Put your school uniforms on folks, we are going to school again. Do you have vague memories of your High School science classes? Does anyone have the fortitude to remember the physics semester? Sometimes key words are a good way to jog unpleasant memories; remember ENERGY? It’s that part of our universe that enables change to take place, or put another way, the capacity to perform some kind of work. Now, some people (not that many probably) had the stomach to sit through the explanation of the concept of POWER! What is power, think now…. no it won’t come back. POWER is ENERGY used per UNIT OF TIME.

Ahhh now we are getting warm, a WATT is a unit for measuring POWER. More specifically the measure of an amount of energy used in one second of time. Because most explantions of energy used per unit time will be longer that one second, we should really think like engineers talk about power and say energy used per second per second.

That’s great, but how do we get this in perspective with sound gear? First let’s do a table of some rule of thumb power levels expressed in watts we run across in everyday life:-

Interesting isn’t it, an amplifier pushing out 500 watts is working harder that a human sprinter running a 100 metres race. An amplifer pushing out 1000 watts is working at a rate of more than one horsepower, in fact for an example it is working about as hard as your average domestic lawn mower. I know what you are going to say now I think…. “I’ve got a reasonable concept of what its like to run flat out for a 100 metres, and I know what happens to a toe caught in a lawn mower blade, if that same power was sound energy shouldn’t it be incredibly loud?” Your’e darn right it would be!

Sorry, back to our High School physics class for this last bit. If we were able to sit through energy, stay awake for power, you were almost certanly nodding off, if not asleep for THERMODYNAMICS. The teacher, bless his/her little cotton socks would have spouted on about the LAWS OF THERMODYNAMICS. There’s three of them if you really really want to know.

It’s the second one that we are interested in here. It says in my humble translation “any process in which energy is converted from one form to another is not 100% efficient, some energy is always lost as heat”. Hmmm… perhaps a little glimer of light here? Our sound system is plugged into a power point and takes in energy in the form of electricity, after many operations sound energy comes out of the speakers. What is going to happen in our 500 watt amplifier, all along the chain of events energy is leaking away as heat. The power cord gets a little warm, the amplifier and all the gizmos attached to it definitely heats up, the leads carrying power to the speakers get warm and finally the voice coils in the speakers heat up. So, our work rate of 500 watts at the power point has partly ended up as sound, and partly as waste heat! Now wait for it…. you average speaker with a paper cone is only a few percent efficient! A horn speaker is more efficient, around 20 percent.

If we get 25% of the power in coming out as sound we are doing real well, in fact we are using top quality audio gear. So, our 500 watt amplifier may give off at the end of the day less than a 100 watts of sound power. You do not by any means, hear coming out as sound all the power that goes in!

There’s more coming very soon!!!

Well, that is very often the case, and there are several reasons why most home audio gear doesn’t perform all that well when worked really hard. However let’s concentrate on the watts issue first. There are three different ways amplifiers and speakers can be rated. Watts RMS (Root Mean Square), Watts PMP (Peak Music Power) and Watts Peak Transient Load.

Because marketers will be marketers, it is most likely your 500 watt hi-fi system is 500 watts PMP, not RMS. Some brands (no names-no pack drill) may even have marketed a system claiming 500 watts peak transient load.

Well, unless you are an electronics head you are probably just as much in the dark after reading that paragraph as when you started; so lets explain the differences between these power ratings.

Maximum watts RMS: This is the TRUE maximum power level the amplifier/speakers can deliver constantly. As a rule of thumb this is about HALF of watts PMP! So your 500 watt system may be in truth be only capable of delivering approx 250 watts constantly.

Watts PMP: This is the maximum amount of power delivered in the form of a short controlled burst of high power signal that the amplifier and or speakers will tolerate for a short time without doing damage. Usually this is around TWICE the RMS wattage of a system.

Peak transient load is the “abuse” a speaker/amplifier system will tolerate for a few milliseconds without blowing up!

Now… we have to stop here for a while, and again go back to our maths class at high school. Remember logarithms? I can vaguely. Logarithms are those horrible calculations where you use powers of numbers, that is multiplying a number by itself a given number of times. Ah yes, through the distant fog I remember, ten to the power of one (101) is ten one time only (= 10), ten to the power of two (102)is ten multiplied by itself twice (10 *10 =100) ten to the power of three (103) is ten multiplied by itself three times (10*10*10 = 1000) and so on.

OK so what, well the Decibel measuring scale is logarithmic and that’s important to get a handle on. Let’s first consider the base measuring unit the Decibel is derived from, the Bel, named after Alexander Graham Bell. A sound pressure that measures one Bel (10 Decibels for the geniuses amongst us) has a certain value, call it anything you like, why not pressure X, everyone else uses X. What would a sound pressure of two Bels equal? Noooooo…. not X*2, it would equal X2 or X to the power of two. If X equalled ten then X2 would equal 100. Two Bels is TEN times stronger than one Bel. We can take that a bit further….. what about 3 Bels? Again assuming X equals 10, 3 Bels would be X3! That’s a HUNDRED times stronger. We do not usually use the Bel (B) as a measuring standard, its numbers are too big for most practical purposes, we use one tenth of a Bel, the Decibel (dB) as the measuring standard.

Let’s leave the maths mostly alone for a while, to everyone’s releif, and ask a basic question. Why use a logarithmic scale to measure sound pressure anyway? Why not use a simple linear scale where you don’t use powers, just ordinary old multipication? The answer is our ears have an incredibly large dynamic range, that is the range from the very softest sound we can hear, to the very loudest sound we can discriminate before our hearing mechanisms are saturated and can no longer discriminate any differences in loudness. If we used a simple linear graph to plot this dynamic range you’d end up with something about as long as ten rolls of toilet paper! A logarithnic scale on the other hand, because its numbers increase so rapidly lets you “squash” that information into something that is reasonable to manage.

We do the same thing with measuring earhquakes by the way, because the amount of energy released in an eathquake can cover an enormous range of values, we use the Richter scale, which you guessed it, is a logarithmic scale. An eathquale of magnitide 2 is TEN times stronger than magnitude 1 etc.

Right students, we are now ready for the nitty gritty.
First, what is the baseline or reference sound pressure we use to measure all other sound pressures against, that is what sound pressure has a value of 0 dB? The intuitive response to that question would be to say “absolute silence”. However that can’t really be tightly defined, so those that know these things have set the reference level as the softest sound a young person with undamaged hearing can hear. You won’t believe how little sound pressure that is .. 20 micropascals!! That is two ten billionths of atmospheric pressure. Incredible isn’t it, our ears are such beautiful acoustic devices that they can detect a difference in pressure that small.

There is a practical “rule of thumb” we can use to describe sound pressure levels that uses everyday metaphors:-
0dB …. The very softest sound a young person with good hearing can discrininate.’
30dB …. A quiet night, with just some distant sounds of chirping crickets, or slight rustling of leaves in a breeze. A very soft whisper.
60dB …. Around the average sound pressure of a person speaking normally.
90dB …. A jack hammer.
110dB …. A rock concert.
120dB …. A jumbo jet on full throtle taking off.
150dB ….. A high powered rifle shot.
250dB ….. A large bomb explosion.

We are all aware that exposure to very loud noises can damage our ears. Damage can occur instantly when sound pressure approaches 175bB or more. Sound wave pressures of those intensities will literally blow our eardrums in. More subtle, but just as nasty is prolonged exposure to sound pressures around 90dB – 110dB. This can cause slow cumulative ear damage, often leading to a loss of the ability to hear high frequencies over a period of time. If your ears are ringing after being exposed to sound the general rule is that is damaging.

This is not the end of the story with the decibel. When Mr Lee DeForest invented the vacuum tube (valve) at the beginning of the 20th century it was possible to amplify very small electical signals. So, a measuring scale of amplification was needed for the first time. Amplification works logarithmically. No problem, a very convenient logarithmic measuring scale already existed the decibel! This measuring scale was adapted for electronics. We talk about amplification in dB (actually dBu dBv but don’t get too worried about that). Anyone who has seen a professional amplifier or mixer will notice that the knobs are marked in dB, not 1,2,3,4 etc. The reason is that those who know the formulas and can work a calculator can work out very precise measurements from those knob settings. One last comment, in electronics, unlike sound, you can have deamplification as well as amplification so in electronics you will see -dB (negative decibels) meaning a signal coming out is weaker than a signal going in.