## 3 Equivalent Circuit of Microspeker  
ECM was used to model the microspeker when it was placed on an infinite baffle in an anechoic chamber for measurement of frequency response. The detailed ECM of microspeker is given in our earlier work [31], with the front chamber and holes in the front chamber after the diaphragm and towards the front. In brief, for quick reference, the ECM of the present microspeker is discussed in brief as follows (Figure 2). The excitation voltage \(e_{g}\) is input to the microspeker that generates Lorentz force for driving VC. The product \(Bl\) is the force factor. The blocked electrical impedance of the VC is represented by \(Z_{eb}\) and the mechanical impedance of diaphragm by \(Z_{m}\). The VC drives diaphragm with velocity \(v\). The ECM circuit, which is shown in Figure 2, includes two transformers, one towards the front side of the microspeker's diaphragm and the other towards the rear side of the diaphragm. The area of the diaphragm is given by \(A\), which acts as a transduction factor between the mechanical domain of the diaphragm and the associate acoustic domains. As shown in Figure 3, the microspeker was placed on an infinite barrier; hence, only the sound waves that radiated in the forward direction were received by the microphone, located 10 cm from the microspeker center. This radiated sound was represented by acoustic resistance (\(R_{a-rad1}\)) and acoustic mass (\(m_{a-md1}\)), collectively giving the radiation impedance (\(Z_{a-md1}\)). The sound waves that radiated backward were propagated through the volume of the rear chamber, which was responsible for the cavity compliance impedance \(Z_{\text{Car1}}\). Additionally, it was estimated that the cavity behind the diaphragm is thin, resulting in a slit-like effect due to acoustic resistance \(R_{\text{qg}}\) and acoustic mass \(m_{\text{qg}}\), leading to impedance \(Z_{\text{qg}}\). Collectively, this cavity was expressed with a T-circuit. Similarly, the sound propagating through the rear openings (vents) of the microspeker demonstrated acoustic resistance \(R_{ab1}\) and acoustic mass \(m_{ab1}\), collectively giving acoustic impedance \(Z_{ab1}\), which was expressed with a T-circuit. The effect of cavity volume of the rear openings was taken care of by the cavity compliance impedance (\(Z_{\text{Cah1}}\)) in the T-circuit. A ventilation \begin{table}
\begin{tabular}{c c c} \hline
**Particulars** & **Details** & **Remark** \\ \hline Impedance & 32 ohm \(\pm\) 20\% & At 1 kHz \\ \hline Sensitivity & 110 \(\pm\) 3 dB & At 1 kHz with V (input) \\  & & 5 mW (400 mV/B\&K 4185) \\ \hline Total Harmonic Distortion (THD) & 100 Hz–200 Hz \textless{} 25\% and 400 Hz–10 kHz \textless{} 15\% \\ \hline Fundamental Frequency (fo) & 80 Hz \(\pm\) 20 Hz \\ \hline Rated Input & 5 mW \\ \hline Maximum Input & 20 mW \\ \hline \end{tabular}
\end{table} 
Table 1: Specification of microspeker DSH 742–005. Figure 2: Microspeker equivalent circuit diagram. cloth covered the rear openings, and their combined acoustic impedance was estimated as \(Z_{ap1}\). Finally, sound that radiated to the external environment was represented as a parallel combination of acoustic radiation resistance \(R_{a\text{-}rad2}\) and acoustic radiation mass \(m_{a\text{-}rad2}\) resulting in acoustic radiation impedance \(Z_{a\text{-}rad2}\). Our group has reported detailed ECM modeling of loudspeaker (woofer) [32], microsp-peaker [31], earphones [33,34], and headphone [35]. The mathematical formulation used in the ECM of the current microsp-peaker is given below in brief:  
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