SOUND DIFFRACTION: WHAT IT CONSISTS OF, EXAMPLES, APPLICATIONS - PHYSICAL - 2025

The diffraction sound is the phenomenon that occurs when the sound curve and scatters around an opening or obstacle. It is something common to all waves: when the sound wave reaches an opening or an obstacle, the points of its plane become sources and emit other diffracted ones.

Sound is precisely a pressure wave that travels through air and also through water and solids. Unlike light, which is also a wave, sound cannot propagate through a vacuum. This is because light works completely differently - it is an electromagnetic wave.

Figure 1. Plane wave incident on the groove and diffracting. Source: pixabay

The key in the phenomenon of diffraction is the size of the obstacle in relation to the wavelength: the diffraction is more intense when the obstacle has dimensions comparable to the wavelength.

In sound, the wavelength is on the order of meters, while that of light is on the order of hundreds of nanometers. While sound has a human scale, light has a microbe scale.

This huge difference in the wavelength scale between sound and light is behind the fact that we can listen to a conversation around a corner without being able to observe those who are conversing.

And it is that the sound is capable of curving around the corner, while the light continues straight. This phenomenon of curvature in the propagation of the sound wave is precisely the diffraction of sound.

Sound

Sound is understood as the pressure waves that travel through the air and that are included in the audible range.

The audible range to the ear of a young, hearing-impaired human is between 20 Hz and 20,000 Hz. This range tends to narrow with age.

Low tones or frequencies are between 20 Hz and 256 Hz. Middle tones between 256 Hz to 2000 Hz. And high tones are those between 2 kHz to 20 kHz.

The speed of sound in air at atmospheric pressure of 1 atm and 0º C is 331 m / s. The relationship between the speed v of propagation of a wave with its wavelength λ and its frequency f is the following:

v = λ⋅f

From this relationship we have that the wavelength has the following ranges:

- Low tones: 16.5 m to 1.3 m.

- Medium tones: 130 cm to 17 cm.

- High tones: 17 cm to 1.7 cm.

Examples of sound diffraction

The open door of an auditorium

An auditorium or concert hall is generally a closed space with walls that absorb sound, preventing its reflection.

However if the auditorium door is open the concert can be heard without problems, even when the orchestra remains out of sight.

If you are right in front of the door you can hear the full range of sounds. However, if you are to the side, you will hear the bass sounds, while the treble will not.

Bass sounds have long wavelengths and therefore can surround the door and be heard behind it. It's all due to the phenomenon of diffraction.

Behind a speaker box

A loudspeaker or speaker emits a wide range of wavelengths. The speaker box is itself an obstacle that casts a sound shadow behind it.

This sound shadow is clear for the high frequencies, which cannot be heard behind the speaker, while the bass and part of the middle can be heard because they are turned around.

The above experiment works best in an open space, because it must be taken into account that sound can be reflected off walls and other objects, allowing all tones to be heard even behind the speaker box.

The band of musicians in the street

A band of musicians playing on the street can be heard from a cross street from which the artists cannot be seen.

The reason, as we said before, is that the direction of sound is able to bend and cross the corner, while light travels in a straight line.

However, this effect is not the same for all wavelengths. Long-wave ones are diffracted or doubled more than short-wavelengths.

For this reason, in the transversal street, from where the musicians cannot be seen, the acute instruments such as trumpets and violins cannot be heard well, while the drums and bass drums are heard more clearly.

Figure 2. Sound diffraction in a street. Source: self made

In addition, long-wavelength low tones attenuate less with distance than short-wavelength high-frequency sounds.

Animals that make use of low frequencies

Elephants emit very low frequency, very long wavelength infrasound waves to communicate with their peers over great distances. The whales do it too, which also allows them good communication at a distance.

Applications of sound diffraction

Increased hearing area

For a loudspeaker to have a large listening area, the width of the speaker must be less than the wavelength of the sound it is emitting.

There is a specific horn design that takes advantage of sound diffraction: it is the dispersion horn.

It is generally believed that the larger the diaphragm of the horn, the more area it covers. However, in the dispersion horn the diaphragm is small and its shape is what makes the sound amplify, taking advantage of the phenomenon of sound diffraction.

The shape of the horn is like a rectangular mouth or outlet horn smaller than the wavelengths it emits.

The correct installation of this type of speaker is done with the short side of the rectangular mouth horizontally and the long side vertically. In this way, a greater breadth of horizontal coverage and directionality of the sound parallel to the ground is achieved.

References

1. Physics / Acoustics / Propagation of sound. Recovered from: es.wikibooks.org
2. Construpedia. Sound Diffraction. Recovered from: construmatica.com
3. Diffraction (sound). Recovered from: esacademic.com
4. The physics classroom. Diffraction of sound waves. Recovered from: physicsclassroom.com
5. Wikipedia. Diffraction (sound). Recovered from wikipedia.com