Acoustics basics (Fundamentals)
1. Concept of acoustics:
- 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).
The sound level pressure in dB (SPL) decrease of 6dB in free field when the distance is doubled.
?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).
Absorption: ability of a material to dissipate the energy of an acoustic wave and thus reduce the level of the reflected wave. All materials have an ability to reflect or absorb sound. This capability is defined by the absorption rate which is between 0 and 1. 0 is a reverberant material and 1 is a fully absorbent material.
Resonance: Ability of an object, a wall or a place to vibrate at a specific frequency. For example a glasses broken by an singer.
Intelligibility: percentage of comprehension of a message. This percentage depends on the ration between direct wave and reverberating wave the reverberation time… It is noted in RASTI, STI or CIS. The higher percentage correspond to the better intelligibility.
Masking effect: This effect appears when the pressure level of the message or the music broadcast is close to the level of background noise. To understand a voice message, we admit a difference of 15-20 dB between these two levels.
Delay: delay is used in the case of multiple speakers are used. The delay time is determined by the distance between speakers and the sound velocity. In case we don’t use delay, phase shift phenomena may occur.
?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.
For loudspeaker more the frequency is high more the directivity is reduced. Tweeters are more directive while a subwoofer is considered omnidirectional.
0% – 30%: Very bad
30% – 45%: Bad
45% – 60%: Correct
60% – 75%: Good
75% – 100%: Excellent
STI conversion table – CIS (common inteligibility scale) :
Experts in the field of PA and VA systems in France and internationally for the past 50 years.
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