Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
requirements could be as simple as limiting peak levels to 10 dBFS or as strict as meeting a
specified dBFS average across the duration of the show.
You might also be putting together equipment that delivers sound to a live audience in an
acoustic space. In that situation you need to know how loud in dBSPL the system needs to
perform at the distance of the audience. There is a minimum dBSPL level you need to achieve in
order to get the signal above the noise floor of the room, but there is also a maximum dBSPL
level you need to stay under in order to avoid damaging people’s hearing or violating laws or
policies of the venue. Once you know these requirements, you can begin to evaluate the
performance of the equipment to verify that it can meet these requirements. Rules of Thumb
Table 4.2 gives you some rules of thumb for how changes in dB are perceived as changes in
loudness. Turn a sound up by 10 dB and it sounds about twice as loud. Turn it up by 3 dB, and
you’ll hardly notice any difference.
Similarly, Table 4.5 gives you some rules of thumb regarding power and voltage changes.
These rules give you a quick sense of how boosts in power and voltage affect sound levels.
change in power, voltage, or distance approximate change in dB
power × 2 3 dB increase
power ÷ 2 3 dB decrease
power × 10 10 dB increase
power ÷ 10 10 dB decrease
voltage × 2 6 dB increase
voltage ÷ 2 6 dB decrease
voltage × 10 20 dB increase
voltage ÷ 10 20 dB decrease
distance away from source × 2 6 dB decrease
Table 4.5 Rules of thumb for changes in power, voltage, or distance in dB
In the following sections, we’ll give examples of how these rules of thumb come into
practice. A mathematical justification of these rules is given in Section 3. Determining Power and Voltage Differences and
Desired Changes in Power Levels
Decibels are also commonly used to compare the power levels of loudspeakers and amplifiers.
For power, Equation 4.6 applies -- ( ).
Based on this equation, how much more powerful is an 800 W amplifier than a 200 W
amplifier, in decibels?
( ) increase in power
For voltages, Equation 4.4 is used ( ( ) . If you increase a voltage
level from 100 V to 1000 V, what is the increase in decibels?
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