Foundations of Physical Science

Unit 4: Sound and Waves

Chapter 13: Sound and Music

• 13.1 Sound

• 13.2 Properties of Sound

• 13.3 Music

Learning Goals• Learn how we hear sound.

• Learn how your brain interprets sound to understand words and music.

• Learn what kinds of sounds we can hear, and what kinds we cannot hear.

• Learn what a sound wave is and how it travels.

• Learn how the loudness of sound is measured.

• Learn the basics of acoustics as applied to the design of buildings and musical instruments.

• Learn to read a sonogram and how a computer recognizes spoken words.

• Learn what supersonic means.

• Learn why a musical scale sounds good, or why it sounds bad.

• Learn how we tell voices and instruments apart from each other.

Vocabulary• acoustics• beat/beats• consonance• cochlea • dissonance • decibel• harmonics• harmony• musical scale• pitch

• pressure

• reverberation

• rhythm

• sonogram

• sound

• supersonic

• ultrasound

• white noise

Sound Waves

• Movement of vibrations of matter through solids, liquids or gases

• There has to be something there to produce a sound wave

How do we know sound is a wave?

1. Sound has a frequency that we hear as higher or lower pitch.

2. Sound has a wavelength that we can construct experiments to show.

3. The speed of sound is frequency times wavelength.

4. Resonance happens with sound.

5. Sound can be reflected, refracted, and absorbed.

6. Sound shows evidence of interference and diffraction.

How do we hear a sound wave?• The eardrum vibrates in response to

sound waves in the ear canal.

• The three delicate bones of the inner ear transmit the vibration of the eardrum to the side of the cochlea.

• The fluid in the spiral of the cochlea vibrates and creates waves that travel up the spiral.

• The nerves near the beginning see a relatively large channel and respond to longer wavelength, low frequency sound.

• The nerves at the small end of the channel respond to shorter wavelength, higher-frequency sound.

The Range of Human Hearing• The eardrum, bones and cochlea contribute to the limited range of

hearing (20-20,000Hz)

• Animals can hear much higher frequencies because they have more sensitive structures in their inner ears

• Hearing ability changes with time

• Hearing can be damaged by loud noise

• Ultrasound can pass through the human body, using refraction and reflection of sound to produce images

Properties of Sound

• Like other waves, sound has the fundamental properties of frequency, wavelength, amplitude, and speed

• Some of these properties have different names

• “Loudness” = amplitude

• Pressure: a measure of the force of molecules being exerted on their surroundings

Properties of Sound

• Pressure is a restoring force: if we increase the pressure in one place, the natural tendency is for the atoms to spread back out again, lowering pressure

• The opposite also holds true

Close-Up Look: Sound Wave

• If you could see the atoms, the difference between high and low pressure is not great

• Here, it is exaggerated.

• A sound wave is a wave of alternating high pressure and low pressure regions of air

The Loudness of Sound• Measured in decibels (dB)

• Related to the amplitude of the sound wave

• Pressure change in a sound wave is very small, so we do not use pressure to measure loudness

• We use the decibel scale, numbered between 0-100

• Every increase of 20 dB, means the pressure wave is 10 x greater in amplitude.

Acoustics• Reducing the loudness of sound is important

in many applications

• The science and technology of sound

• The library might want to absorb sound to maintain quiet

• A record studio might want to block sound from the outside from mixing with the inside

• Sound reflects from a smooth surface: the angle of incidence is equal to the angle of reflection

• Reflected sound makes it sound lively and full

Acoustics

• Reverberation: Sound is too reflective and becomes garbled

• Absorption: Sound can be too absorbent and the room is dull and lifeless

• Concert halls must balance reverberation and absorption

Echo

• The reflection of sound

• Large reflection: a surface that is rigid and smooth

• Small reflection: a surface that is soft and irregular

• Sound energy that is not reflected is transmitted or absorbed

Soundproofing

• The ear is sensitive; it is difficult to block sound

• Sound can move around corners and through gaps

• To soundproof, thick materials are used (concrete, brick, thick curtains and rugs)

The Frequency of Sound• We hear frequencies of

sound as having different pitch.

• Our ears are more sensitive to sounds between 100 - 2,000 Hz.

• Most frequencies that make up speech are also between 100 and 2,000 Hz.

The Frequency of Sound• Complex sound is made from many frequencies

• You can do the reverse-take a complicated sound and break it down into different amounts of pure frequencies

The Frequency of Sound

• Sonogram: a special kind of graph that shows how loud sound is at different frequencies

The Frequency of Sound• White noise: an equal mixture of all frequencies,

like white light is a mixture of all colors

• Because all frequencies are at the same level there is no pattern the brain can recognize

• The lack of pattern is helpful for relaxing; it drowns out more distracting noises

Ultrasound

• Multiple reflections and refractions of ultrasonic waves used by physicians for seeing inside the body without the use of x-rays

Ultrasonic Sound/Echolocation

• When ultrasound (high frequency sound) enters the body, it is reflected more strongly from the outside of organs than from their interior

• A picture of the outline of the organs is obtained

• Used by bats, dolphins, etc.

Refraction of Sound• On a warm day, air near the ground may

be warmer than the air above

• Speed of air near the ground increases

• Sound waves bend away from the ground, resulting in sound that does not seem to carry well

• Opposite on a cool day

Doppler Effect

• The change in frequency of wave motion

• Caused by the motion of the source (or receiver)

• Approaching, the frequency is higher because the wave-fronts are closer together in time.

• Departing, the frequency is lower

Example

• When a wave source moves toward you, do you measure an increase or decrease in wave speed?

• Neither! It is the frequency of the waves that changes for a moving source, not the wave speed.

Doppler Effect

• A stationary bug on water is jiggling, causing circular wave fronts to spread out

• The frequency of the disturbance is the same for observers at A and B

Doppler Effect

• If bug is moving, wave-fronts arriving at B are closer together in time

• The reverse is true for the observer at A

The Wavelength of Sound

• Speakers that have great bass (low frequency) are large

• Speakers that have good treble (high frequency) are smaller

• This is due to the wavelength and energy of the different frequencies of sound

Speed of Sound

• Depends on wind, temperature and humidity

• Does NOT depend on the loudness or the frequency of sound

• All sounds, loud or soft, high or low-pitched, travel at the same speed in a given medium

• The faster moving molecules bump into each other more often

• For each degree rise in temperature above 0°C, the speed of sound in air increases by 0.6 m/s

• Sound travels at about 340 m/s in air at a normal 20°C room temperature

Example

• What is the approximate distance of a thunderstorm when you note a 3-s delay between the flash of lightning and the sound of thunder? (Use 340 m/s for the speed of sound)

• In a 3 s the sound travels (340 m/s x 3 s)

• 1020 m; slightly more than 1 km (0.6 miles) away!

The Speed of Sound

• 340 m/s (660 mph)

• Passenger jets fly slower than sound (400-500 mph)

• Supersonic: faster than the speed of sound

Sound Waves

• Most sounds travels through air

• Any elastic substance (solid, liquid, or gas) can transmit sound

• Many solids and liquids can conduct sound better than air

Shock Wave

• The cone-shaped wave made by an object moving at supersonic speed through a fluid

• Three-dimensional wave

• Produced by overlapping spheres that produce a cone

Sonic Boom

• The loud sound resulting from the incident of a shock wave

• We don’t hear a sonic boom from a slower-than-sound or subsonic aircraft

• The sound waves reach our ears one at a time and make one continuous tone

• Only when the craft moves faster than sound do the waves overlap to reach the listener in a single burst

Music: Pitch and Rhythm

• Pitch: how high or low we HEAR a frequency.

• The way we hear pitch can be affected by the sounds we heard before and after.

• Rhythm: is a regular time pattern in a sound.

Pitch

• Our subjective impression about the frequency of sound

• High-pitch sound: high vibration frequency

• Low-pitch sound: low vibration frequency

Consonance, Dissonance, and Beats

• Beats: occur when two frequencies are close, but not exactly the same

• The phase of the two waves changes in a way that makes the loudness of the sound seem to oscillate

Beat

• A periodic variation in loudness

• A tone of distinct pitch and duration

Consonance, Dissonance, and Beats

• Consonance: when we hear more than one frequency of sound and the combination sounds good

• Dissonance: when we hear more than one frequency of sound and the combination sounds bad or unsettling

Harmonics and the “Color” of Sound

• The same note can sound different when played on different instruments.

• Instruments make mixtures of different frequencies.

• Compare these voices saying hello: