Two numbers worth understanding before you buy

Walk into any hi-fi shop or scroll through a spec sheet and you will meet two figures that look technical but matter enormously: sensitivity, usually quoted in decibels, and power handling, usually quoted in watts. Get them roughly right and your system will sing at sensible volumes with headroom to spare. Get them wrong and you will either buy an amplifier that runs out of puff, or one that quietly cooks your tweeters. Neither is a disaster, but both are avoidable with ten minutes of arithmetic.

Sensitivity: how loud you get for each watt

Sensitivity tells you how efficiently a speaker converts electrical power into sound. The standard measurement is taken at one metre, on axis, with a pink noise or swept tone input, and expressed as dB. Most domestic hi-fi speakers land somewhere between 84 dB and 92 dB. The differences sound small but they are not, because decibels are logarithmic.

Every 3 dB increase needs double the amplifier power. Every 10 dB increase needs ten times the power. So a speaker rated 87 dB needs four times the wattage of one rated 93 dB to reach the same loudness. If you own a 10-watt valve amplifier, a 93 dB speaker will feel ample while an 85 dB design will feel strained and thin. If you own a 150-watt solid-state amplifier, the 85 dB speaker is no problem at all.

Rough categories are useful. Anything at or above 90 dB is efficient and friendly to low-powered amplifiers. The 86 to 89 dB band is the crowded middle ground and wants a reasonable 40 to 80 watts. Below 85 dB is thirsty territory, and small amps need not apply.

The 2.83-volt trap

Here is the detail that catches people out. Many manufacturers measure sensitivity with 2.83 volts rather than 1 watt. Into an 8-ohm load, 2.83 volts is exactly 1 watt, so the figure is honest. Into a 4-ohm load, those same 2.83 volts deliver 2 watts, which flatters the number by about 3 dB. A 4-ohm speaker quoting 90 dB may only manage 87 dB for a genuine watt, and it will also demand more current from your amplifier. When you compare two speakers, check whether the impedance rating matches before trusting the decibels.

Power handling: a safety range, not a loudness rating

Power handling tells you what the speaker can survive, not how loud it will go. You will see it written either as a maximum figure or, more usefully, as a recommended amplifier range such as 30 to 120 watts. Two things are worth knowing.

• Clipping kills more tweeters than excess power. A small amplifier pushed past its limits produces harsh, squared-off waveforms full of high-frequency energy. That is what burns out voice coils, not clean wattage from a larger amplifier.
• The minimum figure matters more than the maximum. If a speaker recommends at least 50 watts, pairing it with a 15-watt amplifier and playing loudly in a big room is asking for trouble.
• Peak and programme ratings flatter. A "200 W peak" claim alongside a "60 W continuous" figure tells you the real story is the lower number.

Matching an amplifier to a real British room

Let us do the sums for a typical sitting room, say four by five metres in a semi-detached house, speakers on stands 2.5 metres from your ears. You have an 88 dB speaker and two of them, which adds about 3 dB. That gives roughly 91 dB at one metre for one watt total. Room gain in a normally furnished British living room adds a couple of dB. Moving back to 2.5 metres costs you around 8 dB.

So one watt gets you roughly 85 dB at the listening seat. Comfortable everyday listening sits at 75 to 85 dB, so a single watt is doing most of the work. Peaks in a well-recorded orchestral climax might touch 100 dB. That is 15 dB above your one-watt level, and 15 dB means about 32 times the power, so roughly 35 watts. Add a sensible safety margin and a 50 to 60 watt amplifier is comfortable. Step up to 92 dB speakers and a 25-watt amplifier does the same job. Drop to 84 dB speakers and you will want 150 watts or more.

Impedance matters here too. Speakers that dip to 3 or 4 ohms ask for current, not just voltage, so favour amplifiers with a robust power supply. Valve amplifiers, typically 5 to 25 watts, want speakers of 90 dB and above, ideally with a gentle impedance curve.

Practical rules of thumb

• Aim for headroom. Choose an amplifier that can deliver at least 3 dB more than your loudest realistic requirement.
• Efficient speakers plus a small amplifier is a classic, refined combination and often sounds wonderfully free.
• Inefficient speakers plus a large amplifier is equally valid and gives you deep bass and dynamic ease.
• Ignore peak power claims and look for continuous, RMS or recommended amplifier figures.
• Never use a tiny amplifier to drive difficult speakers at high volume in a large room. That is how tweeters die.
• Trust your ears as well as the numbers. If the sound turns hard and shouty before you reach your preferred volume, you are running out of either power or speaker capability.

Understand these two figures and you stop guessing. You buy the amplifier your speakers actually want, play them at the volumes you enjoy, and let the system do what it was designed to do.