The physics of hearing and other odds and ends

The range of human hearing The range of average human hearing is from about 15Hz to about 15,000Hz. Though it is generally true that frequencies in the middle of the range can be detected at lower volumes, by most people, than the frequencies at the top and bottom of that range. It is also generally true that people lose the ability to detect the extreme ends of the range, particularly the higher end, as they age. This is partially the result of exposure to loud noise and partially the result of the normal aging process.

Hearing ranges of other animals Cats can hear frequencies from 45 Hz to 64,000 Hz Dogs can hear frequencies from 40 Hz to 60,000 Hz Bats can hear frequencies from 20 Hz to 120,000 Hz Mice can hear frequencies from 1000 Hz to 70,000 Hz

How the ear works The outer ear acts to collect and direct the sound into the ear canal. At the end of the ear canal is the ear drum, which is vibrated in and out by the compressions and rarefactions of the incoming sound wave.

The Middle Ear Three tiny bones, the malleus, incus and stapes, transmit and amplify the vibrations from the ear drum to the inner ear.

The Inner Ear The inner ear primarily consists of the cochlea. For simplicity, this can be thought of as one long tube. Inside the tube are two membranes, called the basilar membrane, attached with fibers, like a ladder. These fibers have different lengths, and therefore resonate at different frequencies.

How does the message get to the brain? Along the basilar membrane, and all throughout the cochlea, are little hairs that wiggle more when a specific fiber near them is vibrating. Each of these hairs sends a signal along a different nerve path to a different part of the brain. Thus, we can tell one tone from another. Louder sound vibrate the fibers more, wiggling the hairs more and sending a stronger signal to the brain.

The Decibel Scale Depending on the person and the frequency, the lowest volumes the human ear can detect are around 0dB. Each rise of 10dBs mean a 10-fold increase in amplitude. This means that 20dB is 10 times louder than 10dB. So, how many times louder is a 50dB sound than a 20dB sound?

Loudness of various environments

Loudness and Hearing Loss The threshold for potential hearing loss is 85dB. Prolonged exposure to noise over 85dB will cause eventual hearing loss. Sound over 140dB can cause instantaneous damage to your ear. USE EAR PLUGS WHEN YOU GO TO CONCERTS!!!! YOU’LL NOTICE YOU CAN ACTUALLY HEAR THE MUSIC BETTER WITH THEM IN.

How does loudness damage hearing? Repeatedly bending the hairs in your cochlea an extreme amount can cause them to become bent permanently. This causes hearing loss in the frequency range that those hairs transmit for and cause tinnitus, an annoying constant perception of a ringing tone that never goes away that isn’t caused by an actual noise.

More on loudness and hearing loss Prolonged exposure can also cause eventual damage to the auditory nerve and other parts of the ear. Extremely loud noise can cause the ear drum to rupture. Remember that sound is a wave made up of fluctuating air pressure. The louder the sound, the more air pressure there is at the compressions of that wave. That high pressure can push so hard on the ear drum, that it will pop like a balloon.

How does a speaker work? Sound is made by a periodic vibration that is pushing on the air. A speaker is basically a diaphragm connected to an electromagnet. Below the electromagnet is a large round permanent magnet. The audio device transmits an electric current to the electromagnet that causes it to switch back and forth between being positively charged and negatively charged. By controlling the frequency at which that switching charge happens, the device also controls the frequency at which the diaphragm vibrates.

Question to ponder What do you think is the difference in the anatomy of the ears cats, dogs, mice, bats and other animals, that makes them able to hear more frequencies and also have a better ability to distinguish between different frequencies than humans can? Look over your notes and write a paragraph explaining your theory.