The subwoofer for the car. Part 1
How will the boxes
The bass in the car is necessary. It is trivial from the point of view of many, including and the author's statement I predposylki all further especially in favor of those who this view is not shared, so that they can feel better to delve into other materials published in this issue of "Salon AB".
And in the meantime we will try, within the given volume of the journal of the bands to decide what to do in order to get their money's bass as long as you need (or how many you want) and this what I want (or what need).
Known to confusion in understanding the principles of formation of the bass link automotive acoustics largely due to advertising information policy, and often reference publications. There is a potential buyer in the first place inform the speaker size, then its capacity, then another mythical "range frequencies" and complete a triumphant chord pictures.
All? There it was! Here begins. In English actually the speaker called driver - drive, and this is very true. Just as the motor car will only enriching themselves with all what has worked out for this humanity and the speaker will be the speaker only in its inherent acoustic design.
With verkhnechoskoye midrange and heads the situation is relatively just tweeters his acoustic design bear, and MF - demand in minimum size.
Other business - basewiki. Here, almost everything is determined by the choice of acoustic design, and depending on this choice the revision will be subject to all given the parameters: and the power and frequency range, and, in a sense - price. For the skillful choice of parameters it is possible to achieve a sickening sound the most expensive bass dynamics and thoroughbred.
The magazine already briefly referred to the main types of acoustic design, now it is time to announce the full list". It is not so long:Acoustic screen Sealed box The bass-reflex Speaker with passive radiator Bandpass loudspeaker (4th order) Bandpass loudspeaker (6-th order) Quasimolecules loudspeaker Three-chamber bandpass loudspeaker (4th order) Three-chamber bandpass loudspeaker (6th order) Acoustic labyrinth A folded horn Aperiodic load (acoustic resistance)
The objective of any low-frequency acoustic design is solved by the ancient the principle of "divide and conquer". "Divide" means that the waves emitted by one side of difusora must be something separate from fluctuations generated return his side, simultaneously and in opposite phase with the first. "Rule" means with clipped so the "extra" sound waves can to act differently.
Historically, the first acoustic design was an acoustic screen. He holds the defense, not letting the vibrations from one side of the diffuser to another and not giving them mutually destroyed up to frequencies at which the shortest distance between front and back side of the diffuser becomes comparable with the length of the half-wave the emission frequency. And below this frequency acoustic screen "signs in complete inability" and provides anti-phase waves cancel each other as they please. To suppress acoustic short circuit at the frequency, say 50 Hz, the shield should be the size of a 3 meter by 3. Therefore, this type acoustic design of practical importance to the long-lost, although used still in use as a reference when measuring the speaker settings.
Structurally, the simplest acoustic design of a practically applicable- a sealed box (sealed or closed in foreign terminology). Here with unnecessary fluctuations act decisively and cool: locked in a confined space behind the diffuser, they will sooner or later fade away, and turn into heat. The number this heat is negligible, but in the world of acoustics all is in the nature of small perturbations, so how does this thermodynamic exchange, care for characteristics of the acoustic system. If you allow the sound waves inside speaker enclosure to hang out without supervision, a significant portion of energy will be scattered on contained within the housing volume of air, he let them and slightly, will heat up and will change the elasticity of the air volume, and upward rigidity. To prevent this from happening, apply filling the internal volume of the sound absorbing material. Absorbing the sound, this material (usually cotton, natural, synthetic, glass or mineral), absorbs and warm. Because are super significantly than that of air, the specific heat absorbent fibers, the temperature increase is much smaller and the dynamics seems to be behind him considerably greater volume than the not very . In practice, in this way it is possible to achieve an increase in the "acoustic" volume compared to the geometrical 15 to 20%. In this case, and not in the absorption of standing waves, as many believe, is the main point of the introduction sound-absorbing material in closed loudspeakers.
A variation of this (and not the previous one, as is often assumed) type acoustic design is the so-called "infinite" screen. In English-language sources, this type of design is called infinite baffle or free-air. All the names given to the same disorient. We're all adults here people and understand that infinite screen in practice can not be. In really is endless screen is considered to be a sealed box with a volume so great that the elasticity of the prisoner inside of air is much less the elastic suspension of the diffuser, so that speaker this elasticity does not notice and speaker specifications are determined only by the parameters of the head. Where passes the boundary, from which the volume of the box is like endless, depends on the parameters of the speaker. However, in solving practical tasks such volume alwaysis the internal volume of the trunk, which, even in a small car will give response "infinitely large" volume even for a big speaker. Another thing is that not every speaker will be to work well in this design, but we will discuss that separately, as we talking about the choice dynamics under acoustic design (or Vice versa).
For all (by the way, apparent) simplicity of the sealed box as acoustic design of low-frequency link automotive acoustics, this decision has many advantages possessed by the other, more intricate designs.
First, the simplicity (or almost ProData) calculation of performance. The closed box has only one parameter - the internal volume. So one can correctly to choose, if you try! Field for error is minimised here.
Secondly, in the entire frequency range, down to zero, the oscillations of the diffuser constrained by the elastic reaction of the air volume inside the box. This significantly reduces the likelihood of congestion dynamics and mechanical damage. Don't know how comforting it sounds, but avid bass speakers in the closed boxes, sometimes burning, but almost never "vypisyvatsja".
Thirdly, only a sealed box is the acoustic filter of the second order, that is, has the AFC decay below the resonance frequency of the system head-box a slope of 12 dB/Oct. Namely, such a slope, only in the opposite sign, has a frequency response of the internal volume of the interior of the car, below a certain frequency. If to guess, calculate or measure (as someone happen) - it is possible to get a perfectly horizontal frequency response at low frequencies.
Fourthly, with proper choice of parameters of the head and volume for her a sealed box has no equal in the field of pulse characteristics, largely determine the subjective perception of bass notes.
The natural question now - so what's the catch? If everything is so good, why need all the other types of acoustic design?
The catch one. Efficiency In the sealed box it is the smallest by compared with any other type of acoustic design. The less we will be able to make the volume of the box, while maintaining Otho same working frequency range, the lower its efficiency. No more insatiable creature in sense input power than a sealed box of small volume, which is why speakers in them, as has been said, though not vypisyvatsja, but they are often...
Next on the prevalence of type acoustic enclosure - bass reflex (ported., vented, bass-reflex), more humane towards the rear radiation the sides of the diffuser. In the phase inverter of the energy, which in a closed box "is placed to the wall" is used for peaceful purposes. To do this, the internal volume box communicates with the outside space of the tunnel containing the some mass of air. The mass value is selected so that, in the combination of the elasticity of the air inside the box to create a second oscillating the system is receiving power from the back side of the diffuser and her radiant need and in phase in the radiation of the diffuser. This effect is achieved in the not too a wide range of frequencies, from one to two octaves, but within its limits, K. p. D. increases substantially, according to the principle of "no waste is niesprzatanie resources".
In addition to the higher efficiency of the reflex port has also another important dignity is near the tuning frequency is significantly reduced amplitude fluctuations of the diffuser. This may at first glance seem like a paradox - how the presence of huge holes in the hull of the loudspeaker can restrain movement diffuser, but nevertheless it is a fact of life. In its working range the bass-reflex creates dynamics for absolutely greenhouse conditions, and exactly on the tuning frequency of the oscillation amplitude is minimal, and most of the sound is emitted by the tunnel. Allowable input power maximum, and distortion introduced by the speaker - on the contrary, minimum. Higher frequency settings the tunnel is becoming less and less "transparent" to sound vibrations, the expense of inertia enclosed inside air masses, and the loudspeaker works as closed. Below the tuning frequency, the opposite occurs: the inertia of undela is winding down and the lowest frequencies the speaker rabotaei almost no load, that is, as if it was pulled from the housing. The amplitude oscillations increases rapidly, and with it the risk of vyplevyvaniya diffuser or damage voice coils from hitting a magnetic system. In General, if not to protect themselves, the campaign for a new speaker becomes a real prospect.
Means of protection from these troubles, in addition to diligence in select the volume, is the use of filters subsonic frequencies. Cutting off part of the spectrum where no useful signal is not contained (below 25 - 30Hz), these filters do not allow the diffuser to go into the dressing at risk for own life and Your wallet.
The phase inverter is much more capricious choice of parameters and configuration since the choice for a specific speaker, subject to three parameters: the volume box, cross section and length of the tunnel. The tunnel is very often make I already finished the subwoofer you can adjust the length of the tunnel, changing the frequency settings.
Because of the presence of two interrelated oscillatory systems is the bass-reflex acoustic filter of the fourth order, that is, it has a theoretical frequency response the decline 24 dB/octave below the tuning frequency. (Really - from 18 to 24). Get horizontal AFC when installed in the cabin is almost impossible. According the ratio of the size of the salon (and, therefore, the characteristic frequency with which begins recovery response internal acoustics) and the tuning frequency of the bass reflex the description of characteristics may deviate from delicate to hump crazy Amur waves. Hump, then there is a smooth rise of the frequency response at low frequencies is often just order what you need for optimal subjective perception of bass in zashumlennym space, but sudden changes in amplitude when poor choice parameters earned the phase inverter, quite unfairly, the nickname boom-box ("booze"). To restore justice, note that thump effect can be achieved from the sealed box - next time I will explain how; and correctly calculated the phase inverter is able to give very clear and musical bass with reasonable input power.
A type bass reflex design is the speaker a passive radiator (or radiator). Foreign language terms: passive radiator, drone cone. Here the second oscillatory system to utilize energy remove from the back side of the diffuser, implemented in the form of a mass of air in the tunnel in the form of the second diffuser, nothing attached, but weighted up to trebanog mass. At the tuning frequency that varies with diffuser the greatest amplitude, and the main - least. With advancement up the frequency they gradually change their roles.
Until recently, this type of acoustic design is not found use in mobile installations, although in home is used quite often. The cause of dislike were unjustified the hassle of obtaining a second diffuser (this is usually the same speaker, but without a magnetic circuit and the voice coil) and difficulties in placing two large diffusers where a conventional bass-reflex necessary to place the diffuser and small tunnel. However, the most recent car subwoofers with passive radiator appeared - needs must. The fact is that in the last time began to appear dynamics of the new generation with a very long stroke the diffuser is designed to work in small volumes. The amount of "blown out" when they the work of the air is very high, and unnel would have to do a fall was entirely in diameter (otherwise the air velocity in the tunnel will rise so that it will sizzle as the locomotive). And the combination of small volume and large diameter tunnel makes to choose for the long tunnel. Here it was found that the conventional inverters designs for such heads adorned with a pipe meter length. To to avoid such unnecessary mishaps, preferred desired collabrasuite a lot of focus in the passive radiator with the speed of the diffuser, the same as the active speaker.
The third type of subwoofer, quite often used in autostroke (although less common than the previous two - a bandpass loudspeaker (bandpass). Sometimes common name - "loudspeaker with symmetrical load" (symmetric loading). If a sealed box and the bass-reflex acoustic filters top frequencies, bandpass, as evident from the title - combines filters the upper and lower frequencies.
The simplest bandpass loudspeaker - single 4-th order (single reflex). It consists of a closed volume, the so-called rear camera and a second, equipped with a tunnel as a conventional bass reflex (front camera). Speaker installed in the partition wall between the chambers so that both sides of the diffuser work the fully or partially enclosed volumes - hence the term "symmetric load".
From traditional designs bandpass loudspeaker, in any case - champion performance. The effectiveness is directly related to the bandwidth of transmittance. The frequency response of a bandpass loudspeaker has the form the bells. By choosing the appropriate volume and frequency settings of the front camera, you can build a subwoofer with a wide bandwidth, but limited efficiency, that is, the bell will be low and wide, with a narrow band and very high efficiency in this band. The bell will be pulled high.
Bandpass - capricious in the calculation and the most difficult to manufacture. Since the speaker is buried inside the case, you have to go to lengths for Assembly box so that you have a removable panel did not violate the rigidity and tightness design. Coordination of the frequency characteristics of the subwoofer, salon and front of acoustics is also associated with a known headache. Pulse characteristics is also not the best, especially when a wide band. What is compensated?
First of all, as mentioned higher efficiency
Secondly - the fact that the entire sound is emitted through the tunnel, and the speaker is fully closed. When building a subwoofer opens considerable opportunities for installer (or Amateur) with imagination. Enough to find a small spot on the junction of the trunk and cabin, where one can sleep the mouth of the tunnel - and the way powerful bass open. Especially for such installations firm JLAudio, for example, manufactures flexible plastic sleeves-tunnels, which it offers (and many agree) to connect the subwoofer output with the salon. Like the hose of a vacuum cleaner only thicker and stiffer.
Even greater efficiency have a bandpass speakers 6-th order with two tunnels. A camera of this subwoofer is configured with a spacing of about the octave. Double bandpass leads to less distortion in the working band, since the speaker is loaded with inverters on both sides of the diffuser, with all the advantages of such a load, but has a steeper, compared to single, the AFC decay below the operating band.
An intermediate position is occupied by so-called quasi-bandpass the speaker, who is a sequential setting, where the rear camera connected by a tunnel from the front, and the front another tunnel with others space.
Three-chamber bandpass speakers are just alternative constructive realization conventional bandpass, and is composed of two normal, then removed the wall separating them.
There are three variants of acoustic design of low-frequency acoustics, which, although sushestvuut, but use almost not found. The first of outsiders - acoustic labyrinth, where the extraction of energy from the back side the diffuser comes in a long tube, usually folded for compactness, but all still increasing the size of the subwoofer to the limits that are not valid in mobile installation.
The second is an exponential horn, which is to obtain a sufficiently low the cutoff frequency must be of cyclopean size, making its rarity use at low level even in stationary systems where space more than the car.
The third type having a single precedent of a loudspeaker in aperiodic load in the form of focused acoustic resistance (aperiodic membrane). We used to be called PASS - acoustic panel sorptivity. The idea is that the load of the cone is closely spaced semi-permeable barrier, for example, dense fabric or layer shaklovity, sandwiched between perforated panels. Theoretically, such a load is inelastic in nature and, as a shock absorber in automotive suspension, extinguishes acoustic energy without affecting the resonant frequency of the speaker. But this is theoretically. But in practice, the presence of air in the volume between the speaker and PASS created a hodgepodge of characteristics and reactions that the results became hard to predict.
So, from a cursory glance at the main types of acoustic design clear that perfection in the world. Any choice will be a compromise. And that creature compromise has become clearer, let's finish this correspondence meeting as it should be - by summing the subtotals. Compare the options considered from the point of view the main factors determining the success of their use in mobile the audio settings.
To these factors should be included:
K. P. D.
The magnitude of the efficiency inherent in any type of acoustic design determines ultimately how powerful the amplifier will need to achieve the desired volume level, and at the same time how difficult life will be the dynamics.
In the most important from the point of view of information reproduction-bass the frequency range 40 - 80 Hz prizes will be: narrowband bandpass the speakers are the Champions in this competition, especially dvuhkanalnyy 6th order. Behind them are broadband doctorally and conventional bass-reflex. And finally, the most eager for input power - closed box and broadband single bandpass.
The distortion introduced
In the bottom octave and a half range of music (30 - 80 Hz) all types acoustic design behave at small power levels. The bass-reflex and bandpass speaker - some better than others, but not by much. But at high powers rivals are stretched along the course. The best results are to be expected from a dual bandpass the loudspeaker. Behind him - the single bandpass and vented. And closes - sealed box, giving the greatest distortion at high signal amplitudes.
The impulse response
Accurate fronts bass instruments is perhaps the most important quality for acoustic bass. A little good low bass vain attempts, if they are oiled and sluggish. In this respect sealed box promises the best results (at the correct account).Transient characteristics of the port can be very decent, but still on average yield a closed-back design. Single bandpass speakers have good characteristics, which, however, deteriorate with the increase in bandwidth. The worst reaction to the impulse the signal has a dual bandpass speaker, again, in particular - broadband.
Coordination in acoustics front
The work of the subwoofer should be starting at a certain frequency, subcontracted the front midbass speakers. For a closed box and the port is not the problem and the system designer has considerable freedom in the choice of frequency partition strips, and since this frequency and slope are determined by external the chains. But narrowband bandpass often have their own downturns, custody starting from 70 - 80 Hz, where not all the midbass can safely catch the song. Requirements for the midbass in this case are complicated, and working with crossover is not getting any easier.
Place all the above in a table based on the usual us a five-point system:Feature Bandpass loudspeaker single double Sealed box The bass-reflex A narrow strip Wide band A narrow strip Wide band Distortion at low power 4 5 5 4 5 4 Distortion at large power 2 4 4 3 5 4 Pulse features 5 4 4 2 3 2 Alignment with the front acoustics 5 5 2 4 2 4 Overload capacity in operating range (above 30 Hz) Transfer interrupted! 4 5 4 5 4 Overload capacity in internetlocation range (below 30 Hz) 5 2 5 5 2 2 Smooth frequency response with regard to the internal acoustics of the car. 5 4 2 3 2 3 The sensitivity to errors for the design and manufacture 5 4 2 2 2 2
Author: Andrew Elyutin, Autosound; Publication: www.cxem.net