(a). Before the bell is struck, the air around it is a region of average pressure (b). Once the bell is struck, however, the vibrating edge creates regions of high and low pressure. The dark areas represent regions of higher pressure; the light areas represent regions of lower pressure

(a). The pressure of the air rises and falls as the sound wave propagates through the atmosphere (b). You can use a sine curve alone to model changes in pressure. (c). Note that the positions of x, y, and z show that the wave, not matter, moves forward. These points remain in the same horizontal position.

Sound travels more quickly through more dense materials – as the energy passes more quickly from molecule to molecule if the molecules are closer!

The human ear is a complex sense organ that translates sound vibrations into nerve impulses that are sent to the brain for interpretation. The malleus, incus, and stapes are the three bones of the middle ear that sometimes are referred to as the hammer, anvil, and stirrup.

Perceiving Sound Pitch: Marin Mersenne and Galileo first determined that the pitch we hear depends on the frequency of vibration. Pitch can be given a name on the musical scale. For instance, the middle C note has a frequency of 262 Hz. The ear is not equally sensitive to all frequencies. Most people cannot hear sounds with frequencies below 20 Hz or above 16,000 Hz. Older people are less sensitive to frequencies above 10,000 Hz than are young people.

Loudness Frequency and wavelength are two physical characteristics of sound waves. Another physical characteristic of sound waves is amplitude. Amplitude is the measure of the variation in pressure along a wave. In humans, sound is detected by the ear and interpreted by the brain. The loudness of a sound, as perceived by our sense of hearing, depends pri­marily on the amplitude of the pressure wave.

The human ear is extremely sensitive to pressure variations in sound waves, which is the amplitude of the wave. Recall from Chapter 13 that 1 atmosphere of pressure equals 1.01 x 10 5 Pa. The ear can detect pressure-wave amplitudes of less than one- billionth of an atmosphere, or 2 x10 -5 Pa. At the other end of the audible range, pressure variations of approximately 20 Pa or greater cause pain and will damage hearing. It is important to remember that the ear detects only pressure variations at certain frequencies. Driving over a mountain pass changes the pressure on your ears by thousands of pascals, but this change does not take place at audible frequencies.

Because humans can detect a wide range in pressure variations, these amplitudes are measured on a logarithmic scale called the sound level. The unit of measurement for sound level is the decibel (dB). The sound level depends on the ratio of the pressure variation of a given sound wave to the pressure variation in the most faintly heard sound, 2 x Pa. Such an amplitude has a sound level of 0 dB. A sound with a pressure amplitude ten times larger (2 x Pa) is 20 dB. A pressure amplitude ten times larger than this is 40 dB. Most people perceive a 10-dB increase in sound level as about twice as loud as the original level.

The Doppler Effect Sound waves travel at approximately 340 m/s (or about 1000 ft/s). If the source of the sound is stationary, the waves move away from it in all directions at this velocity. If the source of the sound is moving, then that velocity is ADDED to the velocity of the sound waves if the source is moving TOWARD you. If the source is moving AWAY from you, the velocity of the source is SUBTRACTED from the sound wave velocity. Waves moving faster cause more of them to hit your ear in a second. This is a HIGHER frequency. Waves moving slower cause fewer of them to hit your ear in a second. This is a LOWER frequency. Demo 1 Demo 2 Demo 3 Sonic Boom

Sources of Sound Sound is produced by a vibrating object. The vibrations of the object create particle motions that cause pressure oscillations in the air. A loud­ speaker has a cone that is made to vibrate by electrical currents. The surface of the cone creates the sound waves that travel to your ear and allow you to hear music. Musical instruments such as gongs, cymbals, and drums are other examples of vibrating surfaces that are sources of sound. Click images for YouTube videos

Corn starch is a shear thickening non-Newtonian fluid meaning that it becomes more viscous (“thicker”) when it is disturbed. When it's hit repeatedly by something like a speaker cone it forms weird tendrils. The video was shot at 30 fps and the speaker cone was vibrating at 30 Hz which is why there is no blur. This is the original video with the actual sound of the speaker. -- Click Image for YouTube video --

Raising or lowering the tube changes the length of the air column. When the column is in resonance with the tuning fork, the sound is loudest. The shortest column of air that can have an antinode at the closed end and a node at the open end is one-fourth of a wavelength long As the frequency is increased, additional resonance lengths are found at half-wavelength intervals. Thus, columns of length.1/4, 3.1/4, 5.1/4, 7.1/4, and so on will all be in resonance with a tuning fork. Sound Resonance

A seashell acts as a closed-pipe resonator to amplify certain frequencies from the background noise.

A string resonates with standing waves when its length is a whole number of half wavelengths. Because strings are so small in cross- sectional area, they move very little air when they vibrate. This makes it necessary to attach them to a sound­ing board, which transfers their vibrations to the air and produces a stronger sound wave.

Sound Quality A tuning fork produces a soft and uninteresting sound. This is because its tines vibrate like simple harmonic oscillators and produce the simple sine wave. Sounds made by the human voice and musical instruments are much more complex, like the wave shown. Both waves have the same frequency, or pitch, but they sound very different. The complex wave is produced by using the principle of superposition to add waves of many frequencies. The shape of the wave depends on the relative amplitudes of these frequencies. In musical terms, the difference between the two waves is called timbre, tone color, or tone quality.

A violin, a clarinet, and a piano produce characteristic sound spectra. Each spectrum is unique, as is the timbre of the instrument.violinclarinetpiano

Consonance and dissonance When sounds that have two different pitches are played at the same time, the resulting sound can be either pleas­ant or jarring. In musical terms, several pitches played together are called a chord. An unpleasant set of pitches is called dissonance. If the combi­nation is pleasant, the sounds are said to be in consonance.dissonance consonance What makes a sound pleasant to listen to? Different cultures have dif­ferent definitions, but most Western cultures accept the definitions of Pythagoras, who lived in ancient Greece. Pythagoras experimented by plucking two strings at the same time. He noted that pleasing sounds resulted when the strings had lengths in small, whole-number ratios, for example 1:2, 2:3, or 3:4. This means that their pitches (frequencies) will also have small, whole-number ratios.

Beats You have seen that consonance is defined in terms of the ratio of fre­ quencies. When the ratio becomes nearly 1:1, the frequencies become very close. Two frequencies that are nearly identical interfere to produce high and low sound levels, as illustrated above.

Beat Frequencies are caused by two nearly identical frequencies combining out of “phase” Applet Example

Go forth and make great music….great music