LO 7 Summarize how sound waves are transduced into the sensation of hearing.

The wind howling. A baby crying. Raindrops pelting the roof of a car. These sounds are perceived so differently, yet they all begin with the same type of stimulus: a sound wave moving through the environment. When we use our sense of hearing, or audition, we are detecting sound waves that enter our ears. Sound waves are rhythmic vibrations of molecules traveling through air or other materials, like water, metal, or wood.

SOUND WAVES Every sound wave begins with a vibration. That could mean a pulsating loudspeaker, a quivering guitar string, or vocal chords fluttering in your throat. When an object vibrates, it sends a pressure disturbance through the molecules of the surrounding medium (usually air). Imagine a very quiet environment, such as the inside of a parked car, where the air molecules are evenly distributed for the most part. Now turn on the radio, and the membrane of the amplified speaker immediately bulges outward, pushing air molecules out of its way. This creates a zone where the particles are tightly packed and the pressure is high; the particles directly in front of the speaker will hit the particles right in front of them, and so on throughout the interior of your car. As the speaker retracts, or pulls back, it gives the air particles ample room to move, creating a zone where particles are spread out and pressure is low. As the speaker rhythmically pushes back and forth, it sends cycles of high pressure waves (with particles “bunched up”) and low pressure waves (with particles “spread out”) rippling through the air (Ludel, 1978). It’s important to note that molecules are not being transmitted when you speak to a friend across the room; air particles from your mouth do not travel to his ears, only the sound wave does.

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Now that we have established what sound waves are—alternating zones of high and low pressure moving through the environment—let’s address the immense variation in sound quality. Why are some noises loud and others soft, some shrill like a siren and others deep like a bullfrog? Sounds can be differentiated by three main qualities: loudness, pitch, and timbre.

LOUDNESS Let’s begin by exploring loudness. We sense the loudness of a sound based on the amplitude, or height, of the wave it generates. A kitten purring generates low-amplitude sound waves; a NASCAR engine generates high-amplitude sound waves. Just remember: The taller the mound, the louder the sound. Loudness is measured in decibels (dB), with 0 dB being the absolute threshold for human hearing; normal conversation is around 60 dB. Noises at 140 dB can be instantly detrimental to hearing (Yost, 2007). But you needn’t stand next to a 140-dB jet engine to sustain hearing loss, as Figure 3.2 illustrates. Prolonged exposure to moderately loud sounds such as traffic and loud music can also cause damage.

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PITCH The pitch of a sound describes how high or low it is. An example of a high-pitched sound is a flute at its highest notes; a low-pitched sound is a tuba at its lowest notes. Pitch is based on wave frequency. We measure frequency with a unit called the hertz (Hz), which indicates the number of wave peaks passing a given point in 1 second. If you are hearing a 200-Hz sound wave, then theoretically 200 waves enter your ear per second. A higher-pitched sound will have a higher frequency of waves; the time between the “bunched up” particles and the “spread out” particles will be less than that for a lower-pitched sound. Humans can detect frequencies ranging from about 20 Hz to 20,000 Hz, but we tend to lose the higher frequencies as we get older. Table 3.1 shows the range of frequencies that can be detected by various species. And just remember, faster peaks mean higher squeaks; waves moving slow make pitch sound low.

When Emma, Zoe, and Sophie began to go deaf, they lost their ability to hear high frequencies first, which means they stopped hearing high-pitched sounds, like female voices. It pains Liz to reminisce about this period because she wonders how her babies felt when it seemed like their mom had stopped talking to them.

TIMBRE We know that sounds can have different levels of loudness and pitch, but how is it possible that two sounds with similar loudness and pitch (Beyoncé and Ariana Grande belting the same note at the same volume) can sound so different? The answer to this question lies in their timbre. The texture of a sound, or timbre (ʹtam-bər), describes its unique combination of frequencies. Most of the “racket” you hear on a regular basis—people’s voices, traffic noises, humming air conditioners—is made up of many frequencies. Timbre is the reason we can tell the difference between two sounds with exactly the same pitch and loudness.

We have outlined the basic properties of sound. Now let’s learn how the ears transform sound waves into the language of the brain. As you will see, your ears are extraordinarily efficient at what they do—transducing the physical motion of sound waves into the electrical and chemical signals of the nervous system (Infographic 3.3).

FROM SOUND WAVE TO BONE MOVEMENTS What exactly happens when a sound wave reaches your ear? First it is ushered inside by the ear’s funnel-like structure. Then it sweeps down the auditory canal, a tunnel leading to a delicate membrane called the eardrum, which separates the outer ear from the middle ear. The impact of the sound wave bouncing against the eardrum sets off a chain reaction through the three tiny bones in the middle ear: the hammer, anvil, and stirrup. The chain reaction of the tiny bones moving each other amplifies the sound wave, turning it into a physical motion that has great strength. The hammer pushes the anvil; the anvil moves the stirrup; and the stirrup presses on a drumlike membrane called the oval window leading to the ear’s deepest cavern, the inner ear. It is in this cavern that transduction occurs.

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FROM BONE MOVEMENT TO MOVING FLUID The primary component of the inner ear is the cochlea (kō-klē-ə), a snail-shaped structure filled with liquid. The entire length of the cochlea is lined with the basilar membrane, which contains about 16,000 hair cells. These hair cells are the receptor cells for sound waves, which are transformed into liquid waves inside the cochlea. When the stirrup pushes on the oval window (a result of sound waves entering the ear), fluid in the cochlea vibrates and the hair cells bend in response. Below the base of the hair cells are dendrites of neurons whose axons form the auditory nerve (Ludel, 1978). If a vibration is strong enough in the cochlear fluid, the bending of the hair cells causes the nearby nerve cells to fire. Signals from the auditory nerve pass through various processing hubs in the brain, including the thalamus, and eventually wind up in the auditory cortex, where sounds are given meaning. Let’s summarize this process: (1) Sound waves hit the eardrum, (2) the tiny bones in the middle ear begin to vibrate, (3) this causes the fluid in the cochlea to vibrate, (4) the hair cells bend in response, initiating a neural cascade that eventually leads to the sensation of sound.

COCHLEAR IMPLANTS The cochlea is an extremely delicate structure. If the hair cells are damaged or destroyed, they will not regrow like blades of grass (at least in humans and other mammals). So when Emma, Zoe, and Sophie lost their hearing around age 2, there was no magical drug or surgical procedure that could restore their hearing. Hope arrived about a year later, when the triplets were outfitted with cochlear implants, electronic devices that help those who are deaf or hard-of-hearing. These “bionic ears,” as they are sometimes called, pick up sound waves from the environment with a microphone and turn them into electrical impulses that stimulate the auditory nerve, much as the cochlea would do were it functioning properly. From there, the auditory nerve transmits electrical signals to the brain, where they are “heard,” or interpreted as human voices, hip-hop beats, and dog barks.

Cochlear implants have enabled many people to understand language and appreciate music, but “hearing” with a cochlear implant is not exactly the same as hearing with two ears. Voices may sound computerized or Mickey Mouse-like (Oakley, 2012, May 30). Liz had no idea how the girls would respond to their first experience hearing with the implants. They reacted the same way they do to most everything in life—each in her own way. Zoe broke into a huge grin; Emma sat still, just listening; and Sophie started crying (Hudson & Paul, 2007).

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CONTROVERSIES

You might be wondering why the triplets were provided with cochlear implants, not hearing aids. Hearing aids work by increasing the amplitude of incoming sound waves (making them louder) so the hair cells in the inner ear can better detect them. If hair cells are damaged beyond repair, as was the case for Emma, Zoe, and Sophie, then the inner ear cannot pass along messages to the brain. There will be no sensation of hearing, no matter how much you turn up the volume.

We know how sound waves are transformed into auditory sensations, but there are many types of sounds. How does the brain know the difference between the yap of a Chihuahua and the deep bark of a Rottweiler, the sweet song of a flute and the bellow of a tuba? In other words, how do we distinguish between sound waves of high and low frequencies? There are two complementary theories that explain how the brain processes pitch.

LO 8 Illustrate how we sense different pitches of sound.

PLACE THEORY According to place theory, the location of neural activity along the cochlea allows us to sense the different pitches of high-frequency sounds. Hair cells toward the oval-window end of the basilar membrane vibrate more to higher-frequency sounds. Hair cells toward the opposite end of the basilar membrane vibrate more in response to lower-frequency sounds. The brain determines the pitch of high-frequency sounds by judging where along the basilar membrane neural signals originate.

Place theory works well for explaining higher-pitch sounds, but not so well for lower-pitch sounds. This is because lower-frequency sounds produce vibrations that are more dispersed along the basilar membrane, with less precise locations of movement.

FREQUENCY THEORY To understand how humans perceive lower pitches, we can use the frequency theory, which suggests it is not where along the cochlea hair cells are vibrating, but how frequently they are firing (the number of neural impulses per second). The entire basilar membrane vibrates at the same frequency as a sound wave, causing the hair cells to be activated at that frequency, too. The nearby neurons fire at the same rate as the vibrations, sending signals through the auditory nerve at this rate. If the sound wave has a frequency of 200 Hz, then the basilar membrane vibrates at 200 Hz, and the neurons in the auditory nerve fire at this rate as well.

Neurons, however, can only fire so fast, the maximum rate being about 1,000 times per second. How does the frequency theory explain how we hear sounds higher than 1,000 Hz? According to the volley principle, neurons can “work” together so that their combined firing can exceed 1,000 times per second. Imagine a hockey team practicing for a tournament. The coach challenges the players to fire shots on goal at a faster rate than one individual. The team members get together and decide to work in small groups and alternate shots on the goal. The first group skates and shoots, and as they are skating away from the goal to return to the back of the line, the next group takes their shots. Each time a group is finished shooting, the next group is ready to shoot. Neurons grouped together in the same way will fire in volleys; as one group of neurons finishes firing and is “recovering,” the next group will fire. The frequency of the combined firing of all groups results in one’s perception of the pitch of the sound.

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ALL TOGETHER NOW Let’s draw this all together. Human beings can hear sounds ranging in frequency from 20 to 20,000 Hz. The frequency theory explains how we perceive sounds from 20 to 400 Hz; the volley principle covers the 400- to 4,000-Hz range; and the place theory accounts for frequencies from 4,000 to 20,000 Hz. At times, both place theory and the volley principle can be used to explain perceptions of sounds between 1,000 and 4,000 Hz.

We have now examined what occurs in a hearing system working at an optimal level. But many of us have auditory functioning that is far from perfect.

At least one third of people over age 65 and half of those over 80 have difficulty hearing (Desai, Pratt, Lentzner, & Robinson, 2001; Gordon-Salant & Callahan, 2009). At the onset of hearing loss, high frequencies become hard to hear, making it difficult to understand certain consonant sounds such as the “th” in thumb (American Speech-Language-Hearing Association, 2012). Everyone experiences some degree of hearing loss as they age, primarily resulting from normal wear and tear of the delicate hair cells. Damage to the hair cells or the auditory nerve leads to sensorineural deafness. Conduction hearing impairment results from damage to the eardrum or the middle-ear bones that transmit sound waves to the cochlea.

But it’s not just older adults who ought to be concerned. One large study found that about 20%, or 1 in 5, of American teenagers suffer from some degree of hearing loss, and the problem appears to be on the rise (Shargorodsky, Curhan, Curhan, & Eavey, 2010). Exposure to loud sounds, such as listening to music at high volume through earbuds, may play a role. A study of Australian children determined that using personal stereo devices increased the risk of hearing loss by 70% (Cone, Wake, Tobin, Poulakis, & Rickards, 2010).

As you know from the triplets’ story, certain medications can also harm the fragile structures of the ear, as can ear infections, tumors, and trauma. Some babies (between 1 and 6 of every 1,000) are born with severe to profound hearing loss (Cunningham & Cox, 2003). How do you think those babies adjust to their hearing deficit? How does anyone with a sensory weakness cope? They find ways to compensate, and often that means fine-tuning other sensory systems. Case in point: Zoe and her exquisite sense of smell.

Question 1

1. frequency

2. timbre

3. amplitude

4. purity

Question 2

1. the volley principle.

2. transduction.

3. the frequency theory.

4. audition.

Question 3

Question 4