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Woofers / Subwoofers Woofers are loudspeakers designed to reproduce the lowest frequencies, or bass end of the audible sound spectrum. They may be anywhere from 4" in diameter all the way up to huge 36" giants meant for highly specialized scientific purposes. For most consumer purposes however, sizes are pretty much standardized at 4, 5, 6, 6.5, 8, 10, 12, 15, and 18 inches, with the 6.5 to 12 inch sizes being the most popular. For a general discussion of speaker operating principles see How Speakers Work.
Because
woofers are specialized reproducers, their design maximizes their potential
for reproducing the lowest frequency. Thus they will have cones that are
suspended in such a way as to promote the maximum back and forth motion,
called XMAX. Middle frequencies by contrast, have smaller cones and less
excursion, or cone travel. Tweeters have cones, domes, or diaphragms that
may seem practically motionless. Even so, they can produce sound in their
frequency range that is just as powerful (and piercing) as their larger
siblings.
Because of the powerful dynamic motions of the woofer cone, the frame or basket is often cast in a heavy metal to reduce the tendency for the speaker to "ring" at certain resonant frequencies. This arrangement also allows for the most precise alignment of the voice coil within the magnetic field and is particularly useful in speakers at or greater than 15" in diameter. In the larger drivers, after years spent resisting the tug of gravity, there is a tendency for the heavy magnet to be pulled out of alignment on the stamped frames of the larger sizes. This is of little practical concern with 12" and smaller woofers.
High
power woofers often have dual voice coils in vertical arrangement on the
cylindrical former. These allow different impedance configurations to
be used. With two 4 ohm coils in two woofers, a parallel circuit can be
used that creates a total mono impedance of 1 ohm. This is a very efficient
arrangement to transfer energy from a compatible amplifier.
For
speakers to truly work well and achieve their maximum audio potential,
they must be housed in an appropriate enclosure. Indeed without some elementary
means of preventing the front radiated wave from interacting with the
back wave, no bass would be heard at all. This is simply because the two
waves are precisely 180 degrees out of phase with each other and cancel
out when they meet. For more on this important subject, see
SPEAKER
BOXES AND ENCLOSURES.
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