Ashly Electronic Amplifier none - Manual (Page 25)

RESONANCE AND DAf^PING
To understand the response characteristic of a particular filter, it is
necessary to consider the factors of resonance and damping. This may be best
accomplished by considering examples of resonance in everyday physical objects
and then seeing equivalent effects in our filter circuits.
If you pluck a guitar string, it will vibrate back and forth at a certain
frequency,
called its resonant frequency. The vibrations will not go on
i ndef i ni tely, since some part of your initial energy will be 1 ost wi th each
excursion. The energy will be dissipated by mechanical and acoustical
friction, and also through the energy of the radiated sound. Eventually, the
string will come to a halt. The process of energy absorption which slows and
stops the string is called damping. Heavy damping, which would result if you
touched the string with your finger, stifles the sound quickly, while a freely
moving string would allow the tone to continue for a long period of time.
A loudspeaker system also has its own resonant characteristics, affected by
the speaker itself, the cabinet it is mounted in, the damping material in the
cabinet and even the room in which the speaker is located. When such a system
is asked to reproduce frequencies which approach or coincide with its own
resonant frequency, it begins to behave in a non-linear fashion. It offers
much less resistance (impedance) to the stimulus, and as a result its output
will be greater than at other frequencies even though the magnitude of the
stimulus remained unchanged. The loudspeaker will be "boomy" or "peaky" at
that frequency. However, if you stuff large amounts of absorbent fiberglass
into the cabinet, that boominess will probably be attenuated somewhat, owing
to the acoustical damping effect of the fiberglass on the cabinet's resonance.
This example of mechanical damping has an electrical equivalent in the filters
in your crossover. That is, a filter has a resonant frequency of its own, and
can actually have a tendency to oscillate if so designed. On the other hand,
the filter can be designed so that its resonance is "heavily damped," thus
stifling any tendency to oscillate. This characteristic will be seen to be a
very important factor in the summed response and overall sound of a crossover.
Let's take another look at a simple passive LC second-order low-pass filter,
shown again in figure 26 below.
(a)
Figure 26.
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