Rules of Design

• There are three indisputable facts to bear in mind when designing a speaker enclosure:
  
  1) Sound is produced by the movement of a speaker's cone material, which is exposed on both sides. This means that fully half of a speaker's sound production issues from the back of the unit. Good sound quality means controlling the sound output.
  
  2) Sound itself is the transmission of kinetic energy through air; the air forms a medium between the speaker and closest object. If the closest object is thin and flexible, it will absorb energy, vibrate and resonate at different frequency than the speaker, emitting a sound of its own. This dulls the audio system's sound and causes "drone."
  
  3) Soundwaves reflect like light, and can be absorbed by soft materials.
  
  

Material

• Forget everything you think you know about fiberglass subwoofer enclosures; they look good, but their thin, flexible walls ensure that they always sound like garbage. For complete control over your sound system's output, you'll need to use a thick, dense material that resists vibration. Use a medium density fiber-board (MDF) to create the enclosure. MDF is heavy, but it has the density of a black hole in concrete, and will do a great job of resisting vibration.
  
  

Shape

• Different interior enclosure shapes will yield different sounds, but one rule remains constant: avoid surfaces that are round or parallel to each other (ex: spherical, cylindrical, cubic and rectangular shapes). Parallel surfaces will bounce sound back and forth, causing wave cancellation and drone. Round surfaces will cause different parts of the sound wave to reflect at different angles, making the output "fuzzy" and indistinct. Cylindrical "cannon" enclosures certainly are loud, but they don't sound very good.
  
  One of the easiest and most efficient of enclosures is the "inverted pyramid." Placing the back of your speaker at the bottom of an inverted pyramid with 45 degree sides will cause sound to reflect directly back toward the speaker hole with a minimal loss of energy.
  
  

Outside Finish

• You can build whatever cosmetic cover for the speaker enclosure that you like, but some types may actually affect sound. Stretched fiberglass cosmetic enclosures might look good when executed tastefully, but can still vibrate when sound bounces back from the car's interior. Consider a flat, wooden exterior covered with acoustic damping material and carpet for the best sound.