SOUND

How is sound created? Sound is produced as molecules of a medium are vibrated. The energy of the disturbance is carried through the medium in a mechanical wave.

Characteristics of the Sound Wave Sound waves are longitudinal waves or compression waves.

Characteristics of the Sound Wave Areas of the wave that have maximum density and pressure are called compressions. Areas of the wave that have minimum density and pressure are called rarefactions

Characteristics of the Sound Wave Rarefactions and compressions spread out (propagate) in all directions from the source of the vibrations. This causes the sound waves to act like ripple waves on a pond.

Characteristics of the Sound Wave Sound waves propagate in all directions. They spread out in three dimensions from their source.

Characteristics of the Sound Wave The distance from one wave front and the next is the wavelength. The direction of wave propagation is indicated by the “rays” which are radial lines perpendicular to the wave fronts.

Characteristics of the Sound Wave The Frequency of a sound wave is defined as the number of cycles per unit of time. (Usually the units are Hertz – Cycles per second) Humans can hear sound waves between 20 Hz and 20,000 Hz. These are called audible sound waves.

Characteristics of the Sound Wave The frequency of an audible sound wave, which determines how high or low a sound is perceived, is known as the pitch. As the frequency of the sound wave increases, the pitch rises. Serway/Faughn Holt Physics Text p406

Characteristics of the Sound Wave The speed of sound depends on the characteristics of the medium, such as temperature, or whether the medium is a solid, liquid or gas. Sound usually travels faster in a solid than a gas because particles are closer together and can transfer motion more quickly.

The characteristics of the medium, therefore, determines the velocity of the sound wave through that medium

The speed of sound is 331.3 meters per second (1,087 feet per second) in dry air at 0 degrees Celsius (32 degrees Fahrenheit). At a temperature like 28 degrees C (82 degrees F), the speed is 346 meters per second. ­The speed changes depending on the temperature and the humidity; but if you want a round number, then something like 350 meters per second and 1,200 feet per second are reasonable numbers to use. - How Stuff Works

The Doppler Effect The Doppler Effect an observed change frequency when there is relative motion between the source of sound and an observer. Moving away from You = lower perceived frequency and lower perceived pitch Moving toward You = higher perceived frequency and higher perceived pitch Serway/Faughn Physics Text p409

The Doppler Effect

The Doppler Effect The same is true if a sound source is stationary and an observer moves towards and away from the sound. Moving towards the sound it appears to be higher pitched than when moving away.

Sonic Boom When an airplane travels through the air, it produces sound waves. If the plane is traveling slower than the speed of sound (the speed of sound varies, but 700 mph is typical through air), then sound waves can propagate ahead of the plane.

If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the "wake" of the plane's sound waves. All the sound waves that would have normally propagated ahead of the plane are combined together. First you hear nothing, then you hear the boom they create.

It is just like being on the shore of a smooth lake when a boat speeds past. There is no disturbance in the water as the boat comes by, but eventually a large wave from the wake rolls onto shore. When a plane flies past at supersonic speeds the exact same thing happens, but instead of the large wake wave, you get a sonic boom.

Sound Intensity The rate at which energy is transferred through a unit area, perpendicular to the direction of the sound wave motion is the intensity of the sound wave. The intensity of the sound wave decreases as the distance from the sound source increases.

Sound Intensity The range of human hearing depends on both the frequency and the intensity of the sound waves. Sounds at high or low frequencies must be relatively intense to be heard, whereas sounds in the middle of the spectrum are audible at lower intensities.

Exposure to sounds above the threshold of pain can cause immediate damage to the ear, even if no pain is felt. Prolonged exposures to sounds of lower intensities can also damage the ear. The threshold of hearing and the threshold of pain merge at both high and low ends of the spectrum.

Decibels A decibel is a dimensionless unit that describes the ratio of two intensities of sound. The threshold of hearing is commonly used as the reference intensity.

Natural Frequency The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed is known as the natural frequency of the object.

Natural Frequency If the amplitudes of the vibrations are large enough and if natural frequency is within the human frequency range, then the vibrating object will produce sound waves that are audible.

Resonance Resonance - when one object, vibrating at the same natural frequency of a second object, forces that second object into vibrational motion.

Harmonics and Sound Vibrations in string musical instruments produce standing waves. The standing wave pattern results when two waves of the same frequency, wavelength and amplitude travel in opposite directions and interfere. Serway/Faughn Holt Physics Text p418

Harmonics and Sound Sounds heard from musical instruments can sound like a single pitch but can be made up of multiple frequencies. Serway/Faughn Holt Physics Text p419

Harmonics and Sound Since the velocity of a wave is equal to the product of the frequency and the wavelength, v = f l and frequency is equal to the velocity divided by the wavelength f = v/ l The frequency of vibration for the first harmonic of the standing wave is f1 = v/ 2l This is called the fundamental frequency.

Harmonics and Sound The fundamental frequency has the greatest possible wavelength and therefore also had the lowest possible frequency. (Remember: frequency is inversely proportional to wavelength.) Serway/Faughn Holt Physics Text p418

Harmonics and Sound Successive frequencies and standing wave patterns are all integral multiples of the fundamental frequency. These frequencies are called a harmonic series. Harmonics above the fundamental frequency are known as overtones (for an open air column, such as a wind instrument, or a stringed instrument).

Harmonics and Sound The general equation for calculating the frequency of a standing wave in a string: Serway/Faughn Holt Physics Text p419

Harmonics and Sound When as guitar player pressed down on a guitar string at any point, that point becomes a node and only a portion of the string vibrates. As a result a single string can be used to create a variety of fundamental frequencies. Serway/Faughn Holt Physics Text p419

Harmonics and Sound Standing waves can occur in a column of air, such as inside a trumpet or saxophone, or pipe organ.

Harmonics and Sound The fundamental frequency can be changed by varying the length of the vibrating air column.

Timbre and Beats Timbre is the musical quality of a tone resulting from the combination of harmonics present at different intensities. It is the mixture of harmonics that produces the characteristic sound of an instrument. Serway/Faughn Holt Physics Text p424

Timbre and Beats When two waves of slightly different frequencies interfere, the interference pattern varies in such a way that the listener hears an alternation between loudness and softness. The variation from soft to loud and back is called a beat. Serway/Faughn Holt Physics Text p426