Hi everybody, this is my first instructables, I hope you'll appreciate. I apologize in advance for my bad english...

Before to start with the introduction, I want to spend some words about the Instructables group of italian authors. If you're italian, please join the group. Contanct me, Darthorso or Lindarose92. We'll provide to add you in the group.

Unfortunely I have already completed the project and I can't provide you photos about the construction of the wood box and the filter. This project is a prototype that I will improve, replacing the actual woofers with other top performing.

The entire project has been build starting from zero, without any base. The low pass filter that is mounted into the box has been realized with inductor and capacitor constructed at the time. The calculations of the filter and the kind of it will be illustrated in the subsequent steps.

P.S. If you have any problem with the realisation, don't be shy, write me.

Step 1: Designing the Box

It may seem strange but the idea was born, observing the shoe that I have in my room. The shape I liked it right away, a rectangular square and compact. Then I equipped with a meter, and I have carried out the measurements of the box to make it happen. Before starting to cut every single panel, I have researched on internet and on DIY magazines what kind of material was better to build the box. At the end I chose the MDF panels.

A lot of audio speakers are made using MDF. This particular kind of wood is made from fine sawdust mixed with adhesive and pressed under high pressure, easy processing, good seal bonding, good availability make it the most widely used material in absolute. But as each material has disadvantages first of all it contains urea-formaldehyde which is a probable carcinogen and may cause allergy, eye and lung irritation when cutting and sanding.When MDF is cut a large quantity of dust particles are released into the air. It is important that a respirator be worn, for the entire time of exposure, and the material be cut in a controlled and ventilated environment. It is a good practice to seal the exposed edges to limit the emissions from the binders contained in this material. I personally use a respirator 3M, model 4079. It's very important for your health. Health is worth more than anything else.

Step 2: Parts and Components for the Realization

If you mount the box as in the photo, the configuration of the panels is as follows.
To realize the box of this subwoofer you will need:

  • 2 MDF 44 x 30.5 (front and back)

  • 2 MDF 46.5 x 29.7 (top-down)

  • 2 MDF 30 x 30 (side)

  • Wood screws M5

  • Polyurethane foam to insulate the box acoustically.

Obviusly some tool are needed for the realisation of the workpieces:

  • A jigsaw
  • Grinder with abrasive disks
  • A drill with bits of different diameters
  • A pair of Goggles
  • A respirator
  • Big clamps for tighten the wood
  • Glue
  • A set of screwdrivers
  • Sandpaper

To realize the low-pass filter will need:

  • 2 inductances 1.8 mH
  • 2 uF capacitors from 56.3
  • Connecting Cables
  • 4 PCB terminal blocks
  • 1 prototyping board
  • An RLC meter for measuring the capacitors and inductances

P.S. All the quotes are in cm not in inch.

Step 3: Impedance of the Speakers

These speaker were bought on ebay in 2009 at the modest price of 0,01 € practically free :D

Before proceeding to the calculation of the filter and the cutoff frequency of the same, we must necessarily measure the impedance of the speaker at different frequencies. This measure will be useful at a later time, to determine the mechanical resonance frequency of the speaker and to know how far the speaker is able to reproduce a signal. For measure the impedance, we need to use an impedance meter and a frequency meter.

Let's start with some theory, I know it's boring, but is needed to understand how it all works.

The impedance

When crossing an inductance or a capacitance any AC voltage encounters resistance whose ohmic value varies with frequency. This resistance to the passage of current is called the reactance and is indicated with XL for induttance and with XC for the capacity. The impedance of a coil, a transformer, a loudspeaker or the input stage of a preamplifier, is always expressed in ohms and indicated by the letter Z. The value of Z takes account of the ohmic resistance of the copper wire, the reactance of the winding, stray capacitance, etc. ...
For this reason, the value in ohms of impedance can not be measured with any tester. To measure this size we need a tool called impedance meter. An impedance meter must be able to indicate the exact impedance value of an inductance, a coil of a transformer, the crossover filters etc. ... To obtain this condition is absolutely necessary that this tool has a low frequency oscillator (BF) can provide a sinusoidal signal of constant amplitude which, starting from a minimum of 20Hz can reach a maximum of 20KHz.

Impedance vs frequency

In this step you also attach diagrams impedance vs. frequency from loudspeakers. The measurements were made in the first analysis, with speakers in free air, then were tested channels of low-pass filter and finally the measures were made with a closed box. I want to clarify that I have not practiced the agreement in bass reflex because as I said at the beginning this subwoofer is experimental. Even if I wanted to I could not make it due to lack of sufficient data on the speakers. These speakers have been sold to me with no data factory, except for the nominal impedance. So I do not have enough data to calculate the bass reflex duct. In the final version will be mounted hose agreement to break down the mechanical resonance frequency and the box will be closed permanently and pitted.

Step 4: Calculation of Filtres

Now we are at good point. After obtaining the measures of speaker impedance, we can proceed to the calculation of the filters. Considering that we are using 2 woofers we should build a low-pass filter, to only pass low frequencies and attenuating high frequencies. Usually the subwoofer reproduce from 20Hz to 250Hz. My sub reproduces from 20Hz to 500Hz this is because during the measurements I noticed that up to 500Hz it had a good response. So I decided to design the filters so that they had a cutoff frequency of 500Hz. The filter in question is passive and has an attenuation of 12 dB per octave. The filter is composed by 2 components, 1 inductor and 1 capacitor.

This is the wiring diagram with the formulas. The values are expressed in microFarad and in milliHenry. The insulation voltage of the capacitor is 500 volts. As regards the section of the copper wire of the inductors, this must be dimensioned according to the power amplifier, the voltage across the outputs of the same and according to the load to be supplied. The section I used is given by the following mathematical relationship.
S = pi * (d ^ 2/4) where S is the section in mm ^ 2 d is the diameter of the cable in mm.

Step 5: Costruction of the Box

As said earlier I could not share the photos of the practical realization of the box. To remedy will tell you the processing steps to follow:

  • 1) Cut the MDF according to the measurements
  • 2) Drill holes in the front panel to the positioning of the woofer
  • 3) Drill the panel at the front of the mounting holes of the woofer
  • 4) Paste the side panels at the front and below. During the period of drying of the glue using clamps to hold the panels blocked.
  • 5) Build the low-pass filter with two channels
  • 6) Drill a hole in the back of the case and enter the terminal.
  • 7) Connect the output of the amplifier input to the terminal blocks.
  • 8) Connect the other end of the terminal (the one inside the box) to the input of the filter.
  • 9) Connect the outputs of the filters to the speakers
  • 10) Paste the sound absorbing material on the walls of the box, except the one where the speakers are mounted
  • 11) Drill the top panel, so as to be able to screw the fixing screws.
  • 12) Connect all the speakers and test

I know, is not a very comprehensive article and I apologize for that, unfortunately without pictures I can not realize the different stages of construction.

Step 6: Test the System

If you did everything right, your subwoofer should work the first time. The unit in question has a power rating of 100W. The amplifier can deliver 50W +50 W per channel. I hope you enjoyed as a first instructables. Since some time I'm working on an amp that I would like to combine these speakers. Thank you for viewing.

This is registered to a few instructables contest, if you liked it please rate it. Thanks

Cià guagliò :D

I'm building a custom box for an 18 inch subwoofer for a home theater system with 3 4 inch ports need to know where I can purchase the 4-inch ports and the wire connector for the back of the box
Thank you all
<p>This is awesome and the explanation is really good!! Remember to take more photos for your next project, I'm looking forward to seeing what it will be! :D</p>
<p>My congratulations Gabriele! A great ible, full of instructions and interesting info!</p><p>Well done! :D</p>

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