The basics of electronics are essential in the practice and application of any sound reinforcement system. This topic is very large, but the scope of the sound reinforcement system is limited, that is, it involves only the interaction between the electronic system and the load.
1, Ohm's law electric power
2, impedance
3, composite load
4, resonance (resonance)
5, the speaker load series - parallel
6. Series connection of resistors, inductors and capacitors - parallel
7, decibel
One of the two speakers has an index of 10 to 50 W, 87 dB/W/m, and the other is 10 to 50 90 dB/W/m. Which one is ringing? The two amplifiers have a frequency response of 10 Hz to 40 kHz. The other mark is 20Hz ~ 20kHz ± 0.2dB, which one is of high quality? If you do not know the exact meaning of "decibel (dB)", you can not say the point.
Simply put, decibel is the unit of amplifier gain. The ratio of the output of the amplifier to the input is the magnification, in units of "times", such as 10x amplifier, 100x amplifier. When using "decibel" as a unit, the magnification is called gain, which is a concept of two kinds of terms.
The conversion relationship between decibel and magnification in electricity is:
AV(I)(dB)=20lg[Vo/Vi(Io/Ii)]; Ap(dB)=10lg(Po/Pi)
The formula for voltage (current) gain and power gain is different when decibel is defined, but we all know that the relationship between power and voltage and current is P=V2/R=I2R. With this set of formulas, the gain values ​​of the two are the same:
10lg[Po/Pi]=10lg(V2o/R)/(V2i/R)=20lg(Vo/Vi)
There are three main benefits to using decibels as a unit.
1, the value becomes smaller, the read and write convenience electronic system's total magnification is often thousands, tens of thousands or even hundreds of thousands, a radio received from the antenna signal to the speaker output, a total of 20,000 times about. Decibel is used to first take a logarithm, and the value is much smaller. The attached table is the correspondence between the magnification and the gain.
Correspondence between magnification and gain Voltage (current) magnification 1/10 1/
1/2 1
2 10 100 103 104 105
Gain (dB) -20 -6 -3 0 3 6 20 40 60 80 100
Power amplification factor 1/10 1/
1/2 1
2 10 100 103 104 105
Power gain (dB) -10 -3 -1.5 0 1.5 3 10 20 30 40 50
2. When the operation is convenient for amplifier cascade, the total magnification is multiplied by each stage. When using decibels as a unit, the total gain is added. If a preamplifier is 100 times (20dB) and the latter is 20 times (13dB), then the total power amplification is 100×20=2000 times and the total gain is 20dB+13dB=33dB.
3, in line with the sense of hearing, the estimated convenience of people to hear the loudness of the sound is positively correlated with the relative growth of power. For example, when the electric power is increased from 0.1 watt to 1.1 watt, the sound heard is much louder; from 1 watt to 2 watts, the loudness is not too much; from 10 watts to 11 watts, there is no People can hear the difference in loudness. If the absolute value of the power is expressed as 1 watt, and the gains are 10.4 dB, 3 dB, and 0.4 dB, respectively, this can more consistently reflect the difference in loudness heard by the human ear. If you pay attention, you will find that the volume knob scale on the Hi-Fi amplifier is the standard decibel, which makes you intuitive when changing the volume.
In the decibel value, two points of -3dB and 0dB must be understood. -3dB is also called a half power point or a cutoff frequency point. At this time, the power is half of normal, and the voltage or current is 1/2 of normal. In an electroacoustic system, a difference of ±3 dB is considered to not affect the overall characteristics. Therefore, various equipment indicators, such as frequency range, output level, etc., may have ±3dB in and out without explanation. For example, the frequency response mentioned above is 10 Hz to 40 kHz, which means that the output amplitude does not exceed ±3 dB in this frequency, that is, at the two end frequencies of 10 Hz and 40 kHz, the output voltage amplitude is only the intermediate frequency range. 0.707 (1/) times. 0dB means the output is as large as the input or the two comparison signals.
Decibel is a relative amount of quantity, there is no absolute magnitude. You can see how many dB of the measured value on the meter or the noise meter on the road, because people have set a benchmark for 0dB. For example, the 0dB of the sound level meter is 2×10-4μb (microbar), so the noise on the road is 50dB, 60dB has an absolute light sound concept.
Commonly used 0dB benchmarks are as follows: dBFS - 0dB in full scale, commonly used on various characteristic curves; dBm - 1mW power (or 0.775V voltage) is 0dB on 600Ω load, commonly used AC level measuring instrument; dBV - 0dB at 1 volt; dBW - 0dB at 1 watt.
After reading out how many dB, it is no longer necessary to turn into physical quantities such as voltage and sound pressure. Professionals can understand. Only in a few occasions is it necessary to fold. At this time, you only need to substitute the formula: 10A/20 (or A/10) × D0. A is the decibel value read, D0 is the reference value at 0 dB, A/20 is used for voltage, current or sound pressure, and A/10 is used for electric power, sound power or sound intensity.
Now you can answer the questions at the beginning of this article. The second speaker doubles the same speaker than the first speaker. If you keep the two speakers ringing, the second speaker only needs to input half the power. The first amplifier is just a very common type, the second amplifier is Hi-Fi, and the output voltage is only ±2.3% over the entire frequency range!
Several expressions of decibels are used in electroacoustic systems:
1), power gain decibel value = 10lg (P1/Po) dB reference power Po = 1W
2), voltage gain decibel value = 20lg (U1/ Uo) dB reference voltage Uo = 775 (dBmV) or Uo = 1 (dBV)
3), current gain decibel value = 20lg (I1 / Io) dB reference current Io = 1 (dBmA) or Io = 1 (dBA)
4) Sound pressure level decibel value (SPL)=20 lg(pre1/preo) dB The reference sound pressure level preo=20×10-6N/m2 is the hearing threshold of human in 1KHZ-3KHZ
5), the sound pressure level is attenuated in free space = 20lg (R1 / Ro) dB Reference distance Ro = 1m
There are also many medium quantities expressed in decibels, such as: level, signal to noise ratio, sensitivity, filter attenuation rate, signal dynamic range, etc. Their multiplication factor is also 20.
8, the transformer actual transformer has some conditions:
1) The power handling transformer is not a lossless device that generates heat during transmission of power.
2) Bandwidth
3) Insertion loss and distortion of the transformer in the mid-band insertion loss is less than 1 dB, usually the insertion loss and distortion of the low frequency and high frequency band will increase.
9, the line loss of power transmission Generally speaking, good engineering requires that the wire is no more than 0.5 dB (10%)
1) Low-impedance transmission line power loss calculation Low-impedance transmission power transmission line for low-impedance output power amplifier and low-impedance speaker (generally less than 16 ohms). The transmission line is characterized by small signal distortion, wide frequency response characteristics and good sound quality. However, the signal current in the transmission line is large, and it is necessary to use a wire with a large cross-sectional area to transmit efficiently, otherwise it will cause great power loss.
Power loss (dB) = 20 lg (RL / 2R1 + RL)
R1=ΡL/S
20 degree density silver = 0.0159 copper = 0.0172 aluminum = 0.0282
2) Calculation of high-impedance (constant voltage) transmission line power loss In large-scale sound reinforcement systems (such as large stadiums, squares or background sound systems, etc.), transmission lines are very long (generally more than 200M, even several kilometers), this If you use a low-impedance transmission line, you must use a large number of very thick wires and add a lot of power loss. To this end, another transmission system with high impedance/constant voltage output (three standard voltages of 50V, 70V and 100V) is used. This can greatly reduce the power loss of the line. However, this method introduces a matching transformer, which significantly affects the low frequency and high frequency of the transmitted signal.
When the line loss is 10%:
The cross-sectional area S required for high impedance is:
S≧(0.37×L×P)/U2
The minimum cross-sectional area S required for low impedance is:
S≧(0.37×L)/Z
L wire length
P transmission power
U voltage across the transmission line
Z speaker load impedance
3) Power matching and impedance matching of fixed voltage transmission system. Adaptation state of each speaker load in fixed voltage transmission system.
Z(SP)=Z(AMP)P(SP)= P(AMP) The best power transmission from the amplifier to the speaker
Z(SP)>Z(AMP)P(SP)= P(AMP) The power supplied to the speaker is lower than the rated power of the speaker and does not cause safety problems.
Z(SP)
Z(SP)= Z(AMP)P(SP)< P(AMP) speaker overload
Z(SP)= Z(AMP)P(SP)≥ P(AMP) Improper power distribution can damage the speaker; normal operation when the power of the speaker is allocated properly
The maximum allowable line length m when the voltage drop is 1 dB in 100V voltage transmission
Wire cross-sectional area transmission power
/mm2 4 10 20 40 100 200 400 1000
0.3 2500 1000 500 250 100 50 25 10
0.5 4000 1600 800 400 160 80 40 16
0.75 6000 2400 1200 600 240 120 60 24
1.0 8000 3200 1600 800 320 160 80 32
1.5 12000 4800 2400 1200 480 240 120 48
2.5 20000 8000 4000 2000 800 400 200 80
4.0 32000 12800 6000 3200 1280 640 320 128
6.0 48000 19200 9600 4800 1920 960 480 192
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