Power is not loudness
Electrical power is measured in watts; acoustic output is measured in decibels. A 100 W amplifier is not twice as loud as a 50 W amplifier. With the same loudspeaker, doubling power adds 3 dB in theory. A 10 dB increase—often perceived as roughly twice as loud—requires ten times the power before compression and room effects are considered.
This is why loudspeaker sensitivity and distance are essential. A sensitive cinema loudspeaker may reach the target with modest power, while a low-sensitivity design several metres away may need far more voltage and current.
Voltage, current and impedance
Voltage swing largely determines the maximum unclipped level into a given impedance. Current demand rises as impedance falls: at the same output voltage, a 4 Ω load demands twice the current of an 8 Ω load. Real loudspeakers vary with frequency and can present phase angles that make the load harder than the nominal number suggests.
A useful amplifier specification therefore states power into 8 Ω and 4 Ω, the frequency range, distortion limit, channel count and test conditions. Protection behaviour and thermal capacity matter when the load is difficult or many channels work together.
Headroom for cinema peaks
Film sound is dynamic. The average power during a scene can be modest while an impact or musical peak briefly asks for much more. Headroom is the gap between the normal operating level and clipping. Three decibels of electrical headroom requires twice the power; 6 dB requires four times the power.
Reserve should be useful rather than reckless. Loudspeakers still have thermal and excursion limits. Correct high-pass filters, crossover settings and limiters can be as important as amplifier size.
Compare ratings honestly
Do not compare a one-channel, 1 kHz figure with a full-band, all-channels-driven result. Check impedance, bandwidth, distortion, duration, mains voltage and whether the number is continuous or a short burst. Dynamic ratings can be valuable for cinema, but they answer a different question from sustained output.
Specification checklist
Before you decide
- Calculate from sensitivity, distance and target level.
- Allow realistic peak headroom.
- Use the loudspeaker’s real impedance information where possible.
- Compare like-for-like test conditions.
- Confirm cooling, circuit capacity and protection behaviour.
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