Diffraction of Sound
Diffraction of sound refers to the phenomenon where sound waves spread out as they travel around obstacles or edges. The degree of this diffraction is heavily influenced by both the size of the obstacle and the wavelength of the sound wave.
When the wavelength of the sound wave is longer compared to the size of the obstacle, the diffraction becomes more pronounced. This means that if the wavelength is long and the size of the obstacle is small, the wave spreads out more widely around the obstacle. As a result, waves that would have originally propagated in a straight line can spread out in a semi-circular pattern around the obstacle.
For instance, the reason we can hear sounds from behind a wall in everyday life is due to this diffraction phenomenon. Sounds with lower frequencies (bass sounds) diffract more effectively around obstacles, so we often experience hearing lower sounds, like the thumping of footsteps, from behind walls. Moreover, when listening to loud music in a neighboring room with the door closed, the bass or drum sounds, which are of lower frequency, often penetrate the walls more clearly.
Since frequency is inversely proportional to wavelength, lower frequencies diffract better than higher frequencies.
Refraction of Sound
Refraction refers to the phenomenon where sound waves change direction at the boundary between two media. This occurrence is due to differences in the speed of sound within the media, which can vary based on the density, temperature, and humidity of the medium. The greater the difference in the speed of sound between media, or the larger the difference in wind speed with height, the more pronounced the refraction.
Sound propagates at different speeds in various media. For instance, the speed of sound in air is different from its speed in water. Sound waves refract at boundaries where there’s a significant difference in the speed of sound. The degree of refraction increases with a greater difference in sound speeds between the media.
In the atmosphere, temperature and wind speed can vary with altitude, influencing the speed of sound. As a result, when there’s a difference in wind speed or temperature with height, sound waves can refract accordingly. In particular, larger differences in wind speed lead to more pronounced refraction.
This principle explains why, during the evening at the beach, sounds from afar might be heard more clearly than usual. This clearer perception is due to the refraction of sound caused by changes in atmospheric temperature and wind speed.
Sound refracts towards areas with lower temperatures, lower sound speeds, where the wind is blowing, and where density is higher.
Snell’s Law
Snell’s law is a principle concerning refraction, describing the angle at which light (or other electromagnetic waves) refracts when it crosses the boundary between two different media. This law quantitatively represents the relationship between the direction of the refracted light and the direction of the incident light.
Snell’s law can be expressed as the equation: n₁ × sin(θ₁) = n₂ × sin(θ₂)
Where:
- n₁ is the refractive index of the first medium.
- n₂ is the refractive index of the second medium.
- θ₁ is the angle of incidence in the first medium.
- θ₂ is the angle of refraction in the second medium.
For instance, when light enters water from air, it refracts because the speed of light in water is slower. Using Snell’s law, one can calculate the precise angle at which the light refracts.
While Snell’s law can apply to other waves (such as sound), it is most commonly used in problems concerning light.
Reflection of Sound
The reflection of sound refers to the phenomenon where sound waves bounce off an obstacle and redirect in another direction. This occurrence can be experienced in our daily lives as the echo effect. For instance, when shouting near mountains or large buildings, one might hear the sound reflected back.
Such reflections play a crucial role in acoustical design. Obstacles with a low sound absorption rate tend to reflect sound more effectively. Therefore, a lot of effort is put into regulating sound reflection and absorption in places like concert halls or auditoriums to provide an optimal acoustic environment.
Upon reflection, the phase of the wave typically inverts (whereas it doesn’t during refraction).