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Have you ever wondered how sound works, why we can hear it or what sound even is? In simple terms, acoustics is the study of sound. Specifically, it is the understanding of how sound is generated, how it travels, and how it affects other things that come into contact with it. So, acoustics can be considered the one stop shop on everything there is to do with sound!
A typical example of a sound wave, with the more rises and falls meaning a higher frequency. This is a wave that travels through the ear to be interpreted by us as a sound. Author: Richard F. Lyon source: Cochlea waves.svg
As a whole, there are five distinct examples of acoustics, and all five are distinct with their own qualities and purposes. Some kinds of acoustics have more purpose than others, and some forms of acoustics are exclusively a detriment to us. Here are the five forms of acoustics:
Environmental acoustics is fairly self-explanatory, it describes the study of all forms of noise that occur in the environment. Specifically, most things in the environment output sound and vibrations as a secondary form of energy. For example, a car driving along a road is mainly making use of kinetic energy, however as everyone knows it will also output noise as it travels, mainly due to the engine. Any version of this kind of noise, typically unwanted noise, is environmental noise. It is a typical goal to attempt to minimise this form of acoustics as much as possible as it serves no use and can cause a nuisance.
On a different end of the spectrum, musical acoustics concerns the study of sound made intentionally. Particularly, it covers how we use specific sounds to generate musical notes, which can involve both sounds created from any kind of instrument, as well as sounds generated from the human voice itself. The study of musical acoustics also takes into account how we listen to and perceive musical sound, and how our ear responds to each form of musical note.
Ultrasound is sound we’re not able to hear because its frequency is above the upper limit of human hearing. Specifically, it’s a sound that has a frequency too high for humans to be able to hear, as humans are only able to hear up to about 20,000 Hertz. Ultrasound, although being no different from standard sound other than in frequency, has a variety of practical uses. For example, ultrasound can be used in the measurement of long distances, as well as in detecting specific objects on the other side of physical barriers.
The name for the technique that uses ultrasound to detect distances is called echolocation. IT involves sending out an ultrasound pulse, and timing how long it takes to come back. Since we know how fast the sound is traveling, we can use time and speed to calculate the distance!
An example of echolocation. Subject A emits a sound towards subject B, and listens for the sound returning back. Its brain is able to determine the distance itself and subject B. author: Marek M source: Chiroptera echolocation's
Infrasound is sound too low in frequency for humans to hear. The lower end of human hearing is less than 20 Hertz, so everything less than that encompasses Infrasound. Infrasound is great for detecting things below the earth's surface For example, earthquakes produce large seismic waves, and these waves have very low frequencies below the human hearing range. Using this Infrasound, we are able to detect the location of earthquakes, their point of origin, and infer their potential severity.
This form of acoustics focuses on the noise emitted as waste energy in mechanical devices, or any devices that have a lot of moving parts. This noise typically results in vibrations, and vibrations in a system that is complex can quite easily stop the system from working as well as intended or damage the system over time. A particular concern is called vibrational resonance, where the vibration will resonate with the surrounding machine and cause even more violent vibrations as a result. This field also concerns earthquakes, as the damage they cause is essentially a result of vibrations on an enormous scale. The field aims to help minimise the damage that these vibrations can cause.
Acoustics and the general study of sound and vibrations have many important roles to play in the field of physics. Something that is vital in many different fields that we’ve already touched upon is the minimisation of noise and vibration. Vibrations and sound cause many problems in many different systems. We’ve already mentioned the damage vibration could potentially cause over a long period of use of a machine or other mechanical system, as well as preventing a machine from working at all. However, other issues involving the health of human beings, such as loud noises that can damage the human eardrum, are also important matters that come under the study of acoustics.
One of the earliest precursors to the study of acoustics is music. Historically, musicians made use of experimentation to develop the instruments and musical scales we still use today. Music theory was created i different cultures in multiple different ways.
Another practical use of acoustics is the analysis of sound and vibrations that occur naturally. Scientists study sound and vibrations that come from the ocean, under the ground in the form of earthquakes, and up in the atmosphere to study weather patterns. The intensity of these vibrations and sounds can be used as guidelines to determine other factors or locate objects that scientists are looking for. For example, knowing the intensity of a sound in multiple locations allows us to figure out the source of a sound.
One major aspect of the study of acoustics is acoustic energy. Acoustic energy is the energy associated with the disturbance of the particles that make up the medium of the sound waves. So, when a sound is made, a force pushes matter in a particular direction, which pushes the adjoining or adjacent matter, and so on. The way this eventual wave hits our ears is how hearing works.
Sound and acoustics are very similar concepts and you could be forgiven for thinking that they mean the same thing; however, they differ subtly. Sound completely depends on human hearing, and how a person will interpret that sound, meaning sound waves can only occur between frequencies of 
and 
, as that is the human hearing range. This however is not definite, and there are exceptions to this rule. In general, however, sound is a wave in air at a specific wavelength that people can interpret.
Humans interpret sound through the eardrum. There are parts in your ear that are incredibly sensitive to the vibrations that make up sound, and will vibrate at the same frequencies of soundwaves that hit them. This phenomenon is known as resonance. These vibrations are then converted into electrical signals that the brain interprets as sound. The resonance of the eardrum works in the same that all solids do in that it has a limit. This is why we are only able to hear up to 
; above this frequency, the eardrum is not capable of resonating with incoming sound waves.
A diagram of a human ear. The various parts that make up the eardrum such as the the malleus, incus, stapes, and tympanic membrane resonate when a sound encounters it. author: Inductiveload source: Anatomy of Human Ear with Cochlear Frequency Mapping.svg
Acoustics is concerned with the physics of pressure waves of all frequencies and studies what happens in between the generation of a sound, and the interpretation by a listener, rather than how sound is interpreted by humans This involves studying how the sound interacts with the medium it is traveling in, the region the sound can travel in, and how the sound will be absorbed, dampened, diffused, or reflected by neighbouring materials.
- 
.Acoustics is the study of how sound is generated, how sound interacts with the medium it travels in and the materials it will interact with.
The types of acoustics are noise, musical, ultrasound, infrasound, and vibrational acoustics.
The importance of acoustics in physics is the many practical uses its study has. Detection of earthquakes and prediction of weather is partly made possible through the study of acoustics. As well as this, ultrasound is used to gain accurate images of the human body in the medical field. These are just a few of many examples.
The difference between sound and acoustics is how they are interpreted. Sound describes how a wave of specific wavelength travelling through the air is interpreted by people, whereas acoustics studies how this sound is generated, and how it interacts with the medium it’s travelling in as well as how it interacts with materials it encounters.
An acoustic field is an area of space in which the acoustics of the environment is studied and known. For example, if noise were to occur within an acoustic field at a specific point and at a specific volume, it would be possible to know how that noise would travel in that field.
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