Noise cancelling on larger scale

[sk8erdude]

Noise cancellation/ Active noise control usually involves the addition of a wave that is "inverted" when compared to the actual wave. So, when these waves interact with each other, they cancel each other and as a result, there will be a reduction in noise.

Now, can we use this technique on larger scale? like completely muting/even reducing the sound of say..a drilling machine? Environmental disturbances are probably a cause for the inability to do this..

What do you think?

 

[TecHNooB]

I think you would just have to build a giant dome of noise cancelling speakers and add a few layers for the noise that escapes between the cracks. Doesn't seem efficient.

 

[Railgun]

No because as soon as you step in any direction for any distance, the sound changes. From where your ears are that tech would need to take a sample from that point, and cancel accordingly. That's the short short version.

 

[Mark R]

The fundamental problem is that the waves will only interact to cancel at certain points. At other points, they may interact to add, and produce a louder sound.

Where you only have to cancel the noise at one point (like in headphones) this works quite effectively. However, if you have to cancel the noise at several places, or over a wide area, it doesn't work.

 

[exdeath]

Sound has to be sampled and canceled at the listener for each listener, not at the source. The whole idea with noise cancellation has to do with what your ear hears, not masking what the noise source produces.

It's like 3D TVs, it's all about controlling the viewer's eyes, it has nothing to do with the screen.

 

[TecHNooB]

can't you do something to do what metals do to cancel all electric fields within it? it will probably suffer to transient effects but it's not out of the question, maybe.

 

[Mark R]

can't you do something to do what metals do to cancel all electric fields within it? it will probably suffer to transient effects but it's not out of the question, maybe.

Metals "cancel" electric fields because they permit charges to move very easily in response to an applied field. As a result, the charges move to oppose the field.

With sound, you don't have a similar "opposites attract" type phenomenon.

The way sound is mitigated is by isolation. You place some sort of barrier, through which sound will not travel around the source. This may be a damped mass (which will absorb acoustic energy, and then dissipate it in dampers), or a sufficiently stiff and massive object to reflect the energy (at the frequency of interest). There are other approaches, certain types of scientific instrument are amenable to being placed in a vacuum chamber (e.g. the switched electromagnets in an MRI machine are installed in vacuum chambers by some manufacturers, in order to prevent them from producing deafening noises from the forces that arise from applying AC signals to a coil in a strong magnetic field).

High, heavy concrete or wood fences can be used alongside highways, and are reasonably effective. The mass and stiffness mean that they transmit little sound. However, they have to be high enough to ensure that you don't get too much refraction of sound over the top - this means 12-18 feet high. Even so, this doesn't work for the low-frequency rumble, which is too effectively transmitted through the ground, and refracts over the top. It's also highly dependent on their being no gaps in the fencing, as sound will travel through the gap and refract, in effect transmitting sound to a large area.

 

[sk8erdude]

So, the materials with good absorption coefficient absorb sound better...like the difference between an auditorium and an anechoic chamber..but i don't think this helps in cancelling out a wave..

 

[TecHNooB]

Metals "cancel" electric fields because they permit charges to move very easily in response to an applied field. As a result, the charges move to oppose the field.

With sound, you don't have a similar "opposites attract" type phenomenon.

The way sound is mitigated is by isolation. You place some sort of barrier, through which sound will not travel around the source. This may be a damped mass (which will absorb acoustic energy, and then dissipate it in dampers), or a sufficiently stiff and massive object to reflect the energy (at the frequency of interest). There are other approaches, certain types of scientific instrument are amenable to being placed in a vacuum chamber (e.g. the switched electromagnets in an MRI machine are installed in vacuum chambers by some manufacturers, in order to prevent them from producing deafening noises from the forces that arise from applying AC signals to a coil in a strong magnetic field).

High, heavy concrete or wood fences can be used alongside highways, and are reasonably effective. The mass and stiffness mean that they transmit little sound. However, they have to be high enough to ensure that you don't get too much refraction of sound over the top - this means 12-18 feet high. Even so, this doesn't work for the low-frequency rumble, which is too effectively transmitted through the ground, and refracts over the top. It's also highly dependent on their being no gaps in the fencing, as sound will travel through the gap and refract, in effect transmitting sound to a large area.

Are these absolute limitations? Or can we do something to mimic the behavior we want? I don't know much about sound waves.