From the last section, Thiele and Small parameters of the loudspeaker can be got by the procedures of measurement. In order to check these values are correct or not.
These values must be taken into electro-mechano-acoustical analogous circuit and the simulation results will be compared with the experiment results. The comparison results are shown in Fig. 15. The testing speaker is the products of Merry Company.
Its data number is DSH456. The type of loudspeaker is the miniature speaker.
From the Fig. 15., the simulation results can approximately fit the experiment results.
Thus Thiele and Small parameters of the miniature speaker can be used to model its dynamic response.
The simple diagram of miniature speaker is shown in Fig. 16. and the real object of miniature speaker is shown in Fig. 17. The basic features can be observed from the Fig. 18. and Fig.19. The voice coil is attached to the diaphragm. A frame is fixed above the diaphragm. Ports in the rear side of the speaker provide ventilation to the rear enclosure. Some damping materials are located on the ports. These features are different from the conventional loudspeaker. What do they cause some effects on the speaker? According to the experiment results, the obvious effects can be
observed from the impedance and pressure frequency response.
In order to analysis these effects, three cases are split and the experiments will be done individually. The three cases are listed below:
Case 1: the original speaker
Case 2: the speaker without the damping material
Case 3: the speaker without the damping material and the frame
Fig. 18. is the back of the miniature speaker and the view of case 1 and case 2. Fig.
19. is the front of the miniature speaker and the view of case 2 and case 3.
The impedance and sound pressure frequency response of the three cases are shown in Fig. 20. and Fig. 21. From the impedance response comparison, the amplitude of the first resonant frequency of the three cases is different with each other.
This phenomenon indicates that the damping material and the frame affect the amplitude of the first resonant frequency. And the damping material causes more effects than the frame. Why does the frame affect the amplitude of the first resonant frequency? There is the mesh material that is located above the frame in the Fig. 19.
It is also the damping material. Frame attached to the diaphragm are important factors that influence the radiated sound field. [5] presents numerical models of miniature loudspeakers in various conditions of frame and presents an analysis of the structure vibration of the diaphragm coupled with the radiated sound field. This paper pointed out that if there are more holes in the frame, the frame will not cause more effects on the sound pressure frequency response.
From the sound pressure frequency response in the Fig. 21., the amplitude of the first resonant frequency of three cases is different. This can illustrates the damping material could decrease the dB of the sound pressure level in the first resonant frequency. In 13k Hz, the sound pressure level of the case 3 is lower than the other
A resonant peak is found in the about 6~7k Hz from Fig. 20. The resonance happened in the 6~7 kHz is caused by the port embedded in the rear side of the speaker and the small space of the cavity in the back of the diaphragm. Fig. 22. is the experiment results that can prove the resonance caused by the port embedded in the rear side of the speaker and the small space of the cavity in the rear of the diaphragm. When the port embedded in the rear side of the speaker is closed, the peak in 6~7k Hz disappeared. And the first resonance frequency is shifted to the high frequency. The speaker is like to be mounted in the closed-box. Thus the stiffness of the speaker is raised, the first resonance frequency rose.
From the concept of electroacoustics, the effects can be explained by this: the port that can be modeled as the acoustic mass and the resistance and the small space of the cavity can be modeled as the acoustic compliance, so the acoustic mass and the acoustic compliance cause the resonance when the imaginary impedance is zero.
The resonant frequency is also called the Helmholtz resonance frequency at which the acoustic impedance is zero. In order to prevent this effect, the dimension of the port can be change large. Then the peak of the resonance will be shifted to more high frequency.
From the discussions above, the damping material plays an important role in the performance of miniature speakers. For the demand of application, the damping material can be selected to change the dynamic response of the speakers that you need.
Figure 23. (a), (b), (c), (d), (e), (f), (g), (h) are the directivity plot of the miniature speaker. Its measuring range of frequency is from 500 Hz to 11k Hz. The speaker is not mounted in the infinite baffle. From the polar plot of the directivity in Fig. 23., the directivity of the miniature is omnidirectiona.
Using Thiele and Small parameters can roughly calculate sensitivity and efficiency for miniature speakers.
The pressure sensitivity of loudspeakers is calculated by Eqs. 72.
1 0
The efficiency of loudspeakers is calculated by Eqs. 75 and 76
AR For a example of merry speaker, its Thiele and Small parameters are listed below
0.3254 0.0000785 2
From above calculation of sensitivity and efficiency, miniature speaker has smaller sensitivity and efficiency than conventional loudspeakers. The efficiency and sensitivity can be increased by increasing the Bl , by increasing the diaphragm area, by decreasing the voice-coil resistance, and by decreasing the total moving mass.