Ask an Acoustic Engineer (me)

[jsalinasii]

Justin,

I don't know if this thread is still open but I do have a question:

Impulse Response in a Car - I have searched and searched the inter web trying to figure out how to run REW (only software I have) to check my tape measured time alignment. Some threads say it is NOT possible with a USB mic (I have a UMIK-1). Is this possible? Is this a technique you use in cars? When I run REW "Measures" using "Use loopback as timing reference" I do get data but the numbers seem really really large for delays. As the internet is dark and full of terrors, trying to find any walk through seems impossible and I am fighting a crossover null between my right tweeter and midrange. To my ears the vocal range appears to be centered but I am afraid I have some type of timing issue between those two drivers.

Thanks

[erinh]

First off: if you are using the USB mic forget the "loopback" method of time reference. You can't do it.

However, REW does have an acoustic timing reference option. Which uses a certain speaker to baseline time deltas of other IRs from. I haven't used it. But here's some info:
https://www.minidsp.com/applications...%20measurement.


Understand that the LPF will impact the IR arrival time peak. High frequencies arrive sooner than low frequencies. If you measure with a HPF enabled or a bandpass filter, you'll get a different IR time than you would if you had the speaker playing with a crossover. That begs the question: do you delay based on IR on measured time. I say measured. Speed of sound doesn't change. The initial arrive of the impulse doesn't peak until it hits the highest frequency (if that's 500Hz, it's gonna take a long time, relative to a 5kHz tone). If anything, pay attention to the step response and try to make sure the handoff from one driver to the next is smooth and even. Use your crossovers and all-pass filters to help flatten the phase response at the crossover. Don't worry about time-aligning for the crossover. Time align for the physical distance. My $.02.

[jsalinasii]

Erin,

Thank you. Clears the madness up and I think I am going to simply abandon ship when it comes to IR and time aligning. Can I ask a follow up?

Do you ever set the crossover frequency differently between each side (L vs R) even if only say 10 hertz? My Bit Ten D does not have all-pass (not that I would know how to use it) and it is only dipping/nulling on my passenger side. Nothing looks totally apparent on my freq response XO curves but I have a dip sure as day at my 5500 hz TW to MR point.

[erinh]

Asymmetrical crossover slopes/points are a necessary evil in car audio.

[Jdunk54nl]

Erin,

Thank you. Clears the madness up and I think I am going to simply abandon ship when it comes to IR and time aligning. Can I ask a follow up?

Do you ever set the crossover frequency differently between each side (L vs R) even if only say 10 hertz? My Bit Ten D does not have all-pass (not that I would know how to use it) and it is only dipping/nulling on my passenger side. Nothing looks totally apparent on my freq response XO curves but I have a dip sure as day at my 5500 hz TW to MR point.

Set your electrical crossovers to whatever you need. The acoustical (measured via REW) crossovers should be the same, as in, the curves of the response should match very close to each other.

[LumbermanSVO]

Instead of impulse response, I use APL TDA to measure arrival times, it gives you a graph like this:

The software allows you to measure points on the graph as well.

You can also import the files to REW:

(Not the same measurement in both pictures)

[Justin Zazzi]

Justin,

I don't know if this thread is still open but I do have a question:

Totally still open!

[ckirocz28]

Justin,

I don't know if this thread is still open but I do have a question:

Impulse Response in a Car - I have searched and searched the inter web trying to figure out how to run REW (only software I have) to check my tape measured time alignment. Some threads say it is NOT possible with a USB mic (I have a UMIK-1). Is this possible? Is this a technique you use in cars? When I run REW "Measures" using "Use loopback as timing reference" I do get data but the numbers seem really really large for delays. As the internet is dark and full of terrors, trying to find any walk through seems impossible and I am fighting a crossover null between my right tweeter and midrange. To my ears the vocal range appears to be centered but I am afraid I have some type of timing issue between those two drivers.

Thanks

HolmImpulse is much more accurate for checking timing. There's a couple of tutorials floating about somwhere.

[Donanon]

Here is a long thread by the developer of HOLMImpulse that covers just about everything the app is capable of.

https://www.diyaudio.com/forums/software-tools/144984-holmimpulse-measuring-frequency-impulse-response.html



https://www.diyaudio.com/forums/search.php?searchid=23396928

[jsalinasii]

I only have a USB mic and in my reading of HOLM it sounds like you have to have a phantom powered microphone in insure you don't have a time lapse. Hoping to change topics if possible and ask another question:

I have summing of my midrange drivers from about 200hz to 1.25khz. After 1.25khz my summing seems to disappear meaning that the individual levels of the left and right side can be greater individually than when combined. I believe it is a phase issue but my crossover point is not until 5000 hz. Is there anyway to address a phasing/cancelation issue than only takes place half way through it's intended range. My tweeters seem to sum but the upper half of the mid range seem to cancel.

I have attached an image. I use a Bit Ten D so I have the look set at 1/3 octave.

Thanks for all of your help each one of you.