Created on 09.02.2007 17:53.
Updated 10.27.2019 22:49.
Author: M. Ephrussi.
The lower limit of the frequency range reproduced by the loudspeaker is determined by the fundamental resonant frequency of the head. Unfortunately, there are very few heads on sale that have a fundamental resonant frequency below 60-80 Hz. Therefore, in order to expand the range of operating frequencies of acoustic systems, the possibility of reducing the main resonant frequency of the heads used in them seems to be very relevant.
As you know, the moving system of the head (diffuser with a voice coil) in the main resonance region is a simple oscillatory system, consisting of the mass and flexibility of the suspension. The resonant frequency in Hz of such a system is determined by the formula:
Where m is the mass of the diffuser, the voice coil and the attached air mass, g, C is the suspension flexibility, cm / dyn.
Thus, in order to lower the fundamental resonant frequency of the head, it is necessary to increase either the mass of the cone and voice coil, or the flexibility of their suspension, or both. The easiest way is to increase the mass of the diffuser by attaching an additional weight to it. However, it is unprofitable to increase the mass of the head moving system, since this will reduce not only the resonant frequency, but also the sound pressure generated by the head. The fact is that the force F created by the current I in the voice coil of the dynamic head is equal to
Where B is the magnetic induction in the gap, L is the length of the voice coil conductor, I is the current.
On the other hand, according to the laws of mechanics, this force is
where m is the mass of the moving system, a is the vibrational acceleration.
Since the force applied to the voice coil depends for a given head only on the magnitude of the current, by increasing the mass, we will decrease the oscillatory acceleration of the coil and diffuser by the same amount; and since the sound pressure generated by the driver in this frequency range is proportional to the acceleration of the diffuser, a decrease in the acceleration is equivalent to a decrease in the sound pressure.
If we tried to halve the fundamental resonant frequency of the head, this would require four times the mass of the moving system, and the sound pressure created by the head would decrease by the same amount at a constant current in the coil.
In addition, an increase in mass would increase the Q-factor of the mobile system and increase the resonance peak, and with it the uneven frequency response, which, in turn, would worsen the transient characteristics of the loudspeaker.
Therefore, in order to reduce the resonant frequency of the head, it is more expedient to increase the flexibility of the suspension of the diffuser and the centering disc, that is, to reduce the rigidity of the fastening of the movable system. This is done as follows.
First of all, peel off or cut off with a sharp scalpel or blade (along the ring of the diffuser holder) the collar of the diffuser. Then the flexible leads of the voice coil are unsoldered, the centering disc ring and the getinax “spider” (if any) are unscrewed, or the centering disc is removed from the diffuser holder.
The flexibility of the centering disc with corrugations is increased by cutting three or four tapered holes evenly around the circumference (see Fig. 1). The total area of these holes should be 0.4–0.5 of the area of the centering disc corrugations.
To protect the magnetic gap from dust, gauze is glued onto the cutouts or the entire disc with ordinary rubber glue or BF-6 glue. If the voice coil is centered by a getinax (textolite) “spider”, then the flexibility is increased by reducing the width of its arms (by filing them with a file or carefully biting them with pliers).
After that, a part of the edge corrugation is cut off at the diffuser so that there is a gap of about 200 mm between the edge of the diffuser and the diffuser holder ring. If, at the same time, a corrugation remains on the edge of the diffuser, then it is straightened over a length of about 10 mm and a suspension in the form of pavinole or soft textvinyl arches is glued to it.
To increase flexibility, remove their textile or knitted backing whenever possible.
Highly flexible and elastic temples can be made using elastosil silicone sealant from thin nylon stockings. The top of the stocking is cut lengthwise and marking is made on the resulting fabric 24–28 cm wide (see Fig. 2). When marking, the temples should be located across the stocking (see Fig. 2), since the elasticity of the stocking is greater in the longitudinal direction.
Then, putting a piece of smooth plastic wrap on a plank or thick cardboard, put a hosiery on it and fix it at the edges with buttons or studs. After that, elastosil is applied with a spatula or the end of a metal ruler, so that the threads of the knitted fabric are not visible. After a day (elastosil polymerization time), the knitwear is turned over and elastosil is applied to the other side.
A cardboard template should be made to cut the temples.
It is desirable to hang the diffuser on no more than three or four bows so that each bow occupies, respectively, a third or a quarter of the circumference of the diffuser.
On the arms and on the edge of the diffuser, the surfaces with which they should be glued are marked with a pencil, the width of these surfaces should be 7-10 mm. The finished bows are smeared one by one with glue and glued to the marked edge of the diffuser with “elastosil” or KT-30 or MSN-7 silicone glue.
Arches made of pavinol or textvinyl are glued with the surface where the textile was located, with BF-2, 88 or AB-4 glue. It is recommended to first check the suitability (conformity) of the glue to the material by gluing a piece of material to thick paper.
The joints between the bows should also be glued so that there are no gaps. It is best to do this with “elastosil”, for pavinol or textvinyl arms it is recommended to fasten the edges with threads and fill in several steps with ordinary rubber glue.
Having finished the suspension of the diffuser, it is installed in the diffuser holder so that the voice coil enters the gap. Then the centering disc ring is strengthened and the voice coil is pre-centered (before the suspension is glued).
Next, the arms of the diffuser suspension are alternately glued to the diffuser holder ring. To bend the arms, when glueing the diffuser ring with glue, it is convenient to use crocodile clips with single-pole plugs inserted into them (for weight).
After gluing the suspension, the final centering of the voice coil is made and the rings of the centering disc or the getinax “spider” are fixed. If the centering disc does not have a metal ring and is peeled off, then first glue, the suspension of the diffuser, and then the centering disc, at the same time centering the voice coil in the gap.
Last of all, the leads of the voice coil are soldered and the support bows made of cardboard, sponge rubber or felt are glued to the diffuser holder.
If the diffuser has a crack (rupture), then it is best to glue it with elastosil glue or fill it in with rubber glue in several steps.
By the described method, it is possible to reduce the frequency of the main resonance of the head by a factor of 1.5–2. For…