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Old 23rd October 2004, 10:29 PM   #1
kneadle is offline kneadle  United States
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Default A little Speaker Workshop help

I'm finally making acoustic measurements with my Wallin Jig II. It's a great program, but...

But I don't know if it's working right! It would be nice if someone could look at this graph and say, "yup, that looks right," or "nope, you done something wrong."

The following is for the Tangband 881S in a TL.

Click the image to open in full size.

Thanks,

Dave

PS--I've crossposted this to the SW forum, but it's not nearly so populated as this one. Sorry if I offend.
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Old 24th October 2004, 12:21 AM   #2
claudio is offline claudio  Italy
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Hello,
I would increase the measurement resolution, since it seems too flat.
Besides, there is a loss of more than 3 dB from 200 to 5K Hz: does the free air response of the driver (manufacturer response) has this rolling look?

Regards

Claudio
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Old 24th October 2004, 12:21 AM   #3
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No, that is probably not correct....

Things that make me say that:
- even nearfield, the response would not likely be that smooth in the upper treble.
- even nearfield, I doubt the response is 3dB down at 20hz.

Try this - hook up your loop cable on both channels and take a nearfield measurement. You should have a perfectly flat response. You should have about the same signal level on both channels (in the VU meters). If that is okay, post more info about your mic, preamp, how you're measuring (where your mic is), levels and settings in SW, etc.
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Old 24th October 2004, 12:39 AM   #4
Sjef is offline Sjef  Netherlands
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It looks like you either have used a lot of smoorthing in the measurements or you have choosen for a very small time window. Anyway, the result doesn't look like a real lif speaker measurement. What is the microphone distance ? According to the high frequency sloop it looks like a free field measurement, but on the other hand acoording to the flatness of the measurement it looks like a nearfield measurement, both with a lot of smoothing and a very short time gate. Are you sure you are looking at the frequency response instead of the phase response ? It sure does look more like a phase reponse plot.
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Old 24th October 2004, 12:41 AM   #5
kneadle is offline kneadle  United States
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Default OK Stay tuned

Thanks Claudio et al. I'll fiddle with it and post back.

Dave
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Old 24th October 2004, 12:48 AM   #6
kneadle is offline kneadle  United States
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Here's how I'm doing it:

Click the image to open in full size.

Microphone: Studio Projects C4 with omni capsule

Preamp: Mackie 1202 VLZ

I have to say, I'm a little confused about the SW settings for acoustic measurement. The "unofficial" manual, as good as it is, and far from me to complain, is not particularly clear here. There are references to non-existent menus, and procedures which are impossible to duplicate. At least for the first 100 times I've tried.

Oh, and Jig II is my jig.

Dave
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Old 24th October 2004, 12:55 AM   #7
kneadle is offline kneadle  United States
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Quote:
Originally posted by John Sheerin
You should have about the same signal level on both channels (in the VU meters).
This is precisely my difficulty. This is a very confusing demand, and I don't know how to make this happen during the acoustic measurement. If you check page 101 of the "unofficial" SW manual, you'll see what I mean.

I'll copy and paste if needed here.

Thanks,

Dave
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Old 24th October 2004, 01:25 AM   #8
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Dave.
I don't think I've read the unofficial manual. Never the less, with both channels looped (ie, no speaker, no mic involved), you should have about the same signal level on both channels (in the VU meters). If one channel does not have the same level, that means something is probably wrong. Ie, if one channel is 16000 max and the other is 100 max when the channels are looped, the 100 channel is probably not getting any signal - just noise or crosstalk between the channels.

For example, open SW, create a driver, make that driver active, have both channels looped, in the View menu have 'VU meter' checked, take a nearfield measurement (so there is no gating to worry about), and the Max and Min numbers in the VU meters should be about the same for the left and right channels. In my case, 32000 is clipping, so I shoot for a max around 16000 to avoid any clipping.

When I actually am taking a measurement, my levels may end up being different between the left and right channel - it depends on how your gain structure is set up, but ideally the levels would still both be about the same and close to 16k or so (in my case) for good signal to noise resolution. For example on my last xover design, I had a compression driver tweeter and two 8" mids where the mids were much less sensitive. I would measure the tweeter first and then when I measured the mids at the same level, the mic channel's level would be much lower than the reference due to the lower sensitivity drivers playing at a lower SPL for the same volume settings. This could be worked around if I cared to take the time, but I do not care - that was good enough for what I was doing.

John
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Old 24th October 2004, 01:39 AM   #9
kneadle is offline kneadle  United States
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Quote:
Originally posted by John Sheerin
Dave.
with both channels looped...have both channels looped,
John
Here's where I could stand a little push. What, specifically, are you referring to by "loop"? Is this a setting in SW, or is this something I'm supposed to do with the jig?

Thanks,
Dave
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Old 24th October 2004, 02:13 AM   #10
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Don't know if your jig will do it or not. I built a Wallin jig a long time ago, but I only use it for Z measurements. But what I'm talking about is running the left output of the soundcard to the left input of the soundcard and the right output to the right input. So there is just a loop between input and output. I do this with a 1/8" stereo to RCA cable and a 1/8" to RCA converter on the other end - all stuff you could pick up at radioshack. If this procedure does not work correctly (response with both channels looped), it is likely that nothing else will work correctly even though you may get results that look half-way convincing.
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