It takes 8 DIY StudioTraps to make one QSF acoustic space
QSF is an acoustic space that saturates the Haas reflection time period. It can be generated by a particular arrangement of StudioTraps that create numerous early reflections the treble range and all the while maintaining an RT60 of about 1/10 second. I have tried other ways to get this sound effect and always end up pretty much with the same setup.
Correction: It is not derived from a hemispherical reflection pattern; it is generated by an array of hemi-cylindrical specular reflectors.
StudioTraps normally list at about $360 each, including stand. The goal being discussed here is how to make a DIY StudioTrap. The next issue should be how to set them up so that they create the QSF effect.
If I were to try to make a DIY StudioTrap out of sonotube and rock wool, following Eathan's not unreasonable suggestion and others, I'd start with an 8" sonotube (not 12"), cut short into 4' lengths. Then I’d cut sonotube in half to make two half cylinders. When cut, sonotube expands, tending to flatten out. We don't want a flattened out half cylinder reflector. We want a real half cylinder.
Side note: The size of the reflecting half cylinder is important. Too big and too much lower treble is reflected, too small and only the high treble is reflected. The diameter of the cylinder is in effect an acoustic cross over, bass pass / treble reflective. The diameter is also related to how loud the reflection is, the larger the diameter, the flatter the reflection and the louder it is. QSF kicks in at about 500 Hz.
Get a piece of fiberglass string reinforced plastic window screen 16" by 60". Hot glue the middle 48" of one long edge onto the outside edge of the 4' cut edge of the sonotube, about 1 1/2" back from the cut edge. Now you have two end flaps and the middle 4' along one edge is glued down at one edge. Wait at least 10 minutes for the hot glue to harden.
Now compress the half tube back into its cylinder shape and pull the screen tight over the other edge. You may need an assistant or some sort of jig to hold things in place. It’s like alligator wrestling. Hold the screen in position, hot glue it about 1 1/2" in from the edge on the other side and hold it for at least 10 minutes, till the glue really hardens. A jig, made of plywood 12"x48" and a couple 4' 2x2s nailed with an 8" space between makes a nice clamp for this job. Glue lines would be set about 2" back instead of 1/2" but that's OK.
Fill the half cylinder, screen faced cavity with some "703" or rockwool or equivalent very absorptive material. Personally, I would go to an upolstery material distributor, or a matress factory and get expensive foam, very soft and dense. Use this material insted of fiberglass or rock wool and make an itch free product but it costs the same as good fiberglass. Pull the end flaps over and hot glue them down to seal the half cylinder.
In studio work, it is very, very important that the talent never gets fiberglass or rock wool fibers stuck in their throat. The slightest fiber can choke the talent and stop a session. We also do not want to see glassfibers floating in the air, landing on CDs, getting sucked into hard drives, tapes, switches or mic capsules. At ASC, we go to great pains to literally seal our products. They just can’t ever leak fibers into the air, no matter how loud the sound gets. By the way, make sure that whatever acoustic material you stuff into your cylinder has a neutral smell.
All TubeTraps products are devoiced; they have no resonance characteristics at all. Their length has nothing to do with their frequency of operation, only the diameter. If you do not use serious absorptive material in the back side of the hollowed out half cylinder, the unit will resonate and blow sonic colors into the mic. That’s not good.
You can make your tube look better by wrapping the screen around the front of the tube, so it gets ride of the spiral groove. Still, the screen does not fiber seal the unit. The window screen only holds the absorptive material in place. We also don’t want the absorptive material to settle, as this would leave a hollow sounding, hollowed out cylinder shape at the top. When glue is under tension, do not reheat it. It will release and things will fly apart.
Get some felt at the craft store and lightly spray glue it down over the entire half-cylinder, both the front and back, top and bottom. It is acoustically transparent enough. You can use different colors, have fun with the covering. The felt will fiber-seal the product you have built. Now it is safe to use in an enclosed room, a studio
We have gone over what it takes to imitate the acoustic properties of a StudioTrap. The cut off frequency and roll off slope are not exactly the same, but close enough for DIY. What we haven't talked about is how to hold the StudioTrap up. I'd start with 3 long and 1 short stick and make a 6' tall tripod with a cross bar at 30" off the floor.
The QSF is created by a horse shoe pattern of StudioTraps, on 18” centers. The center of the reflecting surface is set about at voice or instrument height. High enough off the floor to backscatter the treble back into the mic and separated apart enough to let the bass vent out of the space. There are lots of ways to hold these half cylinders up off the floor. But we do not want to create a giant domino effect, where with one bump, the whole setup tumbles down.
With reflectors rotated away from the talent, flat side in, you will recreate that old familiar Studio-Dead sound. With reflectors rotated toward the talent, flat side out, you get that oh so desirable Studio-Live sound. The better the room the farther apart the StudioTraps can be set. As you raise the mic you can acoustically mix the QSF direct with a little room ambience for sweetning.
Bruce works in the Hit Factory and sets his 14 Traps about 3 feet apart and randomized the reflection pattern. He has been recording in QSF for over 12 years.
A regular engineer in a regular studio likes 10 Studios and will use a tighter pattern. At home, 8 Studios are used and the Trap pattern tightens down to 4 feet in diameter, with the individual Traps about 18” apart and all reflective.
Pete Townshend endorsed the QSF sampling booth something like the one shown in F Alton Everest book back in 87. The QSF booth specifically pictured in his book was used to develop deep space voice recognition algorithms.
The QSF is acoustically sorta like taking a direct signal and splitting it into two tracks. One track stays direct. The other track gets processed; the bass is rolled off and the treble gets turned into a random set of low level, time delayed signals stretching out about 30 ms. These two tracks are mixed back together, acoustically, to get the sound of the QSF effect.
What is missing with the DIY Studio is good looks, cleanlyness, convenience and most important, durability. Working engineers are always handling their Studios and what this DIY is missing is toughness.
By the way, whenever you want to know about TubeTraps, or bass traps in general, just call me or email me. I haven’t forgotten that one of my earliest acoustic projects was a DIY TubeTrap. Come to think of it, I’m the one who invented the TubeTrap.
Arthur Noxon
Acoustic Engineer
www.TubeTrap.com
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It's been a big help. Actually had a bit of trouble finding it this time... but finally did.
