Like all logarithmic quantities it is possible to multiply or divide dB values by simple addition or subtraction. Decibel measurements are always relative to given reference levels and can therefore be treated as absolute measurements.
That is, if a particular reference value is known then the exact measurement value can be recovered from one of the equations shown above. The dB unit is often qualified by a suffix which indicates the reference quantity used, some examples are provided in the following section.
The decibel is commonly used in acoustics to quantify sound levels relative to a reference. This may be to compare two sound sources or to quantify the sound level perceived by the human ear. The decibel is particularly useful for acoustic measurements since for humans the ratio of the loudest sound pressure level to the quietest level that can be detected is of the order of 1 million.
Furthermore, since sound power is proportional to the pressure squared then this ratio is approximately 1 trillion. This is about the limit of sensitivity of the human ear. Note that since the most common usage of the decibel unit is for sound pressure level measurements it is often abbreviated to just dB rather than the full dB SPL. The human ear does not respond equally to all frequencies it is more sensitive to sounds in the frequency range from 1 kHz to 4 kHz than it is to low or high frequency sounds.
For this reason sound measurements often have a weighting filter applied to them whose frequency response approximates that of the human ear A-weighting. A number of filters exist for different measurements and applications, these are given the names A,B,C and D weighting. The resultant measurements are expressed, for example, as dBA or dB A to indicate that they have been weighted.
In electronics and telecommunication , the decibel is often used to express power or amplitude ratios in order to quantify the gains or losses of individual circuits or components. One advantage of the decibel for these types of measurements is that, due to its logarithmic characteristic, the total gain in dB of a circuit is simply the summation of each of the individual gain stages in dB. In electronics the decibel can also be combined with a suffix to indicate the reference level used.
For example, dBm indicates power measurement relative to 1 milliwatt. The following are some common decibel units used in electronics and telecommunications. If the numerical value of the reference is undefined then the decibel may be used as a simple measure of relative amplitudes. As an example, assume there are two loudspeakers, one emitting a sound with a power P 1 and a second one emitting the same sound at a higher power P 2.
Assuming all other conditions are the same then the difference in decibels between the two sounds is given by:. Any practical measurement will be subject to some form of noise or unwanted signal. In acoustics this may be background noise or in electronics there are often things like thermal noise, radiated noise or any other interfering signals. In a data acquisition measurement system the system itself will actually add noise to the signals it is measuring.
The general rule of thumb is: the more electronics in the system the more noise imposed by the system. In data acquisition and signal processing the noise floor is a measure of the summation of all the noise sources and unwanted signals generated within the entire data acquisition and signal processing system. The noise floor limits the smallest measurement that can be taken with certainty since any measured amplitude cannot on average be less than the noise floor.
Figure 2 shows that only signals above the noise floor can be measured with any degree of certainty. In this case the signal level of dB at 20KHz could be measured. If however, the noise floor increased above the dB level then it would become more difficult to measure this signal. For example, it is possible for the human ear to hear a very low sound such as a pin drop or a whisper. However, this is only possible if the noise floor or background noise of the particular environment is very low such as in a soundproof or quiet room.
It would not possible to hear or discriminate such low levels in a noisy room. Various techniques are employed by the Prosig P data acquisition system in order to ensure that the noise floor of the equipment is kept as low as possible. These include signal-processing functions as well as practical features such as the ability to disable cooling fans during acquisition scans.
Dynamic range is a term used to describe the ratio between the smallest and largest signals that can be measured by a system. The dynamic range of a data acquisition system is defined as the ratio between the minimum and maximum amplitudes that a data acquisition system can capture.
Sometimes amplification may be applied to signals before they are input to an ADC in order to maximize the input voltages within the available ADC range. The resolution of a measurement system is determined by the number of bits that the ADC uses to digitise an input signal. Most ADCs have either bit or bit resolution. For a bit device the total voltage range is represented by 2 16 discrete digital values. Therefore for a bit ADC the dynamic range is 96dB. Using the same calculations the dynamic range of a bit ADC is dB.
The noise floor of a measurement system is also limited by the resolution of the ADC system. In these cases, it is of no use to have an ultra-low noise radio receiver. However in applications such as VHF and UHF fixed or mobile radio communications systems where the levels of received noise are much lower, then a low noise radio receiver is more useful. In order to reduce the levels of noise and thereby improve the sensitivity of the radio receiver, the main element of the receiver that requires its performance to be optimised is the RF amplifier.
The use of a low noise amplifier at the front end of the receiver will ensure that its performance will be maximised. Whether for use at microwaves or lower frequencies, this RF amplifier is the chief element in determining the performance of the whole receiver.
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