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S
YSTEM
D
ESIGN
G
UIDELINES
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SYSTEM PLANNING
The largest possible impact on any audio system (home or car) is the tonal quality of the loudspeakers, their respective
placement, and their overall efficiency (loudness). A 4x20 watt amplifier and four dual-cone speakers is never going to make
an impressive performance. The first major improvement comes with the addition of a strong low frequency performance
which give the emotional sensation of “strength” to the audio system. Often many newer cars have acceptable coaxial
speakers in acceptable mounting locations. By adding a subwoofer speaker and amplifier, the system, although only
moderate performance, becomes surprisingly pleasant for most listeners. To move to the next step up in performance you
must install some good component speakers for the mids and highs (“satellite” speakers) then add a separate subwoofer
speaker for the lows.
SYSTEM IMPLEMENTATION
Speaker configurations are a common problem in autosound installations. We want to achieve a sound field in front of us
(like a live concert) as compared to sound partly from the front and partly behind us. This dictates good midrange and
tweeter speakers in front, usually mounted in the doors for good left/right balance, with high-pass crossovers set greater than
80-100 Hz. The best stereo image will occur when the front speakers are spaced as far forward as possible attempting to
achieve nearly equal distance from the speakers to the listening position. For deep bass a subwoofer is required but is nearly
always located behind us in a rear trunk or rear hatch area. If the subwoofer crossover is too high in frequency male voices
can be heard “gurgling” out of the subwoofer speaker and therefore pulls the sound-stage to the rear of the car, which is very
unnatural and therefore undesirable.
MULTI-CHANNEL SYSTEMS
Moving up to amplifiers around 4x50 watts on the satellite channels with component speakers is a common and acoustically
rewarding step. The improvements in acoustical left/right balance and stereo imaging are quickly obvious. By adding a
dedicated subwoofer amplifier the perceived “strength” of the system is
often dramatic.
In order to achieve such a multi-channel system a dividing network must
be installed that keeps the low frequencies out of the midrange speakers.
Such a “crossover” network directs the higher and lower frequencies to
their respective drivers. Such a crossover can be “
passive ” (simple coils
and capacitors) that limit the operating frequencies seen by their
respective speakers. A crossover can also be “
active ” where an
electronic box must be installed in front of a dedicated amplifier and
process the signal via small signal RCA cables before its amplification. In
general, active crossovers are most common for the low/mid transition
area because they best filter the low frequencies and are physically small
compared to the passive equivalent. Passive crossovers are most often
used where they provide adequate filtering in the mid/high frequency
transition with satellite midranges and their tweeters. Passive crossovers
at these higher frequencies require only small coils and capacitors so
they are easy to install and perform well.
AMPLIFIER POWER
Amplifier choice and power is important but less so compared to speaker choice and placement. Matching the rms
(continuous) power capability to that of the speaker is important but it should be noted that “under-powering” a system can
often damage more tweeters than providing slightly more power than stated by the speakers. If the speakers are rated to 50
watts rms, you can often run 60-80 watt rms amplifiers without concern IF the amps are not driven into clipping (deep
distortion). IMPEDANCE is the electrical resistance to AC current flow and is typically 4 ohms for most car speakers.
Impedance loads should not fall below the manufacturer’s recommended minimum impedance or the amp will heat up and
sometimes shut down (2 ohms/channel is common).
SOUND QUALITY VS. LOUDNESS
All well designed sound system can provide good sound quality and still play loud. Above about 120 dB (decibels) the sound
isn’t perceived as getting much louder due to the non-linearities of the human ear. A four loudspeaker system with the per-
speaker efficiency rating of 90 dB (1 watt/1 meter) will often achieve about 110-115 dB if driven by 100 watts per speaker
channel.
(Although often debated, this is more than enough sound pressure level for most humans to enjoy and can easily
cause hearing loss if listened to at such levels for hours at a time.)
Tuner
Passive X-over Sat Amplifier (4x100 W)
Sub Amplifier (2x100 W) Subwoofers