In the 1950s, a Danish amateur acoustician named Christian Volf conducted several listening tests that were to cure hearing disabilities. The listening tests were controversial in many respects.
Volf carried out experiments on patients suffering from hearing loss. The patients were to listen to records with sine waves generated from a home build tone generator. Each record contained tones that equalled an octave on a piano. When a client was tested, he had to listen to the record containing tones in the frequency spectrum where he had perfect hearing ability. Playing tones in this specific area in very loud amplitude, Volf could create a provoked hearing loss that levelled out the total hearing range of the patient. The client would now be equipped with hearing aids that could amplify the whole hearing spectrum. The procedure of damaging the nerve cells inside the ear in order to reach uniform hearing ability were never fully scientifically approved, but nevertheless the tests were carried out – even in a national test on school children in Denmark during the 1950s.
The sounds that Volf used in his hearing test has been re-released by the Danish avantgarde label BIN:
https://www.discogs.com/Christian-A-Volf-Volf-Acustic-Records/release/1639172
TUNING THE EAR 