The Subwoofer DIY Page
"Extended Bass Shelf" systems
last updated: 13 April 2011
The Subwoofer DIY Page

"Extended Bass Shelf", or EBS, systems are ported systems that are usually characterized as follows:
  • The volume of the box is larger (sometimes significantly larger) than that of a maximally flat ported system using the same driver.
  • The tuning frequency of the enclosure is at or close to Fs, the driver's resonance frequency
  • The power handling of the EBS is lower in the midbass frequencies when compared to the maximally flat systems. However, the power handling at lower frequencies is usually better.
  • As with the standard ported systems, almost any driver can be used. However, the best results will be obtained with drivers of Qts <0.35.

One of the main reasons given for using an EBS system instead of the usual "maximally-flat" systems is to take advantage of "room-gain", the increase the bass response at lower frequencies, starting at around 1dB at 30 Hz and increasing to 9dB at 20 Hz, that you get in most rooms. This "room-gain" can tend to produce a boomy response with speakers designed to maximally flat to frequencies below 30 Hz.

To design an EBS system, use any box frequency response modelling software (like my spreadsheet) and model the response of a system using your driver, where the box size is 160% to 180% the volume suggested for a maximally flat response. Then, set the tuning frequency to near Fs for the driver.

If the cabinet is too large, the frequency response curve will be peaked below the shelf. Too small, and the shelf will be attenuated. Adjust the cabinet size to suit.

If the tuning frequency is too low, the shelf will roll off prematurely. Too high and the response peaks at the bottom of the shelf and the premature rolloff occurs again. Adjust the tuning frequency accordingly.

When tuned just right, the shelf level is not too low, and reasonably extended, with a smooth low end rolloff. Note also that if the box is very big compared to what's called for in a maximally flat alignment, there's a loss in midrange efficiency that may be more important than the small increase in low frequency output.

The above notes are based on correspondence sent to the BASS and DIY Loudspeakers Mailing List by the contributors listed below. You may wish to contact them for further details and assistance.

Roger Andersson
Peter Basel
Aaron Bohnen
Dan Wiggins
Mike McCall

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