Speaker Sensitivity, Impedance and What Your Amplifier Actually Needs

Choose the amplifier by the impedance curve and the sensitivity, not by the wattage figure on the front panel. Two speakers with the same nominal impedance can demand very different current, and a nominal 8-ohm rating hides a curve that drops to 3 ohms at the worst point.

What sensitivity tells you

Sensitivity is quoted as sound pressure level at one metre for 2.83 volts of input, which equals one watt into 8 ohms. A figure of 88 dB is typical for a small bookshelf speaker; 92 dB is efficient; 84 dB is demanding.

Each 3 dB of extra sensitivity is worth roughly double the amplifier power. A 91 dB speaker needs half the power of an 88 dB speaker to reach the same level.

Nominal impedance changes the meaning. A speaker rated 4 ohms produces a lower sound pressure for the same 2.83 volts, so a 4-ohm speaker with a quoted 88 dB figure is actually less efficient than an 8-ohm speaker at the same figure.

Read the impedance curve, not the number

The nominal rating is an average, and the minimum is what matters. Most manufacturers will state a minimum impedance in the manual if you look for it.

A curve that stays above 6 ohms across the band is an easy load for almost any amplifier. A curve that drops to 3 ohms in the bass region demands current, and a modest amplifier will struggle or protect itself.

The phase angle matters alongside the impedance. A low impedance combined with a large phase angle is the hardest load a speaker can present, because the amplifier has to deliver current out of step with the voltage.

Do the arithmetic once

Take the sensitivity, subtract it from your target listening level at one metre, and the difference is the dB of gain required. Convert dB to power with the rule that 3 dB doubles the power and 10 dB multiplies it by ten.

A speaker at 88 dB needing 100 dB at one metre requires 12 dB of gain, which is about 16 watts. Add 6 dB of headroom for peaks and the requirement becomes roughly 64 watts.

In a small room, listening at two metres rather than one adds about 6 dB of loss, so the numbers go up rather than down. In practice, a speaker at 88 dB in a normal room is comfortable with 50 to 100 watts.

Why the wattage figure misleads

Power ratings are usually quoted at a single frequency into a resistive load, which no real speaker presents. A rating of 100 watts into 8 ohms says nothing about behaviour into 3 ohms.

Look instead for a rating at 4 ohms, and for a stated current capability. An amplifier that doubles its power when the impedance halves is behaving like a good voltage source, and that behaviour is more useful than a large number into 8 ohms.

Class D amplifiers complicate the picture because efficiency and output depend on the load. Their ratings are usually honest but conditional, and the manual often states the conditions.

Room gain changes the equation

A room boundary reinforces the low end by 3 to 6 dB below roughly 100 Hz in a typical domestic setup. That means the amplifier’s low-frequency demand is partly met by the room.

Where speakers sit close to a wall, the effective sensitivity in the bass rises and the power requirement falls. Where they sit well out into the room, the opposite happens.

This is why the same amplifier and speaker pair can sound entirely adequate in one room and strained in another without anything changing in the electronics.

Matching for headroom, not for loudness

Headroom is what keeps transients clean. A system that is just loud enough at normal levels will compress peaks and sound harsh on orchestral recordings and percussion.

A practical target is at least 10 dB of headroom above your normal listening level. That costs power but it is the difference between an amplifier that sounds relaxed and one that sounds busy.

If you cannot afford the power, choose a more sensitive speaker. Sensitivity is the cheapest route to headroom, and it costs nothing to run.

What to ignore

Ignore peak power claims that have no stated condition. Ignore marketing terms for amplifier classes and power supplies unless a measurement is published.

Ignore the advice to match a speaker with an amplifier of the same brand. There is no technical basis for it, and the impedance and sensitivity figures are the only compatibility question that matters.

A short procedure

Find the minimum impedance. Find the sensitivity. Estimate your listening distance and level. Then choose an amplifier that can deliver the required power at the minimum impedance with 10 dB of margin.

If the arithmetic is uncertain, choose the more sensitive speaker and the more capable amplifier. Both choices are cheaper than replacing a pair after the fact.