Audio Amplifier Power
How much power do you need?

Definitely, a much debated issue among home theater enthusiasts is amplifier power. Yet few have a true understanding of how the power output delivered by an audio amplifier or AV receiver relates to the end loudness level you will hear in the home theater.

Many simply relate loudness with amplifier power. Yet the sound we hear is not just a function of the amplifier output power and the audio source only, but also of the home theater speakers and the room environment.

Speakers impact the sound loudness and also the overall tonality while your room size and room decor impact the absorption of sound energy.

In other words, while amplifier power is the driving force behind home theater speakers, the resultant sound you hear depends also on both speaker efficiency and your home theater room. All three are equally important and we add crucial factors which need to be taken into account when planning a home theater audio set-up.

So the whole issue is: How much amplifier power do you need - taking into account speaker efficiency and room environment - to achieve a level of loudness that best matches your listening habits?

Audio Amplifier Power - Article Index:

An Introduction to amplifier Power: A few basics

Sound Loudness: Loudness levels in the home theater

Speaker Efficiency: Understand speaker sensitivity

Room Size: How does it all relate to an audio amplifier power output?

Conclusion: A few points worth taking note of

Amplifier Power

There is no simple and straight forward answer to amplifier power. This partly explains why audio amplifier power requirement is such a controversial topic.

Audio amplifier power basics

Before delving into how amplifier power, speaker efficiency and room environment influence the resultant sound quality and loudness, it is important to first have a better understanding of a few amplifier power basics.

A very basic concept here is that no matter how one specifies power, as long as all other things are equal, frequency range, load impedance, distortion, dynamic headroom, etc., having more amplifier power is better. It makes your system play louder, sounds better, and less likely to cause speaker failure.

The latter may be a surprise to many but in reality, speaker failure as a result of insufficient amplifier power is one of the primary failures associated with home theater speaker systems. Insufficient amplifier power leads an audio amplifier into clipping, thus generating harsh distortion well outside the speaker frequency response. Any electrical energy within the amplifier output signal falling outside the frequency range supported by a home theater speaker would not be converted into sound and ends up as heat. If this is sustained for a sufficient length of time, it may lead to speaker thermal failure.

This issue of clipping arises because it is impossible to reach a sufficient level of loudness with a small audio amplifier. In other words, with a small amplifier for your speakers and room size, you will not get the perception that the desired level of loudness is being attained. The tendency for many is to turn the amplifier volume towards the maximum, pushing the amplifier into clipping to compensate, thus endangering the speaker integrity due to excessive heat dissipation in the voice coil especially during loud passages during a movie playback or music listening.

If your system is not able to play as loud as you would like it to be without the signal becoming distorted, then you need a bigger, more powerful audio amplifier.

This does not mean that you should go out and get the biggest amplifier you can afford. The power rating of your amplifier should be related to your speakers. Inasmuch as insufficient amplifier power can hurt your speakers, too much amplifier power may also lead to both thermal failure as a result of excessive heat dissipation in the voice coil, and mechanical failure due to excessive diaphragm movement; the latter often leads to a shorted and even deformed voice coil due to possible rubbing along the coil housing and the coil hitting the magnetic pole bottom piece.

So... how much amplifier power do you need?

In general, audio amplifier power needs to be larger than the speaker's rated power because an amplifier is capable of delivering its rated power only when driven by a sinusoidal, or sine wave signal.

In a real life situation, the audio signal driving a speaker consists of a multitude of frequencies all playing at the same time rather than a pure single frequency sine wave signal; as a result, an amplifier will deliver less than its rated power.

As a general guideline, many professionals would advise to allow for a 30%, even 50% increase in amplifier power over the rated speaker's average, or 'RMS' power. Thus if your speakers are rated at 100W, the amplifier should be capable of delivering close to 150W when rated for a sine wave signal.

But there is more to audio amplifier power than allowing for the fact that amplifier ratings are based using a sine wave signal - which in itself is a far cry from what you will be throwing at it.

In our introduction to this article, we indicated that the resultant sound loudness you hear depends not only the audio amplifier power output, but also on both speaker efficiency and the room size.

All three are equally important and we add crucial factors which need to be taken into account when planning a home theater audio set-up to ensure that the resultant sound loudness will match your desired volume level.

Sound Loudness:  

Sound levels are expressed in decibels SPL, or sound pressure level. One decibel represents the minimum perceptible change in volume by human hearing. Normal speaking stands at around 65 to 60dB while a concert climax may reach 105 dB.

In the home theater, the THX reference level for loudness is 85dB for each of the audio channels except for the LFE which is set 10dB louder as measured at your seating position.

Additionally, if you are after a reference audio system, you need to allow for 20dB dynamic headroom - implying that in the home theater, sound levels may reach up to 115dB (95dB for the LFE + 20dB headroom) when setting sound to THX reference. Setting your system to THX Ref for a properly set home theater system means that you will be hearing the movie soundtrack at a level the producer intended it to be.

However, the fact that different people perceive loudness differently means that not everyone will want to listen to music or watch a movie at the THX reference of 85dB. People who like to listen at lower levels may reduce this by 3dB - thus requiring half as much amplifier power while those who would like to listen at very loud levels may have to double their audio amplifier power to get a further 3dB increase in loudness above the THX reference.

Speaker efficiency:

The level of sound at your listening position you get for a specific level of amplifier power - all other factors being equal - is a function of the speaker efficiency.

Remember that a speaker is a device that transforms electrical energy into sound energy. But not all energy from your home theater receiver or amplifier is transferred into sound; the sound level you get depends on the speaker efficiency.

Speaker efficiency, also known as speaker sensitivity, is a measure of the speaker's sound output in decibels, measured at one meter away from the speaker when feed by one watt of power (2.83 volts into an 8 Ohm speaker load) in a non-echoing test environment.

This means that a speaker with say 88dB efficiency will require half as much amplifier power as one rated at 85dB efficiency to produce the same loudness level. In a similar manner, one rated at 95dB will sound twice as much louder than one with 85dB efficiency. That's correct, every 3dB increase in loudness require doubling of the audio amplifier power output, while it takes an extra 10dB of power to double the loudness level you here.

Conventional speakers (drivers using cones and domes with a magnet and voice coil) support reasonable efficiency, usually in the range of 85 to 90 dB/watt/meter, while horn  loaded speakers may reach levels of up to 100dB/watt/meter and even more.

However... In most cases, high levels of speaker efficiency are normally obtained only by trading off sound quality.

Room size and environment: How does these relate to amplifier power?

Your room will influence the quality and loudness of sound you hear and therefore will have an impact on the required audio amplifier output to achieve a desired level of loudness. For example, too many heavy furnishings in the room will absorb more of the reflected sound than desired - thus reducing the amount of reflected sound reaching the listener.

However, while the environment within your room will impact the sound quality, yet it is the room size that is the key factor here when it comes to determining audio amplifier power.

A large room will require typically twice as much amplifier power as an average size room, while a small room will require half as much power as an average size room. The actual amount of amplifier power required depends on your desired level of loudness and the speaker efficiency.

In practical terms...

For the reasons already expressed above, there is no strict and fast rule as to the exact audio amplifier power required for a specific room size. However, the rule of thumb is that for an average medium-to-large room size of 3000 cu. ft (15ft x 20ft x 10ft high), average speaker sensitivity of 87 dB/watt/m and a listening level that is in line with the THX reference of 85dB, an amplifier capable of delivering anything between 50W and 80W average power per channel should be able to deliver more than this desired level of loudness.

Taking this indicative figure for the average room and what we have stated when discussing loudness, then a small room of say 1000 cu. ft would require approximately 25W per channel average power, while a much larger 6000 cu. ft room needs 150W/Ch and even more, to achieve the same level of loudness using the same speaker sensitivity.

But as stated, these figures assume average speaker sensitivity and average listening loudness levels. If you fall within that category that like to listen at high levels of loudness - even if it is for just a small portion of the time - then you will have to possibly double the required audio amplifier power and even more. The reason is that as further expressed earlier on, doubling the amplifier power will only lead to a marginal increase in the effective loudness; you need a 10dB increase to perceive sound twice as loud.

In addition, the resultant sound level is very much affected by the room environment. For example, if your room has a large opening leading to another adjacent room, then you will have to factor this into account - though to a lesser extent than the combined volume of the two rooms. In a similar manner, a suspended ceiling made of light structure material may present itself as almost acoustically transparent; in this circumstance, you will have to add the volume above the suspended ceiling in your calculations when it comes to room size.

Conclusion

Whatever is the case, matching the required audio amplifier power with your desired level of loudness is critical for an enjoyable listening experience.

At the same time, keep in mind that your volume level is also a function of the source signal; not all system components outputs are set at the same level - so for the same audio amplifier setting, some audio sources may not sound as loud as others. Having enough amplifier power reserve will enable you to match the audio amplifier output to your desired listening level even with low level output system components.

Last but not least, having adequate amplifier power reduces the risk of you literally frying your speakers out of your attempt to get more power from your audio amplifier by sending it into clipping.