Improving the head of direct radiation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers
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The recoil of the head of direct radiation, that is, the sound pressure it creates, is known. proportional to the oscillatory acceleration of its diffuser. The acceleration of the diffuser is proportional to the force created by the current I in the voice coil. This force can be determined by the formula F=BIl (B is the magnetic induction in the head gap. l is the length of the voice coil conductor). The product Вl is a design parameter of the direct radiation head.
The current flowing through a voice coil moving in a magnetic field, at a constant voltage amplitude at its terminals, is inversely proportional to its electrical impedance and the back EMF that occurs in the voice coil when it moves in the gap. The inductive component of the impedance of the voice coil and the back EMF are frequency dependent, so the current flowing through the coil, and therefore the output of the direct radiation head, depend on the frequency of the audio signal.
When playing frequency. coinciding with the main resonant frequency of the moving system of the head, there is a sharp decrease in its return. This is explained by the increase in the back-EMF of the voice coil as a result of an increase in the speed of its movement in a magnetic field (the influence of the active and inductive resistances of the voice coil can be ignored in this case).
It is impossible to increase the return at the resonant frequency by increasing the inductance in the gap or the length of the conductor, since the dependence of the return on parameter B1 is not the same at different frequencies.
On fig. 1 shows the frequency response of the head sound pressure at frequencies up to 500 Hz. The y-axis shows the change in the recoil of the head compared to the nominal one (curve b).
Ris.1
Curve a (compared to curve b) is the frequency response of the sound pressure of the head, which has an increased value of B1, and curve c is reduced (with a constant active resistance of the voice coil). It can be seen from the figure that in the frequency range from 70 to 500 Hz, the recoil of the head is directly proportional to the product B1. But near the resonant frequency, the picture changes dramatically. An increase in B1 leads to a decrease in the return at lower frequencies, and with a decrease in the value of B1, the return increases. This happens as a result of the action of the mentioned back-emf, which, other things being equal, is directly proportional to the parameter B1.
It is possible to increase the return of the head at the resonant frequency if more current flows through the coil at this frequency. The way to do this is based on the use of an additional voice coil L2 (Fig. 2).
Ris.2
The second voice coil is wound on top or bottom of the main one. It is connected through a serial LC circuit (L3C1), the resonant frequency of which is chosen equal to the main resonant frequency of the moving head system. Due to the low resistance of the LC circuit at the main resonant frequency, the additional coil is connected in parallel with the main one. In this case, the force F, which ensures the acceleration of the moving system of the head, will be determined by the total current flowing through both coils. Thus, the output of the loudspeaker at the resonant frequency will increase.
At frequencies that differ by at least one octave from the resonant one, the impedance of the L3C1 circuit is large enough, which excludes the influence of an additional coil on the operation of the head.
Literature
- "Audio" (USA). 1974. vol. 58. no. 12 "Wireless World" (England), 1975. vol. 81, no. 1472
Publication: N. Bolshakov, rf.atnn.ru
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