Loudness Recruitment

Loudness recruitment is an abnormally-rapid growth in loudness with increases in suprathreshold stimulus intensity. Recruitment is a common characteristic of hearing losses that result from damage to the sensory cells of the cochlea. Such sensory hearing losses are the most common type. They result from such causes as noise exposure, treatment with certain drugs, and advancing age (a condition called presbycusis).

Another common type of hearing loss -- one that does not exhibit recruitment -- is conductive. Conductive losses result from impaired sound transmission through the outer, middle or inner ear. The main consequence of a conductive hearing loss is that sound stimulating the sensory receptors is simply attenuated. Because there is no loudness recruitment with a pure conductive loss, the presence of recruitment can be used as a clinical sign that differentiates sensory from conductive losses.

Loudness recruitment can be measured with loudness balance techniques. This technique is easiest to perform when a patient has hearing loss in one ear only, with normal hearing in the other ear. A tone is presented at varying sound levels to the impaired ear, and the patient is asked to adjust the level of the tone presented to the good ear to be as loud as the tone in the impaired ear. The resulting plot of equally-loud levels at the two ears is typically like the idealized plot in the figure, constructed for an example hearing loss of 50 dB. The red line for a recruiting hearing loss and the blue line for a conductive loss both emerge above 0 dB on the y-axis at 50 dB on the x-axis, appropriate for a 50-dB threshold shift. Signals at levels below 50 dB will not be heard. (The dotted black line for normal hearing, showing equal levels at the two ears, is given as a reference). As signal level increases above 50 dB the slope of the conductive-loss line is constant at 1 dB/dB, which reflects the simple signal attenuation for this type of loss. The recruiting curve, on the other hand, increases rapidly for signal levels above threshold. This recruitment continues until a point where recruitment is complete, about 20 dB above threshold, above which loudness increases at the normal rate. Because people with recruiting hearing impairment perceive high-level sounds at ‘normal’ loudness it has been said that they have hearing loss only for low-level sounds.

HeLPS uses the simple representations of recruiting and conductive losses, shown by the example in the figure, in its hearing loss simulation algorithm.

Click here to open the HeLPS Loudness Recruitment Demo in a new window.