9-may-15 physics 1 (garcia) sjsu chapter 20 sound

Apr 18, 2023 Physics 1 (Garcia) SJSU

Chapter 20 Sound


Origin of Sound

Sound is a wave that is produced by the vibrations of material objects.

Tuning fork

Guitar stringDrumhead

Nature of Sound in Air

Sound in air is a longitudinal wave created by compressions and rarefactions.


Demo: Sound is not Wind

With sound, air molecules oscillate in place.

With wind, air moves from place to place.

Smoke rings are not sound because the air moves from place to place.


Demo: Light & Sound

Sound waves can only travel through a material, such as air, water, etc.

Light and radio waves can travel through vacuum.

See the cell phone ringing inside vacuum chamber but don’t hear any sound.

Radio

Wav

e


Check Yourself

Do light waves have energy?

What do we call the type of heat transfer that occurs when light transfers energy?

Do sound waves have also have energy?


Media That Transmit Sound

Sound travels better through elastic liquids and solids, such as water and rocks, than through air.

This is due to the close proximity of the atoms as they vibrate.

Hear richer, louder sound transmitted by string


What Your Voice Sounds Like

Your voice sounds different to you when you hear it from a recording.

This is because when you are speaking aloud, most sound waves reach your ear traveling through the solid flesh and bone of your skull. Leave yourself

a voice-mail

Human Ear

Pressure variations of sound waves push the eardrum, whose vibrations are transmitted by the ossicles (ear bones) to the cochlea (hearing canal)

CochleaVibrations transmitted by the ear bones create oscillations

in the fluid with the cochlea (snail in Latin), which is a spiral-wrapped tube.

These oscillations within the cochlea cause the basilar membrane to ripple, like a waving flag.

Organ of Corti

The organ of Corti forms a ribbon of sensory epithelium that runs lengthwise down the entire cochlea.

The hair cells of the organ of Corti selectively transform the oscillations of the basilar membrane into nerve signals.


Loudness & Amplitude

Loudness depends on amplitude of pressure and density variations in sound waves.

DecibelsLoudness of sound depends on the amplitude of pressure variations in the sound waves.

Loudness is measured in decibels (dB), which is a logarithmic scale (since our perception of loudness varies logarithmically).

From the threshold of hearing (0 dB) to the threshold of pain (120 dB) the pressure increase is a million times higher.

At the threshold of pain (120 db) the pressure variation is only about 10 Pascals, which is one ten thousandths atmospheric pressure.


Demo: Make Some Noise

Let’s experience the loudness of sound like by clapping at various decibel levels.

SoundMeter

Start clapping softly and slowly increase or decrease loudness, as I direct you using the sound meter.

Hearing by Age & Sex

Hearing acuity decreases with age, especially in the high frequencies.In general, women have greater acoustic sensitivity than men.

Absolute thresholds of hearing by age in males and females

Male, Age 20

Male, Age 30

Male, Age 40

Male, Age 50

Male, Age 60

Female, Age 60

Hearing Loss

Hair cells that respond to high frequency sound are very vulnerable to destruction, and loss of these neurons typically produces difficulty understanding human voices.

Much of this type of permanent hearing loss is avoidable by reducing exposure, such as to loud music.

The hair cells that line the cochlea are a delicate and vulnerable part of the ear. Repeated or sustained exposure to loud noise destroys the neurons of the Organ of Corti.

Once destroyed, the hair cells are not replaced, and the sound frequencies interpreted by them are no longer heard.

What?


Speed of Sound in Air

Speed of sound in air is about 340 m/s.

Sound travels about one kilometer in three seconds, about one mile in five seconds.

Light is a million times faster than sound.


Demo: Helium Voice

Sound speed in helium is higher than speed in air.

Wavelength of sound unchanged (size of vocal cords is unchanged).

Frequency of voice is higher since

He

Talk like me!

(Wave speed)(Wavelength)(Frequency) =

Breath Helium…


Reflection of Sound

Sound reflects strongly from rigid surfaces.

Softer surfaces absorb sound.

Quiet after a fresh snowfall because the soft, irregular surface of the snow absorbs sound instead of reflecting it.


Check Yourself

When crowded, which restaurant will be quieter?


Singing in the Shower

Multiple reflections from the hard walls create reverberation.

Hear your voice from several sources, slightly shifted in time.

Reverberation extends each note and smears (smoothens) the pitch.

Your voice sounds better when singing in the shower


Refraction of Sound

Sound speed can vary by material or conditions.

This causes the sound to bend in direction, in the same way that light bends when it passes through a glass lens.

Fig. 20.8


Ultrasound

Ultrasound is high frequency (Megahertz), short wavelength

(0.1 mm) sound.

Reflections and refractions of ultrasound by flesh and bone allow “seeing” inside the human body.


Forced Vibrations

Vibrating guitar strings force the vibration of the guitar’s body, producing most of the sound.

553 Hz 731 Hz

Circular rings indicate where the surface is vibrating up and down


Demo: Tuning Fork & Sound Box

Tuning fork by itself is not very loud.

Sound is much louder if it is held against a sound box, such as the body of a guitar or any similar rigid surface.

The tuning fork forces the surface into oscillation at the same frequency.


Natural Frequency

Metal wrench and wooden bat sound very different when dropped to the floor.

Different materials and shapes vibrate at their own natural frequencies.


Demo: Singing Rod

Stoking an aluminum rod with rosin-covered fingers induces loud vibrations at the rod’s natural frequency.


ResonanceResonance occurs when forced vibrations match an

object’s natural frequency.

Oscillations grow in amplitude due to synchronized transfer of energy into the vibrating object.

Acoustic Resonance

Sound at an object’s natural frequency can produce resonant vibrations.

If the amplitude of the sound is sufficiently large, resonant vibrations can shatter a wine glass.

As shown by Myth Busters, this may even be achieved by exceptionally powerful singers (and by average singers using electronic amplifiers).


Tacoma Narrows Bridge

In 1940, the first Tacoma Narrows bridge was destroyed by resonance.

First Bridge

Second Bridge


Movie: Tacoma Narrows Bridge

9-may-15 physics 1 (garcia) sjsu chapter 20 sound

Documents

sound slide

sound sound travels

air sound

light sound sound waves

sound meter

nature of sound

louder sound

decibels loudness of