How to reverse a wave in the whole length

Audacity Help Forum Windows
1

Hello ,

I have a mono wave that i would like to inverse completely , which mean if the the wave at second 1 was value 2 i want the result in second 1 to have value -2 , and this on the whold length.

Thanks for your help

2

“Effect menu > Invert” http://manual.audacityteam.org/o/man/invert.html

3

Thank you for your answer , i have tried using the invert function before , but i didn’t hear anything in terms of the audio , the sound was exactly the same as before inverting it , is that normal behavior ?
Thanks again

4

Here is what I want to do, seperate the audio and set the left speaker and the right speaker opposite in terms of the wave to test the audio cancelation, when i did invert the right audio and played both waves at the same time, I was still able to hear the sound , is that normal ?
Many thanks

5

When played through speakers the effect will be very subtle.
For a more prominent effect try using headphones.

You could try running this code in the “Nyquist Prompt” effect.
Select part of a stereo track, then open the Nyquist Prompt effect from the Effect menu, and paste this code into the text box.
What it does is to make the first half of the selection mono (both channels set to the original left channel), then the second half of the selection has the original left channel remaining unchanged and the right channel becomes an inverted copy of the left channel.

(vector
  (sim (extract 0 0.5 (aref s 0))
       (at 0.5 (extract 0.5 1 (aref s 0))))
  (sim (extract 0 0.5 (aref s 0))
       (at 0.5 (mult -1 (extract 0.5 1 (aref s 0))))))

The effect that you should hear through stereo headphones is the first half of the selection will sound “inside your head” and the second half “different”, possibly “outside your head”. The effect of the inverted channel is hard to describe - try it and hear for yourself.

6

Here is what I want to do, seperate the audio and set the left speaker and the right speaker opposite in terms of the wave to test the audio cancelation, when i did invert the right audio and played both waves at the same time, I was still able to hear the sound , is that normal ?
Many thanks

If you mix the left & right channels digitally or electrically, any sounds that are identical and in-phase in both channels will cancel. Sounds that are different will remain. Out-of-phase sounds will be boosted (you are subtracting a negative, which is addition). If you do this with a 2-channel “mono” file (where both channels are identical), you’ll get dead silence. The Vocal Remover effect will subtract the left & right channels digitally, so that’s an easy way to demonstrate the above effects.

If you have one channel inverted and you mix the sound waves acoustically (in the air) it gets more complicated and the sounds won’t cancel completely, but you should notice that most of the bass cancels. Random acoustics in the room cause the sound waves to bounce-around and mix unpredictably. At mid/higher frequencies, the shorter wavelengths in relation to the space between your ears can result in cancelation in one ear and in-phase addition in the other ear, and different frequencies will add/subtract as you move around the room.* This creates a “spacey” sound and it’s hard to tell where the sound is coming from. This is sometimes used as a “stereo widening” effect.


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  • At very high frequencies (say wit a 10kHz constant tone) you can hear addition/subtraction without inverting one channel. Move your head an inch or so and you can hear the loudness go up-and-down.

P.S.
You might try some test-tones at different frequencies. Generate some tones, and invert one channel them at the 5 or 10 second mark so you can hear the change. At 100Hz you should get a LOT of cancelation. At 1kHz you may not notice the cancellation, but you may notice the spacy stereo widening effect where the sound no longer seems to come from the center-point between the speakers. At 10kHz, you probably won’t notice any difference at all. (Of course, you can try other frequencies, and you can also try some pink or white nose.)

7

You may find this “Channel Mixer” plug-in useful for experimenting. It can (amongst other things) invert one channel of a stereo track: http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Channel_Mixer
Instructions for installing: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#Installing_Plug-ins