Unit 15 ­ Waves
Chapter 26. Sound 26.1 The Origin of Sound pitch​
how high or low a frequency appears to be, high pitch = high frequency, low pitch = low frequency infrasonic ​
too low for the human ear to hear (<20 Hz) ultrasonic ​
too high for the human ear to hear (>20,000 Hz) All sounds are produced by the vibrations of material objects Q1. Find the frequency of a sound wave that has a wavelength of 1.5 m. Could you hear this sound? v = 340 m/s frequency = speed/length = 340 m/s / 1.5 m = 227 Hz. Yes. ☐
I can relate the pitch of a sound to it’s frequency 26.2 Sound in Air compression ​
pulse of compressed air rarefaction​
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disturbance in the air, when pressure is lowered, ​
the opposite of compression Q2. How far away is a storm when you hear the thunder 13 seconds after you see lightning? velocity = distance/time d=vt
340m/s*13s = 4420 m (we’re safe!) ☐
I can describe the movement of sound through air 26.3 Media That Transmit Sound Sound cannot travel through a vacuum (like space), because there is nothing for the waves to vibrate!Sounds needs a medium. Because of this vibrational requirement, sound travels slowest through gas (like air), then liquid (like water), and then solids (like a brick wall). Solids have the most atoms in the same amount of volume = more media. Q3. If the speed of sound in seawater is 1530 m/s, find the wavelength of an underwater sound wave whose frequency is 7 Hz. v = ᵰf ᵰ = v/f
1530 m/s / 7 Hz = 218.6 m ☐
I can compare the transmission of sound through solids, liquids and a vacuum. 26.4 Speed of Sound Not only does the speed of sound depend on the state of matter, but also the temperature and a property called ​
elasticity.​
Sound travels faster through warmer air, than cooler air. Sound also travels faster through elastic materials. An elastic material is able to regain it’s shape after having an applied force. In an elastic material, atoms are closer together, allowing for faster transmission of the vibrations. Q4. What is the speed of sound? 340 m/s Q5. An echo bounces off a mountain and returns 6 hours later. How far away was the mountain? v = 340 m/s, t = 6 hours*3600 s = 21600 s d = vt = 340 m/s * 21600 s = 7.34E6 m ☐
I can describe the factors that affect the speed of sound 26.5 Loudness The intensity of a sound is proportional to the square of the amplitude of a sound wave. It can be measured by an oscilloscope. Loudness is a subjective sensation by the brain, so it’s different for everyone. The intensity can be measured in decibels (dB) ☐
I can describe loudness as sound intensity 26.6 Forced Vibration ☐
I can give an example of forced vibration 26.7 Natural Frequency natural frequency ​
frequency at which an object will vibrate ☐
I can describe natural frequency 26.8 Resonance resonance ​
when a forced vibration matches an object’s natural frequency, resulting in an increased amplitude ☐
I can describe resonance 26.9 Interference Sound waves can experience interference, as well. Crest of a sound wave = compression, trough of a sound wave = rarefaction example, noise cancelling headphones ☐
I can describe how waves interfere with one another 26.10 Beats beats ​
variation in the loudness of a sound caused by interference when two tones of slightly different frequencies occur together A special case of interference! Beat frequencies of over 10 Hz are too rapid to be distinguished, normally Q6. What is the beat frequency when a 262­Hz and a 266­Hz tuning fork are sounded together? 266 Hz ­ 262 Hz = 4 Hz, 4 beats per second Q7. What is the beat frequency when a 262­Hz and a 272­Hz tuning fork are sounded together? 272 Hz ­ 262 Hz = 10 Hz, 10 beats per second ☐
I can describe beats