A friend is remastering a recording for another taper and he states:
Here is a strange one. I am remastering a show from a taper well known to me. I have remastered many of his shows and never run into this particular issue before. During a 1 hour recording there are 5 brief periods lasting 15-20 seconds each, during which a substantial part of the bass drops out of the recording. Analysis shows that this is mostly in the 200-300Hz range but the drop extends up to 400Hz. Interestingly the sub-bass below 150Hz is unchanged. This drop in bass makes the recording sound like a 1970's telephone call. I have attached a 30 second MP3 sample. The first 5 second and the last 10 seconds are "normal" with the middle the result of this dropout. Here is his gear: OKM2mics > Zoom H2n (at WAV 16/44) > SDcard > HD.
Any ideas as to the cause?