Sound Waves 13.4-13.6

Sound Travels in Longitudinal Waves Let’s say the air is made up of tiny randomly-moving ping pong balls. If you vibrate a ping pong paddle in the middle of them, the balls then vibrate in sync with it. In some regions they are momentarily bunched up (compression) and in some they are spread apart (rarefaction). This is what happens to make sound waves! The air molecules work like the ping pong balls.

Parts of a Sound Wave Compressions – more dense area of the wave Rarefaction – less dense area of the wave Wavelength – distance between successive compressions or rarefactions

Pitch Pitch is our subjective impression about the frequency of sound. High pitch = High frequency Low pitch = Low frequency We can usually hear frequencies between 20 and 20,000 Hz. This changes as we age. We can hear waves of frequencies you can’t.

Where does sound travel? Sound usually travels through air, but any elastic substance (solid, liquid, or gas) can transmit sound. Many solids and liquids conduct sound better than air. Examples: Sound of a distant train approaching when placing your ear against the rail When swimming, have someone click two rocks together beneath the water while you’re submerged. Can you hear me?

Speed of Sound Did you ever wonder why you see lightning before you hear thunder? Or why you can see a person at a distance hammering before you hear it? This happens because sound takes time to travel from one location to another. It actually travels at about 330 m/s in dry air at 0ºC.

Speed of Sound What doesn’t affect it? What affects it? Loudness or softness High or low pitched What affects it? Wind conditions Temperature Humidity How do they affect it? Temperature – faster in warm air Humidity – faster in water vapor In water, sound travels about 4 times faster than its speed in air.

Reflection We call the reflection of a sound wave an echo. Reflected sound in a room makes it sound lively and full. Being too reflective, makes things sound garbled. Being too absorbent keeps the sound level low and dull. In the design of an auditorium or concert hall, there has to be a balance between the two. The study of sound properties is called acoustics.

Refraction Sound waves bend when parts of the waves travel at different speeds, like when traveling through uneven winds or varying temperatures. Refraction is the bending of sound. Depending on the weather, sometimes sound doesn’t carry well. Can you hear me?

Refraction On warm day, air near the ground may be warmer than the air above, so the speed of sound near the ground increases. Therefore, sound waves tend to bend away from the ground, making it seem like it doesn’t carry very well.

Reflection and Refraction Multiple reflections and refractions of ultrasonic waves are used by physicians to “see” into the body without X-rays! This is what we more commonly know as an ULTRASOUND!! The ultrasound echo technique is also use by bats and dolphins to locate objects around them. I can find my way in the dark!!

Dolphins!! Sight is not very useful for dolphins in the murky and dark ocean, so sound becomes a very useful sense. Distance is sensed by the time delay between sending sound and receiving its echo. Direction is sensed by the differences in time it takes for the echo to reach each ear. Since a dolphin mainly eats fish and fish only hear at low frequencies, they don’t realize they are being hunted! Don’t eat me!!