# Vision senses electromagnetic energy

Vision senses electromagnetic energy– light–while hearing senses mechanical energy in the form of sound. Sound waves must beat upon the eardrum,  and its vibrations must be transmitted across tiny bones called the ossicles, to

set fluid in the cochlea vibrating.  When hair cells within the cochlea pick up the vibrations and stimulate fibers of the auditory nerve—transduction!—we hear sound. You might visit this sitefor a quick review of the ear.

Now two big questions of psychophysics arise: How do sounds vary in intensity and loudness? How do they vary in frequency and pitch?

Sound & Decibels

First we want to understand how our sensory loudness varies with sound intensity. We can measure sound intensity as air pressure changes, since sound is alternating waves of compression and rarefaction. We would be dealing with fractions of a Pascal, which is a unit of pressure. However, the threshold for hearing is so low that it’s near the pressure of sunlight on the eardrum or the random motion of molecules because of heat. At the other end, we can hear sounds so intense that they may rupture cells in the ear–altogether, a pressure range of about 10,000,000,000,000 to 1. That’s at least thirteen powers of ten. In any case, rather than using such large numbers, it’s easier to count one unit of sound intensity for each power of ten added to the air pressure. Letting units stand for powers of ten is a logarithmic scale, the Bel (or more usefully, tenths of Bels called decibels). And how big is a decibel? Just click on a triangle or “enter” or scroll down to the green bars to find out.

There was just a bit of sleight-of-hand there. How did we go from Pascals to decibels? There are several ways of measuring sound. It’s common to express the threshold of hearing as 0.0002 dynes/cm2. But that’s a unit of force. Psychologists stopped expressing the threshold in force units about 40 years ago because pressure is what we’re really talking about; pressure is force per unit area. So our hearing threshold is now 20 μPascals, or simply 20 μPa. Acoustical engineers talk about sound pressure level, or SPL. It’s a somewhat arbitrary pressure that corresponds to an average youngster’s threshold. We measure sound levels above or below that in decibels. In physics, a 10-decibel increase means a tenfold increase in sound pressure. With SPL, a 20-dB increase represents a tenfold increase in pressure.

Decibels take some getting used to. You can ratchet the discussion down a notch in complexity at this earlier site or up a notch at this website if you wish.

Either way, it’s useful to acquaint yourself with how sound works in hearing. Hearing is basically a mechanical sense like touch and balance. You can find a brief account here or an overall review in this excellent presentation (optional, of course). The former lasts 6 1/2 minutes and the latter almost two hours, but even the long one is worth the time, if you have it. It covers disorders as well as normal function, and why hearing aids will be cool to wear in a couple of years. Just skip the first eight minutes of introduction!

The physical intensity of a sound registers as the subjective impression of loudness, while a sound’s frequency creates the impression of pitch.

Two Mechanisms for Pitch Discrimination

How do we tell one sound frequency from another? We do it by telling the brain either which nerve fibers in the cochlea are firing or how the fibers are firing. The latter process uses phase locking.

Phase-Locking

Phase-locking is a mechanism for responding to higher sound frequencies that all tend to stimulate the apical end of the basilar membrane. One nerve fiber can’t respond thousands of times per second to, say, a 5,000 Hz tone, but many fibers acting together can! If each fiber in the eighth cranial nerve responds to a different sound wave in a series (say the second or fifth wave) and to a particular part of that wave (say the rising part or the trough), the pattern of response among all of the nerve fibers will be distinct for each frequency. On that basis the brain can distinguish among sound frequencies.

In other words, phase refers to the part of a sound wave to which a nerve fiber responds (indirectly, of course, since it has to be stimulated by hair cells). The time of stimulation doesn’t matter. It’s the phase that is important. Here’s an analogy: If you always go to bed at 10 p.m., your sleep is time-locked to each day-night cycle. If you travel to Tokyo or Moscow you will find yourself trying to sleep at odd times of the day that correspond to 10 p.m. stateside. Instead, if you always go to bed when it is sunset, your sleep is phase-locked to each day-night cycle. If you move to Tokyo or Moscow, your sleep won’t be as badly disrupted.

Music

Musical ability appears early. Peretz and Zatorre mention that infants begin to sing by their first birthday and reach competence by five. Music appears as a social interaction; it may support attachment and contribute to group bonding.

It is hard to say what distinguishes music from other sounds, but we can usually tell the difference. There are good sounds and bad sounds, and then there is music. You can watch a video about the physics of music, but it won’t tell you what music is, though music’s resemblance to speech may provide a clue, since a kind of music is present in speech when its ups and downs occur in minor thirds.

Melody

Melody is the part of music in which the pitch changes, note after note. We can change the pitch by varying the resonance of a musical instrument or the human voice. (You can review resonance at sites onetwothreefour, and five.)

Not surprisingly, your ability to appreciate melodies depends on your ability to discriminate different pitches. (Test yourself here (cancel the sign-in) or there.) Closely related to melody is harmony, or the pleasantness of different pitches in combination.

One challenge for psychology is to explain why one melody is more pleasant than another. This is partly a matter of consonance. Dissonant pitches may draw our attention or be unpleasant, but they are not constant across cultures. Cultures differ in the musical scales they use, which tell the listener what pitches to expect in a melody. If you doubt this predictive value of a scale, watch this amazing demonstration of the pentatonic scale (3 minutes). Melodies from other cultures tend to sound out of tune or off-key. Exposure to different kinds of music has good effects if it’s started early; later, it alienates. We don’t like music of older or younger people, either. Millennials aren’t keen on classical music, as a rule, and some people just don’t like music at all.

Of course our attitudes have changed over time as well; for example, we no longer fear the Devil’s Interval*.

There are individual variations in the ability to respond to melodies that also vary with culture. Absolute pitch provides one example. It is rare in Western cultures, though not beyond learning. Its appearance varies with culture and language. Whereas the average Westerner might distinguish just the do-re-mi notes of a scale, the person with absolute pitch may discriminate among 70 pitches and be able to identify by letter each note of the scale, no matter which instrument plays it.

Rhythm

Besides melody, the other important part of music is the rhythm, which is time-based. The rhythm is the beat, while tempo is the overall speed, or the number of beats per minute. Some writers have played up the resemblance between a musical beat and the human pulse rate (about 72 per minute) as significant; but rhythm might have other origins. As with melody, the listener’s expectations are an important part of our enjoyment of rhythm. After the predictable rhythms of centuries-old dances like the Schottische or a minuet, the syncopation of ragtime is a refreshing surprise, when a note appears earlier than we expected it. (Does syncopation improve this lullaby?) Even more complicated rhythms arise in Latin music such as salsa (continued here), enlivened by an Afro-Cuban clave rhythm that may sound complicated to northern ears. Clap your hands as you listen to the clave beat to test your understanding of it.

Beyond generating (and violating) expectations, an amazing feature of rhythm is its ability to synchronize our movement, which is sometimes called entrainment. The effect is so powerful that it raises a chicken-and-egg question: Is music the cause or the result of our desires—to move in synchrony, to bond, and thereby to feel intense emotion?

Are North Americans rhythm-challenged**? There’s one test of rhythm here and a limited test of rhythm memory at this site if you scroll to the bottom.

Putting It Together

The psychology of music has a long history with few significant discoveries. We still don’t know for sure what role music plays in behavior. This topic began with the link between personality and music. The evidence is limited and uncertain and these kinds of correlations reveal little of the way music works in our lives. You are welcome to test yourself further at this site.

Speech Perception

You can quickly review the subtopic of speech perception with try-it-yourself demonstrations at this site. Three phenomena worth knowing about are the following:

• The segmentation problem of distinguishing one word from another. Listen to yourself say “We were away a year ago.” Now look at a sitethat shows such a sentence’s sound pattern. (It’s near the bottom of the Web page.) There are no word boundaries! This is typical of our speech.
• Categorical perception
• The phonemic restoration effect

1. There can’t be sounds without frequencies, but are there sounds without pitch?
2. As you know, one important feature of sound is its frequency. Are you able to hear the 10,000 Hz sound?

Now put frequency and amplitude together to find out which frequencies you are most sensitive to at this website. Try to judge the frequencies to which you are most sensitive and share the result with us. You might want to confirm your results at this site. How did you make your decision?

1. Going through the following links in order, would you say that you have perfect musical memory? Perfect rhythm perception? Perfect pitch discrimination? Perfect (absolute) pitch? What turned out to be your strongest auditory skill, and why?
1. Music isn’t always entertainment. In surgery and religion what is more important: melody or rhythm (or both)? Why?

*You’ve heard it in the music of Black Sabbath and other groups. Or listen to the first two notes of the Simpson’s theme or play C followed by F♯ (G♭) on this piano keyboard.

**Unstable web page. It seems to stabilize when you use this link and right-click on the page as if to “save” it, then hit “cancel” to return to the page.