Recording - September 2017 - 72

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THE ROOM-IN-ROOM EFFECT
Guest editorial by Scott Dorsey

When you put a microphone up in a room and you record
a sound source, what comes out of the microphone is the
original sound plus the room reverberation, and the farther
the microphone is from the source, the more that ratio is slanted
toward room reverberation. But when you play it back on
speakers, you are hearing the sound source, plus the original
room reverberation, plus the reverberation of the playback
room. Consequently the original recording needs to be more
dry than the sound that you want, because it will have the
playback room reverb added to it.
This is why one idea that I often hear from audiophiles is
very incorrect: the notion of putting up a microphone to record
an ensemble in such a way that the sound on playback will be
exactly the sound at the microphone position. This isn't possible
with normal playback (although it is possible with binaural
recording and headphone playback, which eliminates the
playback room effects). Attempting to make recordings this
way is misguided.
This is also why the control room that we mix in needs to
reflect the same basic acoustics of the final listening room.
Sometimes you can predict this accurately, sometimes you
have to guess. Traditionally films have been mixed on large
dubbing stages which have roughly the same ac oustics as a
small movie theater. However, as theatrical release has become
less important and more movies are aimed at the small screen,
it's become increasingly popular to mix movies in smaller mixing
suites that more accurately reproduce a home listening space
like a living room. Likewise, conventional control booths tend
to have the same general reverberation time as a small living
room-more or less-although hopefully the reverb itself is
much more even.
Since the 1960s, people have employed predelay
on artificial reverberation, because both the recording and
playback rooms tend to have short-term echoes coming
back soon after the original signal. Predelay means the artificial reverb picks up as those die out, and it helps make
the effect sound more natural when played back in typical
small rooms.
You can't mix on headphones and expect to get anything
like the same balances that you do on speakers, because
headphones eliminate that playback reverberation and so the
perception of distance from the listener is totally changed.
That said, if you're making something intended to be listened
to on headphones (ambient music is one obvious example),
it's best to mix on headphones. The key is to try your best
to match the mixing environment to the expected listening
environment, whatever it may be.
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RECORDING September 2017

Now, these effects are very well-known. They were not only
known before stereo recording existed, they were known even
before electrical recording existed. In the acoustical recording
era, singers and bands sang and played into a large horn that
captured sound for the cylinder or disc being cut. The studio
was made as dead as possible, because not only was the
reverberation of the playback room added, but the recordings
themselves were subject to severe reverberation from reflections
inside the larger horns used for recording and playback. So
we've known about this effect for well over a century, and
even if you might not think about it, if you've done recording
work you probably know it intuitively.
In September 2016, Richard Hughes, Trevor Cox, Ben Shirley, and Paul Power from the Acoustics Research Center at the
University of Salford in the UK published a paper at an AES
Convention called "The Room-In-Room Effect And Its Influence
of Perceived Room Size in Spatial Audio Reproduction." (This
is AES Preprint 9621 if you actually want to look it up.). These
people actually measured reproduction rooms of various sorts
as impulse responses, and then used computer modeling to simulate various playback spaces over headphones with a variety
of recordings. They found that listeners tended to notice most the
more reverberant of the source and reproduction rooms when
they judged room size, which is a perfectly reasonable result.
In fact, it's so reasonable that a number of people made
disparaging comments about this study, saying that the results
were obvious and that the authors were wasting their time
studying this effect. The thing is: one of the ugly but important
tasks of science is to systematize and measure things that are
"obvious"... because sometimes obvious things turn out to be
wrong, and sometimes they are right but are hiding something
interesting that's not obvious at all.
You need to try obvious things in a controlled environment,
just to make sure that you truly understand them. The chances
that you'll get a Nobel Prize for doing so are slim, but it's one
of those things that needs to be done in order to move ideas
from common sense into science.
Quantifying and explaining the "common sense" features of
the world around us is one of the most important things scientists
do. We all know the sky is blue, but why the sky is blue turns out
to be a fascinating and quite complicated physics problem, and
understanding it has led to lots of other worthwhile knowledge.
We don't all have to be scientists, but in the end we all wind
up taking advantage of their work.
Scott Dorsey (dorsey@recordingmag.com) is, among other
things, a scientist.

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