Chapter 15 - Sound Sound wave is a longitudinal wave

LT #1 Relate the properties of sound waves to the way we perceive sound.

What is sound? Sound waves are longitudinal waves Sound waves is simply a pressure change that is transmitted through matter. Produced by vibrations Transmitted through matter, they need a medium. Sound is a mechanical wave

Interesting Fact…. What is wrong with this video clip? (Video clip: space balls) Sound CANNOT travel through a vacuum! (Video clip: Bell jar)

How do we hear? Your eardrum is a pressure detector. Eardrums vibrate when there is a pressure change. These vibrations send electrical impulses to the brain that decipher it as sound.

Parts Compression = high pressure rarefaction = low pressure Wavelength = C to C or R to R Amplitude = length of compression or rarefaction

Speed of Sound The speed of sound depends on the medium and temperature Equation v sound in air = T – Sound in air at 20°C (72°F) = 343 m/s – Sound in air at 0°C (32°F) = 331 m/s The speed of sound increases with increasing temperature.

Speed of Sound Sound travels fastest in solids, then liquids, slowest in air. Ex: 11 times faster in steel!! Sound travels fastest through materials with the highest compressibility not the highest density. Waves (slinky, sound, light, etc.) slow down in materials with higher densities.

Loudness The loudness of sound depends on the amplitude of the sound wave. Small amplitude = soft Large amplitude = loud Loudness is measured in decibels (dB) We perceive each increase of 10 dB as twice as loud

LOUDNESS

Pitch The frequency of a sound wave is the pitch of the sound. Higher Frequency = Higher Pitch video clipvideo clip Lower Frequency = Lower Pitch video clipvideo clip Longer Wavelength =Low Note (Pitch) Shorter Wavelength =High Note (Pitch)

Interesting Fact Only a small range of frequencies are audible: we can hear from about 20 Hz – 20,000 Hz depending on age! We can’t hear: – Ultrasonic Waves: above 20,000 Hz – Infrasonic Waves: below 20 Hz LET’s take a test:

Beats Beats occur when 2 frequencies (very nearly identical) interfere and produce high and low sound levels. The frequency of the beat = difference between the 2 frequencies

LT #1 Quiz

Problem Solving Solve problems relating to frequency, wavelength and velocity of sound. V=fλ - Speed of sound in air depends on the temperature of air. - Speed of sound in air at 20°C is 343m/s. - Speed of sound increases at the rate of 0.6m/s for every 1°C increase in temperature. -Sound can travel through solids, liquids and gases. It travels the fastest in solids.

Standing Waves A wave appears to stand still when the incident and reflective wave interact. Node – the point that doesn’t move when two pulses meet. Created by destructive interference

Standing Waves A wave appears to stand still when the incident and reflective wave interact. Anti-node- the largest amplitude. Created by constructive interference.

Free and forced vibrations When an object is tapped, it will vibrate with its own natural frequency - this are free vibrations. (This is why different objects sound different when tapped.) When there are vibrations around an object it is forced to pick up that particular vibration – these are forced vibrations.

Resonance When the forced vibrations match the natural frequency of the object, the object easily picks up the energy and starts to oscillate. This is called RESONANCE. Examples of resonance are swings, Tacoma bridge collapse, breaking wine glasses.

Resonance or Standing waves in Strings When waves are continuously sent (incident waves) and they hit a rigid boundary (reflected wave) then they come back out of phase. These incident and reflected waves interact to form standing waves as shown. The faster the vibrations, the more the nodes and antinodes.

Resonance or Standing Waves in Strings Knowing the length, the actual wavelength of the standing wave can be calculated. 1 st Harmonic is also called the fundamental frequency. Other harmonics are called overtones or octaves.

Resonance or Standing Waves in Strings Over all, we can see that since the medium (string) is the same, changing wavelength results in changing frequency! Just like we saw in the slinky lab If the wavelength halves, the frequency doubles.

Resonance or Standing Waves in Open Pipes Open pipes are open at both ends. You can produce notes in open pipes when air is moved through pipe with a particular frequency based on the pipes length. These frequencies are capable of producing standing waves.

Resonance or Standing Waves in Open Pipes Resonance occurs when anti-nodes are formed at the open ends. Anti-nodes occur at the open ends because there is more room for air to Vibrate. The wavelength can be calculated as shown.

Resonance or Standing Waves in Closed Pipes Closed pipes are closed at one end. Based on the length of the pipe, certain frequencies are capable of producing standing waves in closed pipes.

Resonance or Standing Waves in Closed Pipes Resonance in closed pipes occur when there are anti-nodes at the open end and nodes at the closed ends.

Speed of Sound Lab When the tube was at the right length then the reflected wave from the water met another vibration coming down from the tuning fork and they interfered forming a standing wave. Sound will amplify = Resonance

LT #3 Quiz

Doppler Effect The change in wavelength caused by the motion of the source or the observer.

A stationary source would produce waves that travel out equally on all sides. The distance between the wave fronts are all the same. Stationary Object

The wave fronts are produced with the same frequency as before. However, since the source is moving, the center of each new wave front is now slightly displaced. As a result, wave fronts begin to bunch up in front of the object Source/Observer Moving Closer

The wave fronts again are produced with the same frequency as before. This time, since the source and/or observer are moving away, the wave fronts begin to spread out. Source/Observer moving Away

Chuck YeagerChuck Yeager was the first person to break the sound barrier when he flew faster than the speed of sound in the X-1 rocket-powered aircraft on October 14, Sound Barrier

Here: fo = apparent frequency as heard by the observer. fs = actual frequency of the source. V = speed of sound (343m/s) Vo = speed of the observer Vs = speed of the source Determining Apparent Frequency

Use +/- when the source and the observer are getting closer. Use -/+ when the source and the observer are getting further away. Determining Apparent Frequency

Source not moving Example: 1. Find the apparent frequency heard by the observer as the observer runs away from a 300 Hz siren, at the speed of 20m/s. 2. Find the apparent frequency heard by the observer as the observer runs towards the 300 Hz siren, at the speed of 20m/s.

Example: 1. Find the apparent frequency heard by an observer as a cop car drives away with a 300Hz siren, at 20m/s. 2. Find the apparent frequency heard by an observer as the cop car drives towards the observer with a 300 Hz siren, at 20m/s. Observer not moving

LT #4 Quiz