1. The Nature of Sound Sound waves are produced by the vibrations of matter. In most cases, the original vibration (string, reed mouthpiece, vocal chords) starts the vibration of something larger, such as the sounding board of a stringed instrument, the air column in a wind instrument, or the air in your throat and mouth. Then this larger vibrating material sends a longitudinal wave through the surrounding medium.

2. The Nature of Sound Most sounds we hear are transmitted through air, but any elastic substance can transmit sound. The closer the molecules of the substance, the faster sound is transmitted. Sound cannot travel in a vacuum because there are no molecules to transmit the longitudinal waves.

3. The Nature of Sound Remember that it is not the medium that travels with the wave, but it is the energy- carrying pulses that travel. In a longitudinal wave, the molecules vibrate parallel to the direction the wave is traveling.

4. The Nature of Sound Sound takes time to travel from one place to another. – Ex: you see lightning before you hear thunder Speed of sound depends on wind conditions, temperature, and humidity, but does NOT depend on intensity or frequency. All sounds in the same medium travel at the same speed. Sound energy dissipates as it travels through air. Waves with a higher frequency transform sound energy into internal energy much faster than waves with a low frequency. This means that low frequency sound waves travel further.

5. The Nature of Sound The reflection of sound is an echo The energy and intensity of the echo depends on the reflecting surface – smooth and rigid surfaces reflect sound better than soft and irregular surfaces. When sound undergoes multiple reflections and persists after the source has ceased emitting, you hear a reverberation

6. The Nature of Sound Sound waves can also travel through a medium and then bend. This is called refraction. Sound bends toward the medium (or part of medium) that is cooler or where the sound waves travel slower.

7. Sound waves refract (bend) towards cooler air.

8. Interference Remember constructive and destructive interference with transverse waves – the same thing happens with longitudinal sound waves. Interference of sound waves can lead to beats – the periodic variation in the loudness of sound. beat frequency = |f 2 - f 1 | beat pitch = ½(f 2 +f 1 ) Beats are useful when tuning instruments

9. Radio broadcasts Radio stations broadcast in AM and FM AM means that the amplitude has been modified (Amplitude Modulation) FM means the frequency has been modified (Frequency Modulation) The radio station broadcasts at the frequency it was assigned (98.8 FM) Changing radio stations happens when you adjust the frequency of an electric circuit inside the radio receiver to match and resonate with the frequency of the desired station.