Sound wave terminology
The size of disturbance of each molecule from its state of rest. The greater the amplitude the louder the noise.
The distance travelled by a sound wave in the time it takes the source to produce one complete oscillation or cycle is termed the wavelength.
The speed or velocity, measured in metres per second (m/s) that a sound wave travels through a medium is dependent on the elasticity and density of that medium, i.e. the material’s properties.
Frequency is measured in cycles per second or Hertz.
The more vibrations or oscillations each molecule makes in a set period of time the higher the frequency.
A high frequency sound is said to have a high pitch.
The wavelength (X), the frequency (f) and the velocity (V) are related by the formula: v – fx where
X – wavelength in metres. F – frequency in cycles/second. V = velocity in metres/second.
|wavelength vs amplitude|
|frequency vs time|
|Subsonic, sonic and ultrasonic The Acoustic spectrum|
Vibration in air molecules can be produced at any frequency but our ears only detect those with in a sonic range or audible range.
The lower limit is taken as approximately 16Hz and upper limit 20 KHz.
Vibrations below the audible range that is below 16Hz.
Vibrations above 20KHz are termed “Ultrasonic Waves” .
It is generally known that dogs can hear sound waves which are too high pitch for us to hear, in fact up 25 to 30 KHz. Bats work at even higher frequencies, between 30 to 50 KHz ,at which they practice their own brach of pulse-echo location. Ultrasonic flaw detection equipment is operated atfr equencies much higher than , from 500 KHz to 20 MHz. the most common test range being somewhere between 1 MHz and 6 MHz.
Some of the incident sound will travel across the interface and continue in a different direction. This change of direction is called refraction. It is a function of the change in speed of sound between two mediums.
Laws of Reflection and Refraction:
The angles of reflection and refraction are governed by Snell’s Law :
|Reflection and Refraction of Snell’s Law|
Since the various types of waves ( compression, shear etc.) have different velocities in any medium when reflection or refraction take place at an interface of the medium, more than one type of wave may be produced. This is known as mode conversion.
For, example, a compression wave incident on a material interface my produce a reflected compression wave, a reflected shear wave, a refracted compression wave and a refracted shear wave.