24/12/2025
Two full-range loudspeakers in a delay system The alignment of two full-range systems in time isn’t so easy, as the real world loudspeakers aren’t so perfect as the one represented by the Dirac in the wrapped phase section above. Most sound reinforcement loudspeakers have a low frequency cut-off around 35Hz or above. The curves above are both filtered by a high-pass filter around 50Hz. The measurement is often influenced by reflections from the room as seen in the blue curve above. The next graph shows the wrapped phase curves from the two systems in the top graph and the ETC in the bottom graph. Very often mistakes are made when trying to match the wrapped phase. The phase can became several turns off, with time smearing and side-lobe errors as results. The reason is often the cut-off frequency, which makes an increasing group-delay at low frequency and will bend the low frequencies of the phase. This is very hard to see in the phase graph, but if we look at the group delay instead we can see how the curves divert below 150Hz
If the high-pass slope or the LF cut-off is very steep its very important to raise the start frequency of the FFT. Otherwise the phase calculation will start down in the noise floor with invalid results. In this graph the FFT start frequency is raised to 50Hz. It’s obvious that it helps when trying to find the match to the wraps. As seen in the wrapped phase above, there are some extra oscillations due to the reflections in one of the IR. If a strong reflection is present close to the desired IR, there is a risk that one aligns to the reflection instead. It is therefore very important to window the time graph so the measurement will be anechoic. In the time graph below Multiple windows are applied to clean the IR.
Bävholm & Grenander ”
