Recruitment compensation as a hearing aid signal processing strategy

Although Fowler is commonly given credit for discovering recruitment [1936], Steinberg and Gardner at Bell Labs [1937] were the first to understand its true significance. Today recruitment is poorly understood, and is generally misdefined as the abnormally rapid growth of loudness. It is not well known that loudness in sones does not grow more rapidly in the recruiting ear; rather it is the intensity of an equally loud tone (i.e., the loudness-level in phons) that grows more rapidly. Regardless of the definition, recruitment is the most basic manifestation of sensory-neural hearing loss. Recruitment is due to the loss of outer hair cell (OHC) function. The sound-detecting inner hair cells (IHC) within the cochlea have a limited dynamic range of less than 60 dB. The OHCs nonlinearly compress the dynamic range of the signal excitation to the IHC, extending its dynamic range. Thus the normal function of OHCs plays a critical role in loudness and speech coding. Distortion product otoacoustic emissions (OAE) are an objective measure of OHC nonlinear compression, while loudness growth is a subjective measure. This report discusses the application of multi-band compression (MBC) to the compensation of loudness recruitment. This technology was “reinvented” at Bell Labs between 1983 and 1987 and is now sold by the ReSound Corporation. Our basic strategy with multi-band compression is to restore the normal dynamic range which is lost due to OHC misfunction. We describe why MBC works and what the hair cells do. We describe how the OHCs might act as compressors. Finally we review the history of the MBC hearing-aid development at Bell Labs