Re: Pseudo-anechoic (windowed impulse) measurement on Smaart
First off, I'm just an old washed-up non-audio engineer learning my way around these things for the fun of it. I do have a background in DSP and Fourier analysis, although somewhat dated, so you can talk dirty to me.
As for my "lab" situation, for low-to-mid crossover work (300 - 500 Hz) I'm putting home made speaker systems (a bad idea, I know, but I'm learning, my next ones will be store bought) up on ladders and other tower like things outdoors and measuring (on axis, so far) at ~2-3m.
For example, on a 12 ft ladder my first ground bounce is at least 21 ms out, so by windowing the impulse response to eliminate the reflection I can get a frequency resolution of 47 Hz, or ~ 1/7 octave at 470 Hz, enough to useful, I think, and better as I go higher.
As has been pointed out, and as I've observed in my limited experience, you can get pretty decent measurements outdoors even in the presence of modest reflections, so maybe my concern with eliminating all reflections is overblown. Furthermore, and correct me if I'm wrong, the combing that results from distant (late) reflections has little effect on the smoothed magnitude at high frequencies, since the ripples get smoothed out. I think the phase still gets scrambled.
For low frequency work, where the speaker system is much smaller than a wavelength, I use either a ground plane or near field (mic ~5 mm from dust cap) technique, depending on what I'm trying to do. A 60 ft high (10 Hz resolution) tower would be cool, but my friends would get me committed if I tried to build one.
I also use Smaart to get detailed impedance measurements of woofers in cabinets, which has proved very useful in detecting and chasing down small, in-band cabinet resonances.
The last couple of days I've been trying to understand, and prove-out, my measurement tools a little better, so I've been simulating various scenarios using a DSP (Ashley NE 8800) in place of the electroacoustic (speaker-room-mic) piece. Fun and educational -- everyone should try it. Thanks for listening. --Frank
