Getting the sound in your mix space right is crucial. But what is ‘right’, and how do you achieve it?
Imagine you are creating the control room of a studio, and have the luxury of employing professionals to design and build the space to have great acoustics. Depending on who you went to, you might get offered one of a number of different design philosophies, such as Live End Dead End (LEDE), non-environment or Reflection-Free Zone (RFZ). But which is best? This guide compares different design philosophies, explaining how each affects the sound in the room and what the engineer hears as they work at the desk. As most of us do not have the budget to use professionals, we will also look at how the design ideas can be adapted for someone creating a studio through DIY.
Rooms have a significant effect on what we hear, and this happens for all types of loudspeaker reproduction — stereo or surround. Acoustic engineers will talk about the room causing ‘coloration’: a change in the frequency balance of the sound, with some frequencies being boosted while others are suppressed. At low frequencies, the inherent resonances of the room are the cause of this coloration, with the most audible effect being a booming of certain bass notes. At mid to high frequencies, coloration is caused by the interference between the sound reflected from the walls, floor and ceiling, and the sound coming direct from the loudspeaker to the listener. This is most evident as changes in the timbre of musical notes. Early reflections can also create problems with imaging, causing the exact location of sounds in your mix to become broad and blurred, and in extreme cases, being pulled away from their intended location in the stereo image. Rooms also have reverberance that causes a sound to linger, adding a ‘bloom’ that, in a well-designed room, subtly reinforces notes. A complete lack of reverberation sounds unnatural, but at the other extreme, too much reverberation can prevent aberrations from being audible to the sound engineer, leading to problems with the mix being overlooked.
Consider a studio engineer mixing in a control room that is smaller than the live room where the musicians are performing. The ideal control room should have a neutral acoustic where the sound engineer can ‘listen through’ to the acoustical environment of the live room. Unfortunately, if the control room is much smaller than the live space and lacks acoustic treatment, this is not possible. Figure 1 shows what the engineer hears in the control room when a short sharp sound is made by a musician in the live room — something like a single hit on a snare drum. The first room effect the sound engineer hears is due to a reflection from a wall in the control room, rather than something from the acoustic of the live space. This is because the ITD, the Initial Time Delay, which is the time between the direct sound (which, in the control room, is coming from the loudspeakers) and the first reflection from a wall, is smallest in the control room. Because our brain prioritises what it hears first, the sound will be perceived as coming from a space the size of the smaller control room (unless the live space is extremely reverberant).
What we need to do is make the sound appear to be coming from the live space by suppressing the early-arriving reflections from the walls of the control room, so that the sound engineer hears the ITD from the larger live space. Figure 2 shows measured impulse responses within a small listening space before and after treatment — in this case as a Reflection-Free Zone control room. Before treatment, the direct sound and sparse early reflections from the room stand out. After treatment, there is an initial time delay gap before the reflections from the control room arrive. The application of diffusers on the rear wall that scatter sound turn the sparse room reflections into something more like the reverberant decay of a larger room, with increased reflection density.
Reflection-Free Zones (RFZ) (D’Antonio and Konnert, 1984) are not the only way of improving small room acoustics. Other approaches include the non-environment (Newell, 2008); live end dead end (LEDE) (Davies and Davies, 1980), and controlled image design (Walker, 1993, 1998).
One approach to controlling early reflections is to suppress them via absorption. A ‘non-environment’ approach goes even further and removes the reverberation as well as the early reflections, to create a quite dead acoustic. This design philosophy has been successfully applied to many control rooms. How this is achieved is shown in Figure 3.
Non-environment rooms have loudspeakers that are flush-mounted, so that the sound emerging from them is not reflected from the front wall, while the rear, sides and ceiling of the room are highly absorbent. The combined effect of these treatments is that sound from the loudspeakers is absorbed instead of being reflected, so that only the direct sound is heard by the listener. There is potentially a reflection from the hard floor, but for the sound engineer, the floor reflection from the sound playing out of the loudspeaker is usually unimportant, because the mixing desk is in the way of that sound path. Like the floor, the front wall (in which the speakers are mounted) is not absorbing. This means that the acoustic environment, although dead from the loudspeaker’s viewpoint, is not perceived as oppressively dead, because there are two hard surfaces that reflect conversation and other sounds made within the room back to whoever is working in the room.
Proponents of non-environment designs say that the lack of anything but the direct sound makes it much easier to hear low-level detail in the reproduced audio due to the removal of any masking reverberation and other room effects. Furthermore, the rooms provide excellent, pin-point stereo imaging; this is almost certainly due to the removal of any conflicting cues in the sound, such as early reflections or reverberation.
On the down side, creating non–environment rooms requires broadband absorbers that can take up a considerable amount of space. In commercially designed non-environment rooms, the absorbent often occupies more than half the room volume! However, it is possible to use membrane absorbers to achieve sufficient broadband absorption with a depth of 30cm. This allows the technique to be applied in much smaller rooms whose area is approximately 15 square metres. Even so, the amount of treatment required makes this impractical for a home studio. Figure 4 shows a comprehensive non-environment room implementation.
A further problem is that because non–environment rooms have no reverberant field, only the direct sound is available to provide sound level — there is no support for the loudspeaker level. In a typical domestic environment at normal listening distances, by contrast, the reverberant field is about 10dB greater than the direct sound. Thus in a non-environment room one must use either 10 times the power amplifier level, or specialist loudspeaker systems with a greater efficiency, to reproduce the same sound levels. Another criticism that has been voiced by some is that sound engineers working in a non-environment have the additional difficulty of translating what they hear in a dead space, where they are mixing, to the livelier acoustic of typical domestic listening environments.
There is also a potential obstacle to the idea of creating a surround-compatible non-environment space. Some have just added rear loudspeakers without additional treatment. A proper adherence to the design concept, however, would require the front wall to be made absorbing so that the surround loudspeakers radiate into a dead space. This further increases the amount of room volume taken up with treatment.
One way of reducing the amount of absorption and mitigating the high power levels needed for a non-environment is to retain the reverberant tail in the control room, and suppress only the first-order reflections that cause the most coloration. This will lengthen the time between the direct sound and the first significant room reflection, so that the room sounds bigger. Many studio designs try to achieve this goal, the simplest one being the Live End Dead End (LEDE) design proposed by Davies and Davies in 1980.
The concept is simple and is shown in Figure 5. Broadband absorbers supress all the reflections from the front half of the room close to the loudspeakers, creating a ‘dead end’. The back half of the room forms the ‘live end’, which is left untreated. In some implementations, with flush-mounted loudspeakers, the front wall can also be left untreated as is done in non-environments. Also, in most implementations, some form of diffuser is placed on the rear wall to improve the performance of the live end of the room by breaking up the strong reflection from the rear wall.
Treating the room this way has a lot of advantages. Firstly, it is cheaper than a non–environment because there is less treatment, and the loudspeakers are driven less hard because there is a reverberant field available to boost the volume. The reverberation time is set by the amount of absorption used in the dead end. This reverberation can be set to be closer to a home listening environment. It is also possible to have loudspeakers mounted away from the front wall if that is desired.
However, there are disadvantages. An LEDE design presents two distinct acoustics — one live, one dead — and so the sound changes as you move around the room. The mix will sound very different to the engineer at the desk, compared to others auditioning the recording elsewhere in the room. The sweet spot where a good acoustic is achieved is quite small.
The LEDE philosophy is easier to implement at home than a non-environment room because less treatment is needed. Also, the absorbent controlling the earlier reflections can be shallower than is used in a non-environment, although you would also need membrane absorbers elsewhere to control the bass (low-frequency control will be discussed in detail later). However, there are ways of controlling early reflections with less absorption to get a much bigger sweet spot. One disadvantage of LEDE rooms is that the philosophy is not easily extended to deal with surround sound reproduction. A different design is needed, like those outlined next.
The two methods described so far use brute force to suppress early reflections. Although this has some advantages, especially in the case of the non-environment room, they are impractical for many rooms due to the amount of treatment that is required. A better approach is to only put treatment where it is needed, and this is the principle behind design philosophies such as Reflection-Free Zone and Controlled Image Design rooms. But how can one tell where the treatment should be placed? Figure 6 shows one way of working out where treatment should be placed to control early reflections. The idea is to imagine that the relevant walls, or ceilings, are mirrors. Then you can create ‘image rooms’ that show the direction of the earliest reflections. By defining a reflection-free zone around the listening position, and by drawing lines from the image loudspeakers, one can see which portions of the wall need to be made absorbent. The absorber in these locations only needs to treat the mid- to high frequencies that have the strongest effect on creating stereo images and coloration; bass frequencies are considered separately (see box). If the room is already built, you can find the locations by moving a real mirror along the walls and finding out where the reflected image of the loudspeakers can be seen from the reflection-free zone to determine the locations for the absorbers. Treatment should be arranged symmetrically, about the centre line of the room, because all control rooms should be symmetrical for correct imaging. The construction in Figure 6 is easy to do for a rectangular room. The concept also works for more complicated room shapes, but the diagram is harder to construct. It can also be applied to surround systems, the only real difference being the number of sources you need to keep track of.
If you are building a room from scratch, rather than applying treatment to an existing space, a reflection-free zone can be achieved by angling the walls. Once you know which regions need to be treated, you angle the walls to redirect the sound away from the listening area. An example of this type of RFZ room is shown in Figure 7, where the speakers are flush-mounted in the front of the room next to splayed walls that redirect the sound away from the listening area, thus removing the early reflections.
The Controlled Image Design uses surfaces to redirect reflections, using more complex curves that allow loudspeakers to be free-standing and not flush-mounted, as shown in Figure 8. One advantage of this technique is that there are places where absorption is unnecessary. This is useful because it shows you where to place doors and windows that are difficult to make absorptive. These ideas can also be applied vertically, to produce a concave faceted curve of the front wall as it rises to the ceiling. A problem with shaping the walls is that doing so makes the room much more complicated to construct: a bog-standard cuboid is much easier to make! A more pernicious problem is that, because the reflections are redirected rather than being absorbed, the listening quality deteriorates rapidly as you move away from the reflection-free zone due to increased number of reflections. To mitigate this, the rear wall can be treated with absorbers to remove reflections or diffusers to disperse and scatter sound.
On any particular surface, there is a choice about whether to apply absorbers to remove a reflection or a diffuser to disperse the sound. This choice is partly down to the desired amount of reverberance in the room, which sets the total amount of absorption that can be used around the space, and so might well limit how many panels can be absorbent. To minimise the amount of absorption needed, one should make the listening area as small as possible because larger reflection-free volumes require larger absorption patches. Moving between absorption and diffusion on a particular surface also affects the size of sonic images. If absorption is used to remove early reflections, especially near the loudspeakers, then the sonic images in the sound stage will be small, as if sound comes from a point in space. If diffusers are used, the sonic images may be broader, more like a typical home listening environment.
In terms of practicality, the Reflection-Free Zone or Controlled Image Design are probably the best options for a home studio, using treatment in a cuboid room rather than attempting complicated shaping of the front and side walls. The pure absorption approach, where the necessary treatment is achieved solely by absorption, is probably the most practical in the home studio where space is limited.
Instead of absorbing every reflection, as in the non-environment room, another approach is to diffuse every reflection by putting broadband diffusers on every surface, except the floor. In such a room, the impulse response looks like that shown in Figure 11, with a dense set of reflections all at a reduced level. Over a large part of the room the reflections are at least 20dB below the direct sound.
In an untreated room with flat surfaces, the sound arrives at a specific time and level having been reflected from a surface. An example was seen in the top of Figure 2 with the reflections from the side wall, floor and ceiling of an untreated room. In an ambechoic, every surface is covered with diffusers. From the point of view of a listener at a specific place in an ambechoic, sound no longer arrives from a single point on the wall, but from all locations across the wall as shown in Figure 12. These many reflection paths are all of different and longer distances than the specular path (shown in red). As a consequence, gaps that used to exist between strong sparse reflections, with flat walls, are filled with a dense set of low–level early reflections. These are less intense, because the diffuser spreads its energy over a hemisphere. This is why using diffusers instead of absorbers can sometimes be beneficial.
The diffuse reflections are low enough in amplitude to have no effect on the stereo image. There is also a large reduction in the coloration that would otherwise occur in an untreated room. This is due to both the reduction in amplitude due to the diffusion and the smoothing of frequency response caused by the multiplicity of time delays present in the sound arriving from the diffuser. The fact that the reflections are diffuse also results in an absence of focusing effects away from the optimum listening position and this should result in a more gradual degradation of the listening environment away from the sweet spot.
Blackbird Studio C is based on these principles and is shown in Figure 13. The experience of this room is that one is unaware of sound reflection from the walls: it sounds almost anechoic, yet it has reverberation! Stereo and multi-channel material played in this room has images that are stable over a wide listening area. This type of room is also good for recording in as the high level of diffuse reflections helps to both integrate the sound of acoustic instruments and give acoustic feedback to the musicians.
Another embodiment is the MyRoom principle by Petrovic and Davidovic, which uses diffusers placed in front of absorption to achieve a similar result. The diffusers are not completely solid and so allow some sound, especially the bass, to reach the absorbent. Yet another variation is the Early Sound Scattering control room (ESS), which reverses the role of absorption and diffusion in the LEDE type of room and instead puts all the diffusion at the front around the stereo loudspeakers and the absorption, if required, at the back of the room behind the listener. Again this should achieve similar results.
In terms of practicality, all these approaches use large numbers of diffusers and so are difficult to achieve in a home studio. Diffusers need to be at least a quarter of a wavelength deep to be fully effective, which means that you would need about 200mm-deep diffusers, about the depth of a bookcase, all around the room to achieve diffusion down to 500Hz. To get to bass frequencies, diffusers have to be even deeper, and the building has to be very solidly constructed to take the weight of the treatment.
When considering which room design philosophy to follow, it is important to consider your own personal preferences. One of the reasons so many different control–room designs have emerged is because there is not one perfect acoustic solution for a control room. If you have access to other studios, then you should do as architects do before building grand concert halls: do a listening tour to find out which style of control room sounds best to you.
Coloration at low frequencies is caused by the room modes. A room mode is a resonance of the air within the space, and its frequency is related to the dimensions of the room. For any loudspeaker configuration, room and listener location, there will be many modes that are excited, and are easy to see as peaks and dips in a frequency spectrum such as the one in Figure 14. With music, these modes cause amplification of some notes and attenuation of others. With transient sounds, it is easy to hear resonances ringing on after the note has ended. This creates muddiness, commonly affecting the precision of bass notes, kick drum or the low-register instruments in orchestras.
Solutions that were popular in the early stages of control-room design included splaying of the walls, and employing room aspect ratios that were supposed to be optimal in sustaining a well-behaved low-frequency response. Splaying the room walls is effective in reducing the problem of flutter echoes — a distinct ringing effect caused by repeated bouncing of sound between parallel walls — but it has very little effect on room modes. To be effective, the splaying of walls would need to be substantial compared to the wavelength of low-frequency sound, meaning rooms would have to be very much narrower at one end than the other, which is obviously not feasible. The so-called ‘golden room ratios’ that defined the optimal aspect ratio of the rooms were also very popular for a while, but recent research has shown that these are of secondary importance relative to other means of control (Fazenda et al, 2005). Unless you are treating a room with the worst modal distribution, such as might happen in a cubic room, it is not worth rebuilding a room to get golden aspect ratios.
The frequency balance of the bass in a room is dependent both on the position of the listener and of the loudspeakers. A simple test for this effect is to get your digital audio workstation (DAW) to generate a single low-frequency tone and walk around. You’ll be able to hear the sound changing. For the very low frequencies, you’ll find a couple of loudest and quietest points around the room. All loudspeakers sound bassier when placed near the walls, so the best starting positions are usually about 1 metre from any wall, as long as room size allows. As a consequence, positions too close to the corners are not advisable. Some of the high-end monitor speakers have built-in filters that allow some control of this interaction and need to be used if the speakers are to be placed close to walls or desks.
The good news is that loudspeaker position is usually within the control of the user of a domestic studio. Low frequencies are generally considered to contribute little to the localisation of sources of sound (although this is not the case when we consider real-life sources, it applies to stereo and surround reproduction systems) and, as many monitoring systems now rely on the use of a subwoofer to generate the low frequencies, you can optimise subwoofer placement to obtain a better response by trial and error. You can try sitting at your usual listening position and getting a friend to move the subwoofer around the room while you listen to a track with a fairly steady bass line. Movements of half a metre or more are most effective. (Using reciprocity, you can make this process less of a workout. Place the subwoofer at the normal listener position and then walk around to find the place with the optimal bass response. This is where the subwoofer should be placed.) Subwoofers should not be placed too far away from your main speakers, in order to avoid large time differences between the low and high frequencies arriving at the listening position.
The same set of principles applies to systems with two subwoofers or systems with full–range loudspeakers (stereo or surround), but the interaction with the room modes is more complex. In general, you will still be able to hear the differences when you move the loudspeakers but the approach needs to be a bit more careful. If using subwoofers, try to position them symmetrically to the line bisecting the room as you look at the mixing desk. You can move them symmetrically in or out of this line or together front-back along the room. If using full-range loudspeakers, say in a stereo system, you will need to relocate your listening position accordingly.
The most typical room acoustic treatment to address low-frequency modes is absorption. Diffusers can, in theory, change the modal behaviour in a room, but would need to be impractically large to do so. A room that allows most energy to escape through the walls — which is quite common with thin plasterboard constructions — has a better behaved low–frequency response than a room where the walls are heavy and made of brick, keeping all the energy reflecting within the space. Having thin walls or no walls at all is the best form of low-frequency control, the problem being that the bass response is solved at the expense of your neighbour’s sanity!
More practical solutions require efficient low-frequency absorbers. Foam panels would typically need to be at least a metre thick to get significant absorption at low frequencies, so a resonant type of absorber is typically necessary, with membrane designs being most common in small rooms. Some manufacturers will sell these tuned to a narrow frequency range around which they will work most effectively. These work best when placed on the walls and more specifically near the corners or cornices of the room. A considerable surface area covered with these is still required to get a significant reduction in booming. One or two are not usually enough even in the smallest of rooms, which makes the bass treatment one of the most expensive aspects of room design. Strips of resonant absorbers on the walls near the floor and ceiling are a good solution that leaves enough remaining wall area at ear height to place the acoustic panels used to deal with early reflections and reverberation. A very typical and comfortable solution is to place a large, soft couch at the back of the room!
- Absorber: A treatment that attenuates sound, such as an acoustic foam panel used to remove a reflection.
- Broadband absorber: Acoustic treatment that absorbs all of the audible frequency range in one device.
- Coloration: An audible change in timbre, usually caused by early reflections.
- Control room: The room where the sound engineer creates the mix.
- Controlled image design: A control-room design philosophy where surfaces are angled and absorbers applied to attenuate or delay the early arriving reflections.
- Dead acoustic: A space with a very large amount of absorption so that there are very few room reflections (antonym: live acoustic).
- Diffuser: A bumpy surface that causes reflected sound to be scattered in many different directions.
- Direct sound: The sound that travels directly from a loudspeaker or other source to the listener. It is not altered by the room.
- Early reflections: Sound that has reflected from one or more room surfaces (such as walls), and in a control room arrives a few milliseconds after the direct sound.
- Impulse response: The response measured on a microphone when a short sharp sound is made elsewhere in a room. Shows the direct sound and the pattern of subsequent room reflections, and so allows the quality of a room to be gauged.
- Initial time delay (ITD): The time between the sound arriving directly from the loudspeakers and from the first reflection of a room surface (such as the floor).
- Live End Dead End (LEDE): A control–room design philosophy where the front of the room is highly absorptive while the rear of the room has little absorption.
- Live room: A space where musicians are playing and being recorded.
- Membrane absorbers: A bass absorber where a membrane, such as a piece of hardboard or vinyl, vibrates against a spring formed by the air inside a box.
- Non-Environment: A control-room design philosophy where large amounts of absorption remove all room reflections that would otherwise arise from the sound radiated by the loudspeakers.
- Reflection-Free Zone (RFZ):
(1) A control-room design philosophy where absorbers and diffusers are used to attenuate or disperse the early-arriving reflections.
(2) The best monitoring region in the control room, where the early-arriving reflections are suppressed.
- Reverberation: the cumulative effect of the many reflections that, for small rooms, arrive tens of milliseconds and more after the direct sound. Reverberation causes sound to linger for a short while after notes have finished.
- Room mode: A resonance of the air within a room that amplifies sounds close to a particular frequency. Most audible when bass notes ring on for too long.
- Sweet spot: The region within a room where the highest-quality monitoring is achieved.
- Davies, D., Davies, C., The LEDE concept for the control of acoustic and psychoacoustic parameters in recording control rooms. J. Audio Eng. Soc. 28 (3), 585595 (November 1980).
- Cox, T. J. and D’Antonio, P. Acoustic Absorbers and Diffusers, second ed, Taylor & Francis, 2009.
- D’Antonio, P., Konnert, J.H., The RFZ/RPG approach to control-room monitoring. Audio Engineering Society 76th Convention, October, New York, USA, preprint #2157, 1984.
- Fazenda, B. M., Avis, M. R., & Davies, W. J. (2005). Perception of modal distribution metrics in critical listening spaces — dependence on room aspect ratios. Journal of the Audio Engineering Society, 53(12), 1128-1141.
- Howard, D. and Angus, J.A.S., Acoustics and Psychoacoustics, 4th Edition, Focal Press, 2009.
- Newell, P., Recording Studio Design, second ed, Focal Press, Oxford, 2008.
- B.Petrovic, Z.Davidovic, Acoustical Design of Control Room for Stereo and Multichannel Production and Reproduction — A Novel Approach, 129th AES Convention, San Francisco, 2010.
- Walker, R., A new approach to the design of control room acoustics for stereophony. Audio Engineering Society Convention, preprint #3543, 94, 1993.
- Walker, R., A controlled-reflection listening room for multichannel sound. Audio Engineering Society Convention, preprint #4645, 104., 1998.