Build a Home Cinema Subwoofer Cabinet


Unless you have a subwoofer, you don't have enough bass. We will work on the basis that you have woofers already and want to build a subwoofer. If you have a normal bass knob and turning it up already makes a decent boom, you are in the running.

Steps

  1. Get some free software to figure out the box size.
  2. Type a speaker's details into the software. These funny numbers are T & S parameters named after some Aussie engineers. Sensitivity of a speaker is a measure of efficiency. Try and find one that is rated at more than 90 dB/watt at one metre and can take about 100 watts RMS of power. Parts express, Jaycar, and eBay are all good for various sub parts. Car subwoofer speakers WILL WORK FINE, as long as the overall ohm impedance is not lower than the receiver's.
  3. See if the software recommends a vented design or not. Try a few speakers until you find one that is suitable. Good ones are vented for efficiency and offer low low bass. This project has no vent [[1]] but is still a solid performer. However, by cutting a round hole in it of the right diameter with the right length tube it would really take off down low.
  4. Figure out how big a box you will be happy with. This may involve consulting your life partner. {{safesubst:#invoke:convert|convert}} is pretty serious for a 12 or 15 inch speaker {{safesubst:#invoke:convert|convert}} for a 10 inch. To calculate the volume you will be using the inside dimensions in cm (that way you will get the volume in milliliters) minus any bracing, amplifier, the back of the speaker and vents. Errors of 5% should not matter much.
  5. Use the software to show you how the speaker will perform in your maximum sized box. You will then be able to adjust the box's "tuning frequency" - don't worry about what this value is too much; just look at the graph and see if the vent is a reasonable length and diameter (see warnings).
  6. Set the box's tuning frequency up, so that the line representing your speaker stays between 0 dB and -3dB for as long as possible towards the left (lower frequencies and therefore bass.) If it is still above -3dB at 35 Hz, you have some decent bass. The tuning frequency should end up somewhere between 20 Hz (wow you have got a decent speaker) and 40 Hz. All other things being equal, go lower. Below the tuning frequency speakers are relatively unhappy, so it pays to go low.
  7. Figure out the sized panels you will need to make a box from particle board or Medium Density Fiber fiberboard (MDF) 18mm thick or more should be fine. (there is no need for any fanciness, just butt the pieces up against each other) Then get your hardware store to cut it. They will be straighter and squarer and more accurate than you (probably) which will make it easy to assemble.
  8. Glue (use pva wood glue in large quantities) and screw your sub together. Screws at least twice as long as your MDF is thick are best. You will need to drill clearance holes to allow the screw to pass through plus smaller holes in the other pieces to allow the thread to go in. Go crazy and glue and screw some bits of wood onto the walls of the box to make it stiff. The box should vibrate as little as possible (the vibrating speaker makes the sound not the box - concrete anyone?) and be leak proof. Don't forget these brace bits (cleats) will change your box volume!
  9. Cut holes for the speaker, the vent (make sure the end inside the box will be 1 vent diameter from everything), and the amplifier. 'Plate' amplifiers designed to mount in boxes are easy. Just remember to make sure that it does not touch the walls of its hole anywhere to avoid damage. Also remember it is a mains voltage device. If you spend 1.5 times on the amplifier what you did (or should have) on the speaker you will be OK. Matching the speaker to the amp watt for watt is good too. With an efficient speaker 200 watts is very very loud.
  10. Launch yourself at the box with the PVA glue again, this time on the inside. Are all your corners sealed?
  11. Attach some plush carpet or proper speaker 'acoustic wool' (resembling the inside of a synthetic doona) to at least two walls of the box. Use glue. Do not stuff the box solid, this technique is for sealed, not vented boxes.
  12. Screw in the components and connect the speaker to the amplifier.
  13. Use a long 'RCA' lead to connect the input on the plate amplifier to the .1 output on your home theatre amplifier. If you don't have one, most plate amps will create their own subwoofer channel and return the rest to your amplifier or direct to your speakers (see its manual).
  14. Listen for problems. If it is a bit quiet and the auto on function does not work right (most plate amps will turn on when there is a signal) and if the plate amp has a stereo input then buy a cable that splits the single channel .1 into two identical wires and connect them to the stereo plate amp inputs.
  15. Read some kind of home theatre magazine on how to set up a subwoofer in a room. Or put it in the front corner and try the phase switch in both directions (don't panic if you can't tell any difference).

Tips

  • If the total price for this project goes over about 250 USD seriously consider whether you should buy a pro unit or get some more detailed help and advice. Don't forget you are paying for a speaker you have never heard.
  • If you have the speaker pointing at the floor and put the subwoofer on legs (at least 5cm off the floor) you won't need a grill. You can also recess the panel the speaker screws onto a few cm to protect the speaker. (don't forget this will change your internal volume)
  • Gluing cotton backed vinyl to the front, sides and back with a piece of laminated wood on the top to cover the joins provides a cheap finish.
  • Select an amplifier whose RMS (continuous) power extends well above that of the subwoofer(NOTE: The rated power is lowered with the increasing size of the box). Not only will the amplifier remain much cooler during operation, this will give the amplifier plenty of headroom to output clean sound instead of distorted. You're much more likely to blow a subwoofer due to a distorted, underpowered signal as opposed to overdriving it with a clean, unclipped signal.
  • Have a look at commercial units for good ideas.
  • Contrary to what is mentioned in this article, you won't necessarily want an amplifier that costs 1.5 times the price of the subwoofer driver. What is going to make the biggest difference in the end is the quality of the subwoofer, not the amplifier. Spend your bucks on a very high quality subwoofer. Later on, a better amplifier can be purchased.
  • The best recommendation for placing your subwoofer is to set it in your seating location, and walk around your room while playing music. When you stumble (no pun intended) upon the area that sounds to your liking, place the subwoofer in that location. Some may like the booming sound of placing it in a corner, others will prefer the smoothness that comes from placing it along a wall or any other area that's away from the corner. Nonetheless, this should prevent you from placing it in a dead zone.

Warnings

  • The vent must have a large enough diameter or the sub will wheeze like a pack-a-day-smoker. If you are using something about 8cm in diameter or larger (or a series of smaller ones) you will be hard pressed to go wrong. Let the software figure out multiple vent size, you can't just double whatever it recommends. Speaker vent (or port) 'mach factor' is calculated by some software and will warn you if it is too small. An older version of the software recommended above will do this - the new version shows an air velocity graph with a danger zone starting at 17 m/sec
  • This is a really really basic guide. Get interested read widely and make something much better than this guide alone will show you. See for example http://www.diysubwoofers.org/ for some amazing information and links.
  • MDF dust is evil, especially in countries (like Australia) where they still use formaldehyde glue. Wear goggles and a dust mask.
  • You may need a qualified technical to wire it inside, many plate amplifiers have exposed components and could shock you or cause a fire if badly installed.
  • Don't turn up the subwoofer so loud it gets all blurred and drummy. This is distortion and it will destroy it eventually. You can destroy a 500 watt speaker with an overdriven 80 watt amp.

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