General purpose modular acoustic signal generator

A general purpose modular acoustic signal generator includes an outer casing unit having a plurality of disengageably connected sections and an insert unit removably positioned in that outer casing unit. The insert unit has a piezoelectric acoustic generator disc mounted thereon and defines a Helmholtz resonator on each side of the acoustic generator disc. A single outer casing is usable with a plurality of different insert units so a single outer casing unit can be associated with a plurality of different Helmholtz resonators.

TECHNICAL FIELD 
The present invention relates, in general, to signal generators, and, more 
particularly, to acoustic signal generators. 
BACKGROUND ART 
Acoustic signal generators have many applications in modern technology. One 
example of such applications is a Personal Acoustic Alarm System described 
in copending patent application Ser. No. 348,245, filed on Feb. 12, 1982, 
now U.S. Pat. No. 4,473,821, the disclosure of which is incorporated 
herein by reference. 
Applications for acoustic signal generators such as the above-mentioned 
Alarm System require a variety of output signal frequencies. For example, 
the system disclosed by the referenced patent application requires a 
plurality of different output signal frequencies in the audio range. 
Many acoustic signal generators utilize a Helmholtz resonator having a 
piezoelectric generator disc associated therewith. These acoustic signal 
generators include circular metallic discs having a thin layer of 
piezoelectric material bonded to one side thereof. When an alternating 
voltage is applied to the piezoelectric material, the resulting mechanical 
forces distort the disc, causing it to oscillate as a diaphragm. If the 
disc is driven at a natural fundamental frequency and the Helmholtz 
resonator associated with that disc is tuned to the same frequency, a high 
acoustic energy output will occur. Each acoustic signal generator is 
unique to the disc dimensions and to the signal frequency of interest. 
Those generators described in the prior art are integral, molded units 
wherein the casing and the Helmholtz resonators are formed to be a single 
integral unit. These units, once formed, are intended for use at only one 
signal frequency and are not intended to be modifiable to produce 
high-energy signals at frequencies other than the one signal frequency 
associated with the generator as orignally manufactured and sold. 
Therefore, if a multiplicity of signal frequencies were of interest to the 
user, that user had to purchase and have available a corresponding 
multiplicity of completely separate generator units. This situation is 
both costly and onerous for the user. 
Another disadvantage of known acoustic signal generators is the lack of 
acoustic signal volume produced. Even though the acoustic generator disc 
will vibrate symmetrically in two directions, currently known acoustic 
signal generators have only one Helmholtz resonator per unit. Often only 
one side of the disc is exposed to the Helmholtz resonator while the other 
side of the disc is exposed to a casing wall. In such a configuration, 
those sonic waves directed from the disc toward the casing wall not only 
are lost, but also may, upon reflection from the wall, actually interfere 
with the sonic waves directed from the disc toward the Helmholtz 
resonator. Furthermore, because only one Helmholtz resonator is associated 
with such signal generator unit, a unit can produce an output signal at 
only one frequency and in only one direction. Thus, these units must be 
carefully oriented to generate a signal in the desired direction. 
It is apparent from the foregoing that currently known acoustic signal 
generators are not efficient and cannot make maximum use of the signal 
producing capability of an acoustic generator disc. For acoustic alarm 
applications, it is imperative that the acoustic transmitter produce an 
acoustic signal having as high a volume as possible. 
DESCRIPTION OF THE INVENTION 
It is a primary object of the present invention to provide a novel and 
improved acoustic signal generator which is modifiable so the same casing 
unit can be used to produce a wide variety of acoustic signal frequencies. 
The signal generator is modular, and a single outer casing unit is 
combined with a plurality of insert units with the insert units being 
changed to provide different Helmholtz resonators in the same outer casing 
unit. 
It is another object of the present invention to provide a novel and 
improved acoustic signal generator having an outer casing unit which 
includes a plurality of disengageably connected sections so the outer 
casing unit can be opened and one Helmholtz resonator insert unit can be 
replaced with another Helmholtz resonator insert unit to change generator 
output signal frequencies. 
It is yet another object of the present invention to provide a novel and 
improved acoustic signal generator having an outer casing which has at 
least two holes defined therein so at least two Helmholtz resonators can 
be provided in the same outer casing unit by a single insert unit. The 
holes are aligned with each other and can be modified in dimension by the 
insert unit with the dimensions and shapes of the various elements 
associated with the casing and insert units being selected in accordance 
with the requirements of Helmholtz resonator design criteria for each 
frequency of interest. 
It is a further object of the present invention to provide a novel and 
improved acoustic signal generator which includes a casing adapted to 
receive interchangeable resonator chamber forming insert units and which 
utilizes piezoelectric acoustic generator discs which are either edge or 
nodal mounted on the insert units. 
It is yet another object of the present invention to provide a novel and 
improved acoustic signal generator which is constructed to maximize sound 
output from the generator. The generator utilizes a piezoelectric acoustic 
generator disc driven Helmholtz resonator, and the outer casing is 
symmetric with respect to both sides of the disc so each side of the disc 
drives an associated Helmholtz resonator. Acoustic signals will be 
generated from both sides of the disc. 
These objects are accomplished by providing a modifiable acoustic signal 
generator having a separable outer casing unit and a plurality of 
different insert units usable in that single outer casing unit to change 
the output signal from one frequency to another frequency. The insert 
units provide at least a pair of Helmholtz resonators, and a piezoelectric 
acoustic generator disc is mounted on each insert with a Helmholtz 
resonator being associated with each side of the disc. Output of the 
acoustic signal generator is thus maximized.

BEST MODE FOR CARRYING OUT THE INVENTION 
Two prior art acoustic generators 10a and 10b are shown in FIGS. 1a and 1b, 
respectively. The generators 10a and 10b include Helmholtz resonators 12a 
and 12b, respectively, and each generator produces an acoustic signal in a 
frequency range uniquely and permanently associated with the overall inner 
and outer geometric dimensions of such prior art generator. 
As shown, the generators 10a and 10b, respectively, include differently 
dimensioned, unitary outer casings 20a and 20b with the outer casings 
including one-piece cover elements 22a and 22b fixedly coupled to 
one-piece base elements 24a and 24b respectively and having openings 30a 
and 30b defined therein. The coupling between the casings and bases of 
these prior art generators is intended to be permanent, and each of the 
one-piece cover elements includes mounting prongs 26a and 26b respectively 
cooperating with mounting prongs 28a and 28b of the one-piece base 
elements for mounting acoustic vibration producing means, such as 
piezoelectric acoustic generator discs 32a and 32b. The mounted discs 
cooperate with the unitized casings to define the Helmholtz resonators 12a 
and 12b for producing acoustic signals, and each generator is uniquely 
designed to produce a particular acoustic signal frequency, such as, for 
example, 3000 Hz for generator 10a and 6000 Hz for generator 10b. 
As discussed above, once assembled, the prior art generators, as 
represented by the generators 10a and 10b are intended to remain 
permanently assembled and are not intended to be modified. Thus since a 
particular prior art generator can produce output signals at only one 
particular frequency, if a plurality of different Helmholtz resonator 
produced acoustic signal frequencies are required, a plurality of complete 
generators, including casings, must be obtained with each of the 
generators being specifically designed to produce one of the desired 
frequencies. 
The present invention provides a single module acoustic generator which, in 
contrast to the teachings of the prior art, can easily be disassembled and 
modified so the same outer casing unit can be used in the generation of 
Helmholtz resonator produced acoustic signals having one or more of a 
multiplicity of different frequencies. A modular acoustic signal generator 
embodying the teachings of the present invention indicated generally at 50 
is shown in FIGS. 2 and 3, and includes a sectional outer casing unit 52 
having separate sections 54 and 56 disengageably coupled together by a 
detachable coupling means, such as snap joint 58. The sections 54 and 56 
may be hinged together or otherwise joined in such a manner that they may 
be moved apart to provide access to a chamber 59 normally enclosed by the 
outer casing unit 52. 
An insert unit is replacably positioned within the outer casing unit 52 for 
providing at least one Helmholtz resonator in association with the outer 
casing unit 52. One or more Helmholtz resonators can be provided by a 
single modular generator and, if a generator has a plurality of Helmholtz 
resonators, these resonators can have congruent and equal geometric shapes 
and can be positioned to produce oppositely directed output signals of the 
same frequency, or the Helmholtz resonators can be of different geometric 
shapes dimensions or orientations so that a single acoustic signal 
generator can be used to produce acoustic signals having different 
frequencies and directions 
As shown in FIGS. 2 and 3, a double insert unit 60 is positioned in outer 
casing unit 52 so the Helmholtz resonators associated with casing unit 52 
and double insert unit 60 will produce signals at a selected frequency. 
The insert unit 60 includes two frusto-conically shaped elements 62 and 64 
having respective bases 66 and 68 contacting inner surfaces 70 and 72 of 
casing sections 54 and 56. Acoustic vibration producing means V is mounted 
by cooperating edges 74 and 76 of elements 62 and 64, and casing sections 
54 and 56 have apertures 78 and 80 defined therein with the structures and 
apertures being designed, positioned and shaped according to Helmholtz 
resonator design relationships so that Helmholtz resonators 90 and 92 are 
defined by the appropriate cooperating structural elements. As shown in 
FIG. 3, the Helmholtz resonators 90 and 92 are congruent, equal sized, 
coaxial and oriented with respect to each other so that symmetric 
vibration of acoustic vibration producing means V produces identical 
oppositely directed signals from both resonators and utilizes both sides 
of the acoustic vibration producing means, thereby increasing the 
efficiency and the versatility of the generator. The vibration producing 
means may be formed by known ceramic metal resonant disc devices of the 
Murata Erie type, PKM 28-3AD. The circular metal disc has a thin layer of 
piezoelectric material bonded to it on one side, and when an electric 
driving signal is applied to the piezoelectric element, the distortion of 
the element due to the magnetostrictive effect will cause the disc to 
oscillate as a diaphragm. This oscillation has been found to be 
substantially symmetrical. 
As shown in FIG. 4, an insert 100 is positioned in outer casing unit 52 so 
that Helmholtz resonator, or resonators, associated with casing unit 52 
and insert unit 100 will produce an output signal or signals at a 
frequency different from the output signal frequency associated with the 
same casing unit 52 using insert unit 60. The insert unit 100 includes two 
one-piece frustoconically shaped elements 112 and 114 having respective 
bases 116 and 118 integral with webs 120 and 122, with the webs 120 and 
122 contacting inner surfaces 70 and 72 of the casing sections 54 and 56. 
Acoustic vibration producing means V' is mounted by cooperating edges 128 
and 130 of the elements 112 and 114, and the webs are sized and shaped to 
define appropriately dimensioned apertures 134 and 136 so that Helmholtz 
resonators 140 and 142 are defined by the appropriate cooperating 
structural elements having dimensions and orientations determined 
according to Helmholtz resonator design relationships to produce the 
desired acoustic output signals. 
As an illustration of the versatility of the device embodying the teaching 
of the present disclosure, a generator including outer casing unit 52 and 
insert unit 60 can be used to produce acoustic signals having a frequency 
of about 3000 Hz and a generator including the same outer casing unit 52 
and insert unit 100 can be used to produce acoustic signals having a 
frequency of about 6000 Hz. Thus the same outer casing unit can be 
associated with a multiplicity of different Helmholtz resonators. One 
resonator is easily replaced by another by opening the casing unit 52 and 
substituting a new insert unit for the previously used insert unit. This 
simple procedure is contrasted with the requirement of replacing the 
entire generator 10a with a totally different generator 10b to change the 
frequency of prior art Helmholtz resonators. 
Preferably, the insert unit is injection molded and the acoustic vibration 
producing means is a piezoelectric acoustic generator disc which is edge 
or nodel mounted by the appropriate insert unit structural element. 
While the modular acoustic signal generator 50 embodying the teachings of 
the present disclosure has been described as being symmetric and having 
two Helmholtz resonators dimensioned to produce the same frequency, it is 
also possible, with the present invention, for a generator to have two 
Helmholtz resonators of a different size to produce a different frequency 
from each side of the generator. To accomplish this, the disc V would be a 
dual frequency disc. Each disc will have a fundamental resonance frequency 
and will have super-and subharmonic frequencies at which it exhibits 
resonant responses generally less pronounced than the fundamental 
resonance. One of the harmonic frequencies can be used as the second 
frequency. The top Helmholtz resonator insert in the casing would be 
dimensioned to provide one of the disc frequencies while a second insert 
at the bottom of the casing would be dimensioned to provide the second 
disc frequency. Thus the casing might include a large top insert, such as 
the insert 62, combined with a smaller bottom insert, such as the insert 
118 to support a disc V. 
The inserts 60 and 100 may be formed to any shape required to form the 
desired Helmholtz resonator, and they need not always be of the frusto 
conical configuration illustrated in FIGS. 2 and 3. The insensitivity of 
the cavity shape factor and the inverse relationship between the cavity 
volume and the aperture of the opening in a Helmholtz resonator provide 
the flexibility in the design of the insert. 
Industrial Applicability 
The general purpose modular acoustic generator 50 may be effectively 
employed to provide an acoustic signal in either the audio or ultrasonic 
frequency range which is of greater amplitude than the signals produced by 
generators having only a single resonant chamber. The frequency produced 
by the acoustic generator may be easily altered by substituting new 
inserts for those previously used to alter the dimensions of the resonant 
chamber and, in most cases, substituting a new disc V.