For my essay, I will be researching on the reflection, refraction and diffraction of sound. In theory, the law of reflection states that the angle of incidence is equals to the angle of reflection, which applies to the reflection of sound.
A sound wave can be reflected on either a polished or rough surface or a solid or liquid surface. When a sound wave is reflected on a smooth surface, it would produce an orderly and regular reflection. Whereas, if when a sound wave is reflected on a rough surface, the reflection on sound would be irregular and diffused. In theory, the reflected sound wave is always weaker than the original sound wave because part of the sound energy will be absorbed. However, the fraction of the sound wave that would be absorbed varies with the type of surface; for instance, if the surface is soft, more sound energy would be absorbed and likewise if the surface is hard, less sound energy would be absorbed. As such, builders of concert halls and auditoriums would avoid the use of hard and smooth materials when they are constructing the insides of these halls. When sound waves are reflected on a hard surface such as concrete, there would be very little sound energy absorbed. This is why most concert halls have their walls and ceilings built using softer materials such as fiberglass and acoustic tiles. These materials all have a greater ability to absorb sound, thus would be able to give the concert halls more pleasing acoustic properties. A real life example can be seen from Singapore’s Esplanade Theatres, where it was constructed with the ‘boxes within boxes’ concept in mind to ensure protection from external noises and vibrations. This particular method includes wide structural air spaces to ensure that vibrations generated in external noisy areas from outside will not be transmitted into the theatre where the performers and audiences will not be affected from inside the concert halls. Solid and dense construction materials are often used to ensure that acoustic sounds from the low organ pedal notes, to the high overtones of stringed and percussion instruments, are preserved and distributed to the listeners evenly.
The repetition of the reflection of sound waves can lead to one of two kinds of phenomena; mainly known as an echo or a reverberation. A reverberation usually occurs in a small room with height, width and length dimensions of approximately 17 meters or less. This is because of the effect of a particular sound wave on the brain endures for more than a small fraction of a second; the human brain keeps a sound in their memory for up to 0.1 seconds. If the reflected sound wave reaches the ear within 0.1 seconds of the initial sound wave produced, then it seems to the person that the sound is prolonged. The reception of multiple reflections off of walls and ceilings within 0.1 seconds of each other causes reverberations – the prolonging of a sound. Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 s for a sound to travel the length of a 17 meter room and back, thus causing a reverberation. Sometimes to prevent echo and reverberation, the acoustic construction team would avoid using hard materials to build the concert halls as they would affect the clarity of the sound produced to a large extent. Hence instead of using these hard materials, slightly curved acoustically transparent panels were used on the ceiling instead. These curved transparent panels not only fulfil the architectural requirements of having a curved ceiling, but also the flat and stepped panels were also carefully designed by the acoustic team to reflect sound to the audience below.
The refraction of sound waves occurs when the speed of sound changes with its position. It involves a change in the direction of sound waves as they travel from one medium to another. As refraction of sound waves is also known as the bending of the sound waves, it is accompanied by the change in velocity and wavelength of the waves. Hence, if the medium or the properties of the medium at which the sound waves would travel through changes, the speed of the sound wave would also change, thus undergoing refraction. For example, sound waves are known to undergo refraction when travelling over water. Even though the sound wave is not exactly changing its medium, it is still travelling through a medium with varying properties; thus the sound wave will refract and change direction. Additionally, the air directly above water is also slightly cooler than air higher above water, therefore sound waves would travel slower in cool air and travel faster in warmer air. When the air is cooler or warmer in certain parts of the area, the speed of sound varies, causing it to travel in curved paths. When the wave fronts are steered away from the ground, a dead or shadow region is left where nothing can be heard.
As shown in the diagram, the portion of the wavefront directly above the water is slowed down, while the portion of the wavefronts far above the water speeds ahead. Subsequently, the direction of the wave changes, refracting downwards towards the water.
Lastly, the diffraction of sound waves is the process whereby sound wave can be spread around a corner or through door openings. Through a narrow opening, waves are able to spread out almost equally in all directions, where the spread of sound waves depends on the relation between the wavelength and other distances like the size of the opening. If the wavelength is equals to or more than the size of the opening, the spread would be ‘equal’. If the wavelength is less than the size of the opening, the spread would range from a little to none. An example would be the possibility of hearing sound coming from the left at your right ear, because the wavelength of the bass notes are larger than your head, hence they diffract in a way such that the strength is almost nearly the same in both ears. This is also because treble sounds are weaker as treble wavelengths are smaller than your head. Another example would be that sounds from one person can be heard by another person not facing him as sound waves spread out from both sides since it is coming from a small opening, in this case it would be the mouth. Applied in our daily lives, treble sounds are usually sent through a smaller speaker. This is due to the fact that bass sounds spread more evenly as compared to treble sounds which are more confined to a narrow cone going forward.
To give a little bit of insight on the history of this theory; Italian natural philosopher Francesco Grimaldi discovered and coined the term ‘diffraction’ in 1660. He had proved that a single beam of light spread out, creating an interference pattern, if it is shone through very small slits. Sound waves act exactly the same way, and eventually this became a very important finding for those that had advocated the theory of light. Prior to this, people had argued that the sharp boundaries created by shadows meant that light around corners in the same way at which water and sound waves can. The diffraction of sound waves can be depicted by the figure shown below.
Another real life example of the diffraction of sound waves can be found in animals like elephants and bats. Scientists have recently discovered that elephants emit infrasonic waves of very low frequency to communicate over long distances to each other. Elephants typically migrate in large herds that may sometimes become separated from each other by distances of several miles. Researchers who have observed elephant migrations from the air and have been both impressed and puzzled by the ability of elephants at the beginning and the end of these herds to make extremely synchronized movements. The matriarch at the front of the herd might make a turn to the right, which is immediately followed by elephants at the end of the herd making the same turn to the right. These synchronized movements occur despite the fact that the elephants' vision of each other is blocked by dense vegetation. Only recently have they learned that the synchronized movements are preceded by infrasonic communication. While low wavelength sound waves are unable to diffract around the dense vegetation, the high wavelength sounds produced by the elephants have sufficient diffractive ability to communicate long distances.
Bats use high frequency (low wavelength) ultrasonic waves in order to enhance their ability to hunt. The typical prey of a bat is a moth – an object not much larger than a couple of centimeters. Bats use ultrasonic echolocation methods to detect the presence of bats in the air. As the wavelength of a wave becomes smaller than the obstacle that it encounters, the wave is no longer able to diffract around the obstacle, instead the wave reflects off the obstacle. Bats use ultrasonic waves with wavelengths smaller than the dimensions of their prey. These sound waves will encounter the prey, and instead of diffracting around the prey, will reflect off the prey and allow the bat to hunt by means of echolocation. The wavelength of a 50 000 Hz sound wave in air (speed of approximately 340 m/s) can be calculated as follows
wavelength = speed/frequency
wavelength = (340 m/s)/(50 000 Hz)
wavelength = 0.0068 m
The wavelength of the 50 000 Hz sound wave (typical for a bat) is approximately 0.7 centimeters, smaller than the dimensions of a typical moth.
This concludes my research on the reflection, refraction and diffraction of sound waves.