Need help measuring Frequency Response

[Randallm]

I'm brand new to building speakers, and I want to measure their performance so I can build crossovers. I've installed ARTA, HolmImpulse and Speaker Workshop.

For hardware, I have a high end Sound Blaster card, and an ultra cheap microphone that plugs directly into the sound card. I've connected the line out from the Card to my Onkyo amp, and the amp to the speaker I'm testing. I've run tests, and I'm measuring something, but I'm not sure what. I'm working in the smallish kitchen, as that's where the computer lives. The mic is in a stand 1 meter from the speaker.

When I measure the new speaker I've built (the DUT), with no crossover, I get the graph in ArtaDUT. Just for fun I measured the big floor-standing 3-way that came with the Onkyo amp 20 years ago, and posted it as ArtaOnkyo.

The Onkyo results are the Black line, and have the Blue and Red Overlays from the DUT graph. The Black line results look to similiar for me to believe I'm accurately measuring the DUT's FR curve. According to these measurements, the newly built DUT, with no crossover, and the Onkyo 3-way both slowly rise in output from 200 hz to 1K, then have a sharp rise up at 5K.

Can anyone shed any light on what I'm doing wrong with my measurements?

[wintermute]

These definitely don't look right, however I am not familiar with ARTA or with the units being used on the Y axis on your graph, so I'm not certain what it is that I am looking at

The thing that I find especially odd is that your woofer and tweeter curves (for the range 1k to 12k) are very similar, even more odd is that the woofer appears to be more sensitive which is rarely the case

It could be your mic perhaps, any chance of trying another one?

If you could try doing a measurement with holm impulse as well and posting that (with the gating info) that might shed some more light.

Tony,

[jogi59]

Originally Posted by Randallm

and an ultra cheap microphone that plugs directly into the sound card.

That is your problem.
Here is the measured frequency response of some of these cheap microphones

[Henry8]

Before doing actual acoustic tests you need to ensure your chosen software/SC/test jig is working properly together. (with speaker workshop this means i get a flat fr when the setup is looped and consistant calibration curves.)

As for mics, its best to use a mic amp to feed line in.
Mics like wm60 etc can have wildly varying f/r above 2k, so for serious testing calibration of that mike is needed (no "generic" curves)

[tuxedocivic]

What mic is it? Also, I had a logitech webcam plugged into my computer when I first started measuring. I got similar problems. The mic on the webcam was over taking my panasonics wm60 and giving me problems. I had to physically unplug it.

[Randallm]

I switched to a Sennheiser mic and got a much more reasonable curve. Now I've got two new questions.

1) If I buy a new mic from Parts-Express (Dayton Audio EMM-6 Electret Measurement Microphone), what do I need as a pre-amp for the mic? I know nothing about pre-amps, so an actual product reference would be useful.

2) Given the new attached curve, it looks to me like the woofers just sort of over-power the tweeter. The Blue line is the woofer, the Red line is the tweeter, both measured at 1". The Green line is both connected with no crossover, the Gray line is both connected crossed over at 3.5K. both measured at about 24".

The addition of a crappy eBay 3.5K xo doesn't seem to really DO anything. If we assume that the speakers aren't worth investing any real money in them (the drivers were free), what's the minimum crossover I need to protect the tweeters from damage? Is that a reasonable question?

I just want to run these 'as-is' until I construct a set of Full Range speakers for comparisson (cheap build of Cyberg's Needles, just to see what full rangers sound like). Once I've done that, I'll either build some Full Rangers, or 2-ways, with component costs targeted at about $400 for the pair.

[AllenB]

The Dayton seems like a step in the right direction, especially as they say it has an individual calibration curve. You'd put this into Arta and off you go...

The Dayton required a phantom power supply. You can make one of these with as little as a couple of resistors, caps and 9V batteries, or you can buy or build something that maybe even has a pre amp built in.

Now, putting aside the numerous small things that could be said about your setup, you should try putting a resistor in series with your tweeter capacitor.

[Randallm]

At the momement, I'm putting a generic eBay crossover on the speaker. I'd like to build something specific for these drivers. Nothing fancy, it's not worth the money. Can you suggest a specific resistor and capacitor, given what you've seen? I'd like to remove the eBay crossover and just put in a couple of simple components to protect the tweeter.

[AllenB]

With the limited info (and off the top of my head) I'd say the tweeter could stand to come down 6dB or so. I'd start with an 8 ohm resistor. It would be better to put one in parallel as well. Rather than getting too complicated, if the series resistor wasn't enough I'd throw a 20 ohm resistor in parallel with the tweeter, and another if required.

The woofer looks as if it could stand to receive some impedance rise compensation. The crossover doesn't look like it is having an effect due to the woofer impedance fighting against its efforts.

Would you care to post details about the ebay crossover? I'd presume to leave the ebay crossover in place and add the resistor to it and the (resistor and capacitor) to the woofer when you can.