Car Audio Basics - 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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