CPR

I am not going to argue this topic with you, but your boys may want to brush up on the Physics of Sound. The idea of creating 'dead spaces' via loss of longitudal or transverse sound waves by method of cancellation or by frequency evasive patterns is lunacy. It is the equivalent of saying if the wind blows hard enough in the opposite direction, the earth will subsequently rotate in tandem....it simply cannot happen. Laws of Physics...they don't bend, they don't break.

I am not busting your *****, just calling to question their means and methods for system building....in these cars at least. Volumetric acoustical inefficiencies do not exist in the 944 and thus are subsequently easily manipulated to achieve almost whatever sound (and quality) one so desires. Even with slightly sub-standard reproduction components and amplification.

I'm out

odurandina

when i was in college, i spent a lot of time consulting white papers on basic loudspeaker design. kef (germany), and b & w (england) where responsible for the much of the modern innovations in crossover designs and sound...

they always designed their systems to come as close as possible to creating a "point source."

every loudspeaker since the early '80s is borrowing or stealing from their original designs.


i eventually built a set of studio monitors (they're in mazatlan at the moment) using professional JBL drivers, esoteric subs and Audio Research preamp and twin Sunfire (Bob Carver) amps.




in home theatre, many compromises are made to create the most spl, and dynamic range, and sound imaging to make the best of all worlds possible for everyone's specific budget...


here's a white paper from Peavy, keep in mind that the principles of bass transducers creating dead sound zones are even MORE relevent for the closed in spaces of our cars.


http://www.peavey.com/support/techno...ncellation.cfm


BOUNDARY CANCELLATION

How to eliminate low frequency noise cancellation with proper woofer placement


By Marty McCann
Copyright 2001

Many local bands and sound companies are unknowingly victims of a type of low frequency cancellation known as boundary cancellation. This phenomenon occurs when a loudspeaker is 1/4 of a wavelength away from a boundary. Assuming that the boundary (like most walls or floors) absorbs little or no energy, the reflection off of the boundary plays against the energy still coming from the speaker source.

Here is a thought experiment that is a good analogy. Imagine a subwoofer that is placed 1/2 of a wavelength, at certain frequency, away from another sub, and the second sub has had its polarity reversed. It is easy to imagine that the loudspeakers will be opposing each other because one will be moving inward while the other moves outward, causing mutual cancellation. A similar thing occurs when a loudspeaker is placed 1/4 of a wavelength away from a boundary. It is as if the boundary is not there, but there is a phantom speaker 1/2 of a wavelength away that has had its polarity inverted......

pontifex4

I'm still in. Here's the gist of it.

Signal:

First off, in the mind of an installer-for-hire who's thinking of two theoretical full-range drivers in your car, one on either stereo channel, facing each other with nothing but car interior and your head between them, the set up is going to be somewhat challenging ONLY because he won't be sure where between them your head will be. The biggest issue for ONLY TWO full-range drivers in stereo is that if he gets the phasing wrong, the stereo will sound "weird" to the listener. This is a risk, yes, but there are only two ways to configure those two speakers in stereo: either both amplifier positive leads go to the same terminal on each speaker, or they go to opposite terminals.

Once you add speakers into the mix which play only low frequencies, you have to keep track of the phasing you used, otherwise the overlapping and complimentary frequencies can (depending on how far they are away from one another) cause destructive interference like that of noise-canceling headphones.



That is really the only variable worth mentioning in terms of using multiple speakers from a signal perspective, and the reason installers still roll out that old chestnut is because they don't want to experiment. That's reasonable enough: testing takes time (albeit not much) and if they're not paying attention, they run the risk of getting it backwards and causing you to hate the sound and tell others.

Standing waves:

Here's where the plot thickens. Stereos which get installed in cars typically use a lot of energy for bass. That means that dedicated speaker(s) are used to turn a lot of energy into sound waves. Sound waves don't care where they go, but because they're in a car, they are going to get reflected somewhat off the confines of your interior.

Because of that, savvy installers will employ one of two strategies to get maximum bass impact: either minimizing or maximizing reflections of those low-frequency waves before they reach the listener.

The aim is not to have both direct and reflected sound heading to the listener because those reflections will create constructive interference in the low frequencies, amplifying some notes and attenuating others, seemingly at random:



This is where we circle back to cross-firing speakers, and, particularly, mid-mounted cross-firing subwoofers in hatchback cars: the best reason not to do this from a sound perspective is that you run the risk of a standing wave caused by constructive interference from the reflection off the hatch and trunk area before that sound reaches you.

Here's the thing with that, though: those waves would have to travel about four times as far to reach you via that route as sound would to go directly from the speaker to your ears, at the mercy of the inverse square law, past an area which is carpeted and designed to attenuate sound, anyway. In practice, the standing wave is barely measurable in a Honda Civic hatchback. There is almost no reason not to try it in a 944, apart from the need to keep track of phasing as you wire it up.

pontifex4

Addendum: the boundary cancellation issue is a reach in a car interior. Why? A 60Hz wave is 16,400,000 feet long. That makes a 1/4 wave 4,100,000 feet long.

CPR

kef is England as well, not Germany. It stands for Kent Engineering and Foundry.

YoungerThan944

Phew. This got interesting haha. I always had an idea there was a lot more to subwoofer placement than it first seems, interesting read

Quick update on my situation:

I've closed things in, and will be modifying the side panels to fit a speaker grille much like the one in the 10-speaker option. The side panels muffle the sound too much to not do anything about it.

I also mounted my amp in the spare tire, and it looks like I'll pick up a 8" sub to build a KLA-style sealed enclosure for, which will be more or less invisible, and able to be covered for trunk use. I wish i could fit my 10" in the side cubby - the light gets in the way unfortunately.

@odurandina

I love the setup, it looks awesome. However, one of my main goals is to have it invisible. The system doesn't have to look completely stock, but I don't want it to be clear that I've installed an expensive aftermarket stereo.

pontifex4

Your amp mounted right in the spare tire? Sweet! You should post some pictures.

Obligatory caution: the gas tank is under an inch ahead of the front of the spare tire well in late cars, and hangs down almost to the bottom of it. Watch out when fastening stuff back there!

odurandina

ummm not quite. great posts, so far but:

midbass waves between about 220 Hz down to about 110 hz are between about 5 and 10 feet long... plenty small enough to do weird things bending and reflecting around the inside of your car, and sounding less than desireable.

subwoofer waves from about 110 Hz and down to 55 Hz range from about 10 to 20 feet in length... still, potentially causing problems.

by the way, when we first connected my subwoofer... first we did it in proper phase with the mid-bass drivers, and then out of phase....

the difference ? the first setup was amazing.....

the other, dead, dull. simply horrible, no matter how much you turned it up.



.

YoungerThan944

It's not the prettiest installation since nobody but me will ever see it. I might do it properly when I finish my subwoofer box, depending how well it holds up to driving. For now, it involves some baling wire, foam to protect the spare, and a strap haha.

I looked at where the fuel tank was, and there is definitely no space to mount it for me. It was pretty much flush with the spare tire well. Thanks for the warning though

EDIT: I ended up re-installing the side panels as they were and after testing the sound, WOW. SO much better than the 6x9's mounted in the rear. Crisp, clean sound all around you rather than noise just coming from the doors and somewhere far behind you. Dealing with the trim was completely worth it, and not really all that much of a hassle really. I think I'll leave the speakers as they are and get going on the subwoofer box tomorrow.

Paul C 944

Does KLA industries still make thier 8 inch sub box for the rear wheel well? I have one in my car and it is compleatly hidden.Works great.http://www.klaindustries.net/

Been a while since Ive been here!

odurandina

i just clicked on products / 944/951 / hidden subwoofer. way cool.


the thing that's also great is with a setup such as that, opposition can easily be measured,



this cab could also be used as the base for building out for the use of a single duel vc 10", to take advantage of using 1 corner of the car as a point source for a mono bass wave, similar to using a single, duel vc bazooka tube, back in the day.

pontifex4

Sorry, speed of sound in air -- 60Hz in sound = 18.75', a quarter of which is 4.68'. I'm not great with imperial units.

odurandina, why would a sub be playing 220Hz? Also, and this is by no means an attack, but I just don't buy boundary cancellation of subwoofers in an automotive environment: there are way too many attenuating materials and convolutions in a car interior to make it relevant. I'm not saying I don't think driver-driver cancellation doesn't happen (just flipping the phase on any two speakers playing the same frequency will show that it does, as you say). I just wanted to explain why installers so often wrongly shy away from drivers in direct opposition.