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Since then, the development (pioneered by Rane and Sundholm) of
the 4th-order (24 dB/octave) Linkwitz-Riley design solved these
problems, and today is the norm.
What this adds up to is active crossovers are the rule.
Luckily, the hardest thing about an active crossover is getting
the money to buy one. After that, most of the work is already done
for you. At the most basic level all you really need from an active
crossover are two things: to let you set the correct crossover point,
and to let you balance driver levels.
That’s all. The first is done by consulting the loudspeaker
manufacturer’s data sheet, and dialing it in on the front
panel. (That’s assuming a complete factory-made 2-way loudspeaker
cabinent, for example. If the box is homemade, then both drivers
must be carefully selected so they have the same crossover frequency,
otherwise a severe response problem can result.)
Balancing levels is necessary because high frequency drivers are
more efficient than low frequency drivers. This means that
if you put the same amount of power into each driver, one will sound
louder than the other. The one that is the most efficient plays
louder. Several methods to balance drivers are always outlined in
any good owner’s manual.
EQUALIZERS
You may have heard it said that equalizers are nothing more than
glorified tone controls. That’s pretty accurate and helps
explain their usefulness and importance. Simply put, equalizers
allow you to change the tonal balance of whatever you are controlling.
You can increase (boost) or decrease (cut) on
a band-by-band basis just the desired frequencies.
Equalizers come in all different sizes and shapes, varying greatly
in design and complexity. Select from a simple singlechannel unit
with 10 controls on 1-octave frequency spacing (a mono 10-band
octave equalizer), all the way up to a fullfeatured, two-channel
box with 31 controls on 1/3-octave frequency spacing (a stereo
1/3-oct equalizer).
There are graphic models with slide controls (sliders)
that roughly “graph” the equalizer’s frequency
response by the shape they form, and there are parametric
models where you choose the frequency, amplitude, and bandwidth
desired (the filter parameters — see diagram) for
each band provided. Far and away, the simplest and most popular
are the 1/3- and 2/3- octave graphics. They offer the best combination
of control,
complexity and cost.
In selecting graphic equalizers, the primary features to consider
are the number of input/output channels, the number of boost/cut
bands, the center-frequency spacing of each, and the bandwidth
behavior. This last one may at first seem a bit odd, but it
is perhaps the most important characteristic.
Bandwidth behavior is either constant-Q or variable-Q
(see diagrams). The quality factor, or Q, of a
circuit relates to its bandwidth in an inverse manner. That is,
narrow bandwidths result from high-Q circuits and wide bandwidths
come from low-Q circuits. In the early ‘80s, Rane developed
the first constant-Q designs to preserve the same shape (bandwidth)
over the entire boost/cut range.
In contrast, variable-Q designs have varying bandwidths (the shape
changes) as a function of boost/cut amount. They start out very
wide for small amplitude changes and become quite narrow for large
changes. Rane’s constant-Q design became the most popular,
and changed the industry.
Using Equalizers
Equalizers can do wonders for a sound system. Let’s start
with loudspeaker performance. An unfortunate truth regarding
budget loudspeakers is they don’t sound very good. Usually
this is due to an uneven frequency response, or more correctly
a non-flat power response.
An ideal cabinet has a flat power response. This means that if you
pick, say, 1 kHz as a reference signal, use it to drive the speaker
with exactly one watt, measure the loudness, and sweep the generator
over the
speaker’s entire frequency range, all frequencies will
measure equally loud. Sadly, with all but the most expensive
speaker Fundamentals-5
systems, they will not.
Click image for larger view. |
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Equalizers can help these frequency
deficiencies. By adding a little here and taking away a little
there, pretty soon you create an acceptable power response
and a whole lot better sounding system. It’s surprising
how
just a little equalization can change a poor sounding system
into something quite decent.
The best way to deal with budget speakers although it costs
more is to commit one equalizer channel for each
cabinet. This becomes a marriage. The equalizer is set, a
security cover is bolted-on, and forever more they are inseparable.
(Use additional equalizers to assist with the room problems.)
And now for the hard part, but the most important part: If
you do your measurements outside (no reflections
off walls or ceiling) and up in the air (no reflections
off the ground) you can get a very accurate picture of just
the loudspeaker’s response, free from room effects.
This gives you the room-independent response. This
is really important, because no matter where this box
is used, it has these problems. Of course, you must make
sure the cost of the budget speaker plus the equalizer adds
up to substantially less than buying a really flat speaker
system to begin with.
Luckily (or should this be sadly) this is usually
the case. Again, the truth is that most cabinets
are not flat. It is only the very expensive loudspeakers that
have world-class responses. (Hmmm ... maybe that’s
why they cost so much!) |
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