Open space distance for measurements?

[head_unit]

I'm trying to recall how to calculate the lowest frequency you can measure reflection free.

For instance, let's say I mount a speaker in a 3m ceiling, and place the mic 1m away. Let's suppose this is a huge empty room, so the only reflection is the floor. Assume the speed of sound is 345m/s.

The direct sound hits the mic after 1/345th of a second, the reflected sound travels 5m and arrives at 5/345ths of a second. So the reflection free zone is 4/345ths of a second.

Then my brain gets stuck and can't remember. If the 4m reflection free corresponds to one wavelength, the frequency would be 86.25 Hz, and an FFT could get you valid data every 86.25 Hz. A swept tracked sine would be good above 86.25 Hz. Is that really correct?

[SY]

Originally Posted by head_unit

an FFT could get you valid data every 86.25 Hz.

Not quite. That will set the lower limit for the measurement (regardless of whether it's swept, FFT, MLS, whatever), assuming you gate out the reflections. The frequency resolution of the measurement depends on the number of samples within the measurement period.

[ente]

... have a look on this paper

http://www.fesb.hr/~mateljan/arta/Ap...d-Rev03eng.pdf

Regards
Heinrich