Humble Power

About Music

This blog will float between a variety of topics but at the center will be music--specifically, enjoyment of music and the technology surrounding recreation of recorded works.

To begin, I want to touch on music from an electronic perspective. Music, being sound, is a vibration; chiefly traveling (at least at some point) through the compression and decompression (fancy word: rarefaction) of air. It is no great leap to imagine music as a vibration of electrical energy.

When a sound moves through air, the air moves locally--back and forth away and towards the sound source--but does not travel in the general sense. Put another way, when sound moves air, it also moves it back. So it is with electrical sound, in the form of alternating current (i.e. AC.) Much like, say, hula-hooping takes energy but ultimately does not move you much of anywhere, AC power is still capable of performing work without the "lightning-bolt" flow of electricity we picture.

With a complex vibration like music the associated power is not evenly distributed, ebbing and flowing with what musicians call the 'dynamics'. We can describe it in terms of peaks, averages magnitudes, and various other terms. For the point I'm building up to, I will simply note that the average power of music is almost always much less than the peak power (even in a highly compressed Oasis song.) Rules of thumb suggest a figure of 10% is likely still generous.

About Watts

Watts are the unit of electrical power. If you've ever bought a speaker or an amplifier, your familiar with them as a metric loosely associated with loudness. A quick Googling will tell you amplifiers can be found with a staggering array of different Watt ratings: 15, 25, 100, 1000... so on.

Speakers, too. I'm sitting next to two--one is a large PA cabinet that says it can handle 2000W and another a '70s model that says 40W. What neither of them say is how loud they actually are, their efficiency.

A look at one of the most popular aftermarket speaker suppliers (Parts Express) will show you that the actual drivers that make up a speaker (formally, drivers are transducers; devices that transform energy from one form to another) advertise their efficiency as some number of dBSPL, anywhere from high 70s to 110 or so. This number, 'decibel sound pressure level', tells you that the speaker creates that much 'sound pressure' given the right one Watt stimulus (although it doesn't tell you what that stimulus is) when measured from 1 meter away.

This is a few steps abstracted from what you'll hear when you actually use the speaker, to say the least.

The first thing to note is that SPL is measured in decibels (hence the 'dB') and that decibels are logarithmic, a math term that doesn't actually matter because our ears are exactly the same. What isn't in decibels here is the Watt rating, leading to great confusion.

If I were king of the world, I'd force audio folks to write power in decibels--but they don't.

To throw a further wrench in it, when electrical engineers write Watts this way, they use a different sort of decibel. The difference in magnitude between 3dBW and 6dBW is the same as the difference between 3dBSPL and 9dBSPL. This is confusing, but fortunately, for a good reason: SPL is a measure of pressure not acoustic power and once the dust settles on the math, it works out that adding 3dBW to a system is enough to add 3dBSPL. It's pretty convenient.

About You

Onward to the question of "how loud will it be, anyways?"

The answer, in terms of SPL, is that it will be as loud as the efficiency of the speaker plus the dBW of the source. One Watt is 0dBW which is why we quote efficiency at one Watt. Does this mean that a 1W amplifier and a 90dBSPL speaker will play your music at 90dBSPL? (which would mean standing one meter from your speaker was like standing ten meters from a jackhammer?)

Sort of.

If you're listening to a sine wave of the right frequency, perhaps. But music is mostly a tenth as loud as it is sometimes. If your theoretical 1W amplifier plays the loudest moment at its full power, mostly you'll be standing over a football field away from the jackhammer.

Please understand I'm not trying to tell you what amplifier to buy. That involves a lot more than 'how many' Watts--turns out it's rather more about 'what' Watts, but that's for the next post, not this one.