Internal Damping for Bass Cabinets - General Guidance

This page offers some general guidance internal damping for your bass cabinet. 

Bass cabinets require internal damping material to absorb midrange energy and to artificially increase the cabinet volume experienced by the driver. 

What We Recommend 

This general guidance is suggested for all bass reflex and sealed cabinet plans listed on our site. 

Fully line the interior walls of the cabinet with 1" thick sound absorbent foam. For reference, we use Mcmaster Carr Sound Aborbing Sheet 5692T13. This particular product just happens to use a washable surface that is hermetically sealed from absorbing liquids. The sealed surface has been chosen for its ability to act as a diaphragmatic absorber. This sound pressure to act on the foam like a piston, rather than transmitting through the foam unaffected. 

Physical Application 

Apply the foam with adhesive spray. We recommend 3M Supper 77 adhesive spray. 
Rear Wall 

We recommend a double layer of foam on the rear wall of the enclosure so that the primary wave is absorbed early. 

Ports

Care must be taken to ensure that the foam does not block the entrance to the port tubes or vents. Leave a decent amount of space around these areas to prevent turbulence noise created from the edges of the foam itself. 

Driver

Care must be taken to ensure adequate 'breathing' of the woofer. Do not block the driver. 

Additional Steps: PolyFill (Pillow Stuffing)

We also suggest adding a pillow or two depending on the cabinet size. The pillow can be stapled into place at the edges of the pillow fabric. The stuffing amount can range from 50% to 75% being careful not to block the ports or touch the woofer's diaphragm. There should be a clear air path from the driver to the port opening(s). 
Final Adjustment and Tuning 
Take some time to critically listen to the following configurations
 
1) No polyfill (pillow stuffing)
2) 50% full 
When listening, play music content with deep male vocals (ie. 
Leonard Cohen) and observe for any 'chestiness' or 'boxiness' sound. If the sound is colored in this manner, add more pillow stuffing until it sounds acceptable. You may need to stuff the cabinet up to 80% to fully damp the internal resonances. Do not compress the pillow when stuffing into the enclosure as it will lose some of its damping properties. 
Too Much Damping 
If you've gone too far with the damping the music will sound somewhat dull and lifeless. This is a personal preference so it is best to decide for yourself what kind of sonic character you are looking for with your music. 
Experimentation 
A little experimentation can go a long way towards achieving your desired sound. Consider these variables to improve bass quality as well as other attributes such as stereo imaging.
  • Distance from front wall 
  • Toe-in 
  • Listening position and height 
  • Speaker Height 
  • 4ohm or 8ohm taps (if using tube amps) 
  • Floor coupling or isolation 
    • We recommend ISO Acoustics Gaia isolation feet.