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Building a DIY Speaker: Cabinet Design

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Based on low frequency response models for the Peerless 83084 XXLS woofer, the desired cabinet volume for the woofer alone is 2.2 cubic feet tuned to 27Hz.  To obtain this target volume, we have to also account for the volume needed for the RS150-8 midrange, volume of the port and the space occupied by the physical drivers in the system.  Cabinet designs can be done by hand or preferably using 3D computer aided design (CAD) software such as SolidWorks or SketchUp.  It is much easier to see what you are working with and make changes in CAD instead of having to recut cabinets.  Books could be written on cabinet design, but here are some basic things to think about before starting. 

  • Rounding over the cabinet edge reduces peaks and troughs in frequency response around the baffle step diffraction frequency.
  • Cabinet material and material thickness play a huge role in reducing the sound a cabinet makes.  Medium density fiberboard is the standard material because it is relatively inert, readily accessible and is pretty easy to work with.  Baltic birch and plywood are also popular choices.  In most cases, 3/4” or greater wood thickness is recommended.
  • Use plenty of internal cabinet bracing to reduce panel vibrations and space the bracing asymmetrically to reduce the effects of standing waves.
  • Countersink all drivers, the diffraction effects of a surface mounted driver might surprise you.
  • Chamfer the back of all open back driver cutouts to allow for better airflow behind the driver.  This is typically required for all drivers except the tweeter.
  • For 3-way systems, placing the midrange in a separate internal enclosure produces cleaner midrange performance.  It is a good idea to slant the back of the midrange chamber to prevent reradiating sound from the back of the chamber through the midrange cone.
  • Build enclosures a minimum of 10% larger than design requires as it is easy to fill dead space but hard to create volume once it is built
  • Determine proper port tuning by starting with a longer than calculated port tube length and reduce the port tube length to meet the target port tuning by measuring either impedance or acoustic output of the port.
  • Use flared port that is flared on both ends and has adequate diameter for the proposed vented enclosure and expected sound pressure levels.
  • Expect to invest a minimum of 3-5 full days of work for a high quality tower loudspeaker cabinet. 

D08_Precision_Port.jpgetermining the port parameters for a cabinet is not complicated but it is best to use a calculator on the manufacturer’s website.  Flared ports are not all the same and reaching the target tuning frequency is easy to determine by plugging in the necessary parameters into the manufacturer’s website.  Just note that it is best to leave the port tubes a little long and trim to reach the target tuning frequency.  Tuning an internal volume of 2.2 cubic feet to 27Hz requires a port tube that is approximately 9.7 inches long for a 3-inch diameter port.  This port occupies approximately 0.04 cubic feet so the internal cabinet should account for the port volume.  Also, it is important to determine the volume of internal bracing and estimate the volume of the drivers protruding into the cabinet. When all is said and done, add an additional 10% to the calculated required internal cabinet volume.  Once the internal volume is determined, it is pretty simple to determine the required cabinet dimensions.  Standing waves are greatly reduced when cabinet internal dimensions are not the same or multiples of the same distance apart. If you are brave, non-parallel walls should yield even better results.  For this project, the midrange is placed in a separate sealed space that is not included in the woofer cabinet volume calculation.  The midrange chamber benefits from non-parallel walls as well.  It is a good idea to run simulations on the midrange driver in the intended enclosure to confirm the enclosure size will not negatively effect frequency response. 

This project was designed in SolidWorks but could easily be done in Google SketchUp.  The 3D CAD programs allow for easy calculation of internal volume and are really handy for test fitting drivers and ports before building the cabinet.  I once designed a simple sub cabinet but didn’t realize the port length would interfere with the driver until the cabinet was finished.  I ended up rerouting the port using a series of bends that ended up causing port noise when the volume was cranked.

In this project, we used 3/4” Baltic birch with a 3/4” MDF baffle and bottom plate.  This was selected because the loudspeaker sides were subsequently covered with album covers and Lexan for a unique finish.  The panels were cut using a table saw and all chamfering and circular cutouts were completed using a quality plunge router.  A circle-cutting jig is a worthwhile investment as it provides accurate results in short order. The bottom of this speaker is removable with weather stripping between the detachable bottom and the rest of the cabinet.  A bottom firing port is attached to the bottom plate, which requires 2-3 inch long feet or spikes for clearance.  Although it is probably obvious, it is highly recommended to test fit everything before final assembly. When test fitting drivers, it is a good idea to drill holes and use hurricane nuts to allow secure mounting and remounting of drivers.  It is also recommended to use biscuit joints when gluing panel walls together although it is acceptable to use screws to hold panel walls together for gluing.  Once the glue is cured, all screws should be removed.  For this project, the baffle, bottom and back of the speaker were spray painted.  Spray painting MDF is time consuming because it requires several iterations of sanding and painting to obtain a smooth finish since MDF absorbs some paint, especially on corners that are rounded over.  Finally, album covers were glued to the cabinet and the covers were protected with a thin layer of Lexan.

 09_Test_Fit.jpg  

Test Fitting Drivers

Here is a list of materials and tools needed to pull off a typical cabinet build.

  • 3/4” Medium Density Fiberboard
  • Wood Glue
  • Wood Screws
  • Hurricane Nuts and Bolts for Driver Mounting
  • Acoustic Stuffing
  • Flared Port Tube
  • Binding Posts or Terminal Cup
  • Carpet Spikes or Feet
  • Electric Sander and Sandpaper
  • Cordless Drill
  • Drill Bits
  • Biscuit Cutting Tool and Biscuits
  • Table Saw with Solid Gate and Sharp Blade
  • Plunge Router
  • Series of Chamfer Bits for Router
  • Series of Round Over Bits for Router
  • 1/4” Straight Cutting Bit for Router
  • Circle Cutting Jig for Router
  • Tape Measure
  • Framing Square

There are a large number of possible finishes for a DIY loudspeaker project.  I have had success with paint and real wood veneer but both require a little practice and much patience. Once you have decided on a finish, do some research to determine a good application method.  I’ve never gotten a finish right the first time, so practicing on scrap material is highly advisable. 

 

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Recent Forum Posts:

utopianemo posts on July 14, 2020 00:06
It’s about time someone revisited this topic. There are so many sites out there now providing great kits! Aside from the previously mentioned Madisound, Parts Express, and others, we have GR-Research, DIYSG, CSS-Audio, Meniscus, and more.
erinpreston27 posts on July 19, 2013 00:27
I just finished a quick read of this.





fuzz092888 posts on July 18, 2013 16:32
It's great software, but don't forget to factor in the cost of LMS as well. You'll need it. If you've never designed a speaker before you might want to go with soundeasy. It's much cheaper and does nearly everything that LEAP does.

audiofox, post: 977984
I always wanted to splurge and get a copy of this software for my speaker building hobby-this is the SW that Madisound uses for their custom crossover and speaker design services.

LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers
audiofox posts on July 18, 2013 13:58
I always wanted to splurge and get a copy of this software for my speaker building hobby-this is the SW that Madisound uses for their custom crossover and speaker design services.

LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers
Joel Foust posts on July 07, 2013 13:10
I've heard the art USB dual pre works in soundeasy and is decent for the money. I have no direct experience so I can't recommend it but it might be worth researching for < $100 interface. Anyone use it?
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