Science UNIT 21 THE NATURE OF SOUND Section

Science UNIT 21: THE NATURE OF SOUND Section 1: What Is Sound? Section 2: Properties Of Sound Prof. Ignacio Anguera Colegio Real de Panamá The Perfect Partner for your Digital Business 4/27

WHAT IS SOUND? Sound waves can be classified as: 1. Mechanical: they require a medium 2. Longitudinal: they vibrate back an forth Rest position Compression Rarefaction Vibration

PROPERTIES OF SOUND The differences between sounds depend on the properties of the sound waves VELOCITY PITCH Speed of sound depends on the medium, and is affected by two factors: • Spacing of the particles in the media: The closer the atoms or molecules are to one another, the faster the sound will travel. • Temperature of the media: At higher temperatures, the particles vibrate faster, so sound waves can travel more quickly The pitch is a measure of how high or how low a sound is perceived to be. It is related to the frequency as follows: • Higher the frequency (or lower the wavelength), higher the pitch. LOUDNESS Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined by the amount of energy. The unit of intensity is the decibel (d. B). Is related to the amplitude as follows: • Lower the frequency (or • Higher the amplitude, louder higher the wavelength), the sound (more decibels). lower the pitch. • Lower the amplitude, softer the sound (less decibels).

Stationary Situation (No Doppler Effect) DOPPLER EFFECT Because the source, observers, and air are stationary, the wavelength and The Doppler Effect is frequency are the same in all directions an alteration in the and to all observers. observed frequency of Observers Move Relative to the Source a sound due to motion of either the source or Motion of the observer toward the source (Y) increases frequency (passes through the observer. more waves than it would if stationary). Motion away from the source (X) decreases. Source Move Relative to the Observer A source moving to the right spread out from the points at which they were emitted. In the direction of motion wavelength is reduced (observer Y hears a higher-pitch) and the opposite for X.

Source Move Relative to the Observer DOPPLER EFFECT The situation where the source is moving is actually a bit more difficult to picture. In the following diagram, the source (the engine of Time = 0 u the car) marked with a red circle is moving to the right with a certain velocity. . At periodic points in time (depending on the frequency), the engine sends off a circular wave. • At the beginning (time = 0 units) the car is in the position P 1, and sends the first wave (W 1) velocity P 1 W 1

DOPPLER EFFECT Time = 1 u P 1 Source Move Relative to the Observer After a certain amount of time (1 unit) the car sends the second wave, however all the elements has changed position during this period of time. This is the new diagram: • Car: The car sends the second wave (W 2), but since it will have moved forward a bit, the center of W 2 is shifted slightly in the direction that the source is moving (P 2). • W 1: Once the wave leave the source, it is no longer affected by the motion of the source – the wave just travels on its own. After 1 second, W 1 will has displaced a certain amount of distance. P 2 W 1 *Time = 0 u

DOPPLER EFFECT Time = 2 u P 1 P 2 Source Move Relative to the Observer After 2 units of time, this is the situation: • Car: The car now is in the P 3, and sends a third wave (W 3). • W 1 and W 2 has displaced a certain amount of distance. P 3 W 2 W 1

DOPPLER EFFECT Time = 3 u P 1 Source Move Relative to the Observer After 3 units of time, the car sends the 4 th wave while the rest of waves are continuing its path. The Doppler Effect can be observed as follows: • At the back of the car, the wavelength is higher, so frequency is reduced. An observer at the back of the source will hear a lower pitch. P 2 P 3 P 4 • In the front of the car, the wavelength is reduced, so frequency increases. An observer placed in front of the source will hear a higher pitch. W 4 W 3 W 2 W 1

DOPPLER EFFECT This constructive superposition produce pressures great enough to create what is known as sonic boom. This effect can be seen in airplane supersonic flights. A vapor cone visually can be seen and you will then hear the sonic boom a few seconds later, as a rumbling thunder or an explosion. Sonic Barrier (v=c) When the speed of a source equals the speed each successive wave is superimposed on the previous one because both source and sound moves forward at same speed. The resulting pile of waves (waves combine by constructive interference) forms a large amplitude "sound barrier“.

Sonic Barrier (v > c) DOPPLER EFFECT Shock waves are also produced if the aircraft moves faster than the speed of sound. When the speed of a source exceeds the speed of sound (v > c) the wave fronts lag behind the source in a cone-shaped region. The edge of the cone forms a supersonic wave front with an unusually large amplitude called a "shock wave". When a shock wave reaches an observer a "sonic boom" is heard. When the source exceeds the speed of sound, no sound is received by the observer until the source has passed. If the aircraft flies close by at low altitude, pressures in the sonic boom can be destructive and break windows as well as rattle nerves. Because of how destructive sonic booms can be, supersonic flights are banned over populated areas of the United States.

ECHO PROBLEMS Mary Devils went hiking at Anton´s Valley Park. At one point, she let out a holler which reflected off a nearby rocky cliff and was detected as an echo 1. 80 seconds later. Determine the distance to the rocky cliffs. Assume a speed of sound of 344. 00 m/s. Drawing: Procedure: d Data: techo = 1. 80 s v = 344 m/s d? (from the person to the cliff) S 2: We need to find the distance from the person to the cliff, which is half of the travelled by the sound wave (we need to subtract the distance of going back) 309. 60 m

ECHO PROBLEMS A deep sea ocean vessel uses SONAR to detect the ocean's bottom. Sound waves are emitted from the surface of the ocean and travel through the water at 1450 m/s. The ocean bottom is 1630. 00 m below the surface. Determine the amount of time that passes before the sound waves are reflected back to the surface. Drawing: Procedure: d Data: v = 1450 m/s d = 1630 m techo ? S 2: Once we have the total distance covered by the wave, we just need to find the time 2. 25 s

ECHO PROBLEMS You look up and see a helicopter pass directly overhead. 3. 10 s later you hear the sound of the engine. If speed of the air is 320 m/s, how high was the helicopter flying? Drawing: d Procedure: S 1: In this case, there is no reflection of sound, so we apply the formula directly Data: v = 320 m/s d=? t = 3. 10 s d = t v = 3. 10 s 320 m/s = 992 m

ECHO PROBLEMS A person standing between two vertical cliffs and 640 m away from the nearest cliff shouted. He heard the 1 st echo after 4 seconds and the second echo 3 seconds later. Calculate (i) the velocity of sound in air and (ii) the distance between the cliffs. Drawing: S 1: Find the velocity using the information of the Procedure: nearest cliff. We have to consider that the time is time, so the time must be divided over 2 to know t (time to cover d 1). the echo time 1 S 2: Now we apply the formula to find velocity d 1 dt techo = 4 s 1 techo = 7 s 2 S 3: In order to find d 2, we need to know the time to know the wave to cover this distance. This is the half that the time it took for the echo. Data: d 1 = 640 m d 2 v=? d=? S 3: Find d 1 with the velocity formula S 4: Find dt adding d 1 and d 2