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Studio Acoustics

How and why does sound move around your studio?  Howard Turner from Studio Wizard explains some theories about sound, and looks at the basic techniques that can be used in any studio to create a great sounding room.

Most of us in the music business have a pretty good grasp of how sound behaves and what that means to us as musicians and recording engineers. We can manipulate sound in a PC or via a desk, but what about when it comes to physically manipulating the sound in a room?  Or perhaps trying to stop the sound travelling from one area to another?  That’s a whole new ball game.

Lets clear something up.  These two issues are not one and the same thing.

Trying to make a studio sound good and easy to mix in:  that’s best described as  ‘room acoustics

Ensuring you don’t keep the neighbours awake whilst tracking / mixing:  that’s ‘isolation’.  They may be related, but they are entirely separate matters physically.  Once you’ve ‘isolated’ and trapped all the sound inside a room; it’s going to rattle around in there and sound awful unless you do something about the ‘room acoustics’.

Isolation.

For anyone with a studio, isolation is probably the first of these to occur to them, as the studio gets busier – and louder (or indeed if your neighbour suddenly decides to join in with some DIY hammering and drilling when you’re in the middle of some important vocal parts!).  So what’s to be done?

The tools available to you are just the same whether you are a multi-million pound pro facility, or a bedroom studio.

Firstly we need to stop sound travelling in or out through the air – so we need to make the room airtight.  Lets seal up all the air holes, put good seals on the doors and windows.  This is very important – knock a one-inch air hole in a 60dB isolation wall and you’ll discover that it has suddenly become a 20dB wall!  It really does make that much difference, so hunt out every crack and get busy with the filler.

Secondly we need to make sure that the walls/floor/ceiling/doors/windows are stiff enough that they don’t vibrate and transmit sound through them.  Now it starts to get a bit trickier:  we’re into double glazing windows, fitting double ‘soundlock’ entry doors and increasing mass by adding extra layers of plasterboard to walls etc.

These two are easy enough to achieve, and will get rid of most mid and high frequency leakage but the third tool at our disposal is the real key to successful wide band isolation, and the hardest to achieve:  Decouple the noisy room from the rest of the building.  This entails building an entirely separate ‘floating room’ – incorporating elements of the first two criteria – which is not fixed to the existing structure whatsoever, and sits upon some bouncy material (such as Rockwool) to keep it isolated even from the floor.

If we’ve done the best we can with the existing structure in terms of air-tightness and rigidity, and then we drop a floating room inside it, we can achieve startling levels of isolation, even at low frequencies.  Just remember that it’s going to be air tight, and probably the best insulated room you’ve ever seen.  So if something isn’t done about ventilation and cooling, it’s a gamble to see whether you suffocate before you cook!

Carry that weight.

Before you try this at home, do note that even a small bedroom-sized floating room will weigh the best part of a metric tonne, so make sure that a structural engineer has checked the floor it’s to stand on to make sure your upstairs studio doesn’t suddenly arrive downstairs…

Shape & Design.

Ok, so the floating room is in – and it sounds awful!  What went wrong?

Well nothing – if you stop all the sound getting out, then it’s just going to rattle around in the room, meaning that we now have to do some serious manipulation of the room’s internal acoustics to get us back to a ‘normal’ sounding room.

Before we start to add acoustic treatments to the room, there are three factors we can take into account in the design of the room shape that will help.

Firstly we can make sure that none of the surfaces are parallel – including floor-to-ceiling.  There are three frequencies that a room will resonate at, which correspond to the length, width and height.  These are the ‘axial modes’.  Now if a pair of surfaces is absolutely parallel, they will ring at exactly one frequency.  Should the surfaces be offset by around 5 degrees or more, rather than producing a distinct note, they will just produce a ‘lump’ in the bass response; and you can knock that out with a ‘bass trap’.

Secondly we can look at the infamous ‘Bolt Graph’ and see if we can make the dimensions of our floating room fall within the Bolt Area.

The Bolt Area.  This graph illustrates room ratios of length to width when height = 1.

Rooms whose ratios fall within the shaded area will exhibit a favourably uniform distribution of modal frequencies ie: the bottom end will be less lumpy…

Bolt assumes our room will be rectangular, but rest assured, applying a 5-degree offset to a Bolt room will further help improve diffusion and axial mode problems.  Sadly, very few real rooms will fit into the Bolt area; so don’t fret if you can’t squeeze into that shaded blob.  Just make sure you aren’t going the other way and building a perfect cube, where all the axial modes will club together to create the mother of all bass resonances at just one centre frequency!

Thirdly, we can look at the layout of the room internally; at this point we are largely interested in the symmetry of the room from the monitors to the engineers ears, and mating this symmetry with an ergonomically useable layout.  I will deal with the symmetry issue in the next section, but this further illustrates how the design of a control room needs to constantly balance the competing needs of isolation, acoustics and ergonomics; after all there’s no point in having a quiet, great sounding room, if you can’t reach the gear to work in it!

Room Acoustics.

Our aim is simply to create a room whose characteristic sound we understand almost instinctively.  Hardly surprisingly, it appears that the sort of room we understand is one whose sound characteristics closely ape those of an idealised domestic sitting room.  In brief this will entail a shortish reverb (or RT60) time (typically in the RT60 range of 0.15 to 0.35 sec)  in the midrange.  It is considered acceptable for high frequency RT60 to be reduced by up to 50 percent from this figure, and also for Bass RT60 to climb to 120 percent of this figure at 125hz and even up to 180 percent at 63Hz.

Just how a nomad who grew up in a tent (Reverb: non-existent) will relate to such a room design I don’t know, but it does generally seem to work for the rest of us.  When I build a control room the best compliment I can get is someone saying ‘Well it sounds just like any other control room’; then I know I got it right!

Now back to the floating room.  You’re about to chuck a load of gear and furniture – almost all of which is comprised of hard surfaces – into a room where the reverb time is already too long by virtue of having stopped all the sound getting out.  Consequently in order to mimic the ‘sound’ of a domestic room, we are going to have to introduce a serious amount of ‘soft stuff’ (generally some grade of Rockwool tm covered by hessian) to absorb higher frequencies and mimic the effect of carpet, sofas and curtains, along with some tuned resonant bass traps to level out any bumps in the LF response and others to generally absorb the bass in the same way as the windows, doors and ceiling etc were before we stopped them.

Good reflections (and bad ones).

We also want to provide a symmetrical acoustic environment from the speakers to the engineers ears.  Why?  Well for example if there’s a window one side, then to the engineer the speaker that side will seem to be ‘brighter’ by virtue of the extra reflected sound coming off the window.  Hence all the mixes done in that room will sound ‘toppy’ on the other side, as the engineer unconsciously compensates for the room imbalance.   A large void to one side will produce the reverse effect.

The reason reflections are bad news off the side walls is due to the Haas effect.  Sound from a speaker bouncing off a wall to get to you has a longer path to travel than the direct sound from the speaker.  However, Hass discovered that unless this early reflection path is long enough to introduce a delay of over 50msec (which it never is) then the listener’s brain will not identify it as a reflection, but as part of the original sound.  Hence the perceived increase in top in the example above.  Also in stereo placement Haas has implications, as the tweeter now appears to be a wide smear of sound stretching from the tweeter to the point the sound reflected off the wall – in other words, you’ll be lucky to merely identify if sound sources are left, right, or centre in such a room, accurate stereo placement would be impossible.

Typical Small Studio Layout showing offset walls, & a symmetrical monitoring environment.

So, the front end of the room will be pretty much all soft trapped for highs and mids.  In the past the tendency was to let the rear of the room ‘liven up’ a bit so as not to end up with too short a RT60, but the advent of 5.1 monitoring requires that the rear speakers also are free from early reflections, so control rooms are consequently tending to sound a little deader at the back than they used to.

Bass Traps.

These come in two flavours: tuned and general.  The tuned ones are designed to knock out any specific ‘lumps’ in the low frequency response the room exhibits.  The general ones are there to absorb bass over a wide range of frequencies and help the low end reverb time of the room return to a suitable low figure.

There are many complex and elegant trap designs in existence, but lets look at two simple ones.

Absorbent Traps.  Basically just a lot of Rockwool or foam.  And when I say a lot I mean it!  If you want to make an effective bass trap centred at 50Hz, then you will be building a trap of solid Rockwool 22 feet deep!  Now that’s not feasible in the real world.  You might also like to ponder on the efficacy of some of the foam ‘bass traps’ currently on the market that are generally about 2 feet deep, ie with a centre frequency of only 550Hz!  Better build your own!

A Limp Membrane Tuned Bass Trap.  A series of these will typically cover the majority of the rear wall of a studio.

Tuned ‘Limp Membrane’ Traps.  Works a bit like a drum.  A flexible membrane (usually plywood) at the front of a solid heavy box resonates centred on a frequency determined by the mass/sq metre of the ply and the depth of the box.  100mm or so of Rockwool at the back of the trap does the absorbing for us.  A 50Hz trap to this design would be around 450mm deep – that’s more like it!

A General Bass Trap.  A pair of these will usually be positioned in the corners facing the main monitors.

Add more Rockwool, and the trap sucks harder, but at less of a specific frequency (a bit like turning the ‘q’ down on a parametric eq).  So if we build the trap in a corner (where the depth varies) and fill it full of Rockwool, then it is now a ‘General’ bass trap, absorbing all low frequencies.

Building Regulations.

As of this July an important new set of building regulations has come into force.  Called Building Bulletin 93, this regulation has to do with acoustics in schools, but in the process it lays down specific strict criteria for the acoustic performance and isolation of recording studios in schools.  This set of specs can be a useful benchmark to aim at if you are building a studio yourself.  Note that currently the vast majority of educational studios do not meet these specifications, a situation which many schools and colleges are unaware of – so don’t be tempted to copy their designs!  Theoretically such studios could be subject to enforcement if found to fail to meet the criteria.  Expect the quality of educational studios to start to improve over the next few years!

And in summary.

So, there you have it.  How to build a studio?  Of course not!  In an article such as this I can only scratch the surface of such a complex subject, but hopefully I have given you some insight, and unravelled a few myths along the way.

If you are about to embark on a studio build yourself, make sure you are fully aware of your construction methods and also fully understand why you are following those specific techniques.  Sadly without the right build methods, it is possible to use all the right materials and get no effective result whatsoever.  Call a specialist studio consultant if you are in any doubt.  Also if there are any heavy construction elements in your studio design, make sure you have enlisted the help of a competent architect and/or structural engineer.

 

 

More Information.

Books:

Master Handbook of Acoustics, F. Alton Everest, McGraw Hill. ISBN 0-07-136097-2

Building Bulletin 93, HMSO, 2003.

Sound Engineers Pocket Book, M. Talbot-Smith, Focal Press. ISBN 0-240-51612-5

Acoustics and Psychoacoustics, D. Howard & J. Angus, Focal Press ISBN 0-240-51428-9

The Author.

Howard Turner has over 30 years experience in the studio business, and for the last two decades his Studio Wizard Organisation has allowed him to stop shouting at musicians and going to sleep on the mixing desk all of the time, instead he gets to design studios and shout at builders for a change…  Further information:  07092 123666 web: www.studiowizard.com

Tech Terms.

RT60:  The time taken for the level of reverberation to fall to 60 decibels below it’s peak level.

Axial Mode:  The resonant note created by two parallel surfaces.

dBa:  A decibel measurement scale commonly used in the building industry.  This scale is heavily weighted towards the frequencies of human speech.  Consequently manufacturers acoustic specs on building materials are usually only good for this limited midrange bandwidth.  As a result, the published figure for a standard wall construction might look good on paper, but when you fire a kick drum at it it’ll probably go through it like it wasn’t there.

dBc  The scale we wish everyone used!  This is unweighted; ie it measures accurately at all audible frequencies.

The myth of the eggboxes.

Materials that absorb sound actually allow the vibrating air into their structure, where the molecules collide with the absorbent material, converting the sound energy into heat energy.  Hence stiff, hairy Rockwool and dense open cell foam (the sort you can blow through) both fit the bill as suitable absorbing materials.

What matters is the thickness of the soft stuff.  If we are generous and say that the lowest frequency a material is absorbing effectively at is one with a wavelength 8 times the materials thickness (1/8th wave) then we can see that 50mm of Rockwool will be effective down to around 850Hz  (divide speed of sound in air: 340m/s by thickness in metres x8:  ie 340/0.4=850); not bad.

On the other hand carpet (thickness 6mm) runs out of steam at 7kHz, and egg boxes (thickness 3mm) are useless below 14kHz!  Forget about them as treatment materials!

NC Curves.

Studio gear is getting noisier.  Fans and hard-drives make it harder than ever to achieve the sort of quiet we need to be able to monitor effectively, or self-op a vocal or acoustic instrument in the control room.  The accepted way of defining noise is by NC curves (illustrated above).  The NC measurement of a room is the curve which the background noise never exceeds.  We should expect to find NC20-25 in a control room, NC15-20 in a live room and as little as NC5-10 in a voice over booth.  With a noisy PC and a fan cooled desk, you’ll be struggling to make NC45-50!  So; you monitor loud to hear the detail over the background noise, your ears get tired, you get deaf and your neighbours get tetchy.  Time to build some silenced cabinets! (but that’s another story)…

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