Positioning of Sounds in a Mix
A Psychoacoustical Viewpoint
A psychoacoustical viewpoint of this interesting topic.
In order to fully appreciate how a sound can be placed at a certain point in space on a stereo mix, it is necessary to consider not only the specifications of certain electronic processors used to these ends, (ie. reverb, delay lines etc) but also psychological factors which are fundamental to a basic understanding of how we hear, ie. if we know that high frequency sound is very directional, then we can cater for this accordingly when trying to position this particular sound on the mixer, for example. This article will deal mainly with the psychoacoustical aspects of sound location.
To a listener, the location of a sound involves four different kinds of information. The sound will be at a certain distance ie. near or far (Figure 1a). It will be anywhere on a horizontal plane - left, right, back, front or points in between (Figure 1b). It could be anywhere in the vertical plane ie. up or down (Figure 1c). Lastly, the sound may be stationary at one place, or it may move from one location to another.
Information as to the distance of a sound, is cued by the amount of reverberated sound in relation to the direct or non-reverberated sound perceived at a given point. This is because if a sound is some distance away, there will be more surfaces which it will be able to reflect off before it reaches the ear. However, moved closer to the listener, there will be more direct sound perceived and less reflected sound, as shown in Figure 2.
The amount of reverberation is then one of the main cues employed in perceiving the amount of space in an environment. Artificial reverb units can be used to good effect to create the illusion of space. Turning up the reverb in relation to the direct signal does make a sound appear more distant. Reverb in natural conditions, however, tends to decay quicker at higher frequencies. Also, in a natural environment, an increase in distance is generally characterised by a gradual loss of low frequencies. The Quantec Room Simulator, for example, is now able to imitate some of these more subtle aspects.
We use two different cues for locating a sound on a horizontal plane ie. whether the sound is left, right, back, front etc. Firstly, intensity. If a sound is louder on a listeners right hand side for example, it will quite obviously suggest to him that the sound is located in this direction. Intensity differences however are only useful cues at higher frequencies. At lower frequencies (below 1 kHz) where the wavelength is about one foot and long enough to bend around the head, the sound reaches both ears with the same intensity despite where it is situated. At such frequencies we use phase as a cue for horizontal location. That is although the sound reaches both ears with equal intensity, ft will reach one ear before the other, indicating the direction from which it is coming. These intensity and phase cues for high and low frequencies are illustrated in Figure 3.
It is also important to note that sound localisation on a horizontal plane is less accurate at intermediate frequencies (about 1500 Hz) for pure tones, or those with little or no harmonic or non-harmonic content. Here the frequency is too low for differences in intensity to occur and too high to permit location from phase differences.
These facts are quite important to the feasibility and success of positioning a sound in a stereo mix, eg. the higher the frequency the more directional it will be, and hence it will be harder to pan it to be located midway between two points (or speakers). Rather the sound will tend to originate at either one side or the other.
Reverberation, can also aid the feeling that a sound is coming from a certain point on the horizontal plane. In a stereo mix for example, if it is desired that a sound is to be placed to the right of centre, then this sound in the right hand channel should have more volume and less reverb, as compared to the left channel. This should create the feeling that the sound is less distant in this channel.
The ability to position a sound at a certain point on a vertical plane is a relatively new area where research work is still being carried out. In stereo, sound can't be positioned at points on a vertical axis. However, one can get a fair sensation of vertical movement, by sweeping a notch filter up and down a spectrum of broadband noise (white/pink). Sweeping the filter from high to low across the spectrum creates the illusion that the sound is moving from an elevated position to a lower position on the vertical axis. This is shown in Figure 4.
The final type of information conveyed to us about the position of a sound, concerns its rate of movement from place to place. Changes in movement on the horizontal axis are perceived as gradual changes in intensity from one place to another. In a stereo field this would possibly mean simple panning from one speaker to the other, subject to the frequency-imposed limitations mentioned earlier.
Explaining the perception of a change in a particular sounds' distance is slightly more complicated. One cue is a change in reverberation characteristics, ie. as a sound moves further away, for example, it will exhibit gradually more reverberation in relation to direct sound. This is due to the reasons suggested earlier in the article. Secondly, as a sound source moves further away the sound pressure waves become more expanded with the result that less wave cycles are perceived in the same amount of time. The result of this is a drop in the perceived pitch, the rate of the drop being proportional to how fast the sound moves. This is called 'Doppler effect', and is the same effect as is heard when a car drives into the distance. The engine noise drops in pitch. Doppler effect is more prominent at closer proximity, and it is only evident if a sound is moving. When a sound becomes stationary again it goes back to it's original frequency. Changes in reverberation characteristics and Doppler effect as would be expected, alter in the opposite way when a sound moves towards us.
The ability and means to position a sound anywhere in three dimensional space, is a new and relatively unexplored area to many. The scope afforded by techniques such as 'ambisonics', for example, as compared to present day stereo systems, creates a whole new array of possibilities in the recording world, and will take some time to become adopted. Although this article didn't touch on the mechanics and instruments used for positioning sound, it is hoped that by explaining the psychoacoustical fundamentals of sound location, it in some way aided an understanding of what the recording engineer needs to cater for when positioning a sound in a mix.
Feature by Martin Graff
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