Phase Shift
What is Phase Shift?
Phase shift is the time difference or delay between two otherwise identical waveforms, typically expressed in degrees (°) or time units. When this shift occurs due to physical distance, it's often called path-length difference.
Measuring Phase Shift:
Phase shift can be measured in multiple ways:
Time Delay (t): t = d/c Where: d = difference in path length c = speed of sound
Phase Angle (θ): θ = (t × f × 360°) Where: t = time delay in seconds f = frequency in Hz
Path Length Difference (d): d = t × c Where: c = speed of sound (approximately 343 m/s or 1125 ft/s at room temperature)
For example, for a 500 Hz tone:
One complete cycle takes: T = 1/f = 1/500 = 0.002 seconds
A 180° phase shift requires: t = 0.001 seconds
This corresponds to a path length difference of:
Metric: d = 0.001 s × 343 m/s = 0.343 meters
Imperial: d = 0.001 s × 1125 ft/s = 1.125 feet
Figure 1: Two microphone setup
In this scenario, sound travels different distances to each microphone which creates a time delay between the signals. The path length determines the phase relationship
Figure 2: Phase Relationships
When using multiple microphones, the distance between them should be at least 3 times the distance from each microphone to its source to minimise phase issues.
Figure 3: The 3:1 microphone placement guideline
D₁ = Distance from sound source to the closest microphone
D₂ = Distance between microphones
