Measuring response

[Charles Warwick]

Hey, so I was looking at selling off some older speakers (making room for a new BFM build! ) and was looking at measuring their responses so I could post a plot in addition to a picture of them.

What's the best way to measure subs and/or tops? I have an ECM 8000 mic, mixer, TrueRTA, and a USB sound card for my laptop. I've also got a few acres of land to use, I think the horses won't mind... I've played with the RTA a bit, but the quick measurements I did didn't look right, i.e. on a 15'' sub, there was very low response below 100hz compared to the rest of the sweep.

What's the best way to do it in order to get relatively accurate results?

[sine143]

I'd say try to sell em, then if it doesnt work, drag them out to the country and plot them!

[Bill Fitzmaurice]

What's the best way to do it in order to get relatively accurate results?

Ground plane below roughly 400Hz, on axis with the cab on its back and the mic suspended above for above 400 Hz, combine the two for a half-space result. There will be a narrow bandwidth near 400 Hz where the two are the same, that's the transition point.

[Charles Warwick]

Ground plane below roughly 400Hz, on axis with the cab on its back and the mic suspended above for above 400 Hz, combine the two for a half-space result. There will be a narrow bandwidth near 400 Hz where the two are the same, that's the transition point.

Awesome, thanks Bill!

So, for mic placement:
<400hz: on ground in front of normally oriented speaker
>400hz: suspended on stand, on axis, above the speaker facing up (on it's back)

Is a 1 M distance sufficient? Or does it not matter a lot how far away the mic is as long as I'm not doing sensitivity measurements?

I'd say try to sell em, then if it doesnt work, drag them out to the country and plot them!

I already put one up on craigslist, but it's been near 2 weeks. I had someone today ask what the wattage of the speaker was (it's a 15'' folded horn based on a Selenium design with a 5.8 ft horn length) and I'm thinking that most people are going to be basing their opinion on how many watts it can handle. I told him the thermal limit was 300w RMS (which is what the driver is rated for), but unless you have a limiter you wouldn't be able to utilize anywhere near 300watts. I bet to many car audio guys and new DJ's that sounds like nothing when you look at amps and speakers 'rated' for thousands of watts. I told him I used it with an old 200 watt peavey amp and explained that when placed in a corner indoors I really don't need more than 100 watts to each painful levels of bass. I linked him to the BFM sticky explanation of how horn loaded subs work, so maybe he'll get it, or maybe I'll have to start advertising them as 2000 watt (peak) speakers.

Charles

[Bill Fitzmaurice]

1m won't give the best results but it won't give any worse results than most manufacturer plots, as they tend to measure at 1m.

[Charles Warwick]

So would a greater distance be better? Is the farther away the better?
I guess this brings up the question of how loud do you guys measure the speakers? Would you just put 2.83 volts into it and measure it, or try to reach a certain dB?
I had been measuring voltage by putting a 120hz sine wave into it and measuring the voltage across the terminals.

[Bill Fitzmaurice]

I usually measure ground plane at 4 meters, axial at 2 meters. I'd go 4 meters axial too but the mic stand won't go that high. For each doubling of distance from 1 meter you need to compensate the result by -6dB. I measure at 2.83v as even with that the neighborhood dogs chime in from 200 yards away.

[Charles Warwick]

Okay, so I'm kind of confused. I am using a Behringer Xenyx 802 mixer, the ECM 8000 mic, and plugging the computer's output into the amp. When I have everything connected (cables, power, computer et cetera) except the mic, RTA shows its usual flat-ish self, but as soon as I plug the mic into the XLR cable:

That is what I get. That spike at 60hz will not go away. I know it must be something to do with the mixer since if I plug in a regular 3.5mm mic into the computer, RTA works perfectly fine and doesn't give me that 60hz spike. I have all the electronics plugged into the same outlet, amp on upper one and mixer on lower one. Whether or not the computer is plugged in makes no difference in the appearance of the 60hz spike. If I run a quick sweep it shows up beautifully.

Any help? Please? I'm becoming slowly more frustrated as my ideas to resolve the problem run out...

[Ron K]

How are you interfacing your mixers output into your computer? are you going through an outboard sound card etc? There's a possibility that on some outboard sound cards you can engage a monitor circuit which allows monitoring the soundcards output. Turn this off if you have it. Use some headphones and see if there's 60 cycle hum present on the mixers output. This must be done at the mixer and not at the computer.

Check that mic cable for any abnormalities and also check to see if it has the casing shunted to pin 1.These cables are ground loops waiting to happen.

I'll add this to help in diagnosing the problem when listening to the signal.

Humm = too many grounds or ground loops and buzz= lack of proper ground.

[jcmbowman]

60hz noise is almost always caused by poorly grounded or connected power, as 60hz is the frequency that A/C power in the U.S. oscillates at.