- Tue Aug 28, 2007 7:12 pm
#21357
***from our local forum
REFLECTIONS
You can make your vehicle sound better if you reduce the amount of reflected sound ... and increase the amount of artificial ambience. To understand why vehicles are acoustical nightmares and learn how to improve things, let's reflect on:
1. ideal acoustics
2. real vehicle acoustics,
3. what you can do to improve the acoustics of your vehicle.
1. Ideal Acoustics
Don Davis of Syn-Aud-Con developed an acoustical design for recording-studio control rooms called Live End/Dead End (LEDE). LEDE design makes sense for spaces that are larger than vehicles. One end of an LEDE room is live (reflective) and the other end is dead ( absorptive).
The live end is away from the speakers---where the listeners are---so late reflections and ambience are maximized. The dead end is where the speakers are. Absorptive materials minimize early reflections that blur images.
Unfortunately, vehicles are smaller than control rooms, so the late reflections are NOT very late.
"The LEDE approach is not relevant in cars," believes CA&E technical consultant Larry Klein, "beacause of the front/rear distribution of speakers and the acoustic environment of the car."
2. Real Vehicle Acoustics
As Klein implies, the reflective and absorptive surfaces in vehicles are not designed for good acoustics, but for visibility and comfort. In fact, vehicle interiors are somewhat Live Top/Dead Bottom, with the top more reflective and the bottom more absorptive.
The small size of vehicles has two acoustical effects---one good, one bad. The good effect of small size is bass reinforcement. When the wavelength of a frequency becomes twice as long as the longest dimension of the vehicle, the entire space is pressurized and depressurized at the same time, boosting the level of tones below that frequency at a rate of 12 dB per octave (longer waves are reinforced more).
The bad effect of small size is that the sound dies away faster than in large spaces. If you shout in a cathedral, your voice might echo for a few seconds. In a vehicle, the reflections die faster because the space is smaller (and because most vehicles are less reflective than typical cathedrals).
3. Improving Vehicle Acoustics
First, replace and reposition speakers to reduce sonic reflections; second, absorb reflections; and third, add artificial ambience.
Solving Face-Off Problems
"When speakers are facing each other." pointed out Keith Lehmann. US president of speaker manufacturer MB Quart, "the ideal solution is to eliminate the reflective condition by angling the speakers to at least a 20 degree angle with respect to the reflective surface of the speaker for the corresponding channel.
"Why do you think angled kickpanel speaker pods have become so popular among serious car audio enthusiasts and competitors?" Lehmann concluded.
Absorbing Reflections
"As far as controlling reflections in a car," commented Karl Hoelscher of Pacific Audio and Alarm, "cloth interiors and cloth upholstery---instead of vinyl---would help, but you still have a few square feet of glass. I would think that highly directional speakers aimed at you would help, at least so you would receive a majority of the primary information first before any reflections"
Neil Janoff, technical support supevisor for Sony Mobile-Comm America, was working with a TEF system recently to measure the benefits of signal alignment when he ran into a reflection problem.
"While I was able to improve my frequency response significantly, I could not get rid of a big dip at about 17,000 Hz... not until I covered my window with a piece of foam! The reflections are much worse when tweeters are mounted high up in the car. That's one reason why I like coax speakers mounted low in the door; the highs typically get absorbed by the carpet and such before they get the chance to reflect. (Ganito ung set-up ni Gary smallboy)
"Generally speaking," continued Janoff, "more directional drivers aimed away from reflective materials work well. However, in a car, almost everything above the dash is reflective, like windows, the dash itself, plastic panels, etc. I've seen some cars with carpet and/or fabric placed near and around tweeters to redirect the sound away from reflective materials." (Ganito naman ginawa ni mojako)
Experiments and Solutions
"I had heard conflictingtheories about reflected sound," reported John Card, national sales manager of Zapco. "To find out myself, I took glass shelf panels and home speakers and started experimenting with various angles and placement of the panels, noting their effects on the sound quality and the measured response of the speakers. Different angles tended to amplify certain frequencies, while other frequencies would cancel with the reflected energy, causing the total frequency response to vary quite a bit.
"Since this was a fullrange speaker," Card continued, "the total sound could be altered quite dramatically. The one fact that remained constant was that the speakers sounded their best with the glass panels removed completely. Putting the panels anywhere in the normal dispersion path of the speaker had a negative effect on sound quality.
"Since then, I have decided that glass is the enemy. Anything that can prevent it from being in the natural dispersion pattern of the speaker is worthwhile. The efforts include changing the mounting angles of speakers and using different types of foam and fabrics. Narrow dispersion patterns work wellin a car (hence the popularity of horns)," concluded Card, "and give you much more flexibility in angling the speakers to improve the soundstage."
Bringing Life To A Dead Vehicle
Even without acoustical changes decreasing the sonic reflectivity of the interior, vehicles sound pretty dead.
To make music come alive, simulate late reflections, increase ambience, and make the vehicle sound as big as a concert hall, you can: 1. install high-quality speakers in the back (matched to those in the front); and 2. give those rear speakers delayed low-pass-filtered sound, as suggested by Joel Cohen, CA&E's director of evaluation for electronic products. Adding Ambience can make dead vehicles sound great---much better than live and reflective ones.
by Wiliam Burton
Car Audio and Electronics, February 1997, Real Time Analysis pages 108-109
***from our local forum
