# how to read an impulse response?



## asylum (Nov 30, 2007)

Hello this is my 1st impulse readout at the mix position at 3 feet from speakers - i,m trying to interpret the reading?
is the high spike at 0 the sound hitting the mic? what do the other peaks and troughs mean, or is it the spike at about 3ms the 1st sound hitting the mic? all help appreciated
regards
jim


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## Anthony (Oct 5, 2006)

I don't know your room geometry, but it looks like 0 is the first incident and the second spike corresponds to a reflection roughly 3.3 ft longer path length. Since you said the mic was 3 ft from the source, I would be looking for any path that is around 6' for the reflection.

Probably floor, although it could also be back wall.

For single speaker measurements it's always best to move everything as far from boundaries as possible in order to measure as long a time without that second reflection. Once you get into that the comb filtering from out of phase room effects starts to intrude. Still an issue, but not a fundamental flaw with the speaker itself.

The trade off is that if you set your gate too narrow, you lose bass information, which is why large open spaces are best for speaker testing. None of those pesky reflections to deal with


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## brucek (Apr 11, 2006)

For impulse you want to use dBFS as the Y-Axis and not %FS.

If you're looking for determining reflections, it's better to use Energy Time plots in %FS.

brucek


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## asylum (Nov 30, 2007)

Thanks for the reply and great info - so would be right in presumong the 1st peak at 0
is the ipulse hitting the microphone, but i am confused should the 1st wavefront hit the mic at 3 ms later due to the speed of sound, also what does the negative side of the waveform mean? thanks again -


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## asylum (Nov 30, 2007)

Hi Bruce thanks for helping a newbie to this software, it really is great, could you point in the direction of any online info on how to read the impluse response, or is there any around here, like what should i be looking for, and in what window what is the difference of dB% vs dBfs?
does that mak sense, thanks?


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## brucek (Apr 11, 2006)

> so would be right in presuming the 1st peak at 0 is the impulse hitting the microphone


Yes, there is no real time reference (unless you use the left channel as reference feature), so the main signal is the large first peak you see. As time progresses you will see any further reflections as small peaks in the signal that you can measure with the cursor from the main peak until the signal drops into the noise floor.

The signal before the main peak reveals system distortion. You may see smaller copies of the main peak there which represent a distortion harmonic. The first peak before zero is the 2nd harmonic and the next peak is the third harmonic, etc.

After the main peak are the reflections that took longer to get to the microphone than the initial main peak. The timing can show how much further thay had to travel to reach the microphone. So, if your mic was 10 feet from the speaker (main peak at zero), and the distance to hit a wall and get to the mic was 15 feet, then you would see a spike at 5msec (given the speed of sound).

Again, these reflections are a bit easier to see with the Energy Time tab plots. Full range speakers as opposed to the lower bandwidth subwoofer responses are far easier to detect, because they'll show as a nice spike rather than a wide lobe.

brucek


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## asylum (Nov 30, 2007)

Yes i am using the left channel feature, the confusing part for me is if the timeline is in ms and with the speed of sound hitting the mic and if the mic is3 feet from the source, should the 1st peak be at 3 ms and not at the 0 position, and after that reflections?


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## JohnM (Apr 11, 2006)

The zero position on the impulse plot is set to the first point the response goes 10% above the peak value, to remove the source to microphone delay.


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## asylum (Nov 30, 2007)

the the 1st peak is really at 3 ms but its all shifted forward to line up at 0 on the time line so the delay is taken out?


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## Anthony (Oct 5, 2006)

That's correct. So the second peak 3ms later corresponds to the first reflection, which is about 5' longer in path than straight speaker-to-mic.


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## asylum (Nov 30, 2007)

why 5' foot longer?


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## Anthony (Oct 5, 2006)

1125 ft/second X 0.003 ms which is about 3.4 ft, not 5. Sorry, I did the calculation in my head before my coffee kicked in. 

That's the speed of sound times the time it took to travel.


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## asylum (Nov 30, 2007)

so if i get this right the 1st reflection is at 3.4' approx, so i can look at that and measure with a tape measure all surfaces around that distance and expect to see something eg a boundary wall/floor/ceiling? so with regard the rest of the tail how should i interpret file? is there any info on this subject on the net?

thanks again for taking the time to discuss this anthony


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## Anthony (Oct 5, 2006)

It would be 3.4' longer than your speaker to mic distance. So if that was 3', you would be looking for a 6.4'. Most likely it would be from speaker to back wall to mic, or speaker to floor to mic.

Glad to help.


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