Speaker stuffing

I thought stuffing would not be needed because a subwoofer operates at low frequencies. It looks like I was wrong.

Because my cabinet was so tall, there was a resonance at 120hz which needed work

The graph below shows the before and after responses of the "Blast Furnace" following treatment for standing waves. As you can see, the results are encouraging, with response being much smoother

The big dip at 200hz is due to the port resonance, although the measurement was very dependent on height of the SPL meter.

The peak at 120hz (driver to floor resonance) is more worrisome and is tackled by adding the stuffing

Points to note

Standing waves occur between the driver and walls, where the distance represents 1/4 wavelength. You can also get standing waves between opposite walls, where the distance represents 1/2 wavelength

Quarter Wavelength Problems

• Driver to Back Wall - 365mm - 235hz
• Driver to Side Wall - 70mm - 1228hz
• Driver Midpoint to Bottom Wall - 716mm - 120hz

  Half Wavelength Problems

• Front Wall to Back Wall - 365mm - 471hz
• Side to Side - 265mm - 649hz
• Top to Bottom - 990mm - 173hz
• Port "pipe mode" - 905mm - 190hz

Standing waves can be attacked on two fronts, as shown in this drawing

• The walls are high pressure, low velocity zones, so closed-cell foam rubber sheeting can absorb some energy. I used the same rubber that I utilised for the speaker mounting gaskets
  
  
• The area in the middle of the cabinet is a high velocity, low pressure zone, so Acoustic wool can be used to absorb energy there. Don't use fibreglass batts for this as the fibres get into your drivers. Spend the money and buy the proper stuff from your audio supplier - when last I checked, Jaycar was selling a piece 700mm * 1m for about $11aus, which will do more than one subwoofer

Update: I recently suggested these solutions on an online discussion forum, and looked at materials available on the internet, which produced the following observations:

• Lining the walls is not overly effective for low frequencies. Closed-cell foam works better than open-cell "egg-crate" style foam. Information on speaker lining materials was scarce, but for treating rooms, the typical thickness used is 100mm. The foam used here is only 10mm thick, however, as the above graph shows, it is still worth using.
  
  
• For this application, acoustic wool is not very effective below 300hz. Placing it across the cabinet as shown won't have much effect on these frequencies. It's still worth doing though, because the harmonics of these frequencies will still be attenuated.

Rod Elliot has also done some experiments to determine the effectiveness of damping ported boxes