Acoustics basics (Fundamentals)

Sound acoustics

1. Concept of acoustics: 

GLOSSARY :

• Sound : A sound is a pressure variation around the atmospheric pressure expressed in Pascal (Pa).
  Human ear is sensitive from 20µPa to 200Pa. A sound wave is propagated at a speed of 340 m/s at 20°C in air. A perfect human ear perceived sounds from 20Hz to 20 000Khz.

• dB : logarithmic unit that expresses a level of pressure (0 dB = 20µPa, 94 dB = 1Pa, 114 dB = 10Pa).

?Free field is an area without sound reflection. In this case, acoustic waves decrease of 6dB per doubling of distance.
Unlike a free field, the reverberating field is an acoustic propagation area where the acoustic wave encounter obstacles that gives reflections (for example an empty apartment behaves like an echo chamber). In this case the decrease of 6dB can’t be applied.

The SPL of a loudspeaker is increasing by 3dB when the power is doubling. If a loudspeaker have a SPL of 90dB 1W/1m, when this loudspeaker is wired on 2W the SPL will be 93dB at 1m and for 8W the SPL will be 99dB at 1m.

dBA : is a SPL unit weighted according to the human ear perception. Indeed the human ear behaves like a filter that enhance human speech frequencies. Below against the weighting curve of dBA.

?Bruit de fond : le bruit de fond correspond au niveau de pression que l’on constate sans diffusion de message ou de musique dans une utilisation normale. Par exemple le bruit de fond dans un restaurant est d’environ de 50 dBA

Temps de réverbération : temps qu’une impulsion met à perdre X dB de niveau de pression, ou X correspond à 60 dB pour le TR60 ou 30 dB pour le TR30. Il est dû à la géométrie du lieu et aux matériaux employés.

Réflexions : La réflexion est un phénomène qui est du à la rencontre de l’onde sonore contre un obstacle. Il y a deux types de réflexion : les réflexions précoces (importantes pour l’intelligibilité) et les réflexions tardives (nuisibles pour l’intelligibilité).

?Phase shift: This is identical to that known in electronics. When two speaker broadcast toward to the same point are not wired in phase, loss of sound pressure level can appear by the addition of the two waves.

Sensibility of a transducer: sound pressure level (SPL in dB) provided by a speaker when is supplied by a power of 1W at 1 meter.

Directivity of a loudspeaker or microphone: vertical and horizontal angle of sound dispersion from the axis of the loudspeaker depending on the frequency. The most used directivity for microphone are: cardioid, hyper-cardioid or omnidirectional. They are given by a polar diagram.