Acoustic matching material, method of manufacture thereof,...

Details Acoustic matching material, method of manufacture thereof,... Acoustic matching material, method of manufacture thereof,...

2001-10-17

2003-04-08

Lobo, Ian J. (Department: 3662)

Communications, electrical: acoustic wave systems and devices

Signal transducers

Underwater type

C367S140000

Reexamination Certificate

active

06545947

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an acoustic matching member used, when a sound is propagated from one object to another object, for matching acoustic impedances of the two objects, a method for producing the acoustic matching member, and an ultrasonic transmitting and receiving device using the acoustic matching member.
BACKGROUND ART
An acoustic impedance of an object is obtained by (density×sonic speed). The acoustic impedance of air Z
AIR
is about 428 kg/m
2
s, and the acoustic impedance of a piezoelectric vibrator Z
PZT
for generating an ultrasonic wave is about 30×10
6
kg/m
2
s.
When an ultrasonic wave is radiated from the piezoelectric vibrator into the air, sound reflection is generated by the difference between the acoustic impedance of the piezoelectric vibrator Z
PZT
and the acoustic impedance of air Z
AIR
, and thus the radiation efficiency of the sound is reduced.
The acoustic matching member is used to alleviate the reduction in the radiation efficiency of the sound by matching the acoustic impedance of the piezoelectric vibrator Z
PZT
and the acoustic impedance of air Z
AIR
.
An acoustic impedance of an acoustic matching member Z
M
is obtained by expression (1) based on theoretical calculation.
Z
M
={square root over ( )}(Z
PZT
×Z
AIR
)  expression (1)
Here, the value of Z
M
is an ideal value at which there is no sound reflection. Using the above-mentioned values of Z
PZT
and Z
AIR
, the value of Z
M
is about 0.11×10
6
kg/m
2
s.
FIG. 29
is a graph illustrating the relationship between the acoustic impedance of the acoustic matching member and the ratio of the sound energy radiated from the piezoelectric vibrator into the air (transmission ratio). It is appreciated from
FIG. 29
that when the acoustic impedance of the acoustic matching member is about 0.11×10
6
kg/m
2
s, the transmission ratio is 1, at which there is no sound reflection.
In order to obtain an acoustic matching member having such an ideal acoustic impedance, a material having a low density and allowing a low sonic speed needs to be selected for such an acoustic matching member.
FIG. 30
shows an example of a conventional acoustic matching member. The acoustic matching member shown in
FIG. 30
is obtained by mixing glass balloons
121
in a resin material
120
and solidifying the resultant mixture.
The glass balloons are hollow and thus have a feature of being very lightweight. A structure obtained by mixing the glass balloons in the resin material and solidifying the resultant mixture has a lower density than that of a structure obtained by solidifying only the resin material. The size of the glass balloons is set to a value which is sufficiently smaller than the wavelength of the vibration (sound) propagating through the acoustic matching member (about {fraction (1/10)} of the wavelength of the vibration or less). The size of the glass balloons is set to such a value in order to make the propagation of the vibration less liable to the influence of the glass balloons.
When glass balloons having a true density of 0.13 g/cm
3
(“Scotchlight™ Glass Bubbles Filler” available from Sumitomo 3M Ltd.) are mixed in a resin material allowing a sonic speed of about 2300 m/s and having a density of 1.2 g/cm
3
, and the resultant mixture is solidified, a structure having a density of 0.56 g/cm
3
and allowing a sonic speed of 2100 m/s is obtained. An acoustic impedance Z
COM
of the structure thus obtained is 1.18×10
6
kg/m
2
s.
Japanese Laid-Open Publication No. 2-177799 describes that an acoustic matching member is formed using only hollow glass spheres. This acoustic matching member is produced by heating the hollow spheres up to a temperature for softening the hollow glass spheres, compressing the hollow spheres, and binding the plurality of hollow spheres at respective contact points. As the hollow glass spheres, “Scotchlight™ Glass Bubbles Filler” available from Sumitomo 3M Ltd. is used. Japanese Laid-Open Publication No. 2-177799 describes that the acoustic matching member thus produced has characteristics of a sonic speed of 900 m/s and an acoustic impedance Z
BG
of about 0.45×10
6
kg/m
2
s. Since the acoustic impedance of an object is represented by (sonic speed×density), the density of this acoustic matching member is 0.5 g/cm
3
.
As described above, the sonic speed allowed by glass is 5000 to 6000 m/s, but the sonic speed allowed by an acoustic matching member is reduced to 900 m/s by producing the acoustic matching member using hollow glass spheres.
An acoustic matching member can be bonded to a vibrator or a case accommodating the vibrator with an adhesive formed of a resin material such as an epoxy resin. Japanese Laid-Open Publication No. 2-177799 describes an example of heating the plurality of hollow spheres up to a temperature for softening the plurality of hollow spheres and binding the plurality of hollow spheres at respective contact points as well as bonding the acoustic matching member to a vibrator. By such a bonding method, the acoustic matching member is formed only of glass, and thus has superior temperature characteristics to those of an acoustic matching member formed using a resin material. The reason is that the thermal expansion ratio of glass is lower than the thermal expansion ratio of the resin material. When the flow rate of a gas is measured using an ultrasonic transmitting and receiving device, the temperature characteristics of the ultrasonic transmitting and receiving device significantly influences the measurement precision. In order to accurately measure a very small flow rate of gas, the temperature characteristics of the ultrasonic transmitting and receiving device need to be small.
Some types of gases are explosive. A vibrator which needs to provide such a gas with an electric signal is required to be accommodated in a case in order to prevent the vibrator from contacting the gas. Conditions to be satisfied by the material of the case include a high strength against breakage and satisfactory temperature characteristics. For this reason, metal is preferable as a material of the case. The thermal expansion ratio of metal is different from the thermal expansion ratio of glass. Therefore, a metal case and the acoustic matching member come apart from each other and cannot be bonded together at the stage of binding the plurality of hollow spheres at respective contact points after the plurality of hollow spheres are heated to a temperature for softening the plurality of hollow spheres as in the method described in Japanese Laid-Open Publication No. 2-177799.
When the acoustic impedances Z
BG
and Z
COM
of the above-described acoustic matching materials are plotted in the graph of
FIG. 29
, Z
BG
is positioned at A and Z
COM
is positioned at ▪. The transmission ratio is 0.21 for Z
BG
and 0.05 for Z
COM
. Thus, the transmission ratio (i.e., the transmission ratio of sound) for Z
BG
is four times the transmission ratio for Z
COM
. However, in actuality, an output which is four times larger is not obtained, but the outputs are of an equivalent level for Z
BG
and Z
COM
. This is considered to occur since the structure having Z
BG
is more likely to cause the sound to attenuate when the sound is propagated therethrough than the structure having Z
COM
. By contrast, the structure having Z
COM
is less likely to cause the sound to attenuate while the sound is propagated therethrough but allows a higher sonic speed than the structure having Z
BG
. Therefore, the structure having Z
COM
has a larger acoustic impedance and causes the sound radiated into the air to be reflected more than the structure having Z
BG
.
In the end, there is no significant difference in the sound outputs of the both types of acoustic matching members. Therefore, an acoustic matching member providing a large sound output is demanded rather than the acoustic matching member formed of the structure having Z
BG
or Z
COM
One possible reason why the structure having Z
BG
causes the sound to significa

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