What are the advantages/disadvantages to two powered subwoofers in home theater setups? Example 7.2 setup?

Question

I have a JBL L8400p sub and also a JBL PB12 sub (both powered) One rated at 600 watts RMS and the PB12 at 250 RMS. Should I use both or one? My room is faily small- 20' long x 12' wide. I have full range towers and center (Polk RTi12's / CSi5), commercial grade carpet and heavy drapes. Any opinion as to the best placement would be? I am using the L8400 right now in the front left corner and the PB12 is just sitting around.Will using two phase each other out?

Answer 1

Your problem with using two subs is that your going to get wave cancellation if they are not co-located (right next to each other). Stacking one on top of the other is the best idea as it makes each sub act like a single subwoofer. Basically set one up and (if you have a Radioshack meter), calibrate your system with just one. Then turn both on and keep backing the volume on the subs down until you get to the same sound level as when you calibrated. You will also need to test the "phase" on the back of the amps and go from 0 - 180 degrees until the subs are at their loudest point on a sound meter...then you know they are in phase.

Best thing to do is play a test wave at different frequencies and plot it out on a chart to find where you nulls are (where you don't get good SPL) in your frequencies so you can try to get it as "flat" of a response as possible.

If you decide to separate the subs then are most likely going to run into a problem at certain frequencies where the subs cancel each other out. So say (for example) your sub produces 85dBs all the way down to 18Hz, but right at 25Hz it drops down to 70dBs, most likely the subs are cancelling each other out right at that point. So anything in that range won't get the same effect as any of the other frequencies.

Also, it is often best to "corner load" a subwoofer where you place it in a corner of a room. This will give you the most sound from your sub, but will also tend to create a slightly less accurate sound (more "boomy"). If you use your subs for a lot of music listening, it may not be your best placement and in that case, it's best to move it more towards the center of a wall (not more into the room, just along the wall). That will give you less sound from your sub -- but will produce a better quality sound (more tight and accurate sounding) for critical music listening. The sound level can be often helped but just turning up the volume on the sub, but it's best to set it at a calibrated level to the rest of your speakers.

Radio Shack Sound meter can be your best friend with a nice Subwoofer test disc (something like Avia).

Answer 2

Since they are both 12's you are best using a stereo preout on your receiver and using L and R for signal discretely into both one L and the other R. Make sure to set the levels by ear and spl meter and make sure you crossover is set the same. Place them along the side of the fronts for best results. They are not exactly the same speaker so some adjustment is to be expected. The receiver is where most of your adjustments should be.

Some like this kind of set up. I use a sub with a discrete output for each of my front speakers and two for the rear. Some would say I have a 7.5 system but really I am sending the base to localized drivers. DTS sounds the best with this kind of set up. It can be a little complicated and requires a little signal audio post processing.

You could also set them facing out from each other and make a bi-pole or dipole sub since you can select phase on the sub. This may be the way to go either way you will need a additional LFE cable and a few splitters. Try both and see what you like.

Answer 3

Hi there.A lot of wrong information is given out about subwoofers, such as :bass isn't directional, you only need one subwoofer ,or you can put it anyware.Let's set the record strait :Bass is directional in a room-the room causes it to be directional(except the very lowest frequencies) So placement is important so that the bass blends with the rest of the music.Since the subwoofer is an extension of the bass of your two main front speakers ,you should have two to get a balanced sound.To integrate the subs. properly into your system you should place the subwoofers at the same end of the room and next to the main speakers.To make the bass less directional and blend better you need to cross it over at a low frequency .Because of their different power requirements you will have to balance them up by using the controls to adjust volume etc.but that should not be a problem.Make sure they are both in phase and you should get a smoother and clearer image.Don't put them in corners or at the back of the room where they may sound "boomy".They will not integrate properly with your stereo speakers if you do.

Wave cancellation will only happen if they are out of phase with each other.