B-2 Physics: Wave Motion and Sound PDF

Module: B-2 Physics Topic 2.5 Wave Motion and Sound  INTRODUCTION On completion of this topic you should be able to: 2.5.1 Describe the nature of wave motion: mechanical waves...

Module: B-2 Physics Topic 2.5 Wave Motion and Sound  INTRODUCTION On completion of this topic you should be able to: 2.5.1 Describe the nature of wave motion: mechanical waves sinusoidal wave motion interference phenomena standing waves 2.5.2 Describe the characteristics of sound: production intensity pitch quality 2.5.3 State the speed of sound and describe factors that affect it 2.5.4 Describe the Doppler effect 30-03-2024 Slide No. 2  WAVE MOTION When energy is transferred by the passage of a periodic disturbance through an elastic medium, it is said to be in Wave Motion. Energy sent down a rope can cause change, (do work by applying a force), at the far end. No rope moves in the direction that the energy is transmitted. However, it does move up and down between minimum and maximum amplitudes of displacement. 30-03-2024 Slide No. 3  SINUSOIDAL WAVE MOTION Sinusoidal wave motion means that type of motion that follows the pattern of a sine wave. Sine waves are a graphical representation of angular sine values. 30-03-2024 Slide No. 4  SINUSOIDAL WAVE MOTION Compression or longitudinal waves are a pulse through a medium which are characterised by compression and rarefaction. They can be mapped as sinusoidal waves. Sound waves are compression waves because they use the mechanical action of molecules to transfer their action through a medium. For this reason, sound waves cannot travel through a vacuum. Light waves are not mechanical waves, so they are able to travel through vacuum. 30-03-2024 Slide No. 5  SINUSOIDAL WAVE MOTION A compression sound wave, for example from a tuning fork. Compression wave mapped as a sine wave. 30-03-2024 Slide No. 6  SINUSOIDAL WAVE MOTION Electromagnetic waves including visible light travel in sinusoidal wave motion. The waves generated by the source have an amplitude, a wavelength, a period, and a frequency. Amplitude Wavelength Frequency - the number of wavelengths occurring per second. 30-03-2024 Slide No. 7  WAVE EQUATION The period is the time taken for one cycle to complete and equals 1/f. The speed of energy propagation is V and given by: V = λf If λ is in metres, and frequency in Hertz, then speed is in m/s (metres per second) 30-03-2024 Slide No. 8  REFLECTION AND REFRACTION A system of waves will change direction when it changes speed for any reason. (Refraction) If the system encounters a solid barrier, the energy will be reflected. 30-03-2024 Slide No. 9  DIFFRACTION When an obstacle has an edge then the wave system will “bend” and start a new system at that point. This explains how we can hear around corners. The nature of light waves has been proven by experiments showing the diffraction of light. 30-03-2024 Slide No. 10  INTERFERENCE PHENOMENA (SUPERPOSITION) When waves converge their effect is algebraically added. If a sine wave is overlaid on another sine wave of identical amplitude and frequency, it will produce a sinusoidal wave of same frequency but double the amplitude. Similarly, if a sine wave was overlaid on another sine wave exactly half a phase out of synchronisation, the result would be the two waves would cancel themselves out. 30-03-2024 Slide No. 11  INTERFERENCE PHENOMENA A light source illuminates slits in a screen and a small amount of light passes through them. The pattern of light displayed on a screen shows the constructive and destructive interference of the converging semi-circles of light waves. Bright bands appear as the light waves coincide. Dark bands are indicative of the amplitude of the waves cancelling themselves out. 30-03-2024 Slide No. 12  STANDING WAVES Standing waves are formed when a fundamental wave (the longest wavelength that can fit in a tube or on a string) is subjected to interference and a harmonic wave is produced (a multiple of the original, fundamental wave). Standing waves can be formed when waves of identical frequency and amplitude are travelling in opposite directions and converge. The blue wave is moving to the right and the green wave is moving to the left. 30-03-2024 Slide No. 13  STANDING WAVES The point where the waves have no amplitude is called the node. The point of maximum amplitude is called the antinode. Fundamental or 1st harmonic 1st overtone or 2nd harmonic 2nd overtone or 3rd harmonic 30-03-2024 Slide No. 14  RESONANCE DAMAGE Tail Boom has a natural frequency of 1 Hz. Since each of the three blades causes a pulse each revolution, the blades need to be rotating at 1 revolution/3 sec. Therefore a rotor speed of 20 RPM would be critical and the pilot would be warned against operating at that speed. Since the boom also has a secondary, or overtone, resonant frequency of twice the fundamental resonant frequency. 40 RPM would also have to be avoided but would not be as critical as 20 RPM. 30-03-2024 Slide No. 15  BEATS If the frequencies of two sounds are reasonably close, they will interfere to produce a beat frequency. The two propellers of a twin engine aircraft will produce an unpleasant “thrumming” if they run at slightly different rpm’s. Pilots “tune” it out by ear. 30-03-2024 Slide No. 16  SOUND- INTENSITY Determined by the amplitude of the sound wave. The larger the amplitude, the louder the sound. Sound intensity is measured in decibels (db). IL = 10 log I Decibels I0 ( An increase of a factor of 10 is 1 db ) 30-03-2024 Slide No. 17  SOUND- INTENSITY EXAMPLE: The intensity of a given sound is 10-5Watts/m2. What is the intensity level (IL) in decibels? 30-03-2024 Slide No. 18  SOUND- INTENSITY The intensity of sound decreases inversely with the square of the distance from the source of sound. Therefore, doubling the distance from a source of sound decreases the intensity to one-fourth of the previous value. Use PPE 30-03-2024 Slide No. 19  SOUND PITCH The frequency of the sound determines its pitch. The faster the air vibrates, the higher the pitch, and the faster the wavelengths pass the listener. Middle C on the piano is 262 Hz. Human hearing ranges from about 20 Hz to 20,000 Hz. 30-03-2024 Slide No. 20  SOUND QUALITY or “TIMBRE” Quality is the distinctive property of sound. Quality of sound depends on the nature of the harmonics present in a complex sound. You will recall that harmonics are numerical multiples of the original frequency. The tuning fork is a pure note, while the other sounds are affected by different harmonics present in the sound. 30-03-2024 Slide No. 21  SOUND – QUALITY or “TIMBRE” 196 Hz is the fundamental, and each instrument has a different sound due to the influence of its own harmonics. 30-03-2024 Slide No. 22  SPEED OF SOUND Sound travels at a speed which varies according to the elasticity of the medium through which it travels. Speed of sound in air varies according to atmospheric temperature. In normal atmospheric conditions at sea level, the speed of sound is about 340 m/sec or 1,224 km/h. 30-03-2024 Slide No. 23  MACH NUMBER At high speeds the aerodynamic forces depend on the Mach number. Speed of sound in a medium depends on elasticity and varies with temperature. For air, the local speed of sound a given by:- Kts. (Where T is the temperature in Kelvin) 30-03-2024 Slide No. 24  SPEED OF SOUND An aircraft creates disturbances to the air which act like sound waves and travel at the same speed, but are not detected by the human ear. LESS THAN SPEED OF SOUND MORE THAN SPEED OF SOUND SUBSONIC SUPERSONIC Aircraft at the speed of sound, then disturbances cannot propagate away from the airframe, They pile up, creating a shock wave, which is intense enough to be heard. The sonic boom. The “Sound Barrier” is the extremely large increase in air resistance which occurs at this point. 30-03-2024 Slide No. 25  DOPPLER EFFECT A B A: Source of sound - not moving, waves radiate equally - no effect on pitch. B: Source of sound moves frequency ahead of the source becomes higher than the frequency behind it. This change in frequency is called the Doppler effect. As it approaches, the sound becomes louder and higher pitched, then decreases in pitch and intensity as the vehicle passes. 30-03-2024 Slide No. 26  CONCLUSION Now that you have completed this topic, you should be able to: 2.5.1 Describe the nature of wave motion: mechanical waves sinusoidal wave motion interference phenomena standing waves 2.5.2 Describe the characteristics of sound: production intensity pitch quality 2.5.3 State the speed of sound and describe factors that affect it. 2.5.4 Describe the Doppler effect. 30-03-2024 Slide No. 27  This concludes: Module: B-2 Physics Topic 2.5 Wave Motion and Sound