HDP: 18 – 03
The Original Internal Resonator Banjo: the Dobson Great Echo
(Dated: August 2, 2018)
C. E. Dobson designed, patented in 1888, produced, and marketed a banjo, the
Great Echo, whose unique feature was an internal resonator. Although the instru-
ments he made had a single-piece spun-over metal rim, his patent acknowledged that
it could be made of wood and achieve the same desired goal: a more mellow sound
and a slight “echo.” In spite of the fact that examples survive to this day, the Great
Echo is never mentioned in discussions of the history internal resonators, which al-
ways begin with F. Bacon’s 1906 patent and ensuing production of his ﬀ Professional.
Nor does the Great Echo appear in any current histories of 19th Century banjo.
Starting with a salvaged 93
00 Great Echo rim, I cobbled together a 2000 scale 5-
string. Sound samples of this and of a full-size instrument are included.
∗email@example.com; http://www.its.caltech.edu/˜politzer; Pasadena CA 91125
Invention of the internal resonator has always been attributed to Fred Bacon. His patent
was granted in 1906. A virtuoso stage performer, Bacon originally contracted out the
actual manufacture. The growing popularity of his internal resonator 5-string banjo, the “ﬀ
Professional,” led him eventually to organize his own production facility. The Bacon Banjo
Company grew to be large and very successful in the jazz age, producing deluxe 4-string
resonator banjos. The internal resonator design continues to have its enthusiasts among
5-string players. Over the years, small companies and individual luthiers have produced
copies and variants.
These are “open-back” banjos. The internal resonator does not make them louder or
given any impression of being louder. This is in contrast to the wrap-around, solid-back
resonator of bluegrass and jazz banjos. Rather, the internal resonator is thought to make
the sound a bit more mellow and bestow some sort of enhanced “presence” to the instru-
ment. In his patent application, Bacon described his invention “whereby a more lasting
tone is produced and the quality of same improved. ...[The design provides] the rim with a
peculiarly-constructed annular chamber within which the partly-conﬁned air can vibrate in
harmony with the strings and cooperate therewith to produce a strong and resonant tone.”
Unlike the many fanciful and hyperbolic design claims from the late 19th and early 20th
Century, this one certainly has basis in acoustical science, both from analysis of recordings
and from basic theory.
But Charles Edgar Dobson, Jr. was awarded a patent in 1888 for the same idea. In
that patent he wrote,
“The hoop B is preferably made of metal such as brass, German silver, or nickel though it may
be made of wood, and is of the usual dimensions. Extending from the back edge of the hoop B and
ﬂaring inward is a second or supplemental hoop or rim, b, which terminates about a half an inch (more or
less) from the head O. The hoop bis made of the same material as the hoop B, and when metal is employed
it is preferably made integral with the hoop B, it being turned inward by the spinning process. The ﬂare
or angle of the hoop bis such as to leave a considerable space between the two hoops, as shown, and it is
found that the eﬀect of the air partially conﬁned in this space between the two
hoops is such as to greatly modify and improve the tone of the instrument by
causing a kind of echo which deprives the tone of much of its harsh and metallic
quality.” (Boldface is my edit.)
FIG. 1. C. E. Dobson’s 1888 patent; see the APPENDIX below for the full text.
Dobson went into production of this design, christened “The Great Echo Banjo.” Judging
by photos on-line of surviving instruments, the rims were all constructed from a single, spun-
over piece of metal. But Dobson emphasized that the same idea would work when fabricated
out of wood. Of the half dozen photos I have seen, the serial numbers range from 188 to
4063. However, there is no telling whether all the numbers in between were Great Echos
nor how many more Dobson actually made.
The half dozen Great Echoes mentioned above are from recent years’ seller’s advertise-
ments and collectors’ photos. They seem to be regarded as rare but not unheard-of. Never-
theless, I do not recall it ever having been mentioned in any discussion of internal resonators
or more general banjo histories. (I’ve read quite a few.) The book that comes the closest is
the ﬁne 1999 history by Philip Gura and James Bollman, America’s instrument: the banjo
in the nineteenth century. The ﬁve Dobson brothers are featured quite prominently, a re-
ﬂection of their renown and impact that spanned the second half of that century. Gura &
Bollman describe many of the Dobsons’ patents and include photos of extant examples and
reproductions of the patent sketches. They describe three of Charles Edgar’s patents and
imply that those three are all he ever ﬁled. They missed the Great Echo.
The Dobson Brothers
The ﬁve Dobson brothers of New York, New York collectively had a huge impact on the
banjo in America. Their ages spanned twenty six years. Charles Edgar was the second oldest.
Starting in the 1850’s and running into the 20th Century, they were renowned performers,
teachers, writers, innovators, and manufacturers. Between them, they were awarded a great
many patents. The metal tone ring, frets, ﬁngering tablature, top tensioning, and a solid
back are among the many things for which one of the Dobsons has reasonable claim for the
innovation. They pursued patent protection. One case, pressed by Henry Clay and Charles
Edgar, the two oldest brothers, famously went to the U. S. Supreme Court in 1893. They
That the U. S. Patent Oﬃce would approve Bacon’s application in 1906 is not extraor-
dinary, but why no Dobson response? In 1906, the eldest Dobson, Henry Clay, was 74, and
he passed away in 1908. Charles Edgar was then 67 and passed away in 1910. And in 1910,
Bacon’s operation was not yet in full swing. He was still contracting out the manufacturing.
Dobson and Bacon tone rings
Henry Clay’s (earliest?) 1867 patent included a top-tensioned head and a resonator back.
Many patents followed. In 1881, he introduced what we now call a tone ring. It featured
a horizontal ﬂange, about an inch wide and concave-down. It remains a popular design
option to this day, oﬀered by many independent luthiers on their new instruments. Bacon
included a similar wide-ﬂange tone ring in his 1906 patent and all subsequent ﬀ Professional
production. This tone ring continues to have its enthusiasts, although maybe not as many as
the Dobson. The salient diﬀerence is that the Bacon ﬂange is concave-up. The resemblance
is obvious and often commented upon. And the simplest acoustics analysis would predict
similar behavior. However, it is not surprising that people with good ears who care a
lot about the sound can hear diﬀerences. Both from basic physics and from experience,
the geometry of the region right under the edge of the head has signiﬁcant impact. The
simple physics observation is that the highest frequency sounds are radiated from a narrow
region near the edge. In practice, many luthiers and their discerning customers are very
particular about that edge geometry. For example, they can hear the diﬀerence between
diﬀerent radii of curvature of the lip at the top of the rim. And they can hear an eﬀect of a
bevel cut into the inside of the rim just below the head.
Curiously, the Great Echo apparently did not feature a Dobson tone-ring-style ﬂange.
FIG. 2. Bill Rickard’s Dobson-style ring (left) and Stewart-McDonald’s Bacon-style
Air inside the annular region, i.e., between the inner wall and the rim, resonates at a
much lower frequency than the lowest resonance of the same size rim without that wall.
That contributes to the more mellow sound. But the very lowest frequency air resonance
within the body of any stringed instrument has a qualitatively diﬀerent origin and behavior.
Named after Helmholtz, the ﬁrst physicist who analyzed it, it involves air going in and out
of a sound hole while all the air inside the body contracts and expands. It’s responsible for
the note you hear when blowing across the mouth of a bottle. The sound hole of an open
back banjo is the opening between the banjo and the player’s body that allows the air to go
The internal resonator design makes that opening smaller. That, in turn, lowers the
frequency of the Helmholtz resonance. The Bacon all-wood design tends to make that
opening smaller than the spun-over Dobson. However, how the player holds the banjo has
an even greater eﬀect on the eﬀective size of the opening. So, that distinction between the
two designs is not signiﬁcant.
Cobbled Great Echo Banjeaurine
I bought a salvaged but battered 93
00 Great Echo rim. The inner wall is an inch in from
the rim at its top and has a 7
00 gap from the underside of the head. The rim is 23
It came with a set of unmatched hooks and nuts reminiscent of the motley crew of the
Hispaniola. I already had a 1925 Vega 4-string neck. (It had come along for the ride when
I bought a 5-string conversion some years ago.) The dowel stick was about an inch longer
than the rim diameter. Rather than cut it down and end up with the bridge rather close
to the tailpiece, I crafted an extension to the neck, a crude block construction. I installed a
5th string tuner and a little side block to support a screw head pip. This sort of bricolage
is certainly not my invention. It’s no problem that the 5th string ﬂoats over thin air if
one never frets that string anyway. However, I am fond of fretting the ﬁrst three strings
above the 17th fret, which was as far as the Vega fretboard went. So I included a fretboard
extension in my design. Again, this is not my invention but is, in fact, rather common on
short scale banjos. This one’s scale is 2000 . It required that I mount a skin head because the
rim had more than one dent and was out of round and not ﬂat.
The neck angle as determined by the set of the dowel stick in the neck did not really
match the rim. The lay of the dowel stick within the rim is non-negotiable. It’s metal and
has a unique channel that surrounds the dowel stick. I’m not equipped to re-set a dowel
stick. So I had to compensate for the discrepancy with the bridge. 100 high was required to
get reasonable string action. (Typical bridge heights fall in the range 1
00 to 11
Needless to say, it sounds like a banjo — or, more precisely a banjeaurine. “Banjeaurine”
is the instrument and spelling invented by S. S. Stewart in 1885. It has a short-scale and is
typically tuned a 4th or 5th higher than standard. Stewart invented it to serve as one of the
lead instruments of the banjo orchestra. (Yes, banjo orchestras were a big thing in his day,
and he was their most enthusiastic promoter.) The peculiar spelling was to suggest an air
of reﬁnement. Stewart’s earliest models had 121
00 rims (to make them louder). For a variety
of reasons Stewart and the many other manufacturers who copied his idea soon settled on
10 to 11 inches.
At ﬁrst I was surprised by the sustain. It was longer than I’m used to. I suspect
that’s what Dobson meant by “echo” (or Bacon meant by “a more lasting tone”). Then
I remembered that the Bacon-style internal resonators I installed on a couple of favorite
regular size open backs also produced extra sustain. In those cases, I trimmed that with
lighter bridges. The ﬁrst 100 bridge I made for my Great Echo came out to 1.9 gm, a weight
which is already very much on the light side of the typical 2.6 gm. So I left it at that —
and appreciate the echo.
Here’s what it looks like:
The actual sound
I found a sound sample on-line from Huﬀ & Meade, a banjo-ﬁddle duet who’ve recorded
some ﬁne tunes that you can ﬁnd on their site. Here’s just a little snippet on a Dobson
Great Echo banjo:
Click here or go to
When I’ve tried to ﬁgure out the physics of some particular banjo design element, I’d
compare the actual sounds with that element altered or removed. For now, I do not have
anything that would serve as an experimental control sample to compare to the Great Echo
banjeaurine that I assembled. The following is just to conﬁrm that it sounds like a banjo:
Click or go to http://www.its.caltech.edu/~politzer/GreatEcho/tip.mp3
APPENDIX: the Great Echo patent text
 Fred Bacon’s 1906 patent: https://patents.google.com/patent/US823985
 D. Politzer, Physics of the Bacon Internal Resonator Banjo ,
http://www.its.caltech.edu/~politzer, HDP: 16 – 02; scroll down to JUNE 2016.
 C. E. Dobson’s 1888 banjo patent: https://www.google.com/patents/US392381
 Bill Rickard manufactures and sells very ﬁne parts and whole banjos; see
 Stewart-McDonald is a major U. S. luthier supply company; see https://www.stewmac.com/
 D. Politzer, A Bacon Tone Ring on an Open-Back Banjo, http://www.its.caltech.edu/~politzer,
HDP: 16 – 01; scroll down to APRIL 2016.
 The sounds radiating from adjoining regions of the head as they go oppositely up and down
tend to cancel. The regions at the edge have the fewest canceling neighbors.
 For Huﬀ & Meade’s music, see https://huﬀmeade.bandcamp.com/