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Making waves

Acoustic diffusers

What they are, how they work, why you need them

There's more to acoustic diffusion than a trip to Laura Ashley or wall-to-wall deep pile. Stuart Litobarski examines the options...

State-of-the-art QR diffuser, designed by German acoustician Manfred Schroeder

The acoustic diffuser is one of the main building blocks of studio acoustics. The principle is not unique to studio acoustics, but don't confuse it with the diffuser vents of an air-conditioning system. Acoustic diffusion is to do with creating myriad rich reflections from all surfaces. It is about controlling the flow of sound energy around the studio. Later in this feature, we'll show you how to construct a simple version of your own.

Acoustic diffusion occurs naturally, or in certain man-made structures when the architectural features are sufficiently varied to break up the sound. For example, churches have good diffuse acoustics because they have a complex architectural shape. Rough stone makes an excellent natural diffusing surface, especially at the higher frequencies, because of its random surface texture and irregular composition. And ornamental plaster work, timber panelling and deeply scalloped alcoves, so often a feature of concert halls, all serve to generate lots of diffusion.

In the best concert halls, architectural frills create much diffusion, resulting in an acoustic well beloved to musicians and audiences alike. It became standard practice in studios dedicated to orchestras, to install diffusing treatment on the walls and ceiling, perhaps in an unconscious effort to recreate concert hall conditions. A number of systems were tried. One method was to construct bass-trapping in such a way that it provided diffusion as well as bass-absorption. The front surfaces of the bass-traps were incorporated into shapes that were trapezoidal, rectangular, pyramidal, and polycylindrical (curvy).

Lateral thinking

Most people who have tried indiscriminately to use these techniques in studios of today have found the results disappointing. I think acoustics have moved on a bit since those days, and we have become more critical, especially since the advent of digital recording with its deeply revealing dynamic range. With the exception of the front end of a "reflection-free" control room, the old style panel diffusers are no longer adequate to cope with modern set-ups. The current vogue is a system using a technical QR diffuser, based on a mathematical maximum length sequence.

These QR diffusers were originally proposed for use in acoustics by a German acoustician called Manfred Schroeder. The Schroeder diffuser was exploited in concert halls around the world, by exponents such as the New Zealand Architect Harold Marshall, following collaborative research with Acoustician Mike Barron. QR diffusers are to be found in concert halls at Christchurch, at Wellington, and now at many other places around the world. They give powerful supporting lateral reflections to the orchestral sound. Research by Mike Barron and Manfred Schroeder had shown that the existence of diffuse lateral reflections, and the resulting increase in bi-lateral dissimularity, was an important feature in enhancing spatial perception. The orchestral sound appeared to envelop the audience when these diffuse lateral reflections were present, which greatly increased audience enjoyment of the performance.

If you read through old books on studio acoustics you will learn that, when radio studios were first established, engineers understood almost immediately that the sound from the microphone would be unacceptable. It was much too echoey. They tried a number of solutions, one of the most practical being to drape the walls in curtains. After a while they found that defects remained in the sound, and they began to look at room modes for an explanation. In larger rooms, the room defects could usually be removed by close miking of instruments. In smaller studios this was not always possible. We now know that a combination of bass-traps and QR diffusers would have solved many of their problems.

Love them or loathe them, QR diffusers are here to stay, and in a big way. The price is coming down all the time and should soon be affordable to the home based musician. A QR diffuser by ARO of Australia is shown above, courtesy of Chris Morton. QR diffusers are state of the art. They are the most diffusing surface known to man, even more diffusing than rough stone, which is in itself much more diffusing than a plain wall. A geometric sound ray striking a QR diffuser from any angle will be dispersed in an omni-directional pattern. This means that the energy, rather than simply being reflected, is scattered into a myriad of lobes, in every possible direction. So the reflection is reduced in intensity by spreading losses.

1-D QR diffusers on the ceiling of the Big Room at Real World

Phased out

The advantage in the control room is obvious. One of the requirements of stereo sound is that the sound from, say, the left speaker should not bounce off the rear wall into the right ear, and be mixed with sound from the right ear, thereby neutralising the stereo image information. With the rear-wall QR diffuser you have a real advantage, because sound scattered off the rear wall induces minimal interaural cross-correlation. Put simply, it has the effect of taking out rear wall reflections. A remarkable breakthrough.

Pioneering work at the BBC looked at another way you can add diffusion. It was installed in a conventional suspended ceiling. BBC Research Engineer R. Walker conducted an experiment whereby ceiling tiles, in a music studio at TV centre, were replaced with 2-D "diffuser" tiles. They succeeded in almost totally eliminating an annoying honk, which had sprung up when the carpet was removed at the request of operational staff. Walker's design was based on a quantised version of a primitive root sequence.

Happily to say, such devices are now commercially available. US giants RPG have just introduced a budget range, with the RPG Skyline diffuser. The RPG Skyline is a full 2-D fabrication, except the separators between the wells have been dispensed with. Skyline may be mounted in a suspended ceiling, or on the wall.

While compact in footprint, the Skyline will effectively diffuse the all-important psycho-acoustic frequencies, making it ideal for vocal use. Initially offered in white, and paintable, they are expected to become a valuable studio item, and should prove quite effective in breaking up troublesome overhead reflections. Adding diffusers to the upper half of the room can give immediate payoffs, by creating a diffuse and therefore neutral acoustic environment for standing musicians, without causing excess vertical absorption. Let's face it, in a home studio the mic is often nearer to the ceiling than the floor. For a budget that may not stretch to a full room, treating a small zone above the vocal mic position is the ideal solution.

You can follow a duck to water, but a studio must be lead. The exterior of the Big Room, clad in lead and good, honest West Country soil

The parallax view

In recording studios, QR diffusers may often be seen over drum cages and vocal booths. Their chief application, however, is in the control room, where they are used to enhance bilateral dissimularity, by being placed down the median plane (an imaginary centre division through the middle of the room, such that the operators' left ear would be one side and right ear the other). The earliest reflections occur on the shortest path between monitors and ears (this is not necessarily the first bounce). With a reflection-free design the earliest reflection usually occurs at the rear wall, and maybe the ceiling. So for maximum efficiency, QR series diffusers are placed on the rear wall and ceiling. They can of course be placed with equal effectiveness on the front wall, although this space is usually taken up by the observation window, so this is not quite so practical unless you have a Sideways Vision configuration. In a reflection-free control room the front wall is effectively acting as a diffuser, except that it diffuses, or diffracts, only at the higher frequencies.

Figures 1 to 6 : What happens when a sound slams against a wall. See 'The parallax view'
Figure 1

When a sound wave strikes a wall of a room it is reflected in a manner similar to light on a mirror. If the sound strikes "head-on", then most of the reflection comes straight back out into the room. See Fig 1, which shows the reflection patterns from a flat panel (dashed curve) and a QR diffuser (solid curve). The plot displays the sound on the top half of what looks like a "wagon wheel," called a polar plot.

The "spokes" indicate direction, and the height of a curve around the circles shows the sound level in each direction. The wall is represented by the horizontal line through the centre of the circles. Notice that the QR diffuser reflects sound equally in all directions, even though the original sound came from one direction only, as marked by the downwards arrow.

Figure 2

In Fig. 2 the sound wave has moved around, and now comes in from 45 degrees, on the right of the plot. Whilst the flat panel reflection obeys the mirror law by bouncing off as the dashed wave front on the left, the reflection from the QR diffuser preserves its omni-directional pattern. Remarkably, the reflection shape is little changed from Fig. 1, when the sound came in "square-on" to the wall. Whatever angle a sound strikes the QR diffuser from, it will always be reflected in this omni-directional fashion.

Figure 4
Figure 3

In Fig. 3 a TEF contour plot shows the response of a flat panel when sound bounces off it. You should view this kind of plot as though you were looking directly down onto the surface of the sea. The "ripples" represent wave excitations. Compare Fig. 3, of the flat panel, to a QR diffuser shown in Fig. 4. Now the reflection is far more complex, waves are being scattered out in all directions. This is especially noticeable in the higher psycho-acoustic frequencies seen to the right of the plot. You are looking at a diffuse sound field that will have a consistent acoustic quality, quite independent of your listening position within the room.

Figure 6
Figure 5

Fig. 5 demonstrates clearly why conventional flat panels should be mainly thought of as reflectors; they are singularly poor diffusers. The peak on the left indicates sound striking the panel. The almost total absence of any non-specular energy, to the right of the plot, means that most of the sound has bounced straight back into the listening space of the room. Contrast that with the high level of rich controlled diffusion produced by a QR diffuser, as shown in Fig. 6. In this situation the reflection is converted to diffuse energy, and drawn well clear of the operator's ears.

Figure 7: DIY diffusion, basic construction. See 'Careful with that saw Eugene'

Careful with that saw, Eugene

The rear wall of the Big Room discourages loitering - as well as diffusing the sounds produced by Peter Gabriel's myriad clients and guests

If you have the merest modicum of school woodworking skills you should be able to tackle this one. We're going to build a diffuser that is 0.8 metres high by 0.4 metres wide by 0.2 metres deep, simply for the sake of economy. This is a convenient compromise size, as you should be able to build two using the material from a standard 8 ft. x 4 ft. sheet of material, with minimal wastage. Don't worry that the size chosen may theoretically compromise acoustic performance, for home studio use the results should be more than adequate. Off the shelf diffusers come in standard architectural sizes, such as 1.2 x 0.6 metres. If you find you need diffusers to fit exactly into given room dimensions, then you should have them individually specified by acoustic consultants qualified in this work. The sketch in Fig. 7 shows dimensions and basic construction. This is how you do it:

1. Mark up your materials. The overall size of the diffuser will be 0.8 x 0.4 x 0.2 metres. The well spacing (the regular distance between each well separator), should be around 0.055 metres.

2. The well depth sequence is crucial to the correct functioning of the QR diffuser. Your well depths are; 0 (half-width), 0.05, 0.2, 0.1, 0.1, 0.2, 0.05, 0 (halfwidth) metres, starting from either end. The zero-depth seventh well is actually split into two half-width flanges, flush with the front casing on either side.

3. Now start the construction. First, you need to build the four sides of a box; 10mm MDF would be an ideal material to use. It would help to have access to power tools and a router.

4. You should have assembled the four sides of a case. You rout in grooves, six off, equally spaced in the top and bottom sections, to take the well separators.

5. You now slot six vertical separators into the case. These should be thin, but reasonably strong, 6 mm plywood would do. Rounding the front edges will reduce specular reflections.

6. Next, you need to install a back, at the correct depth for each well, in each of the slotted wells that you have created, see Figs. 1 and 9. Pin and glue them into position. Again, MDF would be ideal for this. Cut each back accurately to size. You may need small blocks of timber battening fixed behind to hold the backs in place. Any gaps should be filled using a mixture of sawdust and woodwork glue, to make an airtight seal.

7. You may fix on a rear panel of 10mm MDF to close off the rear for free-standing use. I would suggest filling the rear voids with rockwool to prevent resonances, but this is entirely up to you.

8. You should now have completed your first diffuser. Build the second, and finish both off by sanding and painting to match your studio colour scheme.

9. Attach the finished diffusers side-by-side in the centre of the rear wall of your studio, using mirror plates and screws, or other suitable fixings.

I have designed this diffuser to give you a usable frequency response from 490 Hz up to 2830 Hz, which should be ideal for the home studio. As a woodworking project it uses some complicated joints. If the task seems a bit beyond you, don't fuss, you can buy diffuser modules off the shelf, at a price that won't give your bank manager kittens. Last but not least, remember that this project is brought to you in the name of entertainment, so don't forget to have fun building it.

By the way, don't go into mass production on QRD diffusers, certain aspects of the manufacturing technology are patented. This doesn't mean that you're going to get dawn-raided, it is done simply to protect the enormous investment involved in tooling up. It ensures quality by preventing any unqualified copying.

"QR diffusers are the most diffusing surface known to man"

The art of installation

Installing QR diffusers in your studio should work wonders for your studio acoustics. You will notice a real improvement as you increase the number installed. Eight or more would be ideal. Feel free to cover an entire wall in this way. They will effectively remove the influence of any surface they are installed on, making the room sound like a much larger one. Install them on the rear wall of your control room, beginning with the centre location. Putting them on the front wall of the control room is okay too, provided your studio features Sideways Vision, and is free from a forward observation window. You'll find that, in combination with some acoustic tiles, this treatment will enhance your stereo imaging. Appearances are important too, and you don't have to make these diffusers out of just MDF or plywood. You could use metal, plastic, mirrors, or glass. You can even build windows in the style of a QR diffuser.

Larger areas of diffuser are built up by stacking diffuser modules above and beside each other. The frequency of operation of a diffuser is determined by its physical dimensions, as with a loudspeaker. To extend the frequency coverage, a fractal configuration may be used. Here, the QR modules are mounted onto a wall that is itself built in the shape of a QR diffuser. The 'recursive' pattern, reminiscent of Russian dolls, is designed using fractal theory. Diffusers that operate down to low frequencies have very deep wells. In the smaller rooms typical of a home studio you shouldn't need deep diffusion.

Did you know that the number of wells in a QR diffuser could be any odd prime number that satisfies the design relations? This number could be eleven, thirteen, nineteen, even forty-three or more. In practice seven is usually chosen, simply because it makes a diffuser with the least depth versus bass cut-off frequency. Seven has other advantages too, as it is not so susceptible to diaphragmatic, or low-frequency, absorption. Ideally, a diffuser should have only diffusion, no absorption. In practice, and because of diaphragmatic absorption, this is not always practical. So when you install a large number of QR diffusers, you should always have your studio tuned-up afterwards.

Sounding out

Once you've installed your diffusers, how do you measure the improvement? Well, there's always the Soundcheck CD with its 1/3 octave test routine, pioneered by this author in Home & Studio Recording November 1993 (and tirelessly plugged ever since - Ed). Try the mic in various positions about the room. In a good diffuse room, the results in each position should be fairly similar. If you do find parts of the room where the results vary wildly, then this shows that you still don't have adequate diffusion at the frequencies in question. The answer may be to spread your other acoustic treatment, such as acoustic tiles, around a bit more, rather than putting all the same type on one wall or surface.

Don't worry if the topic of studio acoustics seems a bit beyond you at this stage; acoustic modules are readily available off-the-shelf. For further information about complete studio acoustic kits and DIY plans, contact me direct on (Contact Details).

Stuart Litobarski is an Electronic and Acoustic Engineer as well as a recording musician. His Bath-based company, Soundwave Acoustics, specialises in studio design and acoustics consultancy.

Case study 1: the Big Room at Real World

Diffusers galore in the Big Room

When you look at this case study you'll realise why we had difficulty in finding another studio with which to compare and contrast it. Real World contains so many different studios. So we have taken a liberty and looked at two studios within the complex. These are entirely separate and quite different, particularly with respect to the question of diffusers.

We visited Real World at Box near Bath, global base of 'world' music and geographical home of international music star Peter Gabriel. We were shown round by Studio Manager Owen Leech, who has been with Real World for over eight years, since he returned from teaching in the Falklands.

One-dimensional QR diffusers

The whole idea of the Big Room was heavily steered by Peter Gabriel. Studio designer was acoustician Neil Grant, who worked in a close team with RPG, architects Field & Clegg, and structural engineers Burro & Hapold. Project Manager was Mike Large of Real World, who came ex-BBC. The design was based on the principle of a combined music and control room.

Besides being large in size, the Big Room also features what must surely be the most powerful array of studio diffusers in the World, as the walls and ceiling are almost totally covered with RPGs. The overall result is the creation of a single space, which at the same time as being truly colossal, is largely free from specular reflections.

As you can see from the photos, the studio rear wall features a QRD diffractal, which presents a striking 'Manhattan'-like appearance from stage level. As Neil Grant proudly describes, this was the first RPG diffractal to be designed anywhere in the world. The separators of the larger fractal component of the QRD are fabricated from lengths of steel-stiffened plate glass, which I think you can just see in the photo. More QRD diffusors are arranged along both stepped side walls, with additional ones being suspended from the ceiling.

The stylistic statement behind the Big Room was clear. Everywhere you go in the studio it should sound the same, with no low-frequency loss or focusing artifacts. The stage setting is intended to allow a group of musicians to play closely together, with a strong feeling of envelopment and achievement of good ensemble. The finished result was really remarkably successful, with no acoustic retro-fits being needed.

From the outside, the studio creates a Gothic impression, with the architectural detailing including mock gargoyles. Soundproofing is greatly aided by the external covering of decorative lead, and walls banked with earth on two sides. The large windows to the outside world, which overlook a small lake populated with ducks, are separated by air-gaps of more than 0.6 metre.

The Big Room is structured in concrete, and whilst it goes some way towards a reflection-free approach, the RT appears more on the dry side. Curved wings rise up on either side to the stage area at the rear, where the vocal booth is situated. Although the appearance is light and airy, large amounts of absorption are concealed in zones around the ceiling and walls, to adequately control the RT.

Recording possibilities in the Big Room are therefore endlessly varied. Even drums may be recorded within the room, usually in the same location as vocals - a booth on a raised platform in front of the rear wall. Now that's what you call diffusion.

Case study 2: the Stone Room at Real World

Aaaaaarrrr (that's enough - Ed) rrgh! 2-D QR diffusers on the ceiling of the Stone Room

Two-dimensional QR diffusers

Don't panic at the phrase '2-D QR diffusers'. The two-dimensional, or 2-D, diffuser is very similar to the 1-D diffuser, except it is built as a surface rather than a line array. With the 2-D type diffuser, the reflections are again omni-directional, except that the diffuse back-scattering is in the form of a hemisphere rather than a cylinder. The design uses a quadratic residue sequence, currently favoured by Neil Grant Associates over the alternative primitive root sequence.

Live rooms rely heavily on good diffusion to achieve their distinctive sound. Diffusion is often achieved in acoustic spaces by hanging great panels, called clouds, from the ceiling. These generally work well, if designed so that they don't cause unwanted absorption.

In the Real World Stone Room they have taken care to use massive clouds, made from solid hand-carved slate, each suspended on four threaded rods. Slate absorbs virtually no sound at the all-important mid to high frequencies. Absorption is provided only at the very low frequencies, where it is carefully controlled by absorptive upper faces to the slates. The result is an extremely live and attractive room, with a dense and rapid decay. This makes the Stone Room ideal for brass as well as drums.

From the point of view of recording, the Stone Room is a particularly interesting drum room, although it's not used exclusively for this. Almost any instrument is recorded in it, and it's also used for vocal overdubs - tie-lined to a small production suite next floor up.

Although the exact set-up may vary according to the preferences of the individual producer/engineer/artist combination, both drums and vocals are more often recorded in yet another space - the Wooden Room, where the grand piano is situated.

If all this seems pretty remote, don't feel left out. Real World is run as a commercial operation, and most of the studios are available for hire. So get practising!

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