|When most persons talk
about audible distortion in a working (as opposed to faulty) subwoofer,
they're usually referring to the gross levels of distortion that appear when
the driver (or amplifier) has reached its limits. In other words, they're
talking about VERY audible distortion.
distortion actually starts occurring at much lower output levels. In
fact if you see the driver's cone moving, it is likely also producing
distortion along with the source signal that it's attempting to reproduce.
This distortion may be at too low a level to be distinctly audible, or it
might be high enough in level to appear to be a subtle but noticeable change
in the tonal response of the subwoofer.
The ideal subwoofer should be able to generate any tone
from DC to the subwoofer's upper cutoff frequency at the required SPL
without any distortion. Unfortunately the ideal subwoofer does not exist,
and when designing a subwoofer, compromises have to be made and accepted
with respect to frequency response, output, and size, amongst other things.
To some extent, all of these compromises affect the distortion
characteristics of the subwoofer.
For sealed alignments, for every octave drop in frequency,
the driver must undergo four times the excursion to maintain the same SPL
level. As a result excursion requirements increase as frequency
Vented and passive alignments feature less excursion than
sealed alignments and as a result distortion is often lower with these
systems at and above the resonance frequency (Fb). However, excursion,
and therefore distortion, rises rapidly below Fb.
With respect to bandpass systems, 4th order bandpass
alignments combine the reduced excursion requirements of vented alignments
with the cone control of sealed alignments. 6th order bandpass systems
reduce distortion within the passband even further, but suffer the same
excursion problems as vented systems below the lower resonance frequency.
If not constructed properly, the subwoofer's enclosure could contribute
significantly towards the overall distortion level. The panels should be
constructed as rigidly and as airtight as possible, and sufficient clearance
should be left behind the driver, or distortion may rapidly increase at
lower frequencies. Cross-bracing should be used wherever possible to
minimize panel vibration. Some persons have even gone as far as constructing
enclosures out of concrete, but that may be a bit excessive, considering the
distortion due to the driver itself.
As expected, the construction of the driver itself contributes to the
overall distortion, and it does so significantly. Rubber surrounds actually
cause more distortion than foam surrounds as they are less linear, but the
latter are less popular these days, quite likely as they've gotten a bad
reputation due to the "foam-rot" that used to affect earlier versions. The
design of the motor itself contributes significantly to the distortion
characteristics, with the the best performance achieved from designs that
see to both extend the magnetic field that the coil passes through as well
as make it as linear and symmetrical as possible. Finally the cone itself
contributes distortion components of its own, with very rigid cones being
the best for subwoofer duty. Unfortunately most subwoofer driver
manufacturers do not provide distortion specifications for their speakers,
so usually the only way to find out how good (or bad) it is, is to perform
your own measurements.
Fortunately, we are not that sensitive to distortion at very low
frequencies (if we were, most speakers and subwoofers would sound like
garbage). When it's present, we usually interpret it as a change in tonality
rather than an unwanted addition to the signal that the subwoofer is trying
to produce. However, in a side-by-side comparison, it's usually not that
difficult to distinguish between a subwoofer that exhibits very low
distortion compared to one that has a somewhat higher level, particularly at