Local Radio overlap with Accelerometer Measurements

[SuyashP]

Hello there,
I work as a graduate engineer at an acoustic engineering firm here in Sydney. This is week 10 into the job and I'm loving it!
Last week I noticed something very strange while analyzing data from one of our ground-borne noise measurements at a residential property.
We had 3 PCB accelerometers (one per axis) setup in the property's backyard and 3 inside the property. While going through the recordings at the office, I was VERY surprised to hear that the accelerometer had recorded a local (maybe?) radio broadcast! So while I could hear the roadheader I was supposed to be hearing, I was also clearly able to hear the music from a local channel playing South Indian music. Now neither me, nor my colleague, heard anything while on site. Moreover, the accelerometers inside picked up nothing, and neither did the SLM we had setup.
While everyone at the office had a good laugh at this, we couldn't explain why this had happened. Loud music playing far away? Recording was way too clear for that. Signal crossover from an unauthorized radio broadcast? Maybe but how?
Have any one of you ever come across this?

 

[Strong]

I would expect signal pickup by your A/D system or SLM microphone (kid on bike or car drive-by) and not the accelerometers. I have picked up radio when conducting a ship cargo hatch integrity leak test using ultrasound meter.

Walt

 

[SuyashP]

Thanks for your reply Walt.
I didn't pick up anything on my SLM or internal accelerometers. Also, the radio signal on the external accelerometer wasn't from a pass by, it was a long event that encompassed almost the length of the recording.

 

[GregLocock]

How long was the cable on the external acc compared with the others?What type of acc?

Cheers

Greg Locock


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[SuyashP]

Hi Greg,

The cable for the external acc was 30m, compared to 5m for the internal acc.
The acc was PCB Model:352C04 single axial accelerometer.

 

[scheah]

This is curious! My first guess was your colleagues are playing a prank on you.

What signal conditioner and accelerometer cables are you using?


Best regards,
Jason

 

[hydtools]

At one time doing some strain gage work the system picked up a radio broadcast signal. I recognized the audio on the oscilloscope trace. An rf filter removed the signal from my data recording.

Ted

 

[GregLocock]

500g acc probably needs a heap of gain at the AD. Anyway congratulations on listening to your signal, it is the easiest and best quality control you can do.

Cheers

Greg Locock


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[Strong]

PCB Model:352C04 accelerometer has 10 mv/g sensitivity. It would be a poor choice if you are measuring ground vibrations. Typically accelerometers with 1.0 or 0.5 V/g sensitivity or seismic velocity sensor (Geophone) would be used for ground vibrations. Your outdoor accelerometer may simply be an antenna for your A/D system, especially if the gain is high to get a signal from ground vibrations!

Walt

 

[SuyashP]

I'm going to ask a couple of really beginner questions that show my inexperience:

How is the gain used in context of the accelerometer?
What do you guys mean by an A/D system?

 

[GregLocock]

A to D is the analogue to digitzl converter, which turns your acc signal into a stream of bits. The gain is a preamp stage that amplifies the signal up to a suitable level. You may use autorange, or explicitly set the maximum voltage to be measured. You should also read up on anti aliasing and sample rates. This forum hss an FAQ on reference documentation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[SomptingGuy]

Add my congratulations to you for listening to your signal. Unwanted noises leap out when you listen. Get into a habit of photographing the test too.

Steve