Hi all, maybe a bit of electrical theory would help to clarify a few misconceptions. It is not a bad thing to have an amplifier capable of delivering more power than the power rating of the speaker.
Power rating of amplifiers: this is what an amplifier is capable of delivering, if the load it is driving tries to draw that much. Try to draw more than that and the amp will start to clip, which means "curvy" sine waves (music signals) get chopped off and start to look like direct current. Clipping and direct current is bad for speakers. It creates excessive heat which will eventually melt the thin layer of insulation (really a coat of varnish) on the wire in the voicecoil, causing failure of the speaker. Depending on where the insulation fails the amp may suddenly see a drastic drop in impedance (iel short circuit), causing the output stages to blow.
The amp does not "push" this power into the load. The load draws from the amp. The volume of the speaker is adjusted by varying the voltage across the speaker. As the voltage increases (by turning up the volume knob), the load (in this case the voicecoil of the speaker) will draw more current.
This is basic Ohms Law: Voltage (V) = Current (A) x Resistance (Ohms)
Readjusting the equation gives: Current = Voltage/Resistance
and for Power, Power (W) = Voltage x Current
Power doesn't really exist, while Voltage, Current and Resistance are real and measurable. Power is a way of quantifying how much work a certain combination of Voltage and Current will create across a certain Resistance (load).
For the sake of this discussion we will talk about continuous, constant sound level, like a constant test signal is being put through (as opposed to a musical signal, which varies greatly in sound intensity and duration due to the nature of music).
Try some numbers in the above equations to try to see what I'm saying:
Say you have a 4 ohm speaker. At a certain volume setting of the stereo the amplifier is applying 8 Volts across the speaker terminals.
8Volts/4 Ohms (Voltage/Resistance) = 2 Amps. The speaker is drawing 2 amps of current out of the output stages of the amplifier.
Power = Volt x Current. 8Volts x 2 Amps = 16 Watts.
The amplifier is providing 16 Watts to the speaker.
Now you turn the volume control up on the stereo. This corresponds to a voltage fo 20Volts across the 4 Ohm speaker.
20Volts/4Ohms = 5 Amps. The 4 ohms speaker wants to draw 5 amps of current out of the amp.
Power this corresponds to: 20Volts x 5 amps = 100 Watts of power. The amplifier needs to be able to deliver 100 Watts of power to maintain the 5 Amp current flow demanded by the 20Volts across the speaker.
What happens with a 2 Ohm speaker? Using same example as above:
8Volts/2 Ohms = 4Amps. The speaker is drawing 4 amps of current out of the amplifiter.
8 Volts x 4 Amps = 32 Watts.
Turn up volume control:
20Volts / 2Ohms = 10 Amps
20Volts x 10Amps = 200Watts.
The same amplifier, going into a 2 ohm load, is drawing twice the power out of the amplifier that the 4 ohm load was, at the same voltage levels across the speaker.This is why most amplifiers that are capable of driving both 2 Ohm and 4 Ohm loads are usually always rated for double the power at the lower 2 Ohm load.
In essence what that rating is saying is that the amplifier can maintain the same voltage levels across the speaker because it can provide the necessary doubling of current when going into a 2 Ohm load.'
What happens if the volume level you set results in a voltage across the speaker that the amplifier cannot maintain the necessary amount of current? For example, to maintain the 20Volt level, a 4 ohm speaker is drawing 5 amps of current. If the amplifier is only rated to 50W at 4 Ohms, it cannot deliver 5 amps at 20V (equiv to 100W). Both the voltage and current signals will start to clip, creating a really ugly, distorted signal to the speaker that will cause it to overheat. Result if left at this level for any period of time? Blown speakers and blown output stage in the amplifier.
How does all this relate to the power rating of the speaker? The power rating of the speaker is just a reflecton of the amount of heat the voicecoil assembly can dissipate. The bigger the voicecoil, and the bigger the magnet around the voicecoil (it acts as a heat sink), the more heat it can dissipate. The power rating of a speaker is typically the CONTINUOUS CLEAN power it can handle. IE. a speaker rated at 60W can handle 12V at 5amps (as an example) forever, as long as it's clean (undistored sine wave).
How many of you listen to a continuous test tone? I don't. The music I listen is composed of notes that vary immensely both in strength and duration. A momentary spike equivalent to 100W of power won't blow the 60W speaker, if it's clean and undistorted signal. You're not playing that 100W signal continously.
Synopsis: If you can afford it, always get the amplifier capable of delivering tremendous amounts of current when needed. This will prevent cliipping and distortion, which is what blows speakers and output stages. Because car amps dont' tell you how much current they can actually deliver, you have to go by the power rating.
You do not need to worry about matching the power rating of the speaker to the amplifier unless you're competing and doing SPL tests with contiuous test tones. Persoally I would err on the side of caution and get an amplfiier more powerful than the rating of the speaker, if I could afford it.
Anecdote:
When I owned a high end stereo store (2 decades ago now) we'd take small bookshelf speakers (PSBs if I remember correctly, about 12" tall) and I'd hook them up to a 200W per channel Krell amplifier. You would not believe how good those speakers sounded, and how loud they could play, when driven by an amplifier that could effortless deliver 200W of power. The Krells were rated to be able to deliver momentary bursts of current up to 50Amps. Power rating of the PSBs? About 60W. Did we ever blow the speakers, driving them with a 200W amp? Never. But boy, did we have to do repairs for people who bought the small PSBs and tried to drive them with a 20W amp. They'd try to play them loud, beyond the capabilities of the amp, and blow something (usally the amp). Yet these customers would always wonder why, when they were only using a 20W amp and the speakers could "handle" 60W.
