The room in which you listen to audio can make or break the experience. Even with the best sound system in the world, a poor set-up and room design can leave you wondering why you spent so much on the audio hardware in the first place.
In this article, we will take a methodical approach to determining the best listening and loudspeaker positions. We will also discuss acoustic treatment (absorption and diffusion), and we will dip into the topic of room modes.
Goals
Early reflections and the 20–20 rule
One of the primary goals in creating a good room acoustic for mixing, listening or home cinema rooms, is to reduce early reflections.
Direct sound is sound which travels in a straight line from the loudspeakers to your ears. This is the shortest path a sound wave can take, and it will always be the first sound to arrive at your location. Shortly after this, sound which has bounced off the walls, floor, ceiling and other surfaces will arrive at your ears. Reflected sounds which arrive within 20 milliseconds of the direct sound are known as early reflections.
Reflected sounds which are heard within 20 milliseconds of the original signal are perceived as a single sound, as opposed to two separate events. Because these early reflections take a longer path to reach your ears than the direct sound, they are slightly out of phase (out of sync), and this results in constructive and destructive interference, meaning you get comb filtering (peaks and nulls at various frequencies). Therefore, this effect should be avoided as much as possible, by eliminating early reflections. The ultimate goal is to reflections shorter than 20 ms to less than 20 dB below the direct signal, as this way, the nulls and peaks of comb filtering will be less than 1 dB (barely noticeable). This is known as the 20–20 rule.
Reverberation time
The reverberation time is the length of time it takes for sound to decay in a given room. If you clap your hands in a church hall, the sound of the clap will die out over a couple of seconds. If you clap your hands in a bedroom, the clap will usually die out almost instantly. The church has a long reverberation time (typically a few seconds on average) while a bedroom has a short reverberation time (typically half a second). When a room is ‘dead’, i.e. has a short reverberation times, this means that the acoustic you do hear as the listener is largely dictated by the audio source and the sound system. The longer the reverberation time, the more the room will add its own colour to the sound. Just how much of the rooms coloration you want is a personal choice. Some audiophiles will opt to make their rooms fairly dead, while some do enjoy the added ambience provided by a room.
Watch this space for a more detailed discussion on reverberation times and its effects on human perception.
Read our article about how humans perceive reflected sound.
Set up
How you set up your listening room will depend on its size, shape and what other fixings or furnishings you have in the room. Most rooms are more like a shoebox than a perfect cube. That is, most rooms have four long walls, and two short walls.
Set up your listening position facing towards a short wall
The reason you want to set up your listening position facing a short wall, is because the on-axis sound from the loudspeakers will be travelling the longest distance to the rear wall, meaning that reflections from this rear wall will have travelled further and be weaker, when they make it back to your ears. This reduces the likelihood of them contributing to early reflections and causing comb filtering.
Symmetry
Left-right room symmetry is crucial for a good stereo experience. If you or your speakers are too close to one wall, this can have a negative impact on the stereo imaging, by exposing the listening position to out-of-phase reflections. While it’s difficult to guarantee symmetry in any room, you should try to achieve at least some. Set up the loudspeakers away from other furniture, bookshelves, fixtures and fittings. While symmetry ensures reflections arrive from both sides at the same time, producing a strong stereo image, it’s also worth noting that the halfway points in rooms typically experience bass nulls. So there is some trade-off in deciding whether to perfectly centre your listening position, or to have it slightly off-centre, in a location which has better bass response, but a slightly less than perfect stereo image. This is a personal choice.
The 38% rule
In a shoebox room, the bass response is poorest at the halfway points along each exis. That is, halfway between the front and rear walls, halfway between the left and right walls, and halfway between the floor and ceiling. In the very centre of the room is the poorest low-frequency response. Avoid placing your listening position or loudspeakers in these locations.
Acoustician and studio designer Wes Lachot demonstrated theoretically, that the best listening position in any room is 38% of the way into the length of the room, when measured from either the front or rear wall. While every room is different, this is a great starting point, from which to find the optimal position. For a stereo listening room, this position offers the best compromise in low-frequency room response (peaks and nulls).
The best position in any room will depend on many other things, including loudspeaker location, room furnishings and wall properties. If you are installing a home cinema, 38% is likely to be too close to the screen for a good visual experience.
Angle your loudspeakers in an equilateral triangle with your ears
Loudspeakers are designed to be listened to on-axis, as this is where they have the flattest response. This means that we want the loudspeaker cones pointing directly at our ears. Additionally, we want the signal from both left and right channels to arrive at our respect ears at the same time. The way to achieve this is to set up your loudspeakers at two corners of an equilateral triangle, with the back of your head in the third corner.
In a home cinema with 5.1 surround sound (where the 5 refers to the number of main channels, and 1 refers to the Low-Frequency Effects [LFE] channel for frequencies 120Hz and lower, handled by the subwoofer), you should position the left and right speakers as above, with the front centre channel in an arc between them (i.e. exactly the same distance from your listening position). The rear two main channels should be positioned at an angle of 110 to 120 degrees from the listening position — this is the same set-up used by surround mixing engineers and ensures you will hear the sound as it is intended.
All loud speakers should be the same distance from your ears, and raised to a height where the tweeters are at the same level as your ears.
Position your loudspeakers close to the front wall
There is a myth that loudspeakers should be located as far away from the front wall as possible, to reduce the effect of reflections. However, this ignores the fundamental truth that there will always be reflections, unless you heavily treat that front wall. This in itself requires lots of space and is not always feasible, for example, if you have windows into the live room. At frequencies 300 Hz and below, most loudspeakers are omni-directional, and so signals at those frequencies will be sent towards the front wall, which will in turn reflect them back towards the loudspeaker and the listening position. When this reflected signal combines with the direct signal from the loudspeaker, it will be slightly out of phase, resulting in comb filtering (peaks and nulls at various frequencies). This effect is called Speaker Boundary Interference Response, or SBIR for short.
The amount by which the reflected signal is out of phase will be proportional to the distance from the front wall. Therefore, by reducing this distance, we can pull the affected frequencies upwards, beyond the 300 Hz where the reflections are more readily dealt with by room treatments. The ideal position would be with the loudspeakers built into the wall (and suitably treated at the rear), so that there is no SBIR.
Absorb early reflections
Now that we have optimised our listening and loudspeaker positions, it’s time to treat the room to reduce the effects of early reflections and comb filtering further.
The first location we want to treat are the locations where the earliest (and strongest) reflections will come from; the side walls. You can find the exact centre-point of these early reflections by sitting in the listening position, and asking a friend to hold a mirror flat against the side wall and moving it until you can see the loudspeaker cone of the nearest speaker to that wall. Repeat this on both sides for both loudspeakers, and you will have found the prime locations for acoustic absorption. If possible, apply absorption to the front wall behind the loudspeakers.
The second location we should look for early reflections will be the ceiling. Assuming your tweeters are at ear level, and the ceiling is level, the absorption should be placed on the ceiling at the mid-way point between your listening position and the loudspeakers.
Thirdly, we want to treat reflections from the floor by providing a rug; the thicker, the better.
Fourthly, we want to consider our desk or mixing console. If we aren’t careful, off-axis sound can bounce off of the desk and to our ears with a phase delay, causing comb-filtering. The easiest way to avoid this is to screen the off-axis sound using the desk or mixing console, so that there is no sound striking the table at the required angle to reflect back upwards towards our ears.
Lastly, let us consider the rear wall. Sound travels at 343 metres per second at 20 degrees celsius. For simplicity, let’s say this is 0.34 metres per millisecond. If our room is less than approximately 3.4 metres long, then the time taken to travel the full journey from loudspeakers to the wall and back to our ears will be 20 milliseconds or less, meaning these are perceived as early reflections. In this case, we will need to provide absorption to the back wall as well as the areas mentioned previously.
Absorbers at early reflection points should usually be effective down to 300 Hz, and this normally requires them to be at least 50 mm thick.
It’s all about the bass
The longer the wavelength of a sound, the more difficult it is to absorb it. Bass frequencies are therefore the most difficult to treat, and the absorption has to be larger in order to be effective.
Small rooms are likely to experience modes at low frequencies, creating peaks and nulls in the low frequency response. Music which is mixed in this type of room can sound too bassy, or not bassy enough, when played elsewhere, depending on whether you were sitting in a peak or a null while mixing. There is a myth that adding bass traps will reduce the amount of bass in the room. However, in reality, bass traps will reduce the effect of modes and provide a flatter room response (higher nulls and lower peaks).
Low frequency sound tends to gather in corners. Therefore, it’s good practice to add thick bass traps to the corners of the room, from floor to ceiling. Some bass traps can also be added to the junctions where walls meet the floors and ceilings, for added effect.
Diffusers or absorbers?
You may be asking yourselves why we’ve only mentioned absorption above, and not the beautiful, geometric diffuser panels we often see in professional studios. Diffuser panels are great for specific uses, but you need to use the right tool for the right problem.
Watch this space for a detailed discussion on the differences between diffraction versus absorption.
TL,DR — diffuser panels can be used on the rear walls instead of absorption if your room is longer than about 3.4 metres, but beware that this will result in a longer reverberation time. Diffusers can be used elsewhere along the side walls where reflection points may be causing peaks and nulls, if you prefer a longer reverberation time. This will be a personal choice depending on your preference for a ‘dead’ or ‘live’ room. Also note that most diffusers are only effective from about 600 Hz and higher.
Summary
In this article, we’ve talked about some general principles for determining the layout of your audio listening room, be it a mixing room, control room or home cinema.Here you can see a summary image showing many of the principles we have discussed above.
Some general rules of thumb are:
- Find your optimal listening position starting at 38% of the way into the long room dimension.
- Position your loudspeakers so that the listening position forms an equilateral triangle with them.
- Keep your loudspeakers close to the front wall.
- Apply absorption to the front walls and side walls near the loudspeakers.
- Treat early reflections at the side walls, ceiling, floor and rear wall.
- Treat the rear wall with absorption if you are in a short room. In a longer room, you could employ diffusion panels.
Precise measurement and design requires specialist equipment, software and knowledge. If you would like to speak to us about the design of an audio suite, feel free to contact us today.
E: tony@timbral.co.uk
T: 0208 504 6129
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References
This article was written with reference to the following material:
Ethan Winer — The Audio Expert (Second edition)
Heinrich Kutruff — Room Acoustics
Niels Werner Adelman-Larsen — Rock and Pop Venues, Acoustic and Architectural Design, Second edition
Related: Room modes and finding the ideal room proportions for audio.
