Lecture Notes Intro: Sound Waves:

Transcription

1 Lecture Notes (Propertie es & Detection Off Sound Waves) Intro: - sound is very important in our lives today and has been throughout our history; we not only derive useful informationn from sound, but we also get pleasure from sound - musical instrumentss have been discovered thatt date back to over 43,000 years ago - other organisms than humans also usee sound; bats use high frequency sound to hunt insects; whales use extremely low frequency sound to communicate over vastt distances Sound Waves: - sound is a series of pressure fluctuations in the mediumm between the source and the listener

2 - for example, in speech, the medium iss air and the air pressure fluctuations are caused by actions in the vocal tract - these air pressure fluctuations are called sound waves - sound waves travel through the air, cause the eardrum to vibrate, which translates s into an electrical signal to the brain - consider a tuning fork; when struck, the arms of the tuning fork swing back and forth, causing the neighboring air to move in a regular pattern - the graph of air pressure variation over time is a waveform - tuning forks are engineered to create waveforms that are sine waves

3 - speech, however, consists of complex waves, as do most sounds Longitudinal Waves: - sound waves are longitudinal waves; in longitudinal waves the particles of the medium undergo displacements parallel to the direction of wave motion - it is more difficult to visualize longitudinal waves than transverse waves becausee the displacements of the elements of the medium are in the same direction as the propagation of the wave - the darkerr regions represent areas where gas is compressed; here density and pressure are above their equilibrium levels; such a region of gas is called a compression

4 - an examplee of compression is when a piston is pushed into a tube - the compression will move down the tube as a pulse, continuously compressing the layers in front of it - the lighter areas of the tube represent areas where gas has expanded beyond its equilibrium level; here, density and pressure of the gas are below equilibrium levels - these low pressure regions are called rarefactions; rarefactions propagate along the tube, following the compressions - both the compressions and rarefactions move with a speed equal to the speed of sound in that medium Describing Sound: - sound waves move through air because a vibrating source produces regularr oscillations in airr pressuree - the air molecules collide, transmitting the pressure variations away from the source of the sound

5 - sound waves share the general properties of other waves (reflection, refraction, diffraction, and interference) - the speed of sound depends upon the medium in which traveling it is - in air, the temperatu ure determines how fast sound waves will travel; specifically, as air temperatures rise the speed of the wave will increase Ex. Air (20º); speed of sound = 343 m/s Ex. Air (0º); speed of sound = 331 m/s - sound can travel through liquids and solids; and in general, it can move more quickly in these mediaa than in gases Ex. Sea Water (25º); speed of sound = 1533 m/s Ex. Iron (25º); speed of sound = 5130 m/s - sound cannot travel in a vacuum because theree are no particles to move and collide

6 Detection of Pressure Waves: - sound detectors are objects which can convert other forms of energy sound energy into - remember, sound energy is the kineticc energy of vibrating air molecules - an examplee of a common sound detector is a microphone; the microphone is made up of a thin disk whichh vibrates in response to air waves and then produces an electric signal - an examplee of a human sound detector is the ear; the ear is an excellent sound detector; it is ablee to hear a wide range of frequencies and amplitudes - remember, pitch is frequency and loudness is amplitude - the ear is a complex organ which transforms sound energy into electrical impulses sent to the brain

7 - sound waves enter the auditory canal, causing the tympanic membrane to vibrate - three tiny bones (the smallest ones in your body) are found in the ear; they amplify and transfer the vibration of the tympanic membrane to the fluid of the cochlea - the three bones of the ear are called the: A) malleus (hammer), B) incus (anvil), and C) stapes (stirrup) - tiny hairs which line the cochlea pick up certain frequencies out of the vibrating fluid - the hairs stimulate nerve cells, which send electrical impulses to the brain; this produces the sensation of sound Perceiving Sound: - sound waves are measured according to three properties: A) frequency, B) amplitude, and C) phase A) Frequency - the number of times a period repeats itself for some standard interval of time; for sound waves, the standard interval is one second 1 1 Frequency ; f period T - remember that the period of the wave is the time it takes for one cycle to be completed - frequency is measured in Hertz (Hz)

8 - a sound wave with a high frequency corresponds to a smalll period, since more cycles aree completed within the time interval - a sound wave with a low frequency corresponds to a large period - frequency is the acoustic component of sound; pitch is the perceptual interpretation of frequency - if the frequency of a sound is high, then the pitch interpreted as high is B) Amplitude - the size of air pressure variation in terms of the maximum and minimum of thee wave - amplitude of a sound wave corresponds to the perceptual property of loudness (intensity) Decaying amplitude of a sound wave

9 - sound wave amplitude is measured in decibels (db) - the organ which detects sound in humans is the ear; the ear is very sensitive to the amplitude of sound waves - due to the wide range of sound detectability, amplitudes are measured on a logarithmic scale called sound level - most people perceive a 10 db increase in sound level as about twice as loud as the original level - the human ear can tolerate sounds as highh as 120 db - exposure to loud noisess over long periods of time will damage the ear, especially to high frequencies

10 C) Phase - the phase of a wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 sound waves which are exactly in produce a stronger wave phase add together to sound waves which are exactly inverted, or 180 degrees out of phase, cancel each other out and produce silence. This is how many noise-cancellation devices work sound waves which have varying phase relationships produce differing sound effects Wave Types: - sound waves are spherical as they propagate across an area (medium); a spherical wave propagates radially outward from the radiating sphere

11 - the energy propagates equally in all directions - these spherical waves stand in contrast to the plane waves we have discussed earlier that travel in a slinky or across the ocean

12 Doppler Effect: - what happens to a sound wave if the emitter is the listener or visa versa moving relative to - firstly, let's think about it from the point of view of the number of waves/second which pass our location as the listener - if the listener is stationary and the sound source is moving towards the listener, the number of waves will increase - likewise, if the sound source is stationary and the listener is moving towards the sound source; ; the number of waves per unit time will increase

13 - remember that when you increase the number of cycles per second you increase the wave frequency; the human ear interprets an increase in frequency as an increase in pitch - conversely, if the listener moves away from a stationary sound source or a sound source moves away from a stationary listener, the frequency will decrease and the pitch will lower - the Doppler effect can be summarized as follows: A) If the objects are getting closer together, the frequency should be higher. pitch B) If the objects are separating, the frequency should be lower. pitch - the Doppler effect occurs in all wave electromagnetic types, both mechanical and

14 - the Doppler effect is used in automobile radar detectors, weather radar, and it is also used by astronomers to measuree distances of celestial phenomenon - Doppler Effect Audio - for both a moving source and a moving observer, the frequency that the observerr hears can be calculated using the equation below - in the Doppler effect equation, v is thee velocity of the sound wave, v d is the velocity of the detector, v s is the velocity of the sound s source, f s is the frequency of the wave emitted by the source, and f d is the frequency received by the detector - this equation applies when the source is moving, when observer is moving, and when both are moving the