What Is a Speaker Efficiency or Sensitivity Rating?

Amplifier power is a misunderstood concept. Many people mistake a speaker amp's power rating or wattage for its loudness. Doubling the power output of a speaker does not double the speaker's maximum volume. There are two ways in which an amp's power rating is relevant: The efficiency of a speaker and the ability of the amplifier to handle volume peaks.

What Is Speaker Efficiency?

Speaker efficiency, also known as speaker sensitivity, is a measure of a speaker's decibel output at a specified amount of amplifier power. For example, a speaker's efficiency is typically measured with a microphone placed one meter from the speaker. As one watt of power is delivered to the speaker, the microphone measures the resulting volume through a decibel level meter. The output level that results is the speaker's efficiency rating.

Speakers range in efficiency or sensitivity from about 85 dB (very inefficient) up to 105 dB (very efficient). To compare, a speaker with an 85 dB efficiency rating takes twice the amplifier power to reach the same volume as a speaker with 88 dB efficiency. Similarly, a speaker with an 88 dB efficiency rating requires ten times more power than a speaker with a 98 dB efficiency rating to play at the same volume.

If you're starting with a 100 watt/channel receiver, you would need 1000 watts of power to double the perceived volume level.

Dynamic Range

Music is dynamic in nature. It's always changing in tone and volume, with tone measured by frequency and volume measured by amplitude or perceived loudness.

The best way to understand audio dynamics is to listen to live, non-amplified music. An orchestra, for example, has a range of volume levels, from quiet passages to loud crescendos and some in-between. This range in volume is known as dynamic range, the difference between the softest and loudest passages.

When the same music is reproduced through an audio system, the system should reproduce or approximate the same loudness range. When played back at an average volume level, the soft and medium passages in the music would require minimal power.

If the receiver has 100 watts of power per channel, the soft and medium passages require roughly 10 to 15 watts of power. However, the crescendos in the music require more power for short periods of time, perhaps as much as 80 watts.

A cymbal crash is another example of the difference between dynamics and amplification. Although it is a brief sound, the cymbal crash demands a lot of power over a short period of time. The ability of the receiver to deliver bursts of power in brief spurts is important for accurate sound reproduction.

Although the receiver may only use a small portion of its maximum output most of the time, it must have the headroom to deliver large amounts of power for short periods of time.