Understanding Sound Waves: Principles and Applications

1. Chapter 6(Sections 6.4-6.6) Sound

2. Sound • The speed of sound in a substance depends on: • the mass of its constituent atoms, and • the strength of the forces between the atoms. • The speed of sound is large when: • the atoms have small mass — they’re easier to move, and/or • the forces between the atoms are larger — an atom pushes harder on its neighbor.

3. Sound, cont’d • Typically we represent a sound wave as a transverse wave (even though it is not). • A region of compression is drawn as a crest. • A region of expansion is drawn as a trough.

4. Sound, cont’d • A waveform of a sound wave is a graph of the air-pressure fluctuations caused by the sound wave versus time. • A pure tone is a sound with a sinusoidal waveform. • A complex wave is a sound that is not pure.

5. Sound, cont’d • Pure tone • Sinusoidal wave form

6. Sound, cont’d • Complex tone • Spoken “ooo” sound

7. Sound, cont’d • Noise • Air rushing over a microphone

8. Sound, cont’d • Noise is sound that has a random waveform. • It does not have a definite wavelength or period. • Sound with frequencies below our audible range is called infrasound. • Below about 20 Hz. • Sound with frequencies above our audible range is called ultrasound. • Above about 20,000 Hz.

9. Production of sound • Sound can be produced by: • Causing a body to vibrate: • e.g., plucking a string. • Varying an air flow: • e.g., buzzing your lips. • Abrupt changes in an object’s temperature: • e.g., a lightning flash creates thunder. • By creating a shock wave: • e.g., flying faster than the speed of sound.

10. Production of sound • A piano produces sound by: • The player presses a key so that the hammer strikes the wire. • The wire vibrates and transmits this vibration to the soundboard. • The soundboard then radiates the sound to the room.

11. Production of sound • A flute produces sound by: • The player blows across the opening to create a varying airflow. • The airflow reaches the end of the flute and radiates to the room. • Or the player opens a note-hole to release part of the wave. • The tube is then effectively shortened.

12. Propagation of sound • Imagine a person talking in the middle of an empty arena. • The amplitude of the sound wave decreases as one moves farther away from the person.

13. Propagation of sound, cont’d • In a room, we have to deal with the multiple reflections off the walls and other objects. • It gets a lot more complicated to determine the amplitude at an arbitrary place in the room. • The process of repeated reflections of sound in an enclosure is called reverberation.

14. Propagation of sound, cont’d • A hand clap in an open field is a simple pulse since there is no echo. • But in a gym, there are multiple echoes which tends to make the sound fade away gradually.

15. Propagation of sound, cont’d • This effect can be useful or detrimental. • Speech might be easy to understand in a field but can become muddled with too much reverberation. • Reverberation is useful in music to help blend sounds.

16. Perception of sound • We have to be careful when we discuss sound. • There are physical properties we can measure. • But our ears do not just measure these physical properties. • We have to deal with the perception of the sound.

17. Perception of sound, cont’d • Pitch is the perception of highness or lowness of a sound. • The pitch depends primarily on the frequency of the sound. • It also depends on the duration. • A very short sound might sound like a click even if it has a definite frequency.

18. Perception of sound, cont’d • Loudness is the perception of whether a sound is easy to hear or painful to hear. • It depends primarily on the amplitude of the sound. • It also depends on whether the sound is played with other sounds (before, after, concurrently, etc). • It even depends on the frequency. • Our ears are more sensitive to higher frequencies and less sensitive to lower frequencies.

19. Perception of sound, cont’d • Our eardrums respond to sound pressure level. • A louder sound creates a larger compression, i.e., higher pressure, than a quiet sound. • We typically call the sound pressure level just the sound level. • It is measured in decibels (dB). • 0 dB corresponds to inaudible. • Normal conversation is about 50 dB. • ~120 dB starts causing pain.

20. Perception of sound, cont’d • The sound level of the quietest sound is called the threshold of hearing. • The sound level at which we start experiencing pain is called the threshold of pain. • The minimum increase in sound level that is noticeable is about 1 dB.

21. Perception of sound, cont’d • For a sound to be judged as “twice as loud,” the original sound must be increased by 10 dB. • To make a sound “twice as loud,” you would need ten equal sources. • Two sounds with equal sound levels cause an increase of 3 dB over a single such sound. • A sound with 10 times the amplitude of another is 20 dB higher

22. Perception of sound, cont’d • Tone quality is a measure by which two sounds of the same frequency and amplitude sound different. • A sax sounds different from a trumpet playing the same note because the two instruments have different tone qualities. • We typically refer to tone quality as timbre or tone color.

23. Perception of sound, cont’d • Any complex waveform is equivalent to a combination of two or more sinusoidal waveforms with definite amplitudes and specific frequencies. • Theses component waveforms are called harmonics. • The frequencies of the harmonics are whole-numbered multiples of the complex waveform’s frequency.

24. Sound, cont’d • The complex waveform (left) is a combination of three pure waveforms (right) • The pure tones (harmonics) have frequencies 1, 2, and 3 times the frequency of the complex tone

25. Perception of sound, cont’d • The specific tone quality of a sound depends on: • the number of harmonics that are present, and • the relative amplitudes of these harmonics. • A spectrum analyzer displays a complex waveform in terms of the constituent harmonics.

26. END

27. Sound, cont’d • A waveform of a sound wave is a graph of the air-pressure fluctuations causes by the sound wave versus time. • A pure tone is a sound with a sinusoidal waveform. • A complex wave is a sound that is not pure.

28. Sound, cont’d • Various different waveforms of sounds

29. Perception of sound, cont’d • We can construct the waveform on the left by adding the three waveforms (harmonics) on the right.