Speaker Cancellation

[Roffensis]

Here's a nice one for you all!!!!!

I have four Celestion Ditton 44 speakers tht I have restored, ie checked for airtightness (which they were not but now are) and replaced caps, new wire, all very nice blah blah blah. Because of the position of doors in my room, I have the rears to my sides 9 feet apart, facing each other square on and but one metre forward of my head postion on the couch. The fronts are not set as wide apart, some 6 feet part and 9 form my head. This gives pretty good seperation all round, and there are no "holes". That said, something interesting happens. Very low bass on organ cancels out, but I invert my decoder (SQD 2010) which reverses the phase to the fronts, the bass is there and how, and the rears suddenly come on a load louder too. I assume this must be cancellation caused by time differences and angles?

Or........

Amplifier inverted phase at either imput or output?

Any takers?


Thanks,

Richard

 

[mtgc]

Richard,

Another possibility is that you have one or more of your speakers out of phase with the other speakers, ie, the positive and negative speaker leads going to your speakers are not all the same. Here is what I do to check on speaker phasing. I take a standard D cell flashlight battery, connect a couple of wires to the battery and then touch the wires to the speaker leads. Whichever terminal that I have the positive battery lead attached to that makes the speaker cone move out, ie away from the speaker magnet, I call this the positive speaker lead. After testing all the speakers it is then an easy matter to connect the positive and negative speaker leads from the amplifier/receiver to the proper speaker terminals.

Sorry to get so windy, but I thought that mis-phasing might be a part of your problem.


MTGC

 

[Doug G.]

Actually, Richard you are right on track with the cancellation idea.

When speakers propagate low frequencies, the waveforms are very long and bounce off walls and every other surface in the room.
Since you have all these waves bouncing around, different ones, some directly from the speaker cones and some from room surfaces, will meet all over the place and in differing phase relationships from meeting in phase with each other to meeting 180 degrees out of phase with each other and everything in between.

When they meet 180 degrees out of phase, they cancel and if you were sitting or standing in the exact place where this occurs, you would hear nothing at that frequency.

Since there are so many reverberations, there are many different situations and it would be rare for you to experience total cancellation but it does vary widely all over the room. That is why bass response can change dramatically just by moving a foot or, sometimes, even inches. You move into a different phase relationship between different waves.

Quadraphonics makes this even more complex because there are four sound sources. When I first set up my quad system in this room, I thought the bass response lacking and so I tried reversing the polarity of the back speakers. Viola! It could be very likely that if the two sets of speaker systems were farther apart or closer together, I would have to change it back.

Mike is correct, of course, that speaker phasing is very important and you do want to make sure of the basic polarities of the speakers from the start before experimenting with changing anything to improve bass response.

And Mike thought HE was windy. \:^)

Doug

 

[Roffensis]

Hi to both of you for your helpful comments. I tried the battery idea on all four speakers and yep, all in phase, cones move outward. I used a 500 volt battery!!! HA!!! ....not!!!

Meanwhile, the cancellation thing you also experiences Doug, well yes, i have thought of reversing the polarity of ther rear drivers only, but this of course will mean the cones are then sucked in rather than out for transients. Will this do any harm before I go ahead and try it.

Also, where the crossover pointrs meet for midrange and bass, there will be a cancellation there also, but does that happen when bass and mid in the same speaker are in phase with other or out, like when I reverse polarity on the bass drivers only? I dont want to destroy imaging by just reversing the polarity of the rear speakers as a whole!!! The decoder has a phase reverse switch on ther back, this works on the fronts only, and the imaging does suffer somewhat doing it that way, where both amps are in phase!!!!!!!

What a carry on!!!

Thanks

Richard

 

[Doug G.]

It won't hurt anything to change wiring to the speakers around. It will just affect how they sound.

Phase interaction happens at all frequencies. It's just that at higher frequencies, the waves are so short that the relationships become much more complex because there are so many more wave fronts moving around and bouncing off things.

The imaging actually improved in my system when I changed the back speaker polarity.

But every room is different.

Also, it wouldn't matter whether you changed the back or front around when experimenting. The results would be the same.

I guess the reason I never liked the idea of the two back speakers being on the sides is because of exactly what's happening to you. Certain low frequency waves directly from each speaker are meeting in the middle out of phase and cancelling.

I guess you could try moving them around just a bit to see if you could reach a compromise.

Doug