A control device for preventing mechanical damage to a dynamic speaker is proposed in U.S. Patent Application Publication No. 2012/0328113.
Mechanical defects of a small scale speaker such as that used in mobile devices occur when the temperature of a coil rises due to an excessive current flowing in the speaker coil and as a result exceeds the heat resistance temperature of the insulation material of the coil wire. Air which is filled in a narrow gap between a magnet wrapped around the coil and the coil itself is used as a medium for dissipating the heat of the coil which is generated towards the magnet. In addition, heat in the air of the narrow gap between the magnet which wraps the coil ad the coil itself is also dissipated by the flow of air generated by vibration of the speaker diaphragm.
The flow of air from the speaker diaphragm stops in a state where vibration of the speaker diaphragm for some reasons is suppressed. As a result, since the heat dissipated effects of the heat generated in a coil decrease, mechanical damage to the speaker can easily occur. In particular, it is necessary to pass a large current when attempting to obtain a large volume by vigorously amplifying a diaphragm. At this time, since the temperature of a speaker coil increases rapidly in a state where vibration (amplitude) of the diaphragm is repressed by an external force, mechanical damage can easily occur.
The speaker aperture in a small scale speaker such as that used in mobile devices is small. Therefore, the aperture part is often blocked by a finger of palm of a hand. Since vibration compliance changes depending on the state of the aperture part, the vibration of the diaphragm is sometimes suppressed. Therefore, it is necessary to prevent mechanical damage to a speaker by feedback control of a current flowing in a coil by dynamically detecting this type of state.
In FIG. 3 of U.S. Patent Application Publication No. 2012/0328113, a method is disclosed in which a serial resistor (34) is inserted between an amp (32) which drives a speaker and the speaker (36), the voltage and current of the serial resistor end is measured, the admittance (impedance) of the speaker is dynamically measured and the amplitude of an input signal is controlled based on the measurement results
In this way, a means of feedback control of speaker vibration by inserting a serial resistor between an amp which drives a speaker and the speaker and measuring the current flowing through a speaker is also proposed in U.S. Patent Application Publication No. 2013/0077796, U.S. Patent Application Publication No. 2004/0086140 and U.S. Pat. No. 7,436,967. In addition, technologies such as those disclosed in U.S. Patent Application Publication No. 2012/0020488 and IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, NO. 8, August 2005 have also been proposed.