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Thermal Analysis and Heat Transfer



Increase of voice coil temperature ΔTv 


Increase of magnet temperature ΔTm 


Bypass factor 


Power Pcoil transferred to coil


Power Pcon transferred by convection cooling


Power Peg transferred by eddy currents


The power flow and heat transfer of the loudspeaker can be modeled by the thermal equivalent circuit shown below. The maximal electrical power which a transducer can handle depends on the following factors:

  1. maximal temperature Tv which the voice coil, voice coil former and glue can handle for some time,

  2. low thermal resistances Rtv, Rtc(v), Rtm which determine the heat flow to the ambience,

  3. high thermal capacities Ctv and Ctm which determine the time constants of the heating process

  4. a high value of power Peg generated by eddy currents in the pole tips bypassing the voice coil,

  5. high velocity v of the voice coil which determines the forced air convection cooling in the resistance Rtc(v).

The air convection cooling represented by Rtc(v) and the direct heat transfer represented by the additional power source Peg contribute to the bypass factor describing the fraction of the input power which bypasses the critical voice coil resistance Rtv. A transducer with optimal thermal properties may have a bypass factor of 20 … 50 %.

Increase of voice coil temperature: The figure to the left shows the heat transfer of a woofer in a loudspeaker system with and without vent in the pole piece. The air below the dust cap will be ventilated through the open vent, and the convection cooling of the coil is low giving a low bypass factor (pink curve in right diagram). After sealing the vent (shown in the left sectional view), the air is pressed through the air gap, and the high velocity of the air particles increases the bypass factor to 50 %.

KLIPPEL R&D SYSTEM (development)



Power Testing (PWT)

PWT measures voice coil temperature, displacement and input power using stimuli generated by the internal generator or provided by an external source.

3D Distortion Measurement (DIS)

DIS provides a special measurement (pilot tone at 130 Hz) which estimates the voice coil temperature at sufficient accuracy to protect the transducer under test. DIS module uses the same two-stimulus as SIM module and can be used to verify the predicted behavior by measurements.

Simulation (SIM)

SIM module can predict the large signal behavior of the transducer by using linear, nonlinear and thermal parameters identified by LSI and PWT and imported into SIM. The temperature of voice coil and magnet as well as the power flow within the thermal model are calculated. The bypass factor reveals the effect of forced convection cooling and direct heat transfer.

Simulation-Auralization (SIM-AUR)

SIM-AUR predicts the large signal behaviour of the transducer using linear, nonlinear and thermal parameters. Those parameters can be imported from PWT or SIM. The temperature of the voice coil, pole plates and magnet, as well as their corresponding power flows within the thermal model are calculated using a dynamic model. The behaviour of the internal states can be inspected for any state signal, to gain insight in forced convection cooling and heat transfer effects.

Live Audio Analyzer (LAA)

LAA measures the voice coil temperature, displacement and input power, using the stimulus generated by the internal generator, or by using a dedicated user defined wave file for the measurement. 

Templates of KLIPPEL products

Name of the Template


Thermal Parameters (woofer)

Analysis of heat transfer in woofers based on identified thermal woofer parameters

Thermal Parameters AN 18

Thermal Parameters measured by using PWT module according Application Note 18

Thermal Parameters AN 19

Thermal Parameters measured by using PWT module according Application Note 19

LSI Woofer Nonl.+Therm. Sp1

Nonlinear and thermal parameters of woofers with fs < 300 Hz at standard current sensor Sp1

DIS Compression Out(in)

Output amplitude versus input amplitude at four frequencies

SIM Compression Out(In)

Output amplitude versus input amplitude at four frequencies using large signal parameters imported from LSI; Simulated results are comparable with DIS Compression Out(In).

SIM Therm. Analysis (1 tone)

Heat transfer based on thermal parameters imported from LSI using a single-tone stimulus

SIM Therm. Analysis (2 tone)

Heat transfer based on thermal parameters imported from LSI using a two-tone stimulus

PWT 8 Woofers Param. ID Noise

Parameter identification of woofers using internal test signal (no cycling, no stepping)

PWT EIA accelerated life test

Accelerated life testing according EIA 426 B A. 4 using any external signal to monitor temperature, power and resistance

PWT IEC Long term Voltage

Power test to determine long-term maximal voltage according IEC 60268-5 paragraph 17.3 without parameter measurement for one device monitoring voltage, resistance, temperature and power


Power test for measuring the thermal time constant of the voice coil using sweep signal with low crest factor

PWT Powtest TIME Const.

Power test for measuring time constant of voice coil using internal test signal with cycling (ON/OFF phase)


Audio Engineering Society
AES2 Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement

Consumer Electronics Association
CEA-426-B Loudspeakers, Optimum Amplifier Power

European Telecommunications Standards Institute
EIA 426B Loudspeaker Power Rating Test CD provided by ALMA International

International Electrotechnical Commission
IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers

Papers and Preprints

Y. Shen, “Accelerated Power Test Analysis Based on Loudspeaker Life Distribution,” presented at the 124th Convention of Audio Eng. Soc., May 2008, Preprint 7345.

W. Klippel, “Nonlinear Modeling of the Heat Transfer in Loudspeakers,” J. of Audio Eng. Soc. 52, Volume 1, 2004 January.

C. Zuccatti, “Thermal Parameters and Power Ratings of Loudspeakers,” J. of Audio Eng. Soc., Volume 38, No. 1, 2, 1990 January/February.

K. M. Pedersen, “Thermal Overload Protection of High Frequency Loudspeakers,” Report of Final Year Dissertation at Salford University.

Henricksen, “Heat Transfer Mechanisms in Loudspeakers: Analysis, Measurement and Design,” J. of Audio Eng. Soc., Volume 35, No. 10, 1987 October.