 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a musical instrument for electrically
generating a musical tone through an electro-acoustic transducer like an
electrical or electronic musical instrument and, more particularly, to a
musical instrument with a compact electro-acoustic transducer and capable
of generating a heavy bass sound and a musical instrument capable of
generating a musical tone with good sound quality over the entire tone
generation range, in particular, in a bass range.
2. Description of the Prior Art
Conventionally, electronic and electrical musical instruments are known as
musical instruments comprising an electro-acoustic transducer (to be
referred to as an acoustic apparatus hereinafter).
An electrical musical instrument comprises a tone generating device which
can directly generate a tone by a mechanical or acoustic vibration similar
to a guitar, drum, wind instrument, or the like. The instrument
temporarily converts the mechanical vibration or tone into an electrical
signal and electrically amplifies the electrical signal. The instrument
re-converts the amplified electrical signal into an acoustic wave using an
acoustic apparatus to produce a corresponding sound. Thus, the instrument
can generate an acoustic wave in a larger tone volume than that when it is
directly generated, or can produce a tone with a special effect.
On the other hand, an electronic musical instrument electrically forms a
musical tone signal using an electronic circuit with an oscillator, a
memory, and the like in accordance with an instruction from a tone
generation instruction means such as a keyboard, drum pad, breath input
device, or the like, and produces a sound corresponding to the musical
tone signal using an acoustic apparatus.
The electrical/electronic musical instrument is generally equipped with a
separate speaker box as a speaker system for the acoustic apparatus. In
this case, the speaker box causes an increase in cost or becomes a design
limitation factor.
In some instruments, a pedal box serves as a speaker box, and the speaker
system is housed in the pedal box. In this case, however, both a box
volume and a speaker diameter are reduced, and a bass sound cannot be
satisfactorily reproduced.
In the electrical/electronic musical instrument, the frequency
characteristic of the acoustic apparatus is fixed. Thus, good sound
quality cannot always be obtained over the entire tone range for an
electronic keyboard instrument having a wide tone generation range.
As the electrical/electronic musical instrument, a portable musical
instrument which incorporates an acoustic apparatus, is easily carried,
and has a shape and size suitable for a hand-held performance is also
known.
The portable musical instrument is required to have a compact main body in
terms of easy carrying and easy standing performance, and a small-diameter
speaker is disposed in the compact main body. For this reason, the
conventional portable musical instrument cannot satisfactorily reproduce a
bass sound and can only produce a poor sound. Some portable musical
instruments include a resonator for producing a bass sound. However, the
musical instrument of this type has a large main body like an acoustic
guitar to obtain sufficient sound quality, and is not suitable for a
standing performance. If the instrument of this type has a size suitable
for a standing performance, sufficient sound quality cannot be obtained.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the conventional
problems, and has as its first object to provide a portable musical
instrument which has dimensions suitable for a portable use and can
produce a heavy bass sound.
It is a second object of the present invention to provide an
electrical/electronic musical instrument which can improve a bass sound
characteristic without particularly increasing outer dimensions or can
reduce outer dimensions without impairing a bass sound characteristic, can
minimize design limitations, and is advantageous in cost.
It is a third object of the present invention to provide an electronic
musical instrument which can obtain good sound quality over a wide tone
generation range.
In order to achieve the above objects, according to a first aspect of the
present invention, a portable musical instrument main body comprises, as
an acoustic apparatus, a resonator constituted by a cavity and acoustic
mass means, a vibrator constituting a portion of the resonator and
including a vibrating body for driving the resonator with one surface, and
vibrator driving means for driving the vibrator to cancel a counteraction
from the resonator to a diaphragm of the vibrator when the resonator is
driven, thereby positively utilizing resonance of the resonator.
In particular, in a portable electrical musical instrument constituted by
assembling an acoustic-electric transducer (pickup) and an acoustic
apparatus in a musical instrument which has a resonator in an instrument
main body like a guitar or drum and can directly produce a musical tone
from the resonator, the resonator of the portable instrument main body is
used for the acoustic apparatus directly or by changing a resonance
frequency using acoustic mass means if necessary. A vibrator which
constitutes a portion of the resonator and comprises a vibrating body for
driving the resonator with one surface and for directly radiating an
acoustic wave from the other surface to the outside the instrument main
body is arranged in the resonator. In addition, a vibrator driving means
for driving the vibrator to cancel a counteraction from the resonator to a
diaphragm of the vibrator when the resonator is driven is arranged. Thus,
resonance of the resonator is positively utilized.
The acoustic apparatus of the conventional electrical/electronic musical
instrument is constituted by a speaker system, and a power amplifier,
whose output impedance is essentially zero, for constant-voltage driving
the speaker system. For this reason, in the conventional musical
instrument, an output sound pressure characteristic is influenced by the
volume of a cavity behind a diaphragm of the speaker unit. Thus, if the
volume of the cavity is reduced to make the resonator or the instrument
main body compact, a bass sound characteristic is impaired.
In the first aspect of the present invention, the driving means drives the
vibrator to cancel an air counteraction from the resonator (cavity) side
to the vibrating body of the vibrator. More specifically, the vibrator is
driven in a so-called "dead" state wherein the vibrator is not influenced
by the counteraction from the resonator side and is sufficiently damped.
For this reason, the frequency characteristic of a directly radiated
acoustic wave is not influenced by a space behind a direct radiation
surface of the vibrator, i.e., the volume of a housing. The volume of the
space can be reduced as long as it can serve as the cavity of the vibrator
and a chamber for the vibrator. When viewed from the resonator side, to
drive the vibrator to cancel the counteraction from the resonator upon
driving of the resonator means that the diaphragm of the vibrator is
converted to an equivalent wall which cannot be driven by the resonator.
Therefore, the Q value as the resonator is not influenced by the
characteristic values (f.sub.o, Q.sub.o) of the vibrator, and if the
resonance frequency is decreased, a sufficiently high Q value can be
assured. Thus, if the housing is made compact, the resonator can generate
a heavy bass sound (resonance sound) with a sufficient level.
In this manner, according to the first aspect of the present invention,
although a small-diameter speaker unit is arranged on a compact instrument
main body, resonance of the resonator in the main body is positively
utilized to obtain sound quality with a sufficient bass sound
characteristic. When the present invention is applied to an electrical
musical instrument, e.g., an acoustic guitar, which is large as a portable
instrument, the resonator can be rendered compact without impairing sound
quality, and the size of the instrument main body can be reduced to be
suitable for a standing performance.
According to a second aspect of the present invention, an acoustic
apparatus comprises a housing which defines a closed cavity when a
vibrator is arranged on its outer wall, a duct formed in a leg portion of
a musical instrument and causing the cavity to communicate with an
external region, a vibrator, arranged in the housing, for driving a
resonator constituted by the cavity and the duct with one surface and for
directly radiating an acoustic wave from the other surface, and vibrator
driving means for driving the vibrator to cancel a counteraction from the
resonator to a diaphragm when the resonator is driven, thereby effectively
utilizing resonance of the resonator.
The resonator, the vibrator, and the vibrator driving means in the second
aspect are operated in the same manner as in the first aspect.
In the second aspect, the duct constituting the resonator is formed in a
leg portion of a musical instrument, thus minimizing a change in
appearance due to an addition of the duct. To form the duct in the leg
portion means that when the leg portion is constituted by a column or a
thick plate, a channel is formed in these members to use it as the duct,
or when the leg portion is constituted by a pipe, the pipe is used as the
duct.
According to the second aspect of the present invention, a heavy bass sound
can be produced using a compact housing, and for a conventional musical
instrument which employs a compact housing and a small-diameter vibrator
(e.g., dynamic speaker unit), a bass sound characteristic can be improved
while using a housing and a vibrator having the same dimensions as those
of the conventional instrument. On the other hand, for a conventional
musical instrument which employs a large housing and a large-diameter
vibrator, the housing and the vibrator can be rendered compact without
impairing a bass sound characteristic. The duct is formed in the leg
portion of the musical instrument to minimize a change in outer appearance
due to an addition of the duct, and the housing can be rendered compact,
thus eliminating design limitations of the musical instrument. Since the
housing is small in size and the leg portion is used as the duct, factors
increasing the cost can be eliminated, thus providing advantages in terms
of cost.
In order to achieve the third object, according to a third aspect of the
present invention, an electronic musical instrument comprises, as an
acoustic apparatus, a resonator constituted by a cavity and acoustic mass
means for causing the cavity to acoustically communicate with an external
region, a vibrator constituting a portion of the resonator and including a
vibrating body for driving the resonator with one surface, vibrator
driving means for driving the vibrator to cancel a counteraction from the
resonator to the vibrating body when the resonator is driven, and control
means for controlling at least one of the resonator and the vibrator
driving means to vary a frequency characteristic of the acoustic
apparatus.
In the third aspect, the frequency characteristic of the acoustic apparatus
is variably controlled. For this reason, a frequency characteristic
according to a performance content is set in advance before a performance,
or a frequency characteristic is automatically set in accordance with a
pitch or tone quality designated during a performance, so that a musical
tone with high quality can be produced and a musical tone effect unlike in
a conventional apparatus can be realized.
In the conventional acoustic apparatus, the vibrator is constant-voltage
driven using the vibrator driving means whose output impedance is
essentially zero. In the acoustic apparatus, a bass range reproduction
limit of an output sound pressure of an acoustic wave directly radiated
from the vibrator (e.g., a speaker) is determined by the characteristic
values (f.sub.o, Q.sub.o) of the vibrator and the volume of the housing
(e.g., a speaker cabinet) to which the vibrator is attached. For this
reason, in order to arbitrarily vary the frequency characteristic, a
large-sized vibrator and housing capable of reproducing a lowest frequency
in a variable range are necessary. In order to make the vibrator and the
housing compact, the lowest reproduction limit frequency of the speaker
system is set to be relatively high, and a shortage of a bass range
reproduction level is compensated for by boosting the input signal level
of the vibrator driving means. However, in the conventional acoustic
apparatus, since the output sound pressure below the bass range
reproduction limit is decreased by 12 dB/oct, it is difficult or
impossible to satisfactorily compensate for the bass range characteristic.
In an acoustic apparatus using a speaker system having a resonator such as
a phase-inversion type (bass-reflex type) speaker system, since a direct
radiation characteristic of the vibrator has mutual dependency with a
resonant radiation characteristic of the resonator, characteristic values
must be relatively strictly set to obtain a flat frequency characteristic.
If the resonance frequency is varied, in particular, if it is extremely
lowered, a drift of 12 dB/oct occurs in a bass range frequency
characteristic, and compensation for obtaining a flat characteristic is
difficult to achieve.
In the third aspect, the vibrator driving means drives the vibrator to
cancel a counteraction from the resonator (cavity) side to the vibrating
body of the vibrator in the same manner as in the first and second
aspects. Therefore, the frequency characteristic of a directly radiated
acoustic wave from the vibrating body of the vibrator is not influenced by
the volume of a space behind the direct radiation surface of a diaphragm
as in the above aspects. The volume of this space can be reduced as long
as the space can serve as the cavity of the resonator and a chamber of the
vibrator. The Q value of the resonator is not influenced by the
characteristic values (f.sub.o, Q.sub.o) of the vibrator. Even if the
resonance frequency is decreased, a sufficiently high Q value can be
assured. Thus, if the cavity, i.e., the housing is reduced in size, a
heavy bass sound (resonance sound) with a sufficient level can be
generated. In this case, a decrease in output sound pressure of the
directly radiated acoustic wave in a range lower than the lowest resonance
frequency f.sub.0 of the vibrator is 6 dB/oct. The resonance frequency of
the resonator can be varied regardless of the direct radiation
characteristic. In this case, even if the frequency characteristic drifts,
it can be compensated for by 6 dB/oct as in normal tone control, thus
obtaining a flat characteristic. The resonance frequency of the resonator
can be varied by varying the volume of the cavity or an acoustic mass. The
frequency characteristic, in particular, the bass range characteristic can
be varied by varying a degree of canceling a counteraction from the
resonator in the vibrator driving means.
In this manner, according to the third aspect, since the frequency
characteristic of the acoustic apparatus can be changed as needed, the
pitch of a performance musical tone is caused to match with the frequency
characteristic of the acoustic apparatus, so that a high-quality musical
tone can be produced. In addition, a musical tone effect unlike in the
conventional apparatus can be obtained in accordance with a combination of
the pitch of a performance musical tone and the frequency characteristic
of the acoustic apparatus or by changing the combination during a
performance.
According to Japanese Patent Application No. Sho 62-334262 filed by the
present applicant, a speaker system (acoustic apparatus) capable of
reproducing a sound to a bass range with a smaller speaker box is
proposed. In this acoustic apparatus, a resonance frequency f.sub.op of a
Helmholtz resonator constituted by a port and a cabinet is set to be lower
than that of a conventional bass-reflex speaker system, and a vibrator
(speaker unit) for driving the Helmholtz resonator is driven to cancel an
air counteraction from the resonator side when the resonator is driven.
The first to third aspects present musical instruments to which such a
speaker system is applied. However, in the speaker system, a decrease in
electro-acoustic conversion efficiency in the bass range is compensated
for by increasing an output from an amplifier circuit such as a negative
impedance generator or an MFB circuit. Therefore, a high-power amplifier
and a high-withstand input speaker are necessary, resulting in high cost
and large power consumption.
It is therefore a fourth object of the present invention to provide a
speaker system for an electronic musical instrument which allows bass
sound reproduction using a small-diameter speaker, can reduce cost, and
can minimize design limitations.
In order to achieve the fourth object, according to a fourth aspect of the
present invention, a back-loaded horn is formed in a side plate portion of
an electronic musical instrument or a side plate or in a side leg of an
instrument stand, and is acoustically coupled to a speaker box of the
electronic musical instrument.
With this arrangement, a back-loaded horn speaker system is formed by the
speaker box of the electronic musical instrument and the back-loaded horn
in the side plate portion. In this back-loaded horn speaker system, a
middle/high tone is directly radiated from the front surface of a speaker
unit attached to the speaker box, and a bass sound is radiated in a
sufficient tone volume from the back-loaded horn driven at the back
surface of this speaker unit.
According to the fourth aspect, since the back-loaded horn is housed in the
side plate portion by utilizing a size plate, design limitations can be
minimized, and cost can be advantageously reduced. In some cases, the
thickness of the side plate may be increased in relation to a storage
volume of the back-loaded horn. An increase in thickness of the side plate
is also advantageous in terms of design since it provides a good
appearance as a high-grade system.
Since the speaker system of the present invention is of a horn-loaded type,
it has a high conversion efficiency, and can output a bass sound in a
sufficient tone volume. In addition, neither a high-power amplifier nor
high-withstand input speaker are necessary. Thus, the speaker system of
the present invention is also advantageous in this respect.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a basic arrangement of a portable
electrical musical instrument according to a first embodiment of the
present invention;
FIG. 2 is a graph for explaining output sound pressure-frequency
characteristics of an acoustic apparatus of the musical instrument shown
in FIG. 1;
FIGS. 3(a) and 3(b) are views showing a detailed arrangement of an
application of the portable electrical musical instrument shown in FIG. 1,
in which FIG. 3(a) is a front view and, FIG. 3(b) is a sectional view
taken along a line 3b--3b in FIG. 3(a);
FIG. 4 is a schematic view showing a basic arrangement of a portable
electronic musical instrument according to a second embodiment of the
present invention;
FIG. 5 is an electrically equivalent circuit diagram of an acoustic
apparatus portion of the musical instrument shown in FIGS. 1 and 4;
FIG. 6, is an equivalent circuit diagram when Z.sub.V -Z.sub.0 =0 in FIG.
5;
FIG. 7 is a basic circuit diagram of a circuit for generating a negative
impedance;
FIG. 8 is a circuit diagram showing a modification of the circuit shown in
FIG. 7;
FIG. 9 is a circuit diagram showing a detailed arrangement of the circuit
shown in FIG. 7;
FIG. 10 is a schematic view showing a basic arrangement of a portable
electrical musical instrument according to a third embodiment of the
present invention;
FIG. 11 is a front view showing a detailed application of the portable
electrical musical instrument shown in FIG. 10;
FIG. 12 is a sectional view of the portable electrical musical instrument
shown in FIG. 11 taken along the line II--II in FIG. 11;
FIG. 13 is a schematic view of an electronic musical instrument according
to a fourth embodiment of the present invention;
FIGS. 14(a) and 14(b) are perspective views showing a duct 6 in FIG. 13;
FIG. 15 is a schematic view showing an electronic musical instrument
according to a fifth embodiment of the present invention;
FIGS. 16(a) and 16(b) are enlarged views of an acoustic coupling portion 94
in FIG. 15;
FIG. 17 is a schematic view of an electrical musical instrument according
to a sixth embodiment of the present invention;
FIG. 18 is a block diagram showing a modification wherein a duct shown in
FIG. 17 is applied to an electronic musical instrument;
FIG. 19 is a schematic diagram of an electrical musical instrument
according to a seventh embodiment of the present invention;
FIG. 20 is a graph for explaining output sound pressure-frequency
characteristics of an acoustic apparatus of the musical instrument shown
in FIG. 19;
FIG. 21 is a schematic diagram of an electrical musical instrument
according to an eighth embodiment of the present invention;
FIG. 22 is a schematic diagram of an electrical musical instrument
according to a ninth embodiment of the present invention;
FIG. 23 is a sectional view showing an arrangement of a speaker system
according to a tenth embodiment of the present invention; and
FIG. 24 is a sectional view of an electronic musical instrument according
to an eleventh embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described below with
reference to the accompanying drawings.
First Embodiment
FIG. 1 shows a basic arrangement of a portable electrical musical
instrument according to a first embodiment of the present invention. In
the electrical musical instrument shown in FIG. 1, the present invention
is applied to a musical instrument in which a tone generating device 2
such as a membrane of a drum, a string of a guitar or the like, a reed, or
the like is arranged on the left surface of an instrument housing 1 also
serving as a resonator. A vibrator (dynamic speaker unit) 4 having a
diaphragm 3 is mounted in a hole formed in the upper surface of the
instrument housing 1. A closed cabinet (cavity) 5 having the upper surface
of the instrument housing 1 as an upper surface is formed behind the
vibrator 4. A duct 6 for causing the interior of the closed cabinet 5 to
acoustically communicate with the outside the instrument housing 1 is
provided in the upper surface of the instrument housing 1. The closed
cabinet 5 and the duct 6 constitute a Helmholtz resonator. In addition, a
tone generating circuit 7 such as a pickup for converting a mechanical or
acoustic vibration of the tone generating device 2 into an electrical
signal and a vibrator driver 8 for driving the vibrator 4 on the basis of
the electrical signal supplied from the tone generating circuit 7 are
arranged.
In the Helmholtz resonator, an air resonance phenomenon occurs by an air
spring in the closed cabinet 5 as the closed cavity and an air mass in the
duct 6. The resonance frequency f.sub.op is given by:
f.sub.op =c(S.sub.1 /l.sub.1 V.sub.1).sup.178 /2.pi.. . . (1)
where V.sub.1 is the volume of the closed cabinet 5, S.sub.1 is the
sectional area of the duct 6, l.sub.1 is the length of the duct 6, and c
is the sonic speed.
The Helmholtz resonator and the vibrator 4 constitute a speaker system (to
be referred to as a speaker system with a resonance port hereinafter)
having a shape similar to a conventional phase-inversion (bass-reflex)
type speaker system.
The vibrator driver 8 drives the vibrator 4 to cancel an air counteraction
from the Helmholtz resonator, i.e., the closed cabinet (cavity) 5 side
when the Helmholtz resonator is driven. The driver can employ a known
circuit such as a negative impedance generator for equivalently generating
a negative impedance component (-Z.sub.0) in an output impedance, a
motional feedback (MFB) circuit for detecting a motional signal
corresponding to am movement of a vibrating body by a certain method and
negatively feeding back the detected signal to an input side, or the like.
The operation of the portable electrical musical instrument shown in FIG. 1
will be described below.
When the tone generating device 2 is operated during a performance of the
electrical musical instrument, the tone generating circuit 7 converts a
mechanical or acoustic vibration in the tone generating device 2 into an
electrical signal. The vibrator driver 8 drives the vibrator 4 on the
basis of the electrical signal supplied from the tone generating circuit
7. Thus, the mechanical vibration of the tone generating device 2 is
directly generated as a sound, and the sound is produced in a tone volume
while being amplified through the speaker system with the resonance port
constituted by the vibrator 4 and the Helmholtz resonator.
In this portable electrical musical instrument, since the instrument
housing 1 serves as a resonance box, a sound can be produced in a tone
volume large enough to tune the musical instrument or to confirm a
performance content by a player himself while the tone generating circuit
7 and the vibrator driver 8 are turned off.
When the vibrator driver 8 supplies a drive signal to the vibrator 4, the
vibrator 4 electro-mechanically converts this signal to reciprocate the
diaphragm 3 in the back-and-forth direction (the vertical direction in
FIG. 1). The diaphragm 3 mechano-electrically converts this reciprocal
motion. In this case, the front surface side (upper surface side in FIG.
1) of the diaphragm 3 serves as a direct radiation portion for directly
externally radiating an acoustic wave. The rear surface side (lower
surface side in FIG. 1) of the diaphragm 3 serves as a resonator driving
portion for driving the Helmholtz resonator constituted by the closed
cabinet 5 and the duct 6. Although an air counteraction from air in the
closed cabinet 5 is applied to the rear surface side of the diaphragm 3
upon operation of the diaphragm, the vibrator driver 8 drives the vibrator
4 to cancel this air counteraction.
In this manner, when the vibrator 4 is driven to cancel the air
counteraction from the resonator when the Helmholtz resonator is driven,
the diaphragm 3 cannot be driven by the resonator side, and serves as a
rigid body, i.e., a wall viewed from the resonator. Therefore, the
resonance frequency and Q value of the Helmholtz resonator are independent
from those of the direct radiation portion constituted by the diaphragm 3
and the vibrator 4, and a resonator driver energy from the vibrator 4 is
applied independently of that from the direct radiation portion. Since the
vibrator 4 is driven in a so-called "dead" state wherein it is not
influenced by the air counteraction from the resonator, i.e., the closed
cabinet 5 side, the frequency characteristic of the directly radiated
acoustic wave is not influenced by the volume of the closed cabinet 5.
Therefore, according to the arrangement of this embodiment, when the
volume of the closed cabinet 5 as the cavity of the Helmholtz resonator
can be reduced to be smaller than that of a conventional portable
electrical musical instrument such as an acoustic guitar, and at the same
time, the resonance frequency f.sub.op is set to be lower than that of the
conventional instrument, a sufficient Q value can be assured. As a result,
if the closed cabinet 5 is considerably reduced in size as compared to a
conventional portable electrical musical instrument, reproduction to a
lower bass sound can be performed.
In FIG. 1, the vibrator 4 drives the diaphragm 3 in response to the drive
signal from the vibrator driver 8, and independently supplies a drive
energy to the Helmholtz resonator constituted by the closed cabinet 5 and
duct 6. Thus, an acoustic wave is directly radiated from the diaphragm 3,
as indicated by an arrow a in FIG. 1, and air in the closed cabinet 5 is
resonated, thus resonantly radiating an acoustic wave with a sufficient
sound pressure from a resonant radiation portion (an opening port 9 of the
duct 6), as indicated by an arrow b in FIG. 1. A frequency characteristic
of a sound pressure, as shown in, e.g., FIG. 2, can be obtained under
conditions that the resonance frequency f.sub.op can be set to be lower
than a reproduction frequency band of the vibrator 4 by adjusting an air
equivalent mass in the duct in the Helmholtz resonator and a sound
pressure with an appropriate level can be obtained from the duct 6 by
setting the Q value at an appropriate level upon adjustment of the
equivalent resistance of the duct 6. In FIG. 2, a curve a represents a
frequency characteristic of a sound pressure of an acoustic wave directly
radiated from the vibrator 4, and a curve b represents a frequency
characteristic of a sound pressure of an acoustic wave resonantly radiated
from the opening port 9.
FIGS. 3(a) and 3(b) show a detailed arrangement of the portable electrical
musical instrument shown in FIG. 1. In the electrical musical instrument
shown in FIGS. 3(a) and 3(b), a small-diameter speaker unit is used in a
so-called semi-acoustic guitar whose thickness is smaller than that of a
conventional acoustic guitar. In the portable electrical musical
instrument, the interior of a guitar body (instrument housing) 1 is
partially partitioned to form a closed cabinet 5. A speaker unit
(vibrator) 4 is mounted in a hole formed in a portion of a top plate 11 of
the body 1, which serves as one surface of the closed cabinet 5. A duct 6
which forms a Helmholtz resonator together with the closed cabinet 5 is
arranged in the top plate 11. In addition, a pickup (tone generating
circuit) 7 for converting a vibration of each guitar string (tone
generating device) 2 into an electrical signal and a negative impedance
generator (vibrator driver) 8 for driving the speaker unit 4 are arranged.
In FIGS. 3(a) and 3(b), reference numeral 12 denotes a sound hole; 13, a
tail piece; 14, a neck; and 15, a bridge.
In this manner, a portion of the body 1 of the conventional semi-acoustic
guitar is used as the resonator of the speaker system with the resonance
port, and the speaker system is driven by a negative impedance, so that a
frequency characteristic with an expanded bass sound characteristic can be
obtained regardless of the characteristic of the speaker unit 4.
Second Embodiment
FIG. 4 shows a basic arrangement of a portable electronic musical
instrument according to a second embodiment of the present invention. In
the electronic musical instrument shown in FIG. 4, the entire interior of
an instrument housing 1 is also used as a closed cabinet 5. A tone
generation instruction means 2 such as a keyboard, drum pad, breath input
device, or the like, and a musical tone forming means 7 for electrically
forming a musical tone instructed by the tone generation instruction means
2 are arranged in place of a tone generation controller consisting of the
tone generating device 2 and the tone generating circuit 7 shown in FIG.
1. Other arrangements are the same as those in FIG. 1.
Therefore, the electronic musical instrument of this embodiment is operated
in the same manner as the musical instrument shown in FIG. 1, except that
a musical tone signal during a performance is formed by an electrical
circuit such as a memory, oscillator, and the like, and no musical tone
can be generated while a power switch is OFF since a sound source does not
directly generate a mechanical or acoustic vibration and the instrument
housing does not resonate with the directly generated vibration.
The second embodiment can be applied to portable electronic musical
instruments such as a portable electronic keyboard, electronic percussion,
electronic wind instrument, and the like. According to this embodiment, a
bass sound characteristic can be improved by utilizing a relatively small
space in these portable electronic musical instruments.
The operation of the acoustic apparatus wherein a speaker system utilizing
a Helmholtz resonator is driven by a negative impedance generator will be
described below.
FIG. 5 shows an arrangement of an electric equivalent circuit of the
portion comprising a negative impedance driver as the vibrator driver 8,
and the speaker system with resonance port constituted by the vibrator 4,
the closed cabinet 5 and duct 6 each shown in FIG. 1 and FIG. 4. In FIG.
5, ref | | |
