4 General Technical Information Page 99
4 General Technical Information
4.1 Decibel
A decibel is a logarithmic scale commonly used to express differences in signal levels. It is useful in audio
because it can express a wide dynamic range with relatively small numbers (or a small movement on a
meter), and it more closely matches how we perceive sound.
The measurement quoted in dB describes the ratio between the quantity of two levels, the level being
measured and a reference.
The absolute quantity of the signal is not relevant. This means that decibels are always comparing one
quantity to another. For example, when we measure gain in dB, we are comparing the output level to the
input level.
To describe an absolute value, the reference point must be known. There are different reference points
defined.
dBV represents the level compared to 1 Volt RMS. 0 dBV = 1 V with no reference to impedance. dBu
represents the level compared to 0.775 Volt RMS on an unloaded, open circuit.
dBm represents the power level compared to 1 mWatt. This is a level compared to 0.775 Volt RMS across a
600 Ohm load impedance.
1 dBV equals +2.2 dBu, +4 dBu equals 1.23 Volt RMS, the reference level of –10 dBV is the equivalent to a
level of –7.8 dBu.
Headroom is a measure (usually in dB) of how much higher the peaks of a signal can be compared to the
nominal level without clipping. That is, it compares the peak level (in volts RMS) to the nominal level (in
volts RMS). The difference between the two (in dB) is the headroom.
4.2 100 V
100V-Systems are referred to as constant-voltage distributed audio systems. The constant voltage system is
the most economical way to install a multi-speaker sound reinforcement system.
The term 100V system relates to the maximum output voltage of the amplifier. 100V is the usual voltage in
Europe, 70V in the United States. A higher voltage up to 200V can also be used for very long cable runs and
higher power requirements.
To generate this high voltage, the amplifier is equipped with a step-up transformer, which transforms the
regular output voltage, in the 15 to 30 Volts range, up to the necessary 100V (or 70V respectively).
A 100V-loudspeaker is equipped with a step-down transformer with a relatively high input impedance. The
transformer’s output impedance matches the included chassis’ impedance (usually 8 Ohms). The ratio be-
tween the low output impedance of the amplifier and the transformer input impedance of the individual
speaker is usually between 1:100 and 1:1000.
Directly proportional to the maximum output power of the amplifier, each 100V amplifier matches a certain
minimum impedance than can be connected to this output. It does not matter how the connected impedance
is achieved. A large number of smaller speakers (with high input impedance at their step-down transformers)
or a small number of larger speakers (with lower impedance) can be connected to the 100V line. Any
c
2016 Bittner Audio Int. GmbH
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