Characteristics of Sound

What Produces Sound??

If a tree falls in a forest and no one is there to hear it, will there be sound?

What produces sound?

• Sound waves originate from a vibrating source• Sound is also known as a “pressure wave”• This is because vibrations cause changes in

pressure near the source

Vibrating Sources• http://www.youtube.c

om/watch?v=iYpDwhpILkQ

• http://www.youtube.com/watch?v=3zoTKXXNQIU&feature=fvw

To Do

• Prove that sound is a pressure wave.....

– In groups of three, light a candle and use the pressure generated by your voice to see if you can blow it out

– Determine the characteristics of sound (frequency/amplitude) that are the most successful

Characteristics of SoundSound travels in longitudinal waves

Has areas of high pressure (compressions), normal pressure, and low pressure (rarefactions)

Bell Jar Experiment

• http://www.youtube.com/watch?v=ce7AMJdq0Gw

Characteristics of Sound

• Bell jar experiment tells us that sound cannot travel through a vacuum.

• Sound requires a medium....which type of wave is this?

• Sound is capable of travelling through different types of solids, liquids, and gases.

• In which medium would sound travel the fastest?

To Do• Fill up one balloon with air and the other balloon

with carbon dioxide (use baking soda + vinegar to produce carbon dioxide)

• Investigate the differences in sound between the two mediums– Hold the balloon between your ear and some source

of low-volume sound (i.e. Ticking watch). Compare the sound to the air filled balloon

– Experiment with various sources of sound & record results

Speed of Sound

• At normal atmospheric pressure and 0 degrees C, the speed of sound is 332 m/s

• If air pressure stays the same and temperature increases, speed of sound increases by 0.6 m/s per degree C.

Speed of sound in air = (332 + 0.6 T)m/s** T = temperature in degrees Celsius

Practice Questions

1) What is the speed of sound at 20 degrees C?2) What is the speed of sound at – 20 degrees

C?3) What is the speed of sound in air at 35

degrees C?4) How much time is required for sound to

travel 1.4 km through air if the temperature is 30 degrees C?

Physics – Speed of Sound Answer Key

1) • V = (332 + 0.6 (20))m/s• V = 344 m/s2) • V = (332 + 0.6 (-20)) m/s• V = 320 m/s3) • V = (332 + 0.6 (35)) m/s• V = 353 m/s4) • V = (332 + 0.6 (30))m/s• V = 350 m/s• 1400 m x 1second/350 m = 4s

Frequency of Sound

Avg. Human can respond to frequencies between 20 and 20 000 Hz.

< 20 Hz are called infrasonic>20 000 Hz are called ultrasonic

Dogs 15 – 50 000 Cats 60 – 65 000Bat 1000 – 120 000 Moth 3000 – 150 000

Frequency of Commonly Heard Sounds

Male speaking voice – 120 HzFemale speaking voice – 250 HzHighest piano note – 4186.01 HzStereo system – 15 – 30, 000 Hz

Dog – 452 – 1080 HzCat – 780 – 1520 Hz

Bat – 10 000 to 120 000 HzSilent dog whistle – 20 000 to 24 000 Hz

Frequency vs. Wavelength