Communications – electrical: acoustic wave systems and devices – Echo systems – Speed determination
Reexamination Certificate
2003-04-16
2004-05-18
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Echo systems
Speed determination
C367S902000
Reexamination Certificate
active
06738312
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the measurement of the speed of sound. More specifically, the present invention relates to a method, and an arrangement for measuring the speed of sound based on the travel time of an acoustic signal through a predetermined distance.
FIG. 1
shows a prior-art arrangement
100
and method for measuring the speed of sound, as described in U.S. Pat. No. 4,926,395.
A control module
101
generates an output electrical pulse signal
102
. The instance when the event starts, i.e. when the output electrical pulse signal
102
is generated, is denoted as t
1
. The output electrical pulse signal
102
is sent to a transmitting transducer
103
.
The transmitting transducer
103
converts the output electrical pulse signal
102
into an acoustic pulse signal
104
.
Next, the acoustic pulse signal
104
output by the transmitting transducer
103
travels through a medium
105
, such as air or water, hits a reflector
106
and is reflected.
The reflected acoustic pulse signal
107
then travels back towards a receiving transducer
108
which receives the reflected acoustic pulse signal
107
and converts it into an electrical pulse signal which will be referred to in the following as reconstructed reflected electrical pulse signal
109
.
In this known arrangement the front ends of the transmitting and receiving transducers lie in a plane parallel to the plane of the reflector
106
, since the receiving transducer and the transmitting transducer are embodied by a single transducer.
Next, the reconstructed reflected electrical pulse signal
109
is sent to the control module
101
.
Based on the instance when the reconstructed reflected electrical pulse signal
109
is received by the control module
101
, the traveling time of the acoustic pulse signal, i.e. the time which was necessary for the output and reflected acoustic pulse signals to travel through the medium
105
is determined. The determination of the traveling time will be described below.
The speed of sound
110
, s, determined and output by the control module
101
is given by
s
=
2
·
d
t
0
,
(
1
)
where
d is the distance between the transmitting transducer
103
(or the receiving transducer
108
) and the reflector
106
, and
t
0
is the traveling time which is the sum of the time needed for the output acoustic pulse signal
104
to travel through the distance between the transmitting transducer
103
and the reflector
106
and the time needed for the reflected acoustic pulse signal
107
to travel through the distance between the reflector
106
and the receiving transducer
108
.
In the following, the determination of the traveling time t
0
according to the state of the art will be described.
The output electrical pulse signal
102
generated by the control module
101
is typically a gated sinusoidal signal that is of a finite time duration, as shown in
FIG. 2
a.
Due to the inherent characteristics of the transmitting transducer
103
and the receiving transducer
108
, the transducers
103
and
104
will, in general, deteriorate the characteristics of the signals in the course of the respective signal conversion processes. This means for example that the waveform of the acoustic pulse signal
104
does not exactly correspond to the waveform of the output electrical pulse signal
102
from which it is generated by the transmitting transducer
103
. Similarly, the waveform of the reconstructed reflected electrical pulse signal
109
does not exactly correspond to the waveform of the reflected acoustic pulse signal
107
from which the reconstructed reflected electrical pulse signal
109
is generated by the receiving transducer
108
.
FIG. 2
b
shows an example of a reconstructed reflected electrical pulse signal
109
which is received by the control module
101
and originates from an input gated sinusoidal signal as the output electrical pulse signal
102
delivered to the transmitting transducer
103
.
When the control module
101
receives a signal, it will decide whether the received signal is the reconstructed reflected electrical pulse signal
109
or just background noise.
For ease of discussion, the received signal is considered to be a digital signal.
When the control module
101
receives one digital sample of the received signal, it compares the amplitude of the current sample of the digital signal with two thresholds
301
,
302
, namely an upper threshold
301
and a lower threshold
302
(see FIG.
3
).
In this context, amplitude is meant to be the voltage level of the digital signal at the current time instance.
If the absolute value of the amplitude of the current sample of the digital signal is larger than the absolute value of the upper or lower thresholds, the decision is that the reconstructed reflected electrical pulse signal
109
is detected, and this current time instance is then denoted by t
2
(see FIG.
3
).
Otherwise, the decision of the control module
101
is that the reconstructed reflected electrical pulse signal
109
is not detected, and the control module
101
continues to take the next digital sample and performs the comparison as described above until the reconstructed reflected electrical pulse signal
109
is detected.
The traveling time t
0
is then estimated to be an estimated traveling time t
0
, which is given by
{circumflex over (t)}
0
=t
2
−t
1
, (2)
where
t
1
is the time instance when the output electrical pulse signal
102
starts to be generated by the control module
101
, and
t
2
is the time instance when the reconstructed reflected electrical pulse signal
109
is detected by the control module
101
.
This completes the description of how the traveling time t
0
is estimated by the calculated time {circumflex over (t)}
0
.
However, the prior-art method described above particularly has two problems that cause the estimation of t
0
by {circumflex over (t)}
0
to be inaccurate.
Firstly, the conversion of the output electrical pulse signal
102
into its acoustic form (the acoustic pulse signal
104
) by the transmitting transducer
103
is not instantaneous, mainly due to physical limitations of transducers. Indeed, there is a considerable time delay between the instance when the output electrical pulse signal
102
starts to be generated by the control module
101
(i.e., t
1
) and the instance when the acoustic pulse signal
104
starts to travel in the medium
105
. If the delay is of a fixed value for different runs/scenarios of the speed-of-sound measurement, one could circumvent this problem by replacing t
1
by t
1
plus this fixed value. However, the delay is not constant for different runs/scenarios of the speed-of-sound measurement.
Secondly, the detection of the reconstructed reflected electrical pulse signal
109
is also not instantaneous. Indeed, the control module
101
can detect the arrival of the reconstructed reflected electrical pulse signal
109
only after a considerable time delay of about a few cycles of the reconstructed reflected electrical pulse signal
109
, as shown in FIG.
3
. If the delay is of a fixed value, for example one cycle for different runs/scenarios of the speed-of-sound measurement, one could circumvent this problem by replacing t
2
by t
2
minus this fixed value. However, also this delay is not constant for different runs/scenarios of the speed-of-sound measurement.
These problems become even worse when using low cost components, like a personal computer with a sound card as the control module
101
, a commercial, rather low-cost loudspeaker as transmitting transducer
103
, a plastic compact disc case as the reflector
106
, and a commercial, rather low-cost microphone as the receiving transducer
108
.
SUMMARY OF THE INVENTION
Thus, it is a first object of the present invention to provide an arrangement for measuring the speed of sound with improved accuracy particularly even when using low cost components, especially for educational purposes like for experimental speed-of-sound measurements in high schools.
To achieve the fir
Goh Zenton
Ho Kwok Chiang
Tan Kah Chye
Addest Technovation Pte Ltd.
Andras Joseph C.
Myers Dawes Andras & Sherman LLP
Pihulic Daniel T.
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