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| 11/23 |
| 2005/4/21-23 [Computer/SW/Database] UID:37310 Activity:nil |
4/21 How do you calculate and determine decibel? Say I have a 20db
device. Does that mean it is 20db from X distance? And what is
the X constant? In addition, if I have TWO 20db devices running
simultaneously, clearly, it doesn't mean 40db. What is it then?
Lastly, how much does db decrease relative to distance? Is it
linear? quadratic?
\_ 0 dB is the threshold of human hearing (humans with good ears).
http://en.wikipedia.org/wiki/Decibel
You determine dB by buying a meter and viewing the digital readout.
Once you have a handle on the wikipedia link, then view:
http://www.kodachrome.org/salt/sunderst.htm
\_ If you have 2 sources that have 20dB noise individually, then you
have 2x, or about +3dB power, giving 23dB. Neglecting the damping
effects of air and any echo effects, moving twice as far away from
a point sound source gives 4x less power, or -6dB. If you're moving
away from a line sound source, such as a long narrow air vent, then
if you're 2x as far away you are also exposed to 2x as much sound
producer, so it would be -6dB+3dB=-3dB quieter.
\_ If you have ten 20dB devices, they become 30dB. If you have one
hundred 20dB devices, they become 40dB. A decibel is 10 times
log-base-10 of something. (A bel is log-base-10, and a decibel is
one-tenth of a bel.) So if you have two 20dB devices, they will be
10 * log((10 ^ (20/10)) * 2) = 23.01dB. Or think of it another way,
20dB + 10 * log(2) = 23.01dB. |
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| en.wikipedia.org/wiki/Decibel squar e of this value), the formula is: 20\log_{10}(\frac{V_1}{V_0}) It can therefore be seen that a ratio expressed in decibels is independen t of whether the measurements are made as field strength or power values . frequency weightings are used to allow the result of an acous tical measurement to be expressed as a single sound level. The weighting s approximate the changes in sensitivity of the ear to different frequen cies at different levels. The two most commonly used weightings are the A and C weightings; leaves 10 Human breathing at 3 m 0 Threshold of hearing (human with good ears) Under controlled conditions, in an acoustical laboratory, the trained hea lthy human ear is able to discern changes in sound levels of 1 dB, when exposed to steady, single frequency ("pure tone") signals in the mid-fre quency range. healthy ear, h owever, can barely perceive noise level changes of 3 dB. On this scale, the normal range of human hearing extends from about 0 dB to about 140 dB. Sound pressure levels are applicable to the specific position at which th ey are measured. The levels change with the distance from the source of the sound; in general, the level decreases as the distance from the sour ce increases. If the distance from the source is unknown, it is difficul t to estimate the sound pressure level at the source. meter is less sensitiv e to very high and very low frequencies. The A weighting parallels the s ensitivity of the human ear when it is exposed to normal levels, and fre quency weighting C is suitable for use when the ear is exposed to higher sound levels. Other defined frequency weightings, such as B and Z, are rarely used. Frequency weighted sound levels are still expressed in decibels (with uni t symbol dB), although it is common to see the incorrect unit symbols dB A or dB used for A-weighted sound levels. attenuators, are connected in series, expressions of power level in d ecibels may be arithmetically added and subtracted. It is also common in disciplines such as audio, in which the properties of the signal are be st expressed in logarithms due to the response of the ear. It can also be combined w ith a suffix to create an absolute unit of electrical power. For example , it can be combined with "m" for "milliwatt" to produce the "dBm". Zero dBm is one milliwatt, and 1 dBm is one decibel greater than 0 dBm, or a bout 1259 mW. Although decibels were originally used for power ratios, they are nowaday s commonly used in electronics to describe voltage or current ratios. In a constant resistive load, power is proportional to the square of the v oltage or current in the circuit. However, voltage-based decibe ls are frequently used to express such quantities as the voltage gain of an amplifier, where the two voltages are measured in different circuits which may have very different resistances. out put resistance may be said to have a "voltage gain of 0 dB", even though it is actually providing a considerable power gain when driving a low-r esistance load. In professional audio, a popular unit is the dBu (see below for all the u nits). The "u" stands for "unloaded", and was probably chosen to be simi lar to lowercase "v", as dBv was the older name for the same thing. Chosen f or historical reasons, it is the voltage level at which you get 1 mW of power in a 600 ohm resistor, which used to be the standard impedance in almost all professional audio circuits. Since there may be many different bases for a measurement expressed in de cibels, a dB value is meaningless unless the reference value (equivalent to 0 dB) is clearly stated. edit Acoustics dB(SPL) dB(Sound Pressure Level) relative to 20 micropascals (Pa) = 210^-5 Pa, the quietest sound a human can hear. This is roughly the sound of a mosquito flying 3 metres away. This is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself. In digital systems, 0 dBFS would equal the highest level (number) the processor is capable of representing. The difference of a filter's response to nominal levels, for instance. edit Reckoning Decibels are handy for mental calculation, because adding them is easier than multiplying ratios. First, however, one has to be able to convert e asily between ratios and decibels. The most obvious way is to memorize t he logs of small primes, but there are a few other tricks that can help. edit 3 dB = power A level difference of 3 dB is roughly double/half power (equal to a rati o of 1995). That is why it is commonly used as a marking on sound equip ment and the like. These numbers are ve ry close to being equally spaced in terms of their logarithms. The conversion for decibels is often simplified to: "+3 dB means two time s the power and 1414 times the voltage", and "+6 dB means four times th e power and two times the voltage ". While this is accurate for many situations, it is not exact. As stated ab ove, decibels are defined so that +10 dB means "ten times the power". Fr om this, we calculate that +3 dB actually multiplies the power by 10^3/1 0 This is a power ratio of 19953 or about 025% different from the "ti mes 2" power ratio that is sometimes assumed. A level difference of +6 d B is 39811, about 05% different from 4 To contrive a more serious example, consider converting a large decibel f igure into its linear ratio, for example 120 dB. The power ratio is corr ectly calculated as a ratio of 10^12 or one trillion. But if we use the assumption that 3 dB means "times 2", we would calculate a power ratio o f 2^120/3 = 2^40 = 10995 10^12, for a 10% error. |
| www.kodachrome.org/salt/sunderst.htm Understanding Sound 1 The Decibel - dB + The Decibel (dB) is the unit of measurement used in sound systems + A Decibel (dB) describes a ration between two quantities expressed as a logarithm. Logarithms are used because our ears hear differences in loudness as a Log function. Could easily degenerate into Woofiness or Puffiness if overdone. Above 10 kHz Brightness - Most generally achieved by a global (shelving) EQ of everything above 10 kHz. Below 10 kHz Darkness - The opposite of brightness (a general lack of hig hs at 10 kHz and beyond). The quietest parts of a performance must still be kept louder than th e room noise (called the noise floor) + The noise floor is typically 50 to 60 dB SPL + The quietest parts must be amplified enough that they can still be heard above the noise floor in the back of the room + Assuming 24 dB of loss from the front to the back of a 50 foot long room, the quietest parts would need to be amplified 24 dB to be heard in the back of the room + Note: It may not be possible to provide 24 dB of gain before feedback (see Feedback Control) + The speakers must reproduce the quietest parts at 74 dB SPL in order to be heard at the back of a 50 foot room at 50 dB SPL 11. The loudest parts of the performance must not be so loud as to be obn oxious or painful + The loudest should not exceed 110 dB SPL + The loudest cannot exceed the capabilities of the amplifier and speaker system (also about 110 dB SPL) + However, since the quietest parts must be amplified 24 dB, the loudest parts (110 dB SPL) will also be amplified the same amount, obviously making them much TOO LOUD 12. The useful dynamic range of the speaker system is limited by: + The quietest part of the program must be amplified to 74 dB SPL to be heard in the back of the room + The loudest part of the program exceed 110 dB SPL in the front of the room + This leaves a useful dynamic range of 110 - 74 or 36 dB, much less than the 60 dB dynamic range of a typical music group 13. Solutions to the Dynamic Range problem: + Make the room quieter (requires expensive sound insulation) + Get a louder sound system (could annoy the audience) + Use a compressor / limiter circuit (expensive, especially in a system with many microphones and instruments in one system) + Have the sound engineer turn quiet parts up and turn loud parts down + ***** Have the performers reduce the demand for dynamic range by getting closer to the mike on quiet parts and backing off from the mike on loud parts, similarly, have instrument players control their own volume according to the dynamic needs of the program 14. Feedback Control + Feedback occurs when the sound from the microphone is amplified too much. The frequency of the feedback is affected by: + Direction the microphone is facing + Distance between the microphone and speaker + Frequency response characteristics of the room + Equalization of the microphone channel + Equalization of the monitor speakers + Equalization of the main speakers 15. Sound systems should be operated no louder than 6 dB before the begin ning of feedback (that is half as loud as when feedback starts) 16. Operating closer to the feedback point causes a "hollow" or "ringing" sound 17. Every time you d ouble the number of microphones, the maximum gain before feedback is reduced by 3 dB. Apparent loudness of each mike is cut in half when the number of mikes is increased 4 times. To go from 1 to 4 mikes halves the maximum volume of each mike before feedback. To go from 1 to 16 mikes quarters the maximum volume of each mike bef ore feedback. Feedback is controlled by: + Using directional microphones and carefully aiming them away from monitor and main speakers to reduce feedback + Performers must be careful not to re-aim mikes towards speakers during performance + Performers must be careful when hand holding mikes not to point them towards the monitor speakers + Decreasing the distance between the sound source (performer) and the microphone (so the microphone does not need to be as loud) + Increasing the distance between the speakers and the microphones + Using equalizers to reduce the system's gain at the frequencies where the feedback occurs + Installing acoustic dampening material in the room to reduce sound reflections back to the microphones (expensive solution) 21. Factors Influencing Clarity and Intelligibility + High monitor levels on stage get into the microphones and muffle the sound because the monitor sound is out of phase with the original sound (don't set monitor louder than necessary) + High monitor levels can also cause sound to be hollow or ringing + Instruments playing music louder than necessary on stage causes too much music to be picked up by vocal mikes, making the music muddy (keep music as quite as possible on stage) + Excessive use of equalization to prevent feedback effects the clarity of the overall sound (again, don't set monitor louder than necessary) + Acoustical characteristics of the room (reverberation) affects the intelligibility of the sound. Large flat hard surfaces reflect sound which is out of phase with the original sound. Acoustical treatment of walls and ceilings is desirable. |