I am interested in analyzing surfaces of objects
to detect natural and man-made shaping of the object surface,
and I have been experimenting,
using a MP3 player/recorder to record the sound,
as I drag various fabrics attached to the player across the objects.

Are you actually MP3-compressing the received data? I would be very
concerned that you're losing important information there. MP3 was
designed to reproduce audio, it wasn't designed as a general-purpose
compression algorithm for analog data capture sessions.

It sounds like each of these samples is probably not very large, can
you capture them without compression, i.e. as raw PCM data?