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Topic: srf08 making audible "chrimp" while ranging (added video of the problem) (Read 923 times) previous topic - next topic

ivartaim

Hi,

I am using an ultrasonic rangefinder srf08 with an arduino and almost everything is working well. Signal is stable and accurate. But for my suprise every time the sensor emitts the 40khz ping it also has an audible, much lower frequecy sound to it.It is quite loud and annoying if pointed to a person. For my purposes any sound under 20khz is unacceptable. IS this a normal thing for ultrasonic rangefinders or to this particular model? Or i have a faulty model?. I can make a sound recording if needed for further analysis.

Any help is much appreciated. I have searhed the forum , and the best explanation I found is that the extra sound comes from the air moving next to the transducer. Is this true? are there quieter models out there if this would be the case?

MarkT

The problem is more fundamental - the 'ping' sent has to be short in order to have good spatial definition (so distance
can be measured accurately), which means the bandwidth is very wide and spills into the audio spectrum.  A typical
sender sends just 4 cycles of 40kHz so the power bandwidth is at least 10kHz wide and spills further than that.  Since
the ping is very high power (to get adequate range), even a tiny fraction of signal can be well above the threshold of hearing.

However it may be that the circuit being used has a DC bias in the ping pulse which would then output a click across the
entire spectrum - I suspect some devices are better behaved than others.
[ I won't respond to messages, use the forum please ]

ivartaim

If DC bias would be the problem, what would one consider while designing a ultrasonic sensor from scratch? better filtering?

MarkT

It would need a digital or analog filter with a brick-wall high-pass characteristic, pass the ping waveform through it,
then amplify the signal linearly (perhaps with step-up transformer).   A lot more complicated than a digital waveform
into a LC resonator.  However a pre-calculated sampled waveform could be used eliminating the filtering.

There might still be issues with non-linearities in the transducer - rectification or intermodulation can generate
low frequencies.
[ I won't respond to messages, use the forum please ]

ivartaim

Thank you very much for your response and explanation. I unfortunately can't even start to comprehend all of the concepts you are talking in. But it opened my world a bit.

My level of expertise right now is about as high as choosing the right part if i know what to look for , and go/learn from there.

Maybe someone has experience with other ultrasonic sensors and can elaborate, if some of them are more silent than others? Whatever the range or accuracy or interface. Or some literature which could be useful on the subject.

Thank you!

michinyon


ivartaim

#6
Feb 18, 2013, 04:08 pm Last Edit: Feb 18, 2013, 04:27 pm by ivartaim Reason: 1
Hi,

Thank you for your smart comment.

No i am not a dog. If you read my original post carefully you will see that my problem is human audible chrimps while sending the ping signal. Which frequency is definately in the human audible range.

Here is a video of my problem:
http://www.youtube.com/watch?v=L57rjEJQDHw

Sound level of the video is quite low. 

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