Hi Pass Audio Filter

[AltAudio]

Hello,
I built a low pass filter from specs in https://www.electro-tech-online.com/threads/low-pass-audio-filter.13192/ and it worked wonderfully. Now I'd like to build a hi-pass audio filter to pass anything above 18KHz. I realize that humans(most) cannot hear this level, but I am putting 20KHz hashmarks in a recording that I specifically do not want heard. I then need to filter these markings.

Also, could you recommend some good books/readings that would enable me to start calculating first/second order differentials such that I can build band pass filters and understand the 'hows' and 'whys', rather than grabbing a bunch of inductors/caps and seeing what works and what doesn't. There is a .jpg image in above referenced hyperlink that shows components and cutoff frequency. Are those images from MatLab?

Many Thanks...Mick

 

[MikeMl]

Here is how I design active filters:

 

[AltAudio]

Wow! and it's free? Pretty cool indeed, I will certainly download. Thank you MikeMl.

 

[crutschow]

Filter design is a somewhat arcane subject and the math can be tedious. Thus the program that Mike suggested is great for active filter design (typically use for audio frequency filters) since it does all the number crunching for you.

If you want to better understand their operation you can read some tutorials such as this.

 

[AltAudio]

Thank you Carl.

 

[canadaelk]

The text suggested by Carl is excellent. May I also suggest, for more detail, Don Lancasters book "Active-Filter Cookbook" E

 

[AltAudio]

Thanks canadaelk.

 

[#12]

Keep your receipt. My copy started coughing up pages within 3 days of buying it. Cheap binding.

 

[AltAudio]

Perhaps there's a kindle version.

 

[audioguru]

If you want to pass 18kHz but attenuate everything higher and if you use a 4th-order filter then 36kHz will be attenuated (not removed) only -24dB. 20kHz will be loud and clear.
Maybe if you used a very high-Q notch filter then 20khz will be removed if the signal and notch filter are extremely accurate but 18kHz will still be reduced.

FM radio stations broadcast a 19kHz stereo pilot tone at only 10% modulation. FM radios have a 15kHz lowpass filter to reduce it but it is still there.

EDIT: OOPs, I got it backwards. If you want to pass everything above 18kHz then 9kHz and 10kHz will be reduced but will still be there.