Diffraction of Sound Waves
The Bending of Pressure Waves Around Obstacles or Narrow Openings
Aug 2, 2008
The well-known ability of sound to travel around corners, called diffraction, provides further evidence that sound is wave-like in nature. Reflection alone cannot account for all the indirect sounds. Diffraction can be explained in terms of the characteristics of waves, such as wavelength, frequency and speed.
Diffraction is the Bending of Waves
Sound has been described as represented by pressure waves. Diffraction of sound is the bending of pressure waves around obstacles in the path of the waves, or the bending of waves as they pass through narrow openings (Fig 1).
In diffraction, the wave remains in the same medium and so, its speed, frequency and wavelength remain unchanged. The only thing that changes is the direction of the wave as it passes around obstacles or through gaps.
As a result of waves bending, higher frequency sounds can be heard more clearly if the listener is directly in front of the source, while lower frequencies can be heard quite clearly from a wide range of angles. This has major implications for the design of sound reproduction systems.
Diffraction and Wavelength
How much a particular wave spreads will depend on its wavelength in relation to the size of the obstacle or gap, as shown in Figure 1 for the case of the aperture. Sound waves passing through an aperture (or past an obstacle) that is larger than the wavelength will not be significantly diffracted, but apertures (or obstacles) that are comparable in size to the wavelength or smaller will cause considerable bending, and the sound will spread out.
As a general rule, the amount of diffraction will depend on the ratio of the wavelength (lamda) of the sound to the width (w) of the aperture or obstacle, i.e., lamda / w.
- For small wavelengths, lamda is small compared to w, and lamda / w < 1. Obstacles will cast smaller sound "shadows" and waves will spread out less.
- For long wavelengths, lamda is large compared to w, and lamda / w > 1. Sounds spread out to fill a space, making it difficult to determine the exact source of the sound.
Worked Example Using the Wave Speed Equation
When sound waves of high frequency, 9000 Hz, strike an obstacle such as a person's head, they leave a distinct sound shadow, in which the sound heard is reduced. If one ear is closer to the source than the other, one ear will also hear the sound louder then the other, because of the diffraction shadow. Explain if this effect will be significant for this frequency of sound.
Using the wave speed equation, v = f x lamda, and changing the subject, lamda = v / f = 340/9000. So the wavelength is 0.0378 m or 3.8 cm. This value is much smaller than the size of a human head of about 20 cm, therefore diffraction is expected to be minimal and the sound will not bend significantly around the head.
Diffraction Demonstrates that Sound Travels in Waves
Diffraction is the bending of waves around the edge of a barrier or aperture. The fact that sound diffracts provides further evidence that sound is wave-like in nature. The amont of diffraction depends on the wavelength (lamda) relative to the width (w) of the opening or obstacle. Significant diffraction ooccurs when lamda is at least the same order of magnitude as the width of the opening or obstacle.
Readers may be interested to find out more details about diffraction.
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