How to make a small box big or something about padding. The art of audio

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How to make a small box big or something about padding

The art of audio

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 the magic ingredient.*, which can help achieve the maximum bass from the minimum size**.

*(We are talking about fiber wool made of dacron-polyester. It is difficult to understand what it is, but with the help of my wife, I found some similarity. This material is called "synthetic winterizer" in the common people and is used as a liner in clothes and stuffing furniture. And speech it is 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 most importantly - let the air in! Let it be a little, but let it in. I advise you to impregnate 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 that it is in a box that is larger in volume than it actually is. And the larger the box, the lower the frequency it can reproduce.

Filling the box is an alternative for people who don't 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 inside 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 the hardness of the air, the author meant its thermal expansion, as a result of which, from the point of view of the speaker, air "behind the back" really gets tougher).

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. (For those living in Russia, please take this as a joke and look for it in more suitable places, such as American mattresses).

To measure the filling efficiency, the author used the MLSSA analyzer and three test boxes - closed boxes of 39 liters and 144 liters, and a bass-reflex box of 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.

closed box 39 liters
Fill density (gram/liter)	resonant frequency (Fsb, Hz)	Effective volume (liters)	Percent increase
0	56	39	--
8	53.0	45.3	14%
12	52.7	48	21%
24	51.7	51	29%
28	50.8	53.8	36%
41.6	50.4	45.3	14%
49.6	52.6	34	-14%

Closed box 144 liters
Fill density (gram/liter)	resonant frequency (Fsb, Hz)	Effective volume (liters)	Percent increase
0	42.0	144	--
4	42.0	144	0%
8	41.2	164	14%
12	40.3	175.5	22%
16	39.4	184	27%
20	38.6	184	27%
24	40.2	158.6	9%

Bass reflex box 39 liters
Fill density (gram/liter)	resonant frequency (Fsb, Hz)	Effective volume (liters)	Percent increase
0	42.0	39	--
6.4	39.1	45.3	14%
13.6	37.2	51	29%
20	35.2	53.8	36%
22.4	34.2	56.6	43%
28	35.2	53.8	36%

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

Some 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 larger boxes, the filling should be approximately 16 grams/litre and the volume increase will be approximately 25%.© ML

Publication: www.bluesmobil.com/shikhman

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