# 18 – 2 The Nature of Sound.

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18 – 2 The Nature of Sound

What is Sound? Sound is a form of energy caused by vibrations.
To vibrate means to move back and forth rapidly. What vibrates are the molecules that make up matter. Can there be sound without vibrations? NO

Tuning Fork Demo If you hear a sound is the fork vibrating? ___________ How come the ball moves when the fork is held near it? __________________________________________ If you were to put a tuning fork in water would anything happen? ______________________________ Strike the tuning fork first, now put it in water what happens? _____________________________________

Sound and Longitudinal Waves
Sound waves carry energy through a medium without particles of the medium traveling along. Sound is a disturbance that travels through a medium as a longitudinal wave.

It moves and pushes the molecules together creating a compression. When the drumhead moves again the molecules move apart creating rarefaction.

Sound Sound can travel only if there is a medium to transmit the rarefactions and compressions.

Sounds Bend Diffraction: sound waves spread out as they go through small holes. Diffraction also makes it possible to hear around corners.

Speed of sound The speed of sound depends on properties of the medium it travels through. Speed of sound is 750 mph Light is 186,000 miles per second. Properties include: Elasticity Density Temperature

Elasticity The ability of a material to bounce back after being disturbed. Solids are more elastic then liquids and gases.

Density The speed of sound also depends on how close together particles of the substance are. Particles of a dense material do not move as quickly as less dense.

Temperature Sounds travel slowly at low temperatures and faster at high temperatures.

Pitch Pitch how high or low a sound is on a musical scale. Pitch depends on how fast an object vibrates in one second. We call this frequency of vibration. Faster or more frequently an object vibrates the higher the pitch. The slower or less frequently an object vibrates the lower the pitch.

Hertz Frequency of vibration is measured with a unit called hertz.
One hertz is one vibration per second.

UltraSonic Waves Sounds with a frequency above 20,000 Hz which people cannot hear. Ultrasonic waves are used in sonar as well as in medical diagnosis and treatment. Sonar is used to estimate size, depth, shape of under water objects.

Infrasonic These waves have frequencies below 20 Hz.
Produced by sources such as heavy machinery or thunder. You can’t hear them you sense the disturbing rumble in your body.

What is loudness? Loudness is the amount of energy a sound has. The greater the energy the greater the loudness.

Decibels Loudness is measured in decibels. The higher the decibel number the louder the sound. They start at 0. A sound of 120 decibels or greater could hurt or damage your ears permanently.

What is intensity? The intensity of a sound wave depends on the amount of energy in each wave. As intensity increases the waves amplitude increases.

Doppler Effect The Doppler effect : as the source of sound moves toward the listener, the waves reach the listener with a higher frequency. The pitch appears to increase because of the Doppler effect. Example: as a police car with its siren on moves toward you the pitch becomes higher. As the car goes by and moves away the pitch drops. The frequency of the siren is not really changing. The apparent change in frequency as the wave source moves in relation to the listener is called the Doppler effect. If the waves are sound waves, the change in frequency is heard as a change in pitch.

What is an echo An echo is a reflected sound.
Sound does not bounce off all surfaces. It will bounce or reflect off hard surfaces but not off soft surfaces. Sound that is bounced off of a hard surface produces an echo. Sound that is bounced off a soft surface is absorbed

Resonance Resonance is the ability of an object to pick up energy waves of its own natural frequency. Every object has its own frequency of vibration – the frequency any object vibrates. This is called natural frequency.

Example: the frequency of an object may be 300 hz, another object may be 325 hz. A sound of a certain frequency will cause an object whose natural frequency is the same to vibrate. The ability of an object to pick up energy of its own natural frequency is called resonance. ( can be annoying – crack windows and other glass objects).

Demo Energy of Sound Resonance: Strike the tuning fork against rubber stopper. Hold it near two tuning forks – one of same frequency and one of a different frequency. What happens?_________________________________ Doppler effect: Swing tuning fork over head, or in circle in front of you. What happens to the sound? ______________________________________