Finally! A topic to cover that I’m actually an expert in, sound!
Apparently there is a problem at the World Cup. Or, many problems. Many loud obnoxious problems with a funny name: vuvuzelas. Apparently they are so loud they are distracting both fans and players. What’s to be skeptical about though?
This guy claims to have made an anti-vuvuzela filter. I doubt it will work. Its a MP3 of some vuvuzela noise sampled from a previous game. He is close to having the right idea, but misses in execution. This if from the “How does it work?” section of his website:
The Vuvuzela sound like all other steady noise is a pressure wave, which consist of a compression phase and a rarefaction phase.
Our specially designed Vuvuzela noise-cancellation sound is a wave with the same amplitude but with an inverted phase to the original sound.
The waves combines to form a new wave, in a process called interference, which effectively
cancel each other out -an effect which is called phase cancellation.
Depending on the circumstances the resulting sound wave may be so faint as to be inaudible to human ears.
He covers his ass in the last line.
This won’t work though. Phase cancellation will only work in very specific situations. Lets look at a sine wave:
The inverse looks like this:
If you play those two waves back at the same time, they will cancel each other out and you will hear nothing. That is called phase cancellation. The sine waves are exactly 180 degrees out of phase. If they are any more or less out of phase, you will hear something. If the two waves are in phase, the sound will actually be louder. Think of it like addition and subtraction. Anything above the x-axis is positive, anything below is negative, When you add two points from the two waves together, they will equal zero.
The biggest problem with the anti-vuvuzela filter is that the sound of thousands of vuvuzelas will make a very complex wave form, and depending on how many are playing at once, where the microphone that is recording the sound is, the starting and stopping points of each person playing, the waveform will constantly be changing. Inversing a sample taken form a few games ago won’t work because it’s not the same sound source.
Playback is important too. This has to do with the wavelengths. Humans can hear sounds from about 20Hz – 20,000 Hz. That translates to a wavelength of about 54 ft for a sound at 20 hz to 0.6 in for 20,000 hz. Let’s say you are playing a sine wave at 1,000 hz. That means that the wavelength is about 13.6 inches. If you play it back through two speakers, and place one speaker 13.6 inches in front of the other, the sound will be almost 180 degrees out of phase and you won’t be able to hear much of the 1000 hz tone. The noise coming form a soccer match contains sounds from the entire frequency spectrum. This makes it much harder to cancel the vuvuzela noise.
Sampling vuvuzela noise from a previous match is more or less useless. At best, you will get minor phase cancellations, but you will probably just end up with more vuvuzelas noise in your room. Phase cancellation only works when you are using the original sound source.
The best way to cancel the vuvuzela noise is to install something between your TV and its speakers that automatically inverses the sound coming from the TV and adds it to the original sound source. But that is basically muting the TV, so just mute the TV if you can’t stand the noise.