The decrease in the resonant frequency of heads
The lower bound reproduced by the loudspeaker frequency range is determined by the basic resonant frequency of the head. Unfortunately, the market is very rarely head having the main resonance frequency below 60-80 Hz. Therefore, to extend the operating frequency range of acoustic systems is very relevant is the possibility of reducing the main resonance frequency of the used heads. As you know, moving head (cone and voice coil) in the area of the principal resonance is a simple oscillating system consisting of the mass and flexibility of the suspension. The resonant frequency of the system is determined by the formula:
where t is the mass of the cone, voice coil and attached air mass, g;
With the flexibility of the suspension, cm/Dean.
Thus, to reduce the primary resonance frequency of the head is necessary to increase either the weight of the cone and the voice coil, or the flexibility of their suspension, or both. Most just increase the weight of the diffuser, reinforcing an extra load on it. However, to increase the mass of the moving system of the head unprofitable, as this will reduce not only the resonance frequency, but also created the sound pressure head. The fact that the force F generated by the current I in the voice coil dynamic head is equal to
where B is the magnetic induction in the gap;
l - length of conductor of the voice coil.
On the other hand, according to the laws of mechanics, this force is equal
where m is the mass of the moving system; a vibrational acceleration.
Since the force applied to the voice coil, depends for the head only on the magnitude of the current, increasing the mass, we at the same time reduce the vibrational acceleration of the coil and the diffuser; and since the sound pressure generated by a head in this frequency region, is proportional to the acceleration of the diffuser, reducing the acceleration is equivalent to a decrease of the sound pressure. If we tried to halve the primary resonance frequency of the head, this would increase the mass of the moving system four times, and as many times would decrease created by the head of the sound pressure at constant current in the coil. In addition, the increase in mass would increase the quality factor of the mobile system and increased resonance peak, and with it the frequency response that, in turn, worsen the transient characteristics of the loudspeaker.
Therefore, to reduce the resonant frequency of the head is preferable to increase the flexibility of the suspension of the diffuser and the centering of the disk, that is, to reduce the rigidity of the mounting of the movable system. This is done as follows. First of all peel off or cut with a sharp scalpel or razor blade (for the ring of DiffuseMaterial) collar of the diffuser. Then otpaivaete wire voice coil, Unscrew the ring centering disk and laminated bakelite <spider" (if any) or peel off the centering disk from DiffuseMaterial.
The flexibility of the centering disk with corrugations increase, cut evenly around the circumference of three or four cone-shaped hole (see Fig. 1). The total area of these holes should be 0,4 - 0,5 square of the corrugations of the centering disk. To protect the magnetic gap of dust on cutouts or whole disk usual rubber adhesive or glue BF-6 bonded to the gauze. If the voice coil is centered laminated bakelite (textolite) "spider", increase flexibility, reducing the width of its handles (sipiliva them with a file or gently biting a pair of pliers). After that cut off at the diffuser portion of the marginal corrugation between the edge of the diffuser and the ring of DiffuseMaterial there was a gap of about 200 mm. If on the edge of the diffuser remained corrugations, it's spread out over a length of about 10 mm and stick to it suspension in the form of arches from posinola or soft textoline. To increase flexibility should delete them or knitted textile substrate.
Very flexible and elastic arch can be made using silicone adhesive sealant "elastosil from thin nylon stocking. The leg of the stocking and cut along the resulting fabric width 24-28 cm makes the markup (see Fig. 2). When marking the bow should be located across the grip (see Fig. 2) since the elasticity of the stocking more in the longitudinal direction. Then, putting on some Board or thick cardboard, a piece of smooth plastic film, overlay the hosiery fabric and fix the edges of the buttons or studs. Then with a spatula or butt metal ruler put on knitwear "elastosil" so that the threads of the knitted fabrics were not visible. Through the night (polymerization time "elastosil") knitwear is inverted and applied "elastosil" on the other side.
To cut arches should make a cardboard template. The diffuser preferably be suspended for not more than three or four arches, each shackle was taken by a third or a quarter of the circumference of the diffuser. On the arches and at the edge of the diffuser pencil mark the surface they must be glued, the width of these surfaces should be 7-10 mm. Ready the bow smear alternately glue and paste them to the marked edge of the diffuser elastosil" or silicone glue CT-30 or the sit-7. The bow of ravinala or textoline glued to the surface, where the textiles, BF-2, 88, or AB-4. It is recommended to check the suitability (compliance) of the adhesive material, gluing a piece of material to thick paper.
The joints between the arches should be glued so that no cracks. This is best done "elastosil", palynology or tekstovinilovye arches is recommended to seal the edges with thread and fill-in divided doses usual rubber adhesive.
After the suspension of the diffuser, it will be placed in DiffuseMaterial so that the voice coil has entered into the gap. Then consolidate the centering ring of the disc and produce a preliminary centering of the voice coil (before gluing suspension). Next in turn is bonded to the ring DiffuseMaterial shackle suspension of the diffuser. For folding handles,
when buttering glue ring DiffuseMaterial, it is convenient to use the crocodile clip with inserted them in single-pole plugs (for gravity). After pasting suspension produce the final alignment of the voice coil and fix the centering ring of the disc laminated bakelite or "spider". If centering the disc does not have a metal ring and unglued, the first paste, the suspension of the diffuser, and then centering the disk, while simultaneously aligning the voice coil in the gap. Least solder the conclusions of the voice coil and glued to DiffuseMaterial arch support made of cardboard, foam rubber or felt.
If the diffuser has a crack (break), then it is best to seal the glue elastosil" or in a number of techniques to fill, rubber adhesive.
The described method can reduce the frequency of the main resonance of the head in 1,5-2 times. For example, in Fig. 3 shows frequency characteristics of the impedance head 4A-18 to (dashed line) and after the alteration.
This head is made Leningrad plant cinematics "Kinap" in 1954; its alteration consisted in cutting the three Windows in the centering disk and the replacement of marginal corrugation palynologie temples, and textile substrate was not removed. The resonant frequency decreased from 105 Hz to 70 Hz, that is, 1.5 times. It is interesting to note that the same decrease of the resonance frequency gives additional weight is 25 g.
Author: M. Ephrussi; Publication: N. Bolshakov, rf.atnn.ru