Mechanical Wave Sound, Ear Eugen Kvašňák, PhD., 3rd Medical Faculty, Charles Uni.

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Mechanical Wave Sound, Ear Eugen Kvašňák, PhD., 3rd Medical Faculty, Charles Uni.

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Mechanical Waves

http://online.cctt.org/physicslab/content/Phy1/lessonnotes/waves/lessonwaves.asp

ACOUSTICS

Sound is a Longitudinal Wave Sound is a mechanical wave which is created by a vibrating object. The vibrations of the object set particles in the surrounding medium in vibrational motion, thus transporting energy through the medium.

A closed pipe resonates when the length of the air column is approximately an odd number of quarter wavelengths long. l = {(1,3,5,7,…)/4} * With a slight correction for tube diameter, we find that the resonant wavelength of a closed pipe is given by the formula: = 4 (l + 0.4d), = 4 (l + 0.4d), where is the wavelength of sound, l is the length of the closed pipe, and d is the diameter of the pipe.

An open pipe resonates when the length of the air column is approximately an even number of quarter wavelengths long. wavelengths long. l = {(2,4,6,8,…)/4} * With a slight correction for tube diameter, we find that the resonant wavelength of an open pipe is given by the formula: = 2 (l + 0.8d), = 2 (l + 0.8d), where is the wavelength of sound, l is the length of the closed pipe, and d is the diameter of the pipe.

Sound velocity = wavelength x frequency … v = x f Sound waves travel in air: 344 m/s in water: 1450 m/s in wood and metal: even faster because the molecules are denser The physical vibrations that make sound can be nearly any frequency. However, human ears respond to only a relatively small range … 20 - 20,000 Hz. Within this range humans are most sensitive to the frequencies between 1000 - 5000 Hz. Wavelength…f = 20 Hz in air: = 17.2 m f = 1 kHz in air: = 34.4 cm f = 20 kHz in air: = 17.2 mm

Transverse waves can occur only in solids, whereas longitudinal waves can travel in solids, fluids, and gases. Transverse motion requires that each particle drag with it adjacent particles to which it is tightly bound. (In a fluid this is impossible, because adjacent particles can easily slide past each other.) Longitudinal motion only requires that each particle push on its neighbors, which can easily happen in a fluid or gas. (The fact that longitudinal waves originating in an earthquake pass through the center of the earth while transverse waves do not is one of the reasons the earth is believed to have a liquid core. )

Most wind instruments need a resonant air column half as long as the wavelength of the fundamental frequency they want to play.

There are two types of waves that cause sound: ---- 1. The Transverse wave (like a violin string) in which the vibration is perpendicular to the wave's travel. ---- 2. The Longitudinal wave (like a wind instrument's air column) in which the vibration is parallel to the wave's travel. ---- All waves in an encompassing medium like air can be considered longitudinal waves ---- There are 4 important attributes that we can manipulate to create or describe any sound. And we can work with these attributes in two different ways: we can measure them and we can hear them. If we measure them, they're called physical attributes: if we hear them, they're called perceptual attributes.

Definition of sound: Mechanical energy in the form of pressure fluctuations in an elastic medium which propagate as waves from a vibrating source. -- Important parameters of sound: Speed, Periodicity, Frequency range, Types of waves, and Wavelengths. ---- 4 physical vs. perceptual attributes of sound: Frequency/pitch, Amplitude/Loudness, Waveform/Timbre, Duration/Time The 4 physical attributes are frequency, amplitude, waveform, and duration. Their perceptual counterparts are pitch, loudness, timbre, and time. The two are not exactly parallel. ---- Frequency (in Hertz) refers to how often the vibration repeats. Pitch refers to our perception of frequency on a continuum from low to high. ---- Amplitude refers to how much energy is contained in the displacement of molecules that make up sound waves. It is usually measured in decibels. Decibels is a logarithmic scale in which each ten number increase actually represents a ten fold increase in energy. On this scale a 10 decibel increase equals 10 times the energy, but a 20 decibel increase = 100 times the energy and a 30 decibel increase = 1000 times the energy; etc. We need this logarithmic scale because the loudest sound humans can hear is about 1 trillion times as powerful as the softest. Each doubling of sound energy can be represented by a 3 decibel change. ---- Loudness refers to our perception of amplitude and is sometimes stated in phons. The least amount of amplitude humans can perceive as sound starts the decibel scale at 0 dB. This is about a trillionth of a watt per square meter. The highest amplitude (where sound perception turns into pain) is about 120 dB-- 1 watt per square meter. Here are some typical loudness levels: ---- Waveform refers to the complexity of one complete vibration cycle. It is an interesting physical phenomenon that all essentially 1 dimensional bodies like strings and air columns vibrate not just as whole entities but also simultaneously in 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, and so forth. These parts vibrate 2 times, 3 times, 4 times, etc. as fast as the whole, or fundamental, vibration. That means that a 440 hertz tone from a musical instrument is also vibrating at 880 cps, 1320 cps, 1760 cps, 2200, etc. Usually the higher the overtone is, the less energy it contains. They fade to inaudibility after 10. Together, the fundamental and its overtones are called harmonics and this whole phenomenon of integer- related harmonics is known as the natural harmonic series.-- Any tone that consists of more than one frequency (and that means virtually all sounds) is said to be a complex tone. Any single frequency of any complex tone, whether it's a tonal sound or noise, is said to be a partial. If the sound is "musical"-- in other words it uses the natural harmonic series-- the partials are called harmonics. The lowest harmonic is the fundamental, and all the other higher harmonics are called overtones. ---- Timbre is how we perceive a waveform's complexity. The multiple vibration phenomenon means that any tone we hear from a musical instrument is almost always a combination of many frequencies even though we identify it as a single frequency. It is the relative strength of the harmonics that allows us to identify the body of a tone as flute, oboe, violin, clarinet, etc. ---- Duration refers to the passage of time during a sound. This is often measured in absolute units of hours, minutes, seconds and parts of a second (called either frames or milliseconds). In musical situations we use relative units of measures, beats, and parts of a beat. These are called relative because the actual time that any beat-expressed event occurs depends on how fast the beat is.--- It is important to point out that all the other attributes of sound usually change throughout a sound's duration. These changes are referred to as a sound's envelopes. Quite often we refer to four parts of the envelope: attack, decay, sustain, and release and sometimes we lump these together as ADSR a term synonymous with envelope ---- Time refers to how we perceive duration. We can hear simultaneous but separate envelopes for pitch, amplitude, and timbre. These envelopes help us identify an instrument much mor precisely than the waveform alone could. Use the three buttons below to hear how your perception of the sound shown above depends on the duration of your exposure to it. Hold the fourth button down to see how your ability to identify the instrument is less precise without its envelope. 150 dB = permanant hearing damage 130 dB = marching band indoors, temporary hearing loss 120 dB = threshhold of pain 90 dB = typical wind instrument 80 dB = classical guitar 60 dB = conversation at 3 feet 50 dB = classroom during a test 30 dB = quiet practice room 10 dB = anechoic sound chamber 0 dB = threshhold of hearing

ORGAN OF CORTI EAR

Pain, Hearing, Speech

The velocity of sound in air depends on the air temperature. The speed of sound in dry air is 331.5 m/s at 0ºC. This speed increases with temperature: about 0.6 m/s for every 1 º C. The human ear relates: amplitude to loudness and frequency to pitch.

SOUND LAB DEMO