Manufacturing of subwoofers, secrets of masters.
High-grade sound transmission is inconceivable today without a low-frequency component of an acoustic signal. Therefore even more often drivers, not indifferent to quality of a sound in the car, supplement a stereoradio tape recorder and pair of columns (as the American fitter, "deck and two" would tell) with the loudspeakers expanding the reproduced sound range as towards top, and in particular of the bottom limit. As the signal in 20 Hz - 150 Hz does not render practical influence on stereoeffect, it is accepted to allocate it to one canal which make out one acoustic case, simplifying thereby the circuit decision and keeping space of salon of the car. And thanks to that the sound in the field of bass frequencies extends, bending around obstacles, to place the sabvuferny case it is possible in any free part of the car. An acoustic box make of different materials: plywood, particleboard, plasticity. There are also such masters who use wood from musical instruments.
Acoustic registration of a subwoofer if it not the set of already ready details of factory manufacturing, usually includes: design and calculation of the speaker system for the concrete order, manufacturing of the acoustic case and system installation in the car. As acoustic registration of subwoofers demands the creative approach, each master fitter finds own key to its embodiment in life. Having coordinated with the customer questions of principle, the master starts detailed calculations of future acoustics. Tens circuits of cases for subwoofers can be reduced to several main types among which the closed, fazoinversny systems and systems with passive loudspeakers are most known.
When designing the sabvuferny speaker system usually aspire to receive good transfer function, that is a good ratio of sound pressure created by it and a complex entrance signal. Transfer functions of three mentioned systems in principle are similar to transfer function of the filter of the top frequencies respectively the second and fourth order, with a steepness of recession of the peak and frequency characteristic towards low frequencies of 12 dB on an octave and 24 dB on an octave, and total AChH in the field of working frequencies of a subwoofer depends on characteristics of the used loudspeaker, the applied filter crossover, a design and a case material, and also possibilities of the amplifier of capacity. Considering all these features in acoustic registration of a subwoofer, masters aspire to reduce amplitude of shift of mobile system of loudspeaker and by that to increase level of input electric power, and also the maximum sound pressure that will help to lower its overloads and signal distortion.
For detailed study of a design can use today special computer programs of calculation, putting "on an entrance" parameters of a subwoofer and receiving "on an exit" recommended characteristics of the case of the speaker system. Sometimes for calculation of some volume characteristics of the case the master fitter should use and "equivalents": if the manufacturer of a subwoofer makes recommendations concerning types of acoustic cases and their volume, to calculate own volume of a subwoofer, that is volume izmeshchayemy to them air, owing to complexity of its geometry happens not easy. Some meticulous fitters sometimes use any homogeneous loose substance (not capable, of course, to damage a loudspeaker surface), filling up it and measuring equivalent volume of izmeshchayemy air.
the Acoustic case - the most important element of all sabvuferny speaker system. It should not only have optimum internal volume for placement of loudspeaker and necessary components, but also possess sufficient durability - not only mechanical, but also acoustic. The matter is that the subwoofer of dynamic type works as a pomp, condensing air before a diffuzor and rarefying it from the back party in the axial direction. Thus sizes of pressure from both parties are equal, but are turned on a phase therefore at absence or insufficient isolation of forward and back surfaces of loudspeaker "acoustic short circuit" will take place. To avoid it, it is necessary to provide phase shift of the acoustic signal radiated from the back party of a diffuzor, for example, on a half of length of a wave. It is reached at the expense of installation of "the sound panel partition", more often closed in the form of the case. Than more this partition (or there is more than a volume of the case), is theoretically lower than subjects a tonality of a signal which will not be acoustically closed. If the case volume (i.e. size of the air mass concluded in it) influences height of its resonant frequency, the form of the case influences first of all formation of standing waves as a result of own resonance of the case arising on this or that frequency. To fight against formation of standing waves, resort to loudspeaker shift from the center of the acoustic case. The most unsuccessful the cubic form of the acoustic case with the loudspeaker located on equal removal from all its walls is considered thus.
Systems of the closed type
The increase in volume of the case is higher than a subwoofer recommended by the manufacturer can increase return on the lowest frequencies, however the master will face still big problems at his installation in the car. The schedule from above characterizes the recommended optimum sizes of cases for subwoofers of various diameter.
in search of more effective circuits of acoustic registration of low-frequency loudspeakers of the master began to use already a decade ago p of the case with fazoinvertor and acoustic air lines of various type. Interest to them especially increased in recent years, with hi-fi-equipment development thanks to that they allow to expand a range of reproduced frequencies. Some masters consider justified to spend additional efforts to going further the closed design and to receive a prize in return on low frequencies.
In fazoinversny cases are applied by p the cylindrical or rectangular pipes which are adjusted usually on certain frequency. Acoustic registration of a subwoofer with use of such cases demands from a master fitter of big theoretical knowledge and experience as for receiving good transfer function it is necessary to coordinate, for example, such factors, as mutual influence of resistance of radiation of a diffuzor and a fazoinvertor pipe through mutually соколеблющуюся mass of air.Mistake
, for example, at good quality calculations, and also designing and control of a fazoinvertor are at the bottom of that the speaker system "mutters" or the bass "is smeared". And even if the pipe of a fazoinvertor is adjusted on necessary frequency, it can become a source of nonlinear distortions if, for example, volume speed of air in it exceeds admissible (it should not exceed 5 % from speed of a sound): in this case the stream of air becomes turbulent. Sensitivity of transfer function (sound pressure) fazoinversny system to a rasstroyka of frequency of a fazoinvertor is very high, and after final assembly there can be a need of exact fine tuning.
Estimating efficiency and quality of sounding of this or that sabvuferny system, experts experts often use the analysis of level of sound pressure, for example, in tretyoktavny strips of a range of its working frequencies. For measurement of this level and other important characteristics of ready sabvuferny system it is possible to use tools of the realnovremenny analysis (see. "The master of 12 Volts" N 3 for 1997). Legality of value judgment of audioexperts admits also.
Heavy boxes for easy basses
It is impossible to tell what to make the case or an acoustic column for an automobile subwoofer extremely difficult. If to follow instructions and recommendations of experts, even in the presence of a limited set of manual tools the capable master in a condition to make the high-quality speaker system. As it was noted above, masters use the drevesnostruzhechny plate of the MDF type possessing necessary mechanical and acoustic properties more often. Use multilayered plywood less often. Depending on qualification of the master in joiner's business for it sawing up and adjustment of surfaces of a box on corners can present difficulty. It is especially difficult to achieve high-quality connection, using only the manual tool." Popular" when manufacturing such designs a joint of surfaces by a principle "larkspur" is not necessary, as the majority of drevesnostruzhechny plates, as well as multilayered plywood, does not possess necessary mechanical durability of structure of glutinous structure and a struzhechny filler. In drawing some types of the angular joints used at manufacturing of boxes of automobile sabvuferny systems are presented.
of the Look of the master
We were convinced also that application of high-quality materials for acoustic cases justifiablly. We apply a melkodispersny struzhechny plate of Jamo. For subwoofers the inch plate which not only is strong is ideal, is well processed, but also has good acoustic properties, without demanding additional superficial processing. For internal furnish special foam rubber is used, and for the general completeness of design of the case become covered by a carpet and special acoustically transparent materials.
our firm has now full color scale of such materials, and it is important, that the customer was better informed on available possibilities and options of execution of acoustics in its car.
In general, the choice of materials directly depends on a financial position of the client.
If in the course of installation it is necessary to deal with the loudspeaker which parameters are in advance unknown or demand more exact recheck, these necessary data can be received by own measurements - by means of an audio-frequency generator. It is considered that this way is more exact, but he more labor-consuming and respectively demands big material and temporary inputs.
In principle for any loudspeaker possibly manufacturing of a box of any configuration. Rigid restrictions are not present - everything depends on the purposes which are pursued by a fitter. Let's take for an example the usual closed box. At its small volume loudspeaker damping very high, and it is respectively capable to sustain big capacity. However at increase in loudness there can be a resounding. Increasing box volume, we reduce resounding, but also the limit of the brought capacity thus decreases. Usually we project a box in which the loudspeaker gives out decent capacity and thus does not hoot.
We make p of a box of high-quality multilayered plywood. And for achievement of the best quality of a wall of a box we make of two layers of 10-millimetric plywood connected by special glue. Thus, the thickness of walls makes not less than 20 mm; for the 10-inch loudspeaker an admissible minimum - 15 mm. From within plywood walls are surely pasted over with a sound-proof material.
the Trapezoid form of a box is chosen by us not casually. Such configuration allows to exclude at once pair of directions of standing waves. It is desirable, that walls of a box were not parallel, therefore the trapeze quite suits us. Besides, such form of a box is more justifiable in view of a configuration of a luggage compartment of the car.