# Fan Noise cancelling Issue

One of my customers is considering developing a fan noise reduction solution.

she wants to reduce fan noise on equipment (air purifier) to reduce noise in work area.

The devices under consideration are planned to be placed next to the already installed equipment.
I think it should be designed in such a way that it receives the fan noise from the side and generates the opposite phase noise.

So I'm trying to design an ADAU1772 ANC part

Question

1. Is the output of the ADAU1772 the opposite phase of the noise signal? Or is it a noise-canceled signal?

2. Is there any other solution available besides the ADAU1772 assuming that I know the noise frequency?

Thank you in advance

## Top Replies

• Hello Andrew,

I think technically this is very difficult to do. Fan noise has a lot of higher frequency signals and they are somewhat random. This is all very difficult to cancel out in an acoustic environment. The wavelengths are short so the amount of cancellation will differ with the angle between the source and the speaker differs depending on where the listener stands.

As far as your questions, The 1772 will output whatever signal you would like to output. Normally this part is used to drive headphones so the output is the noise cancelled signal. However, nothing is stopping you from just developing the cancellation signal and sending that out without the original signal.

We have the acoustic noise canceller and the noise reduction algorithms that we license for the ADAU1452 family of parts. It is available in trial form in SigmaStudio. The 1452 had PDM inputs so it can receive audio from a PDM output microphone. Then you can interface it with a codec for other inputs and outputs. The ADAU1372 would be a good choice. It is basically the converters from the 1772 without the DSP.

Dave T

•    Dave is quite correct; a look at this Wikipedia article demonstrates what such a noise reduction problem is up against.  The finite speed of sound (about 1K feet/second) means that sound added to cancel the noise at a particular location, may actually reinforce the noise a few inches or feet away.  Noise cancellation is easier if when when operating right at the listener (headphones), or at the source (inside the fan's duct).  Anywhere else is extremely difficult.  Also it's a lot easier when the undesired noise is a tone or hum of one or a few frequencies.  It's much harder for a fan sound, which typically covers a wide frequency band as does pink noise.  Thus, anything you can do to make the fan quieter in the first place would be more effective than an add-on cancellation system.  Mechanical features such as motor shock mounts, deadening material in ducts and enclosures, and the lowest fan speed that does the job usually provides sufficient noise reduction.

Best regards,

Bob

• This is a pretty common problem that arises anytime something needs both ventilation and sound isolation (e.g. a recording studio). The most conventional approach is: (1) Make the enclosure rigid and damped. Nothing else will work if you don't do this first. Also, as mentioned above, mechanically isolating vibration sources and using good quality bearing will greatly reduce the level of noise that needs to be attenuated. (2) Use high-volume, low-velocity air, which means a big, slow fan. This will minimize turbulence noise at all frequencies. (3) Channel the intake and exhaust into folded duct work, and line the inside of the ducts with sound absorptive material. This will damp the high frequencies considerably and is limited only by the amount of material you can use. (4) If necessary, consider using active cancellation to eliminate just the remaining low frequencies. Since the wavelengths are so large compared to the duct, the sound becomes approximately a planar (1D). Such a wave will not spread as sound does in 3-space, but it is relatively easy to cancel with a woofer, which brings the topic full circle. Active noise cancellation works well with a plane wave at low frequencies.

Ken

• I understand

Thank you very much everyone

• Hello Andrew,

You know, this reminds me of an article I read about active noise cancellation in automobiles a few years back. This was to cancel road noise inside the vehicle.

For decades tire manufacturers have been going to great lengths to make tires that do not produce a few set or tones but instead, they produce a wide band pseudo random noise so that it is less noticeable and easier to cancel with damping material.

Then when you try to use active cancellation techniques the noise from these tires make it much more difficult. So a tire that is poorly designed and produces resonate tones can more easily be cancelled inside the car! However, to the neighborhood, it is more noisy but I am guessing the Deer and other animals like that. They can hear the car coming. :)

Dave T