[Things your Mom warned you about] How much power do we really need?

It is not the first time I tackle the topic of how to match amplifier and speakers. However, I believe it is necessary to delve a little deeper into this issue, because over the years confusion, rather than decrease, has increased and, in the meantime, components that were once expensive and complicated to use, such as digital sound level meters, have become easily available to everyone.

Output power vs power handling

As a premise, let me summarize the concept of power output vs power handling. An amplifier does nothing but boost the level of the electrical signal it receives from the source (CD player, computer, streamer, turntable etc.) and sends it to the speakers, which convert it into an acoustic signal which finally reaches our ears. Therefore, the component that generates power is the amplifier, not the speakers! More properly, it generates voltage (measured in volts) and current (measured in amps) which, once transferred to a load (the impedance of the speakers) becomes power (measured in watts). It is therefore obvious that the latter, at most, are “forced” to handle a certain power supplied by the amplifier, and have a limit beyond which they break or distort. Loudspeaker manufacturers generally supply info both on the continuous power (often expressed in watts RMS) and on the peak power that their speakers can handle without trouble. Consequently, there is no problem pairing a 20-watt amplifier (rated into a certain load) with a speaker of that load that claims to be able to handle much more power. And vice versa, just keep reading to understand why.

What really counts: speaker sensitivity!

The truly interesting datum that is needed to understand the correctness of any amp/speaker pairing is the sensitivity of the speaker, i.e. how much sound pressure it produces with a given amount of power. The higher the sensitivity, the higher the sound pressure. The datum is conveniently expressed in dB/w/m (decibels per watt per meter) and measures how many dB of sound pressure the speaker produces with one watt of power at one meter distance. Quite often, this datum is overestimated by the manufacturer, but it is a useful estimate nevertheless.

Medium sensitivity speakers, like most of those on the market, are capable of transforming one watt of input into a sound pressure level between 85 and 90 dB at a distance of one meter. This might not mean a lot, unless you have a way to understand how loud such a sound pressure is. Simply put, most audiophiles have no idea. Since it is generated by just one watt of output, they tend to think it is a barely perceptible sound pressure, a whisper. Too bad, the exact opposite is true: 90 dB of sound pressure, at the listening point, corresponds to such a loud sound, that two people sitting next to each other need to raise their voice to converse.

Sound pressure and how to measure it

To fully understand the meaning of dB and how a certain amount of dB's transforms into a certain “listening level”, all you need is a sound level meter. Modern smartphones can help, just use one of the many apps that measure sound pressure, for example the free suite JBL Tools that we reviewed some time ago, or an equivalent one. Even better, for €20-30 you can purchase a real digital sound level meter which measures the sound pressure in our listening position with acceptable precision. It is the best investment you can make for your system.

Hence, my first advice, before investing money in unnecessarily powerful amplifiers, is: purchase a sound level meter! It's very easy to use, just press a switch to read the dB level on the display, both constant/average and peak. Thus, you will find that, if you have tolerant neighbours, you will be able to exceed - sometimes - 90 dB peaks at the listening seat. You will be able to achieve 100 dB peaks only if you live in an isolated house and your speakers allow it. Exceeding 100 dB undistorted peaks, is difficult, very difficult. Bear in mind that 100 dB is the sound pressure generated by a jackhammer trashing the road 3 meters away from you. Now figure that into your listening room. On average, in an apartment, what we call "high" listening level corresponds to average sound pressures between 75-85 dB at the listening seat. Depending on your relevant other, this value may vary, and generally it will be lower. I know many people who can hardly exceed 80 dB peaks!

Power vs sensitivity vs distance of the listening seat

From what is written above it is clear that just one watt seems enough to get a high sound level. This is partially true, because the other factor of the dB/w/m measure must be taken into consideration, i.e. the meters of distance between the speaker and the listening point. Let's keep in mind that the dB scale is not linear, but logarithmic. Without going into mathematical details, it is enough to know that a doubling of the power corresponds to a 3dB increase in sound pressure. So: an 85 dB/w/m speaker produces 85 dB with one watt at a distance of one meter, but it produces 88 dB with two watts and 91 dB with 4 watts. With 8 watts you get 94dB! That is LOUD!

Now, how does the sound pressure vary if we move away from the speakers, since normally we keep them more than one meter away from our ears? In a somewhat crude way, one can say that at two meters of distance you lose around -6dB of sound pressure, which becomes -9dB at three meters and -12dB at four metres. These are values in open field i.e. without walls, which is the worst case scenario. With reflections off the walls the situation is a little different, actually it improves (i.e. SPL increases).

Therefore, knowing the usual sound pressure at which you listen, and the distance between the listening point and the speakers, it is very easy to calculate how many of your amplifier's watts you are using. Doing the experiment and the maths, you will discover that those hundreds of watts that you thought were indispensable, were only useful to those guys who produced the amp and to those who sold it. The table below shows an example with a speaker of 95 dB/w/m sensitivity. With just 4 watts you can reach 101 dB at a distance of one meter, the equivalent of the jackhammer mentioned above. With just 4 miserable watts!

Going stereo! Two speakers vs one single speaker

What happens using two speakers instead of one? The sound pressure doubles (two sources instead of one), i.e. +3dB are added to the equation. If everything was additive and simple, two 90 dB speakers playing together should produce 180 dB!!! :-) And evidently this is not the case.

A practical example

If a speaker of 90 dB sensitivity alone produces 90 dB with 1 watt at 1 meter distance, then a stereo pair produces 93 dB. It is therefore easy to understand what happens: if there are two speakers, with 10 watts you get around 103 dB, from which the effect of the listening point distance must be subtracted, say typically 2 metres, therefore -6dB. You are still at 97 dB, a typically intolerable value in any domestic environment. And even if it were 97 dB peak, the power required from the amplifier would still be those paltry 10 watts - or even less. In the table below you can find the sound pressure losses as a function of the distance of the speakers from the listening point, in steps of 1 metre each.

A final remark

N. B. It is worth remarking that everything we have discussed and calculated above applies to an ideal world scenario, that is, an ideal amplifier connected to a fixed non variable impedance (say, an 8&Omega resistor). A real speaker is normally a harder load for an amp, so it can happen that an amplifier claimed to deliver 50 watts on 8Ω generates less power on a real speaker that is particularly hard as impedance (both in terms of module and phase) changes. This was certainly true decades ago, when real load measurements were ignored by amplifier manufacturers. Today, any good amplifier is capable of driving a standard speaker without significant power loss.

Conclusions

How much power should an amplifier deliver to play loud enough? It all depends on your needs (especially on where you live), on the sensitivity of your speakers, on your concept of loud, and on the distance between the speakers and the listening seat. Once you have made the measurements explained above, you will be able to understand how many watts you normally use during your listening sessions. A possible objection could therefore be the following: “Are you saying that very powerful amplifiers are useless?”. Yes and no, in the sense that what matters is quality, not quantity. There can be tens of thousands of euros of difference between two amplifiers of equal power, which should translate into greater precision, tonal coherence, transparency and dynamics. So it makes sense to buy bigger and more expensive amplifiers, but not because they have a higher power output. Let's not forget that Paul W. Klipsch lamented the lack of a "good 5 watt amplifier" (“What the world needs is a good 5-watt amplifier”) and that several famous audio authorities have insisted on the quality of the first watt, because this is what does most of the "work". Among these, the legendary designer Nelson Pass (not surprisingly he created the company First watt) and the journalist Dick Olsher (“The first watt is the most important watt”).

Hence, before any purchase/upgrade, evaluate whether the power you have is sufficient or not: if it is, just look for a better amplifier.

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Things your Mom warned you about: amp power is never enough!

Debunking audiophile myths - power output vs sound pressure

Foreword