Mechanics of Hearing & How the Brain Processes Sound
Did you know where the smallest bones in your body are? They’re not in your fingers and toes; they’re actually in your ear, and they make up a part of the complicated mechanism that allows our ears to turn vibrations in the air into the sounds that we hear.
Sound is made up of molecules vibrating in patterns called waves. When I bang on a drum, the drum’s vibrating surface disturbs the air in patterns that, once they reach my ear, can be interpreted as a sound. Unlike light, which can travel in a vacuum through space, sound waves need material to travel–they need some sort of matter to disturb.
Sound waves that are shorter hit our ears in more rapid succession; they are said to have a higher frequency. Frequency is related to the idea of pitch in music, or the relative highness or lowness of a given sound. When an opera singer hits a high C, she is producing sound waves that are very short and with a high frequency. When a tuba plays a low C, its sound waves are much longer and with a lower frequency.
Loudness is related to another feature of the sound wave, called amplitude. Amplitude is basically the size, or height, of the sound wave. The bigger the wave, the louder the sound. You may have heard the term decibel ; that’s a scale, rather like degrees for temperature, for saying how loud something is. Unlike degrees, however, the decibel scale is logarithmic. This means that if one sound is 10 decibels louder than another, it’s actually ten times as loud. A 60 decibel conversation is ten times as loud as 50 decibel rainfall; a 110 decibel rock concert is ten times as loud as a 100 decibel snowmobile. A 90 decibel lawnmower is 100 times louder than a 70 decibel vacuum cleaner–ten times ten. The scale starts at the threshold of human hearing, so zero decibels represents the point at which a sound becomes so quiet that humans can’t detect it. Prolonged exposure to noises above 85 decibels can cause hearing loss, either by physically damaging the ear or by damaging the nerves that transmit signals to the brain.
Interpreting Where Sound Is Coming From
You know how when you hear a noise, you can usually tell where it’s coming from? This is because we have two ears, spaced apart on either sides of our heads. This means that if a sound is coming from the left, it reaches the left ear sooner than it reaches the right. While the difference in time might be really small, your brain automatically interprets this to help you determine where the sound is coming from.
Now that we’ve learned about sound, let’s take a look at how the ear processes sound and turns it into signals that can be interpreted by the brain. Sound first enters the ear and reverberates around the pinna, or folds of cartilage in the outermost part of the ear. Then it travels down the auditory canal, which amplifies the sound until it hits the eardrum. The eardrum rests up against the ossicles, which are those tiny bones we were talking about at the very beginning. There are three of them, and they help transform the sound from vibrations in the air to vibrations in the fluid inside the nearby cochlea. The cochlea looks kind of like a twisty seashell; it’s filled with fluid and with small hair cells that support bundles of cilia, small fibers that can sense vibrations in the fluid. These hair cells send nerve impulses to nearby neurons. These signals then travel down the auditory nerve and into the brain. There are a lot of parts of the ear to remember, so I like to use a kind of mean acronym to remember them: if you don’t know how hearing works, Please Exit Our Cool Crowd. Pinna, eardrum, ossicles, cochlea, cilia. This is the order in which soundwaves enter the ear and are processed; Please Exit Our Cool Crowd.
There are a few theories on how this mechanism transmits a sense of pitch. The frequency theory of hearing says that the neurons attached to the cilia will fire off at the same rate as the frequency of the sound entering the ear; so they fire off more quickly for a higher pitch sound with a short wavelength and more slowly for a lower pitch sound with a longer wavelength. This is an elegant theory but one that’s problematic when it comes to higher-pitched sounds; neurons actually can’t fire fast enough to match the frequencies of some high-pitched sounds humans definitely can detect. Another theory, called the place theory of hearing, takes care of this problem by proposing that different parts of the cochlea react to different frequencies of sound. So more neurons firing from the opening of the cochlea would indicate a higher pitch, and more neurons firing from the end of the cochlea would indicate a lower pitch.
So we’ve learned that sound travels in waves, and our ears process the vibrations these waves create and turn them into what we hear as noise. Higher pitched sounds have shorter wavelengths with higher frequencies, while lower pitched sounds have longer wavelengths and lower frequencies. Our ears transmit this information either by firing more neurons for higher pitched sounds or by firing neurons from different places inside the ear depending on the pitch. Loud noises can do damage to the ears, and our ear placement makes it possible to tell where sounds are coming from.
By the end of this lesson you’ll be able to:
- Explain how sound waves work
- Define frequency
- Understand how sound waves influence loudness
- Know how the brain processes sound
- Recall how we can figure out the direction of a sound
- Remember how different theories hypothesize the mechanics of different pitches on the ears