In the preview section, the electro-mechano-acoustical analogous circuit and the concept of electroacoustics are discussed. In acoustical system for loudspeaker, something must be cared is ZAB andZAF. The ZAB and ZAF model the acoustical impedance on the front and rear of the diaphragm. The example of the mobile phone is shown in Fig 24. The ZAB and ZAF are the complex-loading conditions. From the Fig 24, the structure of the mobile phone can clearly been realized. The structure in the rear and front of the speaker that causes some effects on the sound field.
These structures of acoustical properties will be lumped to the analogous circuit in the acoustic system. These analogy circuits are composed of the acoustic components like duct, slit, port or cavity, etc.
The front of the diaphragm is covered by a method perforated cover. The acoustic impedance of the mesh screen can be modeled by a series of acoustic mass and resistance. The value of the acoustic mass and resistance can be calculated by the Eqs. 39, 40 and 41. We can image that the volume velocity emitted by the diaphragm flows through the mesh screen. When the speaker is placed in mobile phone, there is a small space in the front of speaker. The acoustic impedance of the small space can be model an acoustical compliance and it can be calculated by Eqs.
27. So the volume velocity will compress the air in the small space, then flow through the port that is placed on the side of small space. The port exhibits an acoustic resistance and mass. The acoustic impedance of the port can be calculated by Eq 42.
The volume velocity flows through the port and flows into the duct. In order to model the duct, lumped parameter oscillator model and the transmission line model can be used to model the dynamic response of the duct. But lumped parameter oscillator model is used here to simulate. The analogous circuit of lumped parameter oscillator model is shown in Fig. 13. In the export of the duct, something can be found that the area of the export of the duct is less than the cross-sectional area of the duct. When the volume velocity flows out the export of the duct, the air will be compressed in the export of the duct.
Thus the dynamic behavior is like an acoustic compliance. After the volume velocity flows out the export of the duct, it will flow through the port and radiates the sound. The port exhibits an acoustic resistance and mass. The acoustic impedance of the port can be calculated by Eqs. 42. Final, there is the radiation impedance between the export of the port and the medium. The analogous circuit of the radiation impedance is shown in Fig 6. The radiation impedance is the radiation impedance of the piston in a tube. Its analogous parameters can be calculated by Eqs. 35, 36, 37 and 38.
The rear of the diaphragm is like a closed box and there are two ports embedded in the rear of speaker. The closed box can be modeled by an acoustic compliance.
The parameters of the acoustic compliance can be calculated by Eqs. 27. The port can be modeled by a series of acoustic mass and resistance. The back volume velocity emitted by the diaphragm compresses the air in the box and flows through the port. The rear of the speaker is like a box that the acoustic impedance is modeled by an acoustical compliance and its analogous parameters can be calculated by Eqs. 27.
The analogous circuit of the electro-mechano-acoustical analogous circuit of miniature speaker placed in mobile phone is shown in Fig. 25. The structure of the
in Fig. 26. (b). The structure of the rear of the speaker is shown in Fig. 27 (a). The analogous circuit of ZAB is shown in Fig. 27 (b). And the circuit elements are defined below:
RAF and MAF : the acoustic impedance of the frame. We can approximately compute the values by the Eqs. (39), (40) and (41).
CAF : the acoustic compliance between the frame and the diaphragm.
We can compute the value by the Eqs. (27)
CFB1 : the acoustic compliance in the front of the speaker. It is like a box. We can compute the value by the Eqs. (27)
RAP1 and MAP1 : the acoustic impedance of the port around CFB1. We can approximately compute the value by the Eqs. (42)
MAL1 : the acoustic mass of the duct. We can compute the value by the Eqs. (29).
MAL2 and CAL2 : Because of the natural resonance of the duct, we can model that by Oscillator Model of a Duct, and compute the values by Eqs. (52) and (53).
RAP2 and MAP2 : the acoustic impedance of the port in the exportation of the duct. We can approximately compute the value by the Eqs.
(42).
CAL : the acoustic compliance in the exportation of the duct. We can approximately compute the values by Eqs. (29)
M1, R1, R2 and C1: the element of the radiation impedance. We can approximately compute the values by Eqs. (35), (36), (37) and (38).
RBP1 and MBP1 : the acoustic impedance of the port embedded in the rear of the
speaker. We can approximately compute the value by the Eqs.
RBP2 and MBP2: the acoustic impedance of the port embedded in the cavity. We can approximately compute the value by the Eqs. (42).
Fig. 28. is the impedance response comparison of speaker and speaker placed in mobile phone and the impedance response is the experiment results. The first resonant frequency of the speaker placed in mobile phone is lower than speaker that is not placed in mobile phone. This is because the acoustic mass of ZAF and ZAB reflected to the electrical system cause the total mass increasingly. Thus the first resonance frequency is shifted to low frequency. This effect can be realize from the below equation.
From the Fig 28, there is a resonance at about 3.5k Hz. The resonance is caused by the compliance of the small space in the front of the speaker and the mass of the duct and the port. The resonance at about 6.5 kHz is caused by the compliance of box in the rear of the diaphragm and the mass of the port.
Figure 29 is the frequency response comparison of speaker and speaker placed in mobile phone. The resonance at about 17k Hz is caused by the natural resonance of
The Pspice is used to construct the electro-mechano-acoustical analogous circuit according to the analysis of the ZAB and ZAF by using the theory of the electroacoustics and do the simulation of impedance and frequency response.
Figure 30 and 31 are the simulation results of the impedance and sound pressure frequency response. The simulation results can approximately fit the experiment results.
Ear-coupling Simulation
In this section, the B&K products of Wideband Ear Simulator for Telephonometry Type 4195 is taken to analyze realistic telephone receive response measurements. The two grades of well-defined leakage make it possible to simulate late the average real loss for telephone handsets which are held either comfortably tight (low-leak pinna) or lossely (high leak pinna) against the human ear.
This product provides the equivalent circuit to model low-leak and high-leak conditions. The equivalent circuit can be used in the frequency range 100 Hz to 8k Hz. The equivalent circuit diagram is shown in Fig. 32. The acoustic impedance simulation results are shown in Fig. 33.. The mobile phone coupled with low-leak and high-leak Pinna simulation results are shown in Fig. 34..
3. Bass-enhanced design for the miniature speakers
In this section, the theories of vented-box and passive-radiator design method are reviewed. Using the concept of the theory, the enclosure of the speaker can be designed to enhance the bass response of the speaker.