Here is my idea, noise cancelling, after I learned about op amp. it's simply invert the input and produce the sound in a speaker.

however, I believe I need to add some more stuff in order to make it to work in real world, so what I can start from the circuit below.

 

This might work for a tone for a ms or so. Sound in speaker cancels sound in. Now no sound in, so no cancelation so sound in. This is a much more complicated job than you envision!

 

That depends on whether the microphone is listening to the speaker. If the speaker is in a headphone where the microphone can't hear it, it has a chance of cancelling external sounds.

 

well, my goal is to reduce/cancel the music from upstairs while I am sleep at night, so as long as it can make my room quite, that's all I need.

if it's likely to work, I like to built it and try it out, I want to sleep in a quite room after 10nish pm

 

Try the broom trick. Wait untill 1/2 hour after the music stops, and then pound on the ceiling with a broom handle. They will soon get the word, but you could ask them to turn the volume down.

 

well, my goal is to reduce/cancel the music from upstairs while I am sleep at night, so as long as it can make my room quite, that's all I need.

if it's likely to work, I like to built it and try it out, I want to sleep in a quite room after 10nish pm

What you are trying is an educational project, but it won't solve your real problem. They sell noise cancellation headphones that will work better than anything you will build, but even these are not perfect.

I've used noise reduction headphones that have a Ipod plug-in. These are not noise-cancellation headphones, but simple noise reduction types used by construction workers. The latest models have an Ipod input plug, so that the wearer can listen to music while blocking out loud noises. Using these with a white noise input is more effective than noise cancelation.

The main problem with this is that you will not hear a smoke detector go off while you are sleeping. So, this is only acceptable if you are sleeping with your spouse who will wake you in an emergency. And, hopefully you are on good terms with her!

 

Actually this circuit would work very well to keep you from hearing noise from downstairs. The more powerful amplifier you use to drive the speaker the better it will be at masking sound.

Note I say "mask" and not suppress. The audio path makes a complete and unstable feedback system and this will whine at one if not multiple frequencies.

Sorry, this just isn't going to work for anything.

 

An opamp cannot drive a speaker. You need an audio power amplifier instead.

But your circuit will not cancel sounds, instead it will just howl and whine since the mic can hear the speaker and the sounds will go around and around.

The phase of a speaker (and maybe the mic) varies a lot at resonant frequencies. Then the total phase is 360 degrees which produces oscillation.

 

Try the broom trick. Wait untill 1/2 hour after the music stops, and then pound on the ceiling with a broom handle. They will soon get the word, but you could ask them to turn the volume down.

I fixed the same problem: move to a house.