S
YSTEM
D
ESIGN
G
UIDELINES
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6
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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 (4x50 W)
Sub Amplifier (2x100 W)
Subwoofers
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