Introduction to Sound Wave Lecture PDF

Introduction to Sound Wave Prepared By Dr. Wagih G. Girgis  Sound is a vibration that propagates as an acoustic wave, through a transmission medium such as 1. gas 2. liquid or 3. solid Only acoustic waves that have frequencies lying between about 20 Hz an...

Introduction to Sound Wave Prepared By Dr. Wagih G. Girgis  Sound is a vibration that propagates as an acoustic wave, through a transmission medium such as 1. gas 2. liquid or 3. solid Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz, the audio frequency range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with wavelengths of 17 meters to 1.7 centimeters.  Sound waves above 20 kHz are known as ultrasound and are not audible to humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges.  Sound is defined as Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation.  The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The number of oscillations per second of the diaphragm (transducer) establishes the frequency of the sound wave. Frequency is expressed in cycles per second, or hertz (Hz). Audible sounds are in the range of 20 Hz to 20 kHz. f= C = speed of sound m/sec = wavelength meter  The wavelength λ is the distance over which a property of a wave repeats itself. =  Physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h) using the formula v [m/s] = 331 + 0.6 T [°C]  Ultrasound refers to any sound whose frequency is above the audible range (i.e., above 20 kHz). Diagnostic ultrasound applications use frequencies in the 1 MHz to 30 MHz (1 million to 30 million Hz) frequency range. Manufacturers of ultrasound equipment and clinical users strive to use as high a frequency as practical that still allows adequate visualization depth into tissue. Higher frequencies are associated with improved spatial detail, or better resolution.  Sound can propagate through a medium such as air, water and solids as longitudinal waves and also as a transverse wave in solids. Longitudinal plane wave  Transverse plane wave  At a fixed distance from the source, the pressure , the velocity, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave. This is obvious for a solid, and the same is true for liquids and gases That is, the vibrations of particles in the gas or liquid transport the vibrations, while the average position of the particles over time does not change.  The behavior of sound propagation is generally affected by three things: 1- density and pressure of the medium 2- motion of the medium 3- Viscosity of the medium  A complex relationship between the density and pressure of the medium. This relationship, affected by temperature, determines the speed of sound within the medium.  Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement.  For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction.  If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind.  The viscosity of the medium. Medium viscosity determines the rate at which sound is attenuated.  For many media, such as air or water, attenuation due to viscosity is negligible.  The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.  Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:  Frequency, or its inverse, wavelength  Amplitude, sound pressure or Intensity  Speed of sound  Direction The END