Posted by Wayne Parham [ 70.234.131.209 ] on June 06, 2008 at 02:05:32:

In Reply to: Re: Measurements posted by Duke on June 05, 2008 at 23:50:23:

I do the same thing, and for a long time didn't publish curves either. Took kind of a beating for it over on Audio Asylum, if you recall. But I definitely understand what you're doing and believe in it. Thiele/Small models are completely reliable below about 300Hz, just about the same place indoors measurements fall apart. So you have complete visibility and can perfect your design with pseudo-anechoic measurements to see the crossover region, and depend on models alone for the bass bin. That's more than adequate, in my opinion.

If you ever feel like it, take your speakers, amplifier and LMS system outside. The back yard will work, as long as you're not going to test at high power. Lay your speakers on their back and elevate the microphone on a boom so it hangs down over the cabinet. If the microphone is carefully placed 2 meters away, then the response chart you get will be 6dB lower than what it would be if measured at 1 meter. Of course, the extra distance will help reduce the angle difference between midwoofer and tweeter. So with microphone 2 meters away, you can set the oscillator for 5.66v (double 2.83v) and the SPL measured will be the same as 2.83v/1M.

Another way to measure that works well at high power is to set the microphone on the ground and put the speaker 10 meters away, also on the ground, of course facing each other. The voltage levels then should be 10x for 1M levels, i.e. 28.3v/10M is the same SPL as 2.83v/1M. Sometimes it works better to stand the speaker upside down so tweeter ground reflections are minimized. The 10 meter method is a good way to measure with high power drive levels, for checking things like distortion, compression and thermal-related parameter shift.

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