BillFitzmaurice.info

Hey I was wondering if anyone has written a "Phase response RTA" piece of software. I was just thinking -> most people that "don't like how a horn sounds" are either talking about delay or phase. What if you could use a microphone to measure the phase and time delay responses and invert them so that a horn (i.e. T60) exhibits linear phase response and compensated delay. Then you would get efficiency + accurate sounding (couldn't think of right word) bass. If there is a paper you could point me to I'd appreciate that as well as comments. I imagine if you want to mess with time delay it would have to connect to your computer or source (Easier if this was simply a computer plugin). Every horn is different - just like every room is different.

Computing power these days is much better then in the past so it must be doable right? I mean heck we have Room Equalizer Wizard for free, what if they came out with this other thing as an add-on.

Is this what Audyssey XT and other Bass management softwares do?

Any good speaker measuring software measures phase. Tools like Audyssey measure time delay, which is related to but not the same as phase. 'Correcting' phase response is unnecessary, because you can't hear phase. Phase is only an issue when you have two sources playing the same signal, which generally means a sub and top at very close to the crossover frequency. If they're 180 degrees out of phase there would be cancellation, but you can fix that with a simple polarity flip of one of the elements. What does matter is time align, if there is sufficient difference in time arrival between the two sources to be audible.

Awesome, good to know. I have never heard a sealed subwoofer before but I've always been told they sound better subjectively due to the time alignment - this is what I would like to achieve. Unfortunately the DSP teachers here at my school are notoriously the worst professors in the department, however this isn't to say I couldn't look into it for senior project... I know a friend who is eager to do DSP work so maybe I could design the circuit/layout (I/O). Thanks for the quick feedback. If I ever produce one of these things I'll let you know.

MissileCrisis wrote:I have never heard a sealed subwoofer before but I've always been told they sound better subjectively due to the time alignment -

No one who is qualified to give you advice would ever make that suggestion.

My response to this is well I use the time alignment in my reciever and my distortion is easily 10 times lower than yours at full tilt - so mine is more accurate! (talking about T60 vs sealed)

I've recently listened to my ported 15 subwoofer (not bad but different ballpark) and I was quickly reminded of why passive > powered, too much 110-200 from distortion/not filtered out. My roomate mentioned how much cleaner it sounded when he pulled down 50-20 at a progressive slope of 6 dB per octave on the eq and my response was - no absorption in a small room combined with direct radiator distortion -> less bass -> less distortion DUH. When people talk about muddy sound the first thing I think of is that it's probably room, THD, or lack of proper low-pass related.

MissileCrisis wrote:When people talk about muddy sound the first thing I think of is that it's probably room, THD, or lack of proper low-pass related.

I have yet to see an RTA of the response in question when someone complains about muddy sound. Of course, someone with knowledge and tools to take an RTA would also know how to correct any response issues, so there would be nothing to complain about.

What if you could use a microphone to measure the phase and time delay responses and invert them so that a horn (i.e. T60) exhibits linear phase response and compensated delay.

MC
Interesting idea.

You don't want phase to end up in a linear form. Phase at any given point is frequency dependant. Therefore phase is intrinsically non linear.

Other than that:
Phase (between say sub cabs and tops) can be altered by positioning. For non install type gigs, unless you only ever play in one room and can guarantee that cabs will be placed in exactly identical positions every time, phase response (as Bill has said, most critically at the crossover point) should be tested before any EQ is applied.

The reason that EQ should be applied after testing phase is that "out of phase" at the crossover point will be exhibited by a null. Applying excessive EQ to a null won't ever fix the problem, and is more likely to cause damage. Since we invariably adjust phase in the subs, with the lower the crossover point, the harder you attempt to drive the subs woofer at that frequency when adjusting EQ.
Result = potentially excessive strain on the sub driver, without changing the null effect.
In the perfect world, you should once again check phase after EQing, as EQing alters phase!

While we often simply invert phase (ie change phase by 180 degrees) if a null is created at the crossover point, since phase is frequency dependant, a 180 degree shift may not be the required amount of phase shifting to completely eliminate a null.
Since most DSP's offer 0 to 180 degrees of adjustment, the correct way to adjust is to use the slider on phase adjustment until you get the loudest response at the crossover frequency.
That may or may not end up with a setting of 180 degrees.

Obviously, time alignment (between subs and tops and when the material exits the cabs due to different path lengths) and compensatory delay, fall into a different category in regard to phase to a large extent...

Grant Bunter wrote: Since most DSP's offer 0 to 180 degrees of adjustment, the correct way to adjust is to use the slider on phase adjustment until you get the loudest response at the crossover frequency.
That may or may not end up with a setting of 180 degrees.

This is what I did when I was testing a T60 and T39 combo, it was interesting to see the results of what a few degrees of phase adjustment did to the SPL. I just sat there with the system on a reasonable volume and clicked through noting the changes in volume through the RTA.

Obviously this would be room dependent but what phase/delay settings worked best for your T39/60 endeavors?

Honestly can't remember, it was an experiment at the back end of last summer. Sorry. I'll probably play with it again though so when I do I'll try and remember to update you.