Lecture 6.2.1 Sound I PDF

Introduction to General Physics II M 6.2.1 Vibrations and Sound Dr Orlaith Brennan  Learning Outcomes On completion of this lecture, students will be able to… Explain how a vibrating object produces a sound wave. Describe the hearing mechanism and frequency discrimination...

Introduction to General Physics II M 6.2.1 Vibrations and Sound Dr Orlaith Brennan  Learning Outcomes On completion of this lecture, students will be able to… Explain how a vibrating object produces a sound wave. Describe the hearing mechanism and frequency discrimination in humans. Discuss, using everyday examples from the physical world the phenomenon of reflection, refraction, diffraction and interference of sound. Explain and determine how the speed of sound varies through different mediums and with temperature. 2  Mechanical Waves There are two classes of waves; mechanical and electromagnetic waves Mechanical waves must have a medium (solid, liquid or gas) to travel through. Mechanical waves travelling through a medium consist of vibrations being passed on from one molecule to another. 3  Longitudinal Waves A longitudinal wave is a wave where the direction of vibration is parallel to the direction in which the wave travels. As the wave passes, the particles vibrate parallel to the direction of the wave. 4  Sound is a Longitudinal Wave We can use a sinusoidal curve to represent a sound wave. The crests of the waves correspond to compressions, the troughs to rarefactions Sound travels as a wave as it exhibits all the properties of waves including reflection, refraction, diffraction and interference. 5  Vibrations and Sound Every source of sound is a vibrating object. To be audible to the human ear a sound must have a frequency between ~ 20 Hz and 20 000 Hz (frequency limits of audibility). Infrasonic waves are waves with frequencies below the audible range and ultrasonic waves with frequencies above 6 the audible range.  How a Vibrating Object Produces a Sound The gas molecules in the air vibrate parallel to the direction in which the compressions and rarefactions travel 7  How a Vibrating Object Produces a Sound 1. Sound wave travels through the air. 2. Enters the outer ear. 3. Travels through the ear canal. 4. Eardrum (tympanic membrane) vibrates with the same frequency. 5. Forces the three bones (ossicles) of the middle ear into motion and sound is amplified. 6. Causes the fluid in the inner ear (cochlea) to move stimulating the hair cells lining the cochlea releasing neurochemical messengers. 8 7. Auditory nerve carries electrical signal to the brain which translates it into sound.  The Human Ear and Hearing 9  Reflection An echo is sound reflection. The further away you are from the wall the longer it takes for the sound to travel back to the person. 10  Refraction Why can sound be heard more clearly on a cold night than on a warm day? Recall the changing direction of a wave when it enters a region where its speed changes is called refraction of waves 11  Refraction Interface Undeflected Fast → Slow Normal Bend towards the normal Cool air: Warm air: Sound travels Sound travels slower faster 12  Refraction On a warm day Normal Sound waves tend to bend towards the normal (away Interface from the warm ground). Refraction is not abrupt, it is gradual. 13  Diffraction Can sound be heard around corners? Recall the sideways spreading of waves into the region beyond a gap or around an obstacle is called diffraction of waves. Sound can be heard around corners because sound waves are diffracted. 14  Diffraction When parallel waves meet a flat obstacle with a hole in it the waves spread out slightly but most pass through. If the wavelength λ of the waves is close in length to the size of the gap the waves spread into the whole region beyond the gap. High pitched sounds tend to be more directional as they do not diffract as much. 15  Interference Two travelling waves which exist in the same medium will interact with each other. If their amplitudes add, the interference is said to be constructive interference, and destructive interference if they are "out of phase" and subtract. When waves arrive out of phase with each other by 180°, one set of waves is λ/2 ahead of the other. Between these two extremes lie an infinite number of variations in which crests and troughs combine together to increase or decrease the 16 amplitude.  Interference Patterns of destructive and constructive interference may lead to "dead spots" and "live spots" in acoustics. 17  Speed of Sound The speed of sound depends on the temperature of the medium. 18  Speed of Sound The speed of sound also depends on the medium. ρ is Density B is Bulk Modulus (indication of how hard it is to compress the material) 19  Speed of Sound - Problem An explosion occurs 275 m above an 867 m thick ice sheet that lies over sea water. If the air temperature is - 7°C , how long does it take the sound to reach a research submarine 1250 m below the ice? The Bice = 9.2 x 109 Pa, pice = 917 kg/m3 and vwater = 1533 m/s. (Neglect any changes in the bulk modulus and density with temperature and depth) Ice 20  Speed of sound in air Time travelling in air vair = 331 (T/273)1/2 v = d/t → t = d/v vair = 331 (273-7/273)1/2 t = 275/327 vair = 327 m/s tair = 0.841 s Speed of sound in ice Time travelling in ice Ice v = (B/p)1/2 t = d/v v = (9.2 x 109 /917)1/2 t = 867/ 3200 vice = 3200 m/s t ice = 0.271 s Time travelling in water Ttotal = Tair+ Tice + Twater t = d/v = t = 1250/ 1533 0.841 + 0.27 + 0.815 t water = 0.815 s = 1.93 s 21