Support Forums › Forums › FIR Creator Support › Magnitude cancellation in phase inversion. › Reply To: Magnitude cancellation in phase inversion.
Hi Bido,
Firstly, the filter length of 400 samples at 48 kHz (8.3ms) is much too short to achieve the magnitude changes and phase correction that you’d like. Take a look at the “FIR Filter Frequency Response” and “Total Error” graphs in your FIR Creator project. To achieve the EQ your desire, particularly at 100 Hz, you need a FIR filter length closer to 2048 samples with a delay of ~400 samples. Take a look at Section 5 on https://eclipseaudio.com/fir-filter-guide/
Secondly, looking at the original measurement – your 1st Smaart screenshot…. The phase rotation at ~760 Hz is an acoustic cancellation – most likely from a reflection – as evidenced by the corresponding dip in the magnitude response and dip in the coherence. No amount of DSP (in magnitude and/or phase) can fix this. Just considering the magnitude response, if you try to boost this with EQ, very little will change when re-measured. In your case, adding the 2nd order phase rotation in DSP is just widening the acoustic cancellation region. (Try removing the 2nd order phase filter and the remeasure, and I think you will see a mag dip similar to the unprocessed measurement.) The other frequencies where the coherence dips – e.g. 90 Hz, 390 Hz and 440 Hz – also correspond to magnitude dips which look like cancellation effects, likely from reflections.
DSP can address phase effects inherent in the loudspeaker, like from crossover filters, but little can be done about cancellation from reflections.
Best regards,
EA Support