# Do impulse response filters create phase and time shift



## peterselby7 (Nov 29, 2008)

I have generated the necessary measurements using REW. I am happy with the results of the exported filters as wav. file. I have measured the response after the convolver wav. file was loaded. It is much flatter and quite satisfactory.

My question is- do the filters that REW implements to change the in room response through the wav. file create a phase or time incoherence? Does this cause ringing? I am not experiencing any problems but the music doesn't seem quite as involving even though the response is much flatter. 

I read somewhere else that all filters introduce phase shift whether active or passive but these filters are digital and with the wav file it is just an imprint on the music if I understand this correctly. (using the convolved wav file) If this is a phase problem how do I correct it?:dizzy:


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## JohnM (Apr 11, 2006)

The filters are not linear phase, but then neither are the modal resonances of your room. The filters generally act in opposition to the effects of the room's modes, so the overall effect is to improve both the magnitude and phase responses (they are actually closely linked in rooms at low frequencies, one cannot be changed without affecting the other). You need some time to become familiar with the new sound of your room, and get used to a flatter response. After a few days listening with the filters in place try listening without them, you may then find the sound to seem much worse than before. If you feel the low end is a little weak after a day or two read up on "house curves".


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hi Pete,

All equalizers introduce some phase shift. In fact, it's necessary for them to work! For your edification might want to check these two articles:

http://www.ethanwiner.com/EQPhase.html

http://www.rane.com/note115.html (see Myth #6)

Regards,
Wayne


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## peterselby7 (Nov 29, 2008)

I am able to align the arrival of the output of my different drivers in my main speakers. By putting a delay on my midrange and tweeter of x milliseconds they can match up specifically in distance with the transient from my bass woofer. My concern is that by implementing corrective filters through the convolver my time coherence is compromised? The signal is output through the cd player into the convolver then into the crossover that handles the delay correction and speaker crossover functions.

So the wav file that carries the REW filter correction is seen by all drivers but not "played" by the midrange or tweeter. I would like to maintain correct phase and time coherence. Are you saying this is not possible? Maybe I don't understand this correctly?


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## peterselby7 (Nov 29, 2008)

Wayne A. Pflughaupt said:


> Hi Pete,
> 
> All equalizers introduce some phase shift. In fact, it's necessary for them to work! For your edification might want to check these two articles:
> 
> ...




That was very interesting! It seems then that by modifying phase- the frequency response changes which makes phase audible. But if you correct phase using the frequency response the phase problems can be masked or are innaudible? 

Then with a REW compensated curve is there a way to measure the corrected sound of each driver, align them by their measured impulse response, and correct using the appropriate time delays? I can set time delays at whatever I want with my output program. 

This should give me a cleaner frequency response then if I did not worry about phase at all. It seems no matter what the filters do to the phase I should be able to delay the other drivers to match up with where they are in a transient? The article also mentioned that we can hear time incoherence but only as it effects frequency response. Is this how I fix it?


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## Wayne A. Pflughaupt (Apr 13, 2006)

I think you're overly worried, Pete! As John noted, typically the result is an improvement. In fact, that's my "golden standard" for equalizing - it should deliver an improvement in sound quality, not necessarily picture-perfect response.

Regards,
Wayne


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