Due to mobility requirements and dimension restrictions, a mobile device (e.g., a mobile phone, a smart phone, etc.) typically comprises one or more small-size or low-cost loudspeakers. Sound quality for audio and speech signals used in mobile devices therefore has been severely limited by not being able to produce enough loudness without introducing damage to the loudspeaker(s), as compared to non-mobile or high-end loudspeaker systems. The widespread popularity of smart phones and of multimedia-intensive mobile applications has triggered demand for better audio quality for mobile devices. Several approaches have been used to achieve better audio sound quality with enough loudness. For example, automatic gain control (AGC) and/or automatic volume control (AVC) have been widely implemented to ease the existing audio quality problem to some extent for mobile devices.
The small loudspeaker in a mobile device can work in a linear mode for small signals, but its linearity would be no longer valid for large signals with high compression. A signal low enough in frequency and/or large enough in level may cause excessive movement of the loudspeaker diaphragm.
Excursion refers to the distance that a diaphragm in a loudspeaker may travel from its resting position. Signals low enough in frequency and/or large enough in level may cause excessive movement of the diaphragm of the loudspeaker in a mobile device. When the loudspeaker is driven by such a high power level signal, the diaphragm movement (i.e., the excursion) consistently exceeds its excursion limit, which leads to poor sound and an unpleasant audio experience for the listener. More particularly, in such a case, the voice coil tends to exit the gap, resulting in the coil rubbing and possibly reaching a break-up mode of the voice coil displacement.
Known prior art diaphragm excursion control techniques use a high-pass or a notch filter to suppress the low frequency contents around the resonance frequency that may cause excessive diaphragm movement. Due to the lack of low frequencies and loss of loudness, these approaches often render an unnatural and tinny sound. Moreover, because the low frequencies in the loudspeaker signal are always filtered out, the unpleasant experience for the listener persists even when the signal is small enough to stay in the loudspeaker's linear range.