Strange sub phasing problem

[HighFidelityGuy]

Hello all,

A few weeks ago I purchased a pair a subs to use with my surround system. I got two as I've got a fairly big room, I read it was easier to integrate two subs than one and they were on special offer.

I decided that as I'd got two subs, I may as well try out integrating them into my system so that I can use them for stereo and surround duties to give me maximum flexibility. So I setup my AV amp so that it thought it didn't have a sub. This made it send the sub channel to the front L + R channels. I then got a couple of little adapters that allow you to connect two amps to one output so that I could connect both a sub and a mono block to each of the front pre-outs on the AV amp. So I now effectively had the subs connected as a pair of stereo speakers along with my front speakers running off the same output. This setup then allowed me to selectively add or remove the subs from the surround system and the stereo system.

I then played some music and fiddled with the crossover and phase settings on the subs until I was happy that they were integrated well. I settled on a crossover frequency of 65Hz and the phase set to 0 on both subs. This seemed to work well for music and movies until I started to try out a wider range of music. I noticed that some albums sounded bass heavy and some sounded bass light. So I immediately though that I must have got the phase set wrong, so I switched both subs to 180. This sounded right again at first but when I then swapped back to the album I'd used to first setup the subs, it sounded really bass light. I've done quite a bit of experimenting now and it seems that some albums need 0 degrees phase and some need 180. I really don't understand this as I would have thought the subs were either in phase or out of phase all the time depending on the setting but for some reason the phase relationship between the subs and speakers seems to change depending on the music I play.

I've also recently purchased a new pre-amp to finally separate my AV and stereo systems and I get the same problems with this and this has two separate lots of outputs designed for using in the type of setup I have. So I know it's not the AV amp or the double adapters that are at fault.

Does anyone know why I'm experiencing this problem or know of any ways to solve it?

I know I don't really need to use the subs and speakers at the same time when playing music and probably shouldn't from a purist point of view but I'd like to get it working properly all the same as it would simplify things and give me lots of flexibility. Also I do really enjoy a good bass fix now and then.

Thanks.

EDIT: I forgot to mention; the subs are positioned directly behind the L and R speakers to get them as close as possible to their partner speaker.

[Primalsea]

I think you will have some difficulty integrating the subs to the stereo speakers. Try turning 1 sub off and get the remaining sub to integrate properly. I presume the sub has various filter roll off settings. Once you get one working turn on the other and see what goes wrong.

My first though is that there has been a mix up when the subs were made and you might need one set to 0 deg and the other to 180 deg.

[HighFidelityGuy]

Thanks Paul, I've tried setting the subs to opposite phase settings and that just sounds the same as having them both set the same when the bass sounds light. I'll try a few other combinations but if I don't make an improvement I'll just connect the subs to the surround system instead. It would be nice to get it working with both but if I can't I'll get over it.

Cheers.

[Primalsea]

Your speakers look like they go fairly sub-terrainian anyway, how low do they go? It could be the bass drivers interfering with the subs.

Just throwing a few ideas, I cant really say anything for sure as I havent been their myself.

[Peter Galbavy]

From the comments I have read on other forums from people who are knowledgeable, the phase controls on most subs only operate on a specific narrow frequency band (think about how pahse works, after all) and do not help integrate across the whole range. I would suggest leaving the phase at 0 and moving the subs - one at a time - in an effort to integrate each physically in the room.

[Primalsea]

What Peter is hinting at can be difficult to understand but is what I was referring to in regards to your speakers bass drivers and their interaction with the subs. depending on the roll off of the subs cross over network the phase will change as the frequency is increasingly attenuated , starting at just before the roll off frequency. from 0 to 90 deg for a single order (-6dB per octave) to 0 to 180 deg for 2nd order (-12db per octave) right through to 0 to 360 deg for a forth order Linkwitz Riley cross over. This change in phase will acoustically interact with not only the bass drivers of the speakers in their linear range but also with the drivers outside of this range which is effected by their cross overs and their phase changes. The result when all things are considered including room interaction and speaker and sub position all sorts will happen. This doesn't even take in to account the frequency dependant time delays of the drivers and cross overs which is yet another headache. Its a wonder speakers work at all and for a large part its only the our brains can compensate for so much it does work at all.

The up shot of it all is that speakers are designed to work well in a mythical listening room that doesn't effect them. Stick them in a normal room and it all goes to pot. Also if you are of a bi-speaker orientation you should expect problems, (a bit like your wife meeting your mistress) they won't get on.

[Themis]

I would suggest leaving the phase at 0 and moving the subs - one at a time - in an effort to integrate each physically in the room.

I agree on this.

[Yiangos]

Dave,in addition,do not forget that not all albums are recorded in-phase.In fact,some songs in albums are recorded in-phase while other out-of-phase.