Sound

All sounds are caused by vibrations

• Vibration – back and forth movement of matter.

• Sound waves are generated by any vibrating object

• The vibrations create molecular motions and pressure oscillations in the air

• The oscillations create a periodic disturbance of the surrounding air

• The effect of these waves is heard as sound

•The human voice comes from vibrations of the vocal cords

•Air from the lungs is what causes the vocal cords to vibrate

•The frequency of vibration is controlled by the singer’s muscular

tension placed on the cords

•The human voice uses the throat and mouth cavity as a resonator

•BRASS

•The lips of the performer vibrate resulting in sound

•REED

•Have a thin wooden strip that vibrates as a result of air blown across it

•WIND

•The air blown across the opening in the pipe (mouthpiece) sets the column of air in the instrument into vibration

•STRING

•A wire or string is set into vibration. The wire is attached to a sounding board that vibrates with the string

Sound waves are longitudinal

 

Sound waves travel faster as medium it travels

through becomes warmer.

• Speed of sound in various mediumsAir 340 m/sWater 1500 m/sIron 5100 m/s

Loudness Vs. Intensity

• Loudness – perception of the rate at which sound energy enters your ear. 

• Intensity – Measurement of amplitude. 

Intensity and loudness decreases with distance 

Intensity

Decibel (dB) Unit of intensity level – compares to lowest possible limit of human hearing

Typical intensity levelsBreathing 10 dBWhisper 30 dBLoud stereo 90 dBJet 150 dB 

Sound waves that enter your ear cause the eardrum to vibrate.

Intensity levels greater than 90 dB can cause hearing loss. 

Frequency and Pitch Pitch – the perception of frequency

Humans can detect a wide range of frequencies and are sensitive to a large range of amplitudes.

Humans (hearing) 20-20 000 Hz Humans (making sounds) 85-1100 Hz

Bat 1000 – 120 000 HzDog whistle 20 000 – 24 000 Hz Waves with frequencies greater than 20,000 Hz are called Ultrasonic. Waves with frequencies less than 20,000 Hz are called Subsonic.

Reflection of Sound Waves

Echo – reflected sound waveFollows the Law of Reflection, where the

angle of incidence is equal to the angle of reflection.

Applications:Sonar – sound navigation and rangingUltrasound – viewing the fetus in the

womb.

Diffraction

– bending of sound waves around barriers (you can hear people in the hallway) 

•It is divided into three parts: outer ear (pinna), middle ear, inner ear.

•The first chamber of the ear contains tiny hairs and cells that produce ear wax

•Sound waves pass by the pinna and beat against the ear drum, causing it to vibrate

•The vibration of the ear drum causes three bones in the inner ear (hammer, anvil, stirrup) to vibrate carrying motion to the inner ear, which is filled with fluid.

•The cochlea converts the vibration of the bones to nerve impulses that the brain receives via the auditory nerve

•The inner ear also controls balance

• Even though two instruments may be playing the same note, their sounds can differ because each sound contains a number of frequencies with different intensities

• Timbre: the quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities

Music 

Shorter strings produce sounds with higher pitch. (higher frequency)Shorter vibrating columns produce higher pitch. (higher frequency)A vibrating surface is responsible for loudness. 

 

The variation from soft to loud and back to soft is called a beat

•The number of beats per second corresponds to the difference between frequencies

•The ability to detect beats depends upon an individual’s hearing and musical training. The average human ear can distinguish beats up to a

frequency of ten beats per second

Beats are formed by the interference if two waves of slightly different frequencies traveling

in the same direction. In this case one beat occurs at T-2 where constructive interference is

greatest.

• The equation for beat frequency is:

F=f2-f1

• EXAMPLE PROBLEM

A 420 Hz tuning fork and a 620 Hz tuning fork are struck at the same time. What beat frequency will be produced?

Solution:620-420 = 200, Therefore the beat frequency produced is 200 Hz.

• Consists of a large number of frequencies with no relationship to each other

• White noise occurs when all frequencies are present in equal amplitudes.

• The human voice uses the throat and mouth cavity as a resonator• The quality of the tone depends on the shape of

the resonator

Acoustics

– study of how materials influence the production and movement of sound