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Feature : Sound Tech 2 - Acoustics
Well designed auditoriums give acoustical directivity from stage to audience,
with a fine balance of reverberation to enhance performance. When recording,
musicians select the right reverberation for the music. However, once recorded,
extra room reverberation detracts from the music. The perfect listening environment
for recorded music is 100% absorbent at all frequencies (free field).
Reverberation is sound reflecting off the floor, walls and ceiling and builds up
to a percentage of the direct sound, and is different at all frequencies. Some
vehicles are absorbent at high frequencies, but reverberant at low frequencies and
vice versa. As we move closer to the speakers, the direct sound gets louder and
clearer. As we move back from the speakers, direct sound diminishes but the
reverberant sound remains constant.
From the point where reverberant sound energy becomes greater than the direct
sound energy, intelligibility and stereo imaging become lost. Acoustics can be
a life-time study, but without understanding 'Critical Distance' all other
information on acoustics has no meaning.
Critical Distance is a point, where direct, and reverberant sound energies, are
equal. Critical Distance is different at all frequencies. The more reverberant a
vehicle interior is, the closer is Critical Distance. The more absorbent a
vehicles interior is, the further is Critical Distance.
Direct sound from the speaker system diminishes in level, as a function of the
distance (inverse square law) whereas reverberation constantly spreads
throughout the vehicle. Because there is new incoming sound from the speakers
reverberation keeps building up until the new incoming sound equals the sound
absorbed (steady-state).
Acoustical Facts:
The more absorbent the vehicle is the further the Critical Distance.
The more reverberant the vehicle is the closer the Critical Distance.
Near field or Direct field is inside the Critical Distance.
Far field or Reverberant field is outside the Critical Distance.
Acoustical absorption of furnishing and fabrics readily absorb high frequencies.
The further fabrics are from walls the better the absorption is to include
lower frequencies. Fibreglass has the highest absorption capacity, converting
molecular air movement to heat.
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