Archive for November, 2009

Why Wattage Does Not Matter

DISCLAIMER

Yes, I know. Some of the technical issues here may be overly simplified. But that’s not the point. I’m just trying to teach beginners and the common bloke how to go about avoiding being lured into marketing hype.


Some of the most commonly asked questions from the common consumer hi-fi purchasers are questions like:

“Is X watts enough?”

“I have Y watt speakers, how thick of a cable do I need?”

“This amp is 150W, so I going to be getting more watts from this 170W one, right?”

Wattage, or the rated maximum power of a hifi component is probably the last thing you’d want to consider when purchasing equipment. Only newbies ask about wattage, much like only newbies compare megapixels in camera gear. Hi-fi doesn’t work the same way as power tools, and even power tools don’t exactly go by a watt-per-dollar comparison. There are good reasons why 40W amps sell for thousands of dollars and 200W amps for $50. Hint: it’s not the wattage.

Speakers

Without going into the really technical bits, let’s assume a simple scenario – you’ve just bought an amplifier and you want to buy some speakers to go with them. Most of us hifi consumers, being males, believe that bigger is better. We often become conned by slick marketing into thinking that a 200W speaker should surely be better than 100W, and if you’d just purchased a 150W amp, then the speakers better be 150W or more, otherwise it won’t be able to handle the power.

The rated power of a speaker is simply the maximum power it is claimed to be able to handle without damage. So even by layman’s logic, you could hook up a 200W amp to a 100W pair of speakers, have the volume up to half way, and all will be fine. (In actual fact, this is even less of a concern, but I will explain later.)

The Log (Exponential) Scale

Before we continue, I’m going to explain the single most important concept in the watt myth: the log scale. The log scale is a mathematical scale based on the logarithm. The inverse of this is the exponential or the power (I don’t mean the power in watts, but the power as in 10^2 = 100). For simplicity and the purposes of this article, I’m going to talk about these things interchangeably as it is just the “direction” to get from one number to another. We are normally accustomed to what is known as a linear scale: 1, 2, 3 …10 etc. I.e. the difference in magnitude as we progress upwards is a constant number. The exponential scale differs in that the difference as we progress is a multiple of the number before. (And the opposite is the log, where we count in a nth root of a number). In other words as we count upwards we need to multiply the previous number by a number, for example: 10, 100, 1000 or 2, 4, 8, 16. As you can see, the magnitude of difference is “increasingly increased”.

A rough graphical indication of this relationship is as follows:

Exponential vs Logarithm

Exponential vs Logarithm

The Decibel

The decibel (dB) is a measure of size which is based on the log scale above. It is frequently used in many fields of engineering to help denote the size of numbers. This is because in nature, the measure of quantity often appears not as a linear relationship, but more as a log one. Human perception of sound (as well as vision, for that matter) occurs in a log relationship. Most of us are born with ears that can detect a tiny fraction of a watt, right across to hundreds or even thousands of watts (although the latter is not recommended). Due to our huge range of hearing, our ears are sensitive not to the absolute value of the energy transmitted into our ears, but the multiple. That is, 2 watts will sound a little louder than 1 watt, and 4 watts will sound louder than 2 watts, but only by as much as what 2 watts sounded louder than 1 watt. Hence, to measure our sense of sound rather than the absolute value, it makes sense to denote this in a log scale. As a guide, every 3dB is approximately double the power.

By now, you can probably see where I am going.

Frequency Response

Frequency Response

Here’s a typical frequency reponse graph. Notice how SPL is linear while the frequency is on a log scale. SPL’s relationship to power is also approximately log, so imagine the frequency response’s axis as power (turn it vertical) and think about the implications of increased power. It doesn’t make that much of a difference unless power is increasing exponentially.

Sensitivity

The purpose of a loudspeaker is to convert electrical power into sound. Ironically, most consumers don’t even look at one of the most important factors in this – sensitivity. Sensitivity is the factor of how efficiently the loudspeaker does what it does. It is measured in dB/W at a certain distance from the front of the speaker, usually 1m (hence dB/m/W). In other words, it is how loud a speaker is for every watt input, a meter in front of the speaker. So if you’re comparing two speakers of exactly the same type, and one rates 3dB more sensitive, then the speaker is twice as efficient as the other. That means it will only need half the amount of power to be equally loud. Most commercial Hi-Fi speakers range between 86-92dB/W in sensitivity, but it could even range a few dBs further than that. Now, the difference becomes apparent. Some speakers can be 8 or more times more efficient than others!

Volume

Most people don’t realise this, but for all practical purposes, even if speaker power ratings were relevant, you’d never use its maximum power. Again, the dB scale applies here. The volume control on most amplifiers are log scale. Assume that the volume ranges from 0 to 100%, and you have a 100W amp. Because it’s a log scale, it means that for low volumes, you will only be getting a tiny bit more power at every additional increment. If we assume that at maximum power, you have 100dB of sound, it means that for every +1 level of volume, you’ll likely receive roughly a 1dB increase in volume. So in theory, at 97% volume you’ll only be getting 50W of power. At 94% volume, you’ll be getting just 25W of power, and so on and so forth. In theory, at 50%, you’ll only be using 1mW of power (1/1000th of a watt). In real life, this is probably less extreme because manufacturers try to limit the amount of power you can get up the top end to prevent equipment damage and skew the volume distribution. Nevertheless, at 50% volume (the “real” maximum volume for using your amp for reasons discussed later), you’ll be using a small fraction of your amp’s maximum rated power.

But I want loud loudspeakers!

For those of us who are happy to spend a modest amount of money on hi-fi, you’ll have a choice. Either go for quality or quantity. For a couple thousand dollars you can buy speakers and an amp which sound accurate and are comfortably classed as “audiophile grade”, but they won’t be able to produce the thunderous (read: muddy) bass or thrilling theatre sound that, although loud, is a poor representation of good sound. I prefer accuracy much over quantity, but hey – each to their own. If you want quantity, make sure the amp is good and the speakers can take a beating.

Use your ears

As I say every time I get asked, this is the factor that really matters. In the end, it’s all about what you hear, and the only way to know what the results would be is to use your ears. Go to a store and listen to a few speakers to get a feel of which brand or type you like the sound of. Make sure you do this under controlled conditions (test two same speakers with the same amp, source, room, volume, cables, etc.).

While judging and comparing power ratings directly between two amplifiers is a little futile, comparing power ratings between two amplifiers made by the same brand, in the same model “series” can give you an indication of relative output between them (although you could probably judge this by the model ranking anyway). There are multiple factors which the more “audiophile” or “expert” consumers would consider benefits in the power department, and some of these include class, transformer size, design purpose, to name a few. Funnily enough, these “specs” are seldom included (or honest) in spec sheets.

Set up the right equipment for your room

As a general guide, what is infinitely more important than most of these factors is getting the right equipment for your room. While there are many room shapes and covering materials which can change the effect of sound produced within it, having the right equipment for the room size is one of the most important decisions you need to make. As a room gets bigger, its volume increases exponentially with its dimensions. If you increase a 4x4m square room by 1m each side (a 25% increase), you’re increasing the volume by 50%, since the 1m extra is squared. Sound propagates evenly in a room and hence, the larger the volume, the more output you will need to achieve the same sound level. Buy an amp with enough grunt for your room. Buy speakers to suit the amp.

Aim for more amp power

“I have a 200W amp, I’ll need a 200W pair of speakers.”

Speaker power is a completely useless figure. Speakers don’t fry because the amp gives too much power – they fry when the amp clips. Amplifiers clip when they are pushed beyond what they can output. A good amp shouldn’t clip for their rated load up to full power. Poor amps will clip when speaker load is too demanding for what they can give. Speaker load, by the way, has absolutely nothing to do with power handling. Buy a good amp, and don’t worry if speakers are rated 70W if your amp is 200W. They’ll never fry unless you push far beyond ordinary limits.

If you want party speakers…buy party speakers

Some people are so obsessed with having a loud hi-fi that they overdrive, and inevitably either damage their equipment or get hearing loss…or both. This is not hi-fi. Hi-fi stands for high fidelity, which means a high level of fidelity to the original sound. Think about it – if you want stuff overly loud, this can’t happen. If you want speakers to shake your neighbour’s knickers, buy party speakers. Not only are they cheaper, they nearly can’t be damaged no matter how hard you drive them.

Manufacturers are liars

All of what I’ve said so far doesn’t even matter in the real world because of one simple fact: manufacturers are liars. Manufacturers these days have little guidance as to what they can put on spec sheets. Power could mean PMPO, short term RMS, long term and anything else in between. They can enter a number and call it whatever they want. There is no governing law or industry standard to measure power, and certainly little standardisation or any internationally enforced system. Power can be measured in a thousand different ways and each with a completely different outcome. A well trained technician can make “10,000W” appear out of a 10W amp, and a guy in the marketing department can easily “mistaken” PMPO for RMS. What incentive does any manufacturer have to be truthful about what they say on a spec sheet of pure marketing? Adding to this is the fact that some reputable manufacturers are honest while others are not, makes comparing power like going with the person who shouts the loudest.

Power doesn’t matter

It’s pretty simple. Power doesn’t matter. Look at power if you want to feel like you’re buying something powerful or if you want to brag about the label to your friends. But it’s no indication of the quality of the product you’re buying. If you want good sound, or even truly powerful sound, use your ears. The sort of spec fudging doesn’t just happen in the hi-fi industry, but in just about every industry which targets the (usually male) ego. They want you to feel like you’ve bought a million watts of fearsome power. Whether it amounts to anything or is of any use, is a completely different matter.

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