For my essay, I will be researching on reflection, refraction and diffraction of sound and how the theory would be applied in real life. The reflection of sound follows the law of reflection which states that the angle of incidence is equal to the angle of reflection. The sound wave can be reflected by either a solid or liquid surface which can be either a rough or polished surface. When the surface is rough, the sound wave would be reflected in a diffused and irregular manner. On the other hand, when the surface is smooth, the sound wave would be reflected in an orderly and regular manner. The reflected wave is always weaker than the original wave as part of the sound energy that is absorbed by the surface. Soft surfaces absorb more sound energy compared to hard surfaces. When there is repeated reflection, reverberation occurs. For example, in halls and auditoriums, there are curved surfaces that are placed in a manner such that the sound sources stay at the focus. This allows a spectator to perceive sounds from every part of the hall or auditorium, making it seem lively and full. For this reason, auditorium and concert hall designers prefer construction materials that are rough rather than smooth. Sound is also more mixed thoroughly when the walls, ceiIings are made up of irregular shapes and protruding edges This causes the waves form to be reflected uniformly. Balcony fronts and chandelier are used in concert halls to break up the sound and distribute it more uniformly. For this reason, auditorium and concert hall designers prefer construction materials that are rough rather than smooth. Another real life example is that in Singapore, Esplanade Theatre is constructed as ‘boxes within boxes’ to insure protection from outside noises and vibrations. These methods include wide structural air gaps to further insure that vibrations generated in "noisy" areas from outside will not be transmitted to the acoustically critical performer and audience areas in the concert hall. Solid, dense construction materials were used to insure that acoustic sounds from the lowest organ pedal notes, to the highest overtones of string and percussion instruments, are preserved and distributed to the listeners evenly. An article on another concert hall, Victoria Concert Hall was refurbished and that a new metal roof was built for the concert hall and a plastered ceiling was constructed beneath it to give good sound insulation. The ceiling has also been fitted with moulding and coffers to reflect sound to the back of the concert hall. In addition, transparent reflectors have also been installed at the stage to let sound to be reflected with the stage area such that the performers are able to hear one another during performances on stage. According to the acoustic team, they prevented hard surfaces as they would cause echos and this would affect the sound clarity to a large extent. Apart from these, slightly curved acoustically transparent panels were used on the ceiling. This not only did fulfil the architectural requirement of having a curved ceiling, but the flat and stepped panels were also carefully designed to reflect sound to the audience below.
Refraction of sound waves involves a change in the direction of waves as they pass from one medium to another. Refraction also known as bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves. So if the media or its properties are changed, the speed of the wave is changed. Thus, waves passing from one medium to another will undergo refraction. Refraction of sound waves is most evident in situations in which the sound wave passes through a medium with gradually varying properties. For instance, sound wave are known to refract when travelling over water. Since it is travelling through a medium of varying properties, the waves would encounter refraction and the change in direction. In addition, air directly above the water is slightly cooler than air which is higher above the water. Due to the fact that sound wave would travel slower in cool air and travel
faster in warmer air, this would mean that the sound wave directly above the water would travel faster compared to the sound wave far above the water. Subsequently, the direction of the wave changes. In another example, speed of sound is greater in hot air than it is in cold air. This is due to the fact that the molecules of air are moving faster and the vibration of the sound wave can thus be transmitted faster. This would means that when sound travels from hot air to cold air or cold air to hot air, it will experience refraction. This is evident on a hot day or cold night. On a hot day, the air near the ground is hotter and thus the sound waves bends upwards from the hot air into the cold air. During night time, the temperature is slightly cooler and that the air near the ground is colder so the sound wave bends downwards. Thus, this is one of the reasons why sometimes people are able to hear sounds from a long way away if the air is cold in the night. In addition, the wind also plays an important part in the refraction of sound and ultimately on the distance they travel. Wind traveling directly into an oncoming sound wave will make it refract upward more sharply. Wind traveling in the same direction as a sound wave will make the sound wave refraction more gradual. In the upper atmosphere a strong wind traveling in the direction of the wave will push the wave further and faster. In the atmosphere, gradients of wind speed and temperature lead to refraction. The wind speed is usually increasing with height, which leads to a downward bending of the sound rays towards the ground. The same holds if the temperature is decreasing with height. If the temperature is increasing with height and the wind speed is low, sound rays are bend upwards.Thus, this means that sound would travel faster in hotter air and that it would travel slower in colder air.
Diffraction means that the bending of waves around objects and the spreading out of waves beyond opening that is smaller compared to the wavelength and that the sound waves are spread around corners. The fact that diffraction is more noticeable with longer wavelength implies that low frequencies sounds are more clearly audible compared to sounds with higher frequencies. The amount of diffraction also increase with increasing wavelength and decrease with decreasing wavelength. When the wavelength of the sound waves is equal or more than the size of the opening, spread is equal. On the other hand, when wavelength of the sound waves is less than opening, the spread is little or none. This explains why we are able to hear sounds when we talk to someone and understands even though we are not facing each other. This is due to the fact that sound emerging from the mouth spreads out to both sides an coming through an opening only a few centimetres wide. This is small compared to the wavelength of most speech sounds and wavelength of the sounds. Another example of diffraction is that bass sounds from a loudspeaker spread more evenly while treble sounds from the same speaker are more nearly confined to a narrow cone in the forward direction. Thus, for the treble sound to diffract more to the people listening to it, treble sound are usually send through a smaller speaker so that it will be diffracted and spread more evenly in all directions. This also explains why our left ear are able to hear sounds that comes from a source on the right. This is because the bass notes have wavelength that are much larger than our head and that they diffract so well that it seems to have the same strength at both the same ears. On the other hand, treble notes form more of a ‘shadow’ and may be weaker at one ear than the other. An illustration would be if a sound source of a treble note is on the right, your right ear would hear the sound first then your left ear. This is because the wavelength of the treble note is smaller than the head. Thus, it could not diffract as well as a ass note. In a real life example, a marching band is performing on the streets, we would most likely to hear the low pitched bass drum first due to its long wavelength compared to the high pitched piccolo. We would not hear the high pitched piccolo as high pitched sounds tend to be more directional and that they could not diffract much.