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Didn't use correct high/low pass filter setting day wasted?

  1. Sep 21, 2016 #1
    Hi, I'm using a hydrophone to detect frequency signals (or more precisely pressure) in water. It involves using a hydrophone to detect the peak to peak voltage of the sinusoidal signal. BUT I forgot to change the high pass - low pass settings!

    It was left to detect between 250kHz (high pass) and 1MHz (low pass), and I have been analysing frequencies at below 250 kHz.

    I have been using the detected peak to peak voltage value of the sin wave, not measuring it's frequency. I got readings that made sense, but now I'm worried that the filter being applied incorrectly will influence results.

    Any ideas? will a high - low pass filter effect the detected peak to peak values?

    Thanks for any advise!
  2. jcsd
  3. Sep 21, 2016 #2


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    Staff: Mentor

    Yes, it will affect the Vpp that you measure. What order is the 250kHz HPF? Even though you were not measuring the sound frequency, do you know what it was? Is it pretty monotonic and stable, or can it vary?

    If you know the order of your HPF and the break frequency, and the frequency of a monotonic signal source below 250kHz, you can factor in the attenuation of the HPF to convert your partially-attenuated. Vpp measurement to the actual Vpp number.
  4. Sep 21, 2016 #3
    Hi, many thanks for the reply! Yes I did a quick test with the filter applied and got completely different results. Day wasted, I'll have to do them all again tomorrow, isn't research awesome!!

    When the signal was outside of the filter, it was mostly noise, but I was able to pick up a sin wave in some instances. But its a cavitational environment (bubbles flying throughout the liquid) so attenuation of the sound wave does occur, which would explain the occational pick up of a readable signal.

    Thanks for the suggestion, I'll give it some thought, but looks like I'll just do the set again tomorrow.
  5. Sep 21, 2016 #4


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    Staff: Mentor

    Agreed. If it's not a known monotonic signal and is composed of a lot of frequencies, the filter effect can't be compensated for and it needs to be set right. Good luck! :smile:
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