Acoustic Couplers and I go back to my days at ADDS, Inc. where, among other things, I helped write the patent application for a new design for acoustic couplers that solved one of their fundamental problems. The mark - space frequencies were harmonics, and the built in sound feedback of the phone handset, put there so you could hear yourself speak, caused problems with acoustic coupler modems.
Our gimmick was to inject a little third harmonic at the appropriate place in the circuit which effectively canceled the problem child. We got the patent all right and got a nice contract from Texas Instruments for evaluation for use in their Silent 700 portable thermal printer terminals. Our competition was Anderson Jacobson and another one, a Novations if I remember rightly.
The test consisted of looping a constant stream of data from Dallas to eastern Long Island (and Dallas to the other places) for SIX MONTHS, day and night to see which one performed the best. The funny part is that we beat the others hands down, but didn't get the OEM order because ours cost slightly more, and cost was the final criteria.
We learned a little later that the real secret of acoustic coupler performance was in the CUPS. If they gripped and cushioned the handset well, the coupler worked well. If not, no amount of circuit magic would make the coupler work well. This of course led to many an insider joke about cup designer qualifications, etc.