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 Multi-Way Conventional loudspeakers with crossovers

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 23rd January 2003, 06:15 PM #21 Bose(o)   diyAudio Member   Join Date: Dec 2001 Location: Ottawa, Ontario, Canada Barber pole is a way out...slight change in the direction. Saw bose 802s on the same pole and was wondering why they all we were at a slight change of angle. This would be the reason. Just a sec here, trying to figure out the phase shift. If for example, you take two sine waves and change the phase shift for one wave by pi/2 (90 degrees), do the two waves add-up? Or, will they require a change of pi (180 degrees). The sine wave represents the sound from one Fostex. I believe that the poor off-axis response is due to the wedge dispersion and lobing effect from using a line-array. This is good however, for use when you want to minimize reflections for PA use in a room with relatively poor acoustics. You get the same power of a larger driver without the reverberations.
kelticwizard
Wizard of Kelts
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Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Quote:
 Originally posted by Bose(o) [ Just a sec here, trying to figure out the phase shift. If for example, you take two sine waves and change the phase shift for one wave by pi/2 (90 degrees), do the two waves add-up? Or, will they require a change of pi (180 degrees). The sine wave represents the sound from one Fostex.
Two sine waves of the same frequency will add together. I believe, but am not sure, that if you add two sine waves together of equal frequency but 90 deg phase difference, that you get a result that is half the amplitude of what it would be if they were in phase together.

If they are 180 degrees out of phase, they cancel out completely.

I think I mentioned once, about anti-noise. Certain airplanes have a system where the noise of the engines is picked up by mikes, sent through an amp, reversed in phase, and played in the passenger compartment. Although there are some time delays, the phase is close enough to 180 degrees that the sound of the jet engines is substantially reduced. Anti-noise = sound waves 180 degrees out of phase.

planet10
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Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Quote:
 Originally posted by kelticwizard I think I mentioned once, about anti-noise. Certain airplanes have a system where the noise of the engines is picked up by mikes, sent through an amp, reversed in phase, and played in the passenger compartment. Although there are some time delays, the phase is close enough to 180 degrees that the sound of the jet engines is substantially reduced. Anti-noise = sound waves 180 degrees out of phase.
Panasonic uses this noise-cancellation technique in their high-end video projector. You can actually be in the same room as the projector!!

It is interesting that the 1st publication of this idea was actually intended to be an April Fool's Joke

dave
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 24th January 2003, 09:24 AM #24 Pan   diyAudio Member   Join Date: Sep 2002 Two drivers in paralell will give 6dB+ for the same voltage in. Two drivers in paralell with a 90degree phase distance will give 3dB+. And obviously at 180degree they cancell out. At 270degree (the case with 3rd order butterworth) the situation is virtually the same as with 90degrees, so the sum is 3dB+. /Peter
peranders
Electrons are yellow and more is better!
diyAudio Member

Join Date: Apr 2002
Location: Göteborg, Sweden
Quote:
 Originally posted by Pan Two drivers in paralell will give 6dB+ for the same voltage in.
Are you sure about that? Double power = 3 dB increase in sound pressure. I think you mixed up voltage and power.
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Join Date: Oct 2001
Location: Queensland, Australia
Quote:
 Originally posted by peranders Are you sure about that? Double power = 3 dB increase in sound pressure. I think you mixed up voltage and power.
We've covered this countless times before. Where the distance between the 2 drivers is small relative to the wavelength, you get a 3dB increase for the same total power input. So for 2 drivers in parallel at lowish frequencies, for a given voltage input, you get 6dB increase in on-axis output relative to a single driver.

 24th January 2003, 10:00 AM #27 Pan   diyAudio Member   Join Date: Sep 2002 Two drivers in paralell doubles the power drawn from the amp. That gives 3dB. An extra magnet with its VC and BL factor gives another 3dB for a net of 6dB. /Peter
 24th January 2003, 01:10 PM #28 Bose(o)   diyAudio Member   Join Date: Dec 2001 Location: Ottawa, Ontario, Canada Great to be back in the special triangles of things eh? Thanks a lot guys I understand now...Unfortunately with noise-cancellation you get a white noise that is evident in almost any noise cancellation (headphones for example). And that, nois-cancellation is always a bit behind in time. So, by reversing the polarity on the tweeter you get a +3dB. increase where the waves are identical because reverse polarity=pi/2 (90 degrees). Thank you! Sorry about the somewhat off topic part of things.
kelticwizard
Wizard of Kelts
diyAudio Moderator Emeritus

Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Quote:
 Originally posted by Bose(o) Unfortunately with noise-cancellation you get a white noise that is evident in almost any noise cancellation (headphones for example). And that, nois-cancellation is always a bit behind in time.
I did not know that, since I never had occasion to experience "anti-noise". Still, I would guess that the residual white noise is much lower in amplitude than the sound of jet engines. I believe that it actually is used on some aircraft-not certain.

Yes, noise cancellation is always a bit behind in time, therefore it is not 180 degrees cancellation. But even 170 degrees out-of-phase goes a long, long way towards cancellation.

Quote:
 Originally posted by Bose(o) So, by reversing the polarity on the tweeter you get a +3dB. increase where the waves are identical because reverse polarity=pi/2 (90 degrees).
If you reverse the polarity of a tweeter, or of anything, you have just done a 180 degree change, not 90 degree.

What you might have in mind is the reversal of a tweeter in a 12 dB symmetrical crossover. But there, at the crossover point, the drivers are already 180 degrees out-of-phase. The crossover components make them 180 degrees out-of-phase. If you left them like that, cancellation would occur at the crossover frequency and you end up with this large hole there.

So when you reverse one driver, (usually the tweeter, but it could be the woofer), you reverse the phase effects of the crossover, and the woofer and tweeter are now back in phase.

kelticwizard
Wizard of Kelts
diyAudio Moderator Emeritus

Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Quote:
 Originally posted by planet10 It is interesting that the 1st publication of this idea was actually intended to be an April Fool's Joke
Details, please. I would really like to find out that story.

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