1. Properties and Interactions of Sound
Mrs. Gergel
Holt Science and Technology: Sound and Light
Chapter 2

2. The Speed of Sound
The speed of sound depends only on the medium in which the sound is traveling.
Medium
Speed (m/s)
Air at 0º C (32º F)
331
Air at 20º C (68º F)
343 (about 770 mph)
Air at 100º C (212º F)
366
Water at 20º C (68º F)
1,482
Steel at 20º C (68º F)
5,200

3. Frequency Frequency is the number of waves per second.
When vocal chords are stretched, they have a higher frequency. When they are relaxed, they have a lower frequency.
Since a frequency of 50 Hz means 50 vibrations per second:
A deep voice can produce about Hz
A high voice can produce frequencies over 1,000 Hz

4. Hearing Frequencies
Most people hear frequencies between 20 Hz and 20,000 Hz
Sound frequencies above the normal human range of hearing are called ultrasound.
Sound frequencies below the normal human range of hearing are called infrasound.

5. Frequencies and Sound
Ultrasonic frequencies are anything above 20,000 Hz, because they are “above sound”
Infrasonic frequencies are anything below 20 Hz because they are “under sound”

6. Pitch When describing how frequencies sound, we use the word pitch.
The pitch of a sound describes how high or low the sound seems to a person.
Sound waves of high frequency have a high pitch, while sound waves of low frequency have a low pitch.
Shorter strings have a higher pitch than longer strings under the same tension.

7. Frequencies Heard by Different Animals
Frequency Range (Hz)
Bat
2,000 to 110,000
Porpoise
75 to 150,000
Cat
45 to 64,000
Beluga Whale
1,000 to 123,000
Elephant
16 to 20,000
Human
20 to 20,000
Dog
67 to 45,000
Low numbers are deeper sounds, higher numbers are higher sounds.

8. The Doppler Effect
The apparent change in frequency as a wave source moves in relation to the listener is called the Doppler Effect.
Imagine a police car driving past you. The pitch of the sound is different coming towards you than it is going away from you.
As a sound source moves toward the listener, the waves reach the listener with a higher frequency. The pitch appears to increase because of The Doppler Effect.

9. Loudness Loudness is a measure of how well a sound can be heard.
A sound wave of greater energy sounds louder.
The harder you hit a drum, the greater the amplitude, which means a greater loudness.

10. Measuring Loudness
The most common unit used to express loudness is the decibel (dB).
The softest sounds an average human ear can hear are at a level of 0 decibels. Sounds that are at 120 dB or louder can be painful.

11. Decibels
Each 10 dB increase in sound level has ten times more energy than before.
Decibel Level (dB)
Sound
The softest sound you can hear 20
Whisper 25
Purring cat 60
Normal conversation 80
Lawn mower, vacuum cleaner
100
Chain saw, snowmobile
115
Loud rock concert, car horn
120
Threshold for pain
140
Jet engine 30 m away
200
Rocket engine 50 m away

13. Tools to help see sound waves
An oscilloscope is a tool that turns sound waves into transverse waves so that the properties can be more easily observed.

14. Interactions of Sound Waves
Reflection is the bouncing back of a wave after it strikes a barrier
Reflection of a sound wave is called an echo.
The strength of an echo depends on the reflecting surface. Think of the walls in the gymnasium vs. the walls in a theater.

15. Echolocation
The use of reflected sound waves to find objects is called echolocation.
Many animals, such as dolphins, bats, and some kinds of birds, use echolocation to find objects in their paths.

16. Echolocation in Technology
People use echoes to locate objects under water using SONAR.
SONAR stands for SOund Navigation And Ranging

17. Echolocation in Technology
Ultrasonography is a medical procedure that uses echoes to “see” inside a patient’s body without doing surgery.
Ultrasonic technology uses waves with a frequency from 1 million to 10 million Hertz
Ultrasonic waves are less harmful to humans than X-rays.

18. Interference of Sound Waves
Interference occurs when two or more sound waves overlap.
Orchestras use constructive interference when several instruments of the same kind play the same notes.
This causes the combined amplitude to increase, resulting in a louder sound.

19. Interference and Standing Waves
Have you ever watched a guitar string after you plucked it? You may have noticed that the string vibrates as a standing wave
A standing wave is a pattern of vibration that looks like the wave is standing still.

20. Resonance
Resonance is a phenomenon that occurs when two objects naturally vibrate at the same frequency; the sound produced by one object causes the other object to vibrate.
Resonance happens inside the hollow body of a guitar, which amplifies the sound.

21. Sound Quality
The result of several pitches blending together through interference is called sound quality.
Each instrument has a unique sound quality that results from the particular blend of overtones that it has.

22. Noise
Noise can be described as any sound, especially a nonmusical sound, that is a random mix of frequencies (or pitches)
Musical notes will produce a repeating pattern.