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The Acoustic Trap

The Acoustic Trap

Acoustic design is perhaps the darkest art of all. And if you listened to every forum post on ‘how to make your room sound amazing’, you might end up with compressed scarab beetle walls held together by the dung of an elephant.

23 April 2012

Story: Andrew Steel

Acoustic design is perhaps the darkest art of all. And if you listened to every forum post on ‘how to make your room sound amazing’, you might end up with compressed scarab beetle walls held together by the dung of an elephant. You can also spend so much time messing about with measurement mics, staring at waterfall graphs (that could represent Niagara Falls for all you know), or building resonators with only a weak grasp of 10th grade maths.

This is not a complete treatise on acoustics or studio design. But rather, a few tips to help you make the most of what you have in the real world with limited means, while pointing out some common things you might already be, or about to do, wrong.

Best of all, you can stop worrying about the acoustic bomb, and get down to making music, isn’t that what we’d all rather be doing?

TECH TIP

If you’re trying to isolate everything from 20Hz up, you might tune its resonant frequency to 5 or 6Hz. After that point it will taper off, so by the time you’re at 20Hz, you might be 10dB down, and falling.

1. WHAT YOU MIGHT BE DOING WRONG

RELYING ON MEASUREMENTS

Just to get it out of the way: This article is not going to tell you to measure anything… acoustic that is. Not to say you can’t, but keeping in the spirit of making good, affordable, and importantly, timesaving decisions that work, it’s just not that important. Especially when our ears discriminate in microseconds.

Reverberation time is the most commonly used measurement in acoustic design, and I don’t think it’s that useful for most studios. Because the typical way we go about it with small studios is: set up the monitors in the right places, get a good listener position, haul in some bass traps, and put in absorbers for the first reflection points on the two side walls and ceiling to get the imaging right. By that time, there’s already a fair amount of absorption in the room. And if we’re not talking about a big room, it’s probably not too bright, maybe just a little bit. We might only put one or two more absorbers until it comes to a nice feel, and maybe one on the back wall.

Then we might have a fairly big plasterboard space between the first incident reflection absorbers and the ones near the rear, and if you clapped your hand in the middle of the room, you would get a flutter, and diffusers will get rid of that.

In that case, if you had the client couch toward the back of the room, you would put a couple of diffusers on either side so when they talk they’re not getting a flutter, or zingy bathroom echo. And that’s it. In a room like that, there’s not much more you could or would want to do.

The other reason for not taking measurements is it’s very hard to know what you’re reading; it’s expensive to get the right measurement equipment; and even more expensive to pay someone that knows what they’re doing. It’s a good few hours to set up and do it properly, and more than an hour (at a bare minimum) to analyse it. At $250 an hour, that’s over a grand just to find out the state of your room, and that’s only once. If you’re really going to hold to the measurements, you need to do it multiple times to ensure you’re actually making an improvement. And most of the time it doesn’t really tell you anything you didn’t already know by listening to the room.

It can also lead to chasing your tail. If it sounds good, get over it. My whole life was based in measurement science, so it sounds dumb, but it’s just not necessary.

2. WHAT YOU MIGHT BE DOING WRONG

ISOLATION INCIDENTS

If you want to isolate anything, be it a generator, record player or studio from a base, you need to know something about the range of frequencies you want to isolate, but it’s mostly about mass. If the weight of the thing you’re trying to isolate rests on the isolating material like a feather on a rhino, it’s going to do no good. Same thing applies if it’s flipped and the element you’re trying to isolate is squashing the isolating material flat like a rhino on a feather — no good. But when it’s just deflecting the isolating material about 5-10% of it’s original height, then it’s in its working region.

The analogy: If you put a Landcruiser on Mini Minor springs, it would squash so flat you would feel every bump in the road. If you put a Mini Minor on Landcruiser springs, they would be so stiff that you’d feel every bump in the road. But when you put Landcruiser springs in a Landcruiser, they deflect a little bit, are in their operating region, and everything is fine.

In a car, we not only have a spring, but also a shock absorber, which we know as a damper. Rubber type materials naturally have both those properties, but we still need to match the mass of the thing we’re trying to isolate with the spring and damper properties of our isolating material to get our isolation. If you put escalator handrails that are extremely hard, or entire sheets of rubber, how are you going to compress its height by 5%? It’s not going to happen.

Even Sheerflex (the material we most often use) needs 700kg/sqm to deflect it adequately. And as it turns out, if you put blocks of Sheerflex at 450mm centres (typical modern day stud separation), there’s not enough weight in a heavy plasterboard wall to squash it the necessary amount. So what we do on the floors is put the Sheerflex element down under a timber stud, then bolt it all down, and tighten it up till the shim measurer says it’s pre-tensioned just right. If you just put the raw material in, or worse, a whole sheet, or some other material that ‘intuitively’ isolates, it might, but all bets are off.

DON’T BE SHOCKED: Make sure the material your using to isolate matches the mass of the thing you’re trying to isolate, aiming for that magic area between 5-10% deflection.

1.Car-diagram-pichi — Landcruiser on Mini Minor springs = Bottomed out, feeling every bump

2.Car-diagram-pichi — Mini Minor on Landcruiser springs = Too rigid, feeling every bump

3.Car-diagram-pichi — Landcruiser on Landcruiser springs = Just right!

CONSTRUCTION MATERIALS

ABSORBERS

Rockwool is fine, but it’s a bit more fragile and more expensive than fibreglass. As with all of these absorptive materials, its absorption characteristics depend on the thickness. Fiberglass is great because it’s a nice stiff regular board that you can cut with a Stanley knife. It just works. Polyester is the ‘doesn’t make you itch version’, but is much harder to cut.

There are a number of properties that dictate absorption, but the most influential one is ‘flow resistivity’. Which is a measure of the pressure drop across the material for the amount of air going through it. Fibreglass and polyester are a much of a muchness, they’re both better than plain old acoustic foam. But foam is so easy to work with, because you don’t have to wrap it in fabric. And egg carton-style designs cut into the foam face? It’s just for show. It doesn’t enhance the absorption properties of the material one bit.

SOUNDPROOFING FOAM?

Probably the biggest thing that people ring up and ask for is ‘soundproofing’ foam. But foam just doesn’t do that. Plasterboard might give you -20dB, foam, maybe 3dB if you’re lucky. But when you stick it to a piece of plasterboard, it wouldn’t even change it 1dB. It’s the wrong thing for the job.

PLASTERBOARD VS MDF

Plasterboard is used over MDF in houses and everything else, because once it’s finished and set, that’s it. MDF may shrink if it dries out, or it may swell if it’s in a humid environment. The place for MDF or plywood is if you put it as your first layer, and then plasterboard or whatever you like over the top. Because it means you can screw anything into it across the wall. But as a finished construction material, it’s not great, unless it’s all veneered or laminated.

OFF THE SHELF

Absorbers are absorbers, there’s not a huge difference. The differences come down to cost, finish, availability, whether they’re robust, and whether they’re combustible. The ones that are difficult to recommend are some of the intricate diffusers, only because they’re expensive. I understand why they’re expensive, because they’re hard to make. It’s a lot of money for something that’s okay, and you could have done the same thing with a nice hanging curve from your ceiling with a downlight in it, and painted a funny colour. It’s pretty hard to justify hundreds and hundreds of dollars, and buying multiple of them. The people that did all the R&D in the first place should be knighted, because it’s a huge amount of work. But it would be nice if they also recognised that they’re frequency dependent and construction dependent. If you make them without walls in the wells, they’re less efficient. So the ones that you see that are just the lumps don’t perform quite so well as those with walls. By the time you get lots of them side by side without walls, it’s doing less and less each time, but they didn’t get any cheaper.

TECH TIP

If you use one 10mm Sheerflex pad, at 450mm centres, tighten them down to 9mm, we know exactly how it will decouple. You will get a tiny bit of transmission through the bolt, but nothing compared to bolting the bottom plate to the floor. If you want to stop transmission waking up your baby or grandma asleep in the next room, this is a proven way where the results are known. If you want to use sheets of fibreglass, it seems to work, but there’s no data for it.

Ceilings require the same arrangement, this time it hangs on a bracket that pulls down on the top of the rubber and compresses it.

3. WHAT YOU MIGHT BE DOING WRONG

PUTTING ABSORBERS ON THE WALLS

Sound is a pressure wave. If you think about air molecules like two billiard balls. The highest point of the pressure wave is when the two balls hit each other, and the velocity at that point is 0. An absorber works because the air molecules have to go through a crazy path inside it, hit the wall and try and find their way back again, which wastes energy.

An absorber does its best work with a high velocity particle, because it’s going crazy through the absorber losing energy. So our absorber works best at high velocity, no pressure. But when we get to a wall, the velocity is zero, until it goes back the other way, meaning our absorber is least efficient. If we sit the absorber off the wall, much more of the time, the particles will have a higher velocity than directly on the wall — making it more efficient. The easiest way is to grab a couple of foam strips, stick them to the back of your absorbers and sit them an inch away from the wall.

Graph2-pichi — At the wall, air molecules creating a sound wave have high pressure and no velocity – it’s the most ineffective place to put an absorber. Try pulling it out from the wall a touch.

4. WHAT YOU MIGHT BE DOING WRONG

LIVE ROOMS

The biggest question I get when people ring up is, “Right, I’ve saved up $10,000, it’s taken me four years, and the wife has finally said I can build my studio. I want to build the best live room I can build for 10 grand.”

The questions start pouring forth. Which instrument are you going to have? A band? Are they all going to stand in the same place every time? As you can see, there isn’t one room that’s going to be able to cover all your needs. You need to be able to adjust it. Put a metal rail around the room so all your absorbers and diffusers can clip on or off, in any arrangement you like. You can screw the rail into each stud, which means you don’t need any MDF in the wall, and be able to change the feel of the room on a whim. You can also buy or build yourself a few gobos to use as isolators, and they double up as removable absorbers.

5. WHAT YOU MIGHT BE DOING WRONG

AIR CONDITIONING

To get air conditioning in an out of a recent room we designed, without noise, we had an in-duct and an out-duct, each one 1.8m wide, 300mm high, and 6.5m long with lots of insulation inside, and two layers of plasterboard and Green Glue all over the outside. That’s because you need more air in and out of the room to bring cooling in (or take heat out), then you do to just ventilate it and stay alive.

It’s obviously much easier to soundproof a 150mm diameter duct purely for ventilation, than it is a 300 or 600mm diameter duct for air-conditioning. Ventilation silencers typically have 6m of roughly 150mm diameter duct built into a heavy wooden box. The sound can either go out through the duct, or into the duct and straight out the wall of the duct. So the wall of the duct has got to be as sound proof as the wall of the building that you’re in.

The only way to make sure, is keeping that airway nice and smooth as you build it. Put a variable speed fan control on it, because the fans all run too fast on 240V. When they’re too fast you can hear them pitching a hum or whir. And when they’re too slow they oscillate. There will be a spot in the middle where the capacity of the fan is matched to the static load of the duct and everything goes quiet. Then don’t touch it, and you’ll get plenty of fresh air in the room.

STUDIO SHIPPED TO YOUR DOOR

If you’re renting, John Sayers recommends a container studio. That way you’ll never lose the investment and you have shippable, long-term repeatability. The original container studio Sayers designed was for a renter, Sparky, who was sick of having to reset every time he moved. The only problem with Sparky’s container is that although it’s technically transportable, there’s no way you’ll get it on the docks. The reason is that Sayers designed it with holes in the back to feed the air-conditioning duct. Sparky has actually been building and selling more container studios. Which are fine if you plan on keeping it landlocked. But Sayers is working on a fully exportable version that he can sell internationally.

“We discovered that you couldn’t modify the container in any way, shape or form externally because they won’t insure the contents,” said Sayers. “It has to be fully lockable and be able to stick it on a container ship. So we redesigned the front to have all the air conditioning built-in. So as soon as you open the doors, you’ve got the entrance to the studio with all the compressors and air conditioning system above it. It’s all completely contained and we’re building them out of steel framing instead of timber so we can use smaller studs.”

At the moment, his partner is in China attempting to build a prototype. And if all goes to plan, it will be ready to ship by the end of the year. The beauty of it, says Sayers, is “you just buy it, open it up, plug a power point into it and it’s away.”

SAYERS’ SAY:

How an Aussie Pioneer Got into Studio Design

John Sayers has been making records and designing studios in Australia for years. He’s also been somewhat of a pioneer on studio design forums, having started his own at www.johnlsayers.com almost a decade ago. These days John designs studios all around the world from the comfort of his home. He took the time out to walk us through how he came to building studios in a potted history of studio development in Australia. Check out The Refinery in What’s On to get a picture of his recent work.

LAYING THE FOUNDATIONS

John Sayers: “I got my first job at Armstrong Studio in Melbourne in 1967 or ’68. Bill Armstrong used to buy these terrace houses and we’d convert them into studios. Then I went and set up Trafalgar Studios in Sydney with Charlie Fisher in ’74. We had a big meeting between the three of us and decided that I should go to America, and I ended up in Los Angeles for three weeks living with a guy called Dean Jensen who was the man responsible for Jensen Transformers and was the tech for the whole of Los Angeles. He did the installation of all of Record Plant, Paramount Studios, Capitol gear, the whole lot, he was a master techo. I watched him go through a console for Michael Pinder from The Moody Blues. He had built a studio in his house at Malibu and Dean went through the whole console and improved the signal to noise by 30dB just by making adjustments. He changed all the curves of all the equalizers and tuned every output transformer.

“One day, while Dean and the chief engineer of a brand new studio in Burbank were in the control room going through the console, I was just wandering around looking at the whole construction because they had acoustic treatment. It was something that we didn’t have in Australia to any degree!

“Also, when we visited The Record Plant’s control room, I put my own tape on and could hear the difference in quality of their monitoring systems. It was like way back in the early days at Armstrong’s when Robbie Porter was producing a Daddy Cool album. I was hanging out assisting Roger Savage to engineer it. Robbie took the tape to America to mix, and a few weeks later he came back and put the tape on. I remember listening to it and saying, “f**k! how the hell did they do that?” There was just a quality of sound.

“So when I got back from spending three weeks in L.A. I rebuilt the control room at Trafalgar and added additional acoustic treatment at the studio.

“First we soffit-mounted the speaker systems, and then we used bass traps, and other things I’d seen in America. At the time, Dave Flett also came up from Melbourne to check out Trafalgar, then went back down and built Richmond Recorders.

“In ’77 or ’78 I built Music Farm up in Byron Bay. That was really successful for four or five years, before the music industry depression in ’82/’83. Then I became the audio director of Beyond 2000. And I ended up designing and building their Twilight Productions facilities in 1986. I built two control rooms with the vocal booth between — one control room for myself to mix and one control room for Murray Burns and Colin Bayley (ex-Misex) to produce all the music. Later on, while I was still working there, I did a studio for a voice-over guy, and another studio for a television firm. So I was really starting to get into it quite a bit.

“Then after I burnt myself out from five years of mixing long, long hours for Beyond 2000, I got a gig designing a studio for Charles Sturt University in Wagga, which was called Riverina Advanced College at the time. They wanted a recording studio they could film television in. Then the college became Charles Sturt University, and because I didn’t have a degree my contract wasn’t renewed so I moved back to the North Coast.

“I was producing Troy Cassar-Daley and other country music for Tamworth. I got a couple of nominations for Producer of the Year at Tamworth and I fixed up the design for Brett Nielsen’s Big Toe Productions [Brett was the face of the Thalidomide disaster — a ’50s morning sickness treatment found to cause birth defects – and grew up without arms — Ed].

“In about 1997 I was starting to get into the internet and created a website called How to Build a Recording Studio. I laid it all out and did all the illustrations, then I put it up on the web for free and it got a lot of attention. So I added a second part to it, which was How to Use a Recording Studio. Tom Misner from SAE saw it and said, “I want that.” So I sold it to him and he put it up as the reference material for the SAE and it’s still there.

“I was also answering a lot of questions on the homerecording.com studio design forum. Then Bruce Valeria from Ottawa sent me an email asking if I’d design a studio for him. We followed it all on the net as he built it and he put the finished studio up on homerecording.com. By then I was spending many hours answering questions on homerecording.com so I decided to set up my own forum. That’s now been going since 2003 and I’ve been designing studios since then.”

REMOTE DESIGN

John Sayers’ recent credits include The Refinery in Melbourne, a studio designed for and built by James Leigh from Pseudo Echo (with the help of his dad), Eskimo Joe’s new studio in Fremantle (which is just waiting on delivery of the console), as well as studios in Hong Kong, India, Colombia, France and plenty more. He’s firmly international, using a combination of Google Sketchup, Skype, and CAD programs to develop the design. Sayer’s ran us through how he manages remote studio design.

JS: “The studio owner sends me the dimensions of the building and photographs, and I design it all in Sketchup, which is a 3D program. We send them back and forth and use Skype a lot. I taught myself draftsmanship, so when the client’s satisfied with the design, I use an AutoCAD-style program called ProgeCAD and I export the model into DWG files and turn them into draft drawings of the studio. When they receive the final drawings, they play it out on the studio floor with chalk marking to make sure it all fits, and then build it.”

REVIEWED

The Acoustic Trap

Acoustic design is perhaps the darkest art of all. And if you listened to every forum post on ‘how to make your room sound amazing’, you might end up with compressed scarab beetle walls held together by the dung of an elephant.