Amplitude Theory

[ameuba10 10+ year member]

i currently have a fiberglass box that fits behind my rear wheel well and i was thinking of building a box just like it on the other side. long story short, i decided not to go with it for time/money issues and the fact i dont wanna hassle with building the other box. but nonetheless i was thinking about it.

a friend of mine told me not to do it because of sound cancellation. he said that having the subs facing each other will cancel out each others sound waves, thus voiding the need for two speakers.

that got me to think. i remember from physics class that two tuning forks tuned to the same frequency that when pinged, and the brought close together, that the amplitude will increase. and i have witnessed this first hand.

now, assuming that the two speakers that are facing each other are playing at the same frequency (which they should), wouldn't the amplitude increase rather than decrease? i guess this would go for any multiple speaker configuration in close proximity.

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[Immacomputer 10+ year member]

Having them on opposite sides is not going to cancel all your output as they are both compressing and decompressing the air in the cabin and thus make sound. You won't have the acoustic coupling effects that you described with the tuning forks though so output would be lower versus having them right next to each other.

 

[ameuba10 10+ year member]

so how would having them face each other result in a lower amplitude rather than side by side?

 

[IDSkoT 5,000+ posts]

I know that the picture is for intro-to-Quantum Mechanics... but it proves your friend's "theory" nul anyway. There will be sound cancellation. However, there's positive and negative cancellations, that is to say, that at some points in said wave, it will be weaker, but at other points, where they diverge, it will be stronger. However, since this happens instantly, assuming that your speaker is working properly, you won't notice it. Now, that isn't to say that you won't get a lower dB level. But, it all depends on the environment, and all of that jargon.

But, simply saying, "D00D! IF U PUT TEH SUBZ FACIN ITCH OTHAR, THEY WILL, LIEK, NAWT BE LOWD!" is incorrect.

 

[ciaonzo 5,000+ posts]

That's a visual depiction of the effects of diffraction.

 

[IDSkoT 5,000+ posts]

That's a visual depiction of the effects of diffraction.

Oh, Geez! You remember that from High School! Congratulations! Would you like a prize?

 

[ciaonzo 5,000+ posts]

Oh, Geez! You remember that from High School! Congratulations! Would you like a prize?

My point was to indicate it's irrelevance to the topic.

Maybe I should have started spewing profanities, intertwined with sarcasm and stupidity so you would've understood it more clearly.

 

[IDSkoT 5,000+ posts]

My point was to indicate it's irrelevance to the topic.
Maybe I should have started spewing profanities, intertwined with sarcasm and stupidity so you would've understood it more clearly.

Irrelevance? I would love to know how that's irrelevance. It shows, visually, positive cancellations... very linearly, albeit, but I'm not gunna give a physics lesson on a forum.

EDIT:: Although I should've stated that that's not EXACTLY what happens. It was just a visual guide.

 

[ciaonzo 5,000+ posts]

Irrelevance? I would love to know how that's irrelevance. It shows, visually, positive cancellations... very linearly, albeit, but I'm not gunna give a physics lesson on a forum.
EDIT:: Although I should've stated that that's not EXACTLY what happens. It was just a visual guide.

OP is asking why two subwoofers might not combine in a constructive manner, not how one subwoofer's sound waves would combine once they traveled through a hole in a wall. Not the same thing at all.

To find whether or not any two drivers will combine in a constructive manner is as easy as taking a measurement between the acoustic centers of the two drivers and finding out the frequency at which that distance represents a half wavelength of that frequency. As long as you stay below that frequency, they will combine and play as a single source. If you go above that frequency, they will begin to interfere and go into comb filtering effects.

 

[ameuba10 10+ year member]

great info there IDSkot. so would facing speakers be the worst possible configuration? i believe that way would have the most possible sound wave interference not counting bouncing around in the trunk and whatnot. so would side by side speakers facing outward at a 45 degree angle each have the least amount of sound wave interference.

 

[IDSkoT 5,000+ posts]

OP is asking why two subwoofers might not combine in a constructive manner, not how one subwoofer's sound waves would combine once they traveled through a hole in a wall. Not the same thing at all.
To find whether or not any two drivers will combine in a constructive manner is as easy as taking a measurement between the acoustic centers of the two drivers and finding out the frequency at which that distance represents a half wavelength of that frequency. As long as the you stay below that frequency, they will combine and play as a single source. If you go above that frequency, they will begin to interfere and go into comb filtering effects.

The speakers are producing the same frequency at the same time (If everything is as it should.) Thus, it's creating the same wave, and while that doesn't show two waves intersecting... I don't feel like finding an image of longitudinal waves that collide with each other. That diagram will be good enough for this application.

great info there IDSkot. so would facing speakers be the worst possible configuration? i believe that way would have the most possible sound wave interference not counting bouncing around in the trunk and whatnot. so would side by side speakers facing outward at a 45 degree angle each have the least amount of sound wave interference.

Honestly... I can't say. It depends. In some applications you might find it will be beneficial. The only way to tell is to try it. Like most hobbies, Car Audio takes a lot of time, and a lot of trying (and money, of course) to figure out what's the best.

Just trial and error.

 

[ciaonzo 5,000+ posts]

The speakers are producing the same frequency at the same time (If everything is as it should.) Thus, it's creating the same wave, and while that doesn't show two waves intersecting... I don't feel like finding an image of longitudinal waves that collide with each other. That diagram will be good enough for this application.

I think I better understand what you were going for now. I just felt that the visual was not an accurate depiction of constructive combination since the black dot on the left represents the driver and the sound waves are traveling through two obstructions, the first with one outlet, and the second with two outlets. What it's showing is destructive interference of the combined sound waves as the bar on the far right shows a comb filter wave front.

 

[jon_k 10+ year member]

a friend of mine told me not to do it because of sound cancellation. he said that having the subs facing each other will cancel out each others sound waves, thus voiding the need for two speakers.

This is a friend who is just repeating bits and pieces of what he's heard from other people.

Certain frequencies will cancel out, while others will be increased in amplitude. Cancellation happens when two opposing waves hit each other at the same wavelength in the same phase, thus attenuating themselves. The factors include frequency output and distance between the drivers; which effect the exact frequency which will feel the most damage. I assume frequencies around that deadpoint will be attenuated at so many db's per octave.

All this means is you'll have a dead spot in a certain frequency range with a bell curve in both directions.

that got me to think. i remember from physics class that two tuning forks tuned to the same frequency that when pinged, and the brought close together, that the amplitude will increase. and i have witnessed this first hand.

This is because in a tuning fork, the waves extrude in the same direction at the part where amplitude is generated causing an audio couple, where the waves couple with each other and intensify. Tuning forks emit waves in all directions unilaterally, while a subwoofer emits from only two directions (front and back of driver; but in one direction if the driver is sealed) Since you'd have two subs emitting in opposing fashion from a linear source, the waves would hit and disperse causing attenuation.

 

[IDSkoT 5,000+ posts]

I think I better understand what you were going for now. I just felt that the visual was not an accurate depiction of constructive combination since the black dot on the left represents the driver and the sound waves are traveling through two obstructions, the first with one outlet, and the second with two outlets. What it's showing is destructive interference of the combined sound waves as the bar on the far right shows a comb filter wave front.

Well, it was only meant to be viewed in terms of the right side. And, even still, it's not showing exactly what's happening. But it's a decent representation.