The Layman's Guide to Listening Environments

In any enclosed space there are a number of factors which affect the way sound is distributed through the space, and therefore the way in which this sound is heard by the listener. In practice these factors can be broken down into four categories; reflection - echoes or reverberation within the room; absorption - how the environment 'soaks up' the sound; deflection - how the sound is scattered; and resonance - how the sound physically affects the environment. These principles apply just as much to the listening environments in recording studios, homes, home theatres, commercial theatres, entertainment venues, churches, meeting places, or anywhere where recorded or amplified sound is distributed.

In the process of recording music the acoustic environment in which the music is created is an integral part of the sound recorded. This is due to the acoustic properties of the environment colouring the sound received at the microphone, and whilst in the past great effort was made to record in acoustically flat environments, today natural colouration is not necessarily considered to be a bad thing. Many recordings have been made in places with specific sound characteristics in order to incorporate these characteristics into the finished product. For example U2 recorded their Unforgettable Fire album in a castle in order to utilise the acoustic properties of the castle halls and Phil Spector once recorded a piano from behind a closed door because John Lennon wanted to use the sound of the piano as he heard it when he was in the corridor outside.

As well as considering the recording environment in the construction of the recorded sound the audio engineer relies upon the listening environment to allow him to hear the recorded sound accurately. He needs to hear this sound exactly as it has been recorded, without colouration from the monitor equipment or the room. In the same way, when the finished CD is playing on your home stereo the audio characteristics of your listening room will affect the way you hear the finished product and directly influence your ability to experience the music as the artist, producer, and engineer intended it to sound.

It therefore makes sense, when constructing any listening environment, to consider what factors will affect the way sound is heard, and to structure the environment in a way which tailors its audio characteristics for best listening and least colouration. Unless you have the luxury of a dedicated listening space you may find that not all of the following measures are practicable but even implementing a few basic changes in your audio environment will almost certainly produce significant improvement.

Reflection:
Imagine you're throwing a tennis ball around a room. When the ball hits a hard surface it bounces off. In fact it can ricochet around the room, from wall to wall, for some time. On the other hand, if it hits soft surfaces such as cushions, drapes, or Aunty Myrtle, it will loose some of it's momentum as this energy is absorbed into the pliable material with which it has impacted.

Oddly enough the same materials which cause your tennis ball to bounce are those which cause the most reflection of sound and by and large those surfaces which might cause your ball to bounce the least are likely to be those which deflect sound the least. In rooms with hard surfaces (including window glass) the sound tends to bounce around for some time and because sound travels at a finite speed this results in the reflected sound being perceived as echoes. Whilst the cumulative effect causes an increase in apparent loudness it also produces a loss in clarity. It becomes difficult to hear the sound source over the reflected sound. Of course no surface reflects 100% of the sound which hits it but despite this, just like in our tennis ball example, sound can continue to bounce around the room for some time. On the other hand a highly absorptive environment will produce little reflected sound allowing the sound source to be heard with increased clarity. 

Absorption:
Absorption is the ability of the surfaces in your listening environment to soak up the sound which impacts them. In professional listening environments walls and ceilings are usually lined with purpose made materials which provide absorption figures approaching 100%. Acoustically this resembles listening to the sound source in the middle of a deserted field.

The problem is that in some environments you may not want to cover your walls, 
windows and ceiling with absorptive material. After all it probably won't really match the decor in your living room. As well as this you will probably find it a rather unpleasant place to be if you reduce it to a completely echoless environment. Neither is absorptive overkill conducive to a pleasant ambience in bars, churches or other sound environments.

The solution is to carefully place sufficient absorptive material in the room to minimise reflection and to combine this with deflective surfaces which scatter the sound - imagine your tennis ball bursting into little pieces. In this way absorptive materials can be incorporated into furnishings, drapes, and decor matched panels.

Deflection:
Deflective surfaces, as we've already mentioned, scatter any sound waves which hit them. To fully understand this we'd have to delve into the physics of sound waves, which is somewhat beyond the scope of this article, so instead let's refer again to our tennis ball analogy. Imagine throwing this tennis ball against a rocky and jagged cliff face. Every time you threw the ball it would be likely to bounce off in a different direction. It holds true that jagged surfaces in your listening environment will scatter sound in a similar way.

To picture how this works with different frequencies (pitches) of sound imagine you have a bouncy ball the size of a marble to represent high pitched sounds and one the size of a bowling ball to represent low pitched sounds. A surface with small irregularities would be unlikely to deflect the larger ball (as the irregularities would be averaged out across the surface) however it could cause significant deflection to the small ball. On the other hand a surface with only large irregularities has the potential to present a flat surface from the perspective of the small ball, resulting in big balls being deflected but small balls merely being reflected. It therefore follows that a deflective area will need to have irregularities of various sizes in order to deflect all sound frequencies.

Resonance:
Resonance is produced when the physical environment in the room vibrates in sympathy with the sound source, or a given frequency of the sound source, causing the resonant frequency(s) to be amplified and re-introduced into the listening environment. This principle is used to good effect in musical instruments such as acoustic guitars and pianos but can cause a number of undesirable results in the listening environment, such as audible buzzing or vibration of fixtures and overemphasis or booming of certain frequencies. Commonly this effect is most audible in the middle and bass frequencies.

In practice resonance can either be prevented by introducing absorptive materials into the environment or counteracted by introducing baffled resonant enclosures to soak up the problem frequencies. The later are effectively tuned enclosures filled with absorptive materials. In studio situations these are commonly built into the walls but can also be constructed to mount in corners or other areas of the room. Rather than damping resonance in the entire room these can be used to absorb sound in the specific frequency range of the resonant frequency.

Standing Waves:
But why does a room resonate? When the distance between two parallel reflective surfaces is an exact multiple of the wavelength of a sound a standing wave will develop.

This can best be understood by imagining our tennis ball leaving wall A at 90 degrees, following through an arc, and bouncing off the floor once before hitting wall B at the same height as it left wall A. If we imagine for a moment that the ball arrives at and leaves the walls at the high point of its upper arc, and that it looses no energy when it bounces, we can picture it returning to its start point on wall A following an identical path to that followed in the first instance.

Once again let's imagine our ball to be a sound wave. Whilst the reflection of a sound wave will cause a loss of amplitude (volume) it will not alter the wavelength of the sound so in this respect our identical path analogy holds true. It can then be seen that any sound which returns to wall A after reflection from wall B will be in phase with (and add to) the original sound. If the sound source is then sustained for any time period this will set up a standing wave (resonance) in the room. The frequency of this sound is known as the room's resonant frequency.

Possible solutions for room resonance are to build the room with non-parallel surfaces, to provide adequate absorption within the room, and to prevent reflection through the use of deflective surfaces.


Sweet-Spots and Your Favourite Chair:
At some stage we've all had the experience of visiting an audiophiles home and being gravely guided to 'The Chair' - the one seat in the house where the stereo sounds just as it should. Whilst this may be an adequate scenario for the lonely audio snob it's absolutely no use for shared environments such as home theatres or for professional recording studios where teams of people work together to create a finished product. So how do we create a wide 'Sweet-Spot' - a listening area big enough for everyone to hear our audio source to full effectiveness? Even in the case of a home listening environment or hobby recording studio it behoves us to ask 'How do the professional studios do it?'

Whilst there is no definitive method the approach most favoured is to deploy a cross-section of the treatments discussed above. With speakers placed left and right ahead of the listening area, the ceiling and the walls to the sides and behind the speakers are treated with absorptive materials. In a home environment this could be achieved with drapes, cushions, carpet, soft furnishings and decor matched absorptive panels. The effect of this is to allow the source audio to reach the ears directly, without any reflected or deflected sound being involved at this point.

If we were to continue the absorptive treatment behind the listening area the end result would be to introduce what would be to the ears an infinite space behind the listeners. In practice this tends to be unpleasant. It also limits the sweet-spot to a position directly ahead and equidistant from both speakers.

To counter this a deflective treatment is introduced to the rear of the room. In a home environment shelves of books and LP records or stacks of CDs can be placed to perform this function. Your audio equipment will also provide a mixture of reflective and deflective surfaces. The more deflective surfaces you add here the wider your sweet spot will become as the sound is scattered throughout the listening area after the first rear deflection. Any areas which remain should be absorptive with one notable exception.

There is one other element sometimes added to this treatment and that is a reflective panel mounted left and right rear. This allows the listener to hear a small amount of the first echo. Any further reflection is deadened by the other three absorptive surfaces. In a home environment this could be a good place to install your windows.

Absorptive resonant chambers can also be added to corners of the room or built into the rear wall. By varying the amount of absorption and the resonant frequencies of these the room can effectively be 'tuned' for a desired frequency response.

Mono Listening Environments:
In situations where a number of people are present, or where room shapes don't allow for adequate stereo distribution, a multi-speaker mono system may produce a significantly better listening environment. For example if a group of people are arrayed through a stereo environment you may find that some are significantly closer to the speakers of one audio channel than to the other. This will result in one half of the program material being heard at significantly lower level than the other. What these people are hearing then has little relation to the mix intended by the audio engineer. A common example of this can be heard in small rock venues when the engineer chooses to place instruments to left or right in a stereo pan. Other situations where a mono listening environment may be more suitable might include office environments, small rooms, outdoor areas, or unusually shaped areas.