A friend told me that he got a bunch of Type IV (Metal)
audio cassettes at the Goodwill a few days ago. They
have audio recorded on them (somebody's LP transcriptions)
and he wants to re-use them.

He ended up with four different brands/models:
Maxell MX
TDK MA
Denon HD-M
Sony Metal SR

And he has tried them on three different cassette
machines (all of which are in alignment/adjustment
and erase other tapes just fine):
Sony TC-611
Sony TC-615
Pioneer 1000

He has even tried bulk erasing them with a big bruiser
he made out of an old power transformer (draws 10 A
and has throroughly degaussed all other formats he has
ever tried including aluminum-flange 10.5 inch reels).

But while he can get rid of some of the high-end,
the LF signals seem impervious to all attempts at
at erasure.

He is going to try a permanent magnet in contact with
the (moving) tape next, but that seems impractical on
several levels.

Anybody know what the deal is here? He says the
cassettes all appear to be labeled as audio tapes, and
not as data (consumer graphics, measured in "minutes"
rather then "metres" or "Megabytes", etc. etc.)