Penurunan daya sinyal selama pengiriman data/informasi berlangsung besarnya diukur dalam satuan decibel

Attenuation is the reduction in amplitude and intensity of a signal. Signals may be attenuated exponentially by transmission through a medium, in which case attenuation is usually reported in dB with respect to distance traveled through the medium. Attenuation can also be understood to be the opposite of amplification. Attenuation is an important property in telecommunications and ultrasound applications because of its importance in determining signal strength as a function of distance. Attenuation is usually measured in units of decibels per unit length of medium (dB/cm, dB/km, etc) and is represented by the attenuation coefficient of the medium in question.[1]

Attenuation coefficient

Attenuation coefficients are used to quantify different media according to how strongly the transmitted ultrasound amplitude decreases as a function of frequency. The attenuation coefficient (α) can be used to determine total attenuation in dB/cm in the medium using the following formula:

\mathrm{Attenuation(dB)} = \alpha\mathrm{(dB/MHz*cm)}\times\mathit{l}\mathrm{(cm)}\times\mathrm{f(MHz)}

As this equation shows, besides the medium length and attenuation coefficient, attenuation is also linearly dependent on the frequency of the incident ultrasound beam. Attenuation coefficients vary widely for different media. In biomedical ultrasound imaging however, biological materials and water are the most commonly used media. The attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below:[2]

Material α(dB / MHz * cm)
Lung 41
Bone 20
Kidney 1.0
Liver 0.94
Fat 0.63
Blood 0.18
Brain 0.85
Water 0.0022

There are two general ways of acoustic energy losses: absorption and scattering, for instance light scattering.[4] Ultrasound propagation through homogeneous media is associated only with absorption and can be characterised with absorption coefficient only. Propagation through heterogeneous media requires taking into account scattering.[5]

[edit] Earthquake

The energy with which an earthquake affects a location depends on the running distance. The attenuation in the signal of ground motion intensity plays an important role in the assessment of possible strong ground shaking. A seismic wave loses energy as it propagates through the earth (attenuation). This phenomenon is tied in to the dispersion of the seismic energy with the distance. There are two types of dissipated energy:

  1. geometric dispersion caused by distribution of the seismic energy to greater volumes
  2. dispersion as heat

In telecommunications, return loss is a measure of power reflected from imperfections in an electrical or optical communications link. It is the ratio PR / PT, representing the power of the wave reflected from the imperfection (PR) to that of the incident, or transmitted, wave, (PT). For best performance, the reflected signal should be as small as possible, meaning the ratio PR / PT should be as small as possible.

Return loss is usually expressed in decibels. The return loss value describes the reduction in the amplitude of the reflected energy, as compared to the forward energy. It will always be a loss, and therefore a negative dB. However one can write -3 dB as simply 3 dB of loss, dropping the negative sign and adding loss. For example, if a device has 15 dB of return loss, the reflected energy from that device, PR, is always 15 dB lower than the transmitted energy PT. When expressed in dB, larger (in magnitude) negative numbers represent larger return losses and thus smaller reflected power.

NeXT Software, Inc. (formerly NeXT Computer, Inc.) was a computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets.

NeXT was founded in 1985 by Steve Jobs after his resignation from the then-Apple Computer (now Apple Inc.). In addition to its hardware, NeXT developed the NEXTSTEP operating system, later retooled as a programming environment, called OPENSTEP, capable of running on several different operating systems, most notably Solaris. This put NEXTSTEP in direct competition with Windows NT and the vaporware object-oriented version of Windows NT, Cairo. In 1993, NeXT withdrew from the hardware business and concentrated on marketing OPENSTEP for Intel based hardware. NeXT also developed WebObjects, one of the first Enterprise web application frameworks. On December 20, 1996, NeXT was bought by Apple for US$400 million.[2] Parts of NeXT’s software were later used as the foundation for Mac OS X, often referred to as OPENSTEP 5.0 by those within the NeXT community

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