Subwoofer for the home, for the family. Part 3. Some tricks. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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How to increase the body of a subwoofer without resizing.

The word "fiber" has firmly entered our lives - we meet it in various manifestations: from fiber-optic cables that have entangled the whole world, to exotic dishes in expensive restaurants, when food is crushed to the state of fibers. But for the mighty box builders, the term "fiber" means first and foremost a magical ingredient* that can help achieve maximum bass from minimum size**.

* (We are talking about Dacron-polyester fiber wool. It is difficult to understand what it is, but I found some similarity. This material is called "synthetic winterizer" in the common people and is used as a lining in clothes and stuffing furniture. And we are talking about cotton wool, and not about sheet synthetic winterizer, although the latter is sold in any fabric store and can be used if it is slightly fluffed).

**(Although for me the term "fiber" primarily means MDF, or, in Russian, fiberboard - fiberboard. It's very convenient to sculpt boxes from it. I can't help but share my own observation - MDF is very hygroscopic and the main thing is that it lets air in! Let it be a little, but it lets it in. I advise you to soak it with some kind of filth, for example, drying oil).

There is no secret why the use of fiber core has been successfully used by experienced installers for many years. Take two exactly the same boxes, insert exactly the same speakers into them, fill one of them, and you will see for yourself that the filled box gives much more convincing low bass. Simply put, the filler tricks the speaker into believing it's in a larger box than it actually is. And the larger the box, the lower frequencies it can reproduce.

Filling the box is an alternative for those who do not want to give up a lot of space for a subwoofer box. Another similar alternative is the isobaric design, where a pair of speakers work. The disadvantage of isobarics is considered to be a more complex design, the need to buy two speakers instead of one, and a loss of sensitivity of almost 6dB, since the mass of the moving system is doubled. True, the latter, as a rule, is compensated by the parallel connection of the speakers.

The physical side of the work of the filler inside the box is no less interesting: The air inside the box during the operation of the speaker is very hot and becomes stiffer*. When the interior of the box is filled with fiber wool, the wiggle fibers dissipate heat, giving the speaker the appearance of a larger box.

Theoretically, filling the box can give a virtual increase in volume up to 40% of the true size. To put it simply, if you have a 20 liter box, it will look like a 28 liter box from the speaker's point of view if you fill it properly.

* (A distinction is made between heating from air compression and heating from the heat generated by the coil, some sources cited data on an increase in air temperature by two degrees for every 10W of power. Obviously, by air stiffness, the author meant its thermal expansion, as a result of which, from the point of view of the speaker, the air "behind" really becomes harder).

Today, three types of aggregate are most commonly used: glass wool, ordinary wool and polyester wool. Polyester wadding is the best choice because it doesn't have the fine particles that can damage your speaker or your skin like glass wool does, and is much less expensive than plain wadding. You can buy it at the furniture department of your nearest K-Mart or Home Depot, or at your nearest furniture factory.

To measure the filling efficiency, the author used an analyzer MLSSA and three test boxes - closed boxes 39 liters and 144 liters, and a phase inverter box 39 liters (port 7.5 cm in diameter and 15 cm long) with different filling densities. For closed boxes, the effective volume was determined from the resonant frequency of the system and the value of Qes. For a bass-reflex box, the tuning frequency of an empty box was compared with the tuning frequency of a filled box using the Speak for Windows program, which allows you to calculate the effective volume from the resonance difference. In each case, the results of the experiment were more than promising: in all three boxes, an increase in effective volume of 25 to 35 percent of the actual volume was obtained, using a filling level of 16 to 28 grams of aggregate per liter of internal volume.

When creating a system based on these data, one must keep in mind that the Qes and, consequently, the Qts of closed boxes are reduced. For a bass-reflex box, the lower impedance peak is reduced, and the upper peak is shifted slightly lower in frequency. Another interesting effect was also found - with an increase in the filling density, the resonant frequency of the system begins to increase again. The reason for this should be considered that the fibers lie so tightly that they cannot move and distribute heat efficiently.

It should also be noted that filling is less efficient in larger boxes.

Morality: the bigger your box, the harder it is to fool the speaker.

A few useful rules:

Fill small boxes - up to 85 liters - with a density of 24 grams per liter of volume and you will get a 30% increase in volume without any serious side effects. For large volume boxes, the filling should be approximately 16 grams/liter and the increase in volume will be approximately 25%. THIS IS THE REAL TRUTH!!!!!!!!! I have repeatedly verified this data in practice, the volume of the case actually increases by 30 percent.

After assembling the case, we calculate the useful volume (not forgetting to subtract the volume of the speaker itself and the beam), it should be at least 3 liters more than the program showed.

Then again, we will use the imagination without forgetting about the wallet and decorate the sub from the outside.

Any subwoofer needs a decent power amp and low pass filter.

In preparing the article, the materials of G.R. Tatevyan were partially used.

Author: Kostyan, akustik13@yandex.ru; Publication: radiokot.ru

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