Phase
09-20-2004, 08:27 AM
#1
1000 Watt CAFz'r
Thread Starter
Join Date: Oct 2003
Posts: 1,783
Just some general observations on phase.
First, ALL drivers do NOT have uniform phase response. That is, they don't reproduce all frequecies in absolute phase. What tends to happen, is that there is increasing delay in phase response as frequency increases. For example, a midrange may reproduce a 300hz tone with a 30 degree "lag" versus the input signal, and 180 degree "lag" when reproducing a 1khz signal. Think of it as a big downhill slope where the driver is further out of phase (with relation to input signal) as freqency increases.
Have I scrambled your noodle yet?
Well it's going to get worse. You see, using 2 drivers with overlapping frequency response also means they have overlapping phase response. This wouldn't be too bad if the energy recieved by the ear did not include near field reflections. Without near field reflections, what your ears would hear would be a summed phase response, with timing cues (what makes your stereo image) summed to provide a fairly stable image. This does not happen in reality. What happens, is that energy in the same freqency range from multiple sources arrives at your ears in a jumble. (out of phase) Your brain does recieve this information, but doesn't know what to do with it, so it throughs it out. This is how poorly "timed" phase response can decrease your systems ability to image. The energy is there, the timing cues have always existed, they are simply unable to be used by your brain to form an acoustic soundscape. So using a midrange and tweeter as an example, it is possible that without any passive X.O. or time alignment functions being used that the drivers are "in phase". It is even more possible that they are out of phase. You see, at 3khz for example, the midrange may be in excess of 360 degrees out of phase with the input signal. That same signal applied to the tweeter at 3 khz may result in the tweeter being just 20 degrees out of phase with the input signal. So while it is very possible to sum the 2 drivers to create a fairly flat frequency response, it is virtually impossible to have them sum completely in phase. And this degree of error is what the brain uses to judge just how "real" something sounds.
This is why RTAs lie.
Flat frequency response is often achieved by making overlaping information from different drivers sum in amplitude evenly. But...this sometimes means increasing the amplitude of a driver that is the greatest degree out of phase with the input signal. And in this case...it sounds bad!
Enough of the theory, I've got some real world stuff to share.
Using my now controversial PXA-H900 to time align my mids and tweeters to my seating possition, I found the following:
I first set up the drivers full range (including the tweeters) to allow me to tune the center image. Starting with the mids, I increased the delay of the closer midrange driver to center voice perfectly over my dash. I did the same with the tweeters. I then summed them together.
The image was not in the same place!
I'll leave you to ponder this one.
Adam
First, ALL drivers do NOT have uniform phase response. That is, they don't reproduce all frequecies in absolute phase. What tends to happen, is that there is increasing delay in phase response as frequency increases. For example, a midrange may reproduce a 300hz tone with a 30 degree "lag" versus the input signal, and 180 degree "lag" when reproducing a 1khz signal. Think of it as a big downhill slope where the driver is further out of phase (with relation to input signal) as freqency increases.
Have I scrambled your noodle yet?
Well it's going to get worse. You see, using 2 drivers with overlapping frequency response also means they have overlapping phase response. This wouldn't be too bad if the energy recieved by the ear did not include near field reflections. Without near field reflections, what your ears would hear would be a summed phase response, with timing cues (what makes your stereo image) summed to provide a fairly stable image. This does not happen in reality. What happens, is that energy in the same freqency range from multiple sources arrives at your ears in a jumble. (out of phase) Your brain does recieve this information, but doesn't know what to do with it, so it throughs it out. This is how poorly "timed" phase response can decrease your systems ability to image. The energy is there, the timing cues have always existed, they are simply unable to be used by your brain to form an acoustic soundscape. So using a midrange and tweeter as an example, it is possible that without any passive X.O. or time alignment functions being used that the drivers are "in phase". It is even more possible that they are out of phase. You see, at 3khz for example, the midrange may be in excess of 360 degrees out of phase with the input signal. That same signal applied to the tweeter at 3 khz may result in the tweeter being just 20 degrees out of phase with the input signal. So while it is very possible to sum the 2 drivers to create a fairly flat frequency response, it is virtually impossible to have them sum completely in phase. And this degree of error is what the brain uses to judge just how "real" something sounds.
This is why RTAs lie.
Flat frequency response is often achieved by making overlaping information from different drivers sum in amplitude evenly. But...this sometimes means increasing the amplitude of a driver that is the greatest degree out of phase with the input signal. And in this case...it sounds bad!
Enough of the theory, I've got some real world stuff to share.
Using my now controversial PXA-H900 to time align my mids and tweeters to my seating possition, I found the following:
I first set up the drivers full range (including the tweeters) to allow me to tune the center image. Starting with the mids, I increased the delay of the closer midrange driver to center voice perfectly over my dash. I did the same with the tweeters. I then summed them together.
The image was not in the same place!
I'll leave you to ponder this one.
Adam
[img]tongue.gif[/img]
09-20-2004, 09:24 AM
#4
1000 Watt CAFz'r
Join Date: Sep 2001
Posts: 1,120
How do you have the intesity set up...what about the on-axis angling of all drivers.
Can you localize any of the drivers when playing all four at once? Are two more dominant than other two?
Just thinking if two drivers most on axis dominate, those two drivers might be one tweet and the other a mid...in which case the natural response of the tweet is different from the mid, so if a tweet and mid combine because of intensity and on axis angling to become the predominant sources of sound that your brain perceives first or as dominant, the image would shift due to the natural frequency response of a tweet & mid being different....they would no longer combine perfectly like a pair of matched drivers would.
Can you localize any of the drivers when playing all four at once? Are two more dominant than other two?
Just thinking if two drivers most on axis dominate, those two drivers might be one tweet and the other a mid...in which case the natural response of the tweet is different from the mid, so if a tweet and mid combine because of intensity and on axis angling to become the predominant sources of sound that your brain perceives first or as dominant, the image would shift due to the natural frequency response of a tweet & mid being different....they would no longer combine perfectly like a pair of matched drivers would.
09-20-2004, 01:57 PM
#5
Guest
Posts: n/a
Originally posted by defro:
you had better redo your install 138 times to eliminate all phase issues before you try to use the 900 assist you or you are bieng a bad person [img]tongue.gif[/img]
you had better redo your install 138 times to eliminate all phase issues before you try to use the 900 assist you or you are bieng a bad person [img]tongue.gif[/img]
anyway..
Does the 900 adjust phase too? I thought it 'just' did time delay [img]graemlins/dunno.gif[/img]
09-20-2004, 04:46 PM
#6
Guest
Posts: n/a
no , i just think redoing your speaker placement as many times as you want will create and eliminate problems at the same time, like phase issues, and frequncy response issues and so on. at some point the compromise will have to be made and some form or processing will be necessary in a car to compensate the limitations of even the best install. dukk, you mean to tell me that not only had you never used or heard a h900 but that you didnt even know the capabilities of the it and were so outspoken about its value, im amazed you cant hear my laughter from wherever you are.
09-20-2004, 07:33 PM
#8
1000 Watt CAFz'r
Thread Starter
Join Date: Oct 2003
Posts: 1,783
To answer the question:
You can adjust delay in 0.1 ms increments. Right next to this adjustment,(on the PC control software interface) is a button that allows you to invert the output 180 degrees. It's a neat feature, I've used it a few times.
Unlike some other processors I've seen on the market, every output on the H900 has T/A.
To further illustrate a point:
Tonight I spent a few hours listening to music in my car with a friend. At the beginning, I had him set up the car with the H900 from scratch. I walked him through a simple setup process optimizing the X.O. and phase of each driver according to his ears. At the end, he had the opportunity to make more adjustments to his setup after listening to a few peices of music. He did, and was happy with the sound. Then, we loaded a preset with my settings. He was astounded at the difference.
There is no replacement for a solid understanding of sound, and how it relates to system tuning. It is an art.
Jamie E. could probably make some notible improvements to my settings as well.
Adam
You can adjust delay in 0.1 ms increments. Right next to this adjustment,(on the PC control software interface) is a button that allows you to invert the output 180 degrees. It's a neat feature, I've used it a few times.
Unlike some other processors I've seen on the market, every output on the H900 has T/A.
To further illustrate a point:
Tonight I spent a few hours listening to music in my car with a friend. At the beginning, I had him set up the car with the H900 from scratch. I walked him through a simple setup process optimizing the X.O. and phase of each driver according to his ears. At the end, he had the opportunity to make more adjustments to his setup after listening to a few peices of music. He did, and was happy with the sound. Then, we loaded a preset with my settings. He was astounded at the difference.
There is no replacement for a solid understanding of sound, and how it relates to system tuning. It is an art.
Jamie E. could probably make some notible improvements to my settings as well.
Adam
09-21-2004, 07:49 AM
#10
Guest
Posts: n/a
i have really found that the dispertion/on off axis can really affect the phase realtionship between my midrange and door tweeter in my system. by adjusting the x-over overlapping and underlapping and phase at the same time, the results have been interesting and mess with the tonality somtimes, somtimes the best setup is the one that makes little or no pratical sense at all, its nice to have thae capabilities of making the adjustments quickly and easily, kind of lets you explore so many more alignments that you normanlly wouldnt get to do without inverting phase manually and such
