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.