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:

Hello Scott

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

-Acoustics Roundtable Featuring Arthur Noxon

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.

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 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
Acoustical Engineer
President of ASC-TubeTrap

Question from a Music professor

Hello Art,

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.

Mike

Art Responds:

Hi Mike,

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,

Arthur

Question from a studio engineer

Hello,

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 products.

Art Responds:

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.

Electrically speaking:

• Acoustic pressure is like electrical voltage.
  
• Acoustic velocity or kinetic energy is like electrical current.
  
• 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. 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.

Arthur Noxon

We 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 also.

What can I do? Please help

Art Responds:

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 or windows.
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 and significant.

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.

Art Noxon

Inquiry 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 time.

Art Responds:

Hi Jack,

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.

Art

We recently recieved this inquiry from a studio in Switzerland

Hi

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 Attack Wall.

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?

Best regards

Art Responds:

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

Hello Arthur,

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 material.

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.

Best Regards
Dror

Art Responds:

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 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.

Arthur Noxon