What's in a Watt? – Speaker Sensitivity and Amplifier Power

Amplifier power expressed in watts is one of the most misunderstood and hyped performance specs used in consumer audio. This brief explanation should help clear up any misconceptions and help you to make better choices when selecting a new audio system.

A common misconception is that the amount of wattage your amplifier has has a direct correlation to loudness or volume. For example, some conclude 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 an important specification relating to two main issues:

  1. Speaker efficiency aka sensitivity expressed in decibels

  2. The ability of the amplifier to handle musical peaks (passages from soft to very loud)

Additional factors to be considered are:

  1. The type of music you listen to

  2. The volume levels you typically listen at

  3. The size of your room

  4. The furnishing in your room, e.g. is it heavily damped with carpets and furnishings or it it a live room with hard surfaces and minimal furnishing?  

  5. Do you want a speaker that can play well during a party? (even more power is needed in that case if you want your speaker to play cleanly above the noise created by all those friends in your home)

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 applied. A speakers efficiency is often measured with a microphone connected to a sound level meter and placed one meter from the speaker. As a standard of reference, one watt of power is then fed into the speaker, followed by the sound pressure meter measuring the volume in decibels. This in turn represents the measure of a speaker’s efficiency and is the first indication of how much amplifier power needs to be matched with your speaker for accurate sound reproduction.

Typical consumer speakers range in efficiency (aka sensitivity) from about 85 dB (highly  inefficient) up to 100 dB (very efficient). For example a speaker with an 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. Keep in mind that a change of 1dB in sound pressure level is the smallest amount of change the human ear can detect and that a 3dB change, while noticeably louder is still a modest increase in volume level.   

Most of today's wireless internally powered speakers have a low sensitivity in the 82-85 range and require quite a bit of power (probably 100 watts minimum) to accurately reproduce music in a moderately sized room.

Dynamic Range

Music is dynamic and constantly changing in volume level and frequency. The best way to understand music's dynamic nature is to listen to live (un-amplified) music. As an example an orchestra presents a wide range of volume levels, from quiet passages to extremely loud ones (think Beethoven’s 9th Symphony). Therefore the difference between the softest and loudest passages is the music’s dynamic range.

When music is reproduced through an audio system, the system should reproduce the full range of loudness – soft to loud. Generally when music is played back at an average volume level, the soft and medium passages in the music require minimal power. If for example your amplifier had 100 watts per channel, the soft and medium passages would require roughly 10-15 watts of power, however, the musical peaks (whether short bursts or sustained louder passages) would require significantly more power (perhaps as much as 80-100 watts).

Although the amplifier 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 to preserve the natural scale of music. This is why a speaker’s efficiency is the only true measure of the amount of power required for accurate sound reproduction when taking into account the list of factors mentioned earlier. In absolute terms, the number of watts does not matter, what matters is that the speakers sensitivity and available amplifier power are well matched and therefore able to effortlessly reproduce the full dynamic range of music.

Have further questions about what’s in a watt? Feel free to reach out to us if you have questions regarding speaker sensitivity and amplification – even if you are considering a speaker other than one made by Modern Fidelity. Our mission is to help you bring home the best possible enjoyment of reproduced music and we are always happy to help.

Look for another blog post discussing a closely related subject: a speaker's ability to accurately respond to its input signal and the negative effects most speakers designed for consumer use impart on the music in the form of dynamic compression.

Vintage Ampere Meter Photo Courtesy of Adamantios CC BY-SA 4.0, from Wikimedia Commons

Gary Leeds