The acoustical properties of enclosed spaces have a
profound effect on the performance of sound systems.
The "three R's" of room acoustics are Reverberation,
early Reflections, and room Resonances.
Early reflections (up to about 60 milliseconds after
the arrival of the direct sound) provide a sense of
texture and source width. The ear/brain auditory system
will combine the early reflections with the initial
direct sound, whereas later reflections become
identified as a property of the enclosed space. The
early reflections, therefore, can be used by the brain
as a part of the decoding process, and most studies
have shown they actually improve sound quality.
Reverberation will enhance music, but it tends to mask
speech intelligibility. The reverberant sound field
consists of sounds from multiple reflections off
surfaces, but because each reflection is attenuated
by absorption, the sound field will fade out within
about a second or so. Many large rooms have too much
reverberation (more than 2 seconds), while most small
rooms have too little (less than 1/2 second).
Bathrooms, with little absorption, are the exception.
On the other hand, the direct sound field is
independant of the room but is proportional
to the directivity index of the sound source.
For instance, a horn speaker will focus the sound
and therefore have a higher directivity index
than a dome speaker, and will tend to improve
speech intelligibility. Music always sounds better
with some reverberation, but the exact right amount
is frequently debated.
Room resonances can be calculated and mapped
using the formula: speed of sound (1130 feet
per second) divided by wavelength equals frequency.
Where the wavelength of the first resonance is 2
times the room length (or width), and subsequent
resonances are integral multiples of that.
Measure a room dimension (in feet), multiply times
2, and divide it into 1130: that will be the first
room resonance. The first resonance for each of
the room dimensions (usually 3) will cause large
peaks and valleys in loudness at those frequencies
across the room at various locations. Contrary
to popular opinion, splaying walls will not
eliminate standing wave resonances, but instead
will broaden the resonant frequency and lessen
the intensity of the peak.