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GIK Acoustics Presents
Acoustic Class Room
April , 2009 - Vol 4, Issue 4
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Welcome to GIK Acoustics classroom!
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From time to time we'll be sending out
newsletters to all of you to keep you up to date on a variety of room
acoustics topics. Our hope for this
newsletter is that it will help you get a
better understanding of how
acoustics work and how you can benefit by
treating your rooms. If you have any questions about the information in this (also ideas for topics to cover)
or any other issue, please feel free to contact us. The more YOU understand
the better YOUR listening environment will be. Also we will interviewing a person in each newsletter, so if you'd like to show off your room to the
world or have something to say, please contact me at glenn.k@gikacoustics.com
Glenn Kuras
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What Are Room Modes
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Room modes are caused by sound reflecting off of various
room surfaces. There are three types of modes in a room: axial, tangential, and
oblique. Modal activity occurs at
frequencies which are directly related to the dimensions of the room.
Axial modes are the strongest and many times, the only ones
that are considered. Tangential and
oblique room modes have less impact per mode but are also more prevalent. A combination of tangential and oblique modes
can cause just as many issues as axial modes can.
Room modes can cause both peaks and nulls (dips) in
frequency response. When two or more
waves meet and are in phase with each other at a specific frequency, you will
have a peak in response. When they meet
and are out of phase with each other, they cancel and you end up with a dip or
null in response.
Dealing with modes is accomplished by abs orbing one of the
boundaries to minimize the reflections off of it so there is nothing to combine
or cancel. While corners are not a
complete solution, they do offer the advantage of being at the end of 2 or even
3 of the room dimensions so there is a lot of benefit in that area. Sometimes there are modal issues which
require treatment of the rear wall or even the ceiling over your head that
treating corners would not solve.
Below is the formula for calculating all room modes, not
just the axials.
F = c/2 * sqrt(p^2/L^2 + q^2/W^2 + r^2/H^2)
F= Frequency
c = speed of sound (1130 feet per second or 344 meters per second)
sqrt = Square Root
^2 = squared
L = Length of Room
W = Width of Room
H = Height of Room
p, q and r represent the mode we're solving for. If you want to know the axial mode for the
room length, p=1, q=0, r=0. If you want
to know the 2nd axial, p=2, q=0, r=0. To find a tangential, use 1 in 2 of the
variables. So, if you want the first
tangential of the length and width, p=1, q=1, r=0.
Generally, as you go higher into the multiples (harmonics) of
the modes, they become slightly less intense, but also occur at more places in
the room.
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SPOTLIGHT:
Bob Katz
Digital Domain
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Bob Katz is an audio mastering engineer who is known for his influential textbook on audio mastering
and his recording of jazz and classical music. Katz has mastered three
Grammy award-winning albums and one nominated album. He has received
high critical acclaim from audiophiles and his book on mastering has received high critical acclaim, and some reviewers consider it the definitive work on mastering. He is also one of the most respected mastering engineers in the business.
He has also invented a proprietary technology called K-Stereo and
K-Surround. These processes recover lost or amplify hidden ambience,
space and ima ging, and generate stereo from mono signals without adding
artificial reverberation.
Katz taught at the Institute of Audio Research from 1978 to 1979. In 1988, Katz joined Chesky Records and began recording jazz and classical artists there, as well as producing the world's first oversampled commercial recordings. In 1990, he founded an audio mastering company called Digital Domain, where he continues to work.
Frank Oesterheld from GIK Acoustics had the chances to talk with Bob for a one on one interview.
Frank:Your name is a
household word in the world of mastering.
How do you improve your skills at this point in your game?
Bob:
By being as open-minded as possible, by
hearing things that may, if I'm honest with myself, even contradict some of the
thoughts I had before, and by being open to the possibility that I may have
been wrong. I'm always learning
things.
I worked on the most purist folk
recording the other week, and I thought that I had acoustic music really down
pat because I've been doing that for most of my career, yet I learned more
during that session with some extremely critical artists than you can
imagine. The artists were the Kruger
Brothers from Switzerland;
they're very well known in the folk music and acoustic music world, and it's
just an acoustic trio: banjo, bass - not upright bass, but electric bass -
guitar and two or three vocals. That
seems really basic and very simple, but when I heard their first mix, which was
done with the finest of equipment and the finest of microphones, I made a suggestion
that opened up one of the biggest cans of worms that I've ever run into: that
the bass instrument they were using had been recorded in a such a way that it
just didn't have the definition...it sounded like it was the output of a pickup
with a very deep low fundamental bass sound, but the harmonics were very tinny,
and it would not translate to a lot of systems in my opinion. I suggested to them that I could, if they
would send me stems (everything but the bass and the bass) instead of a full
mix, re-mike the bass. I had done this
before with a jazz group and the bass came out excellent.
Now, in my mastering room I don't have
a mic and a studio, but I had purchased an excellent plugin called
"Speakerphone" (by Audio Ease) that has an uncanny representation of an Ampeg
B-18 bass amp cabinet miked superbly, that can enhance the sound of a
pickup. So I said, "this is going to be
a cinch!" It turned out to be a
nightmare. The problem turned out to be
one on my part, maybe a bit of arrogance, assuming that just because I'd done
it before that I could do it just as well again.
That's the learning part: never
assume. Every circumstance is unique;
there are only general rules, and as I pointed out in my book (Mastering
Audio: the Art and the Science) I learn from my mistakes, so maybe I don't
make the simple  mistakes anymore...now I make the big, ugly ones! I wasted about ten hours of my time, which I
did not charge the Kruger Brothers for, trying to get that bass to sound
great. It turns out, the Audio Ease
software, and probably a real Ampeg cabinet, exaggerates, which worked really
well for the bass pickup in the jazz piece that I'd done, but this naked bass
with just banjo and guitar just came out so naked that the resonances of the
Ampeg cabinet were negative. It wasn't
just the resonances of the cabinet that were naked, but the artists' choice of
a piezo pickup in the bass. In the end,
the Kruger Brothers admitted their problem when I admitted mine, that they
should not have so casually recorded the bass with a piezo pickup, and they
went back and re-recorded the bass from scratch with another instrument that
had an electromagnetic pickup and a preamp built in. Suddenly things came back to life.
So,
I'm always learning; things that I've done hundreds of times, maybe the hundred
and first time doesn't fall into the same role.
Hopefully I can apply the things that I've learned before, but I'm
always running into something new.
Frank: Right...so every
circumstance, every mix is different.
Bob: Absolutely, and you can follow some general trends when
you're mixing bass is to boost the presence range of the instrument so that the
bass will cut through because in a complex mix it's the harmonics that help to
define what notes he's playing. We all
know this. But in this particular mix,
every time I tried to boost, I don't know, 700Hz, 800Hz or 1.2Khz, it detracted
from the wonderful, smooth, warm tonality of this bass, which was coming
through just fine. And if you don't have
accurate monitoring, you're going to think that you're doing something helpful,
but no...it won't be.
Frank: That actually
leads right into my next question: as studios downsize, how much work are you
getting from small-room or home studios?
Bob: A lot...a tremendous amount. I think this is a general trend. I think you should ask Bob Ludwig that
question to see just how much he's getting that's coming that way. Everybody's downsizing, and it's not always
for the better. How much of that is
happening? A lot, and an interesting
corollary to that is, a good mastering engineer can help project studio
projects proportionally more than projects that are coming from large
professional studios with top-notch engineers.
So I might only have to do a tiny polish to a recording that was done,
or nothing at all perhaps, to a recording that was done in a good professional
room, versus something that was done in a project studio, and I think that bass
is the final frontier. That's probably
the first thing that we notice. I'm
always surprised; I listened to a mix yesterday...it's usually one of the more
common trends that people work too
brightly, and a lot of "S's" come through, sibilance and high frequency
distortions that they don't notice, in this recording the engineer had
over-reacted to that frequency range, and I'm trying to think, "what monitors
was he listening through to have done that?"
But bass is usually where people have the biggest problem, and this was
not the case twenty years ago...not so much.
I think that more people were mixing with wider-range monitors, and I
cannot understand the trend to work with little tiny monitors whose response
drops off a cliff at 100hz. What is
going on in their minds? It really
doesn't help to translate anywhere. Now
you can say, if it's going to get played back on an iBook, you'll never hear
that problem.
Frank:
Sure, I hear
that a lot.
Bob: Well, but what about an iPod, which has
some bass on those headphones. So
there's a lot of that going on. And what
about cars? There's a ton of that going
on, and a lot of cars have subwoofers and decent bass, or indec ent bass with
tremendous resonances down there. So
you're not helping to translate to a very wide section of environments that
your recording is going to be auditioned in, and depending on how far off you
are, in the mastering I can either help that or not, because it's not just the
bass instrument, it's the bass drum.
Often they get the bass drum right, and I can't figure out how they get
the bass drum right and not the bass instrument right, but if they get the bass
drum right and the bass instrument wrong, I probably will lose some of the bass
drum while I'm cutting some of the excess bass fundamental.
So, I get a lot of work from project
studios and the best advice I can give to anyone is establish a relationship with
your mastering engineer as soon as you have made your first mix. Let him or her listen to your first mix and
tell you in his/her opinion if it's ready for mastering, or if there's some
issues that he/she could help clear up, because the better your mix is, the
better the master I can produce is, and the less work I have to put it through
(ironically) the better it will sound.
Frank: Of the issues
you've seen, and we were talking about getting definition between kick and bass
specifically, how much do you think is due to poor listening environments?
Bob: I think over 90%. Now yes, there's a learning curve when you're
just learning how to become a mix engineer, where your skills don't come up to
the quality of the monitoring you have, but if your monitoring doesn't come up
to your skills, you can second-guess it by putting your finger on the NS-10
woofer to see if there's enough bass drum there, but that's not really the
long-term solution.
Frank: Sure...can you
hear in the mixes you work on where there's a big bottom end that somebody just
couldn't hear accurately?
Bob: Oh, absolutely, and if you asked me if
it's the most common problem that comes in, yes. Bass
is the final frontier.
 Frank: We've talked a
little bit about the non-ideal spaces, but in your own experience, what do you
look for in a good mastering room?
Bob: I have a very wide experience, enough
to be able to pick a space, to know if it's going to cause problems, and it
comes to a great extent from my audiophile experience over the years. As an audiophile I was always setting up good
audio reproduction systems and learning how to get good, wide response across
the spectrum. A good mastering room
should have solid walls, solid floors and solid ceilings, and should be at
least 20' long, preferably 30' long.
Now, you as a consultant in acoustics know that you can get good bass in
a 10' room, but it's almost impossible.
You can hear a 30Hz tone someplace
in that room. To encourage the room to
have an even response in the lower octaves, the room should be longer to
support the longer wavelengths, but I think that "support" may not be the right
word...perhaps a better way to say it would be, to play back more wavelengths
more evenly with fewer peaks and dips.
Then we talk about the golden mean, to make sure that none of the dimensions
are integer-related, and there are some online calculators that will help you
avoid issues if you decide that you want to work on a pre-existing room.
Frank: Even you've
encountered an acoustic issue or two over the years that you've had to correct
in your own rooms, right?
Bob: Yeah, and the more I learn, the more
frightened I get. There are many bass
traps in my studio A and there are many bass traps in my studio B (including
some from GIK Acoustics), and there probably aren't enough of them either.
Frank: Bob, we want
to thank you for taking the time to be interviewed for our newsletter.
Bob: It
was my pleasure...thank you.
Bob Katz:
Digital Domain
931 NSR 434 Suite 1201-168
Altamonte Springs, FL 32714
Phone (800) DIGIDO-1 [344-4361] or +1 407 831-0233
http://www.digido.com/
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Bringing Your Sound to Life.
Sincerely,
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Glenn Kuras
GIK Acoustics
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