Speaker Power Compression & Coil Temperature Calculator
Enter the cold and hot DC resistance of a loudspeaker voice coil to estimate coil temperature and the output lost to thermal power compression. The model also shows the compressed SPL and the mathematical power increase that would be needed to recover the loss.
Estimated voice-coil temperature (deg C)117.26
- Voice-coil temperature rise (deg C)
- 97.26
- Resistance increase
- 37.93%
- Current and motor-force remaining
- 72.5%
- Estimated thermal power compression (dB)
- 2.79
- Ideal cold SPL at 1 meter (dB)
- 121.99
- Estimated compressed SPL at 1 meter (dB)
- 119.2
- Acoustic-equivalent power after compression (W)
- 262.81
- Theoretical power needed to recover the loss (W)
- 951.25
Uses alpha = 0.0039 per degree C for copper.
Assumes ideal constant-voltage drive, unchanged force factor, and resistance-dominated behavior.
Parallel voice coils must be measured in the same wiring configuration for the cold and hot readings.
How to use this calculator
- Choose the voice-coil conductor listed by the driver manufacturer.
- Enter the cold DC resistance after the coil has stabilized at the reference temperature.
- Enter the hot DC resistance measured during operation or immediately after the test.
- Enter cold sensitivity and the input power used for the comparison.
- Read the temperature, compression, SPL, and recovery-power results with the notes below the tape.
How resistance turns into coil temperature
Copper and aluminum voice-coil resistance rises almost linearly over the temperature range normally used for loudspeaker thermal checks. If Rc is the cold DC resistance, Rh is the hot DC resistance, Tc is the cold reference temperature, and alpha is the conductor temperature coefficient, the relation is:
Rh = Rc × [1 + alpha × (Th − Tc)]
Solving for the hot coil temperature gives:
Th = Tc + [(Rh ÷ Rc) − 1] ÷ alpha
This calculator uses alpha = 0.0039 per degree C for copper and alpha = 0.0043 per degree C for aluminum. The resistance ratio is not rounded before the temperature, decibel, SPL, or power outputs are calculated.
How compression is estimated
The SPL estimate uses an ideal constant-voltage loudspeaker model. When the hot coil resistance is higher, current falls in proportion to Rc ÷ Rh. Under the simplified assumption that force factor and acoustic loading stay unchanged, that same fraction represents the remaining motor force.
The thermal compression is therefore a voltage-like level loss:
C = 20 × log10(Rh ÷ Rc)
The ideal cold output at 1 meter is:
Lcold = S + 10 × log10(P)
The compressed output is Lhot = Lcold − C. The acoustic-equivalent power after compression is Peq = P × (Rc ÷ Rh)^2, and the theoretical power to recover the loss is Precover = P × (Rh ÷ Rc)^2. That last value is a comparison only, not a safe amplifier or driver rating.
What moves the result most
The hot-to-cold resistance ratio dominates both the temperature estimate and the compression estimate. Cold reference temperature moves the absolute coil temperature, but not the temperature rise. The conductor coefficient also matters: for the same resistance rise, aluminum implies a slightly lower temperature rise than copper because its coefficient is higher.
What this leaves out
Real loudspeakers are not fixed resistors. Impedance changes with frequency, enclosures alter cooling, crossovers add resistance and heat, magnets lose flux with temperature, suspensions change, and nonlinear excursion effects can change measured SPL. A delayed hot reading also understates the peak because DC resistance starts falling as soon as the coil cools. Use four-wire measurement or carefully compensated test leads when small resistance changes matter.
Worked example
For a copper coil with Rc = 5.8 ohms at 20 degrees C and Rh = 8.0 ohms hot, the resistance ratio is 8.0 ÷ 5.8 = 1.37931. The inferred temperature rise is (1.37931 − 1) ÷ 0.0039 = 97.26 degrees C, so the estimated hot coil temperature is 117.26 degrees C.
With 95 dB sensitivity and 500 W, the ideal cold SPL is 95 + 10 × log10(500) = 121.99 dB at 1 meter. The compression is 20 × log10(8.0 ÷ 5.8) = 2.79 dB, so the compressed estimate is 119.20 dB. The model says the acoustic-equivalent power is about 262.81 W, while recovering the loss mathematically would require about 951.25 W.
Common questions
How can DC resistance reveal voice-coil temperature?
The resistance of copper and aluminum rises predictably with temperature. If you know the cold resistance at a known temperature and measure the hot resistance in the same wiring configuration, the resistance ratio gives the coil temperature rise. The method is most useful when lead resistance has been removed or measured with a four-wire setup.
Should hot resistance be measured during playback or immediately afterward?
During playback is best when the test system can do it safely because the coil begins cooling as soon as the signal stops. An immediate post-test reading can still be useful, but it usually understates the peak temperature and compression. Keep the measurement timing consistent when comparing drivers or test levels.
Why is thermal compression calculated with 20 log instead of 10 log?
The simplified model starts with constant voltage. Hot resistance reduces current and motor force by the ratio Rc divided by Rh, and force behaves like an amplitude quantity in this approximation. Amplitude ratios are converted to decibels with 20 log10; power ratios use 10 log10.
What coefficient should copper-clad aluminum use?
Copper-clad aluminum wire does not have one universal coefficient because the effective value depends on the aluminum core, copper cladding thickness, and current distribution. If the manufacturer publishes a coefficient, use that outside this calculator for the most accurate work. Without that data, run both copper and aluminum settings to bracket the likely temperature estimate.
Can extra amplifier power safely overcome thermal compression?
The recovery-power result is only the mathematical power increase needed to restore the lost level in the ideal model. It is not a power-handling recommendation. Extra power can push the coil, adhesive, former, suspension, or cone beyond safe thermal or mechanical limits.
Why can measured SPL compression differ from this estimate?
The resistance estimate isolates one thermal mechanism. Measured SPL can also change because impedance is frequency-dependent, force factor can fall with heat, the magnet and suspension can shift, and the enclosure may cool the coil differently at different signals. Microphone position, bandwidth, and test signal crest factor also affect the measured result.