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The “mid” section forms a bandpass filter,
since it only passes a specific frequency band. Note from the diagram
that the high frequency
passband and low frequency passband terms are often shortened to
just high-pass and low-pass. A 3-way system allows
optimizing each driver for a narrower band of frequencies, producing
a better overall sound.
So why not just use passive boxes?
Problems
The single biggest problem is that one passive cabinet (or a pair)
won’t play loud enough and clean enough for large spaces.
If the sound system is for your bedroom or garage, passive systems
would work just fine — maybe even better. But it isn’t.
Once you try to fill a relatively large space with equally loud
sound you start to understand the problems. And it doesn’t
take stadiums, just normal size clubs. It is really difficult to
produce the required loudness with passive boxes. Life would be
a lot easier if you could just jack everyone into their own headphone
amp — like a bunch of HC 6 or HC 4 Headphone Amps scattered
throughout the audience.
Let them do the work; then everyone could hear equally
well, and choose their own listening level. But life is hard, and
headphone amps must be restricted to practice and recording. Monitor
speakers on the other hand most likely have passive crossovers.
Again, it’s a matter of distance and
loudness. Monitors are usually close and not overly loud —
too loud and they will feed back into your microphone or be heard
along with the main mix — not good.
Monitor speakers are similar to hi-fi speakers, where passive designs
dominate — because of the relatively small listening areas.
It is quite easy to fill small listening rooms with pristine sounds
even at ear-splitting levels. But move those same speakers into
your local club and they will sound thin, dull and lifeless.
Not only will they not play loud enough, but they may need
the sonic benefits of sound bouncing off close walls to reinforce
and fill the direct sound. In large venues, these walls are way
too far away to benefit anyone. So why not use a bunch
of passive boxes?
You can, and some people do. However, for reasons to follow, it
only works for a couple of cabinets. Even so, you won’t be
able to get the high loudness levels if the room is large. Passive
systems can only be optimized so much. Once you start needing multiple
cabinets, active crossovers become necessary.
To get good coverage of like frequencies, you want to stack like-drivers.
This prevents using passive boxes since each one contains (at least)
a high-frequency driver and a low-frequency driver. It’s easiest
to put together a sound system when each cabinet covers only one
frequency range.
For instance, for a nice sounding 3-way system, you would have low-frequency
boxes (the big ones), then medium-sized mid-frequency boxes and
finally the smaller high-frequency boxes. These would be stacked
— or hung, or both — in some sort of an array.
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A loudspeaker array is the optimum stacking shape for each set
of cabinets to give the best combined coverage and overall sound.
You’ve no doubt seen many different array shapes. There are
tall towers, high walls, and all sorts of polyhedrons and arcs.
The only efficient way to do this is with active crossovers. Some
smaller systems combine active and passive boxes. Even within a
single cabinet it is common to find an active crossover used to
separate the low- and mid-frequency drives, while a built-in passive
network is used for the high-frequency driver.
This is particularly common for super tweeters operating over the
last audio octave. At the other end, an active crossover often is
used to add a subwoofer to a passive 2-way system. All combinations
are used, but each time a passive crossover shows up, it comes with
problems.
One of these is power loss. Passive networks waste valuable power.
The extra power needed to make the drivers louder, instead boils
off the components and comes out of the box as heat — not
sound. Therefore, passive units make you buy a bigger amp.
A couple of additional passive network problems has to do with their
impedance. Impedance restricts power transfer; it’s
like resistance, only frequency sensitive. In order for the passive
network to work exactly right, the source impedance
(the amplifier’s output plus the wiring impedance) must be
as close to zero as possible and not frequency-dependent, and the
load impedance (the loudspeaker’s characteristics)
must be fixed and not frequency-dependent (sorry — not in
this universe; only on Star Trek).
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