Diffraction of Sound

What is Diffraction?

Diffraction is the bending of waves around obstacles or through openings. It occurs when a wave encounters an obstacle or a gap that is comparable in size to its wavelength. The wave can bend around the edges of the obstacle or spread out after passing through an opening, reconstructing itself to some degree on the other side.

Measuring Diffractions:

Diffraction effects are typically observed and measured in terms of sound pressure levels (SPL) and frequency response at different points around or behind obstacles. The extent of diffraction depends on the relationship between the wavelength of the sound and the size of the obstacle or opening.

While there isn't a single formula to quantify diffraction in all scenarios, we can use the following relationship to understand when diffraction is likely to occur:

Significant diffraction occurs when: λ ≈ d

Where: λ (lambda) = wavelength of the sound

d = size of the obstacle or opening

For example:

• A sound wave with a frequency of 100 Hz has a wavelength of about 3.43 meters (11.25 feet) in air at room temperature.

• If this wave encounters an obstacle or gap of similar size (around 3-4 meters or 10-13 feet), significant diffraction will occur.

Figure showing diffraction in different scenarios 

Small Obstacle: When the obstacle is small compared to the wavelength, the sound wave bends around it with minimal disruption.

A larger obstacle causes more significant diffraction. The wave bends around the edges and begins to reconstruct itself behind the obstacle.

When sound passes through a small opening, it spreads out in all directions on the other side, acting like a new point source.

If the opening is large compared to the wavelength, most of the wave passes through with minimal diffraction.

In practice, the effects of diffraction are often combined with reflection and absorption, creating complex acoustic environments.