JPH0721426B2 - Volume meter - Google Patents

Description

The present invention relates to an apparatus for measuring the volume of a container or the volume of an object contained in a container, and more particularly to an acoustic volume meter that utilizes a change in resonance frequency of an acoustic system.

As one method for measuring the volume of a container having a complicated shape and the volume of a liquid or solid object contained in the container, there is an acoustic method using the resonance phenomenon of Helmholtz resonator. That is, when one Helmholtz resonator is constructed by connecting an acoustic tube to a container, its resonance frequency changes depending on the difference between the volume of the container and the volume of the object contained therein, that is, the co-product. Then, the volume of the container or the object put in the container when the container is empty is known.

At that time, the most problematic point is how to compensate the change in the resonance frequency caused by the change in the temperature or the change in the humidity. The temperature sensor measures the temperature of the working gas inside the Helmholtz resonator, usually air,
It may be possible to compensate for the above resonance frequency change, but since the resonance frequency changes not only with temperature but also with the components of the working gas such as humidity, it is difficult to achieve precise volume measurement with the above temperature only compensation. . The present invention compensates for changes in the resonance frequency of the Helmholtz resonator, which is the main acoustic resonator, by the resonance frequency of the auxiliary acoustic resonator.

That is, the present invention, the auxiliary acoustic resonator is coupled to the Helmholtz resonator consisting of the container and the acoustic tube, or the acoustic tube of the Helmholtz resonator is also used as the auxiliary acoustic resonator,
An electronic circuit is connected to a sound source or a microphone coupled to this acoustic system to measure the resonance frequency of the Helmholtz resonator or its period and one of the resonance frequencies of the auxiliary acoustic resonator or its period, or The resonance frequency ratio is not directly measured, and the volume of the container or the volume of the object placed in the container is calculated from the ratio of the two resonance frequencies.When the temperature or humidity changes, the auxiliary acoustic resonator Compensation is performed by changing the resonance frequency at the same rate as the resonance frequency of the Helmholtz resonator.

Therefore, a first object of the present invention is to simply and precisely measure the volume of a container having a complicated shape using a sound resonator or the volume of an irregular object such as a fruit or a human body placed in the container. To provide the means to do so. The volume of liquid stored in a tank of complex shape is also measured by the present invention.

A second object of the present invention is to perform an accurate measurement by compensating for the influence of the change in the resonance frequency caused by the temperature change when the volume is measured by the above acoustic method, by using the same acoustic method. It is to provide the means to make the possible.

In FIG. 1, reference numeral 1 denotes a container having an empty volume V ₀ , in which the liquid 7 is contained by the volume V. Further, an acoustic tube 3 having an internal cross-sectional area S and a length 1 penetrates the upper plug 2 of 1. A sound source 5 acoustically drives the inside of the container 1 through a short conduit 9. As the sound source, an earphone or the like is used when the container 1 has a small volume, and a speaker or the like is used when the container 1 has a large volume. 6 is a microphone of a sound detector, which detects the sound inside the acoustic tube 3 through the short conduit 4. If the sound to be detected is below the audible frequency, a low pressure sensor or the like is used as the microphone 6.
Reference numeral 8 is an outlet valve, through which the liquid 7 is discharged to the outside.

The coproduct of the upper portion of the container 1 is V ₀ -V, but the acoustic compliance C due to this volume is C = (V ₀ -V) / γP ₀ (1). Here, γ is the specific heat ratio of the gas inside the container 1, usually air, and P ₀ is the atmospheric pressure. The acoustic inertance L due to the mass of the gas inside the acoustic tube 3 is L = ρl / S (2). Where ρ is the gas inside the acoustic tube 3, usually air,
Is the density of. The above-mentioned two acoustic elements constitute one acoustic resonator, which is a so-called Helmholtz resonator whose resonance frequency r is Is. Where c is the speed of sound, Is. As shown in the equation (3), r is a coproduct V ₀ −V
If V ₀ is known, the volume V of the object 7 placed in the container is known from the resonance frequency r. Also, if there is an empty container of unknown volume, V = 0, so by attaching a stopper or lid to the mouth of the container to which an acoustic tube, sound source, microphone, etc. are connected, as shown in FIG. The volume V ₀ is known. However, in practice, r, which is a change in the sound velocity c due to a change in temperature, changes, so a compensating means is required for that.

Here, in this embodiment, the above compensation is performed by utilizing the parasitic resonance of the Helmholtz resonator. The acoustic tube 3 in FIG. 1 is one acoustic resonator by itself, and its resonance frequency is that of a tube whose both ends are open and has a frequency of r'c / 2l (5) and an integral multiple thereof. Any of these resonance frequencies is proportional to the sound velocity c and can be used for compensation, but in the following description, the lowest resonance frequency r'is used.

FIG. 2 shows a frequency characteristic G () (indicates frequency) from the input voltage e _S (t) (t indicates time) to the sound source 5 to the output voltage em (t) of the microphone 6. The gain characteristic | G () | has resonance peaks at two frequencies r and r '. Then, of these two resonance frequencies, r changes according to the coproduct V ₀ -V, and when the sound velocity c changes due to temperature change or the like, both r and r'change at the same rate. Therefore, by taking the ratio of these two resonance frequencies, the influence of the change in the sound velocity c is canceled. That is, if the ratio of equation (5) and equation (3) is taken, And the speed of sound c is erased. Then, V ₀ −V = (lS / π ² ) (r ′ / r) ² (7), and the extra product V ₀ −V is obtained from the frequency ratio r ′ / r. Therefore, if the volume V _{0 of the} container 1 is known, the object 7
The volume V of is known. If V = 0, the volume V _{0 of the} container is known.

In the volume meter of the present invention, as will be described later, a Helmholtz resonator, which is a main acoustic resonator, has a form in which an auxiliary acoustic resonator is further connected, but the embodiment described above is a Helmholtz resonator. A part of the acoustic tube is also used as an auxiliary acoustic resonator. The main cause of fluctuations in the resonance frequency r of the Hertzholtz resonator due to ambient conditions is that the gas density ρ inside the acoustic tube changes due to temperature and humidity. In this embodiment, since the fluctuation of r is canceled by the resonance frequency of the acoustic tube itself, as compared with the case where the auxiliary acoustic resonator is separately connected,
More precise compensation can be performed.

There are various methods for measuring the resonance frequency of these acoustic systems or the ratio thereof, but the most effective method is to construct an oscillator including this acoustic system to oscillate at the resonance frequency and Or to measure their ratio.

FIG. 3 shows an electronic circuit that causes oscillations at two resonance frequencies r and r ', and a signal processing device that measures the ratio of the oscillation frequencies and performs the calculation to obtain the volume from the measurement. The output em (t) of the microphone 6 is amplified by an amplifier 20, where it is separated into two components by a low pass filter 21 and a high pass filter 21 '. The cutoff frequencies c of these two filters are set in the middle of r and r'as shown in FIG. 2, and the component of the frequency r related to the resonance of the Helmholtz resonator, which is the main acoustic resonator, is 21. To become a signal er (t), and a signal er '(t) through a component 21' of the frequency r'relating to the resonance of the acoustic tube 3 of the auxiliary acoustic resonator. 22 is a phase detector,
Together with the voltage controlled oscillator 23, it constitutes one phase locked loop (PLL). That is, 22 is the signal er (t) and 2
DC current Ed is generated in proportion to the phase difference with the output e ₀ (t) of 3 and the oscillation frequency of 23 is adjusted so that e ₀ (t) has a constant phase difference with respect to er (t). Control. Similarly the phase detector 22 'and the voltage controlled oscillator 23' also constitute one of the PLL circuit, 23 'of the output e _0' (t) so that a constant phase difference with respect to er '(t) 23 The oscillation frequency of 'is controlled. The two outputs e ₀ (t) and e ₀ ′ (t) are added by the adder 23 and amplified by the amplifier 25 to become the input voltage e _S (t) to the sound source 5. Where 22, 23 and 22 ', 2
If 3'is adjusted appropriately, the phase characteristic ∠G () in Fig. 2
The phase is locked at the points A and A'on the curve, and the oscillator oscillates continuously at the resonance frequencies of r and r ', respectively.

Reference numeral 24 is a signal processing device, which is e ₀ ′ in the above oscillation state.
The ratio r ′ / r of the frequencies of (t) and e ₀ (t) is directly measured, and subsequent necessary calculations are performed. Although the specific means of the frequency ratio measurement is known various methods, for example, as shown in FIG. 4, e ₀ to the frequency Tei fold circuit 240 '
(T) is input and a pulse having a frequency of Nr '(N is a positive integer) is generated, and this pulse is regarded as a clock pulse.
In the counting circuit 242 through the gate 241 controlled by e ₀ (t)
When counting only during the period 1 / r of e ₀ (t), Nr ′ /
A measured value of the frequency ratio r is obtained. An arithmetic circuit 243 calculates the co-product V ₀ −V from the frequency ratio r / r measured as described above according to the equation (7), and further calculates V or V ₀ , and outputs the output. The signal Ec is sent to an indicator (not shown) to display the volume or to control the opening / closing of the outlet valve 8. Contrary to the above method, e ₀ (t) is multiplied in frequency to form a pulse having a frequency of Nr, and this is used as a clock pulse, e ₀ ′.
The frequency ratio r /
It is also possible to calculate r ′ and use it to obtain V ₀ −V.

Equation (7), which gives the relationship between frequency ratio and volume, is theoretically derived under idealized conditions.
If the volume is calculated according to this equation, the accuracy may be insufficient. In such a case, for example, (7) V plus a correction term to equation _{0 -V = k 0 + k 1} (r '/ r) + k 2 (r' / r) 2 + k 3 (r '/ r) ³ + ... Operates a polynomial of the form (8) to obtain V ₀ − from r ′ / r.
Find V. Here, k ₀ , k ₁ , k ₂ , k _3, etc. are constants experimentally determined. Alternatively, let x and kx be constants that are experimentally determined, and let V ₀ -V = k ₀ + kx (r '/ r) ^X (9) be a power function of r' / r and have a value of V ₀ -V. Sometimes I ask. Furthermore, r '/ r and V ₀
Instead of expressing the relationship with −V by a mathematical expression, the relationship between V ₀ −V obtained by calibration and r ′ / r or r / r ′ is stored in the arithmetic circuit in the form of a table, r '
Each time the measured value of / r or r / r 'is obtained, this table may be drawn to obtain V ₀ -V. The calculation for calculating the volume performed by the calculation circuit 243 includes an operation for calculating the volume by drawing such a table. What is important in any case is that the sound speed c cancels each other out by taking the frequency ratio, and the change does not affect the final result.

In the above description, the oscillator is configured to use the PLL circuit, but this is merely one means, and various known means other than this can be used to configure the oscillator including the resonance system. is there. For example, in FIG. 3, an oscillator can be constructed by replacing 22 and 22 'with phase shift circuits and 23 and 23' with amplifiers and removing the feedback connections from their outputs to 22 and 22 '. it can. Further, the oscillating waveform does not have to be a perfect sine wave. For example, in the device of FIG. 3, it is possible to generate triangular waves of frequencies r and r'as outputs of 23 and 23 ', respectively.

In the above, it is assumed that the oscillations occur at the frequencies r and r'simultaneously, so that the sound of these two frequencies is present inside the container 1. However, when the values of r and r'are close to each other, it becomes difficult to separate these two frequency components by a filter and guide them to their respective oscillation circuits. In such a case, oscillations at r and r'are alternately performed in time.
That is, the adder 26 in FIG. 3 is replaced by the changeover switch 27 to form a circuit as shown in FIG.
Signal e ₀ (t) of frequency r ′ and signal e ₀ ′ (t) of frequency r ′ are alternately switched and guided to the amplifier 25 and emitted from the sound source 5.

In this case, while the oscillation of one frequency is being performed, the oscillation of the other frequency is stopped, so that the ratio of those frequencies cannot be directly counted and measured as in the previous example. Therefore, in this case, for example, as shown in FIG. 6, a clock pulse having a constant frequency output from the crystal oscillator 244 is controlled by the gates 245 controlled by the signals e ₀ (t) and e ₀ ′ (t), respectively. And through 245 ',
By counting with the counting circuits 246 and 246 ', e
₀ the period of (t) 1 / r and e ₀ 'period 1 / r of the (t)' is measured independently. These counting operations are performed by the control circuit.
Controlled by the timing signal Et sent from 28,
Alternately in synchronism with the switching of the oscillation. However, although the description has been made here to measure the periods 1 / r and 1 / r ′, it goes without saying that this is equivalent to measuring the frequencies r and r ′ that are their reciprocals.

Since the values of 1 / r and 1 / r 'thus counted and measured are held in the counting circuits 246 and 246', respectively,
At this stage, the ratio of the two can be calculated and the co-product V ₀ -V can be obtained by using equation (7), etc., but in the apparatus of FIG. The value of / r 'is further sent to the arithmetic circuit 247, and the sound velocity c at that time is calculated as c = 2lr' (10) based on the equation (5). On the other hand, in the arithmetic circuit 248, the co-product V ₀ −V is V ₀ −V = based on the equation (3) from 1 / r held in the counting circuit 246 and the value of c calculated above. It is calculated as (c ² S / 4π ² l) (1 / r) ² (11). In this method, the frequency ratio r '/
The signal directly corresponding to r does not appear anywhere, but (10)
Substituting equation (11) into equation (11) results in equation (7). As is clear, finding the volume by the frequency ratio is no different from the previous example. That is, determining the volume by the frequency ratio, which is the essence of the present invention, does not always mean that the signal representing the frequency ratio must be present explicitly. There is no reason why the signal processing device 24 also needs to exist together in one device together with the oscillation circuit. For example, the two outputs e ₀ (t) and e ₀ ′ (t) can be output to an external general-purpose computer. You may make it input and let a computer perform measurement of those frequency ratios and calculation of a volume. 7th
The figure shows a means for measuring the resonance frequency by a manual method. 29 is a variable frequency oscillator, and its output e ₀ (t) is amplified by the amplifier 25 and becomes an input signal e _S (t) to the sound source 5. The output signal em (t) from the microphone 6 is an amplifier
After being amplified by 20, the amplitude is detected by the synchronous detection circuit 30, and the result is displayed by the meter 31. Here, if the oscillation frequency of 29 is adjusted so that the reading of the 31 needle becomes maximum, it is nothing but the resonance frequency, and the frequency at that time is counted and measured by the counting circuit 32. The resonance frequencies r and r'measured in this way are read from the display of 32, and the volume is calculated by the equation (7) using a calculator or the like. One of the features of this method is that it is possible to use a synchronous detection circuit or a tracking filter, etc., so that only the driving frequency component of the sound source is extracted from the microphone output em (t) to obtain the signal-to-noise ratio. It is possible to improve and make a precise resonance frequency measurement. Another feature is that there is no part corresponding to the arithmetic circuit, and in some cases, even without using a calculator, the volume is calculated from the values of r and r'measured and displayed on the counter circuit 32 by writing or a table. Sometimes.

FIG. 8 shows an embodiment in which dedicated microphones are used for the Helmholtz resonator and the auxiliary acoustic resonator, respectively. That is, the microphone 6 attached to the end of the airtight terminal 10 mainly detects the sound inside the container 1, while the microphone 6'attached to the acoustic tube 3 by the short conduit 11 'also serves mainly as an auxiliary acoustic resonator. The sound inside the acoustic tube 3 being detected is detected. The sound source 5 is connected to the acoustic tube 3 by means of a short conduit 11, which drives the acoustic tube 3 at its resonance frequency r'and at the same time drives the Helmholtz resonator consisting of the container 1 and the acoustic tube 3 at its resonance frequency r. . These sound sources and microphones are connected to the circuit shown in FIG. This circuit has 6'output em 'in the circuit of FIG.
The output em '(t) is mainly the frequency component of r', while the amplifier 20 'for amplifying (t) is only added, while the output em (t) of the microphone 6 is mainly the frequency component of r. Therefore, the filters 21, 21 '
May have a much lower frequency component separation capability than in the case of FIG.

One of the effects of using a plurality of microphones in this way appears when the resonance frequency r of the Helmholtz resonator is equal to or lower than the audible frequency. That is, in this case, since the microphone 6 only needs to detect the frequency component of r, a pressure sensor capable of detecting pressure fluctuation in the frequency range of 0 to several tens Hz can be used as the microphone 6. On the other hand, if the resonance frequency r'of the auxiliary acoustic resonator is set in the audible frequency range, only the component of r'is required to be detected as the microphone 6 ', so that a normal electret microphone or the like can be used. When only one microphone is used, an expensive microphone has to be used because the one microphone has to detect the sound from the very low frequency to several kHz.

FIG. 10 shows an embodiment in which the acoustic tube 3 which is a part of the Helmholtz resonator is not used as the auxiliary acoustic resonator as in the previous example, but a dedicated acoustic system is used as the auxiliary acoustic resonator. That is, the auxiliary acoustic resonator in this case has a length l'supported by the support frames 13 and 13 'inside the acoustic tube 3.
The resonance frequency of the acoustic tube 14 is r '= c / 2l' (12) and its integral multiples. The microphone 6 is connected by a short conduit 12 near the center of the acoustic tube 14. On the other hand, the sound source 5 drives the inside of the container 1 through a short conduit 9,
It also drives the acoustic tube 14. Microphone 6 and sound source 5
The circuit of FIG. 3 is connected between the two, and oscillates simultaneously at two frequencies r and r '. Here, the reason why the acoustic tube 14 that is the auxiliary acoustic resonator is arranged inside the acoustic tube 3 is that the temperature and components of the gas inside 14 are made to match those of the gas inside 3 as accurately as possible. This is because they want to make compensation. Thus, one of the effects of using the dedicated auxiliary acoustic resonator is that the resonance frequency r'can be set freely regardless of the length l of the acoustic tube 3.

In FIG. 10, the acoustic tube 14 is drawn so as to be arranged concentrically in the acoustic tube 3, but it is not necessary that neither 14 nor 3 have a circular cross section, nor is it necessary to arrange them concentrically. Absent. First
FIG. 11 shows an example in which a partition plate 33 having a cross-shaped cross section and a length of l'is inserted into the acoustic tube 3 to form an auxiliary acoustic resonator. That is, the auxiliary acoustic resonator in this case has a length l'consisting of one of the fan-shaped cross sections partitioned by the partition plate 3.
The sound inside the sound tube 34 is detected by being guided to the microphone 6 through the short conduit 12 connected to the wall of the sound tube 3 through a hole. The actions of the microphone 6 and the acoustic tube 34 having a fan-shaped cross section are the same as the actions of the microphone 6 and the acoustic tube 14 in FIG. 10, respectively.

FIG. 12 shows an example in which a dedicated sound source and a set of microphones are used for the Helmholtz resonator as the main acoustic resonator and the auxiliary acoustic resonator, respectively. The auxiliary acoustic resonator is an acoustic tube 14 provided inside the acoustic tube 3 and having a length of l ', which is closed at one end.
Its resonant frequency is r '= c / 4l' (13) and its odd multiples. 14 is a sound source 5 ′ near the closed end
And a microphone 6'are connected by short conduits 12 ', 12, respectively. This sound source and the microphone respectively drive the acoustic tube 14 and detect the sound inside 14 at that time. The amplifier 20 ', the filter 21', the phase detector 22 ', the voltage controlled oscillator 23' and the amplifier shown in FIG. It is connected to a circuit composed of 25 'to form an oscillator that oscillates at a frequency of r'. On the other hand, the Helmholtz resonator is driven by a sound source 5 attached to a short Y-tube 15, and a sound inside the container 1 is detected by a microphone 6 attached to the Y-shaped tube 15. This sound source and microphone are the amplifier 20 shown in FIG.
The oscillator is connected to a circuit composed of the filter 21, the phase detector 22, the voltage controlled oscillator 23, and the amplifier 25, and oscillates at the resonance frequency r of the Helmholtz resonator. Similar to the previous example, the signal processing device 24 measures these two oscillation frequencies and obtains the volume from the ratio thereof. In this way, the advantage of using independent sound source and microphone sets for the main acoustic resonator and the auxiliary acoustic resonator is that the characteristics of these two resonators are less likely to affect each other, and the design freedom is increased accordingly. It is to increase the frequency.

In the device of FIG. 12, since the acoustic tube 14 is a tube whose one end is closed, the internal cross-sectional area of the acoustic tube 3 is not constant,
Due to the acoustic tube 14, the internal cross-sectional area of that portion is smaller than that of the other portions. However, the acoustic tube 3 can vary in its internal cross-section with respect to the length of the tube, as long as it acts as one acoustic inertance. That is, if the acoustic inertance of an acoustic tube of length l with a non-constant cross section is L, then this tube is equivalent to a tube of the same length with a constant internal cross section Se = ρl / L (14). ,
Therefore, the resonance frequency r in this case is given by substituting the above equivalent sectional area Se instead of S in the expression (3).

In the device shown in FIG. 12, the upper end of the acoustic tube 3 is closed, and instead, a plurality of holes 16 are formed near the upper end, which are openings. The reason for adopting such a structure is to prevent dust and the like from settling in the container 1. For the same reason, there is a case where a J-shaped tube is used as the acoustic tube 3 and the opening to the outside air is directed downward. Function is no different from straight pipe. The acoustic tube of the auxiliary acoustic resonator may be a tube whose cross-sectional area changes in the lengthwise direction or a bent tube.

FIG. 14 shows an embodiment using a small Helmholtz resonator as an auxiliary acoustic resonator. A lid 17 with an acoustic tube 3 is covered on the container 1 containing the object 7, and the acoustic tube 3 and the container 1 form a Helmholtz resonator of the main acoustic resonator. The lid 17 is also provided with an acoustic tube 19 having an internal cross-sectional area S'and a length l ₁ , the end of which is connected to a container 18 of volume V '. 18 and 19 compose one Helmholtz resonator, which acts as an auxiliary acoustic resonator, whose resonance frequency r'is Is. The sound source 5 is connected to the container 18 by a short conduit 9 and drives the interior of the container 1 via the Helmholtz resonator of this auxiliary acoustic resonator. A microphone 6 is also connected to the lid 17 by a short conduit 4,
The circuit of FIG. 3 is connected between the two, and oscillates simultaneously at two frequencies r and r '. It is also possible to measure these oscillation frequencies and obtain the volume from their ratio.
It is the same as the case of the figure.

The auxiliary acoustic resonator according to the present invention is not limited to the acoustic tube or the Helmholtz resonator as in the above-described embodiments, but may be any type of acoustic resonator.
This is ensured by the fact that the resonance frequency of an acoustic resonator is generally proportional to the sound velocity c of the gas under normal conditions where the effect of the viscosity of the working gas is negligible.

[Brief description of drawings]

1 is an embodiment of the present invention, FIG. 2 is a frequency characteristic of an acoustic system, FIG. 3 is an example of an electronic circuit used in the present invention, FIG. 4 is an example of a configuration of a signal processing device, and FIG. Is an example of an electronic circuit in the case of alternately oscillating at two resonance frequencies of an acoustic system, FIG. 6 is an example of the configuration of a signal processing device of the circuit of FIG. 5, FIG. 7 is a manual resonance frequency measuring circuit, 8 is an embodiment using two microphones, FIG. 9 is an electronic circuit connected to the apparatus of FIG. 8, FIG. 10 is an embodiment using an acoustic tube as an auxiliary acoustic resonator, and FIG. Is an example of constructing an acoustic tube of an auxiliary acoustic resonator using a partition plate, and FIG. 12 is an example in which separate sound sources and microphones are used for the Helmholtz resonator of the main resonator and the auxiliary acoustic resonator, respectively.
FIG. 14 shows an electronic circuit connected to the apparatus of FIG. 12, and FIG. 14 shows an embodiment using a Helmholtz resonator as an auxiliary acoustic resonator. 1 ... container, 2 ... stopper, 3 ... acoustic tube, 4 ... conduit for microphone, 5, 5 '... sound source, 6, 6' ... microphone for sound detector, 7 ... for container Inserted object, 8 ……
Outlet valve, 9 ... Conduit to sound source, 10 ... Airtight terminal, 11,1
1 ', 12, 12' ... Conduit, 13, 13 '... Support frame, 14 ...
Acoustic tube, 15 ... Y-shaped conduit, 16 ... hole, 17 ... lid, 18 ...
… Small container, 19 …… Sound tube, 20,20 ′ …… Amplifier, 21,2
1 '... filter, 22, 22' ... phase detector, 23, 23 '
...... Voltage controlled oscillator, 24 …… Signal processing device, 25, 25 ′…
… Amplifier, 26 …… Adder, 27 …… Changeover switch, 28 ……
Control circuit, 29 ... Variable frequency oscillator, 30 ... Synchronous detection circuit, 31 ... Meter, 32 ... Counting circuit, 33 ... Partition plate, 34
...... Sound tube with fan-shaped cross section, 240 …… Frequency multiplier circuit, 241
...... Gate, 242 …… Counting circuit, 243 …… Arithmetic circuit, 244
...... Crystal oscillator, 245, 245 '... Gate, 246, 246' ...
… Counter circuits, 247, 248 …… Calculator circuits.

Claims (1)

[Claims] 1. An acoustic system constructed by acoustically coupling an auxiliary acoustic resonator to a Helmholtz resonator obtained by connecting an acoustic tube to a container, or by using the acoustic tube also as the auxiliary acoustic resonator. And a microphone of at least one sound source and at least one sound detector acoustically coupled to this acoustic system, the resonance frequency of the Helmholtz resonator connected to the sound source and the microphone, or the period thereof and And a means for measuring one of the resonance frequencies of the auxiliary acoustic resonator or its period, or for measuring the ratio of the two resonance frequencies, the volume of the container or the container depending on the ratio of the two resonance frequencies. A volume meter characterized by compensating for the effects of temperature changes, etc. by determining the volume of the placed object.