Calculation of an acoustic phase inverter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Amateur radio calculations

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The proposed calculation method is based on the simplest measurements carried out with a well-defined loudspeaker instance installed in an acoustic phase inverter and on the nomographic determination of the latter's dimensions.

First of all, guided by Fig. 1 and the table, it is necessary to make a "standard volume" - a sealed plywood box, all joints of which are carefully fitted, glued and coated with plasticine to avoid air leaks.

Fig. 1

Next, measure the natural resonance frequency of the loudspeaker located in free space. To do this, it is suspended in the air away from large objects (furniture, walls, ceiling). The scheme of measurements is shown in fig. 2.

Fig. 2

Here ZG is a graduated sound generator, V is an AC tube voltmeter and L is a resistor with a resistance of 100-1000 ohms (for large resistance values, the measurement is more accurate).

By rotating the frequency adjustment knob of the sound generator in the range from 15-20 to 200-250 Hz, the maximum deviation of the voltmeter needle is achieved. The frequency at which the deviation is maximum and is the resonant frequency of the loudspeaker in free space Fv.

The next step is to determine the resonant frequency of the loudspeaker Fv during its operation at the "standard volume". To do this, the loudspeaker is placed with a diffuser on the "standard volume" hole and pressed lightly to avoid air leaks at the junction of the surfaces. The method for determining the resonance frequency is the same, but in this case it will be 2-4 times higher.

Fig. 3

Fig. 4

Knowing these two frequencies, with the help of nomograms, the dimensions of the phase inverter are found. Depending on the diameter of the loudspeaker diffuser, the nomogram shown in Fig. 3 (for diameter 200 mm), in fig. 4 (for diameter 250 and 300 mm) or in fig. 5 (for diameter 375 mm). According to the selected nomogram, the volume of the phase inverter is determined, for which purpose the points corresponding to the frequencies found are connected by a straight line on the axes "Resonance frequency

Fig. 5

Fv (see Fig. 4 point A) and "Resonance frequency Fl (point B). Mark the point of intersection C with the auxiliary axis and from here draw a second straight line through point D to the axis "optimal volume". The value corresponding to the new point of intersection E , and is the desired volume.

If there are no special considerations for designing a box of a special configuration, then the calculation of its internal dimensions for a given volume can be made using the nomogram shown in Fig. 6. The width of the phase inverter will be equal to 1,4 heights, and the height - 1,4 depths. The use of the nomogram is not difficult: a straight line is drawn between the extreme axes on which the values ​​of the volumes are plotted. The points of intersection of the line with the axes A, B, C will determine the width, height and depth of the box. The diameter of the cutout for the loudspeaker is taken equal to the dimension C indicated in the table.

Fig. 6

Further, having given the diameter of the tunnel, it is necessary to determine its length and check whether it fits into the bass reflex box. The length of the tunnel is found from the graphs shown in fig. 7 for three inner diameters: graphs A for 50 mm diameter, B for 75 mm diameter and B for 120 mm diameter. Having selected the appropriate graphs, the length of the tunnel is found from the frequency Fv and the volume of the phase inverter determined earlier (an example in Fig. 7, B). It should be 35-40 mm less than the inner depth of the drawer. If this does not work, you can change the configuration of the box at all, while maintaining its volume, or take a different diameter of the tunnel.

Fig. 7

The phase inverter is made of plywood with a thickness of about 30 mm. If there is no such thick plywood, then to increase rigidity, you need to glue diagonally or crosswise bars of 25X75 mm inside the box. The box is assembled with screws and glue, and all seams are sealed. It is recommended to fasten the back wall with screws (five pieces per side) with a felt pad. The tunnel is made from a thick-walled cardboard tube.

Having made a phase inverter and installing a loudspeaker in it, they begin to dampen it. To do this, it is recommended to completely cover the loudspeaker from the back side with a layer of glass wool 25-50 mm thick, attaching it to the board around the diffuser holder using a ring screwed on with screws or screws.

Fig. 8

Damping sufficiency is checked using the circuit shown in fig. 8. The resistance of the resistor R is taken about 0,5 ohm. If the damping coefficient K of the amplifier with which the unit will work, and the resistance of the voice coil of the loudspeaker to alternating current r, is known, then it can be determined from the formula R \uXNUMXd r / K ohm. Move the switch from one position to another, listen for a click in the loudspeaker. If it is quite distinct and there is no "mumbling" or "ringing", then damping is sufficient. The final decision is made after listening to orchestral music with well-defined basses and upper cats.

Author: Yu. Lyubimov; Publication: N. Bolshakov, rf.atnn.ru

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