Damping of acoustics with bass reflex


Created on 19.12.2006 00:15.

Updated 04.10.2020 10:43.

Author: M. Linovitsky.

The word “fiber” has firmly entered our lives – we meet it in various forms: 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 powerful box builders, the term “fiber” refers primarily to the magic ingredient * that can help maximize bass from minimum sizes **.

* (We are talking about fiber wool made of dacron-polyester. What it is is difficult to understand, but I, with the help of my wife, found some similarity. This material is called in the common people “synthetic winterizer” and is used as a liner in clothes and upholstery. we are talking specifically about cotton wool, and not about sheet padding polyester, 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 allows air to pass through! Let it pass a little, but it does. I advise you to saturate it with some nasty thing, for example drying oil).

There is no secret why the use of fiber filler has been successfully used by experienced installers for many years. Take two completely identical boxes, insert exactly the same speakers into them, fill one of them, and you will see with your own eyes that the filled box gives much more convincing low bass.

Simply put, the filler tricks the speaker into thinking that it is in a box larger than it actually is. And the larger the box, the lower the frequencies it can reproduce.

Box filling is an alternative for people who do not want to give a lot of space for a subwoofer box.

Another similar alternative is the isobaric design, where a pair of speakers are used. 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 by almost 6 dB, since the mass of the moving system doubles. True, the latter, as a rule, is compensated by the parallel connection of the speakers.

The physical side of the damping operation inside the acoustics is no less interesting: the air inside the box heats up a lot during operation of the speaker and at the same time becomes tougher ***. When the interior of the box is filled with fiber wool, the wiggling fibers dissipate heat, giving the speaker the appearance of a larger box.

Theoretically, filling the box of acoustics can give a virtual increase in volume up to 40% of the true dimensions. Simply put, if you have a box with a volume of 20 liters, then, if filled correctly, from the point of view of the speaker, it will look like a box with a size of 28 liters.

*** (Distinguish between heating from air compression and heating from the heat emitted by the coil, some sources cited data on an increase in air temperature by two degrees for every 10W of power. Obviously, the author meant its thermal expansion by air hardness, as a result of which, from the point of view of dynamics, the air “behind the back” really becomes tougher).

Today, three types of filler are most commonly used: glass wool, just cotton wool, and polyester wool. Polyester wool is the best choice because it has no small particles that can damage your speaker or your skin like glass wool and is much cheaper than plain wool. You can buy it from the furniture department of your nearest K-Mart or Home Depot, or from your nearest furniture factory. (For those living in Russia, please take this as a joke and look for it in more suitable places for this, for example, in American mattresses).

To measure the filling efficiency, the author used an MLSSA analyzer and three test boxes – closed boxes of 39 liters and 144 liters, and a phase inverter box of 39 liters (bass reflex 7.5 cm in diameter and 15 cm in length) 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 one using the Speak for Windows program, which allows calculating the effective volume from the difference in resonances.

In each case, the experimental results 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 fill level of 16 to 28 grams of aggregate per liter of internal volume.

Table 1. Closed box 39 Lt

Filling density (gram / liter) Resonance frequency (Fsb, Hz) Effective volume (liters) Increase percentage
0 56.6 39
eight 53.0 45.3 fourteen%
12 52.7 48 21%
24 51.7 51 29%
28 50.8 53.8 36%
41.9 50.4 45.3 fourteen%
49.6 52.6 34 -fourteen%

Table 2. Closed box 144 liters

Filling density (gram / liter) Resonance frequency (Fsb, Hz) Effective volume (liters) Increase percentage
0 42.0 144
4 42.0 144 0%
eight 41.2 164 fourteen%
12 40.3 175.5 22%
16 39.4 184 27%
twenty 38.6 184 27%
24 40.2 158.6 nine%

Table 3. Phase inverter 39 litas

Filling density (gram / liter) Resonance frequency (Fsb, Hz) Effective volume (liters) Increase percentage
0 42.0 39
6.4 39.1 45.3 fourteen%
13.6 37.2 51 29%
twenty 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, it should be borne in mind that Qes and, therefore, Qts closed boxes goes down. For a bass reflex box, the lower peak of the impedance decreases, and the upper peak is shifted slightly lower in frequency. Another interesting effect was also discovered – with an increase in the filling density, the resonant frequency of the system begins to increase again. The reason for this should be considered the fact that the fibers are so tight that they cannot move and efficiently distribute heat.

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

Moral: The bigger your boxing, 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 boxes, the filling should be about 16 grams / liter and the volume increase will be about 25%.

Translation of the article “Fill’er up” or “Make a small box act like a larger one with polyester fiberfill” by Tom Nousaine, published in 1995 in Car stereo review®, 1999.