Inside Audio: Dave Moulton
The Brave New World: Loudspeakers All Around!
As I noted last month, audio authority John Woram has said, "Audio
is a hallucination where the sufferer believes that he or she hears
music coming from a small wooden box."
Woram went on to observe, "Stereo is a more serious form
of the hallucination, where the sufferer believes that he or she
hears music coming from a point in space between two small wooden
boxes."
Alert readers will recall that last month we talked about that
quirky but quintessential audio device, the loudspeaker. We noted
that it is, in and of itself, singularly quirky and difficult. And
by that quirkiness, the loudspeaker makes a mockery of the idea
of precision or accuracy of reproduction. Too many problems with
drivers, crossovers, directionality and, dare I even say it, rooms!
But there’s also a lot more to it. Loudspeakers exhibit,
when played in pairs or groups, a remarkable and unprecedented behavior
that is at such odds with nontechnological acoustics that it absolutely
confounds our hearing. What’s even wilder is that we seem to
be oblivious to the existence of that behavior, right in front of
our noses, even while we’re wallowing in the pleasure we derive
from it! Go figure!
Consider it this way. All normal sound sources including all conventional
musical instruments vibrate freely in space, emitting waves of pressure
change into the space. When I say they vibrate freely, I mean exactly
that. Each source vibrates as a function of its mass, tension and
size. Each vibrates independently of all others, and there is essentially
no linkage between the vibration of one source and the vibration
of any other source.
Our hearing makes use of this behavior. We are immensely sensitive
to the minute variations in frequency that occur between sources,
which we call beating, chorusing, phase-shifting and flanging. This
behavior, whatever we call it, is the basis for how we hear that
there are multiple and different simultaneous sound sources.
"PHASES LOCKED"
When we listen to a single sound in a reverberant room, all the
reflections of that sound have the exact same rate of vibration
(albeit later in time) as the source. The term we use to describe
this is "phase-locked," and it has important implications
for our hearing. In fact, we identify all "early" (less
than approximately 50 ms) reflections of a given source with that
particular unique source because of their particular unique phase-locked
relationship with that source.
When, on the other hand, there are two sound sources, we have
two families of phase-locked sounds that are "beating"
with each other. Even if they have "identical" timbre
and loudness, we have no trouble determining (a) that there are,
in fact, two sources, and (b) where they are in the reverberant
space, thanks to those independent families of phase-locked sounds.
This is where the behavior of loudspeakers gets weird. Loudspeakers
are, of course, driven by an electrical signal that consists of
alternating current flow in a wire (actually two wires). The direction
and magnitude of current flow determines at any given instant in
time the position and motion of the loudspeaker’s drivers.
It is no problem at all to send such a signal (we usually call
it a "mono" signal) to multiple loudspeakers in parallel.
When we do, all the loudspeakers will move in phase-locked synchrony,
unlike any group of non-technological sound sources.
The effect of this is both stunning and outrageous, from a cognitive
standpoint. Think about it! The cognitive "signature"
of any given sound is derived from the phase-locked relationship
of the direct sound and its early reflections. With an array of
phase-locked loudspeakers, however, we now have a multitude of physical
sound sources spread out in space that collectively have the psycho-acoustic
"signature" of a single sound source. The result is that
we perceive a "single" sound whose perceived location
is determined by the collective relative positions of the loudspeakers.
This "single" sound is what we call "the phantom
image."
There is another way of thinking about this: the loudspeakers
serve as the first "early reflections" of a (phantom)
sound source whose direct sound we don't hear. Because our brain
is good at filling in the missing blanks, it "infers"
where that phantom source must be and that "inference"
is what we actually perceive, or think we "hear."
This means that loudspeakers have two different behaviors. As
single sources, similar to any non-technological sound source, they
behave in an entirely logical and predictable way – as discrete
sound sources. However, when we have an array of loudspeakers –
meaning two or more speakers hooked up in a stereo or multichannel
system – those loudspeakers will behave as "the first
early reflections" of any recorded sound source that is sent
in common to multiple members of that array.
THE PHANTOM OF THE OPERA
It gets especially wacky and wonderful when we include recorded
early reflections, ambience and reverberance in our signals. Now,
we have information abut not only a "phantom" sound source,
but also a "phantom" sound space. Our stereo or multichannel
array of speakers, in combination, serves as a kind of "cognitive
window" into that exotic imaginary space. It gets even better
when we get the playback room interface right – then the playback
room also supports the illusion of a "phantom space."
Doesn’t get much better than that!
This is, of course, key to both stereophony and surround audio
production, both for film/video and for music. It helps to understand
how the speakers do their part in this, and to focus on the qualities
of speakers that maximize this effect, to wit:
• First, the speakers need to be well matched in sound quality.
• Second, they need to have really good frequency response.
• Third, they also need to have at least reasonably good power
response, especially in the horizontal plane. There are two parts
to this: low frequencies depend on room absorptivity and symmetry
in placement, while high frequencies depend on special strategies
for gaining wide dispersion, plus room symmetry as well.
• Finally, the speakers all need to be equidistant from the
listener and the listening sweetspot, as well as symmetrically distributed
around the room.
This is what I’ve been working on recently, trying to design
and build loudspeakers that do all these things well while also
remaining affordable. Over the next several months, I'll try to
explain the details of that work, to the extent that I can. What
I can say for sure is that loudspeakers are getting better; I know,
because I've been there and heard them!
Next month, though, we’ll take a fresh look at the quality
of real broadcast audio. It ain’t pretty!
Thanks for listening.
Dave Moulton is trying to do too many things at once. You can
complain to him about that or anything else at either www.moultonlabs.com
or sawonline.com.
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