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There is a scenario: There is a certain box which emits acoustic signals at 30kHz and you are trying to detect these signals using some sort of detector.

I was wondering, would it be more efficient to make a band pass filter for this or some sort of high pass filter to recognise the existence of these signals?

In regards to a high pass filter, this is easily done for example you can use a second order high pass filter since it attenuates more sharply than first order e.g:

enter image description here This could be used to find values we need.

The magnitude response could look something like:

enter image description here

However, how exactly would you be able to make a band pass filter which incorporates both a high pass and low pass element such that only a certain range of frequencies could be used?

Many thanks for any help.

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Building a bandpass filter by combining a high pass section and a low pass section is feasible if the desired bandpass is relatively high compared to the center frequency. This simplifies the design since it means that the high pass and low pass sections will not interfere with each other since they will be relatively far apart in frequency. In terms of Q (which is defined as the center frequency divided by the bandwidth), this is a low Q filter. You didn't provide any information about the bandwidth of the signal you are trying to detect. This is important in determining the bandwidth of your filter. You also didn't give the expected level of your signal. This is important in determining what sensitivity your receiver needs. The lower the expected level, the more significant is background noise. The purpose of the filter is to lower the background noise level so the required bandwidth is function of these parameters. Assuming the signal level is not buried in the background noise, you can probably get away with using separate high and low pass filters. If the signal level is low then you may need to build a much narrower bandwidth filter (high Q) which will require a more sophisticated design.

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