# Measuring acoustic phase difference between two receivers

I am trying to measure the acoustic phase difference between two sensors 30cm apart each other. The sampling frequency is 20kHz, and my interest frequency is about 1700 Hz. The below is how I proceeded.

1. Take FFT of T = 1 sec time domain signal. In spectrum, there are several peaks including the interest frequency.
2. Bandpass filter around the interest frequency (e.g. 1700+-100 Hz)
3. Reconstruct(or IFFT) the signal in time domain.
4. Take Hilbert transform and get phase in time domain
5. Do the same thing for the other receiver.
6. Compare Hilbert transform phase between two signals in time domain

When I did this, the phase difference between two signals is not constant, but changes with time, which is out of my expectation. Shouldn't the phase difference be constant if they have the same frequency peak? Is there any issue with the above procedure?

• It's not clear to me why you need the Hilber transform. See here: dsp.stackexchange.com/q/20067/11256
– MBaz
May 24, 2018 at 21:58
• what's your environment ? unless you are in an anechoic chamber or a quiet environment with the speaker close up, the phase WILL fluctuate due to background noise and time variant room acoustics May 25, 2018 at 11:38

$$\frac{1130~\text{feet}/\text{s}}{1700~\text{cycle}/\text{s}} \approx 0.66~\text{feet/cycle} \approx 20.26~\text{cm}/\text{cycle}$$