What I call Hyper-Recruitment, others may refer to as Hyper-Acusis. It is the tendency for sounds to appear to grow loud too quickly with increasing presentation levels. There are hints of it in one’s audiology when there is a depressed Maximum Comfortable Level (MCL), also known as Uncomfortable level (UCL).
The paradox is that in order to properly treat this condition, it requires a decrease in the signal level at those frequencies, even while your hearing may need considerable help in order to hear properly.
This graph shows the variations in the condition of recruitment hearing, all for the same degree of threshold elevation. The black diagonal line represents normal hearing, where one perceives exactly what is presented in sound levels.
Recruited hearing is indicated by an elevated threshold of hearing. You cannot discern the presence of any sound until it exceeds your elevated threshold. This is what is measured in the isolation boot during an audiology exam, independently for each ear, and across multiple frequencies, spaced at half-octave frequencies from 250 Hz to 8 kHz.
As sounds drop away, recruited hearing senses them dropping away much more rapidly than for normal hearing. But at very loud levels, recruited hearing becomes more like normal hearing.
Obviously, recruited hearing will damage one’s ability to hear sounds in the same way that normal hearing does. The consonant sounds of speech become lost. And since hearing recruitment tends to worsen toward the treble region, while musical instrument harmonics fall away in amplitude at higher frequencies, the sound of music becomes very dulled without help. Instruments have altered timbre and no longer sound like themselves. You lose the distinction between Oboes and Trumpets.
Oddly, in order to treat hyper-recruitment, the proper thing to do is to send the sound through a dipping EQ at those frequencies, ahead of any corrective actions taken to overcome elevated thresholds. How odd! In order to hear anything we need compressive gain. Yet at hyper-recruited frequencies we need to overtly dim the sound before applying this correction. How paradoxical.
If you fail to attenuate the sound at hyper-recruited frequencies, then the listener will begin to hear distortions in the music, and start to experience discomfort at overly loud sensations.
All that the audiologist can tell you is that you have a measured hearing loss, as indicated by elevated thresholds vs frequency, and some measure of depression in your Uncomfortable level (MCL or UCL). They don’t measure for the possibility of hyper-recruitment at each frequency.
And so, the only way to discover that you have hyper-recruitment at some frequencies is to listen for yourself to music with wide and loud spectral content – high strings or female choral music. When you start to discern distortions, try using a peaking EQ and scanning it across the band with boost at first, looking for where the distortions increase. Then at that frequency, make the EQ dip by some amount until the distortions cease. Trial and error will get you the proper amount of EQ for good listening.
The more unusual aspect of this is that a listener with hyper-recruitment doesn’t hear the sound as being attenuated by the EQ. It sounds more normal to them, as long as the sound is being corrected with proper nonlinear compressive gain.