Matching the right speaker and amplifier to one another isn't complicated. In fact,
most speakers will work with most amplifiers; but if you know what to look for, you can
coax even better performance out of the combination. Before we go there, lets first
look at how a loudspeaker works.
To keep things simple, well discuss only the most common type of loudspeaker here
and leave the more exotic types for a future column. The first thing you notice when you
look under the grille-cloth is that not all speakers use the same number of drive units.
However (with very few exceptions), they all use a minimum of two. Why is that?
Sound is measured in Hertz (abbreviated as Hz), which is a unit of frequency
measurement. If you take a string and stretch it between two points and then pluck it, it
will make a sound because the string is vibrating. Each of those vibrations takes the
string above and below its resting point, and one complete trip up and down is considered
a cycle. One cycle per second is measured as 1Hz. High-frequency sounds are
measured in the thousands of Hertz (also known as kilohertz or kHz), while deep sounds
have far fewer cycles -- a low organ tone might measure 30Hz.
The range of human hearing extends from approximately 20Hz
to approximately 20kHz. Actually, 20Hz is so low that we dont so much hear it as
feel it, like a rumble. Very few instruments can play this low (organs, synthesizers,
certain very big drums). On the other end of the spectrum, a 20kHz tone/squeal/whistle is
so high that many of us cant quite hear it anymore. This depends on age and other
factors like hearing abuse (high-level rock concerts or work-related noise pollution).
"20 to 20" is the frequency spectrum a loudspeaker is supposed to reproduce.
Note the word supposed. Very few speakers actually manage to reproduce this entire
range. While that is a subject for future investigation, we can already mention that no
single loudspeaker driver can reproduce this whole range except for headphones -- because
they need to produce sound only inside the very tiny cavity of your ear.
Since a single loudspeaker drive unit cant reproduce the whole frequency spectrum
of sound, the full spectrum is usually divided into at least two parts: high and low.
Specialized high-frequency and low-frequency drivers are each assigned specified portions
of the speaker's response range. High-frequency drivers are called tweeters (in
reference to the noises birds make), and low-frequency drivers are woofers (for
the low barks and growls of a dog). The device that separates the frequencies is called a crossover.
So we now have a loudspeaker box with two round drivers attached to its front: one
small tweeter and one bigger woofer. This basic type of loudspeaker is called a two-way
design because the whole frequency spectrum has been divided into two parts. Thats
the meaning of "-way" -- how many ways has "20 to 20" been divided.
Its also true that the speaker only has two drivers. However, it could have more
drivers and still be a two-way. A designer might decide to use multiple drivers for
each portion of the frequency spectrum. Thats hardly ever done with the tweeter, but
it is done quite frequently with woofers. A designer might double up on the woofers, using
two (or even three) of them. In a two-way speaker with multiple woofers, these woofers are
said to operate in parallel. This simply means theyre doing exactly the same
thing, like dual tires on the back of semi trucks.
Some speakers that use three drivers divide the frequency spectrum into three sections
-- then they are called three-way speakers. And, not surprisingly, the names for the three
drivers, corresponding to their low/mid/high responsibilities for the different frequency
portions, are woofer, midrange (or mid) and tweeter. A speaker with more than three
drivers could either be a three-way with multiple woofers, or a four-way or five-way. But
most speakers are two-way or three-way designs.
Weve talked about drivers quite a bit. In a conventional box speaker, the moving
parts that produce sound are called drivers or transducers. They
literally push the air back and forth like miniature pistons, to create the sound waves we
hear as music. Remember what we learned about frequencies -- higher-frequency sound waves
have more individual ripples, or up and down cycles, per second than lower ones. A 20kHz
note has 20,000 peaks and valleys per second. To reproduce it, a driver has to push out
and back 20,000 times per second. Thats much faster than a human eye can perceive.
Watch a two-way speakers tweeter while its playing really loud -- it won't
seem to move at all. But watch the woofer. Play the music loudly enough, especially with
bass notes, and you will easily see the woofer move.
So far, we've dealt with the different drivers as though they were the same, but of
course they aren't. Tweeters are usually small and dome-shaped, whether they are made from
treated silk or ultra-thin metal, while woofers are much larger and cone-shaped or flat.
Since tweeters deal with the high frequencies, they need to move back and forth very
rapidly. A small, lightweight dome is perfect for this. But the lower frequencies of the
midrange and bottom-end of the spectrum require both greater in-and-out motion (also
called excursion) and greater surface area on the part of the driver. While the
cones (or sometimes flat diaphragms) used for woofers look simple, they are very
carefully designed from their shape to the type and thickness of rubber surrounds used to
anchor them to their frames. Woofers can also be made from a wide range of materials --
from paper to metal to high-technology ceramic.
Next time, we will take a look at the internal parts of the driver and begin to talk
about the basic loudspeaker specifications we will need to know to consider that
all-important speaker/amplifier interface.