Deutsche Version 
In sound engineering there is no Impedance matching or Power matching. In audio we use only high Impedance bridging or Voltage bridging. 
A resistance is a DC resistance, which can be measured with an ohmmeter. If there is a
capacitor in the signal path we measure nothing. With a voice coil of a speaker we can
measure a DC resistance. With a digital multimeter DMM resistances can be measured easily,
but we cannot measure input impedances and output impedances. The capacitance and the
resistance build as frequencydependent form a complex resistance, the socalled impedance
Z. The nominal impedance is in electrical engineering and electroacoustic (audio), the
frequencydependent impedance at the input and / or at the output of an electrical device,
which is specified in the middle frequency range at 1 kHz of a technical data sheet. In electrical engineering and acoustics alternating quantities are always described with its effective value (RMS). 
Output impedance 
Voltage measurement at the points at OUT: V_{1} = Opencircuit voltage (R_{load} = ∞ Ω, that is without R_{load}, switch S is open) R_{load} = Load resistance (R_{test} is resistor to measure Ω value) V_{2} = Loaded circuit voltage with resistor R_{load} = resistance R_{test} Z_{source} = The output impedance can be calculated 
When the voltage V_{2} is equal to half of V_{1}, then the measured resistance value R_{load} (that is R_{test}) is equal to the output impedance Z_{source}. 
The output impedance of a device can simply be determined. We use a
load resistance R_{load}, to load the signal source impedance Z_{source}. The
output voltage is open initially without load as opencircuit voltage V_{1}
(Switch is open, that means R_{load} is infinity) and then measured as V_{2}
under load with R_{load} at point IN (Switch is closed). Then the found
values V_{1}, R_{load} and V_{2} are entered to calculate the output impedance. The load resistance R_{load} should not be too small, because the output is too heavily burdened and should not be too large, as this will change the voltage very little and leads to measurement errors. For output impedance of a normal power amplifier to operate a speaker a R_{load} resistance of about 10 ohms is favorable. For other linelevel R_{Load} a resistance of 2 kilo ohms is useful. 
Internal resistance of a power amplifier
"Measuring the output impedance by means of a burden": Suppose there is a 100 watt amplifier. Then the output voltage at half power is P = 50 W = V^{2} / R. Loudspeaker impedance = 8 ohms. V = √(P × R) = √ (50 × 8) = 20 volts. (You can also use 10 V.) Give a sine voltage of 1 kHz to the amplifier input, until we get 20 volts at the output. Now we apply the "90% method", that is when we put an output resistance R, until there appear 90% of the open circuit voltage, in this case 18 volts. The internal resistance is then calculated with the 90% method: 
The 90% method R_{internal} = R / 9 
At the output fix an oscilloscope, because the wave form should not show
any distortion. For example, if R is measured 1 Ohm, then R_{internal} = 0.11 Ohm. 
Input Impedance Measurement and Calculator
Input impedance 
Voltage measurement at the points IN or at OUT: V_{1} = Generator signal voltage (at R_{s} = 0 Ω, that is without series resistor R_{s}) R_{s} = Series resistance (R_{test} is resistor to measure Ω value) V_{2} = Voltage with series resistor R_{s} = resistance R_{test} Z_{load} = The input impedance can be calculated 
When the voltage V_{2} is equal to half of V_{1}, then the measured resistance value R_{s} (R_{test}) is equal to the input impedance Z_{load}. 
The input and output impedance of a fourterminal network can be
determined by measuring the alternating current strength in amperes and
the AC voltage in volts. The measurement of input impedance typically
occurs as follows: The voltage is measured across the input terminals IN. Then, the current in the circuit is done by the device in series with the signal generator. For circuits with high input impedance the current is very small and difficult to measure. R = U / I. Therefore, we choose for the measurement of highimpedance circuits, a better method. It puts a series resistor R_{s} in the input circuit. First, we measure the input of the device at point IN with V_{1}, the AC voltage, if the resistor R_{s} = 0 Ohm. Then we measure the R_{S} series resistor, the voltage V_{2}. Then these found valuesV_{1}, R_{s} and V_{2} is entered in the above calculator to find the input impedance to be calculated. Search for a suitable measuring resistance value R_{s}. For typical audio equipment that will be about 10 to 100 kiloohms. You can use the digital voltmeter instead at the measuring point IN and at point OUT to measure because the amplifier delivers an output voltage that is proportional to the voltage at its input. 
The impact of input impedance and output impedance of
studio gear for bridging in audio engineering − Z_{source }<< Z_{load}
Impedances of analog audio engineering for
impedance bridging or voltage bridging Z_{source} << Z_{load}
Studio parts  Output impedance Z_{source} 
Input impedance Z_{load} 
Microphone  35 Ω to 200 Ω  − 
Microphone preamplifier  −  1 kΩ to 2 kΩ 
Power amplifier  0.01 Ω to 0.1 Ω  − 
Loudspeaker  −  2 Ω to 16 Ω 
Studio gear (mixer)  40 Ω  10 kΩ to 20 kΩ 
Fortunately, there are no amplifiers with an output impedance of
4ohm or 8ohm which have to fit to speakers with these values. We have no impedance matching (power matching), we use impedance bridging (voltage bridging), whereby the power amplifier often has an output impedance of only one hundredth of the speaker's input impedance. At power amplifiers for musicians usually we can read at the output plugs: 4 ohms to 8 ohms − to tell the user that a 4ohm speaker or an 8ohm speaker has to be used and not to give the "correct" output impedance value, which is around 0.1 ohms. This is often not known by users. 

Loudspeaker input impedance Z_{in} = DF × Z_{out} Amplifier output impedance Z_{out} = Z_{in } / DF Damping factor DF = Z_{in} / Z_{out} 
Output impedance Z_{out} = input impedance Z_{in} / damping factor DF
Please enter two values, the third value will be calculated.
Effect of the input impedance on guitar amps
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