“Huh? What?”

The top 10 most commonly occurring words in English are “the,” “of,” “and,” “a,” “to,” “in,” “is,” “you,” “that,” and “it” (Burch, 2000). Yet, if you listen to people conversing—above the sounds of music, traffic, video games, washing machines, and other people conversing—it may seem like the most common words are “huh” and “what.” People often struggle to make themselves heard.

It’s remarkable that we can hear any meaningful sounds at all. The physical stimuli that reach your ears are sound waves. Sound waves are variations in pressure that travel through some substance, such as the air. Your auditory system can convert these variations in air pressure into signals that provide information about the world. The auditory system can create meaning from air pressure even when multiple sound waves occur simultaneously. So you shouldn’t always need to say, “Huh?”

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From an evolutionary perspective, auditory perception has great advantages. The visual system is sufficient to detect friends and foes, predators and prey—as long as they’re not hiding! The capacity to detect environmental threats and opportunities that are out of sight enhances organisms’ chances of survival.

Let’s look at the types of information the auditory system delivers. Then we’ll examine the biological mechanisms that make auditory perception possible.

When you listen, you perceive meaningful sources of sound: a friend’s hello; a fire truck’s siren; the rhythmic thump of a bass line. Yet psychologists who study perception generally focus not on the perception of specific sound sources, but on auditory qualities that accompany any source of sound (McLachlan & Wilson, 2010). We’ll look at four such qualities: loudness, pitch, timbre, and location of the sound source. Then we will examine psychological processes that enable you to identify what you’re listening to.

LOUDNESS. Sometimes you can hear a pin drop. Sometimes you can’t hear yourself think. Auditory experiences differ in loudness, the intensity, or strength, of an auditory experience. Loudness thus refers to something subjective: the degree to which a perceiver experiences a sound as intense (Plack & Carlyon, 1995).

Your psychological experience of loudness is determined primarily by the physical properties of sound waves. Sound waves with more physical energy are perceived as louder. Scientists measure the physical intensity of sound waves in decibels (dB); in other words, just as one measures physical length in meters, one measures the physical intensity of sound waves in decibels. Figure 5.35 shows the number of decibels associated with a range of sounds. On a decibel scale, a sound of 10 dB is 10 times louder than a 0-dB sound; a 20-dB sound has 10 times the intensity of 10 dB or 100 times the intensity of 0 dB; and so forth. (In the figure, the car, at 70 dB, thus is 10 times louder than the 60-dB conversation.)

Although the physical intensity of sound waves and perceptions of loudness are closely related, it is not an exact relationship. Two research findings show that physical intensity and psychological experience do not correspond precisely:

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PITCH. A second property of sounds is their pitch, the sound experience that we usually describe with the words “low” and “high.” The keys on the far left of a musical keyboard produce a “low-pitched” sound; as you move to the right side of the keyboard, the tone becomes increasingly higher in pitch.

The physical property of sound waves that produces variations in pitch is their frequency. The frequency of a sound wave is the number of vibrations that occur during any fixed period of time. As shown in Figure 5.36, waves of higher frequency produce the perception of higher pitch.

As with loudness, pitch is a psychological experience that is not determined solely by the physical sound waves reaching the ear. A perceptual illusion again illustrates how physical stimuli and psychological experience can diverge. The psychologist Roger Shepard (1964) created a series of tones—now known as Shepard Tones—that produce a bizarre psychological experience. When listening to these tones, people experience an infinitely rising scale; one after another, the tones appear to go up and up, each one higher than the last. Yet, physically, the sound waves do not become infinitely more frequent. Instead, the physical stimuli “circle back” on themselves, rather like—in the visual domain—the stairs in an M. C. Escher drawing that appear to go up and up and yet circle back to where they started (see the accompanying image).

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TIMBRE. A third sound quality is apparent if you compare the sound of a flute with the sound of a violin. Even when played at the same pitch and loudness, their sounds differ. You might say that the violin’s sound is “richer.”

This richness is part of the violin’s timbre (pronounced TAM-ber). Timbre is the “signature” of a sound that enables you to distinguish it from other sounds, even when they have the same pitch or loudness. Variations in timbre generally reflect variations in the complexity of physical sound waves. The sound quality of a violin is richer than that of a flute because the sound waves emanating from the violin are more complex (Rasch & Plomp, 1999).

In the everyday environment, timbre is critical to auditory perception (Menon et al., 2002). Consider your ability to recognize voices. In person and even on the phone, you can instantly recognize the voices of numerous friends and family members. The recognition is not based solely on loudness and pitch. Your phone is designed to deliver different voices at similar levels of loudness. Different friends may have voices whose pitch is similarly high or low, yet the quality of their voices—the voices’ timbre—differs, and this enables you to recognize them.

LOCATION. An animal in its natural environment that hears a predator wonders first and foremost, “Where is it?” Organisms need to hear not only the qualities of sounds—loudness, pitch, timbre—but also where those sounds are coming from. They need, in other words, to localize sounds: to identify the location of the sound source.

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To understand auditory localization, we must distinguish between two dimensions: (1) left/right and (2) up/down. [The third dimension, distance, is determined primarily by loudness (Coleman, 1963); sounds that are less loud are perceived as more distant.] A bit of reflection on human anatomy suggests that left/right localization might be easier because your ears are on the left and right sides—not the top and bottom—of your head.

If a sound comes from your left, and then another from your right, you’ll usually be able to localize the sounds. To provide left/right information, the auditory system capitalizes on an obvious fact: Your ears are on different sides of your head. Sound waves thus travel different routes to reach one versus the other ear (Middlebrooks & Green, 1991). They travel directly to the ear on the near side, but must also make their way around your head to reach the ear on the far side. Two cues to localization result from the layout of your head and ears: timing and pressure. For a sound from your left:

Your auditory system automatically processes and combines these cues, which makes left/right sound localization easy.

The same cues, however, can make another type of sound localization difficult: front/back localization. Suppose two identical sounds come from two locations—directly in front and in back of you. The sounds have the same location on the left/right dimension (in the middle). Their timing and the air pressure they exert are the same at your left and right ears. When people hear such sounds, they often experience front/back confusions: They think a sound came from in back of them when, in reality, it came from in front (or vice versa; Middlebrooks & Green, 1991). You may have experienced this when driving and hearing the distant sound of a siren (e.g., from an ambulance or fire truck); it can be hard to tell if the sound is coming from in front of you or behind.

What about up/down sound localization? Suppose that two sounds come from straight in front of you, one from up in the air and the other from down on the ground. It might seem impossible to distinguish them, since their timing and pressure at the two ears are identical. Yet people often can make up/down distinctions accurately (Middlebrooks & Green, 1991). How? The key factor is the shape of the ear, which affects the physical signals that make their way into your nervous system. This effect differs, depending on whether sound waves enter the ear from above or below, because your ear is not symmetrical from top to bottom; that is, the shape of the top and bottom parts of your ear do not match. Sound waves thus “bounce around” the ear in different manners, depending on whether they enter from the bottom or the top. Such differences can enable people to perceive whether a sound came from above or below.

WHAT’S MAKING THE SOUND? In addition to loudness, pitch, timbre, and location, you want to know what’s actually making the sound. Was that cry from a baby or a cat? Was the rumble thunder or an explosion? These are questions of auditory recognition, the identification of a sound’s source.

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In general, people are quite good at auditory recognition. In one study (Gygi, Kidd, & Watson, 2004), participants heard 70 sounds ranging from A (airplane flying) to Z (a zipper). Each sound was presented for just a few seconds. Many were uncommon (e.g., the sound of a helicopter). Yet participants recognized stimuli accurately the majority of the time.

To recognize sounds, people need preexisting knowledge. Without it, recognition is impossible; if you aren’t already familiar with the Indian Purple Frog (which sounds a bit like a chicken), you won’t be able to identify it no matter how loudly it croaks. To recognize a sound, people must maintain the sound in short-term memory, activate knowledge of sound stored in long-term memory (see Chapter 6), and compare the two (McLachlan & Wilson, 2010).

Now let’s look at the biological machinery that makes auditory perception possible. You won’t be surprised to hear that the story begins at the ear.

AUDITORY PROCESSING IN THE EAR. In everyday language, “ear” refers to those visible structures appended to the sides of your head. But in the study of auditory processing (Moore & Linthicum, 2004), the term ear refers to a complex biological mechanism with three overall sections: an outer ear, middle ear, and inner ear (Figure 5.37).

Sound waves first reach the pinna, the portion of outer ear that captures sound waves and directs them down the auditory canal to the eardrum. The eardrum is a thin membrane that vibrates when struck by sound waves. Sound waves of different intensity and magnitude create different patterns of vibration at the eardrum.

The eardrum’s movement activates mechanisms in the middle ear. Specifically, vibrations of the eardrum cause motion in an interconnected series of small bones in the middle ear called ossicles. Motion of the ossicles, in turn, creates activity in the inner ear, where transduction—the conversion of sound wave vibrations into neural signals—occurs.

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Transduction occurs within a spiral-shaped, fluid-filled body in the inner ear called the cochlea. The cochlea contains auditory hair cells, which are the auditory receptor cells responsible for transduction (Gillespie & Müller, 2009). The hair cells get their name from their shape (Figure 5.38). Motion of the ossicles causes vibration of the cochlea’s fluid, which in turn causes the hair cells to move. Finally, the movement of the hair cells sets off neural impulses that travel to the brain.

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THE AUDITORY CORTEX. Auditory signals leave the inner ear along the auditory nerve, a bundle of thousands of nervous system cells that originates at the cochlea and carries auditory information to the brain. As with the optic nerve, there is a crossing of nerve fibers, with signals from the left ear reaching the right side of the brain, and vice versa.

The high-level processing required to make sense of signals from the cochlea is done in the auditory cortex. The auditory cortex is a brain region located on the temporal lobe (see Chapter 3) that is primarily devoted to the processing of auditory information. It takes up about one-eighth of the overall cortical surface of the brain (Woods & Alain, 2009).

The auditory cortex is therefore a substantial brain region. It contains tens of millions of neurons. Does this brain matter have any overall organization? Recall that the visual cortex is organized systematically; physical stimuli that strike neighboring areas of the retina are processed by neighboring areas of the visual cortex (Figure 5.31). The auditory cortex is similar, except that the organization is based on pitch (Wessinger et al., 1997). Sounds that are similar in pitch—that is, sounds whose waves have similar frequency (Figure 5.36)—are processed in neighboring areas of the auditory cortex (Figure 5.39). Sounds that differ greatly in pitch are processed in locations farther apart (Woods & Alain, 2009).

The auditory cortex deals with other information in addition to pitch. Brain-imaging studies with human and nonhuman primates have identified specialized regions of the cortex that process information about (1) the location of sounds, (2) the identification of the source of a sound, and (3) vocalizations by members of one’s own species (Rauschecker & Tian, 2000).