# 1 All About Sound Rob Shaffer Stoklosa Middle School, Lowell June 11, 2013 Note: this lesson utilizes the Audacity computer program to create and analyze.

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1 All About Sound Rob Shaffer Stoklosa Middle School, Lowell June 11, 2013 Note: this lesson utilizes the Audacity computer program to create and analyze sounds. It is available for free download from: www.audacity.com

2 Attributes of Sound Loudness (volume) Pitch (frequency, tone) Tonal structure –pure tone or combination of several or many tones –timbre Duration Source Direction A mechanical wave with a certain propagation speed

3 Waves What is a wave –A disturbance in the properties of a medium that propagates through the medium –Transfers energy from one point to another Particles in the medium return to their previous position after the wave passes Types of waves –Mechanical waves –Electromagnetic waves

4 Examples of Waves Ripples in a pond Explain wave curvature –Close to the source –Far away from the source Sinusoidal waves –Concept of phase difference between two sound waves – Audacity example

5 How is sound created? Impulsive sounds –Clap two boards together Longer term sounds –Continuous vibration of a sound source such as a speaker, string, column of air –Have the kids come up and feel the speaker cone vibrate

6 Sound (in air) is a rapid fluctuation in the air pressure As a speaker cone vibrates, it pushes on the air molecules, increasing the air pressure. Then it moves the other way, and the air pressure is reduced. The greater the change in air pressure, the louder the sound is. How fast the vibration is determines the frequency, or pitch, of the sound –1 Hertz (Hz) = 1 vibration cycle per second Healthy ears are extremely sensitive to very small changes in air pressure. –Most people can hear sounds with frequencies between 20 Hz and up to about 20,000 Hz –As you get older, this range is reduced –If you listen to very loud sounds a lot, your hearing range will be reduced at a younger age!

7 Some units of pressure The standard unit of pressure is the Pascal 1 Pa = 1 newton / m 2 Normal air pressure at sea level = 1 atm 1 atm = about 100,000 Pa = 14.7 lbs/in 2 If your hearing is good, you can hear sounds with a pressure fluctuation of about 20 μPa (micropascal) =.000020 Pa.000020 / 100000 =.0000000002 atm = 2 x 10 -10 atm That’s about how loud a mosquito is 3 meters away from your ear A painfully loud sound has a pressure fluctuation of about 20 Pa =.00020 atm = 2 x 10 -4 atm

8 Table of Sound Levels in Air

9 How does sound propagate? Mechanical wave: molecules are pushed closer to adjacent molecules, increasing pressure Sound is a Pressure Wave Experiment: –Kids line up far apart (too far to reach) from each other –First kid touches the next one –As soon as you’re touched, touch the next one –Time how long it takes for the “wave” to propagate through the entire line –Repeat with everybody closer together –Did the wave travel faster?

10 Speed of sound in various materials Depends on how close the molecules are to each other, and how energetic they are (temperature) –Air: 340 m/s (761 mph) at sea level –Water: about 1500 m/s (3355 mph) –Metal (steel): about 6000 m/s (13,421 mph) In air, that’s approximately 5 seconds per mile –When you see lightning, start counting until you hear the thunder. If it’s 10 seconds, then the lightning struck 2 miles away

11 Pressure increases Temperature gets colder How does pressure and temperature affect the speed of sound? Air Ocean Water

12 Direction of Sound Experiment: –Everyone close your eyes –I walk around the room making noise –Point at me while I move How do your ears and brain figure out the direction of the sound?

13 Demonstration of Pitch (Frequency) Science and Music Listen to tones of various frequencies: –50, 100, 300, 800, 3000 Hz Generate 2 tones: 440 Hz, 441 Hz –Show the sine waves –Show how they go in and out of phase –Play one at a time – can you hear the difference? –Play both together, creating beats –This is the most accurate way to tune an instrument Use microphone, have students sing to try to match the tone

14 Tone Generation example in Audacity Note: use the Audacity program here, not just this slide. Explain and illustrate the sine waves and how they look at different frequencies, especially very close frequencies.

15 More with tones Generate harmonic tones –440, 880, 1760, 3520 –Play each tone individually –Mix at various gain settings to create sounds with different timbre More advanced stuff if time permits: –Spectrum analysis of the tones (with Audacity) –Spectrum analysis of voice (with Audacity) –Explain what the spectrum plot shows (amplitude vs. frequency)

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