Selected studio acoustics
Correspondence From the Founder of ASC
to an article in Electronic Musician magazine
Mr. Noxon read Scott Wilkenson's article, "Got
Modes?" in the September'08 issue of EM Magazine.
He then sent Mr. Wilkenson the following letter:
6400 Hollis St, Suite 12
Emeryville, CA 94608
I caught your article in EM which introduces DSP
signal correction into the recording studio.
I’ll phrase the question that is on most anyone’s
mind, the one not answered in your article: How can an engineer
trust the mix when he is sitting in an electronically defined neutral
Engineers today still don’t even know what
to do with subwoofers and now you’re asking them to adding
a mysterious signal distortion device into the audio chain? I’m
sure all this is a hot topic in and of itself but that’s for
1) Today, I wanted bring you up to date, and add
some information to your library for reference when you prepare
for the next article you write on DSP.
This is a reprint of an interview between a DSP manufacturer,
me, a bass trap manufacturer and a product reviewer.
2) The age old question goes like this. When it comes
to powerful low frequency audio in small rooms, the traditional
sound control device is the Bass Trap. Now we have a new tool, DSP
Signal Correction. Which do we use, when and why? BassTraps, DAP
Signal Correction, neither or both?
The short answer is “Both”
The next shortest answer is “Bass traps first
and DSP last.” And I am quoting DSP manufacturers, not me.
They all say the same thing. It’s just that this time it’s
Why? The bottom line for DSP is computer crunching
power. The less they have to do, the better they can do what they
have to do. The more Bass Traps in the room the less DSP correction
is needed and the less correction needed means better results.
3) We always have to remember that EQ, in any form,
does not change how the RT60 works in a room. EQ does not change
the value of the RT60 in the room. Changing the RT60 is only done
by absorbing the reverberant bass energy circulating in the room.
This is the job of Bass Traps. They don’t change how loud
sound is as much as they change how quickly the sound leaves the
room. They are specialty devices that convert low frequency sonic
energy into hot air, by means sonic friction. Like break pads on
EQ by any name, including DSP, just regulates how
loud the sound is at some location in the room. Sorta like automatic
speed control on a car. It has nothing to do with how quickly you
can stop the car because it does not actually remove any acoustic
energy from the room. Now, it is true, that if the sound level is
lower in the room then bass traps can drop the sound level more
quickly down below the noise floor. For the given RT60, the louder
the sound, the longer it takes to disappear.
But, here’s something very true about sound
canceling. It takes acoustic energy to “cancel” acoustic
energy. And DSP signal correction is actually a sound canceling
process. It corrects by canceling. When sound is cancelled in one
place, you will find that it is even louder someplace else. Energy
plus energy equals more energy. The more acoustic energy there is
in a room, even if it happens to be sounding fairly quiet in one
location, the longer it takes for the whole room full of energy
to die out.
And finally, we all learned in music acoustics class
that music is a sequence of complex tonal attacks, releases, sustains
and decays, just like the synth says. EQ and DSP both relate to
how loud the sustain becomes. And how loud the sustain gets is important.
But what people listen to when they are judging the musical quality
of a sound is not how loud the sustain is but how accurate the attack
transient. What we want to know is what happens to the attack transient
when the signal is being DSP’d. Does the pluck of a guitar
string still sound like the pluck.
Some things, once established, do not change. Recording studios
still have to have an RT60 of about ½ second no matter whether
if the frequency response at the mix position is lumpy or smooth.
The only way to get that ½ second RT60 is to adjust the absorption
in the room.
I am always available for a visit when you are working
on a new assignment. There isn’t much in acoustics that I
don’t have some direct experience with and as well, an opinion
Thanks for your patience. I hope you check out QSF
and Awall recording systems. With them you don’t need bass
traps or DSP or even a recording studio and you can still get professional
tracks and mixes. There is a reason that the top recording engineer
in the world, Bruce Swedien, has been recording inside the QSF and
mixing on the Awall for about 12 years. And the reason is that it
works, every time. And, it’s not even all that expensive.
Arthur Noxon, PE
President of ASC-TubeTrap
from a Music professor
I have a student, Lizzy Tanzer, that is very interested
in becoming an Acoustic Engineer. She has taken several acoustic/music
related classes at Portland Community College and just graduated
from CGCC this past June. Lizzy has done some home recording in
the past, as well as being a member of a band and a local DJ.
Her goal is to become an Acoustic Engineer and work in the recording
industry. I am just helping her research possible sites where
she could get the appropriate training. Your company looks like
a perfect fit for her, however I am just not sure of the options
available there. Any help would be appreciated.
Thank you for your time.
Let's be sure we are talking about the same thing.
There are recording engineers, sound engineers and acoustic engineers.
Recording engineers work in studios and make records, sound engineers
hang speakers and run sound boards. Recording and sound engineers
can go to trade schools that specialize in this area. These are
technical or trade schools. There is no trade school for acoustic
techs that I know of.
Acoustic engineers fix and create sound sometimes
for the music industry but mostly for the rest of the world. They
make things like restaurants, offices, churches and so on sound
good, and in the community, to help it be a quiet community. They
work with OSHA for occupational noise and DEQ for environmental
noise and HID for residential noise. They work like a detective
might with lawyers in legal battles that involve noise and sometimes
as a consultant on a crime that involves noise.
There are only a few schools where a person can get
an acoustic engineer degree. It is a 4 year engineering degree and
also a master's degree. Usually the acoustic engineer gets a BS
in physics, mechanical or electronic engineering and takes a masters
in acoustical engineering. Also, it is possible to get the BS in
some engineering field and self study and apprentice for about 5
years and get an Acoustic Engineering license, only in Oregon. Later
in life, after doing lots of jobs, a person might evolve into being
an acoustician, someone who voices halls and other rooms.
I love my work. So, let's double check, does she
really want to become an acoustic engineer?
By the way, the people who work here are musicians
and recording engineers who have to have a real job during the daytime
and who love to be working in the music industry, while after hours,
they work on their avocation, some other, more personal involvement
with the music industry.
I hope to hear from you guys,
from a studio engineer
I have been recommending TubeTraps as acoustic treatments
to clients and friends for years. Recently, a
technical issue has come up in that I referred to them as "cylindrical
pressure zone traps".
A fellow who uses a competing (flat) product said "TubeTraps
as sold by ASC are not pressure absorbers. They are based on rigid
fiberglass and act on wave velocity. Wood panel traps made using
a sealed box with a vibrating front membrane are pressure absorbers."
Is his statement accurate?
Thanks for your help with this and for your wonderful
We are all "right".
To begin the explanation, let's first remember that
what we are talking about is an acoustic wave. An acoustic wave,
like all waves has two components of energy, one is pressure and
the other is kinetic.
We hear the pressure part of an acoustic wave and
mistakenly call it a sound wave. We also feel the kinetic or velocity
part of an acoustic wave, in the low bass, as it brushes our hair
and sometimes even our clothes.
A kinetic bass trap absorbs kinetic energy out of
an acoustic wave. A pressure bass trap absorbs pressure energy out
of an acoustic wave.
Acoustic pressure is like electrical voltage.
Acoustic velocity or kinetic energy is like electrical
Electrically speaking, the TubeTrap is a resistor
in series with a capacitor.
It takes pressure to force current through the resistor.
Yes, current is ultimately what creates the friction and how the
energy is absorbed within the walls of the resistor. To make a useful
acoustic resistor, fiberglass densities in the range of 4 to 7 #/cuft
must be used. This is about 100 times more dense than the density
of air. If the density is heavier, sound bounces off and if it is
lighter, sound tends to just go right through it.
So the TubeTrap takes pressure energy, converts it
to kinetic energy in the walls of the Tube and then absorbs the
energy. This is why it is a pressure zone bass trap, it operates
because of bass pressure.
But let me make my point even more clear.
A kinetic bass trap is a bass trap designed to remove
kinetic or velocity energy from a sound wave.
A kinetic bass trap is typically a large block of
fuzz, very lightweight fiberglass, like building insulation. Typically
the density of acoustic fuzz is about 0.2 #//cuft. It is just about
3 times more dense that the density of air itself, which about 0.08
#/cuft. It interacts directly with the movement of air as an acoustic
wave goes by. It does not use pressure to get work done.
If we have a vertical standing wave in a corner of
a room, we have big pressure down low, at the floor corner and big
pressure zone up high, at the ceiling corner. Half way between we
have a sound or phase cancel zone where the sound of the resonance
is silent. Inside this phase cancel zone all the energy of the vertical
resonance is in its "velocity" form, the acoustic kinetic
energy form. Here is where you put a large loose pack block of insulation
to absorb energy from the kinetic energy part of the wave.
In either corner there is no kinetic energy, just
pressure changes. A big block of fuzz doesn't work as a bass trap
in the tri corners.
Put a TubeTrap in the tri corner and it does work
because the pressure there is strong and pushes air into and pulls
it out of the TubeTrap.
Put a TubeTrap in the kinetic energy part of the
standing wave, half way between the floor and ceiling, and you'll
see that it does not absorb bass energy. A TubeTrap is too hard
and too small and the air movement does not run through the Tube,
but just goes around it, as if it were a tree trunk or a pillar.
So, yes, the only way a TubeTrap works is by absorbing
velocity, but it is not a velocity bass trap, (a big ball of fuzz)
it is a pressure bass trap, because it only absorbs pressure energy
out of an acoustic wave. And yes, it does use pressure to create
the velocity within the walls of the Trap.
Now when people talk velocity and waves, there are
two types of "velocity" an we need to make sure we are
all talking about the same thing. With "sound waves" the
main velocity people know about is the "speed of sound",
which is about 5 miles per second or a little more than 600 mph.
This is officially called the "wave velocity" and it does
not have anything much actually to do with sound absorption.
There is another "velocity" in waves, which
is the speed that the air sloshes back and forth when a "sound
wave" passes by. This depends on the frequency and pressure,
but roughly it calculates to be about 1/2 foot/second for real loud
sound at very low frequency. This is what wiggles your hair or your
clothes when you are near a bass driver out in the open. And yes,
this is the "velocity" or kinetic energy part of the sound
wave that interacts with a ball of lightweight fuzz. TubeTraps do
not absorb energy out of this type of acoustic energy, the velocity
or kinetic energy part of a "sound wave" .
A thin wood panel faced box with fiberglass inside
is also a pressure bass trap, usually getting a 30% efficiency or
less, (compare to the efficiency of a TubeTrap that is upwards of
150%). It takes pressure to move the panel. So it's a pressure bass
trap, like a TubeTrap. But what happens when the panel moves? It
sloshes air around inside the box, like an ole time plunger washing
machine. Only here, the sloshing air takes place inside the fiberglass
that is packed inside the box. The air sloshes because the middle
of the panel is free to move and the edges are fixed, so air moves
back and forth from the center of the wood panel in and out, towards
the fixed edges.
By the way, the IsoDamp
Wall system is a giant membrane bass trap. And it does
not absorb energy due to either of the two velocities associated
with sound. It absorbs energy due to displacement and what drives
displacement? Yes, pressure. Pressure pushes the wall in and WallDamp
gets distorted and energy is absorbed. The IsoDamp wall and ceiling
system is also a pressure zone bass trap.
We have been working on a membrane bass trap box
product using WallDamp instead of fiberglass to absorb the energy.
Now, a carpet is a kinetic trap. It is a large flattened
out ball of fuzz. As bass energy circulates around the room, pure
pressure on the carpet produces no distortion and no air flow, which
means, no energy absorption. But the velocity or rubbing part of
the circulating energy rubes against the carpet and looses energy.
As the kinetic part of the bass wave hits the floor and scrubs the
fibers of the carpet, friction absorbs energy out of the wave.
I hope I have cleared up how it is that we are all
correct on this one, I'm glad to say...
Sometimes the words we use get in the way of what
we are talking about. I like science because we can always go back
to first principles and figure out in slow motion what the heck
people are talking about.
Thank you very much for giving me an opportunity
to review and discuss this issue. I look forward to your next brain
recently recieved this inquiry from a studio engineer
Hi, first sorry about my English. I am
living in Turkey and I have my own project studio here.I am a
very big fan of BRUCE SWEDIEN, so I found you. I need a good acoustical
enviroment for recording vocals or guitars. I know your great
product Quick Sound Field but dont want to disturb my neighbours
What can I do? Please help
The QSF is not a vocal booth. It does reduce the
amount of bass and treble that gets into the room and it diffuses
the bass and treble before it gets to the room. By reducing the
energy hitting the walls, the amount of sound leaving the room is
reduced. By diffusing the direction of energy hitting the walls,
less sound hits the walls square on and secondly, the wavefronts
are small, weak so they cannot deliver strong impacts to the walls
Yes, the QSF reduces the sound level transmitted to your neighbors.
I estimate that the sound reduction should be in the 5 to 7 dB range.
This will be noticeable to your neighbors. Reduction of 3 dB is
typically just noticeable for people. The neighbor's perception
of sound reduction will be something like it was reduced by 60 to
70%. This may not be enough for your neighbor but it is noticeable
Send photos of your studio and photos to help me
understand where your neighbor is. Possibly I can help you with
sound containment. We have many accessories for studio work.
Here's another thing about QSF. It provides lots
of information coming back to the singer about how they sound. You
will find they will not be getting loud for effect, replacing effect
for accuracy. When a singer is in sync, when the sound in their
mind matches the sound in their ears they have no more unmet needs
and they do not try to find their personal power with power as they
have found it with quality.
It's subtle a very real effect. Trust the QSF
and just do it. It is much more than you can imagine. Don't over
intellectualize it, trust it and do it and discover what it brings
to you. Discover why Bruce wants you to use it. You'll never know
it until you hear it.
from a music student
Hello, My name is Jack and I'm currently
studying a BA in Music Technology. As part of one of my third
year modules I have to produce a detailed and professional report
of the modal responses of a control room and a suitable suggestion
for acoustic treatment. The report itself has to take the form
of a commercial consultancy, therefore I was wondering if it would
be possible for you to email me one of your old consultancy sheets
so I could see how it is laid out and presented. Any help you
could offer me would be very greatly appriciated. Thanks for your
We don't exactly fix modal responses, although people usually think
we do. Tweaking room modes is not as easy as it seems and doesn't
give the results wanted in high performance rooms. Room mode adjustment
is all about steady state acoustics.
Music is not steady state. We work the dynamic part of music. Essentially,
we try to get the attack transient to be as undistorted as possible.
We work in a time scale that is very small compared to room modes.
To develop a room mode, it takes a continuous play of the same
sound, lasting about 1 second or more, (whatever time the RT60 of
the mode is). Except for Bach organ music, most musical moments
have come and long gone well before a room mode could ever get developed.
Typically each distinct musical sound last about 1/4 second which
means modes don't really exist in real music. wow, who'd have thought.
Still, dreaming about modes is fun and a good mental exercise.
I know you're in school and I'll be glad to help you through your
assignment, even if it isn't very relevant to how real rooms are
recently recieved this inquiry from a studio in Switzerland
Thank you very much for the floor plan of the Attack Wall setup.
I placed 9" paper templates of the 17 StudioTraps and of
the 2 Monitor Stands on the ground as positioned on your plan
and it fits exactly to the room dimension. So it's OK for the
Now what about the Live End of the room itself? Would it be possible
to improve the rear wall, the side walls or the ceiling? What
would you suggest?
There is no LEDE (LIVE END DEAD END) as you are imaging
it with the ATTACK Wall. In traditional studios the DE is up front
and the LE is behind. In the Awall system, the immediate area around
you is the DE and the area outside of the Awall is the LE. The LE
surrounds you and the DE surrounds you. Sound that isn't absorbed
by the wall escapes over the top and under the bottom of the wall
and is reflected back off the floor and ceiling and the walls, right
back onto the outside of the wall. Between the outside of the Awall
and the room is very diffusive. All the reflectors of the Awall
are facing out which creates the diffusive tail. The time delayed
diffusive energy spills into the Awall zone, from under the traps
and over the top of the traps
Usually there is no need for extra acoustics. The
Awall is complete. Just wait to hear the Awall system and see how
well it works. If you need something extra, then we'll deal with
from an Israeli studio acoustician
I was very happy to receive your mail.
I intend to focus my business in israel in the industrial and
commercial area's (live music venues, offices, library's, home
theaters etc.) since the music recording industry here is not
so strong and most of the small studio owners will not invest
thousends of dollars on acoustic products. I already got some
response from studio owners and the most common questions i got
was - "how many of these traps i need for my studio?"
or"whats the minimum number of traps i need?". In order
to make any primary order from you i would need a price list so
i can give potential costumers a rough estimation of costs. I
will also appreciate if you could send me some catalogs / promotional
In my studies i examined the effect of
acoustic diffusers (slotted, perforated and bass trap) on small
room acoustics. I built one diffuser of each kind, all from the
same materials and of the same mass and checked which combination
of two out of three was the most efficient in eliminating the
specific acoustic deficiency in the room (all resonators were
tuned to eliminate the specific frequency). The results suggested
that the combination of bass trap and perforated resonator were
the most efficient.
1) It is good to build acoustic devices and experiment
with them. With the TubeTraps, all the building and experimenting
has already been done about 15 to 20 years ago. We are now hoping
to export TubeTrap products and technology to Israel through you.
2) Building and experimenting. Importing and selling. These are
two very different topics.
3) We used to build resonator traps. They work well
for steady state but do now work for transients. We stopped building
and converted our resonator products to a form of product that could
fully absorb transient pressure pulses. Resonator products do not
absorb energy out of transient pressure pulses.
We learned long ago that controlling the RT60 is
not what makes good sound. Working with RT60 control means resonator
devices have enough time to get charged up and start working. However,
our customers play lots of transient pulses, not tones. We needed
products that removed energy immediately upon initial contact with
a pressure pulse waveform, long before the room reverberation was
even created, let alone began to die out.
As a TubeTrap dealer, we will teach you about TubeTrap
technology. You will then understand why your early experimenting
was a good experience. It gives you the background you need in order
to understand what we are doing. And I can assure you, that we are
not successful and well known because we do the same thing everyone
else does. We are successful because we operate on sound long before
it becomes the sound that most people think is the sound of room
4) We asked that question in the early days. If you
read my AES papers (1985) you will find the answer. You need a minimum
of one TubeTrap for every 500 cubic feet of room volume. Acoustics
is naturally much more complicated than "how many TubeTraps
do I need" but that is a good question.
5) You need to provide corner bass trap/treble diffusion
stacked up in each and every vertical corner of the room, floor
to ceiling. Your work-horse product is the 16" TubeTrap.
6) You will find that ASC has become very loyal to
the sound of curved surface diffusion. We do not use slotted or
perforated bass traps. These are typically used for resonant absorbers,
not broad band absorbers. We like the sound of poly-cylindrical
diffusion. We use suspended limp mass combined with perforated sheets
to create out cross overs that make up the TubeTrap frequency response
7) We do have a shaped frequency response curve.
We do have an LRC circuit, but the R is so large that it is not
a resonant (LC) circuit. It is more like a parametric circuit, with
the low (RC) frequency roll off typically set around 60 Hz and the
midrange roll off (L) set at about 600 Hz.
I look forward to working with you. By working together,
I am sure we can develop into a good team. The most important thing
is to purchase a pallet of TubeTraps so you can begin to experiment
with this next era of acoustics for you.