Young’s modulus (E), Poisson’s ratio (v) and loss factor (η). The method is based on the dynamic compression of two samples of different shape factors (s1 and s2). This enables the simultaneous characterization of the three elastic properties. The method is valid for frequencies from 20 to 100 Hz.
The viscous and thermal characteristic lengths are average macroscopic dimensions of the cells related to viscous and thermal losses, respectively. The former may be seen as an average radius of the smaller pores, and the later as the average radius of the larger pores.
The tortuosity, or structure factor, is a geometrical measurement of the deviation of the actual path followed by an acoustical wave in a porous material from a direct path in the air.
The efficiency of an acoustic material is determined by measuring its absorption coefficient, denoted α. The absorption coefficient is defined as the proportion of acoustic energy that has been absorbed in the material over the total energy.
The static airflow resistivity (SAR), often represented by the Greek letter σ (sigma), is a very important parameter for acoustic material modeling. In fact, one of the first acoustic models presented by Delany & Bazley in 1970  was only relying on this parameter.
An acoustic absorbent material is a material in which sound waves penetrate and then are dissipated. There are two main families of absorbent materials: fibrous materials and porous materials.