avia test disc question

[Buckle-meister]

...it seems either i can put the mains in phase with the sub by setting the phase knob to 0-179(i notice no difference between these two ranges)

I wouldn't worry about the fact that you may not hear a difference. I have a switch on my sub, and I cannot hear a difference between 0 and 180 degrees.

...im assuming anyways that [the subs] are out of phase by the drop in sound level and the dips in the frequency sweeps around the crossover points... is that correct?

If the subs and speakers are out of phase and happen to be cancelling to some degree, the level would drop. How do you know the dips corresspond with the crossover points in your speakers? It is possible that if you are simply hearing the frequency sweep swell sometimes at certain frequencies, those are the room's modes that are being excited.

Regards

 

[3db]

First of all Shamus, sorry for the delayed response; I had guests.



Why would you use the phase control to achieve a maximum SPL? Why not just use the volume knob on the sub itself?


In practice, true, but in principle, two speakers facing each other wired 180 degrees out of phase generating the same sound, would cancel each other at the point of collision of sound.


Regards

By varying the phase of teh sub woofer with respect to a fixed location, ie "Your chair, no body elses, the sweetspot", you are effectively changing where in the wave form passes by your ears. You may be sitting in a location where all you get is the null of the waveform, and hence not much bass, By tweaking the phase, you are actually shifting the wave form bacwards or frontwards so that its no longer a null at the listening position. You therefor increase the impact of the bass at your listenign location without making sthe sub have to work extra hard.

Theoretically you are correct about the cancellation of sound with two speakers 180 degrees apart in phase. Infinity and Bose used that in the late 80s to make their cars even quiter by playing road nosie 180% out of phase. Don't know how successful they were in that endeavour.

In reality, in HT, the sub should not be playing the same frequencies as the rest of the speakers in the ensemble so the chance of cancellation is very very slim

 

[Buckle-meister]

By varying the phase of teh sub woofer with respect to a fixed location, ie "Your chair, no body elses, the sweetspot", you are effectively changing where in the wave form passes by your ears.

I agree.

You may be sitting in a location where all you get is the null of the waveform, and hence not much bass

I disagree. A pure tone will consist of peaks and troughs, but sitting at a certain location where the trough coincides will not cause you to hear a 'null'. It is only when a frequency or frequencies from other sources sum to a peak of equal intensity at that specific location that a null would be heard from the cancelling of all the sources.

By tweaking the phase, you are actually shifting the wave form bacwards or frontwards so that its no longer a null at the listening position.

I both agree and disagree. I agree that by adjusting the phase, you are shifting the wave forms position (but not its form), but for the same reason as described above, I believe it is shifted not to place a null from the sub alone, but rather not to detrimentally combine with other sounds at the listening position.

In reality, in HT, the sub should not be playing the same frequencies as the rest of the speakers in the ensemble so the chance of cancellation is very very slim

In principle, I agree, but the rolloff of the crossover from speakers to sub is not a brick wall filter. Therefore in practice, there will be some small duplication of sound occuring.

This is reaching the limit of my present understanding of such matters. If anyone else would like to chip-in, or correct me if I am mistaken, please feel free.

Regards

 

[shamus]

thanks guys... i will keep messing with it...thanks for pointing me in the right direction.

 

[3db]

I agree.



I disagree. A pure tone will consist of peaks and troughs, but sitting at a certain location where the trough coincides will not cause you to hear a 'null'. It is only when a frequency or frequencies from other sources sum to a peak of equal intensity at that specific location that a null would be heard from the cancelling of all the sources.

You'll still here bass, agreed. Its not totally cancelled. But moving the phase to a peak be it plus or minus will cause you to here more bass. Its smiiliar to moving the sub around a room to get a better bass response. Thats more amplitude and boundary effect I know but phase also plays a factor in that due to the overly long wavelengthof bass. I've heard it 1st hand when calibrating my sub to the room.

I both agree and disagree. I agree that by adjusting the phase, you are shifting the wave forms position (but not its form), but for the same reason as described above, I believe it is shifted not to place a null from the sub alone, but rather not to detrimentally combine with other sounds at the listening position.



In principle, I agree, but the rolloff of the crossover from speakers to sub is not a brick wall filter. Therefore in practice, there will be some small duplication of sound occuring.

This is reaching the limit of my present understanding of such matters. If anyone else would like to chip-in, or correct me if I am mistaken, please feel free.

Regards

Exactly You are shifting teh waveform with respect to time or distance relative to the seated position. You are not altering the waveform shape itself. And with respect to the other frequencies, the sub isn't playing them so the effect is only going to be taking place at the roll off where the mains are dropping them and the sub is beginning to play them.

And the other thing we are both forgetting is that the signal to the sub is most likelt being provided by the LFE of the receiver which is usually a phono plug output so it will it should be in phase with respect to the othe signals going to the speaker outputs.