search for keywords Balanced Armature (BA) and diaphragm,
currently the receivers are BA (one of the main manufacturer is Knowles), and the higher power UP receivers have a bigger diaphragm to produce louder volume at lower frequencys, and the bigger diaphragm is, the lesser is the ability for producing very high frequencys.
i suggest you read this thread
Quote: “hearing aids are mainly designed for SPEECH, therefore receivers (Look at the Knowles Tech bulletins, data-sheets and support notes ) are designed around the same principle. Most speech occurs in the 250Hz to 6Khz range”
Frequency lowering aka. compression like “Sound Recover2” (Phonak) , “Speech Rescue” (Oticon) is for lowering/shifting down higher frequencies and copy that into areas of lower frequency where most of the people have better hearing. Most manufacturers have some variation of frequency lowering.