Lecture Notes (Properties & Detection Of Sound Waves)

Intro: - sound is very important in our lives today and has been throughout our history; we not only derive useful information from sound, but we also get pleasure from sound - musical instruments have been discovered that date back to over 43,000 years ago - other organisms than humans also use sound; bats use high frequency sound to hunt insects; whales use extremely low frequency sound to communicate over vast distances

Sound Waves: - sound is a series of pressure fluctuations in the medium between the source and the listener

- for example, in speech, the medium is 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 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

- 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 because the displacements of the elements of the medium are in the same direction as the propagation of the wave - the darker 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

- an example 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 regular oscillations in air pressure

- the air molecules collide, transmitting the pressure variations away from the source of the sound

- sound waves share the general properties of other waves (reflection, refraction, diffraction, and interference) - the speed of sound depends upon the medium in which it is traveling

- in air, the temperature 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 media 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 there are no particles to move and collide

Detection of Pressure Waves: - sound detectors are objects which can convert sound energy into other forms of energy - remember, sound energy is the kinetic energy of vibrating air molecules - an example of a common sound detector is a microphone; the microphone is made up of a thin disk which vibrates in response to air waves and then produces an electric signal - an example of a human sound detector is the ear; the ear is an excellent sound detector; it is able 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

- 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 1Frequency ; period

fT

- remember that the period of the wave is the time it takes for one cycle to be completed - frequency is measured in Hertz (Hz)

- a sound wave with a high frequency corresponds to a small period, since more cycles are 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 is interpreted as high - the frequency and therefore wavelength of sound waves is controlled by the frequency of the vibrating source - the human ear can detect frequencies from 20 Hz to 20,000 Hz; however, the ear does not respond in the same way to all frequencies - the ear is most sensitive to frequencies of about 3000 Hz - as people age, they find high frequencies more difficult to hear - human speech ranges from 600 Hz to 4800 Hz

- other body organs can detect sound waves, especially vibrations of low frequencies; for example, the intestines and stomach are susceptible to low vibrations, with a maximum response at about 3 Hz - your stomach wall actually moves in and out in response to such vibrations

- this property was used in cinemas during the late 1970's to produce an effect called ‘sensurround’ in which low frequencies are generated by banks of large ‘woofer’ speakers to make the effects of bomb blasts and earthquakes more realistic as you feel the effect as well as hear it - physicists call such low frequencies infrasound (Latin infra = ‘below’) - while it may be safe under controlled conditions, infrasound can also cause nausea and dizziness, such as in car sickness; death can occur in extreme cases when internal organs rub against each other and rupture

- ultrasound is sound at frequencies above that of human hearing range, that is, above 20,000 Hz; in Latin ultra means ‘beyond’ - artificial ultrasounds are produced by vibrating quartz crystals, which are induced to vibrate by high- frequency alternating currents - the uses of ultrasound waves are increasing very rapidly especially in the medical profession - reflection and refraction of ultrasound waves are used to see unborn babies, tumors and body organs - high-frequency ultrasound is used to make particles of objects vibrate at such a rate as to make them shatter; this is used by dentists to remove plaque from teeth and by doctors to break up kidney stones - ultrasound has been used for a number of years in sonar equipment developed during WW II to detect enemy submarines - sonar comes from the term sound navigation and ranging - because ultrasound waves have shorter wavelengths, they are less diffracted by water than sound waves are and they are not absorbed by sea water as much as microwaves are; they can therefore penetrate to great depths in water

- bats use ultrasound to assist their poor eyesight; they are able to produce ultrasound pulses 0.010 s apart and of approximately 100,000 Hz

- because the wavelength is so small these waves can reflect from small objects; bats’ ears are also concave to concentrate the reflected sound waves

B) Amplitude - the size of air pressure variation in terms of the maximum and minimum of the wave - amplitude of a sound wave corresponds to the perceptual property of loudness (intensity)

Decaying amplitude of a sound wave 

 

- 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 the decibel scale

- the decibel scale measures relative intensity level where I is the intensity of the sound in W/m2; I0 is the reference level, taken as the least audible sound (10–12 W/m2); β is the relative intensity level in dB - therefore the relative intensity level of a whisper in dB is:

- 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 high as 120 dB - exposure to loud noises over long periods of time will damage the ear, especially to high frequencies

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 phase add together to produce a stronger wave

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 - 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

Doppler Effect: - what happens to a sound wave if the emitter is moving relative to the listener or visa versa - 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

- 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 types, both mechanical and electromagnetic

- the Doppler effect is used in automobile radar detectors, weather radar, and it is also used by astronomers to measure distances of celestial phenomenon - Doppler Effect Audio

- for both a moving source and a moving observer, the frequency that the observer hears can be calculated using the equation below

- in the Doppler effect equation, v is the velocity of the sound wave, vd is the velocity of the detector, vs is the velocity of the sound’s source, fs is the frequency of the wave emitted by the source, and fd is the frequency received by the detector - this equation applies when the source is moving, when the observer is moving, and when both are moving