Joined: Jul 2011
Location: Eugene, Oregon
Lots of questions here, thanks for asking for my input. If I leave something out or confuse anyone, just speak up.
The TubeTrap is a pressure zone bass trap. However it does not directly convert sound pressure into heat, it transforms sound pressure into air flow or movement through an "acoustic resistor" something like but not exactly Owens Corning 703, which has a density of 4#/cuft, and is more like what we use in the wall of the TubeTrap.
TubeTrap is not a velocity trap. By velocity I mean acoustic velocity. This is not the velocity of sound, which is over 1100 feet per second. Acoustic velocity is the speed that the air moves back and forth when a sound wave passes. It is typically just a few feet per second.
Incidentally, a sound wave does not exist. It is just the pressure part of an acoustic wave. The other part of the acoustic wave is the velocity part, the kinetic energy part. That's why when we "cancel sound" we don't really cancel sound, we transform the pressure part of the sound wave into kinetic energy or velocity, which we don't hear. Condenser mics measure pressure, while ribbons measure velocity, the kinetic energy part of the acoustic wave.
Velocity traps use low density fiberglass, 0.4#/cuft, something like fluffy building insulation, which is located at a 1/4 wavelength position. To get to 55 Hz, this device would be (1128/55)/4 = about 5' away from the wall. Velocity traps are certainly the most talked about bass trap but no one much makes them. To dampen the 1st harmonic down the length of a room we'd have to fill the middle 1/3rd of the room with building insulation. That's not how it's done in the real world.
Don't get me wrong. I love them. I wrote an article on all the bass traps I could find for dB magazine back in maybe '86. It's posted on ASC website. There you can read about many types of bass traps. Here's an example of a hybrid velocity bass trap. Take a 4x8 sheet of plywood and hold it perpendicular to the direction of the sound wave. A sound pressure difference is found between the two sides. That pressure difference causes air to flow around the edge of the plywood, back and forth. Add building insulation around the outer perimeter of the panel and that air movement in the fiberglass causes friction, which results in heat. You could call this a pressure gradient bass trap, in that something causes a pressure difference and air flows because of the difference, causing friction and energy loss.
TubeTrap is like that except it works in the corners or at the walls of the room, places in the room where there is only pure pressure. Take a pile of fuzz and put it in the corner, there is no velocity gradient, and no velocity, just pressure in the corner. No velocity means no absorption for fuzz in the corner. TubeTraps are not a ball of fuzz. They do not work due to the pressure difference along an acoustic wave. They create their own pressure difference. They operate only in response to the pressure part of the sound wave. That's why I call them a pressure zone bass trap.
The pressure builds outside the wall of the trap, which pushes air through the wall of the trap, which causes pressure to build inside the trap. When there is a pressure difference, the air flows and the trap absorbs. When the air pressure fills up the air stops flowing. That's why bigger diameter traps work to lower frequencies, they take longer to fill up.
Yes, pull traps out of the corner and you move them out of the pressure zone and that's why they absorb less. TubeTraps provide air movement in response to air pressure. This means they have acoustic impedance. Impedance is like electrical resistance, the ratio of current to voltage. In acoustics is air flow velocity divided by air pressure. We set our impedance to that of sound when in the open air. Since TubeTraps are impedance matched, they do not reflect as they absorb. They are like an impedance matched resister at the terminal end of a wave guide, like a 300 ohm resister attached to the open end of a 300 ohm coax line.
The walls of the traps are sealed in two ways. One is that they are pneumatically sealed, so that only when air moves in and out of the wall of the trap can air get into or out of the interior air volume. Even the threaded holes at each end of the TubeTrap, in the end caps are sealed air tight. The second way is that they are sealed fiber tight. No fiberglass dust ever escapes from any ASC product, TubeTrap or flat panel or anything else. So they are air tight and fiber tight sealed.
As far as DIY traps, it's awful to get covered in fiberglass. Just about no one does it more than once. We have created a fiberglass dust-free TubeTrap factory so that no one has to breath fiberglass dust in order to make TubeTraps.
We do not use foil reflectors on TubeTraps. Rigid round air conditioning duct comes covered with foil. DIYers cut half the foil away and call the other half a half cylinder diffuser. Unfortunately, they don't sound right. Too muted. And they choke off air flow, because they seal half of the surface of the tube. We don't use that type of duct, because it does not have the correct flow resistance. Our reflector is a suspended sheet of vinyl, that has the proper weight and a proper hole pattern so it creates a good sounding acoustic crossover, letting bass and mids pass right through the hole pattern and reflecting the upper treble, anything above 600 Hz. It is an engineered acoustic crossover (12 dB per octave slope) set at the proper frequency. The suspended vinyl diffusion sheet covers 1/2 of the cylinder surface that makes the TubeTrap.
Rotate traps so the chrome button faces forward, or into the room, to keep the treble range ambient diffusion as bright as possible. Rotate the reflector away from the room to darken their effect.
Side note on diffusion: There is a huge difference between QRD (random phase) class diffusion and TubeTrap (specular, phase aligned) class diffusion. They sound nothing like each other. TubeTrap diffusion sounds exactly like what went in, a crystal clear replica of the incident sound, except quieter in level because it is more spread out. QRD reflections take the incident sound and converts it into whistle tones beamed in different directions. The difference is shocking, once you take the time to beam treble range pink noise into them, first one and then the other, while walking around in the reflection zone and listening to the sound of the reflection.
There is much more we could visit about here, but maybe later.
Next topic here is the AttackWall. First, read up on it on the ASC website. Here's what it does. It's like a soffit loaded monitor setup, except it's freestanding in the midfield. The frequency response in a room is flat in the top end and gets very droopy in the bottom end. AttackWall creates a slight, uncolored bass boost which results in a flat frequency response in the room. Use pink noise to adjust the EQ of your AttackWall setup. The tighter the traps the more sealed the Awall becomes and the more bass boost is delivered. It's like an acoustic EQ. Open the spaces, more bass leaks out and through the wall and the bass boost is reduced.
The goal for the engineer is to work on monitors that have a flat frequency response, flat without electronic EQ. The Awall performs an acoustic EQ to the system, colorlessly adding a few dB of low pass bottom end, whose low pass gain, ranging between 0 and +5 dB,Acoustic, is adjustable.
All TubeTraps used in the AttackWall are set up with the reflector pointed to the outside, and only the dead side is facing the engineer. Sound that is emitted from the monitors, which passes over the top or under the bottom of the Awall, bounces back against the walls of the room and back again into the diffusive reflectors of the StudioTraps. This begins the LE part of the LEDE studio criteria.
As for TubeTrap styled bass traps and their use in the Awall....some confusion shows up here. I recently gave a talk at the grand opening of our show room project at RSPE in Universal City where an engineer asked.."those StudioTraps are only good to 110 Hz but here (in the AttackWall) you are using them to develop and support sound down to and even below the 50 Hz range. How is that possible?" Great question, same asked here.
The Awall is not using the StudioTrap as a bass trap. It is using StudioTraps to create a complex acoustic wall. For sound to get past this wall it has to enter one side of the StudioTrap, pass thru the air cavity and exit the other side. It had to pass through two walls of the TubeTrap. Here the TubeTrap is being used as an element in the construction of a wall.
Side note: Put speakers in the corner of the room and you load them, and get more bass. But you also get a megaphone effect. The Awall slightly loads the speaker but does so on a colorless way. Then it does many other things at the same time resulting in a legitimate LEDE recording studio. Only this one is something you can pack up and take with you.
Yes, as few as 2 Studios on each side (4 total) of a speaker gets you pretty close to the Awall sound. That's how we do the other 2 speakers in 5.1 mixing environments. The MonitorStand Traps are very special TubeTraps, nothing like normal TubeTraps. They help to weaken and mismatch the floor and ceiling bounce so these two bounces don't feedback with the monitor.
Well, what did I leave out? Let me know....
Thanks for the interest
Inventor of TubeTraps
President of ASC