Sound diffraction is the bending or spreading of sound waves as they encounter an obstacle or pass through an opening. When a sound wave meets the edge of a barrier, a wall opening, or a doorway, it does not simply stop or continue in a straight line — it curves around the edge and spreads into the region behind it, reaching areas that would be in acoustic shadow if sound travelled only in straight lines.
Diffraction is the reason you can hear someone talking around a corner, why noise barriers along highways are less effective than their height might suggest, and why low-frequency sounds seem to penetrate everywhere while high-frequency sounds are more easily blocked.
Real-World Analogy
Watch ocean waves approaching a harbour entrance. As the waves pass through the gap in the breakwater, they do not continue as a narrow beam the width of the gap. Instead, they spread out in a fan shape, curving around the edges of the breakwater and eventually reaching the entire harbour basin. A wider gap relative to the wavelength produces a narrower fan; a gap close to the wavelength produces nearly circular spreading.
Sound waves do the same thing at doorways, window openings, and barrier edges. A sound wave passing through a doorway spreads into the room beyond, filling the space rather than projecting as a narrow beam. The degree of spreading depends on the relationship between the wavelength of the sound and the size of the opening or obstacle.
Technical Definition
Diffraction is explained by Huygens' Principle: every point on a wavefront acts as a source of secondary spherical wavelets. The new wavefront is the envelope of these wavelets. When part of a wavefront is blocked by an obstacle, the wavelets at the edge continue to propagate into the shadow zone behind the obstacle.
The critical factor is the ratio of wavelength (lambda) to the size of the obstacle or opening (d):
- When lambda >> d (wavelength much larger than the obstacle), sound diffracts strongly and effectively wraps around the obstacle. The obstacle has minimal effect.
- When lambda << d (wavelength much smaller than the obstacle), diffraction is minimal. A clear acoustic shadow forms behind the obstacle.
- When lambda is approximately equal to d, diffraction is moderate and frequency-dependent effects are most pronounced.
Barrier Diffraction: The Maekawa Formula
For noise barriers, the insertion loss due to diffraction is commonly estimated using the Maekawa formula:
IL = 10 x log10(3 + 20N)
Where N is the Fresnel number, defined as N = 2 x delta / lambda, and delta is the path length difference between the diffracted path (over the barrier) and the direct path (through the barrier if it were absent). This formula is referenced in ISO 9613-2:1996 for outdoor sound propagation calculations.
A barrier with N = 1 provides about 13 dB of insertion loss. Doubling the Fresnel number (by increasing barrier height or frequency) adds approximately 3 dB.
Why It Matters for Design
Diffraction has practical consequences in several design scenarios:
Noise barriers. Every highway noise barrier, construction hoarding, and office screen is limited by diffraction. Sound bends over the top edge and around the sides, particularly at low frequencies. This is why doubling a barrier's height does not double its effectiveness — the improvement follows a logarithmic curve. Effective barrier design requires understanding diffraction to set realistic expectations.
Open-plan offices. Desk dividers and partial-height screens provide meaningful attenuation only at frequencies where the screen is many wavelengths tall. For speech frequencies (500 to 4000 Hz), a 1.5-metre screen can provide 5 to 15 dB of attenuation depending on frequency and geometry. Below 250 Hz, the screen is acoustically almost transparent.
Door and window openings. An open doorway is a diffraction aperture. Sound passing through spreads into the adjacent room, and the degree of spreading is frequency-dependent. This is why closing a door blocks high-frequency sound far more effectively than low-frequency sound — even before considering the door's mass and transmission loss.
Room partitions. Partial-height walls in open offices and classrooms allow sound to diffract over the top. The effective sound reduction depends on the gap between the partition top and the ceiling, the frequency, and the absorption on the ceiling above the partition.
How AcousPlan Uses This
AcousPlan's room acoustic calculator models enclosed rooms where diffraction at boundaries is implicitly handled by the statistical energy approach (Sabine and Eyring methods assume a diffuse sound field). For rooms that are well-enclosed, diffraction at edges is not a separate calculation — it is part of the reverberant field assumption.
For sound insulation calculations, AcousPlan's STC/Rw calculator accounts for flanking paths that include diffraction effects — sound travelling over, under, and around partitions rather than only through them. The flanking transmission module considers the path length differences and frequency-dependent attenuation associated with diffracted paths.
Understanding diffraction helps users interpret results correctly: if a partial partition shows lower-than-expected noise reduction in practice, diffraction over the top is likely the cause.
Related Concepts
- What is Sound Refraction? — Bending through gradients, not around obstacles
- What is Sound Wavelength? — The dimension that controls diffraction behaviour
- What is Frequency in Acoustics? — Higher frequency means less diffraction
- What is Transmission Loss? — How barriers block sound that does not diffract
- What is Flanking Transmission? — Indirect paths that include diffraction
Calculate Now
Understanding diffraction helps you set realistic expectations for barriers and partitions. Model your room's sound insulation and acoustic performance with the AcousPlan Room Calculator.