|
How does a watt relate to the output level (SPL) of a speaker
(or "why watts can be meaningless without context").
First off SPL stands for sound pressure level. Remember that term, it is used a lot around here. It is a basic measure of how LOUD something is.
SPL is measured in a unit call the decibel. Decibels are a logorithmic scale not a linear scale. ie a doubling of the output level is a 3dB increase while a factor of 10 increase in output level is a 10dB increase (if it was a linear scale, a factor of 10 increase would be a 30dB increase). Bottom line is the higher on the scale you go the larger the difference between the levels.
For a person to easily perceive a difference between the intensity of two sounds they need to differ by approximately 3dB. To percieve a doubling of the intensity, two sounds need to differ by approximately 10dB.
With this info you are ready to see how a watt relates to a speaker's output. A speaker has a sensitivity rating. This rating is given as dB/W/m. This number can range from the low 80's to some very efficient horn loaded speakers with ratings over 100. For the purposes of this explanation lets assume we have a speaker with a sensitivity rating of 91dB/W/m. This means that with one watt of power applied to the speaker, measured one meter away from the speaker, you should read a SPL level of 91dB.
Now remember the dB scale definition above, to increase the output by 3dB we have to increase the input power by a factor of 2. So it would go something like this:
dB - watts
91 - 1
94 - 2
97 - 4
100 - 8
103 - 16
105 - 32
108 - 64
111 - 128
114 - 256
Now as you can see power requirements for a meaningful increase in output go up very very quickly. It also becomes obvious that the difference between a 80watt and a 100watt per channel amp is not going to be all that great. Finally you can also see that a 91dB/W/m speaker will require half as much power as an 88dB/W/m speaker to reach the same output level.
Now there is one other thing you have to consider. These numbers are only valid if you are listening 1 meter away from the speaker. Very few people listen to there speakers at a distance of one meter. However, you can apply the same sort of rule to distance. Every doubling of distance will cause a 6dB decrease in output. So listening to our 91dB/W/m speaker at a distance of 4 meters (about 13') the scale changes to this:
dB - watts
79 - 1
82 - 2
85 - 4
88 - 8
91 - 16
94 - 32
97 - 64
100 - 128
103 - 256
Now it won't quite drop this much as room gain will increase the level a bit, but it's the jist of it all. The watt is meaningless without also considering the speaker being driven and the room the speaker is in.
Also, don't take this as meaning higher efficiency speakers are always better. Only use the sensitivity ratings as an indication of how much power you will need to effectively drive the speaker. Listen to the speaker to determine whether you like it or not. But don't forget about the power handling rating. It is a guide line for how much power the speaker can take. Short bursts above this power rating won't damage the speaker. In fact it can be a good thing to have more power available than the speaker can handle since amp clipping (running out of power) is much more harmful to speakers than over driving them. Amp clipping gives no warning, but over driving your speakers makes them sound bad warning you to turn it down. Still, do remember running a speaker continously above its' rated power handling will eventually damage it.
As a side note. Also remember the watt rating on an amp isn't always an accurate one. Things to watch for are amps that give their rating at 1khz instead of from 20hz-20khz. Amps that give THD numbers higher than 0.1 in their power rating. Amps that don't specifiy all channels were being driven when this power output rating was obtained. Amps that rate into 6ohms but say not to drive 6ohm speakers. So as an example take these two specs:
1) 110W at 1khz, with 0.7%THD into 6ohms (on the front of the box they also claim 110Wx5 and in the manual say not to drive 6ohm speaker with it)
2) 70Wx5 (all channels driven) from 20hz-20khz, with 0.08THD into 8ohms (manual says you can drive 6ohm speakers)
2 should actually have 70W for each channel. 1 if measured under the same conditions as 2, will in all likely hood have less than 40Wx5.
|