# Physics of waves in acoustics and electromagnetic field

## Basic science

This part of the article is pure basic science. tinnitus, hyperacusis or meniere's disease sufferers would not have any practical benefit from this part, but those who decide to read it may enrich their scientific knowledge.

The initiation, propagation and reception of a matter in a forum of waves and vibrations is the main part in the study of acoustics. In optical waves, or in more comprehensive concept, electromagnetic waves, there is no mechanical medium for the waves, and it is possible that the electromagnetic waves will pass though complete vacuum.

## Acoustic energy

The origin of acoustic energy can be in many other forms of energy that were transduced to sound. For example: a guitar player is applying mechanical energy on the string, and the vibrating string is inducing sound waves. In other words, the transduction procedure transforms energy from some other forms into acoustic energy, producing the acoustic wave.

## Acoustic wave propagation

There is one basic mathematical equation that describes acoustic wave propagation, but the variety of phenomena that emerge from it is complex and rich in types and sub types of sounds. The acoustic wave carries energy via the propagating medium. Transduction into other forms of energy may be natural, forced or a combination of both. The final effect may be purely physical, biological or other domains. The central part in the mechanic of sound is wave propagation.

## Branch of physical acoustics

Propagation of a sound wave in a medium is a branch of physical acoustics. Pressure waves are the mechanism in fluids, gas and solids, sound propagates primarily as a pressure wave. In solids, the pressure waves can take forms of longitudinal wave, transverse waves and surface waves. Acoustics deals with the degree of pressure and distribution of frequencies in the sound wave.

## Sound waves

Transduction processes are also of scientific interest. In gases and fluids such as air and water, sound waves travel as disturbances in the degree of pressure in the matter. Even when this deviation from the equilibrium is of low energy, it can be detected by the human ear. The lowest energy of sound that a healthy person can hear is called: the threshold of hearing. It is nine orders of magnitude smaller than the ambient pressure.

The slang of the acoustic technicians and the semi professional people is calling loudness of sound: "decibels". The basis for it is that the sound pressure level (SPL) is the term for the energy of sound, and is measured on a logarithmic scale in decibels.

## Physics of sound

Other experts in acoustics such as sound engineers and investigators in physics of sound tend to formulate sound pressure levels in terms of frequencies. Electronic equipment for broadcasting or receiving and human auditory system interprets sound waves.

What we identify as "high tones" or "low tones" are pressure vibrations of air having a large or few numbers of cycles per second. Sound waves can be divided to units in time, and then presented in a more friendly way to the human ear such as octave bands or time frequency curves. These two ways of presentation are used to analyze sound and monitor acoustic phenomenon.

## Spectrum of frequency

The spectrum of frequency, in relation to the human hearing system, can be classified into three sections: audio, ultrasonic, and infrasonic.

The audio range includes the spectrum of 20 Hz to 20,000 Hz. This part of the range is important to us because its frequencies can be heard by the human ear. The audio range is considered as the critical part of the spectrum for humans. It includes speech communication and music.

The ultrasonic range refers to the very high frequencies: 20,000 Hz and over. This part of the spectrum has shorter wavelengths which allows better resolution in imaging technologies. Imaging machines for medical utilizations such as ultrasonography and elastography (non invasive test for suspected soft tissue pathology under pressure) rely on the ultrasonic frequency waves.

The lowest frequencies are known as the infrasonic range. These unheard sound waves are used to study geological events such as earthquakes.

## Acoustic signals

Special microphones are used to identify sound. A computer program can control the work of the spectrum analyzer and facilitate both visualization and measurement of acoustic signals. The new advance in computation enables to analyze their properties. The spectrogram of a recorded sound signal, produced by such an instrument provides a graphical display.

The observer can see a time scale of varying pressure level and frequency profiles. The dynamic display of the components of the sound waves gives the specific acoustic signal its defining character.

## Transducer in acoustics

A transducer is a device for converting one form of energy into another. The common transducer in acoustics is converting sound energy into electrical energy, such as done in a microphone or electrical energy to sound energy which is the function of the speakers.

Most acoustic applications based on electronic instrumentation, include few types of acoustic transducers. Today, only a singer who is singing serenade to his girlfriend at midnight, under her window should not use acoustic transducers, because one of her neighbors may wake up angry. Among the acoustic transducers you can find drivers, electric microphones, hydrophones and sonar projectors. Amplifiers are not included because their input and output is an electric signal.

These acoustic transducers convert an electric signal to a sound pressure wave or translated a sound wave to electric signal. The transducers in the typical loudspeakers are electromagnetic devices with the capacity to propagate sound waves; Their structure includes a suspended diaphragm activated by an electromagnetic voice coil, and producing output of pressure waves.

Microphones employ electrostatics. As the sound wave strikes the microphone's diaphragm, it moves and induces a voltage change. The ultrasonic systems used in medical imaging based on ultrasonography is using piezoelectric transducers. Ultrasound of cervical arteries is important for tinnitus sufferers who complain on pulsating sound or vertigo sufferers (including meniere's disease) with suspected narrowing or obstruction of these blood vessels.

The physical aspect of electomagnetic waves will be written under the title of laser physics, but the "wave mechanics" can be applied to other fields of the spectrum of electomagnetic radiation as well.

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