Gary, can we get a good estimate of group delay in a sealed subwoofer in a limited frequency range, say 20 - 80 Hz?
Lee
Lee
Hey Gary, in any event, no where near as much research as been done as is needed to understand reliably the pscychoacoustic side! Here is hoping for a Floyd Toole 2 to magically appear and get funding to do lots of in depth listening experiments to understand better what is happening.
Link to pages for anyone interested http://www.physics.sc.edu/~kunchur/Acoustics-papers.htm
Gary,
As I have no experience setting subs I do it the "logical" way - I take a sinusoidal signal generator, set it at the frequency of the crossover and visualize it at channel1 of the oscilloscope, using it as trigger. Using a microphone at a fixed position I measure the time delay of my main speakers at this frequency versus channel1, switch it off, and then set the sub-woofer at exactly the same time delay, so that the two curves at channel2 overlap. Anything wrong with this technique?
Gary, can we get a good estimate of group delay in a sealed subwoofer in a limited frequency range, say 20 - 80 Hz?
Lee
Subtract 7ft and that is where the subwoofer needs to be for perfect phase/time alignment with the loudspeaker at any frequency.
Nothing - it's perfect if you have the instruments and the ability at the subwoofer to control the delay fine enough. (...)
Am I subtracting 7 ft from the distance of the sub to the microphone or the sub to the speaker? Am I using 7 ft because that is half the wavelength of 80 Hz? If so, then if using a different frequency (i.e. 100 Hz) the distances used in calculation will be different, too.
It sounds like it would be easier to use the RTA function of Room Equalization Wizard while playing an 80Hz tone and adjust the distance setting during playback to find the lowest SPL. Not everyone can adjust their distance setting, but it is easy enough when using a PC as the source.
Subwoofer integration into a 2-channel system, or home theater system is hugely complex. IMHO, it cannot be simply distilled into "My subwoofer has a group delay of 9ms" - sorry, Mr Ober. Hopefully you'll jump in here - but he may just have tried to over-simplify things for Lee. Mark Seaton might also want to leap in too!!
Gary,
As I have no experience setting subs I do it the "logical" way - I take a sinusoidal signal generator, set it at the frequency of the crossover and visualize it at channel1 of the oscilloscope, using it as trigger. Using a microphone at a fixed position I measure the time delay of my main speakers at this frequency versus channel1, switch it off, and then set the sub-woofer at exactly the same time delay, so that the two curves at channel2 overlap. Anything wrong with this technique?
Now for the last step. The first step is to find the relative group delay between the loudspeaker and the subwoofer. As I mentioned in the last post, there are several factors that are out of our control. If we have the tools to measure, it becomes pretty easy, but let's say we have a Radio Shack spl meter, and hopefully a test disc with test tones or a signal generator like:
http://www.performanceaudio.com/cgi/product_view.cgi?products_id=4115
Pick a frequency with a convenient wavelength - this will be a wavelength that is short enough to move your subwoofer around in your room. I usually pick 80Hz so that the wavelength is 14ft. It doesn't have to be absolutely precise unless you really want to know the numbers.
First, position your spl meter 10.5ft or 17.5ft (3/4 wavelength or 1 1/4 wavelength) from the main loudspeaker. Play the signal to get about 70dB. You don't want to play too high a level to keep room interactions down.
Place the subwoofer right in front of the loudspeaker. Play the signal with the crossover set at the frequency of the signal and adjust the gain to again get about 70dB.
Now, play the signal through both the sub and the speaker and move the sub towards the spl meter until the sound level is a minimum. It won't completely cancel because of room interactions. Also, because the sub will be very much nearer the meter than the speaker, it should measure far louder once the speaker is turned off.
Subtract 7ft and that is where the subwoofer needs to be for perfect phase/time alignment with the loudspeaker at any frequency. Now, inscribe an arc with diameter between the listening seat and the subwoofer - you can put the subwoofer anywhere on that arc and still the "correct" alignment. Use this arc placement to find the place in the room where the subwoofer sounds best.
The difference in the group delay between the sub and the speaker at 80Hz in seconds can be calculated as distance between the speaker and sub divided by 1126. The inherent group delay caused by a 80Hz 2nd order low pass filter is 3.125ms - which translates to about 3.5ft. Any difference in the distance can be attributed to residual group delay caused by something outside the low pass filter.
If you don't want the sub at that spot so far out into the room and it's a home theater, you can then use the relative group delay in feet to do what Lee suggested in the OP. This process should work with all subwoofers.
In short, room interaction with a subwoofer dominates the measurements, and group delay/phase is directly related to most changes in frequency response. As such, you want/need to measure the subwoofer in the chosen location, as the room's effect on the frequency response can change the group delay as much as the absolute distance.
Thanks, Mark. Glad you had a chance to enjoy a vacation.
I was answering the OP where JL had stated to Lee that all subwoofers had a 9ms group delay, and hence use 10 feet as the offset. I attempted to give a more generalized solution with simple instruments.
I know that there are better ways to integrate using proper software and instruments, but this is not my area of expertise, and I'll defer to the real experts
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