U.S. patent number 3,892,159 [Application Number 05/494,662] was granted by the patent office on 1975-07-01 for soundboard-bridge configuration for acoustic guitars.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Adrianus J. M. Houtsma.
United States Patent |
3,892,159 |
Houtsma |
July 1, 1975 |
Soundboard-bridge configuration for acoustic guitars
Abstract
The bridge of a lute type instrument, such as a guitar, is
supported by a bridge plate under the top of the instrument. The
bridge plate is mounted to a stiff, rigid bracing in a manner which
allows the plate to rotate about an axis in the direction of the
neck of the instrument while preventing it from rotating forward
under the tension of the strings. The bracing does not touch the
top of the sound chamber except permissibily near the edges of the
top. The thickness of the bridge plate varies monotonically
transverse to the string direction to achieve optimal sound energy
transmission over the frequency range of the instrument.
Inventors: |
Houtsma; Adrianus J. M.
(Cambridge, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
23965435 |
Appl.
No.: |
05/494,662 |
Filed: |
August 5, 1974 |
Current U.S.
Class: |
84/307;
84/291 |
Current CPC
Class: |
G10D
1/08 (20130101) |
Current International
Class: |
G10D
1/08 (20060101); G10D 1/00 (20060101); G10d
003/04 () |
Field of
Search: |
;84/291,298,307 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tomsky; Stephen J.
Assistant Examiner: Miska; Vit N.
Attorney, Agent or Firm: Smith, Jr.; Arthur A. Santa; Martin
M. Shaw; Robert
Claims
What is claimed is:
1. A stringed musical instrument of the lute type having a hollow
body consisting of a top, a back and sides, a neck, strings tuned
to increasing pitches from low on one side to high on the other
side running generally parallel to said neck and terminating on a
bridge attached to the top, the improvement comprising said bridge
being also flexibly attached to a structure of at least one brace
extending between said sides, neck or tailblock, said brace
structure being separate from said top leaving a space between said
brace structure and said top over the entire, or nearly entire,
surface area of said top.
2. The invention as claimed in claim 1 in which a bridge plate is
rigidly attached to said top underneath said bridge and in addition
said bridge plate is rotatably attached to said brace structure at
least two points which are along the intersection of said bridge
plate and the plane bisecting the instrument, so that the bridge
plate is free to rotate about an axis extending between said two
points.
3. The invention as claimed in claim 2 in which some of said braces
are attached to both said top at their extremes near said sides and
to said sides in order to obtain a better bond between said top and
said sides.
4. The invention as claimed in claim 2 in which the thickness of
said bridge plate varies monotonically from side to side in a
direction perpendicular to said strings so that its mass is largest
at the side where the lowest tuned string terminates and smallest
where the highest tuned string terminates.
Description
This invention relates to stringed musical instruments of the lute
type, having steel, gut or nylon strings, such as the lute, the
classical guitar and the folk guitar. In this class of instruments
the strings terminate on a device, called the bridge, which is
attached to the top or soundboard of the instrument.
In prior art instruments, the top was reinforced by braces glued to
the top resulting in an instrument which was inefficient for the
bass frequencies. Reinforcing was required to make the instrument
sufficiently strong to hold the tension of the strings.
Accordingly, the objects of this invention are:
First, to increase the instrument's efficiency of sound radiation
without making the instrument overly fragile.
Second, to provide a soundboard that radiates equally efficient at
all audible frequencies, resulting in a better balanced tone
quality.
Third, to support the bridge assembly and prevent it from tilting
in the direction of the force exerted on it by the strings,
resulting in a much better tuning stability.
Fourth, to provide virtually unlimited structural stability to the
instrument without adversely affecting its sound quality.
For an understanding of the nature and objects of the invention,
reference should be had to the following detailed descriptions,
taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a top plan view of a stringed instrument in which the
invention has been incorporated.
FIG. 2 is a perspective view from the rear side of the instrument
with the back removed.
FIG. 3 is a detailed side view of one of the large cross braces
shown in FIG. 2.
FIG. 4 is a fragmentary sectional view of the bridge and its
suspension, taken on the line 4 -- 4 in FIG. 1.
FIGS. 5 and 6 are sectional views of the bridge suspension taken on
lines 5 -- 5 and 6 -- 6 in FIG. 2 as seen from the tailblock.
Referring to the drawings, FIG. 1 shows an acoustical musical
instrument of the lute type, in this case a guitar. The guitar has
a body 3 consisting of a top 7, a back (not shown), sides 9, and a
neck 10 which ends on a head 11 and is covered with a fretted
fingerboard 12. At the bottom end, the sides top and back are held
together by a tailblock 13. Strings 14 are stretched from the head
where they are fastened to tuning machines 15, over the nut 16,
approximately parallel to fingerboard 12, terminating on bridge 17
where they pass over saddle 18 and are secured by bridge pins 19.
Linings 20 are glued along the top and bottom of sides and form the
main connection between sides, top and back. Braces are anchored in
the top linings 20, providing structural support for sides 9 and
additional bonding between top 7 and sides 9, as shown in FIG. 2.
The particular brace pattern shown in this figure is not of very
great importance; other equally effective layouts are possible. The
braces are cut away at the top, entirely or partially, as shown in
FIGS. 2 and 3, so that only a small part of the bracing network,
near lining 20, are attached to the top 7. Referring to FIG. 2, the
top clears most of the bracing network 21 by at least 4 mm and is
glued only to the extremes of braces 21, linings 20 and tailblock
13. The bridge 17 and bridge plate 22, both made from a strong
hardwood, are glued on either side of top 7 as shown in FIG. 4, and
are pinned together by two or three small dowels 23. The bridge
plate 22 is connected to braces 24a, 24b at top and bottom center
by two metal or plastic brackets 25, 25' and pins 2, as shown in
FIGS. 2, 4, 5 and 6. Although these brackets should generally be in
the center of the bridge plate to obtain tone and mechanical
balance, they may in some cases be off center. The top bracket 25,
the one closest to sound hole 1, is notched into the bridge plate
22 to form a compression joint (FIG. 6), while the bottom bracket
25', shown in FIG. 5, is anchored as a tension member. Both
brackets are connected with braces 24a, 24b by plastic or metal
pins 2. Both pinjoints (brackets and pins) should be sufficiently
strong to withstand the bending movement produced by the tension on
the strings.
The structural demands on string instruments of the lute type are
very particular because the strings terminate on the bridge.
Especially when steel strings are employed, the force exerted on
the bridge by the strings can be as high as 400 pounds. Referring
to FIG. 1, the tension in strings 14 causes a shear force and a
forward bending moment on the joint between bridge 17 and top 7. If
the total string tension is 400 pounds and the strings clear the
top by 0.3 inches (these figures are typical for a 12 string folk
guitar), the shear force is 400 pounds and the bending moment is
400 .times. 0.3/12 = 10 ft-pounds. The shear force causes an
equally large tension force in the wood fibers of the top 7 between
bridge 17 and tailblock 13. Wood fibers are usually strong enough
to hold such a tensile force. The bending moment, however, would
cause severe distortion of the top, leading to stress
concentrations and eventually breakage, if the top were not
properly reinforced with braces.
Acoustically the guitar top 7 is the principal sound radiating
element. Mechanical energy stored in the vibrating strings causes
the bridge 17 to move in a variety of possible modes, e.g., up and
down, forward and backward, and rocking from side to side. The
amount of motion is determined by the vibration frequencies, and
the mass and stiffness of the bridge - bridge plate assembly 17,
22. Generally speaking, vibration frequencies that coincide with
natural mode frequencies of the top 7 will cause relatively much
motion, resulting in a strong sound. At other frequencies the top
is less willing to oscillate, and these tones will sound
weaker.
Conventional lute type instruments have the braces glued over their
entire length to the underside of the top. In such designs
acoustical and structural considerations always interfere. For a
well balanced response of the lower frequencies, the top needs to
be much more compliant than structural requirements allow. Hence
one always ends up with an instrument that may be structurally
strong but has little or no bass response, or an instrument with a
well balanced tone that is too fragile.
The present invention allows one to satisfy and optimize both
structural and acoustical requirements without interference. When
the guitar is strung up, the bending moment on the bridge caused by
the strings is counterbalanced by reaction forces in pinjoints 2.
Braces 24a, 24b and the entire bracing network 21 can be made
sufficiently strong to withstand such reaction force. Because the
braces are not touching the top 7 except in a limited area very
near linings 20, they will not cause any stiffness effect on the
top and render sufficient compliance for maximum acoustical
efficiency at low frequencies. Because of pinjoints 2 the bridge
and bridgeplate assembly 17, 22 is free to rock from side to side,
and due to the special bracing the entire bridge and top assembly
17, 7, 22 can also move up and down to a large degree. Other modes
of motion play a role only at higher frequencies where stiffness is
an asset rather than a liability. Extra stiffness braces may still
be applied locally to top 7 for optimum high frequency response,
but in the present invention their proper sizes and locations are
derived entirely from acoustical considerations and they play no
structural role. Stiffness braces have been omitted from the
drawings.
Another critical factor which determines the efficiency of sound
radiation is the mass or weight distribution of the bridge - bridge
plate assembly 17, 22. Because the bridge is the only visible part
of this assembly, its size and shape can be determined largely
using aesthetical considerations; the thickness of bridge plate 22
can be tapered, as shown in FIGS. 5 and 6, so that the total mass
distribution of the entire assembly will be optimal for best
balanced sound transmission over the entire frequency range of the
instrument.
* * * * *