Power & Porkers
Power specs are the most abused of all audio specs. Nearly all of them are fantasies, and Marketeers know that humans are suckers for numbers, and the bigger the better.
The truth is that there is virtually no relationship between the electrical power of a speaker and how loud it goes.
What's a Watt?
Power rating is measured in Watts (W). and as with everything there are many ways to rate an amplifier. But for a portable speaker it’s not really important. You just need to know how loud the speaker is.
Most speakers have batteries that will supply about 4-6W inside, but the actual amplifier chip may be rated at 15 or 20W. And you can bet that the figure they will promote will be the higher one.
Because we’re conditioned to think that bigger is better, the Marketeers exploit power figures to boost a speakers perceived quality. Things got so bad in 1974 that the US FTC authority had to step in to stop people claiming that a 4W amplifier was capable of 1000W music power!
Sadly we seem to have slipped back to the 70’s now with one prominent manufacturer of portable speakers claiming they were rated to 300W. To most people, this speaker would be obviously better than a 30W one. But of course the 300W figure is a useless marketing figure rather than a true measurement of the speakers actual power output.
The only way to quote a power figure is to specify the continuous sine wave or pink noise RMS power the system can put into a speaker without the speaker blowing up or distorting. Not some weird peak power, or music power measurement.
Lets have a look at some factors.
RMS and peak power
The most useful measurement for a speaker is RMS power. This is a measure of the mathematical average power fed into a speaker. You can feed a speaker lots of power for a very short time, but this doesn’t really have any bearing on anything. For a speaker to be useful it has to play music at a loudness level continuously.
RMS power is always less than peak power, sometimes a lot less.
Maximum electrical levels
The amplifier in your portable speaker will have a maximum power capability. This should be matched to the maximum level of the audio, so the amplifier doesn’t clip. Most modern music has an average to maximum level difference of about 10dB. So this means that the average power level is going to be 10dB or 1/10th of the peak level. So suddenly our 300W amp is actually going to be running at 30W for most of the time.
Efficiency and power
Loudspeakers are very in-efficient. Most of the power you put into a speaker turns into heat instead of sound. Typically a small speaker is about 0.1% efficient. This means for every Watt you put in, you get 0.001W of sound out! In comparison, a 1000W electric heater is 100% efficient.
Now you can improve the efficiency of speakers by using lighter materials, stronger magnets or acoustic transformers like horns. The best speakers are about 2% efficient, not great, but 20x better than a small speaker. The problem is that in order to get this level of efficiency, you have to sacrifice audio quality, as every mechanism you use, makes the sound weaker.
So you are left with two options:
- Use linear in-efficient speakers, but with big amplifiers
- Use not so linear efficient speakers with smaller amplifiers
Electrical vs acoustic power
Physically, to make a sound twice as loud, you need to increase the acoustic power by 10dB, which is the equivalent of 10x the electrical power – an increase from 4W to 40W. That’s quite a lot of extra power, and that power has to be delivered by the speaker’s batteries. If the speaker can run at this level, the batteries certainly won’t last for more than a few minutes. Let’s look at our magic 30oW speaker.
Portable batteries use Li-ion cells, which are based on 3.6V cells. The lowest impedance of a loudspeaker drivers is usually 4Ω, so using simple maths, the power at V²/R = 3.6/4 = 3.24W for one cell,. 2 cells = 13W, 3 cells =29W, and to get the 300W magic figure we’d need to have 10 cells in series ( 36V supply). There are a few problems with this:
- This is peak power which is √2 bigger than RMS power, the actual useful measurement
- Li-ions max capacity is about 3000mAH. This means that the batteries could deliver this 300W peak power for only 20 minutes. Not very useful.
- The batteries are expensive, most speakers (90%) have only 1 or 2 cell power systems
As with everything in audio, unless the figures are specified with conditions, ignore them and question why the manufacturers are not being honest with you!