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Old 04-21-2008   #1
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Default Opinions interpreting impulse response plots.

Here is my typical measurement setup used to grab impulse response from a single mono mic.


Sound card is a M-Audio Mobile pre. Channel 1 is the microphone. Channel 2 is looped back to provide timing reference. Using ARTA analysis software. Software MLS settings:

ARTA limits me to a minimum of a 340mS measurement window. However, I don't believe that plays a factor here with raw timing info. It comes into play when looking at frequency response. For frequency response I could chose a "gated" slot in time to do the analysis to eliminate the affects of late reflections.

Here is a impulse response plot from my drivers side midbass:

From what I have read you are supposed to grab your timing off the first large peak. I guess in this instance it would be labeled "b" in the diagram. However, what is the small negative spike before it labeled "a"? may I assume the next peak labeled "c" that occurs some 10ms later is a reflection?

It gets a little more surley when we look at the passengers side midbass:

My best guess tells be negative peak "b" is what I need to look at and that the drivers are out of phase. This still brings up the question, what is positive peak "a", and now positive peak "c"? But, even more important, what is positive peak "d". It is equal in amplitude but opposite in polarity to peak "b". Peak "d" occurs about 5ms later.

Just when I thought things couldn't get more confusing I took a measurement of the drivers side midrange:

Lesser peaks occur at "a" and "b". The largest peak "c" is immediately followed by an equal in amplitude but opposite in polarity peak "d". As you can see by my marker, I chose peak "c" as the one to estimate delay from. Was this a good assumption?

Now for the most confusing of them all sofar. The passengers side midrange:

Peaks occur all over the place. As you can see, I chose "b" as the one to derive timing off of. If correct, the drivers are in phase. But, what on earth are peaks d,e, and f? I'm starting to second guess choosing peak "b" as the proper one to deriveing timing from. If this were true, then the passengers side Speaker (16.1ms delay) would be closer to me that the drivers side Speaker (16.2ms). This physically does not make sense. The drivers are kick mounted and path lengths have been optimized. However, there still is a notable path length difference. The passengers side midrange speaker is definitely further away. Perhaps I should have chosen peak "d" or "e" instead?

The drivers side tweeter plot is a little hard to read. ARTA will not allow me to zoom up close enough to get a good look:


Now the passengers side:


The two tweeter graphs both show a small positive peak at "a" followed by a larger negative peak at "b" followed up with an equal in amplitude but opposite in polarity peak at "c". The way I read this is that I should derive my timing off peak(s) "b". Both tweeters are in polarity with each other but out of polarity with the rest of the front stage.

The midbasses are door mounted and band limited from 63Hz to 300Hz. I expect to see some nasties in the impulse response due to this mounting location.

The midranges are kick panel mounted tucked way up under the dash. Go here for a look:
2004 Dodge Durango - simple system
The midranges are band limited from 500Hz to 3KHz. I assume many of the nasties I see here are due to reflections off surfaces under and around the underside of the dash. I intend on doing some work with sound absorbing material in the near future to try and tame this down a little.

The tweets are also kick mounted right next to the midranges. They are band limited from 4KHz and up. It is hard to determine the nasties present there due to lack of resolution in my graph.

Did everything I mentioned in my observations make sense? Can you make sense out of what I could not determine? Any useful/helpful input?

Thank you for your time.

Ge0

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Last edited by Ge0; 04-21-2008 at 02:28 PM..
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Old 04-21-2008   #2
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Default Re: Opinions interpreting impulse response plots.

agreed. your midbass are out of phase. All other drivers appear ot be in phase.

I think you chose wrong peaks to time the midranges. I would have used peak "a" as my goal is to align the first waves.
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Old 04-21-2008   #3
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Default Re: Opinions interpreting impulse response plots.

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I think you chose wrong peaks to time the midranges. I would have used peak "a" as my goal is to align the first waves.
In one respect I'd say your are true. This is the way I have done it 3 times in the past. However, I can't ignore the higher amplitude, dominant, reflections. Can I?

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Old 04-21-2008   #4
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Default Re: Opinions interpreting impulse response plots.

From the reading I've done here, I agree with what you've said about the phase. From what I've gathered, the first peak is what you pay attention to. A valley means out of phase, a spike is in phase.

I'll stay tuned for further discussion...

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Old 04-21-2008   #5
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Default Re: Opinions interpreting impulse response plots.

Nobody else wants to take a stab at this?

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Old 04-22-2008   #6
 
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Default Re: Opinions interpreting impulse response plots.

I would probably choose peaks "a" for the midranges. I wish I could suggest more but I have the same questions.
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Old 04-22-2008   #7
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Default Re: Opinions interpreting impulse response plots.

Taking physical measurements and deriving time delay out of it seems to coincide with taking the first initial "pulses" on the graph as my time reference.

I can see I have definite problems when the amplitude of later reflections SWAMP out the original signal. This will confuse the imaging something fierce.

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Old 04-22-2008   #8
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Default Re: Opinions interpreting impulse response plots.

i would go with the first peak as well. that the first signal that is being received by the mic so it would be the first sound you hear.

and what is with this tiny reply box?

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Old 04-22-2008   #9
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Default Re: Opinions interpreting impulse response plots.

My work blocked photobucket pics so I can't really see anything. My beef with Arta is that you can't really zoom in. Otherwise it's a great tool.

One good thing to see is how important or destructive the space is the sound. Kind of makes putting uber expensive drivers and equipment in a Car when the space is killing it.

Can you practice with this in the house on a set of speakers? You will see how much different the impulse will look and it's easier to interpret results. Then move this into the Car and try to tackle it.

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Old 04-22-2008   #10
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Default Re: Opinions interpreting impulse response plots.

Quote:
Originally Posted by Ge0 View Post
In one respect I'd say your are true. This is the way I have done it 3 times in the past. However, I can't ignore the higher amplitude, dominant, reflections. Can I?

Ge0
for the purposes of aligning the initial wavelengths? (aka the arrival times) Why not?

turn up the volume on one side, or install padding everywhere, or fill the entire footwell with polyfill and take the measurement again. Does it change the dominant wave peak? Does it change the overall amplitude? Does the microphone affect the overall amplitude?

What can we really compare? what data is real?

On the other hand, we know the very first waveform input is directly comparable as an absolute data point.
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Old 04-22-2008   #11
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Default Re: Opinions interpreting impulse response plots.

Can you postprocess it to an etc curve? It's a bit easier to read.


A speaker is only as good as the room you put it in.
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Old 04-22-2008   #12
 
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Default Re: Opinions interpreting impulse response plots.

If he FFT'd the damn thing it would be very readable.
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Old 04-22-2008   #13
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Default Re: Opinions interpreting impulse response plots.

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If he FFT'd the damn thing it would be very readable.
Looking for time domain info there slappy, not frequncy domain. Thanks for the hint though .

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Old 04-22-2008   #14
 
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Default Re: Opinions interpreting impulse response plots.

Understand which matters more slappy.
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Old 04-22-2008   #15
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Default Re: Opinions interpreting impulse response plots.

I have no way to save files in the ARTA eval software so had to re-run the experiment and save the Impulse Response Envelope data to the clip board.

Here it is:

Drivers midbass:


Passengers midbass:


Drivers midrange:


Passengers midrange:


Drivers tweet:


Passengers tweet:


Hardware loopback check:


Nearfield (approx 4") on drivers midbass:


Nearfield (approx 4") on drivers midrange:


Hopefully this lends some insite. I'm starting to think my original readings were valid. My vehicle is just a horror for reflective surfaces. Leather seats, hard plastic trim, kick panel mounts...

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Old 04-23-2008   #16
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Default Re: Opinions interpreting impulse response plots.

Quote:
Originally Posted by Tommythecat View Post
Understand which matters more slappy.
Heh heh. Agreed. I spend most of my time futzing with frequncy response. Cripes, see numerous other posts I have put up like this one. I have a handle of freq response for the time being. Now I'm wondering how I can clear up the image a little. Will identifying and taming major reflections help out?

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Old 04-24-2008   #17
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Default Re: Opinions interpreting impulse response plots.

Ge0,
Thanks for continuing to try to take a technical/analytical approach to solving problems in your vehicle! (always interesting to see data!)
Looking at the plots, it seems to me that your problem is with the length of the impulse you are using to excite these speakers. The impulse length is way too short to be appropriate for the bandwidth of the given speakers. (notice how much better the midrange looks vs. mid bass?) If ARTA allows you to adjust the impulse width and/or shape, you are likely to see much better results. Haven't tried to do this in ARTA myself though.
Hope this helps.... GL.

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Old 06-10-2009   #18
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Default Re: Opinions interpreting impulse response plots.

Did the OP managed to tackle the problem?

Anymore infos?

Thanks,
Kelvin

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Default Re: Opinions interpreting impulse response plots.

Hey Geo,
Welcome to the world of enlightenment. Once you've made the measurements you've made (which look like every Car I've measured), entered the appropriate delay settings and verified that they sound pretty close, it's time to close the impulse response measurement window and forget it exists. Trying to tame all of those reflections will be like playing Whack-a-mole. The Car IS reflective and since you have to be able to see out in order to drive, attempting to remove them is futile. It's also unnecessary, because they all happen so close to the initial sound that we simply perceive them as part of the first sound. The best you'll do is fix their effects on the sound in the frequency response and live with the rest. THe worst reflections of all are the ones from the passenger and driver's windows. If you want to find them, pick one, determine the time in milliseconds that the reflection takes and go searching in the car with a tape measure. You can find the frequency response of those reflections by analyzing only that part of the inpulse response, but the times are so short, it's unlikely that you'll have enough resolution to see much below 10k or so.

Remember, that the steeper the peak in the impulse response the more high frequency content in the measured signal. This is one reason that locating midbass drivers and subwoofers this way is very difficult unless you eliminate low pass filters.

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Old 06-10-2009   #20
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Default Re: Opinions interpreting impulse response plots.

Quote:
Originally Posted by Andy Wehmeyer View Post
The worst reflections of all are the ones from the passenger and driver's windows.

I’ll x2 on this due to personal experience. The last time I really had an up-and-up tune (last December), I remember having a huge reflection issue off the driver’s side window.

I would listen to the setup with the seat nearly upright. Everything was gravy. Lean the seat back a bit and all of a sudden it seemed like the reflection made a b-line for my ear canal. I would lean up and the problem was gone… lean back and it was right back. I had to really work the EQ to tame some things (particularly 2k). I got it fixed, but learned a lesson.

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Old 06-10-2009   #21
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Default Re: Opinions interpreting impulse response plots.

Quote:
Originally Posted by subwoofery View Post
Did the OP managed to tackle the problem?

Anymore infos?

Thanks,
Kelvin
This is what I think I have learned. Don't treat this as fact until you have verified it for yourself.

It's not really possible to SOLVE the problem. The best you can do is work with what you've got and make minor improvements here and there. Compare your measurements to what physical measurements would dictate in regards to time delay. Pick the peak in impulse response that most closely matches what you've calculated the propegation delay should be. This more often than not corresponds to the first positive or negative peak you see. The rest are either constructive or destructive interference caused by reflections. You can try to treat your vehicle to tame some of the higher frequency reflections (steeper slopes) but there is not much you are going to do about the low frequency reflections (shallower slopes). Acoustical treatment for frequencies in the sub and midbass range just aren't practical for a car.

But remember, reflections that occur within 20mS to 30mS of the original are masked by your brain anyway (temporal masking). Anything beyond this is percieved as an echo and can add to the ambience, or perceived acoutsical space, of your vehicle.

Fix frequency response anomalies caused by reflections. Once you have your drivers in acoustical alignment (via impulse response 1st peak) forget the time domain stuff.

Hope this helped.

Ge0

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Old 06-10-2009   #22
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Default Re: Opinions interpreting impulse response plots.

Quote:
Originally Posted by Andy Wehmeyer View Post
Hey Geo,
Welcome to the world of enlightenment. (snip) Trying to tame all of those reflections will be like playing Whack-a-mole. The Car IS reflective and since you have to be able to see out in order to drive, attempting to remove them is futile.
Mind if I use the "Whack-a-mole" statement in a future sig? Can't agree with you more.

Quote:
Originally Posted by Andy Wehmeyer View Post
It's also unnecessary, because they all happen so close to the initial sound that we simply perceive them as part of the first sound. The best you'll do is fix their effects on the sound in the frequency response and live with the rest.
Correct. Reflections that occur within 20 to 30 mS of the original are masked by the brain. This is called temporal masking. It helps us determine the point of origin of a sound. Without it, our auditory system would be chaos. Everything you hear would have wild echos and could not be localized.

Quote:
Originally Posted by Andy Wehmeyer View Post
The worst reflections of all are the ones from the passenger and driver's windows. If you want to find them, pick one, determine the time in milliseconds that the reflection takes and go searching in the Car with a tape measure. You can find the frequency response of those reflections by analyzing only that part of the inpulse response, but the times are so short, it's unlikely that you'll have enough resolution to see much below 10k or so.
Stepping away from time domain interpretation for a moment. You can actually use the impulse response graph (or physical measurements) to help correct problems in frequency response.

For instance, I had a sucking hole at 800Hz and another at approximately 1600Hz in my kick mounted midrange response on the passengers side. The wavelength of a 800Hz sound wave is roughly 16inch. The wavelength of 1600Hz is, believe it or not, 8 inch. I know that sound that reflects off hard surfaces does so 180 degrees out of phase. Sound reflecting off soft surfaces stays in phase. Sound must be reflecting off a hard surface to create a null at this frequency. Hmm, what was a hard surface within 8 inches (or 1/2 wave of 16 inches) of the loudspeaker? DAMN, my HVAC blower motor housing! I wrapped the housing in open cell foam ( a soft surface) and that null sure as shit went way.

Quoting my buddy Foxy "well slap me in the ass and call me Charlie!!!" This math and physics stuff really does work...

Ge0

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Old 06-10-2009   #23
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Default Re: Opinions interpreting impulse response plots.

this is a 'my dad' moment, but I just wanted to reiterate that the MLS window should be varied depending on what frequencies you plan on monitoring... correct?

Tweeters need a lower sampling rate, while midbasses need a higher... or no?

Probably covered somewhere on this forum, but it's something that was lodged in my brain and seems to fit the format of this conversation.

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Old 06-10-2009   #24
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Default Re: Opinions interpreting impulse response plots.

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Originally Posted by Ge0 View Post
For instance, I had a sucking hole at 800Hz and another at approximately 1600Hz in my kick mounted midrange response on the passengers side. The wavelength of a 800Hz sound wave is roughly 16inch. The wavelength of 1600Hz is, believe it or not, 8 inch. I know that sound that reflects off hard surfaces does so 180 degrees out of phase. Sound reflecting off soft surfaces stays in phase. Sound must be reflecting off a hard surface to create a null at this frequency. Hmm, what was a hard surface within 8 inches (or 1/2 wave of 16 inches) of the loudspeaker? DAMN, my HVAC blower motor housing! I wrapped the housing in open cell foam ( a soft surface) and that null sure as shit went way...
Sometimes it's the most simple things that are overlooked. Good catch, and awesome fix.
I really like your scientific approach to finding solutions (or work-arounds, more accurately). Most people slap band-aids all over the place in hopes of one of them working, but I can appreciate the planning and thinking behind your process and I'm impressed by it.
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Old 06-10-2009   #25
 
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Default Re: Opinions interpreting impulse response plots.

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Originally Posted by bikinpunk View Post
Tweeters need a lower sampling rate, while midbasses need a higher... or no?
Or no. Higher frequency, higher sampling rate. See Mr. Nyquist for the reason.

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