# Filtering below 20hz

Hi there. I’m doing some stuff with sub frequencies where its possible to generate frequencies below 20hz. I think 12hz is about the lowest I’ve seen. Do I need to filter these out? I have a DC blocker in the chain. Is that what this does? If not, how does the alpha on the high pass filter work in terms of calculating a frequency cutoff?

Cheers

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The DC blocker is a high pass filter with a cutoff around 20 Hz so it should be enough to block any unwanted subsonics. Here is a low pass and high pass filter node with the necessary formula to calculate the alpha value from Hertz. These are 6 dB per octave filters so the cutoff is fairly gradual.
low-pass and high-pass filter.audulus (3.6 KB)

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Perfect, thankyou!

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When you ask if you should filter those frequencies out, unless hardware designs have changed, frequencies that low require a lot of power and can cause power amps and speakers to generate dangerous amounts of heat. Maybe modern gear has built in brickwall filters somewhere below 20 Hz. But if not you wouldn’t even need to hear it for it to have a very negative affect on the amp.

I imagine an amp from 20 years ago would catch on fire at some point with a sustained signal of 12 Hz being fed through an amp that’s already working hard.

Which reminds me! I was playing a gig one summer evening in Texas and the singer or guitarist looked at me during the middle of a song and nodded to signal me to turn around. My amp! Smoke was pouring out of the top. It then caught on fire with flames coming out the top. And the amp was still working! Not for long though. A few seconds later it went silent, forever. Luckily, I guess, it was the last set of the night. The amp was in such bad shape I had to buy a new one. I was so surprised how long that amp kept working while in full meltdown. I was very impressed. Wasn’t even mad that I lost my amp.

Anyway, that’s what overheating can do to an amp. 12 Hz could certainly destroy anything that could reproduce it.

If things have changed, like maybe Class D amps have some sort of protection that would prevent it from a 12 Hz death, please let me know.

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Welcome to the forum! You are absolutely correct. Subsonic signals can not only cause amps to overload but can cause speaker failures as well. Modern solid state amps often have over current protection which would prevent a total meltdown but you could certainly still cause some damage.

Scary stuff!