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Using Phase‐0 Frequency For Loudspeaker Voice Coil Voltage and Current Measurement Disclosure Number: IPCOM000245644D
Publication Date: 2016-Mar-23

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Using Phase‐0 Frequency

Loudspeaker Voice Coil

Voltage and Current Measurement


A loudspeaker design for mobile application is highly restricted in physical size due to the nature of hardware requirements. However, there is an increase demand to play back louder sound.

This meant that a loudspeaker amplifier have to deliver higher power. Serious concern due to applying high power to the loudspeaker that is to increase the voice coil temperature, and an excess amount of membrane excursion. Both could lead to damage the loudspeaker prematurely. In order to avoid these problems, various speaker protection methods were developed.

Typical speaker protection method is to monitor both voice coil temperature and amount of membrane excursion in real time, and using these data to control and modified input signal of the loudspeaker amplifier. This is a type of feedback system rely on accuracy of the voice coil temperature and membrane excursion data.

This paper focuses and explains new method to measure a voice coil temperature.

Loudspeaker's electrical impedance characteristics are well understood for various acoustical models. Typically, there is a parallel resonance at mid‐low frequency range, in the range of several hundred Hz, and the maximum impedance is approximately a few times as much as of the DC resistance at the loudspeaker's (parallel) self‐resonance frequency. At high frequency range (10 ~ 20kHz), the impedance will gradually increase due to the voice coil inductance.

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Figure 1 show a method to obtain both voice coil voltage (V) and current (I). I is obtained in the form of voltage across reference resistor Rref. Therefore I is Ispk = Vref / Rref. Once the resistance R is determined, the voice coil temperature can be derived by equation (1) below.

∆T = ∆R /(R0 * α) + T0 … (1).

∆T = T - T0, ∆R = R ‐ R0
α is a temperature co‐efficient of the voice coil material.

R0 is the resistance at the reference temperature T0.

R is a measured resistance at temperature T.

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Figure 1 ‐ Loudspeaker Impedance

1. Measuring Voice Coil Voltage (V) and Current (I) Using DC

A device exhibit complex behavior electrically as well as acoustically such as a loudspeaker, the determining the voice coil voltage (V) and current (I) during its operation would benefit for analyzing the device's behavior.

A method of applying small but sufficient amount of DC current to the voice coil during audio playback is described by M. Buck (*). However, this method adds membrane excursion offset to the voice coil that may not be desirable especially for small loudspeaker commonly used in mobile application.

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(*) AES Paper, 1999 106th Convention, Munich, Germany

2. IV Measurement Using AC Signal

Using AC signal to measure the I and the V are more desirable in many aspects, such as to add no membrane offset by the AC signal, and be able to measure frequency dependent behavior of the loudspeaker li...