question about cross correlation alignment

[darklight]

So I finally understand that cross correlation alignment uses the reference peak to align the different measurements. What I don't understand is why are the reference peaks in some measurements so far away from the beginning of the waveform and in others it is almost at the beginning.
Can someone explain this? Also does this affect the usefulness of the cross correlation alignment?

 

[darklight]

Anyone has idea?

 

[John Mulcahy]

You'd need to attach an mdat file so we have some idea what you are describing.

 

[darklight]

You'd need to attach an mdat file so we have some idea what you are describing.

like in this exaaple that I attached "test measurement" SL 2 has the impulses almost right at the beginning while SL 0 and SL 1 have their respective impulses further into their waveform. When I do a cross correlation alignment of the 3 measurements the beginning of the waveforms dont match up like I've seeing in others examples. In the auydyssey test which are measumens that were converted by a script provided by OCA the differences are even bigger. My questions why does this happens and does this affects the result of the of cross correlation alignment?

 

[John Mulcahy]

SL 2 has the impulses almost right at the beginning while SL 0 and SL 1 have their respective impulses further into their waveform

At the beginning of what? The time which you have chosen for the left side of the graph? The relative times of the impulse responses reflect their timing compared to the reference channel, the left speaker. If they arrive before that they are in negative time, if they arrive after they are in positive time.

Your front L and R measurements have lots of very large reflections, larger than the direct signal from the speaker, and long decay times. Those big reflections are a significant problem, the room seems to be very reflective. It will be very difficult to get meaningful results until the reflections are tamed.

 

[darklight]

At the beginning of what? The time which you have chosen for the left side of the graph? The relative times of the impulse responses reflect their timing compared to the reference channel, the left speaker. If they arrive before that they are in negative time, if they arrive after they are in positive time.

Your front L and R measurements have lots of very large reflections, larger than the direct signal from the speaker, and long decay times. Those big reflections are a significant problem, the room seems to be very reflective. It will be very difficult to get meaningful results until the reflections are tamed.

I am sorry if I am not explaining myself very well. I am very new to REW and all of this. What I meant is why does the measurement on the first picture have the reference impulse response almost at the beginning of the waveform while the one in the second picture the reference impulse is much further away from the beginning of the waveform thus when cross correlating both waveform their start dont line up.

Do you think the very large reflections are due to the speakers being bipolar design or just the room being very reflective?

 

[John Mulcahy]

The reference time for the IR window is the peak of the IR, the later reference time is because there's a large reflection there. It is rare for reflections to be larger than the direct sound from the speaker. One cause can be curved surfaces focussing reflections, another is an environment so reflective that the reflections sum to be greater than the direct sound. The large reflections will cause issues with cross correlation alignment and time delay estimates.

 

[darklight]

Thank you for taking the time explain all this to me. I really appreciate it.

 

[moedra]

Yes, your room is extremely reflective. You'll need to put up some treatment before you can get useful results.

 

[darklight]

Yes, your room is extremely reflective. You'll need to put up some treatment before you can get useful results.

It's a concrete wall room I guess that's why it's so reflective? Also my speakers are Definitive Technology Bipolar speakers with drives in front and back maybe that also has something to do with these reflections?

 

[moedra]

It's a concrete wall room I guess that's why it's so reflective? Also my speakers are Definitive Technology Bipolar speakers with drives in front and back maybe that also has something to do with these reflections?

It's very likely that the reason is bare walls. You'd need to put up some acoustic treatment to absorb those reflections.