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Acoustic bases (Basic)

The acoustics for the sound 

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).

The sound level pressure in dB (SPL) decrease of 6dB in free field when the distance is doubled.

sounds presaure
Sound preasure 2
Image perte 6dB

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.

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Image Courbe de pondération A
  • 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.
  • Background noise: The background noise is a sound pressure level that we can ear with any message or music broadcast or particular noise in normal use. For example, background noise level in a restaurant is about 50 dBA.

 

  • Reverberation time: time that an impulsion pressure level decrease of XdB. X corresponds to 60dB for RT60 and 30dB for RT30. It is due to the geometry of the location and the materials used.

 

  • Reflections: is a phenomenon that is due to the encounter of an acoustic wave against an obstacle. There are two kind of reflections: early reflections (important for intelligibility) and late reflections (harmful to intelligibility).
Image Réflexion
  • 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.
Image Retard
  • 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.
Image Déphasage
  • 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.
Image Directivitée

For loudspeaker more the frequency is high more the directivity is reduced. Tweeters are more directive while a subwoofer is considered omnidirectional.

STI Index: 

    0% -  30%:   Very bad
  30% -  45%:   Bad
  45% -  60%:   Correct
  60% -  75%:   Good
  75% - 100%:  Excellent

STI conversion table - CIS (common inteligibility scale) :

Image tableau STI