An earphone includes an earplug of sound insulating material which is insertable in the external auditory meatus of an ear, and an elastic vibration generator responsive to an electric signal supplied thereto for generating and applying an elastic wave corresponding to the supplied electric signal to an outer end of the earplug inserted in the external auditory meatus. The elastic vibration generator may be held in or out of contact with the earplug, so that the elastic wave generated by the elastic vibration generator can be applied directly or indirectly to the earplug. The earphone may be combined with a helmet or a headband. The earplug is effective to prevent external noise from being transmitted to the ear drum of the ear, and also allows desired sound to be transmitted in the form of an elastic wave reliably and clearly to the ear drum.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an earphone, and more particularly to an 
earphone suitable for use with a radio receiver in a noisy environment 
such as an automobile racing circuit, a construction site, or the like. 
2. Description of the Prior Art 
In automobile racing, conversations between the drivers of racing 
automobiles and pit members or directors are usually transmitted and 
received typically through transceivers. The driver hears transmitted 
conversations with a small-size loudspeaker, a headset, or an earphone 
which is incorporated in a helmet that the driver wears to protect his 
head. The noise produced by a racing car while it is running has a very 
high level of up to 100 through 120 dB. While the helmet has a certain 
noise insulating capability as it covers the driver's ears, such a high 
racing noise level is excessive enough to make the helmet ineffective as a 
noise insulation. Conventional earphones are designed for use with audio 
systems or in low-noise environments, and cannot be used in noisy 
environments as the transmitted information that is reproduced by the 
earphones is masked by the noise. 
In view of the aforesaid problems, there have been developed earphones with 
a noise insulating capability as disclosed in Japanese laid-open utility 
model publications Nos. 2-21891 and 2-75890, for example. 
The earphone disclosed in Japanese laid-open utility model publication No. 
2-21891 has an acoustic passage extending from an electroacoustic 
transducer toward an end to be inserted in an external auditory meatus of 
the user, the acoustic passage being in the form of an air vibratory 
system. Since sound produced by the electroacoustic transducer is 
propagated through the air in the acoustic passage by means of wave 
motion, external noise may leak through a vibratory plate of the 
electroacoustic transducer and the acoustic passage into the external 
auditory meatus. 
Japanese laid-open utility model publication No. 2-75890 discloses a 
headset having a vibration damping material for insulating sound. The 
headset includes pads for covering the user's ears. When the pads are not 
properly held against the ears, external noise tends to leak through the 
headset into the external auditory meatus. 
Inasmuch as the conventional earphone or headset is designed to propagate 
sound waves through air, its noise insulating capability is not sufficient 
in noisy environments such as automobile racing circuits, construction 
sites, engine compartments on ships, or the like. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an earphone which is 
capable of reliably insulating noise when used in noisy environments, and 
also of clearly transmitting desired information to the user of the 
earphone. 
According to the present invention, there is provided an earphone including 
an earplug of sound insulating material which is insertable in the 
external auditory meatus of an ear, and an elastic vibration generator 
responsive to an electric signal supplied thereto for generating and 
applying an elastic wave corresponding to the supplied electric signal to 
an outer end of the earplug inserted in the external auditory meatus. 
According to the present invention, there is also provided an earphone 
including an earplug of sound insulating material which is insertable in 
the external auditory meatus of an ear, and an elastic vibration generator 
held in contact with the earplug and responsive to an electric signal 
supplied thereto for generating and applying an elastic wave corresponding 
to the supplied electric signal directly to an outer end of the earplug 
inserted in the external auditory meatus. 
According to the present invention, there is further provided an earphone 
including an earplug of sound insulating material which is insertable in 
the external auditory meatus of an ear, and an elastic vibration generator 
held out of contact with the earplug and responsive to an electric signal 
supplied thereto for generating and applying an elastic wave corresponding 
to the supplied electric signal indirectly to an outer end of the earplug 
inserted in the external auditory meatus. 
According to the present invention, there is further provided an earphone 
and helmet assembly including an earplug of sound insulating material 
which is insertable in the external auditory meatus of an ear, an elastic 
vibration generator responsive to an electric signal supplied thereto for 
generating and applying an elastic wave corresponding to the supplied 
electric signal to an outer end of the earplug inserted in the external 
auditory meatus, and a helmet shell, the elastic vibration generator being 
attached to an inner surface of the helmet shell at a position 
corresponding to the external auditory meatus. 
According to the present invention, there is further provided an earphone 
and headband assembly including an earplug of sound insulating material 
which is insertable in the external auditory meatus of an ear, an elastic 
vibration generator responsive to an electric signal supplied thereto for 
generating and applying an elastic wave corresponding to the supplied 
electric signal to an outer end of the earplug inserted in the external 
auditory meatus, and a headband, the elastic vibration generator being 
attached to the headband at a position corresponding to the external 
auditory meatus. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention shown by way of illustrative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Like or corresponding parts are denoted by like or corresponding reference 
characters throughout views. 
FIG. 1 illustrates the principles of an earphone according to the present 
invention. 
As shown in FIG. 1, an earphone according to the present invention has an 
earplug 4 insertable into the external auditory meatus 2 of an ear 1, the 
earplug 4 being made of a sound insulating material, and an elastic 
vibration generator 5 responsive to an electric signal V.sub.IN applied 
thereto for generating and transmitting an elastic wave V.sub.E directly 
to an outer end of the earplug 4 remote from the inner end thereof to be 
inserted in the external auditory meatus 2. The elastic vibration 
generator 5 is held in contact with the outer end of the earplug 4 for 
transmitting the elastic wave V.sub.E directly to the earplug 4. 
Since the earplug 4 is made of a sound insulating material and inserted in 
the external auditory meatus 2 of the ear 1, external noise is prevented 
from entering the external auditory meatus 2 through the earplug 4. The 
earplug 4 inserted in the external auditory meatus 2 also serves as a 
medium for propagating sound, i.e., the elastic wave V.sub.E produced by 
the elastic vibration generator 5. Therefore, sound, typically voice, from 
the elastic vibration generator 5 can reliably and clearly be transmitted 
through the earplug 4 to an ear drum 3. The earplug 4, which serves as an 
elastic wave propagation medium, is effective to block external noise, and 
also to propagate the elastic wave V.sub.E efficiently. 
The elastic wave V.sub.E generated by the elastic vibration generator 5 can 
be transmitted highly efficiently to the earplug 4 because the elastic 
vibration generator 5 is held in contact with the earplug 4 for direct 
transmission of the elastic wave V.sub.E to the earplug 4. 
1ST EMBODIMENT 
FIGS. 2 through 5 show an earphone according to a first embodiment of the 
present invention. As shown in FIGS. 2 and 3, the earphone according to 
the first embodiment comprises a cylindrical or rod-shaped earplug 4 that 
can be inserted into the external auditory meatus 2 of an ear 1, and a 
cylindrical exciter 5 (elastic vibration generator) coupled to an outer 
end of the earplug 4 for generating and applying an elastic wave V.sub.E 
directly to the earplug 4. 
The earplug 4 has an outside diameter slightly larger than the inside 
diameter of the external auditory meatus 2, and is made of a sound 
insulating material such as an elastic foamed polymer, e.g., urethane 
foam, which should preferably have a very high internal loss. When 
compressed, the earplug 4 is elastically restorable to its original shape. 
The earplug 4 is as hard as an ear lobe, and has such a degree of 
elasticity that when in use, it can be compressed by fingers, and after 
being inserted in the external auditory meatus 2, it will elastically be 
restored to its original cylindrical shape within an appropriate period of 
time. When the earplug 4 is restored to its original cylindrical shape 
after being inserted in the external auditory meatus 2, the earplug 4 has 
its outer circumferential surface held in intimate contact with the inner 
wall surface of the external auditory meatus 2 under pressure. Therefore, 
the earplug 4 is placed in the external auditory meatus 2 tightly enough 
to acoustically isolate the external auditory meatus 2 from outside of the 
ear 1 for the prevention of entry of external noise into the external 
auditory meatus 2. The earplug 4 may also be made of silicone resin, clay, 
or the like. 
The exciter 5 is in the form of a vibrator for generating elastic vibration 
on a vibratory surface 8. The exciter 5 may comprise a dynamic exciter 5 
as shown in FIG. 4 or a magnetic exciter 5 as shown in FIG. 5. 
The dynamic exciter 5 shown in FIG. 4 has a bottomed cylindrical casing 10 
of synthetic resin with one axial end open, and a circular vibratory plate 
8 of metal or magnetic material such as iron mounted on the open axial 
end, closing the casing 10. The casing 10 houses a bottomed cylindrical 
yoke 11 suspended therein with a suitable degree of stiffness by dampers 
12. The yoke 11 has an open axial end direction in the same direction as 
the open axial end of the casing 10. An axially extending cylindrical 
magnet 14 is disposed centrally in the yoke 11, with a circular center 
pole 13 mounted on the tip of the magnet 14. The inner circumferential 
surface of the yoke 11 and the outer circumferential surfaces of the 
magnet 14 and the center pole 13 define a ring-shaped gap therebetween. In 
the gap there is disposed a ring-shaped voice coil 15 out of contact with 
the yoke 11, the magnet 14, and the center pole 13, the ring-shaped voice 
coil 15 having an axial end fixed to the vibratory plate 8. The voice coil 
15 is electrically connected to leads 7 (see FIGS. 2 and 3) that extend 
from a transceiver (not shown). When an electric signal V.sub.IN is 
applied over the leads 7 to the voice coil 15, the yoke 11 vibrates at a 
frequency corresponding to the frequency of the applied electric signal 
V.sub.IN through the interaction between a magnetic field produced in the 
gap by a magnetic circuit composed of the yoke 11, the magnet 14, and the 
center pole 13 and an alternating magnetic field induced by the voice coil 
15. The vibration of the yoke 11 appears as elastic vibration on the 
vibratory plate 8. The outer end of the earplug 4, which is remote from 
the inner end thereof inserted in the external auditory meatus 2, is held 
in mechanical contact with the vibratory plate 8, so that an elastic wave 
V.sub.E is propagated from the vibratory plate 8 through the earplug 4, 
thereby vibrating the inner end thereof inserted in the external auditory 
meatus 2. 
The dynamic exciter 5 shown in FIG. 5 has a bottomed cylindrical casing 16 
of synthetic resin with one axial end open, and a circular vibratory plate 
21 having an outer circumferential edge fitted in a ring groove 23 defined 
in the open axial end, closing the casing 16. The casing 16 houses a 
bottomed cylindrical yoke 17 fixedly mounted on the bottom thereof and has 
an open axial end directed in the same direction as the open axial end of 
the casing 16. A cylindrical magnet 18 and a cylindrical center pole 19 
which extend axially are disposed centrally on the bottom of the yoke 17. 
A ring-shaped voice coil 20 is disposed coaxially with and around the 
center pole 19 out of contact therewith, the ring-shaped voice coil 20 
having an axial end fixed to the vibratory plate 21. The voice coil 20 is 
electrically connected to leads 7 (see FIGS. 2 and 3). When an electric 
signal V.sub.IN is applied over the leads 7 to the voice coil 20, the 
Vibratory plate 21 vibrates at a frequency corresponding to the frequency 
of the applied electric signal V.sub.IN through the interaction between a 
magnetic field produced in a gap defined by a magnetic circuit composed of 
the yoke 17, the magnet 18, and the center pole 19 and an alternating 
magnetic field induced by the voice coil 20. The vibration of the 
vibratory plate 21 is elastic vibration. The outer end of the earplug 4, 
which is remote from the inner end thereof inserted in the external 
auditory meatus 2, is held in mechanical contact with the vibratory plate 
21, so that an elastic wave V.sub.E is propagated from the vibratory plate 
8 through the earplug 4, thereby vibrating the inner end thereof inserted 
in the external auditory meatus 2. 
The exciter 5 is not limited to the structures shown in FIGS. 4 and 5, but 
may be of any structures insofar as they can produce elastic vibration. 
As shown in FIGS. 2 and 3, the earplug 4 and the exciter 5 are joined to 
each other through the outer end of the earplug 4, which serves as a 
vibration receiving surface 9, and the vibratory surface or plate 8 of the 
exciter 5. The earplug 4 and the exciter 5 may be integrally fixed to each 
other in advance, but should preferably be separate from each other so 
that they can easily be joined to each other when in use. 
As shown in FIG. 1, the electric signal V.sub.IN applied to the exciter 5 
is converted into mechanical vibration by the exciter 5. The mechanical 
vibration produced by the vibratory plate 8 is transmitted as an elastic 
wave V.sub.E from the vibratory plate 8 through the vibration receiving 
surface 9 into the earplug 4. The elastic wave V.sub.E is then propagated 
through the earplug 4 toward the inner end thereof. When the elastic wave 
V.sub.E reaches the inner end of the earplug 4, the inner end vibrates at 
the same frequency as the frequency of the applied electric signal 
V.sub.IN, radiating a sound wave V.sub.A into the external auditory meatus 
2. Since the external auditory meatus 2 is acoustically isolated from the 
space outside of the ear 1, at this time, the intensity of external noise 
which may enter the external auditory meatus 2 is very low. Almost all 
acoustic energy that reaches the ear drum 3 at this time is the sound wave 
V.sub.A radiated from the inner end of the earplug 4. Consequently, the 
user of the earphone can clearly hear or perceive the sound reproduced 
from the sound wave V.sub.A with a low noise background. 
2ND EMBODIMENT 
FIG. 6 shows an earphone according to a second embodiment of the present 
invention. The earphone according to the second embodiment includes an 
exciter 5 having such an outer size or profile that it is snugly fitted in 
the concha 26 of an ear of the user and retained in place against removal 
by the tragus 25 of the ear. 
The outer surface of the exciter 5 is covered with a material having a 
certain degree of resiliency and a coefficient of friction. Therefore, 
once placed in the ear of the user, the exciter 5 is securely held in the 
ear against dislodgement. The material, structure, and shape of the 
earplug and the internal structure of the exciter 5 are identical to those 
of the earphone according to the first embodiment. 
3RD EMBODIMENT 
FIG. 7 shows an earphone according to a third embodiment of the present 
invention. The earphone according to the third embodiment has an earplug 
4A and an exciter 5A that are detachably coupled to each other. 
The exciter 5A has an outwardly extending protrusion 27 on the center of 
the vibratory plate 8, and the earplug 4A has a recess 28 defined in the 
center of the outer end or the vibration receiving surface 9 thereof, for 
receiving the protrusion 27 therein. The inside diameter of the recess 28 
may be slightly smaller than the outside diameter of the protrusion 27, or 
the protrusion 27 may be progressively larger in diameter toward its tip 
end and the recess 28 may be progressively smaller in diameter toward its 
open end, so that the protrusion 27 that is received in the recess 28 is 
securely retained therein against forces tending to separate the earplug 
4A and the exciter 5A. 
The earplug 4A and the exciter 5A that are detachably coupled to each other 
make the earphone usable conveniently. More specifically, when the 
earphone is to be used, the earplug 4A is first inserted into the external 
auditory meatus 2, and then the exciter 5A is joined to the earplug 4A. 
Since the earplug 4A and the exciter 5A can be handled independently, the 
earphone can be handled with ease when it is placed in the ear which is of 
a relatively complex structure. The material, structure, and shape of the 
earplug 4A and the internal structure of the exciter 5A are identical to 
those of the earphone according to the first embodiment. 
4TH EMBODIMENT 
FIG. 8 shows an earphone according to a fourth embodiment of the present 
invention. The earplug 4 and the exciter 5 of the earphone according to 
the fourth embodiment are also detachably coupled to each other. 
The exciter 5 has an adhesive tape 29 applied to the vibratory plate 8 in a 
position where the vibration receiving surface 9 of the earplug 4 contacts 
the vibratory plate 8. In use, the earplug 4 is attached to the exciter 5 
by the adhesive tape 29. The adhesive tape 29 should preferably be capable 
of maintaining its adhering ability even after the earplug 4 is attached 
to and detached from the exciter 5 a number of times. Inasmuch as the 
earplug 4 and the exciter 5 are detachably coupled to each other, the 
earphone according to the fourth embodiment can also be handled with ease. 
The material, structure, and shape of the earplug 4 and the internal 
structure of the exciter 5 are identical to those of the earphone 
according to the first embodiment. 
5TH EMBODIMENT 
FIG. 9 shows an earphone according to a fifth embodiment of the 
present'invention. The earphone according to the fifth embodiment has an 
exciter 5B and an earplug 4B which are designed to enable the earplug 4B 
to be held in intimate contact with the inner wall surface of the external 
auditory meatus 2. 
The exciter 5B has an outwardly extending tapered protrusion 30 on the 
center of the vibratory plate 8, and the earplug 4B has a recess 48 
defined in the center of the outer end thereof, for receiving the 
protrusion 30 therein. The tapered protrusion 30 is slightly larger in 
diameter than the recess 48, so that when the protrusion 30 is inserted in 
the recess 48, joining the exciter 5C and the earplug 4C to each other, 
the recess 48 and the portion of the earplug 4B which surrounds the recess 
48 are spread radially outwardly. When the earphone is worn by the user, 
and the exciter 5B and the earplug 4B are coupled to each other, the outer 
end of the earplug 4B is spread radially outwardly into intimate contact 
with the open end of the external auditory meatus 2. Therefore, when used 
in the ear of the user, the earphone according to the fifth embodiment 
provides an increased sound insulating capability against the entry of 
external noise into the external auditory meatus 2. The material, 
structure, and shape of the earplug 4B and the internal structure of the 
exciter 5B are identical to those of the earphone according to the first 
embodiment. 
6TH EMBODIMENT 
FIG. 10 shows an earphone according to a sixth embodiment of the present 
invention. The earphone according to the sixth embodiment includes a 
conically tapered earplug 4C that can easily be inserted more intimately 
into the external auditory meatus 2. 
The earplug 4C has a recess 49 defined in the outer end thereof. The 
earphone also includes an exciter 5C which has an outwardly extending 
tapered protrusion 30 on the center of the vibratory plate 8, which is to 
be received in the recess 49. The tapered protrusion 30 is slightly larger 
in diameter than the recess 49. When the protrusion 30 is inserted in the 
recess 49, joining the exciter 5C and the earplug 4C to each other, the 
recess 49 and the portion of the earplug 4C which surrounds the recess 49 
are spread radially outwardly. When the earphone is worn by the user, and 
the exciter 5C and the earplug 4C are thus coupled to each other, the 
outer end of the earplug 4C is spread radially outwardly as an expanded 
portion 32 which is pressed against held in intimate contact with the open 
end of the external auditory meatus 2. Therefore, when used in the ear of 
the user, the earphone according to the sixth embodiment provides an 
increased sound insulating capability against the entry of external noise 
into the external auditory meatus 2. The material of the earplug 4C and 
the internal structure of the exciter 5C are identical to those of the 
earphone according to the first embodiment. 
7TH EMBODIMENT 
FIG. 11 shows an earphone according to a seventh embodiment of the present 
invention. The earphone according to the seventh embodiment comprises an 
exciter 5D and a conically tapered earplug 4D for easy insertion into and 
intimate contact with the inner circumferential surface of the external 
auditory meatus 2. 
The earplug 4D has a bottom, i.e., the outer end thereof, bonded to the 
vibratory plate 8 of the exciter 5D by an adhesive or the like which 
prevents the exciter 5D and the earplug 4D from being detached from each 
other once bonded together. Because the earplug 4D and the exciter 5D are 
firmly joined to each other, the earphone can be worn by the user in one 
operation, or the user is not required to attach the earplug 4D and the 
exciter 5D separately, i.e., to insert the earplug 4D into the external 
auditory meatus 2 and then attach the exciter 5D to the earplug 4D. The 
conically tapered shape of the earplug 4D prevents itself from being 
inserted into the external auditory meatus 2 as deeply as the cylindrical 
earplug such as shown in FIGS. 2 and 3, and hence has a lower sound 
insulating capability against the entry of external noise. However, the 
earphone with the conically tapered earplug is much better at noise 
prevention and sound perception than conventional earphones in medium 
noise level. When the earphone with the conically tapered earplug is used 
with an audio system, the leakage of reproduced sound from the earphone 
into the space outside of the ear is quite low. Therefore, the earphone 
can effectively be used with a portable cassette recorder. 
8TH EMBODIMENT 
FIG. 12 shows an earphone according to an eighth embodiment of the present 
invention. 
The earphone shown in FIG. 12 has an exciter 5F and a conically tapered 
earplug 4F which are integrally joined to each other, the earplug 4F 
having a central axis X.sub.1 displaced off the central axis X.sub.0 of 
the exciter 5F. It is known that the central axis of the external auditory 
meatus 2 is usually not aligned with, but displaced from, the central axis 
of the concha of the ear. Based on the average distance between the 
central axis of the external auditory meatus and the central axis of the 
concha among possible users, the central axis X.sub.1 of the earplug 4F is 
displaced off the central axis X.sub.0 of the exciter 5F for allowing the 
earphone to be fitted neatly in the ear. Another advantage is that since 
the exciter 5F may be increased in size by the distance between the 
central axis X.sub.1 of the earplug 4F and the central axis X.sub.0 of the 
exciter 5F, the exciter 5F may have an increased driving capability for 
better sound reproduction. 
9TH EMBODIMENT 
FIGS. 13 and 14 show an earphone according to a ninth embodiment of the 
present invention. 
The earphone shown in FIG. 13 has an exciter 5G and a conically tapered 
earplug 4G which are integrally joined to each other, the exciter 5G 
having a central axis X.sub.0 inclined a certain angle to the central axis 
X.sub.1 of the earplug 4G. The exciter 5G which is thus inclined to the 
earplug 4G can snugly be fitted in the concha 26 (see FIG. 14) of the ear, 
and, after fitted, is less liable to be detached from the concha 26. Since 
the exciter 5G is inclined with respect to the axis of the external 
auditory meatus 2 when placed in the ear, the exciter 5G is positioned 
clear of projecting portions of the ear. Accordingly, the exciter 5G may 
be increased in size for a higher driving capability. 
In the above embodiments, the earplugs are of a uniform hardness, density, 
or material throughout their cylindrical or conical shape. However, the 
earplug of an earphone according to the present invention may be of an 
internal structure having a plurality of regions of different hardnesses, 
densities, or materials, as shown in FIGS. 15 through 19. 
FIG. 15 shows an earplug 4H which is heavier and harder in a region 34 near 
the outer end or the vibration receiving surface 9 held against an 
exciter, and which is lighter and softer progressively or stepwise in a 
region 35 toward the inner end. Since the exciter is much heavier and 
harder than the earplug 4H as a whole, an elastic wave transmitted from 
the exciter into the earplug 4H is subject to a transmission loss. The 
heavier and harder region 34 of the earplug 4H serves to reduce such a 
transmission loss because the weight and hardness of the earplug 4H in the 
region 34 near the outer end which contacts the exciter are closer to 
those of the exciter. The reduced transmission loss results in an 
increased elastic wave transmission efficiency for an increased intensity 
of sound reproduced by the earphone. The earplug 4H of composite 
properties may be made of either a single material that is processed to 
provide different densities in different regions of the earplug, or 
different materials of different hardnesses, densities, and weights that 
are arranged in different regions of the earplug. 
FIG. 16 illustrates an earplug 4I including a conical harder member 36 
fitted in the outer end or the vibration receiving surface 9 thereof for 
reducing a transmission loss. 
FIG. 17 shows an earplug 4J including a hard member 37 of greater hardness, 
density, and weight attached to the outer end thereof. The hard member 37 
may not necessarily be of the same material as the earplug 4J insofar as 
it can reduce a transmission loss. 
FIGS. 18A and 18B show an earplug 4K including a cylindrical hard core 
member 38 disposed therein and extending axially therethrough between the 
inner and outer ends. The hard core member 38 is harder, heavier, and 
denser than a surrounding softer sleeve member. 
FIG. 19 shows an acoustic transmission characteristic (spectrum) of an 
earplug made of a single material. When the earplug is made of a single 
material, upper frequency limit of the sound wave transmitted through the 
earplug is substantially determined according to an equivalent mass of the 
vibratory plate in the exciter and a Young's modulus of the earplug. In 
this case, the earplug nearly functions as a first dimensional low pass 
filter. An earplug made of a soft material such as a foamed polymer still 
retains its softness after inserted into an external auditory meatus of an 
ear, the transmittance of sound waves (pressure) vibrated at a 
predetermined acceleration decreases at the rate of 6 dB per octave over 
the frequency of 200 Hz, as shown in FIG. 19. Since a spoken human voice 
has frequency components in the frequency range over 200 Hz, an earplug 
having such transmission characteristic provide an indistinctness of the 
spoken words. In view of this, the hard core member 38 is provided in the 
earplug 4K so as to reduce the transmission loss through the earplug. 
Other modifications of the earplug having a hard core member are shown in 
FIGS. 21 through 25. In an earplug shown in FIG. 21, the hard core member 
38a is covered with the softer sleeve member so as to be inserted into the 
external auditory meatus without pain or injury thereof. An earplug 4M 
shown in FIG. 22 has a softer sleeve member surrounding a hard core member 
38 and being formed tapering at inner edge thereof to make a smooth 
insertion possible. An earplug 4N shown in FIG. 23 has a softer sleeve 
member surrounding a hard core member 38a and covering and tapering at 
inner edge thereof. An earplug 40 shown in FIG. 24 is provided with a core 
member 38b comprised of a plurality of thick core members to produce a 
flexibility. An earplug 4P shown in FIG. 25 is provided with a core member 
38c having a plurality of notches at an outer circumferential surface 
thereof to produce a flexibility. 
As a material of the core members 38, 38a to 38c, a foamed rubber sponge, 
etc., having a flexibility of hardness of a vinyl chloride polymer, having 
the Young modulus of 5 to 20, can be used. Alternatively, an urethane 
foam, a vinyl chloride foam and a polypropylene foam, etc., having an 
applicability in density and hardness thereof can be used. 
FIG. 20 shows an acoustic transmission characteristic of an earplug made of 
complex materials, as described above. As can be seen from FIG. 20, the 
transmission characteristic is remarkably improved over the frequency of 
200 Hz. An elastic wave from the exciter is propagated primarily through 
the hard core member 38. External noise can be insulated by the softer 
sleeve member surrounding the hard core member 38. The earplug 4K is 
therefore effective to increase the intensity of reproduced sound. 
FIG. 26 shows an earplug 40Q including a sol body 39 disposed therein and 
extending axially therethrough between the inner and outer ends. The sol 
body 39 may be a sol of silicone oil or the like. The sol body 39 is 
encased in a flexible sleeve of high-strength plastic material. The sol 
body 39 is progressively tapered from the outer end to the inner end of 
the earplug 4L. As with the earplug 4K shown in FIGS. 18A and 18B, the sol 
body 39 serves to propagate an elastic wave from the exciter therethrough. 
10TH EMBODIMENT 
FIG. 27 shows an earphone according to a tenth embodiment of the present 
invention. 
The earphone shown in FIG. 27 is arranged to improve acoustic impedance 
matching between an exciter 5 and an earplug 4M. 
As shown in FIG. 27, the exciter 5 of the earphone has a disc-shaped thin 
large-diameter elastic member 40 bonded to the vibratory plate 8 thereof. 
The elastic member 40 is of the same material as the earplug 40R, and has 
a radially larger outer profile or area than the earplug 40R. Since the 
earplug 40R and the elastic member 40 have the same acoustic 
characteristics, acoustic impedance matching is achieved between the 
earplug 40R and the elastic member 40 and hence improved between the 
earplug 40R and the exciter 5. Consequently, the efficiency with which the 
elastic wave is transmitted from the exciter 5 to the earplug 40R is 
increased. The wide area of the elastic member 40 allows the earplug 40R 
to be positioned relatively freely with respect to the exciter 5. 
Therefore, the earplug 40R and the exciter 5 may be joined to each other 
without strict positional limitations, and hence may be handled with ease 
when they are joined to each other. 
FIG. 28 shows the principles of another earphone according to the present 
invention. 
As shown in FIG. 28, an earphone according to the present invention has an 
earplug 4 insertable into the external auditory meatus 2 of an ear 1, the 
earplug 4 being made of a sound insulating material, and an elastic 
vibration generator 5M responsive to an electric signal V.sub.IN applied 
thereto for generating and transmitting an elastic wave V.sub.E indirectly 
to an outer end of the earplug 4 remote from the inner end thereof to be 
inserted in the external auditory meatus 2. The elastic vibration 
generator 5M is held out of contact with the outer end of the earplug 4 
for transmitting the elastic wave V.sub.E indirectly to the earplug 4. 
As with the earphone shown in FIG. 1, the earplug 4, which serves as an 
elastic wave propagation medium, is effective to block external noise, and 
also to propagate the elastic wave V.sub.E efficiently. The elastic wave 
V.sub.E generated by the elastic vibration generator 5M is transmitted 
indirectly to the earplug 4 which is held out of contact with the elastic 
vibration generator 5M. Because the elastic vibration generator 5M and the 
earplug 4 are separate and independent from each other, they can be 
handled and used freely with ease. If the earphone shown in FIG. 28 is 
used with a helmet worn by a racing car driver, then the elastic vibration 
generator 5M is connected to the helmet and the earplug 4 is put in the 
ear of the driver. The elastic vibration generator 5M and the earplug 4 do 
not need to be accurately positioned with respect to each other when the 
helmet is worn by the driver. In addition, the helmet can be put on or 
taken off quite easily as the elastic vibration generator 5M and the 
earplug 4 are not joined to each other. 
11TH EMBODIMENT 
FIGS. 29 and 30 show an earphone according to an eleventh embodiment of the 
present invention. 
As shown in FIG. 29, the earphone comprises an earplug 4 and an exciter 5M 
which are held out of contact with each other. An elastic wave generated 
by the exciter 5M is transmitted indirectly (more specifically, 
magnetically) to the earplug 4. 
The earplug 4 is in the shape of a rod or cylinder, and made of a sound 
insulating material such as an elastic foamed polymer, e.g., urethane 
foam. When compressed, the earplug 4 is elastically restorable to its 
original shape. The earplug 4 includes a circular vibratory plate 6 
attached to its outer end remote from the inner end to be inserted in the 
external auditory meatus of an ear, the vibratory plate 6 having a 
diameter which is substantially the same as that of the earplug 4. The 
vibratory plate 6 is made of a metal or magnetic material such as iron. 
As shown in FIG. 30, the exciter 5M comprises a magnetic generator having a 
closed hollow cylindrical casing 42 of resin, a bottomed cylindrical yoke 
50 disposed in the casing 42, a cylindrical magnet 43 and a cylindrical 
center pole 44 which are axially joined to each other and disposed 
centrally in the yoke 50 in the axial direction of the casing 42, and a 
ring-shaped voice coil 45 placed in a gap defined between the inner 
circumferential edge of the open end of the yoke 50 and the outer 
circumferential surface of the magnet 43. The casing 42 has a vibratory 
surface or plate 8 facing the vibratory plate 6 of the earplug 4. The 
voice coil 45 is attached to the vibratory plate 8. The magnet 43, the 
center pole 44, and the yoke 50 jointly make up a magnetic circuit for 
generating a direct magnetic field, in which the voice coil 45 is placed. 
When an electric signal V.sub.IN is supplied to the voice coil 45, the 
vibratory plate 8 of the exciter 5M produces an alternating magnetic field 
which is biased by the direct magnetic field and represents the applied 
electric signal V.sub.IN. 
The magnetic excitation of the exciter 5M can be transmitted to the earplug 
4 which is held out of contact with the exciter 5M. More specifically, in 
use, the earplug 4 is inserted into the external auditory meatus of an ear 
of the user such that the vibratory plate 6 faces outwardly of the ear. 
Then, the vibratory plate 8 of the exciter 5M is placed near the vibratory 
plate 6 out of contact therewith. When an electric signal V.sub.IN is 
supplied to the voice coil 45, the vibratory plate 8 of the exciter 5M 
produces an alternating magnetic field V.sub.M (see FIG. 28) corresponding 
to the electric signal V.sub.IN. The alternating magnetic field V.sub.M is 
applied to the vibratory plate 6. The vibratory plate 6 is vibrated, i.e., 
attracted toward and repelled from the exciter 5M, at a frequency 
corresponding to the frequency of the alternating magnetic field V.sub.M. 
The vibration of the vibratory plate 6 is propagated as an elastic wave 
V.sub.E through the earplug 4. When the elastic wave V.sub.E reaches the 
inner end of the earplug 4, the inner end vibrates at the same frequency 
as the frequency of the applied electric signal V.sub.IN radiating a sound 
wave V.sub.A into the external auditory meatus 2. 
Since the exciter 5M and the vibratory plate 6 are magnetically coupled to 
each other and the elastic wave V.sub.E is produced and transmitted 
through the earplug 4 based on such magnetic coupling, the earplug 4 can 
insulate external noise and transmit sound clearly from the exciter 5M 
without being physically joined thereto. The exciter 5M and the earplug 4 
can easily be handled and are not required to be positionally adjusted 
strictly with respect to each other as they are separate and independent 
from each other. The earphone with the exciter 5M and the earplug 4 being 
separate from each other is advantageous when used in a helmet to be worn 
by the user because the user can easily put on or take off the helmet with 
the exciter 5M attached to the helmet and the earplug 4 left in the ear. 
12TH EMBODIMENT 
FIG. 31 illustrates an earphone according to a twelfth embodiment of the 
present invention. 
According to the twelfth embodiment shown in FIG. 31, the earphone is 
incorporated in a helmet. The earphone comprises an earplug 4 and an 
exciter 5M which are separate from each other. The earplug 4 and the 
exciter 5M are identical to those shown in FIGS. 29 and 30. The exciter 5M 
is attached to an inner surface of a helmet shell 46 at a position 
corresponding to the external auditory meatus 2 of an ear of the user. In 
use, the user inserts the earplug 4 into the external auditory meatus 2 
with the vibratory plate 6 facing outwardly thereof, and then puts on the 
helmet shell 46. The helmet includes cushioning pads 47 of vibration 
damping material attached to the inner surface thereof by adhesive bonding 
and which, when the helmet is worn, contact the head of the user and holds 
the exciter 5M spaced from the vibratory plate 6 in the vicinity thereof. 
The vibratory plate 6 and the exciter 5M are thus maintained out of 
contact with each other, but magnetically coupled to each other for the 
transmission of reproduced sound. The earplug 4 can therefore transmit 
reproduced sound while insulating external noise. The earphone shown in 
FIG. 31 is particularly suitable for use by the driver of a racing car. 
13TH EMBODIMENT 
FIG. 32 illustrates an earphone according to a thirteenth embodiment of the 
present invention. 
According to the thirteenth embodiment shown in FIG. 32, the earphone is 
constructed as a headphone. The earphone comprises an earplug 4 and an 
exciter 5M which are separate from each other, the earplug 4 and the 
exciter 5M being identical to those shown in FIGS. 29 and 30. The exciter 
5M is attached to one end or each end of a headband 52 with a spacer 51 
mounted on an inner surface thereof. When the headband 52 is worn by the 
user, the exciter 5M is spaced from the vibratory plate 6 by the spacer 
51. Therefore, the vibratory plate 6 and the exciter 5M are kept out of 
contact with each other, but magnetically coupled to each other in use. 
The earphone shown in FIG. 32 is particularly suitable for use by a pit 
member in a car race paddock. 
14TH EMBODIMENT 
FIG. 33 shows an earplug for an earphone according to a fourteenth 
embodiment of the present invention. 
The earplug, generally designated by 4N in FIG. 33, is substantially 
cylindrical or rod-shaped and elongate axially. The earplug 4N has a 
radially outwardly enlarged portion 52 on its outer end. The axial length 
of the earplug 4N is such that when the earplug 4N is inserted in the 
external auditory meatus of an ear, the portion of the earplug 4N, 
including the enlarged portion 52, which extends out of the external 
auditory meatus is about 5 mm longer than that of the earplug 4 according 
to the previous embodiments. The earplug 4N is therefore reliably held in 
contact with an exciter 5N shown in FIG. 34. The radial size of the 
enlarged portion 52 may be selected such that when the earplug 4N is 
inserted in the external auditory meatus, it can easily be handled and 
nearly fitted in the ear, and the enlarged portion 52 provides a large 
area of contact with the exciter 5N without strictly positioning the 
exciter 5N with respect to the enlarged portion 52. The material of the 
earplug 4N is the same as that of the earplug 4 shown in FIGS. 2 and 3. 
As shown in FIG. 34, the exciter 5N is attached to an inner surface of a 
helmet shell 46 through a cushioning pad 53 of vibration damping material 
at a position corresponding to the external auditory meatus 2 of the ear 
of the user. The cushioning pad 53 serves to acoustically isolate the 
helmet shell 46 and the exciter 5N from each other for preventing unwanted 
vibratory noise from being transmitted from the helmet shell 46 to the 
exciter 5N. The cushioning pad 53 also allows the exciter 5N to apply 
elastic vibrations generated thereby to the earplug 4N efficiently without 
being adversely affected by the helmet shell 46 which is heavy. The helmet 
also includes cushioning pads 47 of vibration damping material attached to 
the inner surface thereof by adhesive bonding. 
In use, the earplug 4N is inserted into the external auditory meatus 2 
before the helmet is worn. At this time, the outer end of the earplug 4N 
projects about 5 mm from the open end of the external auditory meatus 2. 
Then, when the helmet is worn, the enlarged portion 52 is brought into 
contact with the exciter 5N. Since the enlarged portion 52 has a large 
area of contact, it is held in reliable and stable contact with the 
exciter 5N for efficient transmission of the elastic vibration from the 
exciter 5N to the earplug 4N even if the earplug 4N is not properly 
inserted or the exciter 5N is not positioned in exact alignment with the 
earplug 4N. Accordingly, the earplug 4N and the exciter 5N can be handled 
with ease, and the elastic wave can be propagated through the earplug 4N 
efficiently. 
The earplug 4N and the exciter 5N with the cushioning pad 53 shown in FIGS. 
33 and 34 may be combined with a headband as shown in FIG. 32. 
In the above embodiments, the exciter and the transceiver are electrically 
connected to each other by the leads 7. However, signals can be 
transmitted from the transceiver to the exciter by a wireless transmission 
device or radio transmitter. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.