Sound ( Acoustics )

Topic includes 

- Reflection

- Refraction

- Absorption of sound waves

- Pitch

- Loudness

- Quality

- Echo

- Reverberation

- Control of reverberation

Reflection

Refection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.
Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.

In acoustics, reflection causes echoes and is used in sonar. In geology, it is important in the study of seismic waves. Reflection is observed with surface waves in bodies of water. Reflection is observed with many types of electromagnetic wave, besides visible light.

The laws of reflection are 

-The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.

-The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

-The reflected ray and the incident ray are on the opposite sides of the normal.

These three laws can all be derived from the Fresnel equations.

Refraction

-The fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density.

-Change in direction of propagation of any wave as a result of its travelling at different speeds at different points along the wave front.

-Measurement of the focusing characteristics of an eye or eyes.

-Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. ... Refraction is responsible for image formation by lenses and the eye.

Absorption of sound wave

-Acoustic absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy.
-Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been 'lost'.

-When sound from a loudspeaker collides with the walls of a room part of the sound's energy is reflected, part is transmitted, and part is absorbed into the walls. Just as the acoustic energy was transmitted through the air as pressure differentials (or deformations), the acoustic energy travels through the material which makes up the wall in the same manner.
-Deformation causes mechanical losses via conversion of part of the sound energy into heat, resulting in acoustic attenuation, mostly due to the wall's viscosity. Similar attenuation mechanisms apply for the air and any other medium through which sound travels.

Pitch

Pitch is a perceptual property of sounds that allows their ordering on a frequency-related scale, or more commonly, pitch is the quality that makes it possible to judge sounds as "higher" and "lower" in the sense associated with musical melodies.

Loudness

In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud." The relation of physical attributes of sound to perceived loudness consists of physical, physiological and psychological components.

Sound quality is typically an assessment of the accuracy, fidelity, or intelligibility of audio output from an electronic device. Quality can be measured objectively, such as when tools are used to gauge the accuracy with which the device reproduces an original sound; or it can be measured subjectively, such as when human listeners respond to the sound or gauge its perceived similarity to another sound

Echo

An echo is a repetition or imitation of sound. When sound waves hit a hard surface they might reflect, making the sound bounce and repeat. If you agree with someone, you might echo his or her statement.

What creates an echo?

Echoes are created when sound waves hit an obstacle and bounce back. The smoother the obstacle the sound wave hits, the clearer and louder the echo – because the sound wave remains more intact when it hits a smooth surface than when it hits a rough one.

Reverberation of sound

Reverberation is the collection of reflected sounds from the surfaces in an enclosure like an auditorium. It is a desirable property of auditoriums to the extent that it helps to overcome the inverse square law dropoff of sound intensity in the enclosure.

What is an example of reverberation?

The definition of a reverberation is a reflection of light or sound waves, or a wide reaching impact of an action. An example of a reverberation is the sound bouncing around in a large speaker. An example of a reverberation is the impact of a no trespassing law at a shopping center on students at a nearby high school.

 Difference between echo and reverberation

An echo is a single reflection of a soundwave off a distance surface. Reverberation is the reflection of sound waves created by the superposition of such echoes. ... A reverberation can occur when a sound wave is reflected off a nearby surface.

Echo vs. Reverberation

An echo is a single reflection of a sound wave off a distance surface. Reverberation is the reflection of sound waves created by the superposition of such echoes.

An echo can only be heard by humans when the distance between the source of the sound and the reflecting body is more than 50 feet in distance. A reverberation can occur when a sound wave is reflected off a nearby surface.

An echo is usually clear and can be easily distinguished because of the distance and time the sound wave travels. Reverberations do not usually have enough distance or time to travel which means they can pile up on each other making it very difficult to comprehend.

Echoes can be used to determine the distance of a reflecting object such as a large building or mountain if the ambient temperature and humidity are known. Reverberation does not allow enough travel time to use it to measure distance.

With reverberation, sound waves pile on top of each other arriving at the receiver or ear at different times making it difficult to comprehend or understand.

The piling up of soundwaves also increases the sound energy in a room. This buildup of energy eventually will excite the structure and increase the overall volume in a room.

Reverberation in a room will cause people to raise their voices in an attempt to compensate for their inability to understand speech.

Adding sound absorption in a space will reduce the reflections and cause the sound waves to decay at a faster rate. In public spaces, a comfortable decay time should be less than 2 seconds.

Control of reverberation

To control reverberation time, acoustic absorption is used. Absorbent materials conventionally take two forms; fibrous materials or open-celled foam. Fibrous materials absorb sound as sound waves force the fibres to bend and this bending of the fibres generates heat.
How do you reduce reverberation?

Reverberation can be reduced by absorbing the sound using some materials as it reaches the wall and ceiling of the room and thus prevent the sound from getting reflected. Some materials which are used to reduce reverberation are fibreboard, heavy curtains, plastics etc.