The subwoofer for the car. Part 2
"Then I began to consider violin volume of the box, and this work was long and exciting. ...Volume cannot be reduced - violin will nasipit, will start muffled chant. If you increase shrill screams, the bass will become dull and weak...", A. A. Weiner, G. A. Weiner, "a Visit to Minotaur".
In the previous part of our conversation it became clear than good different types acoustic design and the bad. It would seem that now the objectives are clear, for the work comrades.." there it was. First, acoustic design, in which not installed the actual speaker is just with varying degrees of thoroughness collected box. And often to collect something it cannot yet be determined, a speaker will be in her set. Secondly, and this is the main fun in the design and manufacture of car subwoofers - features subwoofer not worth much outside of the context characteristics, at least the most basic, car, where it will work. There is also third. Mobile acoustic the system is equally adapted for any music - rarely achieved ideal. A competent installer, you can usually know what, "deposing" the client requesting the audio sound, he asked to bring samples of what the client will listen on delivery of the system after its completion.
As can be seen, the factors influencing decision - and a lot to reduce everything to simple and unique recipes, there is no possibility that makes building mobile audiostrobe is much activity in the related art. But some General guidelines outline is still possible.
Timid, lazy and humanitarian educated I hasten to warn - formulas will not. As long as possible, try to do even without calculator - forgotten through oral accounts.
Subwoofers are the only link automotive acoustics, where the dimension of harmony algebra - it is not hopeless. Straighter say - without calculation to design the subwoofer is simply unthinkable. As the same source of data for this calculation are the parameters of the speaker. What? Yeah, not the ones you hypnotize in the store, to be sure! For the calculation, even the most approximate, characteristics of the woofer must know Electromechanical parameters, which is darkness. This resonance frequency, and the mass of the moving system, and the induction in the gap of the magnetic system and at least two dozen indicators that are understandable and not. Upset? It is not surprising. As upset was about twenty years ago, two Australian - Richard Small and Neville Thiel. They suggested that instead of mountains of figures to use universal and quite compact set of characteristics, perpetuated, deservedly, the names of. Now that you will see in the description of the dynamics of the table entitled Thiel/Small parameters (or just T/S) - you know what it was about. And if such the table you don't find one, go to the next option - this is hopeless.
The minimum set of characteristics that you will need to figure out is:
- Self-resonant frequency Fs dynamics
- Full quality factor Qts
- Equivalent volume Vas.
Natural frequency is the frequency of resonance of the speaker without any acoustic design. It is measured speaker suspended in the air on the greatest possible distance from the surrounding objects, so now its resonance will depend only on its own characteristics - mass a mobile system and suspension stiffness. There is a perception that the lower resonant the frequency, the better will the subwoofer. This is only partly true, for some designs too low resonance frequency - interference. For reference: low this 20 - 25 Hz. Below 20 Hz are rare. Above 40 Hz is considered high, for the subwoofer.
Full figure of merit
The figure of merit in this case is not the quality of the product, and the ratio of elastic and viscous forces existing in the moving system dynamics near the resonance frequency. The mobile system dynamics in much akin to the suspension car, where there's a spring and a shock absorber. The spring creates an elastic force, there are stores and releases energy in the process of oscillation and shock - source viscous resistance, he does not accumulate, and absorbs and dissipates in the form of heat. The same thing happens when vibrations of the diffuser and everything to it attached. A high value of q means that is dominated by elastic the forces. It is like a car without shock absorbers. Enough to run over a pebble and the wheel starts to jump, no inhibitions. Jump on the resonant the frequency of this oscillatory system.
In relation to the loudspeaker means the emission frequency characteristics at the resonance frequency, Bali, the higher the complete quality system. The high q-factor of thousands - bells, which as a result, neither at what frequency, in addition to the resonant sound is not willing, the benefit is also that from this him and no one calls.
Popular diagnostic method of the suspension of the machine by rocking - nothing like the measurement of q pendants artisanal. If you now lead suspension in order, to attach parallel to the spring shock released during the compression of the spring energy is not all back, and in part will be ruined the shock absorber. This is reducing the quality factor of the system. Now again back to the dynamics. Nothing we walk back and forth? This, they say, is useful...With a spring in dynamics all, like, okay. This pendant diffuser. And a shock absorber? Shock absorbers - two running in parallel.
The full dynamics of merit consists of two: mechanical and electrical. Mechanical the quality factor is mainly determined by the choice of material of the suspension, and mostly centering washer and not the external corrugation, as is sometimes thought. Large losses here usually does not happen and the contribution of the mechanical quality factor in not full exceed 10 - 15%. The main contribution is owned electric figure of merit. The the hard absorber employed in the oscillatory system dynamics is the ensemble from the voice coil and magnet. Being by nature an electric motor, and relies motor can operate as a generator and this near busy the resonance frequency when the speed and amplitude of movement of the voice coil - maximum. Moving in a magnetic field, the coil produces a current, and load for this generator is the output resistance of the amplifier, that is, almost - zero. It turns out the same electric brake, which is equipped with all of the train. There, too, during braking, the traction motors are forced to work in mode of generators and load their batteries braking resistors on the roof.
The magnitude of the generated current will, of course, the more, the stronger the magnetic field, which moves the voice coil. It turns out that the powerful magnet speaker, the lower, ceteris paribus, it the figure of merit. But, of course, as in the formation of this magnitude are involved and the length of the wire winding, and the width of the gap in the magnetic system, the final output only on the basis of the size of the magnet, it would be premature to do. And preliminary - why not?...
Basic concepts - is considered to be a low full q-factor of the dynamics of less than 0.3 - 0,35; high - greater than 0.5 to 0.6.
Most modern heads loudspeakers based on the principle of "acoustic suspension". We sometimes called "compression", which is wrong. The compression head is a different story associated with the application in the role of acoustic design loudspeakers.
The concept of acoustic suspension is to install dynamics in this volume of air, elasticity which is comparable to the elasticity of the suspension dynamics. It turns out that in parallel to the existing one in the suspension spring set one. Equivalent volume is one in which winavi appeared equal in the spring elasticity is already available. The size of the equivalent volume is determined by the stiffness suspension dynamics and diameter. The softer the suspension, the greater will be the magnitude airbag, the presence of which will begin to disturb the speaker. The same occurs with the change of the diameter of the diffuser. Large diffuser at the same the offset will be stronger to compress the air inside the box, thereby experiencing greater return elastic force of the air volume.
This circumstance often determines the choice of speaker size, based on from the available capacity to accommodate its acoustic design. Large diffusers create the preconditions for high performance subwoofer, but require large the volumes. The argument from the repertoire of the room down the school corridor "and I more" here should be applied with caution.
Equivalent volume of interesting family ties with a resonant frequency, without awareness are easy to miss. The resonant frequency is determined the stiffness of the suspension and the mass of the moving system, and equivalent volume - the diameter of the diffuser and the same stiffness.
As a result of this situation. Suppose you have two speakers the same size and with the same resonance frequency. But only one of them this frequency is happened due to heavy diffuser and hard suspension, and the other - on the contrary, a light diffuser for a soft suspension. Equivalent volume at such a couple, with all the apparent similarities may vary very significantly, and when installed in the same box the results will be dramatically different.
So, finding that the mean vital parameters will finally start to choose her spouse. Model unless you believe that you have determined, on the basis of, for example, the materials of the previous article of this series, with the type of acoustic clearance and now we have to choose for him speaker of the hundreds of alternatives. Having mastered this is the inverse process, i.e. the choice of appropriate design selected under the speaker will be given to you easily. I mean, almost without effort.
As mentioned in the article, a sealed box is the simplest Akusticheskie design, but not primitive, on the contrary, having, in especially in the car, a number of important advantages over other methods. Popularity its mobile apps does not quenched, because it's a start.
What happens to the dynamics characteristics when installed in a sealed box? It depends on a single value - the volume of a box. If the amount so great that speaker hardly notices, we come to option infinite screen. In practice, this situation is achieved when the volume of the box (or other enclosed volume behind the diffuser, or simply what is there to hide the trunk of a car) exceeds the equivalent volume dynamics three times or more. If this ratio is, the resonant frequency and full quality factor of the system will remain virtually the same as they had dynamics. And that means them and choose accordingly.
You know, what speaker system would have the most smooth frequency for full value of q equal to 0.7. At lower the values are improved impulse response, but the decline of custody begins quite high in frequency. Under high - frequency response becomes rise close to resonance and transient performance is slightly worse. If do you focus on classical music, jazz or acoustic genres the optimal choice will be somewhat pretentiously system with q 0.5 to 0.7. For the more energetic genres will not hurt underline, bottom, is achieved when the quality factor is 0.8 - 0.9. And finally, fans of the rap would have a blast the full program if the system will possess a quality factor equal to one or even higher. The value of 1.2 is necessary, perhaps, to recognize the limit for any genre, applying for musicality.
It must be kept in mind that when you install a subwoofer in the car happens the rise of low frequencies, starting with a certain frequency, due to the size the salon. Typical values for a start lifting frequency response 40 Hz for big machine, like jeep or a van; 50 - 60 for medium, like eight or "loin"; 70 - 75 for a small, Taurus.
Clearly, for installation in endless mode screen (or Freeair if you do not mind that the last name patented Stillwater Designs) need speaker with full quality factor of not less than 0.5 or greater and a resonant frequency not below the Hertz commercials 40 to 60, depending on what will be put. Such parameters typically indicate a fairly stiff suspension, the only thing that saves speaker overload in the absence of acoustic support" the closed volume. Here is an example - firm Infinity releases in the series and Reference Kappa versions of the same heads with indexes br (bass reflex) and ib (infinite baffle).
Parameters Thiel-small, for example, ten inch Reference differ so:The Parameter T/S 1000w.br 1000w.ib Fs 26 Hz 40 Hz Qts 0,28 0,77 Vas 83 l 50 l
It is seen that option ib on the resonant frequency and quality factor - ready to work "as is", and based on both the resonance frequency and is equivalent to this modification is much stiffer than the other, optimized for the phase inverter, and, therefore, more likely to survive in difficult conditions Freeair.
What would happen if, ignoring small letters, you are driven into these conditions are similar as two drops of water speaker with an index of br? Here's what: due to the low quality factor of the frequency response starts to be filled up already frequencies around 70 - 80 Hz, and unchecked the "mild" head itself to feel very uncomfortable at the lower edge of the range, and overload it there - easier than ever.
For use in the "infinite" screen you need to select a speaker with high overall q-factor (not less than 0.5) and resonant frequency (not less than 45 Hz), specifying these requirements depending on the predominant type of music material and size of the cabin.
Now about the "finite" amount. If you put a speaker in volume, comparable with its equivalent volume, the system will have characteristics significantly different from those with which this system was the speaker. First of all, when installation in closed volume will increase the resonant frequency. Stiffness- increased, and the mass remains the same. Will increase the quality factor. Judge for yourself - having put in using the stiffness of the suspension stiffness is small, that is, unyielding air volume, we thus put a second spring and a shock absorber left the old one.
With a decrease of the quality factor of the system and its resonant frequency grow in the same way. So if we saw a speaker with q, say, of 0.25, and want to have a system with a factor of, say, 0.75 l, the resonance frequency is also will triple. And what is it there at the speaker? 35 Hz? So, right, with the shape of the frequency response, the amount of it will be 105 Hz, and this, you know, not a subwoofer. So - on fit. "You see, the calculator does not needed. Watch another. The resonant frequency of 25 Hz, a factor of 0.4. The result is a system with a factor of 0.75 and a resonance frequency of about 47 Hz. Gangbusters. Will try immediately, without departing from the counter, to estimate what the volume will need a box. It is written that Vas = 160 l (or 6 cu.ft, more likely).
(There would be a formula to write - it is simple but not promised). Therefore, for calculations at the counter ladies cheat sheet: copy and put in your wallet, if buy bass dynamics included in the plans of your shopping experience:The resonant frequency and quality factor will increase in If the volume of the box is Vas 1.4 times 1 1.7 times 1/2 2 times 1/3 3 times 1/8
We have about half, so it's a box with a volume of 50 litres - 60. Too much will....Let us next. And so on.
It turns out that to make out a conceivable acoustic design, the parameters of the speaker is not enough that should be in any particular the corridor values, but also be linked to one another.
This linkage experienced people brought in indicator Fs/Qts.
If the value of Fs/Qts is 50 or less, speaker born for the sealed box. The required volume of the box will be the smaller, the lower Fs or less than the Vas.
External data "natural born hermits" is heavy diffusers and soft suspensions (which gives a low resonant frequency), not very large magnets (so the quality was not too low), long sound the coils (because the speed of the diffuser at the dynamics of working in a closed box, can reach quite large values).
Another type of popular acoustic design - phase inverter, with all burning desire at the counter you can not count, even approximately. But to estimate the suitability for him dynamics. And as for the calculation we are going to talk separately.
The resonant frequency of this type of system is defined not only one resonant frequency dynamics, but also the setting of the phase inverter. The same applies the quality factor, which can vary significantly with a change in the length tunnel even for the same volume of the housing. Since the phase inverter may be, in unlike the sealed box, tuned to a frequency close to or even lower than that dynamics, resonance frequency of the head "allowed" to be higher than in the previous case. This means that with the right choice, easier diffuser and, as a consequence, the improvement in pulse characteristics, what is the phase inverter needs, because its "innate" transient characteristics is not the best, worse than the sealed box, at least. But the figure of merit is desirable to have perhaps below, not more than 0,35. Keeping it in the same ratio Fs/Qts, the selection formula the dynamics for the phase inverter looks like this:
To work in a bass-reflex dynamics approach, in which the indicator Fs/Qts is 90 and more.
The outward signs of a phase-breed: light diffusers and powerful magnets.
Bandpass (very briefly)
Bandpass speaker, with all its loud advantages (in this the sense of maximum efficiency, in comparison with other types) is the most complex in the calculation and manufacture, and the harmonization of their characteristics with internal acoustics car with insufficient experience can turn into a living hell, so this kind of acoustic design it is better to go on the stones and take advantage of the recommendations of the manufacturers of speakers, though it ties his hands. However, if the hands remain in a decoupled condition and are itching to try: single bandpass fit almost the same dynamics as for of inverters, and for double or quasimolecular - they are the same or, more preferably, the head with the index Fs/Qts is $ 100 and above.
How to calculate already moving away from the counter next time.
Author: Andrew Elyutin, Autosound; Publication: www.avtozvuk.com, www.cxem.net