Dear friends,
I am a research student in the uk and i am trying to generate ultralow frequencies. Basically its a Non destructive testing but using ultralow frequency instead of ultrasonic. I would like to know if it would be possible to generate 0-1000Hz frequencies. Can a mixer circuit achieve this kind of low frequencies.

Basically i looked into piezoelectric transducers but most of the off-the-shelf transducers in the market are of higher frequencies (1-50kHz...0.5-30MHz). The idea is to send a ultralow frequency pulse wave to one end of a steel rod (25mm-150mm dia. with lengths ranging from 2m - 100m) the wave would travel to the other end and reflects back. By measuring the time delay between the sent pulse and the reflected pulse (assuming we know the wave velocity travelling in the rod) we can then calculate the total length of the rod. So essentially i am trying to work in a pulse-echo mode using this low frequencies.

On another note, this is just out of curiosity, would it be possible to generate even lower frequencies 1X10e-6Hz (0.000001Hz) with an intensity of 120dB-200dB. Would a mixer circuit work for this kind of application



Thanking you in advance

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Well for either application (but particularly this second one), you could use a micro-controller - like a PIC - feeding a digital to analogue converter. You store a data table of a sinewave in the PIC (I'm presuming you want a sinewave?) and simply output it one point at a time through the analogue converter. To alter the frequency you simply alter the time intervals between each data item being plotted.

The maximum frequency will depend on the speed of the PIC and the number of plotted points in the table (a maximum of 255 makes things easier) - your 1KHz maximum should be easily achievable. The minimum frequency could be as low as you like - 1 plot per hour, 1 plot per week, 1 plot per year - anything you want.

As for your intensity requirements - for a start you don't really mention what it is! - dB's are a ratio, and you don't mention what it's with respect to. BUT!, presumably you are looking for high powers, this is going to be large and expensive - you will need a fully DC coupled amplifier (the low frequencies are near enough DC) - and it will need high power handling and big heatsinks, as it's likely to dissipate a lot of heat.

Presumably you have access to some kind of transducer able to handle these high powers?, as with the amplifier it will be generating a lot of heat.

 

Do you know the speed of propagation in the medium? In one sentance you use the term "pulse" in another you use the term "low frequency". I assume you mean low frequency pulse, or do you know? A pair of 555 timers will produce a low frequency pulse train. Zero frequency is not possible, but you can get below 1 Hz.

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Zero frequency is easy :lol: it's just DC!.

 

OK, Nigel, you got me!
One way to generate a very low frequency is using a motor and cam a'la the Klaxon horn.

The RADAR principle is so simple, I don't know what the low frequency is about.

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Hi Nigel and Russlk,

First of all thanks for the input. Basically this rods are embedded in concrete/ground. The reason i am using low frequency is that high frequency (ultrasonic) used in conventional NDT has high 'resolution' but the energy content is low and hence the signal dissipates before any decent reflection is detected, that is why i am looking for low frequency waves. A piezoelectric transducer converts a electric signal to a mechanical wave, which is what i need. I need an elastic wave of low frequency which ideally should have a higher energy content and when sent down at one end of the rod it would travel all the way down the other end and be reflected. Since it has a higher energy content the reflected signal would be detected with ease. The problem with conventional ultrasonic is that it can only travel for short distance (2-3m rods) but since it has a higher frequency(higher resolution) it would be able to detect small defects along the rod(cracks,corrosion) but in my application i am only interested in determining the total length of the rod.

Russlk wrote:
One way to generate a very low frequency is using a motor and cam a'la the Klaxon horn. The RADAR principle is so simple, I don't know what the low frequency is about

What is the radar principle, do you think it would be applicable to my case?
Where can i find more info on this "motor and cam a'la the Klaxon horn"
Yes, what i am trying to achive is a low frequency(high energy) pulse.

Nigel,
Yes what i meant by 120-200dB is (1-10e8 W/m2) but that is just out of curiosity...i don't think i would need that kind of very low frequencies at the moment. I assume that a frequency between 1-400Hz should have enough energy content for my application.

If you have any ideas besides what i am looking for please just throw it in...i would appreciate any kind of input

Thank you for your efforts guys...really appreciate it.

regards

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The science of sonic

 

Take a big low frequency loudspeaker en feed it with via a power transformer out of the power net (outlet) and you have a sinewave of 50/60 Hz.

 

Hi guys,
What i need is basically the same concept as a radar like this one
http://hamjudo.com/sonar/

but instead of pulsing 40kHz i am trying to pulse 1Hz to 1000Hz. The signal should be short and sharp. The emitter/receiver would be coupled to one end of the rod. The pulse having low frequencies should have a higher energy content and hence would be able to travel longer distances with minimum energy dissipation so that a decent reflected signal can be identified from the far end of the rod.

I have contacted a Piezoelectric manufacturer but they have not got back to me yet. If they are able to manufacture a piezoelectric transducer that produces low frequencies 0-1000Hz, then in order to focus/generate P-waves(longitudinal waves) more efficiently a cone would utilized to bond a waveguide onto the surface of the piezoelctric transducer disc and to behave as a mechanical transformer, converting lateral displacements at its base into longitudinal displacements at its apex.Theoretically by coupling this to one end of the rod we would be able to send a low frequency short,sharp pulse which would have higher energy contents thus would travel to the other end of the rod and be reflected back up and consequently detected by the receiver.

Am i right in my assumptions or am i missing something here....

cheers for the feedback

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Can you just strike the end of the rod with a solenoid or some other sort of hammer?

 

Well Ravide, I think Ron H has hit the nail or rather rod on the head here.
A pair of 555 timers and a good old fashioned large discharge from a capacitor through a solenoid should be more than sufficient to strike the rod with enough force. Though using a pic micro does make everthing a lot simpler to prototype, sandwich the flat disc piezo trandsducer between the solenoid and the end of the rod under test and hammer away.
For a simpler solution just mount the transducer on the face of 5lb lump hammer (best use a soft faced "dead blow" type) and put the measuring circuit in the handle....

 

Dear Ron H and tansis,
Please, you have to pardon my ignorance (i am electronically inapt). Could you provide a sketch of the schematic (the experimental arrangement) not the detail design. I will include a schematic of my current experiment as well as my plans. Basically If the piezoelectric transducer is able to produce freq. in the range of 0-1000Hz, then there would be no need for a hammer. The piezoelectric transducer would be able to convert the electrical signal to a mechanical elastic wave. The transducer will be coupled via a 'waveguide' and a cone.

tansis wrote:
For a simpler solution just mount the transducer on the face of 5lb lump hammer (best use a soft faced "dead blow" type) and put the measuring circuit in the handle....

the transducer you mention here i pressume is a Force transducer....so if i understand it right then what we have is the commercially available impact hammer test...or am i missing your point altogether

I am essentially trying to find an alternate method, instead of mechanical impact (and the common ultrasonic method, using the piezoelectric transducer to create a mechanical wave) is there any other way of sending a pulse down the rod and measure the reflection. This is similar to the TDR (time-domain reflectometry) method, but instead of cables i am diagnosing the rods for defects.

best regards,

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You seem to be hoping for total opposites here - you mention 1Hz and then want 'short and sharp'. Sound waves travel in air at (very roughly) 1000 feet per second, in solids it's considerably faster - so by the time you've transmitted a single wave it's already gone at least a thousand feet.

I think you need to look further into the situation!.

 

I see several problems here:
1. You don't have the piezoelectric transducer, I don't have one either.
2. You don't have a receiver sensitive enough to detect the reflected wave.
If I am wrong, what is your problem?

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Hi Ravide,

Low frequencies will not do this.
A low repetition rate of med freq to high freq pulses could.

To use an echo from the distant end in this way you would
need to apply a physical burst of longitudinal frequency of
a fairly high frequency, not too high as it has to 'bounce'
off the other end and if the frequency is too high then a
poorly defined end will give too much scatter.

The speed of travel through the steelwork is not a function
of the frequency applied.

20 k/c travels well through steelwork, and would be a low
enough frequency to ignore small imperfections and to echo
well from a poorly defined end. I would suggest using a
frequency of about 70 k/c because this is within the spec
of most audio equipment and amps to drive the transducer
(and for the receiver)
should therefore be easy to obtain.

I would suggest using two transducers one to send and one
to receive, switching arrangements for using only one are
troublesome. Arranging for the receiving end to be quenched
during the send signal is not difficult.

Circuitry listening for a frequency signal can be made very
much more sensitive than listening for a simple click or
pulse.

I don't think you will get steel rods embedded in concrete to
respond at their natural resonant frequency, which in essence
is what you appear to be describing. But bursts of high
frequency signals will travel through rods like that, and
yes they will echo back from the end, but if the end is
welded or bolted to steelwork it may give spurious effects.

Best of luck with it, John

 

Ok John,

My assumption that an operating frequency of a transducer of 1-1000Hz, if pulsed (assuming just one pulse is sent) would not do the trick. The reason i assumed this is that it was stated that common ultrasonic method with frequencies higher than 20kHz were of low energy and hence the energy reflected was not sufficient to be detected.

So if i understand you right, i need a transducer with an operating frequency of 20kHz-70kHz pulsed at a low repetition in order to get a decent reflection from the far end of the rod.

The 'natural frequencies' that i mentioned are not the natural frequncies of the rod it self but rather the rod embedded in concrete/ground. We have developed a mathematical model that can accurately predict the natural frequencies of rods (diff length and diff dia.) embedded in concrete,ground or any other medium. The initial objective was to correlate the natural frequencies to the total length of the rod but the natural frequencies are predominantly dictated by the length which is 'free' in other words not in contact with the surrounding medium(ground,cocncrete..etc) hence it was difficult to accurately determine the total length based on its frequency response.

Once again, thank you for your time in entertaining my quiries

best regards,

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The science of sonic