Selected General Acoustics Correspondence From the Founder of ASC
question about Glass between control room and live room
We are building our first small studio in Berlin and I have a question regarding the window construction between the recording and the control room. I searched the internet but could not find out how it is done best. It would be great if you could help me... here are my questions:
A - Which and how many glasses (and the type of the glass)
would you recommend? Do I buy 2 very thick single glasses or 4 thinner
glasses (2 double glasses with vacuum in between) in an "A"
or "V" shape?
First, it needs to be laminated glass. Automobile window glass is curved laminated glass. You want flat laminated glass in your studio. It costs twice as much as regular glass because there are two layers of glass. Probably use 8, 9 or 10 mm thick laminated glass. The clear laminating adhesive between the sheets of laminated glass damp out the vibrations of the glass just like WallDamp does for walls. Technically different thickness laminated glass is better than same thickness, as in 5mm + 4mm will give a 10mm asymmetrical laminated glass window.
Do you really need a double glass isolation window? Are you going to be making loud sound in the control room when you have an open mic and are laying tracks? Is your mixing in the control room going to be bothered by some muffled noise coming through the window from the live room? My guess is that a V window is an overkill.
If you aren't using a double V window then leave the window vertical, like the walls.
A large square window looks great but it drums, thunders. A set of tall, narrow windows look great, not quite as open, but they don't drum or thunder.
If you are making a V window, you have to put one glass in one wall and the other glass in the other wall and leave a gap between the walls, also open to the air cavity between the windows. Close the edge of the window with black felt, so the gap is visually closed but acoustically open. We don't want a sealed V window going into to wall, we want it edge vented so the pressure that builds up inside the V is vented to the wall cavity. Because the top side, the wide end of the V window is open to the wall cavity, no sound damping is necessary. It's already in the wall cavity.
If a double window, always do a V shape, not upside down. Dust collects on the slant surface. Also add 2" sound panel to the window ledge at the bottom of the V outside of the glass so you get rid of that corner reflection.
Single laminated window has STC about 20 and double
laminated window that is end vented will get STC about 35 to 40.
question from a home studio with a sub-woofer
I just finished reading your "Subwoofer Placement Article "and was very impressed. Your reasoning as to where to place the sub just makes so much sense to me; it's seems a brilliant way to avoid the grueling trial and error method. I had a question about the sub placement in my 13.25' L X 10' W X 7' H home studio.
What I understand from your article is I need to place the speaker center at 40" from the front or rear wall, 30" from the side wall and 21" from the floor. You state that the speaker center is the only location on the sub that matters and that the outside edges of the sub are not to be considered. If the sub is firing down the length of the room the speaker face is a one dimensional point that I can see would result in the 40" front to back spacing. However when the speaker center is supposed to be 21" off of the floor the bottom edge of the speaker cone would then be only 15" from the floor. It seems to me that the sound wave coming from the speaker is 12" wide and this would make the edge of that wave 15" from the floor, would this excite one of the nodes? Should I add half of the speaker diameter to the 21" in order to raise the sub high enough avoid this problem or is there something incorrect in the way I am thinking about this? I would appreciate any information you could give me to help answer this question. Thanks.
You are using 0.25 times the length, width or height for your subwoofer cone location. My recommendation is 29% not 25%.
This means for your room, which is 13.25 x 10 x 7, your cone should be located 35.8" in from the side wall, 24.4" off the floor and 46.1" off the front wall. You are thinking to use 30", 21", 40".
There are two parts to this topic. One is to not stimulate MODES and the other is to deliver attack transients that are free from any drone-tone.
You room is 7' high, 84".
The first resonant mode for this height has a 14' wavelength, which is 80 Hz. The second resonant mode has a 7' wavelength, which is 160 Hz. And the third is a 4.7' wavelength at 240 Hz.....
Your sub is rolled off at least at 80 or 90 Hz, possibly lower depending on the power and extension of the woofers in your mains. For good mixing you should have big powerful mains in the midfield and your sub should only be used to support energy 50 and below. Mixing on little mains, and using your sub to deliver energy up to 80 or 90 Hz is good for the studio budget but bad for the mix. The sub certainly does not stimulate 160 Hz or above. So we don't have to worry about height positioning for these upper harmonics of the room's height.
If you are rolled off at say 50 or 60 Hz, we don't even have to worry about vertical placement of the sub because we aren't even going to be playing 80 Hz. As far as MODES are concerned.....
However, we always have the THUMP vs THUMM effect, which converts non tonal attack transients into tones, the old "one note bass" type of sound in rooms, coined by J Gordon Holt in Stereophile back in about 1987. This is when we mic a great kick drum that has been damped out with a good drum pillow so there is nothing but a good thump and then you hear a thumm sound during the mix.
My suggestion to make the woofer most mode free and atonal is to locate it either 29% or 42% between each set of parallel surfaces. For an 84" ceiling and 29%. this is a height of 24 1/3rd inch. This should be the height of the center of the woofer above the floor. You came up with 21" and I'd like to know where that number came from. 21" is 1/4 of 84" and that is not something I would have suggested.
The main thing is to get the sub off the floor and put a bass trap underneath the sub. There is nothing better, almost, than a musical subwoofer. Until you hear it, you will never have even imagined how music could actually extend down into the subwoofer region This is because we have all gotten used to the awful sound we get by sitting subs on the floor. And the reason we sit subs on the floor is because people think it looks good down there and people want to design, make and sell what other people think look good, instead of what sounds good.
I'd like to see somebody in subwoofer audio just be real for once. Design and make a subwoofer with a built in bass trap that is upward firing, sits on the floor and whose height is 29% of the room height.
Response 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 mix environment?
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 another day.
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.
Room Acoustics: Audio's Final Frontier
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 in print.
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 a car.
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.
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 about.
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
question 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,
question from a studio engineer
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.
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. Stay tuned.
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 teaser.
We recently recieved this inquiry from a studio engineer
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
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.
inquiry from a music student
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 set up.
We recently recieved this inquiry from a studio in switzerland
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 that.
question from an israeli studio acoustician
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.
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 acoustics.
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 curve.
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.