Arthur noxon

Selected studio acoustics Correspondence From the Founder of ASC

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

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