Sound

3.14.13

Sound is a wave that carries vibrations. It is mechanical, longitudinal, and a pressure wave.

These are two tubes with a speaker at one end. The tubes are filled with air. Assume the speed of

sound traveling in these tubes is 350 m/s.

What’s the frequency, Kenneth?

• The relationship between the frequency of the two waves is the same (equal). – Why? Because they have the same

wavelength of 0.5m. Therefore, their frequency, even though you haven’t calculated its value yet, will be proportional.

– Don’t forget, wavelength and frequency have a linear, inversely proportional relationship.

– Both have a liner, direct, proportional relationship with wave speed.

Doppler effect

• The sound frequency will be LOWER BEHIND the moving object.

• The sound frequency will be HIGHER INFRONT of the moving object.

• According to the Doppler Effect, what is the relationship between their frequencies?

• First think about 1 and 2. The train is moving to the left, so which hears it lower and which hears it higher? – Doppler Effect says Student 2 is higher. The relationship is that

1 is less than 2. f1<f2• Then think about 2 and 3.

– Doppler Effect says student 3 hears it higher. Their relationship is that 3 is greater than 2. f3>f2

• Frequency of sound are described as pitch.

1 Hz= 1 vibration/second

• Amplitude of sound is described as loudness. The Loudness is directly proportional to the amount of energy the wave is carrying.

• The speed of sound only changes when in a different medium. vsolids > vliquids > vgases

Therefore, speed is faster in water than air. (~1480 m/s) The temperature also slightly affects the speed.

Does sound travel faster or slower as temperature increases? Why do you think?

Common Sounds and their Intensity

Decibels measure the “intensity” of sound, which relates the energy of the wave. It shows the amount of energy transported over time.

This is measured on a logarithmiclogarithmic scale, which is by factors of 10. For example, a 20dB sound is 100x greater than a 10dB sound. A 30 dB is 1000x greater than a 10dB sound.

FYI, 160dB would carry enough energy to pierce an eardrum.

• We use the term supersonic to describe motion at speeds faster than the speed of sound (about 340 m/s in air).

• A shock wave forms where the wave fronts pile up.

• The pressure change across the shock wave is what causes a very loud sound known as a sonic boom.

• The human ear can hear vibrations from 20-20,000Hz

• The human ear is most sensitive to sounds between 300 and 3,000 Hz.

• The ear is less sensitive to sounds outside this range.

• Most of the frequencies that make up speech are between 300 and 3,000 Hz.

• The human voice is a complex sound that starts in the larynx, a small structure at the top of your windpipe.

• The sound that starts in the larynx is changed by passing through openings in the throat and mouth.

• Different sounds are made by changing both the vibrations in the larynx and the shape of the openings.

Recording sound• A common way to record sound starts with a

microphone. A microphone transforms a sound wave into an electrical wave with the same pattern of oscillation. The electrical wave carries the energy and the sound information.

• The electrical signal is amplified until it is powerful enough to move the coil in a speaker and reproduce the sound information.

Hearing sound• 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 that detect and transmit information to the brain are located in the cochlea.

• They nerves near the opening respond to longer wavelength, low frequency sound.

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

Music• The pitch of a sound is how high or low we hear

its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected by the sounds we heard before and after.

• Rhythm is a regular time pattern in a sound.• Music is a combination of sound and rhythm that

we find pleasant (maybe).• Most of the music you listen to is created from a

pattern of frequencies called a musical scale.