Speaker characteristics on the oscilloscope screen. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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You can determine the resonant frequencies of the moving loudspeaker system using any electronic oscilloscope. The method is based on the property of such oscilloscopes to very accurately record the phase shift between current and voltage in electrical circuits. In the presence of a phase shift, an inclined ellipse will be observed on the screen of the tube, as this is shown by the dotted line in Fig. 1. With a decrease in the phase shift angle, the ellipse will narrow and, in the absence of a shift, will turn into a straight inclined line (solid line in Fig. 1), the angle of inclination and length of which can vary compared to the length and inclination of the ellipse. The moment of transition of the ellipse into a line fixes the onset of resonance.
Ris.1
The circuit for switching on the voice coil of any loudspeaker for measuring phase shift is shown in Fig. 2. The outputs of the loudspeaker Gr are connected to the output of a sufficiently powerful (at least 2-3 W) sound generator ZG through the active resistance R, the voltage from which is supplied to the horizontal deflection plates. Voltage is applied directly from the voice coil to the vertical deflection plates. With this inclusion, it is possible to observe the phase shift between current and voltage in the voice coil circuit and it is possible to fix the moment of resonance. In this case, the internal horizontal sweep in the oscilloscope should be turned off.
Ris.2
Figure 3 shows a typical frequency response of the internal resistance of a cone loudspeaker and indicates the likely figures on the oscilloscope screen for different points of this characteristic. The resistance value R can be any in the range from one to 100-200 ohms. The greater the resistance of the voice coil, the greater this additional resistance must be. It is more convenient to carry out measurements in a mode in which the amplitude of the current in the voice coil circuit remains constant in order to be able to determine the amplitude of the voltage on the voice coil along with the phase and frequency of the resonance. To do this, the resistance R must be 20-30 times greater than the resistance of the voice coil, and in any case not less than 100 ohms.
Ris.3
The proposed method of using an oscilloscope can be very useful for determining the quality and identity of individual loudspeakers (heads), especially when selecting them to work in various installations and acoustic systems, as well as for setting up phase inverters and correctly matching heads with acoustic design.
Literature
- Radio No. 3 1964, c.59.
Publication: N. Bolshakov, rf.atnn.ru
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