The Physics of Hearing

Physics of the Body.

My hearing is not what it used to be. It’s not terrible now; I still get along okay. But I find myself asking my wife “what?” a lot. So, I borrowed a copy of Physics of the Body—by John Cameron, James Skofronick, and Roderick Grant (1999)— and read Section 11.9: Deafness and Hearing Aids. It covers much of the information that Russ Hobbie and I discuss in Chapter 13 of Intermediate Physics for Medicine and Biology, and more. Below I quote a paragraph from Physics of the Body, with references removed and my comments in brackets.

In 1985 it was estimated that 21 million persons in the United States were either deaf or hard of hearing. The frequency range most important for understanding conversational speech is from about 250 to 3000 Hz. [The figure below shows the hearing response curve for a young adult from IPMB, with the range from 0.25 to 3 kHz shaded.] A person who is “deaf” above 4000 Hz but who has normal hearing in the speech frequencies is not considered deaf or even hard of hearing. [I was taking a walk with my daughter Kathy a few months ago. As we passed one house she grimaced said “what is that terrible high-pitched noise?” I said “what noise?”] However, that person should not spend a lot of money on good stereo equipment. [Music sounds the same now as I remember it back when I was a teenager. Nevertheless, I need no additional encouragement to not spend money; I’m a cheapskate.] Hearing handicaps are classified according to the average hearing threshold at 500, 1000, and 2000 Hz in the better ear [I haven’t noticed any difference between my left and right ears]. A person with a hearing threshold 30 dB above normal would probably not have a hearing problem. People with hearing thresholds of 90 dB are considered deaf or stone deaf. [According to Table 13.1 in IPMB, 30 dB is the “maximum background sound level tolerable in a broadcast studio” and 90 dB is the sound “inside a motor bus.” I am definitely not stone deaf. Is my threshold below 30 dB? I don’t know.] About 1% of the population have thresholds for speech frequencies greater than 55 dB [IPMB says 55 dB is between “office” sounds and “speech at 1 m.” I definitely can hear speech at 1 m. It’s speech from my wife calling from another room that I have trouble with.] and should use hearing aids [Both my mom and dad used hearing aids when they got older. My health has generally parallels my father’s. I fear it is just a matter of time]. About 1.7% have a slight hearing handicap; they have problems with normal speech but have no difficulty with loud speech [I think I am better than that]. Hearing problems increase with age [Yes, that’s the problem. I’m getting old.]

The average sound level of speech is about 60 dB. We adjust the sound level of our speech unconsciously according to the noise level of our surroundings. Speech sound levels in a quiet room may be as low as 45 dB; at a noisy party they may be 90 dB. A person with a hearing loss of 45 dB in the 500 to 2000 Hz range may do all right (hearing-wise) at a cocktail party but hear very little of speech in a quiet room. [I don’t know. It seems to me that I have more of a problem distinguishing speech from background sounds than just hearing speech. I suspect I would do worse at the cocktail party even with people speaking at 90 dB than chatting at 45 dB in the quiet room.]

Now that I’m on Medicare, my wife is encouraging me to have my hearing checked. I suppose I should.