The nominal power rating on the data sheet of a loudspeaker is not the acoustical power delivered by the loudspeaker, but the maximal undistorted sinus power of the amplifier which drives the speaker with the waveform defined in the standard. Since the efficiency of most of the drivers used in home audio loudspeakers is only 0.2-2%, in case of dynamic speakers the majority of the power fed by the amplifier only heats the voice coil of the speakers (except for mechanical and eddy current losses). If too high a power is applied to a speaker, the voice coil may get so hot that it comes away from the carrier and so it is damaged. However it may be that the voice coil excursion gets too high even at a lower power and the voice coil hits against the inner structure of the speaker and it peels off of the cone due to the serious forces coming into play. So there are two limiting factors for the applied input power: a “temperature limit” and a “mechanical limit” (or “excursion limit”).
It is customary to interpret the power rating specified by the manufacturer that it is the “recommended maximal amplifier power”, which may be used to drive the loudspeaker without damage. This interpretation seems to deal with the problem of both the temperature limit and the mechanical limit, but it is only true as long as the amplifier in not overdriven into clipping. Unfortunately if a lower power amplifier is used which doesn’t have enough supply voltage to deliver the required signal in its integrity and is therefore overdriven, then this clipping increases the RMS relative to peak voltage of the signal. Therefore a lower power amplifier may deliver comparable power to a higher power amplifier (at least at musical signals with high crest factor), which could have amplified the required signal without clipping. Moreover as the clipping increases the high frequency content in the output signal, an undersized power amplifier in clipping may damage the tweeter more “efficiently” than a higher power amplifier!
So we don’t have a cushy time. The IEC 60268-5(2003)/EN 60268-5 standard includes specification for “rated noise power”. But this is not the only standard in use. Making use of this chaos, manufacturers like to present a showy high value for the power rating of their loudspeakers, it may well be that they specify the result of some non-standard measurement method, one that has an “easy” waveform (e.g. the peak power of short burst signals with long breaks) differing from the pink noise with a defined crest factor and defined frequency distribution appearing in the standards. We have to conclude that what the different manufacturers simply call “rated power” may not be comparable. Therefore it is expedient to inquire from the manufacturer, what exactly covered by their “rated power,” and by what standard it is measured. The next figure shows the frequency distribution of the noise signals of the most frequently used standards for measuring rated power: the IEC 60268-5 (previously 268-5), the AES2-2012, the EIA RS-426-A, and the EIA RS-426-B. You can see the significant differences between them, which makes comparing impossible.
Further information on different standards of loudspeaker rated power: http://www.doctorproaudio.com/doctor/temas/powerhandling.htm
What is the minimal loudspeaker rated power for home listening? The rated power must be evaluated together with sensitivity and impedance, and it depends on room size, and even the listener distance from the loudspeaker. A larger room generally requires a higher loudspeaker power for the same perception of loudness, when one listens to the loudspeaker from afar. Sensitivity is generally measured with a 2.83 V RMS voltage, which imposes 1 W on a 8 Ω loudspeaker, 1.5 W on a 6 Ω loudspeaker, and 2 W on a 4 Ω loudspeaker. Therefore if different impedance loudspeakers are to be compared “power proportionately,” relative to a 8 Ω loudspeaker, the sensitivity of a 6 Ω loudspeaker must be decreased by 1.8 dB, while the sensitivity of a 4 Ω loudspeaker must be decreased by 3 dB. In this way the sensitivity will be given in dB/W. After applying this correction, any loudspeaker with a 3 dB higher sensitivity needs half the electrical input power for achieving the same sound pressure (loudness). It works vice versa: a loudspeaker with 3 dB less sensitivity needs double electrical power to achieve the same loudness.
Taking the rated power of the IEC 60268-5(2003) standard for the basis, the normal listening demands in a 20-25 m2 room containing a usual amount of sound absorbing appointments and assuming a 87 dB SPL/ 1 W sensitivity loudspeaker listened to at a distance of 2 m is more than adequately served by an only 10…20 W rated loudspeaker for almost any kind of music. Naturally, a loudspeaker with 3 dB higher sensitivity (90 dB/W) needs only half of this power, that is 5…10 W for the same max. loudness, and a loudspeaker with 3 dB less sensitivity (84 dB/W) will need 20…40 W. But beware! If the bass or treble needs to be boosted with the tone control, then a 3 dB boost may call for up to two-fold increase, a 6 dB boost may call for up to four-fold increase of the rated power, depending on the actual construction of the loudspeaker!
Decibel (sensitivity) Firstly, we may talk about what a decibel is. When talking about power or intensity measurements, then the measured signal level (P1) as compared to an agreed base level (P0) can be expressed in decibel as follows: