It has to guarantee that the cable has 3dB or less attenuation to the customer, giving the customer at least 0. Assume the load is 1 ohm. A technician goes to make measurements to verify, he measures in voltage and gets 0.
So if he multiplies that by 10 and calls it decibel, it would be 1. Making it difficult to compare to the specification. Multiplies it by 20, he gets 3dB and can make direct comparison to the specification. In your question you say you want to calculate the voltage or current db ratio. This is where you are getting confused. For instance if I said the "signal" across a resistor increased by 6. The basic problem is that you believe that there is such a thing as "voltage or current db ratio" and that somehow this is independent or different to power.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. For example a voltage ratio of 1,, could be expressed as dB. Since the definition of the decibel involves logarithms, it is appropriate to review some of the properties of logarithms. The common logarithm log of a number is the power to which 10 must be raised to equal that number.
As can be seen from the above, multiplication of numbers becomes addition of their logs. This can be very useful for EMC measurements. For example, let's assume that we want to know the voltage that would be measured by a meter connected by a long cable to an antenna exposed to an electromagnetic field. If all these numbers are expressed in dB, however, all we have to do is add them together -- a much simpler task.
Remember dB's are always ratios of numbers. The filters used for dBA and dB C The most widely used sound level filter is the A scale, which roughly corresponds roughly to the inverse of the 40 dB at 1 kHz equal-loudness curve. Using this filter, the sound level meter is thus less sensitive to very high and very low frequencies. Measurements made on this scale are expressed as dBA. The C scale varies little over several octaves and is thus suitable for subjective measurements only for moderate to high sound levels.
Measurements made on this scale are expressed as dB C. There is also a rarely used B weighting scale, intermediate between A and C. The figure below shows the response of the A filter left and C filter, with gains in dB given with respect to 1 kHz. For an introduction to filters, see RC filters, integrators and differentiators. ISO Loudness, phons and sones, hearing response curves The phon is a unit that is related to dB by the psychophysically measured frequency response of the ear.
At 1 kHz, readings in phons and dB are, by definition, the same. For all other frequencies, the phon scale is determined by the results of experiments in which volunteers were asked to adjust the loudness of a signal at a given frequency until they judged its loudness to equal that of a 1 kHz signal. To convert from dB to phons, you need a graph of such results. Such a graph depends on sound level: it becomes flatter at high sound levels.
Recording level and decibels Meters measuring recording or output level on audio electronic gear mixing consoles etc are almost always recording the AC rms voltage see links to find out about AC and rms. So what is the reference voltage? Intensity, radiation and dB How does sound level or radio signal level, etc depend on distance from the source? So, if we double the distance, we reduce the sound pressure by a factor of 2 and the intensity by a factor of 4: in other words, we reduce the sound level by 6 dB.
If we increase r by a factor of 10, we decrease the level by 20 dB, etc. For fresh water, the specific acoustic impedance for water is 1. So a sound wave in water with the same pressure has a much lower intensity than one in air. For many cases in communication, isotropic radiation is wasteful: why emit a substantial fraction of power upwards if the receiver is, like you, relatively close to ground level.
For sound of short wavelength including most of the important range for speech , a megaphone can help make your voice more anisotropic. For radio, a wide range of designs allows antennae to be highly anisotropic for both transmission and reception. Example problems A few people have written asking for examples in using dB in calculations.
All else equal, how much louder is loudspeaker driven in its linear range by a W amplifier than by a 10 W amplifier? I am standing at a distance R from a small source of sound size much less than R , at ground level out in the open where reflections may be neglected. The sound level is L.
If I now move to a distance nR n a number, and nR still much greater than the size of the source , what will be the new sound level? If, in ideal quiet conditions, a young person can hear a 1 kHz tone at 0 dB emitted by a loudspeaker perhaps a softspeaker?
An amplifier has an input of 10 mV and and output of 2 V. What is its voltage gain in dB? What is the difference, in dB, between the irradiance light intensity on earth 8. Occupational health and safety Different countries and provinces obviously have different laws concerning noise exposure at work, which are enforced with differing enthusiasm. Some FAQs How loud is an aircraft? A train? A person singing? A dog barking? A power tool? The answers to this question vary considerably.
It depends strongly upon how far away you are, whether you are indoors or not, whether there is reverberation, how strong the particular source is and what its spectrum is. To give values, without being very specific about the conditions, would be somewhat misleading.
Because the rest of this page is intended to be reliable, as far as it goes, I'd rather not give values here. How does one "add decibels"? If the sources are coherent which usually means that they ultimately come from the same source , then there may be complicated interference effects. In most cases, where the sources are independent, one can add the intensities and then convert to decibels.
However, if you are given the sound levels in dB A , it is not so easy to go back to intensity, and one must know something about the spectrum of the sound. If you know the distribution of the sound in different frequency bands, you can use the applet on this link. Some of the questions on our FAQ in music acoustics relate to decibels, including Why do the sounds of two musical instruments always reinforce, and never cancel out?
How much does one more violin add to the sound level of a section of n violins? Does adding two equal sounds give an increase of 3 or 6 dB? What is the loudest sound? Is there really no sound in space? Asked 5 years, 3 months ago. Active 5 years, 3 months ago. Viewed 1k times. Improve this question. Add a comment. Active Oldest Votes. Improve this answer. Mark H Mark H I, for one, are usually working with dB for amplitudes, which means I am multiplying the logarithm with
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