The sensitivity tells you how loud a sound you'll get out of the speaker when you put a given amount of electrical power in. To understand this value you need to know what the decibel means. Decibels are a logarithmic way of representing relative power levels. In fact, the basic unit here is the Bel, although you almost never see it mentioned. A deci-bel is one tenth of a Bel. Two power levels, P1 & P2 , are said to differ by, B, Bels where

This means that the same power ratio can be described as

in decibels (usually written “dB”).

As an example, consider two power levels, P1 = 6 Watts, andP2 = 3 Watts (i.e. P1 is twice as powerful as P2). Using the above we can say that

i.e. P1 is 3 dB's greater than P2. Rearranging equation 2 we can say that

If you look through electronics or engineering books you'll often see powers quoted in units like, “dBa”, “dBW”, “dBm”, etc. These are used to specify power levels compared to a specific reference level. We can use the above equation to work out what power a given value in “dB-something” means. For example, “dBm” is used to refer powers to a level of 1 milliwatt. Hence a signal which is said to have a power of, say, 16 dBm, has an actual power level of

Sound levels are usually measured with reference to an intensity of 10-12 Watts per square metre. This is regarded as about the faintest sound humans can hear. The “a” of “dBa” means that a specific kind of filter is used to remove sound frequencies which humans don't hear very well. Hence a sound level in dBa indicates how loud something sounds rather than saying how much power is around at inaudible frequencies.

A loudspeaker sensitivity of 85 dBa/W at 1 metre tells us that, when we place a microphone (or an ear!) one metre in front of the speaker and drive the speaker with one watt electrical power from the power amplifier, the sound level at the microphone (or ear) will be

To give you some idea how loud this is, the sound level of traffic on a busy road is typically between 70 & 80 dBa, and a pneumatic road drill 3 metres ways is about 90 dBa. Def Leppard/Led Zepplin/etc tend to play rock concerts at 120 dBa. (If you're really interested, a nuclear warhead exploding 500 metres away produces around 220 dBa! Note that, being logarithmic, a 10 dB increase in level means ten times more power, a 20 dB increase means one hundred times more, etc. So a 220dBa bang is 30 million million times more intense than our typical speaker driven with one watt of music!)

From the above figures you should be able to see that, using a speaker like the one considered above, we need an amplifier able to provide about 1 watt into an 8 Ohm load to produce moderately loud music & about 25 watts into 8 Ohm to play loud music peaks. Since V = IR and P = IV we can say that

i.e. for 25 watts into an “8W” speaker we need

In fact, most Hi-Fi power amplifiers have their maximum output specified in terms of the biggest continuous sinewave level they can produce. Since the peak/rms ratio for a sinewave is root two (1·414...) this means that we'd need an amplifier able to provide peak levels of 2·5 Amps, and 20 Volts. Hence a typical power amplifier driving a typical loudspeaker has to be able to provide output levels of up to a few amps and a few tens of volts.

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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.