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From "Physics, Waves, Music": a website about the physics of waves and sound, acoustics of musical instruments, musical scales, harmony and music.
What is diffraction?
When a wave encounters an obstacle in its path or passes a small opening, under certain physical conditions that we will investigate later, it manifests a peculiar behaviour: it (along with the energy it is transporting) can reach points that would not be reachable if propagation occurred following straight rays. It is as if the wave "breaks up" (hence the name of the phenomenon - from the Latin "diffractus", the past participle of diffringere - which was coined for the first time by F. M. Grimaldi in 1665) and then recomposes while spreading beyond the obstacle or opening.
Sound diffraction
As sound is a wave, it is subject to the diffraction phenomenon in certain conditions. Those conditions are illustrated in general on the page about diffraction and can be summarised by saying that:
- a wave can pass through an opening without significantly modifying the shape of its wave fronts if the size of the opening is much greater than the wavelength of the wave. In cases where the opening is narrow (or much smaller than the wavelength), the wave front of the incident wave is deformed and becomes almost spherical. If you wish to see these effects, visit our 2D Wave virtual laboratory and try the experiment described on this page:
- a wave can "go around" an obstacle if the size of the obstacle is less than (or the same as) the wavelength of the incident wave. For this case as well, visit our 2D Wave virtual laboratory and try the experiments described on these pages:
On this page, we illustrate the specific phenomena of sound diffraction. Since the criterion for establishing the presence or absence of diffraction is related to wavelength, we invite you to read the page on sound wavelength. For sake of readability, here are the data reported there: sound has a wavelength from 1.7 cm (very high sounds at 20,000 Hz at the upper threshold of audibility) to 17 metres (very low sounds at 20 Hz at the lower threshold of audibility).
Sound directionality
When a sound is irradiated from a source (e.g. a loudspeaker), we realise that high-pitched sounds (high frequencies and short wavelengths) come out mainly forwards, like a beam, while low-pitched sounds (low frequencies and long wavelengths) spread in all directions: moving to the side of the loudspeaker, or behind it, the sound becomes duller. This is completely consistent with the first criterion previously illustrated: in this case, the source is the loudspeaker (instead of the opening). If you have tried the experiments, you will have seen these two situations:
| large opening | small opening |
|---|---|
| with respect to wavelength | |
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| In this case, sound (with high pitch and short wavelength) maintains the direction given by the source (either the opening or the loudspeaker). It is for this reason that loudspeakers for high frequencies must be pointed at the listeners. | In this case, sound (with low pitch and long wavelength) does not have precise directionality: this is why a sound system needs only one loudspeaker for the bass (subwoofer), which can be placed almost anywhere in the room. This has an important consequence for voice as well; since our mouths are small compared with the wavelengths of speech, emitted wave fronts are almost spherical. |
Ability of sound to go around obstacles
When a sound encounters an obstacle, its ability to go around it depends on the ratio of the obstacle size to the sound wavelength, as established by the second general criterion previously stated. In the case of sound, we could say that, with the same obstacle size,
- lower sounds (of longer wavelength) can more easily go around obstacles
This fact has a series of important consequences:
- low sounds can go beyond an obstacle such as a person's head. This means that:
- a low sound goes around our head and reaches both ears with almost the same intensity: the head casts no "shadow". By estimating the delay between the arrival at one ear and the other, our auditory system can nevertheless get information regarding the localisation of a sound source. For high sounds, instead, the head casts a shadow and the ear farther from the source receives a weaker sound: this intensity difference is also used to localise the source.
- if we are standing behind a person and listening to them, it will be difficult for us to understand what they are saying. The fact is that only low-frequency waves can go around the head of the person speaking and reach our ears. This, as explained on the page about the voice, makes it difficult to recognise the formants of the vocal sounds.
- in any case, everyday obstacles (a tree, wall, column, etc.) are small or comparable with respect to the wavelength of the majority of sounds used in speech and music. It is for this reason that sound goes beyond them so easily! This does not happen with light, which has much smaller wavelengths than the size of a tree; light is stopped and behind the tree a shadow is formed, under which we often seek refuge on a sunny day.
Check your understanding
A person is talking around the corner of a building: you hear them but cannot see them. Why does sound go around the corner while light does not?
Answer. The wavelengths of the voice range from a few centimetres to a few metres, comparable with the size of the obstacle and of the passage: sound is diffracted and also reaches the "shadow" region. Light has wavelengths of less than a thousandth of a millimetre, millions of times smaller than the building: it continues in a straight line and the shadow remains behind the corner.Why is it harder to understand a person who speaks with their back to us?
Answer. The speaker's head is an obstacle about 20 cm across. The low components of the voice, with longer wavelengths, go around it; the high ones, with wavelengths of a few centimetres, are largely blocked. So mainly the low frequencies arrive, and the higher formants, which are needed to distinguish vowels and consonants, are weakened.In a sound system the loudspeakers for high frequencies must be pointed at the listeners, while a single loudspeaker for the bass can be placed almost anywhere. Why?
Answer. A loudspeaker behaves like an opening from which the sound comes out. For high sounds its size is large compared with the wavelength and the sound comes out as a beam directed forwards. For low sounds, with wavelengths of metres, the loudspeaker is small compared with the wavelength and the sound spreads in all directions.A sound of 100 Hz and one of 5000 Hz meet a column 50 cm wide. Which of the two goes around it more easily?
Answer. The 100 Hz sound, which has a wavelength of about 3.4 m, much larger than the column. The 5000 Hz sound has a wavelength of about 7 cm, much smaller than the column: behind it there is a shadow region for this sound.In-depth study and links
- A very helpful principle to understanding what happens to a wave when it encounters an obstacle or an opening, if only qualitatively, is that of Huygens. If you wish to learn more, visit this page.
- The phenomenon of sound diffraction plays a fundamentally important role in designing closed environments with "good acoustics". This topic is further discussed on the page about architectural acoustics.
- The phenomenon of sound diffraction is in some way complementary to that of sound reflection. If you wish to have a more complete picture of what happens when a sound wave encounters an obstacle, visit the page on sound reflection.
- On the page about musical instruments, and in particular the one dedicated to the trumpet, you can listen to an interesting recording done with a microphone placed in front of or behind a trombonist. The perceptible change in timbre of the instrument is due to the ability of various frequencies to go around the obstacle created by the trombonist's head.
- Related questions and answers:
