Mic distance for driver measurements

[tedwilt]

How far should I place my ECM8000 from a driver using STEPS for minimal problems from measurement room reflections? I assume this should change with frequency. I am using a Xenex 802. What level to measure without overdriving the mic or mixer? Advice for measurement techniques would be greatly appreciated. I will be measuring a North D25 tweeter to compare with the factory measurement paper to calibrate my mic. Also will be measuring drivers from 4" to 10". Thanks,
Ted

[diy speaker guy]

Ok, first off, it's a really really bad idea to calibrate your mic by using a manufacturer's measurement of a tweeter. The tweeter they measured in the factory could be substantially different than the one you have at home even though they are exactly the same model. They probably measured in an anechoic chamber of unknown properties with the tweeter on some type of infinite baffle. Can you recreate those conditions?

To eliminate room reflections you need to use gating, or simply don't measure in a room. (I'm not familiar with STEPS, maybe gating is on by default? Some of the more respected manufacturers measure outside, well away from any boundaries (including the ground), 10w at 10m. For the hobbyist, flying the speakers and mic for measurements is not really necessary, and the actual distance between speaker and mic is flexible, depending on what you are measuring, and what you are measuring for. Industry "standard" is usually 1w (or 2.83V)/ 1 m. Regardless, even without gating or being outside, the closer the mic gets to the speaker, the greater the ratio of direct vs reflected sound, so extreme nearfield measurements will be less influenced by the room.

Personally, I use my built in laptop mic, uncorrected and uncalibrated, and measure everything outside when possible unless I want to see actual response at the listening position, which naturally includes room effects. So far my measurement equipment does everything I need it to but I don't require ultra accurate measurements yet.

[tedwilt]

Thanks for your reply. I have 10 North D25 tweeters that each have impedance and frequency response graphs for that particular driver. All are quite different with the matched pairs somewhat closer.

I cannot measure outside so I need the mic as close as possible to minimize room reflections. Steps is gated and takes a long time for a measurement but is much more accurate than the quick sweeps. I am primarily interested in on-axis response. I plan to use some 4" thick mattress foam for isolation and perhaps some reinforced cardboard to deflect unwanted reflections. I guess I will have to stop reading soon and start measuring. Thanks,
Ted

[r-carpenter]

If you can place your tweeter in a medium size panel say 2x2 and have it away a meter or two from all the boundaries (walls etc) then you can get accurate
results 800hz and up depending on your set up. Which is more then enough for a tweeter. You can also do a "Near Field" with the microphone 3 inches or so away from the tweeter if the "on axis" is all you after.
Chad Gray has a great video on his web page for sound easy but the principle is the same for other software.
http://chadgray.info/soundeasy/

Roman.

[tedwilt]

Thanks for the good help Roman. This is just what I needed.
Ted