The Watt is a measure of electrical power, and in HiFi this is commonly used to specify the amount of power an amplifier is able to provide and the amount of power a loudspeaker is able to tolerate.
Although power can be an indicator of loudness, the efficiency of the loudspeaker will dictate how loud the output will be at a given power level. Volume (or Sound Pressure Level) is defined in decibels (or dB), which is a logarithmic scale, whilst power in Watts is a linear scale. So why does this matter?
A 10dB increase in SPL will double the loudness and require 10 times the power!
Increasing power has diminishing returns.
A relatively efficient loudspeaker might deliver 90dB from 1 watt at measured from 1 meter away, this roughly approximates to the SPL from your listening position*. This is about the level of a lawnmower, a shouted conversation or a bass drum. This also means that to hear the same signal at half the loudness (80dB) you’ll need only 0.1W and for double the loudness (100dB) you’ll need 10W.
It’s worth stating at this point that prolonged exposure to sound levels above 80dB can result in permanent hearing damage.
Fig 1
*The SPL measurement was taken in an anechoic chamber (which simulates an open space), and in this setting
loudness drops away sharply as you move away from the source. We listen to Hi-Fi at home and a lot of the
sound that would dissipate in an open space is reflected back by the walls, floor and ceiling. So for a rough rule
of thumb if you’re listening in a domestic setting then the SPL at 1m is close to what you’ll hear from where
you’re sitting.
Fig 1 shows the graph of a loudspeaker output level against the power required to drive it. This is based upon a 180w speaker with an efficiency of 90dB/w/m.
So if you read all that and got a bit lost in the numbers, you’ll be pleased to hear the only take away you need is this:
- The more power you add, the less you get back in return.
- You probably need a lot less power than you think you do.
- Efficient speakers are great!