detection and analysis of low frequency acoustic wave

Hi friends
On **broken link removed**, it asserts that "It is also much more difficult to determine the exact location of a slowly changing pressure wave than a quick one."
I think that detection of acoustic wave is performed based on intensity of wave pressure received by sonic sensor. so can anyone tell me why reduction of frequency (as asserted on above website) makes challenges for detection and analysis.
I consider that long wavelength of received pulse compared to dimension of circuit makes these challenges. Is it true?
which DSP techniques must be employed for solving these problems.
by the way i apologize if my english is poor.

I consider that long wavelength of received pulse compared to dimension of circuit makes these challenges. Is it true?

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Not particularly "dimension of circuit". The article is obviously describing a (low frequency) echo-sounding apparatus, mpst likely using a single transducer generating and receiving sound pressure.

It's not specifically the dimension of the transducer and even less the circuit that makes the difference to high frequency, e.g. ultrasonic waves. Instead it's in the received waveform and the difficulty to derive pulse time-of-flight with a precision far below a wave period.

It's by the way not obvious that the measurement uses pulses, FMCW might be more promising. A DSP sound generation and receiving unit would provide all options to tune the method for best results, in contrast to a simple echo tof system.

Thanks for answer
but what is the advantage of using CW and why sending pulse in to the well is not appropriate.
Ill appreciate if you specify the useful methods for this task. in some literature wavelet transform for noise cancellation. any idea?

I don't know if the implemented instrument uses a "pulse" (in ultrasonic echo sounding, a single frequency burst is commonly used), a more complex designed wavelet, a chirp or a continuous wave with slowly varying frequency.

What's your specific application?

I want to design sonic well sounder which sends low frequency acoustic wave in to the deep well and determines the level of water in the well based on reflected wave.

AliBahar said:

more difficult to determine the exact location of a slowly changing pressure wave than a quick one."

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I think this is saying a handclap is better for sonar echo detection, rather than broadcasting, say, a sine waveform.

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low frequency acoustic wave in to the deep well

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It is conceivable you can adjust the frequency until you find the correct wavelength which generates standing waves (vertical) in the well. You'll need to place a cover over the well to do this.

The frequency is likely to be lower than 20 Hz, too low to be picked up by ordinary audio microphones.

Spacial resolution is proportional to wavelength, 50 Hz in the air is about 7 meters. Don't mean to be rude, but referenced article is BS. Who needs a measurement device with accuracy +-7 meter?

Spacial resolution is proportional to wavelength, 50 Hz in the air is about 7 meters. Don't mean to be rude, but referenced article is BS. Who needs a measurement device with accuracy +-7 meter?

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How do you derive that a hypothetical echo sounder operating at frequencies in a 50 Hz range has only 7 m resolution?

I don't see a specification of the Enoscientific well sounder operating frequency, by the way. But it's reasonable to assume that a claimed 0.05 feet resolution corresponds to a small fraction of wavelength. But that's in no way surprizing, rather a common feature of most acoustical or electromagnetic wave "radar" methods.

How do you derive that a hypothetical echo sounder operating at frequencies in a 50 Hz range has only 7 m resolution?

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Oppps, my mistake, resolution for reflected wave is limited different parameters. I was thinking in terms of XY dimension.
Anyway, I worked with RF 26 GHz level measuring in oil industry, and all clear to me as long as wavelength is smaller than pipe's diameter. Acoustic isn't quite my area, but quick search on Google just confirmed my remark to an article.

In principle, there are echos from the remote however, for narrow pipes, the visco-thermal losses at the walls reduce the magnitude of the echo by ~ 80 dB.

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https://newt.phys.unsw.edu.au/jw/acoustic-impedance-measurement.html

Waves just don't travel in small waveguides, doesn't matter if it's RF or acoustic. Small comparatively to wavelength, of course.

Anyway, I worked with RF 26 GHz level measuring in oil industry, and all clear to me as long as wavelength is smaller than pipe's diameter.

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what is the range of length that you can measure with your instrument and how do you eliminate the effect of multipath & noise?
by the way I need to make level measuring for depth of 250m.

Waves just don't travel in small waveguides, doesn't matter if it's RF or acoustic. Small comparatively to wavelength, of course.

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Have to disagree again. Electromagnetical and acoustical wave behave different. Electromagnetical waveguides have the wellknown lower cut-off frequency for the lowest order mode that can propagate. Sound propagates as pressure wave and has no cut-off frequency in tubes (acoustical waveguides).

The dispersion and attenuation mechanisms are apparently the reason why low frequency has been chosen for the well sounder system. Time ago I did some experiments with ultrasonic waves "guided" in small tubes at D < λ, they can be well used e.g. for flow measurements. If you scale the setup to well pipes, you end up at 100 to 1000 Hz approximately.

In other words, I'm quite sure that the enoscientific well sounder and similar instruments can work as claimed.

In other words, I'm quite sure that the enoscientific well sounder and similar instruments can work as claimed.

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I agree. If the high frequency is chosen the multipath, scattering and the other unwanted phenomena will be unavoidable and If the low frequency (infrasound) is chosen these challenges become negligible but the localization process becomes complex. therefore the second way is more reliable but what should I do with localization problem?? Ill appreciate if anyone represents helpful guidance.
by the way, as you know I cant use electromagnetic wave because in low frequencies the dimension of antenna is very enormous.

Electromagnetic wave might work well in metal well tubes, but it's probably more effort. Antenna size would be manageable at required GHz frequencies because low frequency doesn't work for EM in pipes anyway (waveguide cut-off frequency, see above, also https://en.wikipedia.org/wiki/Waveguide_(electromagnetism) )

but what should I do with localization problem?

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In other words you didn't yet study existing methods for high resolution echo sounders.

See below a wave detection scheme based on a programmable threshold detector.



Digital signal processing gives access to general correlation methods.

To start with practical experiments, I would chose a suitable wideband transducer (e.g. speaker + optionally a separate electret microphone), define some stimulus signals and use a notebook sound system for playback and echo recording. Then analyze offline.

Another things to consider. Last summer I did an ultrasonic 3D radar (sonar) with arduino board and cheap distance meter HC-SR04. Transducer emits 8 periods of wave, and I'm pretty sure, that all other models sold on a market emit close to 8 period. Not ready to support by theory, but engineer (thousands of them) who work in the field of acoustic didn't chose a random number. Because it can't be less. So there is we have a minimum detectable distance, and with 40 kHz its equals to 340 m/s x 8 x 25 us / 2= 6.8 / 2 = 3.4 cm. In echo location transmission has to stop, in order to echo to be discriminated, except chirp, but its not the case. And 340 x 8 x 20 ms (50Hz) /2 = 27.2 m. You can't measure a distance less than 27 m. On the other side, for distances above that, -80 dB attenuation.
P.S. I wander, why nobody notice lack of technical details in the article?

The parameters of air ultrasonic systems are primarly enforced by the small band transducers. 8 periods is a kind of compromise. I believe that the cheap processor based distance sensor designed are more or less copied from HC-SR04, in so far it's not surprizing that you see similar parameters. Technically the burst would be a least long enough to achieve full settling of the transducer voltage. Longer bursts don't offer an advantage if you don't have advanced signal processing options like correlation (cheap microprossors obviously don't have).

Minimal detectable distance is more determined by crosstalk than burst length, at least for a dual transducer system like HC-SR04.

P.S. I wander, why nobody notice lack of technical details in the article?

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If you mean the enoscientific publication, that's more a product advertizing than a technical article. An yes, I would also like to know more parameters. Maximum distance and resolution/accuracy is however in the specification, but not the implementation details. It's not intended as a contsruction manual!

Could some one enlighten me what is :

Generating a clean low frequency pulse is more difficult.

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I used to think in term of frequency and time, never heard about low-frequency pulse.
They claimed:

MEASUREMENT:

Resolution - .05 ft
Accuracy - .1 ft
Range – 9 to 2000 feet.

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Minimum distance 9 feet? it's close to a period, so transducer emit not 8 but 1? Could be.

Technically the burst would be a least long enough to achieve full settling of the transducer voltage.

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Agree, so the question is what kind of transducer they used, to get voltage setting time in 1 period?

Doubts about 2000, add up to losses over big distance, temperature isn't constant :
https://en.wikipedia.org/wiki/Geothermal_gradient

As previously mentioned, I don't agree with your assumption that you can't detect distances with time-of-flight below the transmitted burst length. It's a matter of receiver dynamic range and applied signal processing methods.

what is the range of length that you can measure with your instrument and how do you eliminate the effect of multipath & noise?
by the way I need to make level measuring for depth of 250m.

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It was RF, you don't care much about multipath, as you calculate distance by whatever first echo arrived. Multipath would just create a few echo after direct beam, and should be easy to clean up. Sorry, don't remember specification for max length.
More research on a subject "acoustic wave losses" brings me 30 dB at 100 m for 1000 Hz. Looks like I was mislead by another site, that I quote earlier. What is more important, losses proportional to sqrt(F), and this is why they used low frequency range. If my new source is correct, than attenuation over 100m at 50 Hz should be 30 dB / sqrt(1000 / 50) = 6.7 dB. Now it looks quite doable.
Take a sound card 24-bits 192 kHz ( the higher rate the better), than run FFT to determine a phase of the echo, and voila, you get accuracy ~1 cm or so
( I was able to get 12 micrometers in my ultrasonic 40 kHz project, though over short ~10 meters distance).
The only problems:
1). temperature gradient, may be fixed over calibration for specific pipe - location;
2) TX transducer, probably, 10W speaker would be just fine.

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FvM said:

As previously mentioned, I don't agree with your assumption that you can't detect distances with time-of-flight below the transmitted burst length. It's a matter of receiver dynamic range and applied signal processing methods.

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No way. As you probably knows, there are 3 basic technology, frequency division (FDMA), time division (TDMA) and code (CDMA). You can not discriminate echo from direct beam, if they are the same in frequency and overlap in time.
They still use duplexers do divide transmitting and receiving path in mobile communications (Frequency Division), or switches to split receiver-transmitter time-slots (Time Division).
Doesn't matter how good DSP technology is developed, and how many billions was invested into market.

No way. As you probably knows, there are 3 basic technology, frequency division (FDMA), time division (TDMA) and code (CDMA). You can not discriminate echo from direct beam, if they are the same in frequency and overlap in time.
They still use duplexers do divide transmitting and receiving path in mobile communications (Frequency Division), or switches to split receiver-transmitter time-slots (Time Division).
Doesn't matter how good DSP technology is developed, and how many billions was invested into market.

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You don't "discriminate echo from direct beam" without prior knowledge. You know what the crosstalking direct beam signal is (from a previous calibration), so you can determine the difference. That's particularly easy in the near range where the echo has the same range of magnitude as the Tx signal.

And there's of course no need to limit the operation mode to single frequency or fixed pulse width. Adaptive signal parameters can be used if appropriate.

You know what the crosstalking direct beam signal is, so you can determine the difference. That's particularly easy in the near range where the echo has the same range of magnitude as the Tx signal.

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It's a dream. Vast majority of the application I know, Rx is in order of magnitude -60 dB or even -190 dB lower than Tx. Crosstalk is a variable, depends on temperature, pressure, operators hands etc. etc. Variation in frequency is also may not be possible. In given example, echo over 4000 ft is expected to be -80 or less, and changing F creates more troubles:
https://newt.phys.unsw.edu.au/jw/graphics/cylinder_cone.gif