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-   -   Time & phase align bas/midrange (http://www.diyaudio.com/forums/multi-way/71791-time-phase-align-bas-midrange.html)

Ultima Thule 15th January 2006 10:21 AM

Time & phase align bas/midrange
 
Hi,

I have wondered for some time how would one time and phase align a bas and midrange driver if it's crossed over at as low as 100 Hz??

The problem as I see is that it would not be physical possbile/realistic to phase/time align the drivers by recessing the midrange as I guess this means fairly big distances which would make the loudspeaker enclosure look a bit funny, or am I wrong?

And how about the electrical part, like filters and their contribution to the time aligning issue?

Also I wonder what should one strive for, which parameters are most important/will our ears perceive well/badly, I mean things like out of phase, time differences etc?

Well, there's couple of Q's as a starter for my issue.

Cheers Michael

MBK 15th January 2006 03:48 PM

Time alignment can be done with allpass delay filters if you use active crossovers. Siegfried Linkwitz uses these for the mid-bass and treble-mid transitions , in both Orion and Pluto.

Actually, I am more interested in the measurement part.

For the treble-mid x-o, by inverting the polarities of just one driver, you create a notch in the FR through cancellation at the x-o frequency, and optimizing for the deepest notch allows you to optimize the time alignment.

But when I tried the same with the mid-bass transition, I didn't get any notch - neither with correct nor with reversed polarity. So, how to measure time alignment in the bass region? Energy time curves of bursts (my software doesn't do ETC)? Burst waveform on oscilloscope (must be iffy to set up)? Burst waveform on RTA software (I get inconsistent software delays related to the trigger function of the burst mode in Audiotester, whih is what I use)?

mazurek 15th January 2006 04:12 PM

My take is that at 100 hz, the wavelength is 3.43 meters. My math says that a 10 cm offset would result in only about 10 degrees phase shift, not a huge problem.

MBK 16th January 2006 01:45 AM

Yes, based on wavelength considerations I never worried too much about offset either at 100 Hz.

But physical offset is just one element, driver behavior is another. Siegfried Linkwitz's take on it makes me reconsider this calculation - if he goes through the trouble of equalizing the delay (if memory serves it comes out to 300-900 usec for Orion and Pluto) I assume there is something to it. 300 usec at 100 Hz comes out at

At least, if feasible, one should attempt to measure the actual acoustic offset. It just intrigues me that the opposite polarity trick doesn't work, I assume because of floor bounce etc which effectively muddles the picture (no chance of windowing this out at 100 Hz).

HiFiNutNut 23rd January 2006 01:30 AM

Hi,

Yesterday I installed an 180 degree adjustable allpass filter on the woofer side of my active XO between the bass and the mid, crossed at 200Hz. I spent an hour listening to it by turning the pot 30 degree at a time and swapped polarity of the speaker wires to get a 360 degree adjustment. There were definitely some differences but they were small.

I had not understood all the maths and formulas so I simply tried the allpass filter for subwoofer application by Randy Slone printed in his book. Probably my XO point is too high that makes that cirucit of little use.

How can I get an active circuit that provides 180 degree phase shift for the woofers with a XO at 200Hz?

Regards,

MBK 23rd January 2006 02:13 AM

Hi,

swapping polarity will get you 180 degrees, not 360 degrees phase shift...

The allpass filters are for *delay* though, which is an (approximately) constant time delay below a certain cutoff frequency, rather than a constant *phase shift* (which would produce a "delay" that varies with frequency if seen in the time domain).

Allpass filters for delay adjustment come in 2 varieties: inverting and non-inverting. The inverting topology will produce delay *and* 180 degrees phase shift.

These allpass filters will only provide a delay below their cutoff frequency, and the amount of delay produced depends on that frequency. THe lower the frequency, the higher the delay. So the effect depends greatly on what allpass filter you exactly used.


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