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Thermal analysis and heat transfer

Characteristics:

KLIPPEL R&D System

Increase of voice coil temperature ΔTv

LSI, SIM, PWT, DIS

Increase of magnet temperature ΔTm

LSI, SIM

Bypass factor

SIM

Power Pcoil transferred to coil

SIM

Power Pcon transferred by convection cooling

SIM

Power Peg transferred by eddy currents

SIM

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 %.

KLIPPEL R&D SYSTEM (development)

Module

Comment

Large Signal Identification (LSI Woofer, LSI Tweeter, LSI Box)

LSI measures the thermal parameters automatically by performing a special thermal identification procedure using different kind of stimuli interrupted by ON/OFF-cycles. During the measurement the input power is controlled to keep the voice coil temperature in a permissible range.

Power Test (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 Module (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.

Large Signal 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.

Increase of voice coil temperature The figure above 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 %.
The figure above 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 fa

Templates of KLIPPEL products

Name of the Template

Application

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

PWT Powtest SWEEP

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)

Standards:

  • IEC Standard IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers
  • AES2-1984 AES Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement
  • CEA CEA-426-B Loudspeakers, Optimum Amplifier Power
  • EIA 426B Loudspeaker Power Rating Test CD provided by ALMA International


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.