What's This Phase Alignment Knob For?

[Dumar]

Hello,

I brought home my very first sub last night ... finally. I am now the proud owner of a Paradigm PS1000, and am really looking forward to playing around with this baby.

I've been doing a lot of reading on this excellent site about how to set up sub woofers, but am having a hard time getting my head around the concept of Phase Alignment. The PS1000 has a continuously variable 0 - 180 degree alignment knob on the amp. Can someone please help me understand what this adjustment is for and how to set it.

Thanks.

 

[gregz]

Sure Dumar,

The subwoofer augments or takes over for the low frequencies where the mains drop off, but there's usually still a little overlap between the two in the upper bass frequencies.

Although both your main speakers and the subwoofer are all getting the electrical signal at the same time, they don't physically respond at quite the same rate due to mechanical and electrical differences, and a delay in one can put them out of sinc.

When your subwoofer is out of sinc with your main speakers, it can actually cancel out the upper bass instead of adding to it - that's called phase cancellation. Without that knob, your choices for tweaking are to reverse polarity on the sub and see which way sounds closer to being right, then to move the whole sub around until you find just the right spot.

By moving the sub, you don't correct the phase mis-match with the mains, but by giving the sound wave a different starting point you can bring the arrival of the sound waves in harmony by giving one a head start. The phase adjust knob on your sub saves you all this trouble, and allows you to electrically vary delay the sub output so you can tune it just right.

From the chair you'll be listening from, you need to sit and direct someone else to turn the knob to different points while listening to either an audio setup CD or a good music CD. If you don't have an assistant, you'll be getting up, moving the knob a bit, sitting down, then getting up again because you can't judge the phase from the sub itself, you'll want to align the phase so it's right at the listening position.

Hope that helped. Happy tweaking!

 

[Dumar]

Thanks a lot for your reply, gregz.

It's really starting to come together now. Bear with me while I reiterate what you said ... writing it down helps me understand what's going on.

With all speakers set to small on my RX-V793, the frequency at which bass management kicks in (according to the spec's) is 90Hz. So I adjust the Cutoff Frequency on the sub to some value near 90 Hz, but not below or I'll be missing some of the signal. Now, because the cutoff point on the sub is above the bass management starting point on the receiver, the two signals overlap each other ... which is OK, right? However, room acoustics and the differing mechanical characteristics of the speakers may cause the two overlapping signals to arrive at my sweet spot at different times. I'm thinking this would sound like the drummer or bass player is not keeping up with the rest of the band ... or is described as "loose" bass as opposed to "tight" bass. Is this what phase cancellation sounds like? Anyway, adjusting the Phase Alignment at the sub allows me to synchronize the overlapping signals with respect to my sweet spot.

Which brings up another question: I haven't paid much attention to this before now, but have noticed my receiver has the ability to introduce time delay to some of the speakers. Assuming I can accomplish synchronization of the overlapping bass signals by having the receiver introduce delay in the sub signal, which is the preferred method?

Thanks again,
Dumar

 

[Leprkon]

time delay is generally used if your speakers are very far apart or if the sound doesn't match the action on the TV (you know, the effect you get from watching an old Kung-Fu movie). you add delay until the words match the person speaking.

to match in a sub, you definitely want to work the phase or the position of the sub. delay is not going to help much.

 

[gregz]

As Leprkon said, the "time delay" option is different than phase select. While they both deal with delay, the "time delay" offered by receivers is a much larger delay than that offered by the phase select knob.

Your re-iteration is close, with the exception that an out of phase sub won't give you a perception of an out of sinc drummer, the arrival time is still too close for that. What will happen is phase cancellation, which will be evident by the absence of bass in the overlap region, as if someone had overstuffed the drum with a bunch of rags.

Bass speakers make sound waves when they move out, but they also make sound waves by pulling in. The sound waves are ripples in the air, just like ripples in water. If you have two sources producing the same frequency, then they must be timed (or positioned) to both push on the up side of the ripple, and pull on the downside of the ripple as it passes by.

 

[Dumar]

Hope you guys don't mind if I keep flogging this one.

Thanks for clearing up the phase cancellation issue and speaker time delay.

So lets assume the overlap is a 10Hz region from 90Hz to 100Hz. Lets also assume a worse case scenario where those frequencies from the sub arrive at the listening position 180 degrees phase shifted with respect to the ones from my mains. In this case they would cancel each other out and I would not be able to here them ... there would be a "hole" in the sound. If they were less than 180 degrees out of phase, less than complete cancellation would occur and it might sound as you mentioned, gregz, like someone stuffed rags in the drum. That's why the knob is variable from 0 - 180 ... in the worse case it would need to be turned all the way up to the 180 mark to bring the two signals into sync.

It sounds like the closer one can get the cutoff frequency to the bass management frequency, the less of an issue this becomes. If they were the same, phase cancellation could not occur since there are no overlapping frequencies.

I think I need to get myself a scope ... just kidding.

Thanks once again you guys.
Dumar

 

[gregz]

You have the jist of it. If there's no overlap, they're pretty easy to match up, and don't really need the knob.

The subs in my system operate with overlap of the mains, and I have no knob since they're all passive speakers. Thus, I start by reversing the connections - essentially giving me either 0 or 180 degrees (and 180 is almost always right). Then, I move the main speakers slightly forward or backward relative to the subs to make the fine adjustment in arrival times until I have just the right sound.

As tempting as it seems to avoid overlap, give a little overlap a try and see if the sound is better or worse. A good audiophile loves to tweak, tweak, tweak! MOhahahahah!!!!!!

 

[Dumar]

I don't know if I qualify as an audiophile or not, but I do love to tweak.

Right now, however, I'm just listening (in two channel) to familiar music to get used to the sound/feel of the sub. My mains no longer have to produce below 90Hz, which relieves them of about 60Hz worth of hard work, and I still have the phase adjustment at zero with the cutoff at about 100Hz.

I have to say right off the bat that the change that has occurred in my system is nothing short of amazing. I can't get over how good my old music is sounding. Beautiful bass is filling the room without sounding boomy, and you can't tell where its coming from. My mains have taken on a whole new character ... which I really like.

I know I have more tweaking to do to get things just right, and it's awesome to think it will probably get even better than it is right now.

Gregz, have you (or any body else out there) had any experience using the Rives Audio disk. Would this be something worth getting and playing around with?

Thanks,
Dumar

 

[gregz]

I don't have the Rives, but audio test CDs are quite useful - I have some older ones I bought a few years back; one Stereophile test CD, and one from Sheffield Labs.

Any reputable test CD is probably quite good, as they all tend to be about the same - two to six well recorded music tracks, followed by test tones and sweeps.

 

[Dumar]

Thanks, gregz, I'll check them out.