negative sound

[Kane2oo2]

hello
i think the right term is negative sound. Where you play the oppostie (or negative) sound to the sound being heard and it cancels it out?

is there anyway of doing this with 2 computers. Say 1 computer plays music and the other computer has a mic and speakers that uses a program to create a 'negative' of the sound, this 2nd computer then replays the negative sound to cancel the original sound out.

would this work? or would the delay between recieving the sound and reproducing the sound be too long ..so that the sounds are "out of sync"

if it would work Where could i find such a program?

thanks alot
Kane

 

[Optikon]

hello
i think the right term is negative sound. Where you play the oppostie (or negative) sound to the sound being heard and it cancels it out?

is there anyway of doing this with 2 computers. Say 1 computer plays music and the other computer has a mic and speakers that uses a program to create a 'negative' of the sound, this 2nd computer then replays the negative sound to cancel the original sound out.

would this work? or would the delay between recieving the sound and reproducing the sound be too long ..so that the sounds are "out of sync"

if it would work Where could i find such a program?

thanks alot
Kane

Not negative sound... sound in the usual context does not have polarity so the term negative is meaningless. I think what you are referring to is actually called "destructive interference."

The concept is simple. Two sound waves, same amplitude, one taht is 180 degrees phase shifter from the other. When they meet, they will "cancel".

you might be able to find a program that delays audio signal but in practice it will be very difficult to achieve even poor (barely noticable) results.. Th emajor problem will be your actual environment. As sound waves bounce around off of everything there will be absorption, scattering and all kinds of non-linear phase delays produced. Unless you have a very controlled environment (like a sound lab) where all the properties of walls and geometries are well known, I think you wont have much luck.

 

[Kane2oo2]

ah ok

just wondered thats all

thanks
Kane

 

[audioguru]

Hi Kane,
Sound that cancels another sound is said to be "out of phase". Noise-cancelling headphones for pilots (you can buy them too) and some pricey cars use that principal to cancel low-frequency noise. They pickup noise with a microphone and simply invert it (a computer or time delay is not necessary, but it probably needs an adjustable variable-phase circuit) and feed it through the headphones or car speakers.
Try it yourself. Have your stereo speakers facing each other, only about 2cm apart. Play mono music and the bass will sound good and loud. Now connect one speaker out of phase by swapping its 2 wires. The bass will almost disappear, and the stereo's balance control will adjust the null. One speaker is cancelling the sound of the other.
The cancellation works only with low frequencies because their wavelength is long and they travel around blocking objects. Microphones and "cancelling" speakers have a huge phase change at high frequencies and also with small changes in distance, so that if you calculate the distance between them to be out of phase then many high frequencies will actually be in phase and cause acoustical feedback howling or squealing.
Even with low frequencies a microphone and speakers would be difficult to set-up. The microphone must be able to hear the noise, but not its own cancelling sound output. So put the microphone near and facing the noise source and the cancelling speaker near you. Since there is a difference of distance between the microphone to the noise source and between the cancelling speaker to the noise source, only low frequencies would be out of phase and cancel.

 

[zachtheterrible]

lol, i saw this concept on a spy show once. I think it was alias. the spy had this tiny little device that she turned on, and then turned on a stereo. when the device was on, the stereo couldn't be heard.

 

[zachtheterrible]

Wow!!! I just tried that little experiment u were talking about audioguru, cool!!

If the speakers arent 2 cm apart, (across the room), will this still impede the sound? I tried it aways apart, and it didn't seem to make any difference. of course, it took me so long to change the wires around that it was kind of hard to compare.

 

[Nigel Goodwin]

Wow!!! I just tried that little experiment u were talking about audioguru, cool!!

If the speakers arent 2 cm apart, (across the room), will this still impede the sound? I tried it aways apart, and it didn't seem to make any difference. of course, it took me so long to change the wires around that it was kind of hard to compare.

It's a well known phenomenon in audio circles, particularly PA use. I once spent ages trying to get all the speakers in a PA stack phased the same, eventually I borrowed a 9V battery out of an effects pedal and tested each speaker to see which way the cone was moving!.

 

[-=GST=- Nemisis (cs/cz)]

wow, that weird but cool makes u think i suppose. ne other cool things u can do with audio(or microwaves)?

 

[zachtheterrible]

eventually I borrowed a 9V battery out of an effects pedal and tested each speaker to see which way the cone was moving!.

lol :lol:

 

[zevon8]

Reminds me of the old trick for checking speaker phasing where you set a FM stereo tuner off station, then listen for when the noise is the loudest, meaning the speakers are in phase, the sound waves not cancelling each other out. The same thing can be done for an "almost" white noise test. Very annoying on large metal midrange horns, lol.

 

[audioguru]

Hi Zach,
Even if the speakers are across a room, they will produce louder bass and sound better if they are correctly wired "in phase". That is why Nigel carefully wired each speaker in a large PA stack.
When I visit an appliance store, I laugh when I hear their stereos wired out-of-phase but with their bass control turned all the way up to try to make some bass. I always re-wire them correctly in phase and turn-down the bass control to nomal. When fairly close to me, I can "hear" speakers that are out-of-phase.

I am glad that you liked my phase cancellation demo. Try another. Connect just one speaker to the "hot" (usually red) terminals of both channels of a stereo. When playing stereo music, it will produce sometimes loud echoes and strange sounds, but the main vocals and bass will be cancelled. That is also a simple "surround sound" method where both rear speakers are wired in series and their end wires connect to the stereo's red terminals.

 

[zachtheterrible]

wouldnt that be 2 much current/voltage for the speaker?

while we're on the subject of speakers, my stereo says that it requires 3 ohm speakers, i have 8 ohm speakers connected 2 it. is that okay? (im pretty sure they're 8 ohm, ill have to look again)

 

[Nigel Goodwin]

wouldnt that be 2 much current/voltage for the speaker?

No, all it's feeding the speaker is the 'difference' between the two channels, as 'audioguru' says it was a crude method of producing 'surround sound'.

while we're on the subject of speakers, my stereo says that it requires 3 ohm speakers, i have 8 ohm speakers connected 2 it. is that okay? (im pretty sure they're 8 ohm, ill have to look again)

You're OK using higher speakers, but not using lower ones (with solid state amplifiers). Most speakers are 8 ohm, 4 ohm are commonly used in cars - to get the most from the low supply voltage. 3 ohms is pretty obscure though? - obviously a cheap and nasty stereo! :lol:

 

[audioguru]

Hi Zach,
A speaker doesn't use all the voltage/current (power) that is available from an amplifier unless you turn up its volume conrtol. In theory, a speaker that is connected between 2 amplifiers like that (the amplifiers are bridged, and the speaker is their bridge-tied-load) can draw up to 4 times the normal power from the amp and therefore cause it to blow-up!
Just use common sense and keep the volume no more than it was with the speaker connected normally.
Use your ears, not just the position of the volume control. A volume control that is turned to half-way does not necessarily make the amp produce half-power. It could be a lot more or a lot less.

I bet your stereo says, "Minimum load impedance is 3 ohms". Then it can safely drive 2 paralleled 8 ohm speakers, but will get pretty hot when played loud.

My stereo says, "High Dynamic Power", which reads good, but actually means that it has a weak power supply that can give massive power but only for a moment until its voltage sags. That used to be called 'Music Power".
It also says, "Low Impedance Drive Capability", which is true. Its rated power into 4 ohms is about 1.5 times its rated power into 8 ohms. But its rated power into 2 ohms is the same as with 4 ohms so that's when its power supply is really straining. Who cares, it sounds great!

 

[zachtheterrible]

ill give it a try :lol: