Unit 10: Sound

Sound waves are longitudinal waves Sound waves are longitudinal waves. When sound waves occur in air, air molecules press together and push apart. Based on what you know about longitudinal waves and what they look like, draw a picture of what you think the air molecules in a sound wave look like.

Sound Waves are Longitudinal High Pressure “COMPRESSION” Low Pressure “RAREFACTION”

Compressions and Rarefactions occur in any Longitudinal Wave

Speed of Sound depends on Medium Sound waves can travel through gases, liquids, and solids. Sound travels faster in liquids than in gases, and even faster in solids. This is due to the closeness of the particles; they come into contact with each other more frequently. Will sound travel faster in air or in water? In air or in steel?

Pitch (Frequency) High pitch sounds have high frequency waves Low pitch sounds have low frequency waves The average human can hear pitches with frequencies from about 20 to 20,000 Hz. Sounds with frequencies from 20 Hz to 20,000 Hz are called audible sounds

Infrasonic and Ultrasonic Sound Sound waves with frequencies below 20 Hz are called infrasonic Sound waves with frequencies above 20,000 Hz are called ultrasonic We cannot hear infrasonic of ultrasonic sound waves, but many animals can. Bats, dogs, dolphins can hear ultrasonic sounds; Elephants and giraffes can hear infrasonic sounds.

Doppler Effect When a fire engine approaches you, you experience the sound waves of its siren more frequently (high pitched sound). But as it moves away from you, you encounter the waves less frequently (low pitched sound). The Doppler Effect occurs whenever there is relative motion between the source of waves and the observer. Either one can move towards the other, or both could be moving at the same time. A person standing in front of the ambulance will hear a higher pitch. A person standing behind the ambulance will hear a lower pitch. There is only a change in frequency; the speed of the sound waves does not change (speed of sound in air is always 340 m/s)

Sound Intensity (Loudness) The rate at which wave energy is transferred through the area in which the sound wave occurs. Power is also the rate at which energy is transferred, so intensity can be described in terms of Power. Units: W/m2

Sound Intensity Intensity = Power 4π (distance from the source)2

Forced Vibration When one vibrating object forces another object to vibrate, producing a louder sound. Examples: Cell phone vibrating on table forces the table to vibrate, and an increase in sound occurs. A guitar string attached to the body of a guitar forces wooden body to vibrate, and an increase in sound occurs.

Natural Frequency and Resonance Every object has a natural frequency - the frequency at which the object will naturally vibrate. When an object’s natural frequency matches the frequency of a forced vibration, an increase in amplitude occurs; this is called resonance. A swing has a natural frequency. When you pump your legs back and forth, you are causing a forced vibration in the swing. When your frequency – the frequency at which you pump your legs – matches the natural frequency of the swing, an increase in amplitude occurs.

(fundamental frequency, or first harmonic) Harmonic Series The series of frequencies of a vibrating string string length and half the wavelength (fundamental frequency, or first harmonic)

String length and wavelength This is called the second harmonic of the vibrating string, because it is the second possible vibration for the string.

The third harmonic frequency is equal to three times the fundamental frequency or the first harmonic f3 = 3f1 The fourth harmonic frequency is equal to four times the fundamental frequency f4 = 4f1 Each harmonic is a multiple of the first harmonic (the fundamental frequency).

Beats When two compressions of a sound wave meet, the sound is at a maximum (at its loudest). When a rarefaction and a compression meet, the sound is at a minimum. These variations in loudness are called beats. Two sound waves that are in phase then out of phase then in phase again and so on, produce beats.

Beat Frequency The beat frequency will tell you how many beats are heard each second. The beat frequency is the difference between the two frequencies. If one sound has a frequency of 200 Hz and another sound has a frequency of 204 Hz, the number of beats heard each second is 4. Therefore, the beat frequency = 4 Hz (beats per second)

A piano tuner using a 392 Hz tuning fork to tune the wire for G-natural hears four beats per second. What are the two possible frequencies of vibration of this piano wire?