Pressure-based measurement methods are widely used in anechoic conditions for noise quantification purposes. The bias error introduced by a p-p probe can be approximated by I ^ ^ n p − − p ≃ ≃ I n − − φ φ pe p rms 2 k Δ Δ r ρ ρ c = I n ( 1 − − φ φ pe k Δ Δ r p rms 2 / ρ ρ c I r ) , where I n is the “true” intensity (unaffected by calibration errors), I ^ ^ n p − − p is the biased estimate obtained using a p-p probe, p rms is the root-mean-squared value of the sound pressure, k is the wave number, ρ ρ is the density of air, c is the speed of sound and Δ Δ r is the spacing between the two microphones. This expression shows that phase calibration errors are inversely proportional to frequency and microphone spacing and directly proportional to the ratio of the mean square sound pressure to the sound intensity. If the pressure-to-intensity ratio is large then even a small phase mismatch will lead to significant bias errors. In practice, sound intensity measurements cannot be performed accurately when the pressure-intensity index is high, which limits the use of p-p intensity probes in environments with high levels of background noise or reflections.