THE BASICS IN 5 MINUTES..
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culprit is bass waves bouncing off the wall behind you. Nulls as deep as 30 dB are not
only common, but typical."
few milliseconds later, reflections arrive after the sound bounces off the side walls and
ceiling. This is known colloquially as time smearing."
packing blankets, egg cartons and non-acoustic foam: These do not work. That's just
another Internet myth."
| By Ethan Winer
electronic gear these days is very high quality, and can create first-rate music that
sounds as good as anything on the radio. Yet many recording enthusiasts, unhappy with the
quality of their productions, wrongly blame their gear. Of course, experience matters, but
so does having accurate acoustics. This "Tip Jar" article from Music Connection
magazine explains the basics of acoustics in a way that everyone can easily understand.
All of the information herein also applies to audiophile and home theater listening rooms.
BASS IN THE PLACE
Bass frequencies are the most difficult to tame in the small
rooms many people use to record and mix their music. One common problem is a mix that
sounds great in your room is too bassy when played anywhere else. The culprit is bass
waves bouncing off the wall behind you, creating deep nulls in the response you hear.
Nulls as deep as 30 dB are not only common, but typical. Most small rooms have several
nulls in the bass range below 300 Hz. The Before / After frequency response graph above
was measured in a typical domestic room. Click the image to see it full size.
Since you hear less bass than is really in the track, you add too
much bass with EQ to compensate. The finest loudspeakers in the world are of little value
if your room skews everything you hear. The solution is bass traps placed in the room
corners. When bass traps are added to a room, the low frequency response becomes much
flatter and tighter, and also changes less around the room. Note that rectangle rooms have
12 corners: four where each wall meets another wall, four where each wall meets the
ceiling, and four more where each wall meets the floor.
ROOM ORIENTATION AND SPEAKER PLACEMENT
In a standard rectangular room, it's best to orient the mix position so the speakers are
firing the longer way down the room. This improves the response at low frequencies by
putting the reflecting wall behind you farther away. Symmetry is equally important, so
center your desk and speakers left-right in the room. Placing the mix position in the
front part of the room also helps the low end response. The ideal speaker height is with
the tweeters at ear level, because the flattest response is on-axis. I prefer speakers to
be level, not angled down to your ears. Otherwise, as you move forward and back slightly
the high frequency response changes. The article How To Setup a Listening Room
AVOID EARLY REFLECTIONS
Another common mixing problem is poor clarity and imaging caused by "early"
reflections. Sound from the loudspeakers travels directly to your ears, but a few
milliseconds later, reflections arrive after the sound bounces off the side walls and
ceiling. This is known colloquially as "time smearing," though the more proper
term is comb filtering. Comb filtering is a specific type of frequency response
error that creates a series of peaks and dips. The cure for early reflections is absorber
panels made of rigid fiberglass or acoustic foam, placed at specific points on the side
walls and ceiling. Once these points have been treated, clarity and soundstage magically
come to life. All of a sudden you can easily hear EQ changes of less than half a dB, and
very small changes in left-right panning. See the article Creating a Reflection-Free Zone for
Diffusion avoids the damaging echoes and comb filter effect caused by reflections off
nearby walls, but without reducing desirable ambience as absorption does. The best type of
diffuser is called a QRD, which stands for Quadratic Residue Diffuser. You don't need to
understand the math behind a QRD diffuser to appreciate how much better it sounds than a
bare reflecting wall! Unfortunately, good diffusers cost more than good absorbers. If the
room and budget are both small, placing absorbing panels on bare surfaces works well. But
when cost is no object and you want to retain as much liveness as possible, diffusion is
the key. Good diffusers also make a small room sound much larger than it really is. But
forget bookshelves - that's just an Internet myth. Our two videos Hearing is
Believing and All About Diffusion let you hear
what diffusion sounds like.
JUST SAY NO TO ROOM EQ
Trying to use an equalizer to fix room acoustics problems does not work very well. Every
location in a room has a different response, so no single EQ curve can help everywhere.
Even if your goal is to correct the response only where you sit, it's impossible to
counter nulls. If you have a 25 dB dip at 60 Hz, adding that much boost with EQ will
increase low frequency distortion in the loudspeakers. And at other places where 60 Hz is
too loud, EQ makes the problem worse. EQ can reduce peaks a little, but it does not reduce
the extended decay time that accompanies most peaks. Our Audyssey Report article explains why
EQ is not a suitable substitute for bass traps and other treatment.
PROFESSIONAL MATERIALS = PROFESIONAL
Forget packing blankets, egg cartons, non-acoustic foam, and carpet: These do not work.
That's just another Internet myth. Egg cartons are too thin to absorb low frequencies
which is needed for music rooms. Likewise, non-acoustic packing foam is not suitable
because it's not the open cell type that absorbs sound. Rigid Styrofoam panels meant for
home insulation have no useful acoustic value either. The best bass traps and absorber
panels are made from rigid fiberglass. Inch for inch, rigid fiberglass absorbs more, and
to a lower frequency, than any other material available.
Most of my tips are about improving your ability to mix well. After all, if you can't hear
accurately, it's impossible to know what mix elements need adjusting. But good acoustics
is just as important where you record instruments and singers with microphones. The same
reflections that reduce clarity when listening through speakers also make live instruments
sound boxy and off-mic. The closer the performer and microphones are to bare walls, the
stronger the reflections will be. Important places for absorption or diffusion in a
recording room are surfaces closer than about ten feet, and especially on the ceiling
above drum overhead microphones.
JUST FOR THE FUN OF IT
In my experience, problems due to poor room acoustics are the biggest cause of
dissatisfaction among home recordists. Sadly, many people consider everything but
acoustics when they're unable to make a mix sound the way they want. Besides making it
much easier to hear what's actually "on tape," high quality acoustic treatment
makes recording and mixing a lot more fun. It's a real eye-opener when you first hear
every note clearly articulated by an electric bass, and are able to hear very small
changes in EQ and reverb settings. It may seem surprising, but acoustic treatment will
improve the quality of everything you record and mix far more than your choice of
microphone, preamp, and sound card. In that one moment, when you first work in a
well-treated room, it's immediately clear what you've been missing all along!
Ethan Winer plays the cello and electric guitar, and loves to
write pop instrumentals. Ethan has, at various times, earned a living as a studio
musician, computer programmer, audio engineer, composer/arranger, technical writer, and
college instructor. Ethan now designs acoustic treatment products and runs RealTraps where
you'll find many more educational articles and videos about acoustics.