Gearslutz.com - View Single Post - acoustically i have done something wrong and need help

Mulmany · 9th April 2009

When you use a sub with any system make sure your phase response at the crossover is correct. That means - 80hz is at 40 degrees phase at the LR speakers but because of the sub displacement (distance) 80hz from the sub is at 70 degrees phase. This kind of combination will cause a comb filter on up though the frequency range. The numbers I picked are random, your system will be different.

Most subs come with a phase switch or knob that you can adjust. You want to send a sinewave though your system at the crossover frequency. (Your crossover frequency is the frequency that is -6dB at the natural low roll off of the speaker) For fun take a walk around the room and listen to the changes. Now invert the phase on the sub 180 degrees and move it around until the level drops at the listening position, this will take two people to make short work of it. Once you have found the position that creates the greatest reduction in level, switch the phase back to zero. If your sub has a phase knob - invert the LR speaker phase, and adjust the knob until you have a level drop at the listening position - correct the polarity on your LR speakers and take a listen. You should have a fuller, richer bass responce, since your Sub is now truly reinforcing the LR speakers. You also want to adjust the level of the sub to create as smooth and flat transition as you can into its range.

Advanced dual channel FFT programs will allow you to do this much quicker and more accurately like SIA's Smarrt.

This is not to say that you don't need more bass trapping. Put as much as you can! It will also help to aline the sub with the LR speakers as you will reduce the ringing in the room, and let you hear when you have cancelation.

Hope this helps

PS make sure that the sub and speakers are level matched before testing the phase.

9th April 2009	#7
Mulmany Gear addict Joined: May 2008 Location: Delaware Posts: 305	Phase When you use a sub with any system make sure your phase response at the crossover is correct. That means - 80hz is at 40 degrees phase at the LR speakers but because of the sub displacement (distance) 80hz from the sub is at 70 degrees phase. This kind of combination will cause a comb filter on up though the frequency range. The numbers I picked are random, your system will be different. Most subs come with a phase switch or knob that you can adjust. You want to send a sinewave though your system at the crossover frequency. (Your crossover frequency is the frequency that is -6dB at the natural low roll off of the speaker) For fun take a walk around the room and listen to the changes. Now invert the phase on the sub 180 degrees and move it around until the level drops at the listening position, this will take two people to make short work of it. Once you have found the position that creates the greatest reduction in level, switch the phase back to zero. If your sub has a phase knob - invert the LR speaker phase, and adjust the knob until you have a level drop at the listening position - correct the polarity on your LR speakers and take a listen. You should have a fuller, richer bass responce, since your Sub is now truly reinforcing the LR speakers. You also want to adjust the level of the sub to create as smooth and flat transition as you can into its range. Advanced dual channel FFT programs will allow you to do this much quicker and more accurately like SIA's Smarrt. This is not to say that you don't need more bass trapping. Put as much as you can! It will also help to aline the sub with the LR speakers as you will reduce the ringing in the room, and let you hear when you have cancelation. Hope this helps PS make sure that the sub and speakers are level matched before testing the phase.