Sound - Explanation and definition of sound

What is sound

The sound is defined as the set of pressure fluctuations that propagates as waves through a solid, liquid, gas or elastic medium. Commonly we define the sound to all sound waves are picked up by our ears and interpreted by the brain, but that definition is incomplete due that in nature there is a wide range of sounds that we can not perceive.

The sound is produced as a result of the vibration of a material body such as the vibration caused by tearing the string of a guitar, hitting the membrane of a drum or the vibrations produced in the vocal folds as we speak, in all these cases the initial vibration is transmitted along all the atoms, molecules and particles present in the medium producing the generation of sound waves. In order that sound can be transmitted is necessary that the medium by which it spreads is elastic in contrast molecules and atoms could not vibrate or transmit the disturbance and therefore the sound will not transmitted.

As we have indicated before the sound like the light has a wave nature and as such has all the features and properties that define a wave as amplitude, frequency, period, length, energy, speed of propagation... A sound will be different from another depending on the values acquires by each of these characteristics of the sound wave, so for example, a sound wave with the amplitude greater than other will perceive more strongly or what is the same with most volume, another sound with the same amplitude but different frequencies or wavelengths will perceive as sounds more bass or treble.

Another feature that has the sound is its propagation speed which depends directly on the medium through which it propagates, in a solid or liquid medium the propagation speed of sound is inversely proportional to the density of the medium, less speed greater density of the medium, on the other hand the gaseous mediums sound speed is proportional to the temperature of the gas, and for example we find that the velocity of sound in air at a temperature of 20C is 344m/s, the same sound nestled in a freezer at -100C would propagate at a speed of 279m/s, on the other hand if we were in an oven at 400 C the speed of sound would be 521m/s.

The propagation speed of sound is higher when it passes through a solid or liquid when propagating through a gas, for example the speed of sound in air is 344 m/s in water is 1.482 m/s and through aluminium is 6.420 m/s.

Unlike electromagnetic waves, sound waves can not propagate through vacuum because sound is transmitted through the vibration of the atoms and molecules that compose the medium, in outer space near absence of such molecules and atoms makes impossible the propagation and transmission of any kind of sound, which is why astronauts can declare they know the authentic silence.

Sound Spectrum

With the sound spectrum we can classify the wide range of sound waves that occur in nature depending on the wavelength, frequency or energy associated.

The sound spectrum is divided into 3 main areas:

• Infrasound

• Audible Zone

• Ultrasound

The audible area includes all those waves within a range of frequency between 20 and 20.000 Hz, that waves can be captured by our ears.

Ultrasound represent the set of sound waves with a frequency above 20.000 Hz, while infrasound represent the set of sound waves with a lower frequency at 20 Hz.

Ultrasound is widely used in different sectors as a method of detection, measurement and display of materials or organisms that can not directly access them, so for example the industrial sector uses ultrasound equipment as detection and verifying tools about quality of many materials as well as the identification of various defects as internal cracks in welds and metals, leaking ducts and pipes, air trapping and delamination in plastic materials, thickness measurements in paints and coatings, etc... Inside the area of medicine is common to use that equipment to perform ultrasound to visualize some organs of a patient or the evolution of a baby in the maternal womb support medical diagnosis.

Moreover thanks of the associated energy that has ultrasound is used as a method to removal of kidney stones, muscle injury recovery, welding for synthetics tissues, cleaning of oxides and contaminants on the surfaces of the materials... In nature we find animals like dolphins or bats that can emit ultrasound which use them as a way to guide and hunt.

Infrasound has the ability to propagate over long distances and although we can not hear that are present in sounds produce in nature as in the wind, the waves, avalanches, in meteorites, in auroras ... infrasound’s are captured by using specific microphones through which we can identify sounds made by animals such as elephants, giraffes, rhinos and tigers allow us know its position, on the other hand the use of these microphones can detect volcanic eruptions located at great distances.

Thanks to the knowledge acquired about the sound spectrum we have developed tools and instruments such as sonar, which exploiting the properties of the sound at different frequencies is used as a means of navigation, communication and detection of submerged underwater objects.

Noise pollution

Today most of the environments in which we move, work and rest are full of noise sources, cities with traffic congested where the sound of engines, horns and noise rolling movement of cars and motorcycles is present at all hours, factories where machines and tools like presses, sanders, grinders or drills continuously emit noise, everyday machines such as elevators, escalators, appliances... generating noise during operation. The reality makes us recognize the existence of various sources of noise from some level become what is known as noise pollution.

Thousands of studies support the source of behaviours and diseases directly related to noise pollution such as lack of concentration, hearing loss, aggressive behaviour, sleep disorders or stress, all this forces us to identify the sound environments to which we are exposed and to define and implement the necessary actions to reduce noise sources.

The sound level meters are the equipment responsible for capturing the intensity of a sound source and represent it by numerical data known as dB or decibel, the greater intensity or loudness of sound higher decibels will have, with this information we can represent the different intensities noise and the consequences of being exposed to them:

Sound	Decibels	Effects
Murmurs, quiet breathing, rustling leaves ...	10-30 dB	Barely audible
Computer, quiet rural area, library ...	30 - 50 dB	Tranquility
Conversation, background music, washing machine ...	50-70 dB	Normal
Dryer, city traffic, crying baby ...	70-90 dB	Annoying
Rock concert, emergency siren ...	90-110 dB	Very strong
Thunder, firecracker, pneumatic drill ...	120 dB	Max Supported
Takeoff aircraft, artillery fire ...	140 dB	Pain
Takeoff rockets, nuclear bomb ...	> 180 dB	Permanent hearing loss

To avoid damage our quality Life and health is necessary to measure and understand the sources of noise generated in our environment in order to do corrective measures such as the use of earmuffs or helmets noise, uses of absorbing noise materials such as polyurethane foam, cork, special anti-sound coatings, rockwool ...

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