The Infinite Baffle Speaker Enclosure
The purpose of the loudspeaker enclosure is to prevent waves emitted from the front and rear of a drive unit meeting and thereby cancelling each other out.
The obvious manner in which to keep the front and rear waves from ever meeting is to extend the baffle. Several designs have been made with an open-back or di-pole format, however the low frequency performance on these models is very poor, and would only be suitable for lead guitar amplification in a PA situation.
Folding the baffle into a full box, thereby completely containing the rear emitted wave is a much more popular design. This is the method of enclosure employed by the majority of speakers within a low budget range, because they are relatively easy to manufacture, especially in bulk, are suitable for speakers of virtually any size; and give an agreeable sound quality. As the baffle has no edges it is commonly referred to as the ‘infinite baffle’. Drive units selected for use with these enclosures need to be carefully selected, with a weaker or more compliant cone suspension than others, as the trapped air within the box acts itself as suspension under the pressure of cone movement. For this reason this type of arrangement is also called ‘acoustic suspension’.
The infinite baffle (acoustic suspension) enclosure
However because of this trapped air behind the cone, the cones movement is modified, resulting in untrue movement of the driver cone. This causes unwanted coloration to the sound waves emitted from the front of the drive unit- i.e. the one that the listener hears. The damping effect of the trapped air on the cone decreases compliance and results in the natural resonant frequency of the enclosure (a point at which the frequency of sound is reinforced by synchronous vibration of the cone and enclosure) being reached. This effect is reduced with larger volumes of enclosure as a given cone excursion exercises less compression on a larger air volume than on a smaller one. So larger boxes have lower resonant frequencies with the infinite baffle enclosure. In the same way a smaller driver cone also compresses a given air volume less than a large cone, hence the cone size also affects the enclosure resonant frequency. The cone mass also affects resonance as smaller masses have a higher resonant frequency to begin with. The object should be to have as low a resonant frequency as possible, preferably outside of human hearing limits (below 20Hz). Hence box size (therefore air volume), cone size and cone mass need to be carefully selected in this type of enclosure.
There are always sacrifices to be made when looking at loudspeaker features, and the consequence of having a large cone mass to achieve low resonant frequency is that the sensitivity of this type of enclosure is low. This is because mass requires power to move it, so more power is needed to do so than for a driver with a lighter cone.
Example of a floorstanding infinite baffle enclosure:
Dimensional Resonances within the infinite baffle enclosure
In addition to enclosure resonant frequencies, there are also dimensional resonance’s to consider. These resonances occur when the dimension is equal to half a wavelength of the sound, and this could cause coloration of the reproduced sound at the corresponding frequencies. Should this happen, there would be a very pronounced peak at the corresponding frequency.
This can be avoided by ensuring none of the internal cone-to-baffle dimensions are the same. Height, width and depth of the enclosure should all be different and the drive unit positioned off centre in the vertical plane. This means that if cancellation does occur at certain frequencies, they will all be at different frequencies and the overall result will not be noticeable.
Off-centre mounting of the drive unit ensures cancellation at unnoticeable frequencies.
Ideally an irregular shape with non-parallel sides would be used, such as a pyramid. However the disadvantages with this shape include the fact that a pyramid has only one third the volume of a rectangle with the same base area and height. Therefore either the base area and/or height must be increased to compensate, or the reduced bass response inherent with a smaller enclosure must be accepted.