Many in-car entertainment systems employ multiple
speakers and possibly multiple amplifiers. Each speaker works most efficiently
when it plays only its specific range of audio frequencies, for example
a large bass speaker will move more slowly than a small tweeter, and hence
it reproduces bass accurately, but can't play the higher treble notes
at all. Not only is it wasteful to try to play the wrong frequencies,
but it can also be damaging to the speaker.
To split up the frequencies a filter is used,
more commonly referred to as a cross-over. There are two basic types,
low-pass which allows frequencies below the cut-off frequency to go to
the speaker, and high-pass which allows frequencies above.
In its simplest form, a high pass cross-overs
consists of a capacitor in series with the speaker, and the blocking of
the lower frequencies increases at the rate of 6dB per octave.
Similarly, a low-pass cross-over consists of a
choke in series with the speaker, again acting as a 6dB/octave filter.
Bass Sub-Woofers
To run a sub correctly, a low-pass cross-over
is essential. Purpose built cross-overs will have more components, and
as a result have a higher slope than the simple choke, or many built-in
filters in amplifiers. If you are using a single amplifier, then this
passive form of cross-over is OK, but it does have disadvantages.
Multi-Amped Systems
Each amplifier is designed to drive a specific
frequency to each speaker pair, with a series of active cross-overs before
each amplifier. Each cross-over has a slope of at least 18dB/octave (eg
Butterworth filter as used by Audio Control) or even 24dB/octave (Linkwitz-Riley).
The latter produce a flat frequency response, and outputs which are in
phase at the cross-over frequency. Adjustment of phase relationship of
the output allows time correction for speakers that are not in the same
acoustic plane.
Checking a System
The performance and spectrum analysis is made
using a real time analyser (RTA), and Road Radio uses the AudioControl
SA-3050A RTA, as used by the IASCA sound-off judges. We can tune
your system just the way you want it.
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