Theory
Judges listening

Sound quality

What it sounds like

There's one thing my car absolutely excels in: sound quality. There is a reason this system brings home trophies, and pure hifi sound quality is why. This car's audio system is built to make HiFi sound; pure, natural, neutral, fresh and clear. The system cannot boom, it won't boom and it shouldn't boom. The objective of the audio system is reproducing sound as naturally as possible.

So, what should it sound like? As naturally as possible. If a blindfolded person took place in a car and would not be able to tell whether a sound were real or produced by the audio system, you reached the goal. In my car this works well with concerts and real life sounds from test CDs. The sound should appear to come from the front of the car ("front staging"), ideally the artists should appear to be standing on the bonnet or at least on the dash. The stereo image should be wide and should appear to be broader than the car itself. The sound must detach from the speakers. Every instrument should have its own place in the sound stage, which should reach far beyond the boundaries of the fysical speakers.

Good Bad

Good: wide stereo image
front staging

Bad: narrow stereo image
center/rear staging

The average CD has such poor sound quality that it is not the system that fails but the recording quality on the CD. When sound quality is less important but music is, I prefer listening to music by Cathy Dennis, Mylene Farmer, Debbie Gibson, Transvision Vamp or Suzanne Vega. But to enjoy the pure quality of the audio system I'm forced to listen to audiophile CDs and special re-masters like gold-CDs. These CDs have more detail and greater dynamics. Some highly recommendable listening material for hifi freaks:

  • Soundtrack from "The Lion King" by Walt Disney
  • "Rhythm of the Pridelands" music inspired by The Lion King
  • "Days of Open Hand" by Suzanne Vega
  • "Let it Go" by Clair Marlo
  • "I'm Breathless" by Madonna
  • "Famous Blue Raincoat" by Jennifer Warnes
  • "The Hunter" by Jennifer Warnes
  • "The Simpsons sing the Blues" by The Simpsons (yes, the cartoon)
  • ... and Ivo's Do-It-Yourself test CD

     

Setting gain

One part of fine tuning the system is setting the gain on the amplfier(s). Gain is not volume control! Gain is dynamics control. With gain set at maximum the signal from the head unit is amplified as much as possible. It is not a good idea to set gain at maximum for these reasons:

  • Noise is also amplified at maximum giving the system ground noise
  • Your speakers may not be able to handle extreme dynamics or power
  • The amplifier may not be able to handle extreme dynamics, because:
    • it has insufficient current
    • the head unit sends a very strong signal which makes the amplifier perform above its limits
    • it simply cannot amplifiy that much
  • The other amplifiers may not be able to play as loud. Now gain is used as a volume control. When doing this, first set the weakest amplier. Now set the other amplifiers so that they are well in their limits and play as loud as the weakest amplifier.

An easy way to set gain:

  1. Set gain at minmum
  2. Play very dynamic music
  3. Turn the volume of your head unit to maximum or near maximum (many head units don't send a clear signal at maximum volume)
  4. Up the gain until you hear the system cannot cope anymore
  5. Turn the gain down a bit
  6. Preferably repeat with some different music
  7. Choose the lowest of these settings
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Practice
Microphone in car

Measuring sound quality

The graphs below show my sound quality from head unit to amplifier to speakers. It gives a purely technical and mathematical display. Pleasant sound is not necessarily the same as sound that has good RTA score (i.e. a good graph). These measurements were done indoors, engine off, with no noteworthy sources of disturbing noise nearby. The 1991 IASCA CD was used to generate pink noise, "L'Daddy" was used for SPL. Click on any of the graphs to download the original graph from the
LinearX PCRTA jr scoring computer.
Head Unit Sound quality of the (old) Alpine 7525R head-unit is nearly mathematically perfect. The graph on the right shows the output of the head-unit playing pink noise (a noise pattern that represents all frequencies). A perfect graph would be an exact flat line (on any level, since the dB on the Y-axis has no meaning here). However I fed the output to the computer via my Harrison F-mods. And it shows, one can see them kicking in at exactly 60 Hz. I highly recommend Harrison F-mods since they do not affect sound quality, do not need power (so no risk of any electrical problem or disturbance), are easy to install, take virtually no room and are cheap. The Alpine head unit is the absolute best when it comes to sound quality from a compact disc. Do not try to listen to the radio tough because your father's old Pioneer from 1972 has better reception! Alpine tuners sound like you have no antenna on your cars and the performance of the RDS decoder is downright shameful.

Graph1
Amplifier This next graph shows the sound quality of the AudioSystem amplifier, sending the amplified signal to the speakers. The blue units are the peak levels of the head unit. Subsequently, any difference between the black bars and blue bars mean affected signal by the amplifier. This measurement was done with the speakers still on the amplifier but on very low volume because we had no idea how much power the computer could handle. Amazing how accurately the amplifier did its work without altering the signal at all. Compared to a SoundStream amplifier this sound is a bit more aggressive and lacks little detail. However stereo image and front staging are near perfect because of speaker locations, no matter what amplifier is driving them.
We did not measure the Audison subwoofer amplifier because of its high power it was likely to blow up the expensive testing computer.

Graph2
Speakers No matter what head unit or amplifiers, it all comes down to the sound quality in the car. The third graph sounds what the speakers make out of the signal from the first two graphs. The blue blocks show output from the head unit including Harrison F-mod (however this graph displays all frequencies, including those from the other pre-out which doesn't have an F-mod). So, this graph shows the complete system playing at 90 dB, including the subwoofer amplifier (which only does frequencies below 60 Hz, explaining the difference between filtered pre-out at sub frequencies) and it's connected subwoofer. The effect from the Harrison F-mod should not be visible or audible. The peak at 1 kHz is said to be a typical thing for the MB Quart 218.X series. You can especially hear it on high notes played on a real piano. It makes the speakers a little more lively and play "mid high in your face", which I personally like very much.

Graph3

For more score sheets, see: 2nd NIASCA challenge of 1995, 3rd NIASCA challenge of 1995 or RS car audio meeting. Or click on any of these icons to go directly to the score form and skip the story of that particular day.



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