Sound, Sound Energy and Speed Sound Creating Sound Moving Sound Inquiry Noise Cancelling Headphones The Speed of Sound

Question – Philosophical or Scientific? If a tree falls in the woods, does it make a sound?

Question - Answered Of course it does! When the tree falls to the ground it generates a vibration which creates sound waves whether someone is there to hear them or not.

Sound Hearing: Humans can generally hear sound waves with frequencies between 20 Hz and 20,000 Hz (20 kHz) Energy: Sound is simply the transfer of energy (caused by vibrations) between particles within and across mediums The Power of Sound Waves

Waves caused by disturbing a medium – results in “noise”, music, vibrations you can’t hear by other animals can. Mechanical Waves – Need a medium (solid, liquid, gas) to travel Longitudinal Waves – Vibrations are parallel to wave motion. Sound Waves

Sound waves need a medium to travel If the medium changes, the speed of sound changes. Average Speed of Sound Air: 340m/s Sound moves the fastest through solids and the slowest through gases. solids  liquids  gases Sound moves the fastest through hot air and the slowest through cold air. hot air  cold air

Sound Waves Notice the frequency and wavelength DO NOT change the speed of sound. Changing the frequency changes the wavelength. Changing the wavelength changes the frequency. A high frequency means it is “high pitch” (alto singer) A low frequency means a “low pitch” (baritone singer) There are sounds people cannot hear. – “Ultrasonic” sounds are pitches higher than human hearing. – higher than 20,000 Hz – “Infrasonic” sounds are pitches lower than human hearing. – lower than 20Hz f λ f λ

Creating Sound - The Tuning Fork When you hit a tuning fork you cause the arms to vibrate back and forth Each outward movement pushes the nearby air molecules away (a compression) When the arms come back together the air molecules move back to their original spot (a rarefaction)

Creating Sound As the arms move back and forth, rarefactions and compressions follow one another as the sound waves travel through the air away from the tuning fork *Note the particles only vibrate, they maintain their original areas while transferring the energy away from the source**

Moving Sound The vibration of particles can be better represented with a graph which shows changes in air pressure (transverse waves)

Creating Sound All sound is created in a similar way A disturbance causes a compression which is followed by a rarefaction followed by a compression and so on as the signal moves away from the source

Moving Sound 1 wavelength is the distance between successful compressions The “harder” the particles are moved away from the source the greater the air pressure change (amplitude) The larger the amplitude, the louder the sound

You can change the loudness of a sound two ways. Change the amplitude (amplify) You can amplify a wave by turning “up” the volume, OR by adding to the wave again and again... And again and again… – Turning “up” the volume is increasing the “decibel level” of the source. Everything is vibrating (every object vibrates, “natural frequency”) – you can amplify the wave if you match its natural frequency. Change the intensity – Intensity can be changed by your distance from the source. – The closer you are to the source, the more intense the sound, aka “louder” Sound Waves

Recall – The Speed of Sound Sound travels faster in – solids then in liquids – liquids compared to gases – denser mediums Why would sound travel faster in steel than in air? Your answer should be scientific and specific. MediumSound Wave Speed Air (20°C)343 m/s Water (20°C)1,482 m/s Steel (20°C)5,960 m/s

People Hearing Sounds Intensity

Both Intensity and Frequency Frequency (Hz)

Infrasonic and Ultrasonic Waves Dog whistles are high pitched so that dogs can hear them, but not humans. Dolphins and bats use “sonar” aka: ultrasonic waves to communicate and “see” barriers. – We have imitated them and use ultrasonic waves for ultrasounds! Whales, elephants, hippos, and other animals use infrasound to communicate over distances—whales can do this over the distance of hundreds of miles! Infrasonic waves can be used to predict natural disasters such as tornados, volcanic eruptions, etc.

Doppler Effect – Sound Waves Why does each observer hear a different sound? This is the Doppler Effect! Each observer hears a different sound because as the motorcycle moves, it “catches up” with it’s own sound waves. Observer B hears a high frequency because the waves are closer together (small wavelength). Observer A hears a low frequency because the waves are farther apart!

The Doppler effect makes sources of sound appear to be higher and lower pitch based on their relative motion. Moving towards the observer – higher frequency Moving away from the observer – lower frequency High frequency means high pitch! Low frequency means low pitch! Doppler Effect – Sound Waves

FREQUENCY IS NOT LOUDNESS! It’s not FLIPPING louder if it is more frequent! … and remember…

Speed of Sound in Air The speed of sound in air depends on the temperature of the air it is passing through Its speed increases by 0.59 m/s for every 1°C increase in temperature The specific speed can be accurately determined using the following equation:

Check Your Understanding Calculate the speed of sound in air when the temperature is 16°C. Answer: 341 m/s

Check Your Understanding A string is vibrating at a frequency of 440 Hz. How many vibrations does it make when the sound produced travels 664 m through air at temperature 0°C? Answer: 880 v =fλ v = d/t

Check Your Understanding A 200-m dash along a straight track was timed at 21.1 s by a timer located at the finish line who used the flash from the starter’s pistol to start the stopwatch. If the air temperature was 30.0°C, what would the time have been if the timer had started the watch upon hearing the sound of the gun? Answer: 20.5 s v =fλ v = d/t