Measurements

This is the place to post those questions to get your up and running with Soundeasy.
Post Reply
Dogsled20
Posts: 1
Joined: Thu Dec 03, 2020 4:06 pm

Measurements

Post by Dogsled20 »

OK, I am sure this has been covered at length, would like to discuss how to obtain "real" spl levels. Calibrating the measurements seems very convoluted or simply not possible. This is critical to match multiple drivers, of course.
I have used the mls measurements, and keep mic positions, levels, etc, the same to measure tweeters , mids & woofers. The results vary all over the place. How does one calibrate?
User avatar
Chad Gray
Site Admin
Posts: 46
Joined: Wed Dec 02, 2020 7:26 pm
Contact:

Re: Measurements

Post by Chad Gray »

Wow that was a coincidence... i just posted to the Wiki John K's method of "calibrating" your system so you can get accurate SPL levels when measuring.

https://soundeasy.chadgray.info/wiki/in ... solute_SPL

You do have to understand that in the past we would just measure woofers and tweeters at the time of design. So you measure out your tweeter first then leave the levels alone. Measure the rest of your drivers and now you have all drivers with *relative* (to each other) levels.

So you know your tweeter is 10dB louder than your Woofer. Then when designing a crossover you know how much to bring the tweeter down to match up with the woofer.

Hope this helps!
Chad
dcibel
Posts: 43
Joined: Sun Dec 13, 2020 6:39 am

Re: Measurements

Post by dcibel »

It is detailed in the manual to some extent, that a requirement for absolute SPL means some SPL calibration device, phsyical hardware involved at additional cost.

The good news is that absolute SPL is rarely needed for any speaker design purpose. All that is needed is relative SPL between drivers which involves simply keeping your mic at the same distance from the baffle for all measurements, and don't touch the volume knobs!

Regardless of the mesured SPL, for design I will adjust the SPL level so that the design ends up around 85-90dB range, as 90dB seems to be the reference level in SoundEasy.
meloV8
Posts: 38
Joined: Thu Dec 03, 2020 10:45 am
Location: Poland

Re: Measurements

Post by meloV8 »

I have a question about Acoustical Distance in the MLS Impulse Response window. Why does the given distance never correspond to the actual distance? No matter what sound card I use, with what resolution, with what MLS signal length.
dcibel
Posts: 43
Joined: Sun Dec 13, 2020 6:39 am

Re: Measurements

Post by dcibel »

Well, relative distance will be accurate, as its simple math based on time per sample and speed of sound, however I believe the discrepancy is that the 0cm location is not exactly at the start of the reference impulse, at least if you try and follow Bohdan's minimum phase document here:

http://bodziosoftware.com.au/Min_Phase_ ... ration.pdf

It states:
the start of the FFT window for the Reference impulse response is 10 sample times from the peak, or 9 sample times from the start of the impulse response
It also states the peak of the reference impulse is at bin 60, which is consistent with my own experience, and 10 sample times back puts the start of the impulse at bin 50. However bin 50 is not the 0cm location, but rather bin 52 is. At 96kHz bin 50 is -0.717cm location, at 48kHz bin 50 is -1.433cm. The 0cm location is actually 8 sample times back from the peak. I have asked the reason for this in the past but the question has remained unanswered.

At the end of the day, it's not necessarily a bad thing that the 0cm location is a bit behind the start of the reference impulse, as it helps to ensure that your FFT window is always behind the start of the measured impulse, however I understand the frustration when the acoustic distance shown in the software is always further than the actual measured distance. Shift the distance shown in SoundEasy by 2 sample times and it appears to agree more with the physical measurement.
meloV8
Posts: 38
Joined: Thu Dec 03, 2020 10:45 am
Location: Poland

Re: Measurements

Post by meloV8 »

What you wrote, everything is correct, only this given acoustic distance is misleading, especially if we take into account the recommendations from the John K tutorial.
Post Reply