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Speaker Power Ratings and Amplifier Power Ratings
This page is intended to help people understand the relationships between speaker power ratings and amplifier power ratings.  A question that comes up in designing a system is "how much power do I need for my subs?" and "how much power do I need to run my other speakers?" I have a BS in Electrical Engineering so I do not know how much of this the average Joe is going to understand. I am also human so there may be mistakes below.

When most people consider how they are going to match their speakers and amps together they usually only consider matching the power levels.  There are many more factors that come into play.  A big factor is the sensitivity rating of the speaker.  The sensitivity (efficiency) rating of a speaker gives you a rough idea of how loud the speaker will play given a certain amount of power.  Let's consider a speaker with this sensitivity rating:

87 dB / 1 watt / 1 meter

What this spec means is that the speaker will produce sound at 87 dB 1 meter away from the speaker when it is given input power of 1 watt.  Typically the input sound's frequency is 1 kHz.  Depending on the type of enclosure and other factors the speaker may not produce 87 dB but it's still a useful spec for comparison with other speakers.

It takes a doubling of input power to produce TDB more sound (assuming the speaker is not reaching its limits).  Therefore we can make a table for how loud the speaker will play given a certain amount of power like this:

Power in watts Volume in dB
1 87
2 90
4 93
8 96
16 99
32 102
64 105
128 108
256 111
512 114

You can see how it starts to take a lot of power to make a speaker play very loud.  Fortunately even 32 watts of power gets us decent volume.

When you ask yourself how much power you need for your system you need to ask yourself how loud you want your system to play and plan accordingly.  Going with higher power amps or more sensitive (efficient) speakers will make your system play louder.

There are some subjective items to consider as well.  Designing a quality speaker is a process fraught with many compromises.  For example, a speaker whose cone is stiffer tends to produce less distortion at high output levels but the added weight of a stiffer cone can smear quick transient response.  Speaker cones have been made out of something as simple as stiff paper (typical of poor factory speakers) to exotic materials like Kevlar (some fairly high end aftermarket speakers).

Sensitivity is another factor when designing a speaker.  Typically factory speakers and aftermarket speakers meant to be driven from a head unit are very sensitive because they must be able to play loudly with only small amounts of input power.  The compromises that are made to create highly sensitive speakers can have a negative impact on the quality of sound the speaker produces.  Some of the higher end speakers have low sensitivities because it was easier to design a high quality speaker that had low sensitivity than one that sounded good and had high sensitivity.  Also, it is presumed that a high end speaker will be driven by a proper aftermarket external amplifier with more power than a head unit.

As for matching power ratings between speakers and amplifiers, it is not necessary.  Most speakers can accept clean input power in huge amounts before destroying themselves.  Any quality amp that can produce enough power for your loudness expectations should work fine.  The only advantage a 200 watt amp holds over a 100 watt (of the same design) is the ability to play 3 dB louder.

Finally, one other item to consider when choosing an amplifier is whether it is 2 ohm stable.  One may not think this matters if you are going to be using normal 4 ohm speakers but it can still be a factor.  When a speaker is rated at 4 ohms, that is just a nominal rating.  The actual impedance will change with frequency and is also affected by the type of enclosure the speaker is in.  There can be frequencies where the impedance dips well below the nominal 4 ohm value.  Having an amplifier that is stable to 2 ohms assures that your amp will be able to provide the current necessary for the speaker to reproduce sound accurately at those frequencies.

So in the end the basic answer to the question of how much power you need for your speakers is based on how loud you want your system to play and how sensitive the speakers are that you are going to use.  Also consider that subs can be less efficient than other speakers so you will probably want to give your subs more power than the higher frequency speakers in your system.  The lower the frequency the more power that is required by the speaker to reproduce it.

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