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Sound Advice

A Shiftin' air affair

Bass Traps

Phone booths into concert halls - if you can ace your bass, that is

Just as a pair of nearfield monitors can sound twice their size, the smallest of studios can overcome their limitations. Stuart Litobarski explains the science of the bass trap - and tells you how to install your own

It may look like a sewer pipe to you, but the aRTs bass trap is a typical and affordable tuned-duct absorber (photo courtesy Karl Brown)

Few components of sound play as much havoc with your studio's acoustics at the bass end. And few things are as good at nullifying its effects as a bass 'trap' - a custom-designed section of acoustic treatment designed to absorb low frequencies and prevent them spinning around the room.

From a scientific point of view, good bass absorption demands a thick, porous absorber. Wide-band techniques can extend bass absorption, but in practice special bass traps are more often used. They are based either on a tuned pipe arrangement, on panel absorption, or on the Helmholtz principle. The way these special bass traps work is by setting up the bass absorber as a resonant system. In this way they can be made more efficient than you would expect from looking at their humble physical proportions.

The average studio room is quite small and will not fully propagate or support low bass. This apparent deficiency can actually be turned to your advantage, because it reduces the number of bass traps you need to balance the acoustics. In home studios, monitoring is quite often nearfield monitoring, which will not reproduce the low bass anyway. On the downside, in a smaller studio the modes are at a correspondingly higher frequency in the upper voice range, so if they are not well-controlled they become clearly audible.

Chamber music

Have you ever seen pictures of an anechoic chamber? This is an acoustic laboratory where all the surfaces - walls, floor, and ceiling - are totally covered in wedge-shaped absorbers, to a depth of several feet. Very little of the sound striking the walls is reflected back.

If you tried to build a studio like that, most sound energy from the loudspeakers would be soaked up before it even got to you. This would mean that you would require extremely high-power monitoring. The resulting studio environment would be quite unnatural to the ear, and working in it would be very claustrophobic.

Plot 1: St George Community Centre, before treatment, showing uncontrolled bass

This is one reason you can pack your studio to the rafters with rockwool and yet still not get the acoustic you desire. While the studio may seem as 'dead as a Dodo', hard walls behind the rockwool will act as reflectors at the bass frequencies, and can spoil the intended result. This is graphically illustrated by Plot 1, showing a TEF measurement for the untreated St George Community Centre. It indicates an imbalance, because while the treble is closely damped, the bass energy remains unchecked. The result is a boomy and muddy bass sound when music is played in the room. Is this the ideal band rehearsal room?

Audiophiles agree that a bass trap is the single most important improvement you can make to listening-room acoustics. There are three main types, as follows:

The tuned duct or tube absorber

One way you can make a bass absorber is to use a tuned duct, or tube, with the opening facing into the room. The way it works is that the inside surface of the tube is lined with porous absorber. A bass sound striking the tube undergoes pipe resonance - which, incidentally, is just what happens in church organ pipes - and is absorbed. The pipes are highly tuned.

The practical effectiveness of these tuned duct absorbers, as measured in Sabins, can be far superior to results that the size of the absorber alone would suggest. Even so, if you are looking at an American version, such as the Tube Trap, remember when checking the spec that US Sabins are in square feet, not square metres. Sabins in feet are about ten times as great for the same absorber!

Although tube absorbers may look a bit like a sewer pipe, they are actually specially designed. The excess bass sound is converted to heat by the cunningly engineered porous lining.

To be effective, tubes need to have a length of Lambda / 4 (for a closed end), where Lambda is the wavelength of sound, which is very long at low frequencies. So you can see that, for bass, very long tube traps are required, since the wavelength at 40Hz is about 8.5metres.

In practice the average home studio is quite small, and fortunately does not sustain these very low and hard to control frequencies. This means that a modestly long tube absorber should be more than adequate. The cylindrical shape of tube absorbers provide some welcome diffusion to the average studio, too. A UK design by aRTs is now available from Absolute Audio (see photo).

The panel absorber

These are the traditional method for controlling room bass response. They are mass-controlled. A rockwool infill is generally used to increase the absorption, and to broaden the resonance peak. The standard formulas tell you the frequency of absorption but not the amount for Alpha, which must be measured. For the technically minded, the simplified formula is:

fpeak = 60 / sqrt(m x d);

where m is the weight of the panel per square metre, d is the depth of the air-gap in metres, and sqrt means square root, something you perhaps learned at school but forgot because you were too busy playing guitar!

You'll be lucky to get any more absorption than about 0.4 for Alpha using this system though, so you'll need to build in plenty of panelling. You may also find that wooden panels are a bit too reflective at the higher frequencies, so you should step and angle them to gain adequate diffusion.

The latest development in panel absorbers is the B.A.S.S. Trap by RPG. This device fully exploits the new Danish wonder material 'SLAM', to give an Alpha of up to 0.8 at 80Hz. More about this later.

The Helmholtz absorber

To get a higher Alpha than the panel absorber you need the Helmholtz version. Helmholtz resonance is what happens when you blow across the top of a bottle (after drinking the contents, naturally - Ed). The resulting absorber is a highly-tuned unit, suitable for controlling specific room modes. However, it is not so suitable for general use, unless a full design is carried out, as the response can be quite peaky. On the plus side, a rockwool infill broadens the resonance peak, and increases Alpha absorption, making the device more adaptable.

The formula for calculating the frequency of operation of a Helmholtz absorber can be found in several books. It is more complex than the previous one, and is based on the percentage perforation. For accuracy, add a small correction for the actual hole size used.

Again, though, the formula doesn't tell you the specific amount of absorption, which must be found out by measurements, or from experience. The classic example in the studio is the BBC 'A' series of acoustic modules.

There are many other forms of this device, based on slots, slats with gaps between them, or special surfaces with complex cavities. They can give quite a striking decorative finish, but their complexity puts them beyond the scope of this feature.

Bass à la mode

If you are a keen recording fan you probably already know from experience about room modes, and how these can affect the sound of your studio room if they're not absolutely right. Room modes are rarely a problem at high frequencies - they are far more likely to give grief in the bass. The addition of just a couple of bass traps can work wonders in taming unruly room modes, which might be giving the room an embarrassing 'honk'.

Take a look at the improvement at Disk Edits (Plots 2 and 3). After only two Aro Technology bass traps have been added, the second peak, which was previously quite prominent, is now noticeably better controlled.

Plot 2: Disk Edits, before treatment, showing prominent bass mode

Plot 3: Disk Edits, after treatment with two Aro Technology bass traps

If you have never read a TEF plot before, then view the curves a bit like mountain peaks, or waves, of sound. The sounds are spread out across the horizontal scale like notes on a piano keyboard, with bass notes on the left and treble to the right. Vertical height tells you how loud the sound is, while longer sustaining notes protrude further towards you, as viewed coming out of the page along the time, or 't', scale. In a balanced room the waves should all be the same height, and be smoothly connected without any obvious gaps. The modes should all decay at the same rate, as indicated by each of the waves stopping at the same point on the 't' scale. This desired result will be more difficult to achieve with a smaller room, because there are fewer individual modes with which to play.

Plot 4: Adelaide School of Audio Engineering control room

Plot 5: Rocking Horse Studio B, before treatment, showing modal problem (all plots courtesy Chris Morton, Aro Technology, Australia)

"Room modes are rarely a problem at high frequencies - they are far more likely to give grief in the bass"

Plot 4, of a control room at Adelaide School of Audio Engineering, shows the sort of smoothness that can be achieved. Contrast this with Plot 5 of Rocking Horse Studio B before treatment. Here the modes were quite narrow and disconnected, with clear gaps. The bass modes ran on for almost twice the treble. This would be clearly audible as a strident bass honk.

Slap up a bass trap

Figure 1: Construction of a panel-absorber type bass trap

Bass trapping is an easy thing for the studio DIY enthusiast to tackle. Primarily, it consists of nothing more than a thin 'pine-cladding' type construction, mounted on battens over an air-space. Figure 1 shows the general idea. When you install the bass-trap cladding, it is also an excellent opportunity to add some further soundproofing. You could treat an entire party wall in this way, to balance your room acoustic and get along with your neighbours at the same time.

1: The battens should be laid edgewise on a grid of 0.5 to 1.0 metres across, with a maximum aspect ratio of 2:1. Alternatively, build separate box-like modules with MDF sides.

2: Glue and pin the panel to the front face at the batten edges, to create an airtight seal to the finished bass trap. Thin plywood is quite suitable for use as panelling.

3: Lay some mineral-fibre in the air-space. Although rather expensive, this will once more broaden the absorption frequency characteristic, and increase the figure for Alpha. Be sure to select a mineral-fibre type that is optimised for acoustic rather than heat performance.

Besides using panels of thin plywood or plasterboard, you could also try out a synthetic material. Roofing felt is definitely out of favour these days. And sheets of rubber could get the neighbours talking. Some of the more modern plastics look promising, but could be outside your budget, at least for the time being.

How do you know how much panel absorber basstrapping to install? Well, if you expect a realistic maximum Alpha of 0.4 for your bass trapping then, as a rule of thumb, you will probably need to budget for twice the area that you have of wideband absorber.

Handle with care

If you feel comfortable with the acoustic design formula I have given you, then you should use it to make the result more predictable. Otherwise, simply suck it and see. If you apply the treatment a little at a time, appraising the results as you go, you will be unlikely to apply too much. The best place to position bass traps is undoubtedly in the corners of your room, where all room modes terminate.

Finally, don't worry if the topic of studio acoustics feels a bit beyond you at this stage; acoustic modules are readily available off-the-shelf.

Case study 1: 'Mr Grin'

The control room, with the massive rear-wall diffuser/trap on the right

Mr Grin studios in Bristol are advertised as having "good acoustics." We visited them at their new Technology Park location, to find out just how designer-owners James Withers and Mick Ward have tackled their bass-trapping.

The studio is used for recording a wide range of music, from rock to jazz and country. Mick is actually a trained drummer, so prides himself on getting the best possible drum sound.

Mick serves as the studio sound engineer. He explained that their studio design had been based largely on magazine articles, with Mick himself doing the boffin calculations, and James providing the practical input.

Custom bass-trapping at one end of the studio room

The studio room is fairly natural in acoustic, so has not demanded elaborate bass-trapping. Simple panel absorbers have proved sufficient. The bulk of bass-absorption is provided by the pine-panelling on the far wall, which also incorporates variable acoustics.

Hinged panels can be opened to add absorbency, and create a 'dry' zone in the local acoustic. The stepped panels along the side walls are designed to provide further bass-absorption, as well as diffusion. Finally, the ceiling adds a large area of valuable bass absorption. This inner room is totally floating.

The various studio screens available for placement around musicians

In the control room, the rear wall features an interesting custom designed unit that includes both bass-trapping and diffusion. The vanes are adjustable, and intended to allow the desired combination of random diffusion and bass absorption to be set up rapidly. The final result is visually pleasing and effective, too.

Studio isolation is more than adequate. Massive 9" and 6" concrete block walls are separated by an air-gap of 18", all under woodwool ceilings. Woodwool slab is far more predictable than sand pugging in case of fire - Mr Grin Studios have been fully approved in this regard.

Sketch plan of Mr Grin Studios, showing the acoustic treatment in detail

The only isolation problem has occurred when heavy-metal musicians, using the rehearsal studios across the corridor, left the soundproofed doors propped open while popping out to the studio snack bar. Although these massive doors are sand-filled, remember that the act of leaving a door open can defeat even the best soundproofing.'On-air' lights have now been installed to guard against this.

The studio screens have been specially designed. They are based on recycled office screens, with one surface faced-off in fabric over a rockwool blanket, low in height so that drummers can see over the top.

Case study 2: The Coach House

Lifting the curtain on the panel-absorber bass traps on the rear wall of the Coach House control room

The Coach House has an enviable reputation, having played host to many successful recording artists, including Massive Attack, Andy Sheppard, and The Blue Aeroplanes, to name but a few. Designer-owner Andy Allen studied his craft with an engineering degree at Bath University before going on to build the Coach House.

The studio features some of the best acoustics around, and Andy has installed plenty of bass-absorbers in the studio room, in the form of panel absorbers that are stepped and angled to provide lots of diffusion. A large panel absorber in each of the four corners more than adequately controls any boominess. Some of the absorbers are what Andy calls High-Q, some are not.

Panel-absorber bass trapping in the studio room at The Coach House; each corner is fitted out like this

Layout of the Coach House

The acoustic quality is reminiscent of a BBC room, with clarity and a well-controlled bass. Variable acoustics are incorporated in the form of a patent rotating acoustic panel system, which is mounted in the ceiling.

The control room has two massive 40Hz panel absorbers fitted under the observation window. Andy is justifiably proud of his great monitoring and control room acoustics. As he says: "After putting in the treatment, you've got something balanced, and you can't half use your monitor power."

We wouldn't disagree with that...

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The Mix - Copyright: Music Maker Publications (UK), Future Publishing.


The Mix - Jul 1994

Donated by: Colin Potter

Coverdisc: Mike Gorman

Sound Advice



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