- For the average person, what is the lowest volume of a pure tone at each frequency they can hear as a function of age?
- I know some people lose hearing faster than other people, because they are exposed to loud sounds. So I am also interested in hearing loss induced by loud sounds as a function of frequency.
- The brain can adapt to the hearing loss, thereby creating the illusion that a sound is just as loud as if nothing happened to their hearing. For that reason, I also wish to know what volume the ear transmits to the brain as a function of frequency and age.
- Hearing sensitivity deteriorates with age, which is called age-related hearing loss, or presbyacusis. Presbyacusis is characterized by elevated hearing thresholds. Especially the high frequencies are affected by age, but eventually even the low frequencies may deteriorate substantially. Hearing sensitivity is typically expressed in an audiogram, which depicts the hearing threshold as a function of acoustic frequency. To express hearing loss as a function of age, however, it is more convenient to plot the threshold at various frequencies as a function of age (Fig. 1).
Fig. 1. Hearing thresholds at various frequencies plotted as a function of age. source: Handbook For Acoustic Ecology.
- Exposure to loud sounds may be detrimental to hearing, referred to as noise-induced hearing loss. Typically this results in a loss of sensitivity at the moderate frequencies (3k to 4k); a mid-frequency dip (Fig. 2). Old age will add a high-frequency hearing loss on top of that.
Fig. 2. Typical audiogram associated with noise-induced hearing loss. source: American Hearing Research Foundation
- The brain will not adapt to elevated thresholds. A threshold represents the minimal intensity level a sensory system can detect. Elevated hearing thresholds due to old age and noise-induced hearing loss are caused by sensori-neural hearing loss, meaning the hair cells in the inner ear (the cochlea) are dying. The brain can never detect a signal that is not transmitted via the auditory nerve and can hence never compensate for elevated thresholds. In turn, when thresholds are elevated substantially, a person suffering from such a hearing impairment may ask you several times kindly to repeat what you were saying, but a little louder. Then, when you start screaming after the third time, the hearing-impaired person returns the dreaded "You don't have to scream at me!". That is because the brain does not adapt to the elevated thresholds; once the sound is transmitted by the degenerated cochlea, it is transmitted at that loudness and perceived at that loudness by the brain. Moreover, a process of recruitment can occur in the hearing impaired, where the perceived loudness is exaggerated. Recruitment may occur at the hair cell level, but it is a not well characterized phenomenon.