Using REW to close mic a sub

[fredk]

I am curious to see what my sub's response is like without room interactions. I seem to remember others doing measurements with the mic close to do this.

So, here is what I think I should do:

Place my RS meter 1' from the woofer centered on the cone. Turn the volume waaaay down and calibrate the meter to 75db as per normal measurements. Fire up the measureing window, check levels and measure.

Is this correct?

[brucek]

I would probably use 12" rather than an inch for a sealed sub.

It's a different story for a ported sub though. You have to choose a point where you feel the mix between the ports output and driver are somewhat equal.

brucek

[Ricci]

I'd go just an inch or 2 from the drivers dustcap at a low to moderate volume for the measurement. I'm not sure if your meter might have a proximity effect. The ECM8000 doesn't. It's harder to get an accurate measurement on a resonant based subwoofer especially if the ports or PR's on on different faces of the sw.

[fredk]

Thanks gents. I forgot to mention its a ported sub, with the port being below the driver. I guess that means I need to take the port into account. I will give it a go and let you know the results.

I am just curious to see how the sub rolls off as I am surprised at how much low end gain I seem to be getting (placed on a side wall).

[bjs]

FWIW, I've compared outdoor free space and ground plane measurements with inroom on sealed systems and get excellent correlation when the mic is about 1 inch or less from the dust cap. 12 inches is too far, room effects become quite noticeable at that distance.

[brucek]

excellent correlation when the mic is about 1 inch or less from the dust cap. 12 inches is too far, room effects become quite noticeable at that distance.

Yeah, interesting. I certainly have no practical experience in this type of measurement, as my interest has always been limited to how the room effects my sub, so I only parrot the information I read in articles on the subject.

I have read mostly that 12" or less is considered the definition of near-field, and that 1" is correct for tweeter measurements (combined with proper impulse gating), but with respect to a sub driver, that too close could cause some wind noise on the mic diaphragm.

I don't doubt that a room resonance may overwhelm at 12", so I suppose closer might be a smart move (for a sealed sub only, of course). Your experience shows 1" to be optimum, and any further is a degrading of the actual output?

brucek

[bjs]

Yes, 1" or less is good and matches outdoor results to a small fraction of dB. Room induced errors increase steadily with mic distance and by 12 inches the error can be several dB (depending on room). In room response at the listener position matters most however, and usually it is wildly different than either of these.