Inertial acoustic pickup

An inertial acoustic pickup (300) for use with a string musical instrument (700) having a soundboard (702) for delivering acoustic energy, comprises a chassis (302) including a coil (304) coupled to the soundboard (702), an armature including upper and lower substantially parallel planar suspension members (310) having planar perimeter regions (308) coupled to the chassis (302), and comprising a plurality of independent planar circular non-linear spring members (312) arranged regularly about a central planar region (314) within the planar perimeter region (305), an inertial mass (316) suspended between the upper and lower planar suspension members (310) about the central planar region (314) and having an axis (342) extending therebetween and including a plurality of permanent magnets (320) arranged regularly about a perimeter of the inertial mass (316), whereby acoustic energy coupled to the chassis (302) from the soundboard (702) is transformed through the planar non-linear spring members (312) into motional energy generated in a direction parallel to the axis (342) of the inertial mass (316) thereby generating an audio output signal in response to movement of the plurality of magnets (320) within the coil (304).

CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
Related, co-pending applications include patent application Ser. No. 
08/297,730 filed concurrently herewith, by Mooney, et al., entitled "Dual 
Mode Transducer for a Portable Receiver" and patent application Ser. No. 
08/297,443, filed concurrently herewith, by McKee, et al., entitled "Mass 
Excited Acoustic Device", both of which are assigned to the Assignee 
hereof. 
FIELD OF THE INVENTION 
This invention relates in general to electromagnetic pickup devices, and 
more specifically to a inertial acoustic pickup for use with a device 
which generates acoustic energy, such as a string musical instrument. 
BACKGROUND OF THE INVENTION 
There are numerous acoustic pickup devices which are available in the 
market today for application to devices which generates acoustic energy, 
such as string musical instruments, as well as for other applications, 
such as devices which are subjected to acceleration, deceleration and 
vibration. The most commonly used acoustic pickup devices include 
electromagnetic pickup devices which are coupled to the strings of the 
musical instrument, and piezoelectric pickup devices, which are coupled to 
the musical instrument soundboard. Within the electromagnetic category of 
acoustic pickup devices, there are further numerous makes and models which 
are available which offer various features, such as providing adjustment 
for the string diameter and dual pickups such as used for hum 
cancellation. All the acoustic pickup devices, either electromagnetic or 
piezoelectric have had to rely on electronic preamplifiers and tone 
controls to provide the audio output quality suited to the needs of the 
musical instrument and the musical instrument user. 
What is therefore needed is an acoustic pickup device which can be readily 
manufactured to enhance the various tonal qualities of a wide variety of 
string musical instruments, as well as to be easily adapted for use with 
other devices which generate acoustic energy. 
SUMMARY OF THE INVENTION 
In a preferred embodiment of the present invention a inertial acoustic 
pickup for use with a string musical instrument having a soundboard for 
delivering acoustic energy comprises a chassis including a coil and 
coupled to the soundboard; an armature including upper and lower 
substantially parallel planar suspension members having planar perimeter 
regions coupled to the chassis, the planar suspension members comprising a 
plurality of independent planar circular non-linear spring members 
arranged regularly about a central planar region within the planar 
perimeter region; an inertial mass suspended between the upper and lower 
planar suspension members about the central planar region and having an 
axis extending between the upper and lower planar suspension members; and 
a plurality of permanent magnets coupled to and arranged regularly about a 
perimeter of the inertial mass, whereby acoustic energy coupled to the 
chassis from the soundboard is transformed through the planar non-linear 
spring members into motional energy generated in a direction parallel to 
the axis of the inertial mass thereby generating an audio output signal in 
response to movement of the plurality of magnets within the coil. 
In an alternate embodiment of the present invention, a inertial acoustic 
pickup system comprises a string musical instrument having a soundboard 
for delivering acoustic energy; and a inertial acoustic pickup which 
comprises a chassis including a coil and coupled to the soundboard; an 
armature including upper and lower substantially parallel planar 
suspension members having ;planar perimeter regions coupled to the 
chassis, the planar suspension members comprising a plurality of 
independent planar circular non-linear spring members arranged regularly 
about a central planar region within the planar perimeter region; an 
inertial mass suspended between: the upper and lower planar suspension 
members about the central planar region and having an axis extending 
between the upper and lower planar suspension members; and a plurality of 
permanent magnets coupled to and arranged regularly about a perimeter of 
the inertial mass, whereby acoustic energy coupled to the chassis from the 
soundboard is transformed through the planar non-linear spring members 
into motional energy generated in a direction parallel to the axis of the 
inertial mass thereby generating an audio output signal in response to 
movement of the plurality of magnets within the coil.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, there is shown a top view of a planar non-linear 
spring member 100 utilized in a inertial acoustic pickup in accordance 
with the preferred embodiment of the present invention. The planar 
non-linear spring member 100 has a planar, substantially circular spring 
member 102 having in one embodiment a circular inner diameter 104 and a 
circular outer diameter 106, and in an improved embodiment an elliptical 
inner diameter 104 and a circular outer diameter 106, as shown in FIG. 1. 
The improved embodiment Of the planar non-linear spring member 100 shown in 
FIG. 1 provides a spring member having a nonuniform width, the width "2X" 
being the widest in the region contiguous to the end restraints 108, and 
tapering to a width "X" about the midpoints 114 of the substantially 
circular planar spring members 102. The circular spring members 102 couple 
through end restraints 108 of substantially uniform width "2.57X" to a 
central planar region 110 and to a planar perimeter region 112. 
FIG. 2 is a cross-sectional view taken along line 1--1 of FIG. 1. As shown, 
the thickness of the improved planar non-linear spring member 100 is by 
way of example "0.43X". It Will be appreciated that the dimension and 
thickness of the planar non-linear spring member 100 affects the resonant 
frequency at which the inertial acoustic pickup 300 resonates, and can be 
changed to accommodate different operating frequency ranges, such as are 
required to provide optimal inertial acoustic pickups for a wide variety 
of devices generating acoustic energy, including string musical 
instruments, such as, but not limited to guitars, violins, violas, string 
bases, mandolins, autoharps, etc. 
FIG. 3 is an orthogonal top view of a inertial acoustic pickup 300 (with 
circuit board 306, shown in FIG. 4, removed). Shown in FIG. 3 is a coil 
form 302 which functions as a chassis, and which by way of example is 
approximately 0.7 inch (17.78 mm) in diameter and which encloses an 
electromagnetic coil 304 (FIG. 4) which functions as a signal generator 
for generating an audio signal in response to the movement of a number of 
magnets 320 within the electromagnetic coil 304, as will be described in 
detail below. The coil form 302 is manufactured using conventional double 
shot injection molding techniques using a plastic material, such as a 
thirty-percent glass-filled liquid crystal polymer which fully encloses 
the coil 304 except for terminals 326 which provide electrical connection 
to the coil 304. It will be appreciated that other plastic materials can 
be utilized for the coil form 302, as well as other configurations for the 
coil form 302, such as a bobbin supporting the coil, and an unenclosed 
wound coil impregnated with an epoxy material to provide structural 
rigidity. The coil form 302 establishes two planar perimeter seating 
surfaces 330 (FIG. 4) about a planar perimeter region 308 on which two 
planar suspension members 310 (an upper and a lower) are supported, and 
further includes eight contiguously molded bosses 332 which are used to 
orient and affix the planar spring members 310 to the coil form 302 using 
a staking process, such as using heat or ultrasonics. 
Each of the two planar suspension members 310 comprises four independent 
planar non-linear spring members 312 arranged regularly around a central 
planar region 314 which is used for positioning and fastening an inertial 
mass 316 to the two planar suspension members 310 also using a staking 
process. The planar non-linear spring members 312 are defined as having a 
circular outer perimeter and a circular or elliptical inner perimeter such 
as described in FIG. 1 above. The planar suspension members 310 are 
manufactured from a sheet metal, such as Sandvik.TM. 7C27M02 stainless 
martensitic chromium steel alloyed with molybdenum, or a 17-7 PH heat 
treated CH900 precipitation-hardened stainless steel. It will be 
appreciated that other materials can be utilized as well. The sheet metal 
thickness is preferably 0.002 inch (0.0508 mm) thick, and the planar 
suspension members are formed preferably by a chemical etching, or 
machining technique. The inertial mass 316 is manufactured using 
conventional die casting techniques using a Zamak 3 zinc die-cast alloy, 
although it will be appreciated that other materials can be utilized as 
well. 
The arrangement of the parts of the inertial acoustic pickup 300 is such 
that the inertial mass 316 can be displaced upwards and downwards in a 
direction normal to the planes of the two planar suspension members 310, 
the displacement being restricted by a restoring force provided by the 
independent planar non-linear spring members 312 in response to the 
displacement. The inertial mass 316 is formed such that there are shaped 
channels 318 for allowing the inertial mass 316 to extend through and 
around the independent planar non-linear spring members 312 during extreme 
excursions of the inertial mass 316, thereby providing a greater mass to 
volume ratio for the inertial acoustic pickup 300 than would be possible 
without the shaped channels 318. The inertial mass 316 includes by way of 
example four radially polarized permanent magnets 320 arranged regularly 
around the perimeter of the inertial mass 316. The radially polarized 
permanent magnets 320 magnetically couple to the electromagnetic coil 304, 
and acoustic energy which is coupled from the soundboard to the chassis, 
or coil form 302 is transformed through the planar non-linear springs 
members 312 into motional energy which is generated in a direction 
parallel to the axis 342 of the inertial mass 316. This produces a 
displacement of the radially polarized permanent magnets 320 within the 
electromagnetic coil 304 which results in the generation of an audio 
output signal from the electromagnetic coil 304 which can be processed, 
such as by amplification, as will be described below. 
The radially polarized permanent magnets 320 are manufactured using 
Samarium Cobalt having a preferable Maximum Energy Product of 28-33 and 
having a N-S radial orientation to produce a coercive force of 8K-11K 
Oersteds, although it will be appreciated that other magnetic materials 
such as Alnico.TM. can be utilized as well with a corresponding 
performance change with regard to the audio output signal amplitude being 
generated. The two planar suspension members 310, the inertial mass 316, 
and the four permanent magnets 320 comprise a resonant armature system 336 
for the inertial acoustic pickup 300, and the resonant armature system 336 
can be customized, as will be described below, for different string 
musical instruments, as well as other devices which generate an acoustical 
energy output. 
An additional detail shown in FIG. 3 comprises four radial projections 322 
projecting in a direction normal to each surface (top and bottom) of the 
coil form 302 for compressively engaging with the planar perimeter region 
308 of the top planar suspension member 310. The projections 322 pre-load 
the planar perimeter region 308 after the planar suspension member 310 is 
attached to the surface of the coil form 302 using bosses 332 located on 
either side of each of the protrusions 322. The bosses 332 are staked 
using heat or ultrasonic energy to secure the planar suspensions members 
310 to the planar perimeter region 308 of the coil form 302. The purpose 
of pre-loading is for preventing audible (high frequency) parasitic 
vibrations during operation of the inertial acoustic pickup 300. 
With reference to FIG. 4, a cross-sectional view taken along the line 2--2 
of the inertial acoustic pickup of FIG. 3 clearly shows an air gap 324. 
The air gap 324 surrounds the inertial mass 316 (partially shown), thus 
allowing the inertial mass 316 to move in a direction normal to the planes 
of the two planar suspension members 310. The coil form 302 and magnetic 
motional mass 336 are enclosed in a housing 344 which is formed preferably 
of metal, although it will be appreciated that other materials, such as 
thermoplastic materials which have been formed by such processes as 
injection molding can be utilized as well. An adhesive material 346, such 
as a high tack film adhesive is selectively applied to the housing 344, 
and provides a convenient means for attaching the inertial acoustic pickup 
to the soundboard. A protective film (not shown), such as a Teflon tape, 
can be applied to the adhesive to provide protection of the adhesive 
during shipping and handling. 
FIG. 5 is a cross-sectional view taken along the line 2--2 of the inertial 
acoustic pickup 300 of FIG. 3 showing an alternate mounting approach. As 
shown, the inertial acoustic pickup 300 is positioned into a pedestal 502 
which comprises a platform 504 and a foot 506 which are formed contiguous 
to each other. A ring 508 having a circular periphery is also formed 
contiguous to the surface of the platform 504 and is used to mount the 
inertial acoustic pickup 300 to the pedestal 502. The pedestal 502 is 
preferably formed using an injection molding process, and any of a number 
of thermoset plastic materials. The inertial acoustic pickup 300 is 
attached at the perimeter 510 of the coil form 308 to the ring 508 and is 
preferably held in place using an adhesive, such as a cyanoacrylate or 
epoxy adhesive. By way of example, the foot 506 is 0.145 inches (3.7 mm) 
in diameter which is substantially smaller than the 0.700 inch (17.8 mm) 
diameter of the platform 504. A threaded screw 512 has a first end 514 
formed as a splined metallic rod and a second end 516 as a threaded rod 
with a wood screw taper. The foot 506 can be molded over the splined rod 
end 514 during the injection molding process described above, or can be 
ultrasonically staked into a cavity formed into the foot during the 
injection molding process. An adhesive material can be placed on the 
surface 518 of the foot 506 which contacts the soundboard to prevent the 
screw 516 from loosening due to any vibration and shock which is imparted 
to the string musical instrument either during playing or handling. 
FIG. 6 is a graph 600 depicting the impulse output as a function of 
frequency for a inertial acoustic pickup utilizing a non-linear, hardening 
spring type resonant system when driven as a transducer. As shown, the 
inertial acoustic pickup utilizing a non-linear, hardening spring type 
resonant system when driven by a Swept driving frequency operating between 
a first driving frequency provides a lower impulse output 602 and a second 
driving frequency provides an upper impulse output 604. The upper impulse 
output 604 is preferably selected to correspond substantially to the 
maximum driving frequency at which there is only a single stable operating 
state. As can be seen from FIG. 6, two stable operating states 604 and 610 
are possible when the driving frequency is set to that required to obtain 
impulse output 610, and as the driving frequency is increased therefrom, 
three stable operating states can exist, such as shown by example as 
impulse outputs 606, 608 and 612. As will be described below, those 
impulse responses which lie on the curve 600 above the operating state 612 
are suitable for providing audible responses as a transducer, and defines 
the minimum operating frequency limit as a inertial acoustic pickup for 
use with a string musical instrument or other device which generates an 
acoustic output. In addition, the response of the inertial acoustic pickup 
300 to audio input energy at frequencies above the operating state 612 are 
enhanced by harmonic responses of the inertial acoustic pickup 300 at 
frequencies higher than operating state 612. 
As was described above, each string musical instrument has a frequency 
range which they reproduce. Table I below lists typical frequency ranges 
for several typical string musical instruments: 
TABLE I 
______________________________________ 
Instrument Frequency Range 
______________________________________ 
Bass Viol 41.20 Hz-246.94 Hz 
Cello 65.41 Hz-698.46 Hz 
Viola 130.81 Hz-1174.70 Hz 
Violin 196.00 Hz-3136.00 Hz 
______________________________________ 
As was described above, the inertial acoustic pickup in accordance with the 
present invention can be customized as to operating characteristics as a 
pickup. By varying the size and thickness of the armature, which impacts 
the size of the parallel planar suspension members, a inertial acoustic 
pickup can be produced which has a single stable operating state 612 which 
lies below 41.20 Hz, thus providing a pickup suitable for use with a bass 
viol. Likewise, a inertial acoustic device can be produced which has a 
single stable operating state 612 which lies below 196.00 Hz, thus 
providing a pickup suitable for use with a violin. 
FIG. 7 is an orthogonal top view of a string musical instrument providing 
details related to utilizing the inertial acoustic pickup of FIG. 3. A 
typical string musical instrument, such as guitar 700, is constructed 
having a body 726 to which is attached a neck having a fret board 720 (a 
finger board in a violin, and such), a nut 724 and a peg head 722. The top 
body plate is the soundboard 702 which includes a sound hole 708 (a pair 
of f holes in a violin and such) and to which a bridge 704 is attached. 
The soundboard 702 is resonant and exhibits different vibrational 
characteristics at different points about the soundboard 702. Positions 
708 and 710 designate preferred mounting locations, while points 712 and 
714 designate alternate mounting locations (tone weighted response). The 
areas within the soundboard 702, and in particular areas 710, 712 and 714 
exhibit low frequency responses having high amplitudes, while those areas, 
such as area 716, which lies on the periphery of the soundboard exhibit 
high frequency, low amplitude responses. The peripheral area is typically 
a band 718 which is with 1" to 2" inches (25.4 to 50.8 mm) of the 
perimeter of the soundboard for a guitar. The preferred mounting locations 
708 and 710 are those most commonly utilized for positioning an acoustic 
pickup device, such as the inertial acoustic pickup 300 in accordance with 
the present invention. When the preferred mounting location 708 is 
utilized, the pickup device is typically mounted in a manner which 
provides ease of removability at a later time. The preferred mounting 
location 710 and alternate mounting locations 712 and 714 are utilized by 
affixing the pickup to the underside of the soundboard using an adhesive 
interconnect as described in FIG. 4, above. 
FIG. 8 is an orthogonal internal top view of a string musical instrument, 
in particular that of a guitar, providing alternate mounting details 
related to utilizing the inertial acoustic pickup of FIG. 3. As shown in 
FIG. 9, the soundboard 802 of a guitar includes a sound hole 804 and is 
braced, such as using the X-bracing pattern 806, 808 which is typical of a 
steel string acoustic guitar. Area 810 on the soundboard 802 is reinforced 
to provide rigidity to pegs 812 which secure the strings below the bridge. 
The inertial acoustic pickup 300 can be affixed directly to the soundboard 
in area 810 using an adhesive, or as shown, affixed to the X-bracing using 
an adhesive, or the alternate screw mounting method described in FIG. 5. 
The audio output from the inertial 2acoustic device 300 couples directly 
to a jack 816, or may couple indirectly after being processed by an 
internally mounted preamplifier 818. 
FIG. 9 is an electrical block diagram of a inertial acoustic pickup system 
in accordance with the preferred embodiment of the present invention. The 
audio output of the inertial acoustic pickup 300 is coupled to the input 
of an audio preamplifier 902. The relative volume is controlled using a 
volume control 904. The amplified output of preamplifier 902 is preferably 
coupled to a tone control amplifier 906 which enables control of the 
frequency/amplitude characteristics of the audio signal. Tone control, 
such as 908 for bass boost/cut and 910 for treble boost/cut are provided 
to adjust the tone of the string musical instrument to suit the instrument 
players needs. The tone control amplifier output is then coupled to the 
input of a buffer amplifier 912 which provides the amplified and 
waveshaped, or equalized, audio signal at an output impedance suitable to 
drive an audio amplifier. 
A inertial acoustic pickup has been described above which can be readily 
manufactured to enhance the various tonal qualities of a wide variety of 
string musical instruments. In particular, the inertial acoustic pickup 
300 comprises, in part, two planar suspension members 310, an inertial 
mass 316, and four permanent magnets 320 which in combination comprise a 
resonant armature system 336 which are customizable, as described above, 
for different string musical instruments, as well as other devices which 
generate an acoustical energy output. The inertial acoustic pickup of the 
present invention utilizes planar non-linear spring members which 
transform acoustic energy generated by a soundboard into motional energy 
generated in a direction parallel to the axis of motion of the inertial 
mass which results in the generation of an audio output signal in response 
to the movement of the permanent magnets within a surrounding coil. By 
designing the inertial acoustic device to have a fundamental response 
below that of the acoustic energy generated by the string musical 
instrument, the inertial acoustic pickup 300 is suitable for use with any 
string musical instrument, or other device which generates acoustic 
energy.