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Old 10th January 2011, 03:45 AM   #1
Fedess is offline Fedess  Argentina
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Default A question about measuring passive filters...

Hi,

I
have been reading the "Design of Passive Crossovers" article from ESP Audio, and today I just started building a 2nd order Linkwitz-Riley crossover, at 5000Hz frequency cut.

I only made the highpass section for the tweeter, which I measured 9,17 ohm impedance at that freq. (Fr of the tweeter is about 1000Hz so I won't use a notch filter).

The calculated values for the filter are:
0,58 mH for the coil,
and 1,74uF for the cap.

I used three 570nF (nominal value) caps connected in parallel so I got 1,75 uF, measured with my tester (beginner's luck!). Using LIMP the value was between 1,72 and 1,77 uF (depending the freq. I set the cursor)


For the coil, I used the online software to calculate diammeter, number of turns, etc, from this web: Pronine Electronics Design - Multilayer Air Core Inductor Calculator

Then the values I could measure were from 560 to 590 uH, again depending where I put the cursor. (I can only measure the coil with LIMP...)

So I thought I have the correct cap and coil value (or the best I can get), and decided to connect them to the tweeter and measure the overall impedance.

At this point I realized that I don't know what to expect from the measurement, because LIMP doesn't measure the electrical response in dB, but it does in Ohms...

So my question is, how could I know if the filter is correctly working using LIMP?





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Old 16th January 2011, 12:42 AM   #2
Fedess is offline Fedess  Argentina
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Nothing here?... =(
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Old 16th January 2011, 01:35 AM   #3
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hello Fedess....

It's good to start with Limp and now that this is done, you have to check the acoustical output, no more Limp but Arta.

Create the target curve with the LRII slope and then measure it, at say 1 meter. If floor bounce or anything else, adjust the window.
Probably you will have to adjust some values to stick to the target...be ready to have some surprises with the natural roll of of your drivers if close from the chosen Fc.
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Old 16th January 2011, 03:17 AM   #4
witwald is offline witwald  Australia
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Quote:
Originally Posted by Fedess View Post
So I thought I have the correct cap and coil value (or the best I can get), and decided to connect them to the tweeter and measure the overall impedance. ... how could I know if the filter is correctly working using LIMP?
You will need to use ARTA with a measurement microphone. You can close-mic the tweeter (say 1cm or 2cm away from the diaphragm) and this will give you the dominant driver+filter response, without significant room effects.

Owing to the reactive nature of the tweeter's impedance curve, and the fact that the formulae that you used assume a constant resistance, you may find that your filtered response does not match as well as you might like with the theoretical response. Amongst other things, this will depend on the location and height of the impedance peak at the tweeter's resonance, as well as the nonlinear impedance rise normally associated with voice-coil inductance effects.
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Old 17th January 2011, 01:36 PM   #5
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Measuring a tweeter near field is going to give inaccurate results. As the frequency rises the size of the tweeter becomes a significant portion of the wavelength. This leads to cancellation by sound emanating from different areas of the diaphragm. Move back to 30-40 cm and you'll avoid this effect without letting too much room effect into the measurement, especially when gated.
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Old 18th January 2011, 05:10 AM   #6
witwald is offline witwald  Australia
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Thanks for the added information, which makes sense to me. At 10 kHz the wavelength of sound is about 34 mm, and for a phase cancellation pathlength differences of about 17 mm need to be present. Is it really necessary to have the microphone as far away as 30–40 cm from the tweeter when measuing the tweeter's response, which will mainly be affected by the filter below about 5 kHz? I would expect that at such a distance the effects of room reflections will start to have a bit of a noticeable effect.

I expect that ARTA is able to do some sort of gated measurements, based on MLS methods. This would make the 30–40 cm distance quite workable I suppose, as one could trim out the section of the measurement that is affected by room reflections. This would reduce the accuracy of the low-frequency portion of the trace to some degree, as there would not be enough data available.

Do you think that a distance of 20 cm might be OK? At that distance away from the top of the dome of the tweeter (assuming it has a dome diaphragm), and if the dome is raised by about 4 mm, the path length difference between the edge of the dome and the middle of the dome is only about 4 mm. This is only about 1/8 of a wavelength at 10 kHz, which is a 1/4 of half a wavelength, so the effects of phase cancellation at that frequency shouldn't be very great.

Last edited by witwald; 18th January 2011 at 05:18 AM.
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Old 18th January 2011, 06:11 AM   #7
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Hello...

IMO at such a distance (around 40 cm), you will not see any difference between gated and ungated measurements, and you use Arta, following the user's manual, better to forget the MLS excitation, Sweep is better.
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Old 18th January 2011, 06:44 PM   #8
ente is offline ente  Germany
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witwald = CALSOD ???

Regards
Heinrich
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