Clamp Power, Real Power, And Reactive Loads

Definition: a clamp-power result is meaningful only when its voltage, current, load, phase, waveform, bandwidth, timing and averaging method are known.

Known Resistor

For a measured resistive load and a suitable waveform measurement:

P = Vrms² / R
I = Vrms / R
P = Vrms × Irms

Use the assembled load's hot resistance, not just its nominal label, and verify that voltage and current remain in phase to the required accuracy.

Reactive Or Nonlinear Load

p(t) = v(t) × i(t)
Pavg = average of p(t) over the declared interval
S = Vrms × Irms

S is apparent power. It equals real power only under the appropriate voltage-current relationship. A loudspeaker's magnitude and phase change with frequency, level, temperature, enclosure, and motion, so nominal impedance does not complete the calculation.

Do Not Multiply Unrelated Peaks

Peak voltage and peak current may occur at different times. Multiplying the largest value from each trace creates a number that may never have existed as instantaneous power.

Dynamic Music And Meter Windows

Two handheld meters can use different update rates, crest-factor capability, bandwidth and averaging windows. On dynamic music, their displayed voltage and current may describe different moments. Use synchronized waveform capture or a power analyzer designed for the required waveform and bandwidth.

What To Report

resistive-load watts, time-domain real watts, or apparent volt-amperes
load and its temperature or impedance condition
waveform, frequency or program, crest factor and duration
voltage and current sensor ranges, bandwidth, filtering and deskew
supply voltage, clipping, limiter, compression, protection and temperature state
averaging interval and number of repetitions

What The Number Does Not Prove

A higher clamped watt result does not by itself prove more acoustic output, lower distortion, safe excursion, safe voice-coil temperature, safe amplifier load, adequate electrical supply, or repeatable sustained performance.