Is One Subwoofer Really Not Appropriate For 2-Chnl

04-18-11: Roscoeiii
Can you point us to a good explanation of how to time align subs and mains?

04-18-11: Stanwal
There is a good set of set up instructions on the Sumiko site.

The Sumiko subwoofer setup instructions can be downloaded here. According to Sumiko...

The optimal position for a REL is in one of the corners behind the main speakers...

No doubt the folks at Sumiko are knowledgeable, but I respectfully disagree with their advice. IMO, placing sub(s) BEHIND the mains may result in good *frequency* response, but it rarely results in good *transient* response.

The reason is because, by placing the sub(s) behind the mains, as Sumiko recommends, the sound from the sub(s) will arrive at the listening position several milliseconds AFTER the sound from the mains. Hence the sub(s) and the mains will not be time aligned.

Some people say that a few milliseconds of time misalignment isn't audible at low frequencies. My experience, and some scientific research, suggest otherwise.

Adjusting the sub's phase will not solve the time misalignment created by placing the sub(s) behind the mains, since the signal that needs to be delayed is NOT the signal for the sub, but rather the signal for the mains. Adjusting sub's phase will do nothing for that.

The time misalignment created by placing the sub(s) behind the mains CAN be solved by digitally delaying the mains. But judging from the systems on A'gon, very few people have that capability. So the most practical way to time align the sub(s) with the mains is...

On the z axis, place the sub(s) roughly coplanar with the mains.

By placing the sub(s) coplanar with the mains, the sound from the sub(s) and the sound from the mains will arrive at the listening position at close to the same time. Hence the sub(s) and the mains will be time aligned, resulting in better transient response. This is audible as better coherence and PRaT.

Keep in mind that subs typically introduce latency into the low-passed signal. Therefore, if you cannot digitally delay the mains, the sub position that results in optimal time alignment will often be slightly IN FRONT of the mains. This is, of course, the *opposite* of what Sumiko recommends.

Another benefit to placing the sub(s) coplanar with the mains is that, in addition to giving better transient response, it will often result in better *frequency* response, for the following reason: Coplanar sub placement minimizes the destructive interference around the crossover frequency. An explanation for why that is so can be read here.

That brings me to Roscoeiii's question about how to time align the sub(s) with the mains. The easiest method is the following…

1. Flip the polarity of either the sub or the mains (but not both).
2. Play a test tone at the crossover frequency.
3. Use an SPL meter to measure the output level.
4. Adjust the sub position (or digital delay, if you have that capability) until you MINIMIZE the SPL at the listening position.
5. When the sub is in place, flip the polarity back so that the sub and the mains are the same polarity.

In steps 1-4, you are essentially maximizing the *destructive* interference between the sub(s) and the mains. In step 5, you are restoring the sub to the correct polarity, which now maximizes the *constructive* interfere between the sub(s) and the mains.

Following this method, the sub(s) should be time aligned with the mains to within about a millisecond or two. You can further improve the time alignment, by adjusting the position of the sub(s) in 1-3 inch increments, searching for the position that results in the best coherence and PRaT. This last step has to be done by ear, so it requires some experience and patience. But it is well worth it, IMO.

You can read more discussion/debate of sub time alignment, transient response, and the limits of human temporal resolution, on another thread.

Hope that helps.

Bryon

Is this a case where one good sub is better than two less costly subs?

just a thought...

For me, I'd opt for a higher quality sub versus two less costly ones, but that's just my opinion. For example, I'm a Velodyne dealer and to have two Optimum series subs or one DD Plus series sub would be an easy choice for me. I'd go with the Velodyne DD Plus single sub in a heartbeat. The Optimum series is good, but the Digital Drive Plus is in a whole other league. I use a DD-10 Plus and, honestly, two Optimum 10 would not play as deep and the sound quality are in different leagues as you can guess by the price difference. Two Optimum 10 subs would also cost more than a single DD-10 Plus. The original poster is looking at using very high quality standmounted speakers, so in that situation, two lesser quality subs may stand out more than a higher quality sub which would match the quality of the speakers better. Once again, just my 2 cents.

Hi Bryon, great post! I have a practical question. As I mentioned above, I don't have a lot of placement options for my sub. So, my sub is aimed directly at my seating position and sits about 4-5 inches behind the mains.

In addition, I summed the channels into mono for the sub, so I don't think I can reverse polarity as you suggested, or perhaps I still don't understand how the method works. Having said that, my sub has a built in phase adjustment capability which I adjust to achieve the loudest bass when playing music that puts out a lot of bass. The thought is that I am trying to reduce phase cancellation at the crossover point.

Anything else I can do to improve the bass with my sub?? Thanks.

Bifwynne - Glad my post was helpful to you. If you can manage it, it would be worth the experiment to move the sub roughly coplanar with the mains and see if it improves the system's transient response. It will be audible as better coherence and PRaT.

The fact that you sum the channels into a mono signal for the sub doesn't affect anything in the procedure I described in my last post. If your sub has a polarity switch, use it. If it doesn't, just reverse the positive and negative leads on your speaker wire for *both* of the mains. Then the mains and the sub will have opposite polarity, assuming that your sub doesn't invert polarity.

Adjusting the sub's phase can be helpful, but not nearly as helpful, IME, as time aligning the sub. As I mentioned in my last post, if the sub is behind the mains, the signal that needs to be delayed is *the mains*. Adjusting the sub's phase will do nothing for that. If you cannot digitally delay the mains, you can time align the system by placing the sub coplanar with the mains, or possibly a little in front of them (to compensate for the potential latency - i.e. delay - introduced by the sub itself).

To sum up, if you can do it, move the sub so that it's roughly coplanar with one of the mains, then follow the procedure in my last post. That should get you pretty close to time aligned. IME, the results of sub time alignment are dramatic, especially when the sub is "fast" enough to keep up with the mains.

Bryon