Posted on Jun 24, 2017
The two surfaces act as separate sources producing the same sound, but with a phase difference of 180 degrees. If we lead this loudspeaker with a signal, the two sources are combined in space and mutually eliminated. The sound produced on both sides virtually in practice loses its intensity. To avoid this phenomenon, we have to isolate the front side from the back and the two sources do not combine in some way.
One solution would be to construct a surface with very large dimensions, infinite, so that the back surface does not communicate with the front and the sources are not combined. The dimensions of such a surface, of course, can not be infinite but finite. The problem of infinite surface leads to a loudspeaker, where one surface, the back is isolated with a wall in an enclosed space. This type of speaker gave the built-in loudspeaker construction. Extension of this type is a second type where a surface allows to communicate part of the energy from the back side with the energy of the front surface.
If the sound is the same, then the sound is amplified, otherwise it fades. This type of speaker gives us a closed speaker with a mechanical acoustic filter. The filter is an audible open tube with a specific response curve. This tube allows the sound energy to pass from the back of the speaker to the outside of the speaker. This action must go in equal phase with the front and add it with it, increasing the performance of the speaker. If careful design of the sound tube leads to amplification of sound at low frequencies.
Contrary to the type of closed speaker, this type gives more intensity at low frequencies. Therefore, the speaker has a higher output and requires a much smaller volume than a closed speaker. An essential complement to the speaker construction is the cross over, which distributes the acoustic energy to the corresponding loudspeakers. Simply cross-over separates the acoustic area into two sub-regions. Of course, the two areas overlap because the filters do not cut vertically, but have a specific call and a different phase versus frequency. This call is defined and measured in dB per octave, ie by doubling frequency.
The cross-over is a necessary network with passive materials for the operation of a loudspeaker combination. This is intended to separate the acoustic frequency range into two smaller ones. If there is no cross oer, two things happen: on the one hand, all the frequencies are simultaneously transmitted to different loudspeakers, and on the other hand unnecessary power is consumed in loudspeakers that can not produce it and convert it into acoustic energy. Cross overs depending on the number of loudspeakers that drive are distinguished on two roads and three roads. The simplest system is that of two roads with a 6dB gradient. The crossover of this construction separates the acoustic area into two sub-ranges to drive two loudspeakers one for the high frequencies and one for the low ones.
The acoustic area is divided into two by two passive filters: a low pass and a high pass. The low pass filter drives the low frequency loudspeaker and the high frequency loudspeaker for the high frequencies. The low-frequency loudspeaker is known as a woofer and speaker for high-frequency tweeters.
The loudspeakers are characterized by several features that make them stand out from each other. The main features that interest us in their choice of construction are the impedance we have and the curve that gives us the frequency-to-noise ratio. The loudspeaker resistance is characterized by a frequency depending on the destination and the type. The loudspeakers are distinguished for their destination in woofers, mid-range and high-frequency (tweeter) loudspeakers. The resistance to most loudspeakers in Ω is 4Ω, 8Ω and 16Ω.
The cross-over we present is for 8Ω speakers. The theoretical circuit is shown in the figure. The construction uses passive materials, coil capacitors and resistors. To make cross-over you need a printed circuit. As we see in the theoretical, the circuit has one input and two outputs. At the input we connect the amplifier output to both outputs from a loudspeaker. One is the high-frequency loudspeaker and the other is the low-frequency loudspeaker. There is a high-frequency filter from the input to the high frequency loudspeaker. In the path to the low frequency loudspeaker there is a more complex filter from a low pass filter. The low frequency loudspeaker filter is a belt filter and allows the passage of frequencies from 110Hz to 3500Hz. In this filter there is a low passage section consisting of a 0.68mH coil and a 15μF capacitor.
The coil is connected in series with the circuit and the capacitor parallel to the path to the loudspeaker. The impedance of these components varies with frequency. The coil impedance value is proportional to the frequency and capacitor inversely proportional to the frequency increase. As the frequency increases, the inductance acquires greater impedance and the capacitor is less. Thus, they prevent high frequencies above 3500Hz from reaching the speakerphone.
In the corresponding part of the high pass filter towards the high frequency loudspeaker, the assembly of the components is upside down. In series, a 2.2μF capacitor and a coil with 6.8mH inductance are placed in series. In this filter, as the frequency increases, the capacitor reduces its composite resistance and the coil parallel to the loudspeaker increases. As the frequency increases, the capacitor impedance decreases and thus allows the high frequencies to pass. Correspondingly, the impedance of the coil increases and so less absorbs the coil power from these frequencies.
In addition to the filter components, there are resistors and capacitors in the circuit that stabilize the loudspeaker behavior. An additional resistor is in line with the high-pass filter to reduce the power passing through the circuit. The line resistance reduces the sound level. To make the construction you will need to make the printed one shown in the figure. In this you will mount the materials in the theoretical circuit according to the figure. The assembly of the materials will start with them, the capacitors and the resistance, while at the end you will install the inductors. If you can not find the cross-over coils, you can make them for yourself. To test the cross-over, you will put 8 ohm resistor for each speaker. At the entrance you will connect a small amplifier, which will amplify the signal of an acoustic generator. The generator will cause it to produce a semiconductor signal. By varying the frequency, we observe each output on an oscilloscope. If everything is good, then as the frequency increases and approaches 3.5kHz, the output voltage for the low frequency loudspeaker will decrease, while the other output will increase.
The cross-over is the electronic part of the construction or better the electrical part of the speaker. The cross-oer is two-way with a frequency of 3.5 kHz. The theoretical circuit of the cross-oer is shown in the figure. It consists of two filters one for the low frequencies and one for the high ones. The Low Frequency section is a bandpass filter to drive the low-frequency speaker (woofer). The filter has a minimum threshold of 1OOHz and a maximum of 3500Hz. The second filter is a high-frequency with a low frequency of 3500Hz. The high-pass consists of a capacitor in series and a coil in parallel. The filter call is 12dB.
The second low-frequency loudspeaker is more complex and consists of two parts that are combined into one. These sections are two separate filters, a low-pass filter and a high-pass filter. The low pass filter consists of a coil in series and parallel to a capacitor. The high-pass filter is the same as the high-frequency loudspeaker with a difference in material prices. Each speaker has its own crossoener.
For each you will use the printed one shown in the figure. In this you will mount the materials. As we have said, the manufacture of speakers is essentially a wood-construction. Each speaker is a rectangular wooden rectangle with small dimensions. The dimensions for the sides of the speaker are shown in the figure. The sides of the speaker are made of 16mm thick MDF. On the face surface you will open three round holes to pass the speaker body and the bassreflex tube.
When you place the loudspeakers you will notice that the loudspeaker body protrudes. If this is a problem then you can dismantle the facade or add an extra plywood surface. The surface will have a thickness of 5mm, as is the body of the loudspeakers that protrudes. In this way the loudspeakers will not protrude and will have a more professional look. On the back surface we'll make another hole. We will secure good quality input plugs. The internal volume of the speaker is 31t. The speakers can be made of various woods or preferably 16mm thick MDF. Wood for MDF sides is available in the market with various coatings.
The surfaces are finally polished with glaze. The same job can be done with a wood preservative. On the edges where there is a discontinuity, that is to say, the veneer with the veneer will stick tape veneer. To stick it needs ironing with an electric iron. Once stuck we cut excesses back, left, right and left. Bonding between the sides is done by glueing and tightening the surfaces with clamps. For best results, we recommend using 6mm or 8mm diameter blades and 2.5cm to 3cm in length.