Radio Shack SPL Meter and justMLS - I'm getting odd measurements...

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Hi all!

I decided to take my speaker building project very slowly, and learning first about what makes a good speaker.

Anyway, I got lspCAD 6, and was happy to see justMLS included. I had never heard about it, so I began playing with it.

The instructions are not ver clear... at all. I'm using a laptop, a Sound Blaster MP3+ card, and the Radio Shack SPL meter as a mic.

I built the reference cable (the one that goes from the speaker to the Line In, with the voltage divider circuit). If I plug it into the sound card, a weird "tick-tick-tick" noise (loud!) is heard on the speaker.

But the weirdest thing is - although I can measure frequency response (it looks terrible, but the shape corresponds to that shown on the tweeter's literature), the phase is all over the place. It crosses 0 10 or 20 times and never settles down. I tried calibrating the sound card, and it's no use...

Is there a FAQ where I can read a bit more on justMLS? I have read the instructions twice, tried measuring using one and two channels, changed drivers, played with the distance, and nothing seems to help. Alternatively, do you think the Radio Shack SPL meter is playing serious tricks with phase?

Any help will be greatly appreciated! :)
I would look at the reference cable you built and also at your soundcard. Soundblaster cards are not that accurate over 10 KHz, the FR oscillates.

Try to keep the imput voltage (or output of the SPL meter) of your soundcard at around 1.5V I guess. Soundblaster cards have a bad habit of saturating their op-amps when you use the standard 2V level.

Something seems to bring the whole thing into oscillation, too much capacitors somewhere? Hehehe!

It could be the RatShack meter too...
Good luck!
I don't know enough of your setup and JustMLS but a few things to consider:

- usually such software calculates phase from the frequency response. It calculates the frequency response by doing an FFT from the MLS signal's time response.
- the FFT is really a discrete calculation. Fitting a discrete calculation onto a continuous spectrum means risking artefacts: Depending on the parameters set for the FFT you do get both "real" terrible looking frequency wiggles, and artefactual wiggles
- those real and artefactual wiggles = mathematically generated noise, will now be included in the calculation of phase... with the result that you get.

Possible solutions:

- more FFT points, though there is a point depending on length of MLS stimulus where more FFT points will not increase precision
- averaging of the frequency response and hoping that the software uses the averaged frequency response for phase calculation
- forget phase and use averaged frequency response for a first order mental approximation of phase ;) (rule: FR goes down->phase becomes negative; FR goes up->phase becomes positive - this is why you get such a mess in phase, it comes from the up and downs of you FR)

Incidentally much of the terribly looking wiggles you get come from the room interaction (if not gated) and the baffle edges (here even gating won't help much).

If you want to test your microphone and soundcard measure the tweeter at 1/2 inch from the dome. This way you'll have essentially just the tweeter and it should look very smooth and flat. If it doesn't this indicates some peaks and dips of your microphone.

But first, test your soundcard. Measure distortion over your reference wire (just the wire, no mic and no speaker - output to input), using a single frequency sine wave signal instead of MLS.Play it continuously and increase input and output levels one at a time, while doing a real time FFT (watch screen while turning knob) until you see weird spikes appearing at the harmonics (multiples of the signal frequency ). This indicates clipping distortion onset. Remember that both outputs and inputs can clip: outputs from weakness, inputs from overload. For the input, overload can occur at the mic level, the mic preamp level, and the coundcard level.

If you want to look any of this kind of measurement to look good, just use octave averaging and a display scale of 100 dB on the y axis - that's what manufacturers do, hey, and you won't see any wiggles at all :cannotbe:

Hope this helps
Sorry, I forgot that the Soundblaster cards are reconverting stuff to 48 kHz most of the time to apply some effects, the conversion engine is really crappy, so use 48 kHz sampling frequency on the tone software generator and the microphone line-in if you can to bypass this issue that might show false results. Check that CMSS is disabled and things like that.

Also, follow the advice of MBK because he knows more than me hehe!
But the weirdest thing is - although I can measure frequency response (it looks terrible, but the shape corresponds to that shown on the tweeter's literature), the phase is all over the place. It crosses 0 10 or 20 times and never settles down. I tried calibrating the sound card, and it's no use...

I don't know your software so I can't tell you the fix. But I can tell you the problem- you haven't corrected for time of flight, i.e., the time it takes to get from the speaker to the mike. If there's any kind of distance variable there, that's what you need to play with.

For example, let's say there's a millisecond of delay because your mike is a foot (30cm) away. At 1kHz, that's a full cycle, so the dumb program thinks that, in addition to the real (minimum) phase, there's another 360° of phase there. At 2kHz, there's another 720°. Since the phase display "wraps" at +/-180°, what you get is a display that has a wildly varying phase curve.
I think SY is right. I have version 5 of Just MLS and it has an input that I think is called distance. It allows you to enter the distance between the microphone and the driver in either meters or miliseconds. You can start by measuring the distance with a tape measure and entering that. Then make a small adjustment - you will see the phase get worse (i.e. it will show more cycles or peaks) or better. Keep making corrections in the direction that reduces the number of peaks. Eventually you will come to a distance that minimizes the number. One warning - if you enter a value that goes too far in the "right" direction, you will see things getting worse again.

I can't help you with the clicks - I've not had that happen (yet). You are not alone in finding JustMLS to be tempermental. I'm getting better at it but I'm still having problems repeating measurements. I'm sure it's something I'm doing (as opposed to the software) but I will let the users' manual share the blame.


Didn't even think of that one too.

Again, I don't know JustMLS but say in Audiotester, you have a time domain diagram and you can tell the software when to start the FFT. That way you don't even need to measure, you see the time point where the impulse arrives, and you click to make this time zero.

JustMLS must have that feature too, because surely there will also be an echo point / gating feature, and for that you need the time domain graph as well.

Your single suggestions is what I have Not been doing and perhaps that is why I never got the phase correct to begin with.. I did not thought of that and I will test it...

when I measured, I set the distance of the microphone but the phase still looked bad and what you just said about fine tuning the distance until corrected, should of been said in the manual in my opinon..

thanks a bunch!

Sy, thanks for explaining what's going on with the phase and the time of fly, you have explained it wayyyy better than the author of the software, you have made it very clear....

My question is, is the Radio Shack meter accurate enough to make measurements?
I never used mine 'cause I did not think so.

I find that, designing XOs is really freaking hard!
way too many variables!

but and then again I am a beginnier..
I bought a Behringer Mike and Mixer to power it so I can't help much with the your meter question. I do have a RS meter and I know that it has a significant rolloff at the low end and a lesser rolloff at the high end. If you google for "Radio Shack meter calibration" I think you will find sites that will give you correction tables. I'm pretty sure JustMLS has the facility to import mike correction table - I don't know about format. There is also a site that gives you detailed directions for making realtively simple modifications to the meter to get rid of both the high and low droop. I've made the correction for the low end and I can confirm it makes a big difference. I've not corrected the high end as I was interested, at the time, in making subwoofer measurements.



Set the meter for "C" weighted, Slow. For best results, set the meter on a tripod to keep reflections at a minimum. Get a Test CD that has 1/3 octave warble tones like Stereophile Test CD #3. Start with the 1 kHz tone and set the meter at 80 dB and set the 1 kHz tone to show 0 dB on the meter. I would suggest setting the meter at you listening position to see what the response is there. Start the test tones and with the chart I am suppling below, mark the level beside the frequency on the chart. You can do the corrections after you finish. Stand as far from the meter as you can when doing this so reflections off of your body does not influence the reading. Also try not to have anything else to near the meter. This will give you a close approximation of what the frequency response is of your speakers at your listening postion. Below is the correction chart. Everything is added.

Test Disk index Hz Correction

The problem with these correction charts is they are quite old and the new Radio Shack SPL meters have the 33-4050 model number and these charts are usually for the previous 33-2050 or something. Another chart from the BassList is floating around the internet, they say the meter is flat almost up to 20 kHz and only the low end need correction. We need someone to publish a new correction chart hehe!
Hello all,

I just re-checked things (thanks a lot for your help!). I still get very, very odd phase measurements, especially on the upper end (2KHz and up), but by inverting phase and playing around with the distance setup on justMLS I finally got something resembling a decent graph. I'm sure it's still wrong, but it's enough to help me with my design.

I also decided to buy a Behringer mic preamp / ECM8000. I measured a couple of drivers I had around, and I see spikes that shouldn't be there and frequency response variations that I know don't exist (besides, if I already got lspCAD, investing in extra equipment is a must, I think).

Simon: I'll do that. I'll try to lower the SPL voltage meter. . I also test using 48K, since I read somewhere the resampling to 44.1 really messed things up. Results weren't that much different.

MBK: I tested my soundcard by looping it, and frequency response and phase are very flat. I also tested a Philips Aurilium PSC805 USB sound card, and I couldn't believe its phase / FR issues. The Sound Blaster looks a lot better in this regard. Now I understand why manufacturers' specs look so good, too! Thanks :)

SY: I did play with the time of flight. But the strange thing is, the distance from microphone to driver was, say, 20 cms., and I had to use something closer to 40 cms. for the graph to look right. I know I'm doing something wrong. Right now, I'm blaming two things: the Radio Shack meter and using the software on a closed room. Maybe I should start taking measurements outside. It's very easy to correct it using justMLS - actually, all I do is enter the delay time in miliseconds, and the distance between driver and microphone.

Denis, I have to agree with you. I'm pretty sure the software is doing the right thing, but the manual is simply of no help. They only describe what should be done, and it leaves you scratching your head wondering why or how you should do it. Thanks a lot for all your help - it's really, really helping me out! :)

Memofer, from my experience the Radio Shack meter is not accurate enough. Not even with the ETF calibration file. I wonder whether this is of any importance, though - maybe since the inaccuracies are on both extremes of the audioband, you could still design a decent 2 way with it. I'm not sure - but to be on the safe side, I got the ECM measurement microphone :)

Once again... thank you so very much for your help. It's really appreciated. I've been reading about designing my own speakers for months now, and I have learned from many of you more than you'll ever know. I'm really grateful for that. :)
Don't feel bad as you are not the only one learning here, warble tone is:
a tone resulting from rapid modulations of frequency within fixed limits around the basic pure tone frequency. Commonly used in real measurements as they are not easily influenced by standing waves from reflective surfaces.
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