Honors Physics - Chapter 15: Lesson 3

Chapter 15: Lesson 3

Effects of Sound

Chapter 15: Lesson 3 - Effects of Sound

Musical Instruments

Physics is behind your sound in musical instruments. While I cannot play a musical instrument, I can tell you the physics behind them!

In brass instruments, (trumpet, trombone, and tuba) the lips of the performer vibrate.

In reed instruments, (clarinet, saxophone, and oboe) have a thin wooden strip, or reed, that vibrates as a result of air blown across it.

In a flute, recorder, organ pipe, or whistle, air is blown across an opening in a pipe.

In stringed instruments, such as the piano, guitar, or violin, a wire or string is set into vibrations.

Louder notes are made by bigger vibrations and softer notes are made by smaller vibrations, although the number per second, or frequency, of vibrations may remain the same. Loudness depends on amplitude, more amplitude equals a louder sound. The loudness of a musical note does not necessarily change its frequency.

Loud High Note vs Loud Low Note

Soft High Note vs Soft Low Note

Resonance and Waves

Changing the length of the resonating column of vibrating air varies the pitch of an instrument. Additional resonances can be heard.

Remember from chapter 14, a standing wave is a wave appears not to be moving. A standing wave has nodes and antinodes.

Open Pipe Resonator

For an Open Pipe Resonator:

The shortest column of air that can have an antinode at both the bottom and the top is one-half a wavelength long. As the air column is lengthened, additional resonances can be heard.

For a Closed Pipe Resonator:

The shortest column of air that can have an antinode at the bottom and a node at the top is one-forth a wavelength long. As the air column is lengthened, additional resonances can be heard.

Closed Pipe Resonator

Sound Interference

Sound waves share the properties that we have studied in chapter 14. They can bounce off hard objects. They can also constructively or destructively interfere. When two waves have total destructive interference, they can cause dead spots where little or no sound can be heard.

Interference affects the loudness of sounds.

If you are equally distant from two sound speakers that simultaneously trigger identical sound waves of constant frequency, the sound is louder because the waves add.

If you move to the side so the waves reach you buy a difference of one wavelength, destructive interference happens.

When sound is in phase, you have constructive interference and the sound is louder. When the sound is out of phase, you get destructive interference and the sound is softer or could cancel out entirely.

Noise Cancellation Headphones

Noise canceling headphones are an example of destructive interference!

Sound Vocabulary

There are some vocabulary words that are key when you study sound.

Pitch is the frequency of the wave. Frequency is the number where pitch is subjective. You can say the frequency is increasing or decreasing and without numbers, you are taking about pitch. When you say that the frequency increased by 200 Hz, that is a frequency.

An octave is when two notes with frequencies are related by the ratio 2:1. If you have a frequency of 400 Hz and a frequency of 200 Hz, they are said to differ by an octave. Same with 1000 Hz and 500 Hz, anything that is related by the ratio 2:1.

Timbre is the sound quality or sometimes called tone color.

A beat is the throbbing vibration in the loudness of sound caused by interference when two tones of slightly different frequencies (7 Hz or less) are sounded together. We will look at the equation to find the beat frequency in lesson 4.

Watch the short video to hear the beat when two tuning forks of slightly different frequencies are struck at the same time.

Dissonance is when two waves differ by more than 7 Hz, the ear detects a complex wave. If the sound is unpleasant, it is called dissonance. Consonance is when two waves differ by more than 7 Hz, the ear detects a complex wave. If the sound is pleasant, it is called consonance. This is also called a chord. The only difference between consonance and dissonance is the sound. Consonance is a pleasant sound and dissonance is unpleasant, think nails on a chalkboard sound.

The fundamental frequency is the lowest frequency making up the sound. Harmonics are the waves of frequencies that are whole number multiples of the fundamental.

Doppler Effect

The doppler effect happens because the frequency is higher when the source of the sound is moving towards you, then suddenly drops to a lower pitch as the source moves away from you.

When the moving sound is moving towards you the frequency at which the waves are hitting you increases, therefore, the frequency of the sound increases. As the object moves away from you, the frequency that the waves are hitting you decreases, and therefore, the frequency of the sound decreases.

The actual frequency of the siren never changes. Only how frequently the waves hit you changes.

Click on the video to see a quick demonstration of the doppler effect with a car horn.

The doppler effect occurs in all waves. Radar detectors, astronomers, physicians, and bats all use the doppler effect.

Watch this quick clip to show how the frequency of sound changes because of the motion of a source.

Do you watch the Big Bang Theory? Is a funny show about a woman who moves into an apartment across the hall from two brilliant but socially awkward physicists shows them how little they know about life outside of the laboratory.

Here is a fun clip when Sheldon dresses up as the doppler effect for Halloween.

Check Your Understanding:

Click on the down arrow when you have your answer to check to see if you are correct.

Does your cell phone have an antenna?

YES! You may not see it, but there is one internally in your phone. It would not be useful without an antenna!

2. Why do you need an antenna for radio, TV, or cell phone transmission and reception?

Antenna's convert electromagnetic waves into electrical currents, which the can be turned into sound, and vice versa.

3. Why is there a time delay between the time we send a radio message to astronauts on the moon and when they receive it?

The sound waves have to be converted to electromagnetic waves then converted back into sound waves.

You have finished Lesson 3!

Be sure to head over to google classroom and fill out the exit pass.

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