Amplifier Power and Speaker Efficiency

The common misconception about wattage and volume

Hand turning stereo dial
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Amplifier power, measured in watts, can be a confusing subject and is commonly misunderstood. A common misconception is that wattage has a direct correlation to loudness or volume. Some believe that doubling the power output will result in a maximum volume that is twice as loud. In fact, power has little to do with loudness. Power output is relevant to two main issues:

  1. Speaker efficiency
  2. The ability of the amplifier to handle musical peaks

Speaker Efficiency

Speaker efficiency, also known as speaker sensitivity, is a measure of the speaker's output, measured in decibels, with a specified amount of amplifier power. For example, speaker efficiency is often measured with a microphone (connected to a sound level meter) placed one meter from the speaker. One watt of power is delivered to the speaker and the level meter measures the volume in decibels. The output level results in a measure of efficiency.

Speakers range in efficiency or sensitivity from about 85dB (very inefficient) up to 105dB (very efficient). As a comparison, a speaker with 85 dB efficiency rating will take 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 will require ten times more power than a speaker with a 98 dB efficiency rating to play at the same level. If you're starting with a 100 watt/channel receiver, you would need 1000 watts (!) of power output to double the perceived volume level.

Dynamic Range

Music is dynamic in nature. It is constantly changing in volume level and frequency. The best way to understand music's dynamic nature is to listen to live acoustic (un-amplified) music. An orchestra, for example, has a wide range of volume levels, from very quiet passages to loud crescendos and some in-between quiet and loud. The range in volume level 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 the same range in loudness. When played back at an average volume level, the soft and medium passages in the music would require minimal power. If the receiver had 100 watts of power per channel, the soft and medium passages would require roughly 10-15 watts of power. However, the crescendos in the music would require more significantly more power for short periods of time, perhaps as much as 80 watts. A cymbal crash is another good example. Although it is a short-term event, the cymbal crash demands lots of power for a short period of time. The ability of the receiver to deliver bursts of power for a short time 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.