Phase Control Switch

[talstarone]

In looking to purchase a new subwoofer(my first and only subwoofer for my home theater)I was wondering how important the phase is.

I see on many subwoofer amp controls a switch to change the subwoofers phase from 0 degrees to 180 degrees.Is this very important?
Should I avoid a sub without a phase switch? Or with my Yamaha RX-V665 Receiver does the YPAO automatic mic calibration take care of this?

Thanks for reading the post,and Any and All help will be Greatly Appreciated...

[VinnAY]

My sub has the switch and I've played with a few times but can't say one way or the other if it has any tangible effect, for me.

[Big Daddy]

Theoretically, the phase control on the back of the subwoofer is important. Practically, the vast majority of people leave it at zero as they cannot hear a significant difference between different phase settings. Many others have no idea what phase is all about.

The animation in the following shows two waves traveling in the same direction. The phase difference between the two waves varies with time so that we see constructive interference when maximum points are aligned (peak) and destructive interference when minimum and maximum points are aligned (null). This illustrates why it is important to adjust the subwoofer’s phase control with respect to the main speakers so that we obtain maximum output.

Animation courtesy of Dr. Dan Russell, Kettering University

If the main speakers are producing bass at the same time as the subwoofer, at some frequencies the waves will reinforce each other. At other points the bass waves will cancel each other. The solution is to allow only the subwoofer to reproduce bass by setting the front speakers to small in the receiver’s setup menu. This can yield a smoother bass response throughout the entire room.

Because modern receivers allow us to do proper bass management in a home theater room, the crossover and the phase controls on the back of the subwoofers have become pretty useless. If you use more than one subwoofer, the phase control may be of some use.

Finally, the answer to your question. If you are planning to buy only one subwoofer, buy a subwoofer that has WAF appeal (if you are married) and can give you quality bass sound with good low frequency extension. The phase control should not be your main criterion in choosing a subwoofer.

[saprano]

I always worried about getting a sub with a phase control big D. i figured i would have to deal with setting the phase right. how would one know whats the the right phase setting? and with most people using the LFE input those settings get bypassed anyway right?

[Big Daddy]

Originally Posted by saprano

I always worried about getting a sub with a phase control big D. i figured i would have to deal with setting the phase right. how would one know whats the the right phase setting? and with most people using the LFE input those settings get bypassed anyway right?

As I said in my earlier post:

If the main speakers are producing bass at the same time as the subwoofer, at some frequencies the waves will reinforce each other. At other points the bass waves will cancel each other. The solution is to allow only the subwoofer to reproduce bass by setting the front speakers to small in the receiver’s setup menu. This can yield a smoother bass response throughout the entire room.

To get the best bass response, you should set the phase of the subwoofer to deliver the highest output at the listening position. This can be achieved with the help of a test signal at the crossover frequency and an SPL meter. You should run this test several times by changing the phase of the subwoofer and measuring the bass response on the SPL meter. Select the phase option that results in the highest bass response. If you don’t have an SPL meter, you will have to trust your ears.

Most subwoofers have a switch to change their phase. If the subwoofer does not have a phase switch, you can change the polarity of the main speakers by switching the positive with the negative speaker wires (the black wire goes to the red terminal and the red wire to the black terminal). Some subwoofers have a “variable phase control”. This control can be set continuously between 0 and 180 and allows for a more precise phase control of the subwoofer.

When setting the subwoofer phase by ear, play some music (not a movie) that has a repetitive bass line. Switch the polarity several times and choose whichever setting sounds “fuller”. If you do not hear any difference, leave the phase switch at “0" or “normal”.

With most modern receivers, the phase problem has become less important as we can direct all bass sound through the subwoofer.

If you are using two subwoofers, put one in the front and the other in the rear opposite from the front sub and set the phase of the rear sub to 180. It may be usefull to experiment with different settings. Again, if you don't hear any difference, leave them at zero.

[talstarone]

A Sincere Thanks Big Daddy!

Now I can stop worrying about the phase switch and just reset the front speaker size as you stated.And actually I imagine the Polk Audio R300 speakers are probably meant to be set to small anyway,as they dont produce a great deal of bass on their own.

And I am single so No wife or kids to effect my sub choice.Just size and budgetary constraints.

[kingofgrills]

Big Daddy, does a room's characteristics influence signal cancellation at all, or is it just competing bass output from the systems speakers? I always thought room characteristics could influence this too, and adjusting the speaker phase could help alleviate some of the signal cancellation.

Also, you mentioned using an 80 Hz test signal to measure SPL and adjust phase (assuming an 80 Hz crossover point, which my system is using). Do you know where a steady 80 Hz test signal can be found?

Thanks

[Big Daddy]

Originally Posted by kingofgrills

Big Daddy, does a room's characteristics influence signal cancellation at all, or is it just competing bass output from the systems speakers? I always thought room characteristics could influence this too, and adjusting the speaker phase could help alleviate some of the signal cancellation.

Also, you mentioned using an 80 Hz test signal to measure SPL and adjust phase (assuming an 80 Hz crossover point, which my system is using). Do you know where a steady 80 Hz test signal can be found?

Thanks

Yes, a room's characteristics as well as the location of your subwoofer and your seat can have an influence on wave interaction and what you actually hear. However, for a phase cancellation to occur, you need two identical frequencies.

Check the Understanding & Using Test Tones thread. There are plenty of links to free test tones. Check bottom of Post #1 and Post #2.