Loudspeakers speech, singing, music, noises and other sounds. The playback quality of sound is mainly determined by the degree of frequency and distortion. With proper selection of speakers and registration of them (boxes, screens and so on. N.) The distortion will be greatly reduced, and the overall sound quality will increase.
The purpose of this article — to help ham-designer to choose the right and most effective use of the loudspeaker.
Electrodynamic loudspeakers diffuser best reproduce the sound and therefore widely used. These disadvantages are low efficiency, is quite significant frequency dependence of distortion and directional characteristics of the frequency.
What should be guided by a radio amateur designer when choosing a speaker? First of all, the minimum capacity must be equal to or greater than the maximum undistorted (within accepted tolerances) the power output stage of the device for which the selected speaker. Overload the speaker makes a very large distortion.
Another important condition is the need for a sufficiently uniform playback of the band, which is intended receiver or a separate low-frequency amplifier. The frequency response of electrodynamic speaker has large blockages in the lowest (below 150-200 Hz) and high frequencies (above 5000- 6000 Hz) and has a considerable unevenness (peaks and troughs) in the operating frequency band. Because of these features are often not available in speaker provides good sound, although the cam receiver or amplifier has a desired frequency response. Using two loudspeakers simultaneously equal power, one can obtain significantly better results. To collaborate loudspeakers necessary that the fundamental resonance frequency of each speaker differed 20-30 cps, and it is desirable to lower the resonance frequency of the order of 70-80 Hz. Improving the quality of audio reproduction when the two loudspeakers due to the fact that the total frequency response becomes more uniform. Aligning characteristics occurs because the different instances of the same type speakers have different frequency characteristics, the dips and peaks are at different frequencies. In addition, doubling the speakers almost doubles the efficiency of the (return), as emitted by the sound energy is increased almost four-fold increase in half thanks to the effective surface of the diffuser, and the consumption of the amplifier
Electric power only doubles. It can be applied even more identical speakers at the same conditions, but in this case, the structure becomes bulky. In installations with an output of 10-15 W should be applied two speakers of 5-8 volts of broadcast receivers first or second class («Peace», «Riga-10», «Belarus», and so on. N.) Or sound kinoperedvizhek. For power plants at suitable speakers with the power of 1-3 watts.
When they need to couple an speakers correctly to phase. This phasing is done visually, with the help of the battery in the 1.5-4.5, which is attached to the conclusions of the voice coil speaker (if the electromagnetic magnetization, the magnetizing coil is necessary to supply power). Switching terminals of the voice coil, are out to the diffuser when the two moving in the same direction. Then, noting the polarity is output voice coils, you need to properly connect them (like poles in parallel connection, and unlike in series).
Proper phasing check and listening, trying to switch ends of the voice coil winding or bias. At the wrong turn on the volume of the midrange considerably reduced. However, this method is suitable only if the couple an speakers. A larger amount of phasing in the hearing becomes difficult.
After you select the speaker to properly determine the size and design of the box in which the receiver is mounted. Bad box can cause rattling sounds or other side, as well as degrade the frequency response of the loudspeaker.
Good sound of the lowest frequencies provide quite difficult, as the speakers sharply reduce the impact on frequencies below their fundamental resonant frequency (60-120 Hz). Therefore, it is desirable to use the speakers with a low enough natural frequency of resonance (of course, not forgetting the tweeter). In addition, the radiation conditions speaker sound energy lowpass very unfavorable and essentially depend on the external design of the speaker.
Diffuser speaker while moving forward compresses the air in front of him and behind him dilution. Such compression and expansion of air evenly distributed front and rear diffuser. Skirting the diffuser, they are «stacked» on top of each other and cancel each other out. At movement of the diffuser turns back the same picture. This effect is called acoustic «short circuit.» Instead of passing sound waves further surpasses the air diffuser on the one side to the other. .
To eliminate this phenomenon strengthens the speaker on the board, delimiting the front and back sides of the radiating cone. If the size of the board is not less than half of the wavelength corresponding to the lowest frequency sound emitted by the loudspeaker, the «short circuit» will not occur at higher frequencies. The box with an open rear wall has the same effect as an acoustic screen.
It may seem that, instead of a large board or box without the rear wall of the box can be used smaller, but with the rear wall, which blocks the way to the back of the emission cone and thus eliminates the possibility of an acoustic «short circuit.» However, it is not. The resiliency of the closed volume of air in the box, folding with the elasticity of the mechanical system loudspeaker increases its own resonant frequency, and thus further reduces the impact at the lowest frequencies. The presence of such a closed volume and causes unwanted resonance effects at higher frequencies. In addition, in closed boxes deteriorates cooling tubes and parts receiving and amplifying part, and therefore such boxes are not used. Amateur radio design, with a powerful stage antics (15-20 W), should be placed in large boxes cantilever. The bigger the box, the better reproduce lower frequencies. Of course, it is not necessary to get involved gigantomania and start from the aesthetic considerations and take into account the size of the room, where there will be radiogram.
Sometimes it is advisable to place the speakers away from the receiver or amplifier, especially if you can use a wall or corner of the room.
At the beginning of this article shows two variants of placing the speakers in a corner. Both increase the effective dimensions of the baffle, and, moreover, the angle forms a kind of large horn also improves reproduction of lower frequencies. The structure is located in the lower corner of the room, it can be implemented as a board affixed to the wall, and a triangular box. Top design should be open and the same as the hole for the loudspeaker tightened rare matter, improving external appearance and somewhat protect them from dust. The trihedral panel for the upper corner of the room also need to provide free access vibrations produced by the rear side of the cone, leaving a gap between the upper edge of the shield and the ceiling. Sami speakers useful wrap one or two layers of a rare material, protected against the ingress of dust into the mobile system.
For better sound at lower frequencies can be used fazoinvertor — closed box with an additional small hole is usually located above the speaker. Dimensions of the box and the holes are calculated so that the sound waves of lower frequencies emitted by the reverse side of the diffuser, out into the environment through the holes in the same phase as the vibrations emitted by the front face of the diffuser. Closed box and mounted in it are two loudspeaker connected speakers: the volume of air enclosed in an additional opening connects the two systems and provides the desired phase relationship. Change the size of the hole, you can emphasize the sound of bass. The inner walls of reflex recommended to cover the sound-absorbing material. This eliminates the undesirable effects of natural vibrations of the air inside the box at higher frequencies.
With proper selection of the size of bass reflex speaker output in the frequency range 80-250 Hz is increased by more than two times (by 6-8 dB), and in addition, reduced harmonic distortion due to the damping of the moving system speaker volume of air trapped inside the box. This damping occurs just at the natural resonant frequency at which the highest harmonic distortion. Of course, the use of reflex can only be placed in the separate receiving and amplifying parts. Reflex is very suitable for the above-mentioned triangular box located in the lower corner of the room.
The natural frequency of resonance of the moving system, the speaker is not always indicated in the model parameters. Often you can find the average value of this frequency, which in different instances of the speaker is often different from the actual to 20 Hz. Determine the natural frequency of resonance is possible, feeding on small voice coil (about 1) the voltage of the audio oscillator. By changing its frequency in the range of 20 ± 200 Hz, to achieve the maximum amplitude of the oscillations of the diffuser (well-marked eye), and the frequency of which indicates the desired value to us.
It is also possible the measurement of the internal resistance of the speaker voice coil voltmeter-ammeter method or by substitution (for resistor). The presence of the peak in the lower frequency indicates the resonance frequency of the moving system. Such measurements are performed twice, before you install the speaker in bass reflex and after. By changing the frequency and the peak height can be judged on the quality of coordination with Bass reflex speaker. With good coordination of the peak should be sharply reduced or even disappear completely, and the frequency response may appear two small peaks, which are located above and below the natural frequency of resonance.
Taking care of providing a good bass sound reproduction, should not forget about the peculiarities of the emission treble. If the speakers are located in the back of the drawer or in the slot made in the wall of the colon (20-30 mm), the tweeter may worsen as diffusers formed before a sufficient amount of air. In this case it is useful for a speaker hole cut in the form of a cone with an angle of 45 ° (between the cone and the wall surface).
Significantly affect the tweeter and a speaker settings as its directional characteristic, which determines the dependence of the sound pressure at a given frequency at a point at a certain distance from the center of the outer surface diffuzoroderzhatelya, the angle made between the working axis of the loudspeaker and the direction to this point. This characteristic is usually represented in polar coordinates, and its appearance depends on the ratio of the length of the radiating surface of the sound wave. With increasing frequency (decrease in wavelength) or at a given frequency with increasing diameter of the diffuser directivity characteristic becomes more acute, as the sound pressure, measured at the same distance from the loud of the speaker, but at different angles to its axis, decreases the faster the higher the frequency and the larger the angle measurement. As a result, the sound quality of the speaker is best for the students, sitting along the axis, and the worst for the audience at the sides. For these reasons, for example, can not recommend placing the speakers on the top or side walls of the box (which, unfortunately, is done, for example, in some of our television sets). Generally tweeter speakers are better suited to small diffuser having a wider directivity characteristic and a better sensitivity at these frequencies.
Placing four speakers, suitable for tabletop and console design of the boxes. This placement of speakers mass production in view of the above recommendations provides a fairly good sound quality, of course, at high quality of all the equipment in general. If the tweeter used separate speakers (in extreme cases or types 1GD5 1GD6), they need to include through appropriate separation filters, better provide for their own single-ended output stage is designed for frequency bandwidth of 2-10 kHz. This method will allow more efficient to choose and agree on how the frequency response and return to the upper frequencies. Of course, in this case, and to phase secondary windings of the output transformer.
For manufacturing phase inverters, as well as acoustic screens (reflective boards and boxes) should be used materials having sufficient rigidity. A good material is plywood thickness of 10-15 mm. Shields and wall boxes should be well-fitting, glued and plastered. No cracks or fractures are not allowed.