The so called "Acoustic bass" register of pipe organs is designed to
improve bass notes. For example, using a 16' note with a fifth above,
you hear a "virtual" note which sounds one octave below, in the 32'
range.  That note, softer than the sound of a real pipe of corresponding
size, is used in small organs, where doubling bass pipes would take too
much space.

You can listen to an MP3 example ("Phantom Bass revisited") at:
<http://www.dogstar.dantimax.dk/acoustic/acoust-1.htm>

This tuning in fifth is similar to the Power chords of Rock guitarists,
that get their "oomph" from the notes perceived one octave lower.
I personally use dulcimers in a medieval ensemble; they are tuned in
the same fifth relationship (C3-G3-C4), which results in a sound more
impressive than one would expect.

"Acoustic bass" in mechanical organs?

In MMD (1998), Ron Schmuck wrote: "I have found over years of tuning
fair organs that the builders often built-up sound by adding one pipe
in a mixture which was tuned a forth (or fifth) higher than the note
being played. (...)  If two notes at an interval of a fifth are played
together (C & G), a note one octave below the lower note will be heard.
Many people feel this combination of two notes played a fifth apart
produce a better sound than one pipe playing the octave below."

My question to organ experts: Do you know small portable organ models
using "bass helpers"?

I simulated this acoustic bass effect with the medieval tune Sumer is
icumen (Summer's comin') in a Midi file (Sumer26G.mid) with the
Alderman 26-note scale going down to G (around 196 Hz, like many busker
organs).  To each one of the 4 basses (G-A-C-D) I added the fifth
(D-E-G-A) voiced at a lower volume.  Even if we perceive just a "dark
hue" around the bass notes, I find it useful.

Of course, arrangers can use the fifth pipes already existing in the
accompaniment section of any scale.  But these notes, sounding at the
same volume as the bass pipes they are helping, add a less subtle
padding.  I use them sparingly, e.g. on the accented beats, in Mancini's
Baby Elephant Walk, on a Raffin 31-note scale: (BabyEl31G.mid)

Roland Tremblay, Montreal