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Old 13th September 2005, 04:30 PM   #1
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Default A couple of questions regarding LSPCad and MTM speakers

Hi all!

Thanks to your input, I'm starting to build my crossover for my speakers from scratch.

I have a couple of questions regarding LSPCad, though...

1) I measured impedance using the Woofer Tester for my woofers and tweeters.

2) I'll be measuring frequency response in-baffle for my tweeters and woofers.

I assume that when measuring frequency response I'll have to place the microphone about 50 cms. away from the baffle, in the center of the MTM (acoustic center of the tweeter). Considering this scenario, should I model the woofer in LSPCad as a driver consisting of two woofers in series, and then using the impedance file from a single woofer and the frequency response for the *two* drivers in the baffle? I think this would be the correct thing to do... am I right?

Thanks!

Javier.
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Old 14th September 2005, 12:55 AM   #2
paulhfx is offline paulhfx  Canada
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No, the impedance for one driver will not be the same as the combined impedance in your mtm.

I'd measure the frequency response individually for each driver, and put them in your lspcad schematic as separate drivers (let lspcad combine them). You can use the same impedance file for each of your mids.

Paul
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Old 14th September 2005, 02:56 AM   #3
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Quote:
Originally posted by paulhfx
No, the impedance for one driver will not be the same as the combined impedance in your mtm.

I'd measure the frequency response individually for each driver, and put them in your lspcad schematic as separate drivers (let lspcad combine them). You can use the same impedance file for each of your mids.

Paul
Got it! I'll then wire them in series and design the Zobel and notch filters for them simultaneously. Thanks a lot, that makes a lot more sense than my idea.
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Old 14th September 2005, 03:54 AM   #4
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what impedance are your midbass drivers?? If they are 4 ohm then series is the way to go, but if they are 8 ohm you probably should consider wiring them in parallel.

I've been measuring my drivers in speaker workshop, but have been having some problems..... my approach was to create a new driver and measure the impedance in box of both drivers connected in parallel, and treat it as a single driver (for crossover modelling).

I also took my freq response measurements as you suggested in your first post (both drivers running, mic at the centre of the tweeter), except at a distance of approx 90cm.... I still don't trust my freq response measurements though...

Depending on your box, (separate enclosures for each driver or a common enclosure) you may or may not be able to successfully measure the freq response of the drivers individually. If there are separate volumes then it should be ok, but if they share the same volume, and you are only driving one driver, the other driver will tend to act as a passive radiator and potentially give you a very different measurement.

Tony.
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Old 14th September 2005, 04:13 PM   #5
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Quote:
Originally posted by wintermute
what impedance are your midbass drivers?? If they are 4 ohm then series is the way to go, but if they are 8 ohm you probably should consider wiring them in parallel.

I've been measuring my drivers in speaker workshop, but have been having some problems..... my approach was to create a new driver and measure the impedance in box of both drivers connected in parallel, and treat it as a single driver (for crossover modelling).

I also took my freq response measurements as you suggested in your first post (both drivers running, mic at the centre of the tweeter), except at a distance of approx 90cm.... I still don't trust my freq response measurements though...

Depending on your box, (separate enclosures for each driver or a common enclosure) you may or may not be able to successfully measure the freq response of the drivers individually. If there are separate volumes then it should be ok, but if they share the same volume, and you are only driving one driver, the other driver will tend to act as a passive radiator and potentially give you a very different measurement.

Tony.
Ouch. You are right. It's the same volume, and yes, the other driver may act as a passive radiator. I may try shorting the other driver so as to slow it down as much as possible, but I don't think it'll do much good.

The drivers are 2.9 Ohms each. What a strange impedance they have (I got them from a pair of speakers I had around). So it's a 6 Ohm speaker right now.

I think I'll try both methods - measuring both drivers and then measuring them individually, and try both approaches on LSPCad. I hope the results aren't that different, and I can seek a compromise.
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Old 14th September 2005, 09:57 PM   #6
paulhfx is offline paulhfx  Canada
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Sounds like you'll need them wired in series (unless you've got amps that can do 1-2 Ohms )


Your best measurement procedure in this case would be to install them wired in series and measure their combined frequency response and impedance. Sorry I didn't think about the effects of the drivers on each other through the box in my post above - I've been working on dipoles for a while now.

Paul
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Old 14th September 2005, 10:22 PM   #7
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is that 2.9 Ohms measured with a multimeter?? If so then they would be 4 ohms nominal drivers, the dc resistance is always lower than the nominal impedance, but yeah you will need to do them in series

Tony.
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