In recent years, the trend toward thinner and smaller mobile phones and other mobile instruments has been accelerating; and in addition, the need for thinner and smaller parts mounted in audio-visual instruments and the like has also been increasing.
As a method for driving a loudspeaker that plays a ring tone or a music signal with a mobile phone and the like, an electrodynamic type has been used conventionally. However, when the electrodynamic type is used, due to the structure thereof, it is inherently difficult to reduce the thickness of a loudspeaker; and when reduction in thickness is attempted, there are problems such as: deterioration of low-frequency sound pressure, necessity for measures to prevent magnetic leak due to the use of a magnetic circuit, and the like.
On the other hand, a piezoelectric type loudspeaker, which has been widely used in an electric appliance or an information instrument for playing audios, is attracting attention as a driving method suited for reducing thickness; and there are increasing number of examples in which the piezoelectric type loudspeaker is mounted in a mobile phone or a small size information terminal.
Conventionally, the piezoelectric type loudspeaker is known as an acoustic transducer that uses a piezoelectric material as an electric acoustic transduction element, and is used as acoustics outputting means for small size instruments (e.g., refer patent document 1).
The piezoelectric type loudspeaker has a configuration in which a piezoelectric element is bonded to a metal plate or the like. For this reason, it is easy to reduce the thickness of the piezoelectric type loudspeaker compared to an electrodynamic loudspeaker that requires a magnet and a voice coil, thereby the piezoelectric type loudspeaker has an advantage of not requiring measures to prevent magnetic leak.
When using the piezoelectric type loudspeaker for playing an audio, it is necessary to pay attention to the following properties.
First, while a speed of a diaphragm is proportional to a voltage in the electrodynamic loudspeaker, in the piezoelectric type loudspeaker, a diaphragm displacement is basically proportional to the voltage. Therefore, the characteristic of the sound pressure of the piezoelectric type loudspeaker during constant voltage driving is a characteristic in which sound pressure level increases as the frequency increases (a characteristic of a constant increase). This sound pressure characteristic is different from a flat frequency characteristic that is generally required for a loudspeaker.
Second, while an electric impedance of the electrodynamic loudspeaker may be considered to be nearly at a constant value regardless of the frequency, an electric impedance of the piezoelectric type loudspeaker reduces inverse proportionally to the frequency, since a piezoelectric element part operates as a capacitor. For this reason, the piezoelectric type loudspeaker has a danger of short-circuiting due to an overcurrent at a high frequency range.
In order to deal with the above described properties, when designing the piezoelectric type loudspeaker, an electrical resistance is normally connected to the piezoelectric element in series to form an RC circuit, allowing an electric current to flow through an electrically resistive part at the high frequency range. This enables suppression of a high frequency range input to the piezoelectric loudspeaker, which is also a capacitor; and allows obtaining desired sound pressure characteristic and electric characteristic.
FIG. 25 is a figure with measurement data showing a sound pressure suppressing effect at the high frequency range when the electrical resistance is connected to the piezoelectric element in series with a bimorph type piezoelectric loudspeaker. The capacitance per one surface of the bimorph type piezoelectric loudspeaker is 210 nF; and the electrical resistance, which is connected to the piezoelectric element in series, is 220Ω. As obvious from FIG. 25, when compared to a case of having only the piezoelectric element itself (a case without the electrical resistance), a case where the electrical resistance is connected to the piezoelectric element in series (a case with the electrical resistance) shows increased electric impedance at the high frequency range, and flat sound pressure characteristic.
Furthermore, the piezoelectric type loudspeaker causes performance deterioration of the piezoelectric material due to reduction in the capacitance by a pyroelectric effect, when used at a condition where temperature shifts extensively. In order to avoid this, there are conventional arts in which an electrical resistance is parallelly connected to the piezoelectric type loudspeaker (e.g., refer patent document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-230193    Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-275190