Categories of Sound Waves Sound waves fall into three categories covering different ranges of frequencies. Audible sound waves In the range of human hearing (20 Hz – 20 kHz) Infrasonic waves Frequencies below the audible range (< 20 Hz) Ultrasonic waves Frequencies above the audible range (> 20 kHz)
Applications of Ultrasonic Waves Widely used in medical applications Diagnostic tool AND treatment Ultrasound Imaging Transducer placed on mother’s abdomen Emits ultrasonic waves Reflected waves are picked up by transducer and converted into an electric signal that forms an image Ultrasound Treatment Ultrasonic waves used to break up kidney stones or promote healing in biological tissues.
Speed of Sound Depends on the density of the air and its temperature. Value increases by m/s for every increase of 1 o C. T = temperature in o C
Sample Problem 1 The temperature outside is 23 o C. What is the speed of sound in air at this temperature? G: T = 23 o C R: v = ? A: v = m/s + (0.606 m/s/ o C) T S: v = m/s + (0.606 m/s/ o C)(23 o C) = 345 m/s S: The speed of sound in air at 23 o C is 345 m/s
Sample Problem 2
Practice Questions Page 393 m/s o C o C
Mach Number Ernst Mach researched sound waves and devised a way to describe air speeds of objects in terms of the speed of sound. Ratio of airspeed to the local speed of sound No units for M Mach number is not fixed – depends on speed of sound in its vicinity
Practice Problems Page 394 x 10 2 m/s = 1100 km/h m/s = 1.0 x 10 3 km/h
Human Perceptions of Sound Intensity The threshold of human hearing ranges from about 1x W/m 2 to about 1 W/m 2 Easier to use decibels The unit of sound level used to describe sound intensity level 1/10 of a bel (B) The decibel commonly gives measurements on a scale of 0 to 100, sometimes exceeding 200 Decibels refer to sound level, not intensity, so an order of magnitude is an increase of 10 decibels.
Typical Sound Levels
Loudness and Distance The farther you are from a sound, the quieter it becomes. As the sound wave expands from the source, the total energy stays the same, but the area of air it acts on is greatly increased. Loudness drops off quickly, but audible levels persist for quite a distance
Sound Safety Any sound levels greater than 100 dB that persist for more than a few minutes will damage hearing. The louder a sound, the less time that can be spent near it without damaging hearing.
Summary Audible sound waves range from 20 Hz to 20 kHz. Infrasonic waves have frequencies below 20 Hz. Ultrasonic waves have frequencies above 20 kHz. We can apply our understanding of the properties of sound to technologies that benefit society. The speed of sound through the atmosphere, in metres per second, is given by the relationship v = m/s + (0.606 m/s/°C) T, where T is the temperature in degrees Celsius. Sound intensity is a measure of the energy flowing through the unit area due to a sound wave. Human hearing can detect a range of sound intensities over many magnitudes in intensity. Loudness levels are usually described on the decibel scale, which is more convenient than the range of values for sound intensity. Loudness levels are dependent on the distance from the source of the sound. Sound levels in industry and recreation must be kept to a reasonable level to avoid hearing damage.