With the increasing adoption of multi-channel audio production monitoring and enjoyment in the home, there is a need for more loudspeakers and a commensurate need to reduce their size! This need is reflected in multi-channel audio's adoption of a specific low-frequency channel (LFE) and the adoption of a corresponding loudspeaker enclosure whose duty it is solely to reproduce frequencies below 100 Hz; the so called "sub-woofer". The enclosure for the sub-woofer loudspeaker may be either of the types previously discussed; that's to say fully sealed enclosure or reflex design. In all cases, the upper frequency limit is controlled by electronic filtering.
However, there is another type of sub-woofer enclosure which is enjoying a good deal of popularity at the time of writing and that is the "band-pass" enclosure. This type of enclosure is illustrated in the figure below.
In effect, this enclosure is a fully sealed enclosure firing into a Helmholtz resonator. This type of enclosure can be thought of as two, parallel loadings of the low frequency load speaker driver; loading from behind by a fully-sealed enclosure and in front by a bass-reflex design. By judicious choice of resonant frequencies and of Q value; this type of enclosure has a substantially band-pass frequency response, even without the need of electronic filtering. Practical designs, however, nearly always contain some form of electronic filtering to stop higher frequencies from "escaping" via the front firing port. Variations of this principle abound such that sometimes you will find that the rear enclosure too is bass-reflex loaded.
The problems with ported enclosures and other thermal effects
As the loudspeaker moves to and fro in its enclosure, it excites air both within the cabinet and the air outside. This agitation causes the air to warm up; albeit very slightly at low operating levels. However at high volume operating levels, such as might be encountered in public address or studio monitoring applications, the air can warm up quite considerably and, in so doing, will expand. The same energy that warms up the air inside the cabinet warms up the air outside. But the air outside is trapped in a much larger enclosure (the listening room) and so the heat rise is commensurably much smaller. This causes an asymmetry and a build up of pressure behind the loudspeaker cone. This leads to harmonic distortion effects and eventually, if permitted to go on long enough, can result in the cone of the loudspeaker “locking” outwards against its end-stop with potentially catastrophic results. These distortion effects are also accompanied at high level by a fairly broad band “chuffing” noise due to turbulence at the ends of the port as the air is forced in out. A diagram of these distortion effects is given in the figure below.
Note particularly the second harmonic distortion whose insidious presence indicates the asymmetric non-linear effect which can ultimately lead to loudspeaker cone "lock-out".
Other wide ranging performance variations occur due to high level operation in moving-coil loudspeakers; although the root cause of these fairly disparate affects is common and very simple. When the loudspeaker is operating at high level, the voice-coil - formed as it is of a few turns of narrow-gauge wire - heats up very considerably. Because the power delivered into any load is dependent upon the relation
P=V²/R
it is easy to see that, as R increases, the power available to be radiated from the loudspeaker for given voltage-swing at the amplifier output terminals is reduced. Simply put, this means that there is disproportional increase in acoustic radiated power for a given change in voltage swing at the loudspeaker terminals; with a doubling of input voltage resulting in a less than quadrupling of acoustic power. The effect is somewhat as if an electronic compressor had been used to modify the signal and indeed this effect is sometimes referred to as "high level compression" in moving-coil loudspeakers.
By itself this compression effect would go relatively unnoticed but, referring back to the electrical analogy of the ported loudspeaker cabinet [given in Chapter 14 - RAB], it was noted there that the voice-coil resistance was a significant parameter in the performance in the overall filter characteristic. We can therefore predict that a change in voice-coil resistance, due to heating effect, would cause a change in the low frequency alignment of a loudspeaker in a ported box and experiments confirm this. This change of base frequency response at high power is one of the most significant effects of operation at high level. The change voice-coil resistance can also have effects further up the frequency scale, whereby there is a change in the performance of the crossover filtering resulting in mid-band performance variations with temperature too.
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