Installation manual

Community S-Series - Operation and Installation Manual - Page 39
bumps, and an overall drop that looks like the inverse of the low-frequency rise. You feel a
little like Alice in Wonderland. When the equalizer was switched in, the filters actually
improved the phase response! You’ve got to get your hands on one of these FFT things,
and soon.
# # #
OK. Let’s put this event into more scientific terminology. Here’s what’s happening: The
loudspeaker is transferring its acoustic energy into the room. This energy presents itself in
the form of pressure waves, causing cyclical pressure and rarefaction in the room’s
atmosphere. Under excitation, the volume of air in the room begins to resonate, as
confined volumes of air tend to do. This is not a particularly large room, so its primary
resonant frequency is quite high at 362 Hz
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. The second harmonic of that frequency is also
present at 725 Hz. Going back and looking more carefully, one would probably see
additional third order harmonic resonant modes, and possibly a sub fundamental mode as
well.
Other parts of the room, particularly if it’s a complex architectural design, might exhibit
their own resonant modes at different frequencies, such as in the underbalcony area.
But why was the phase response improved merely by applying frequency equalization? The
answer is simple. The peaks in amplitude at 362 and 725 Hz that were removed by the
equalizer were caused by systemic resonance (the ‘system’ being the sum of the
loudspeaker and the room). Because it takes time to complete a period of resonance, this
time period alters the systemic phase response as well as the frequency response. If one
could precisely cancel out the variation in phase response with an FIR filter, the result
would be the inverse, or a perfectly flat frequency response curve. It’s a wholly organic
process in which phase response and frequency response are intrinsically linked.
The ideas and techniques described above can be extended to arrays, clusters, delay
systems and distributed systems. Managing the various zones of a large-scale sound
system is, of course, much more complicated, but the basic techniques remain the same.
Properly applied, equalization can be a powerful tool with benefits extending even into the
time domain, as we’ve illustrated above. The potential for radical improvement in both the
phase and frequency response, through the use of precision equalization, can even make a
large, reverberant room sound significantly ‘smaller.’ This is because the reverberant field
in a room is typically longer and higher in amplitude at frequencies where it exhibits
excessive resonance, than throughout the remainder of the audible spectrum. By reducing
the energy from the sound system at those resonant frequencies, the room may no longer
sound particularly reverberant at all.
When using precise measurement equipment, additional useful processes can be brought to
bear. For example, instead of flattening the ancillary underbalcony and over balcony
systems, first look at the spectral content of the energy that’s arriving in those areas solely
from the main array(s) located far forward in the room. Typically you’ll see that there’s
already too much low-frequency content. You might also see a local zone resonance that
wasn’t noticeable in the forward section of the room. And there might be an excess of
energy at some particular mid-spectrum frequency.
By shaping the delay system to add only the portion of the spectrum that’s lacking from the
main house array(s), and precisely delaying it to within a millisecond of the true
propagation time, these ancillary systems can wonderfully improve the listener’s experience
in what are often called the ‘cheap seats.’ Additionally, when an ancillary delay system is
additively aligned as described above, its overall energy contribution is lower and therefore
it’s far less prone to reflecting energy back into the room, which could quite possibly
corrupt the sound in the forward seating areas.
This ‘additive’ technique can be applied to front fill loudspeakers, down fill loudspeakers,
and any other area where multiple systems overlap in a shared acoustic space.
Precedence (The Haas Effect)
The Haas Effect, or precedence effect, is named after Helmut Haas who first described it in
his doctoral dissertation. It states, in part, that one sound source may be as much as 10
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In a real life situation the primary room resonant frequency would tend to be much lower, but it’s easier to illustrate the principal
in a range where the graphic equalizer has more available bands.