We live in a multi-unit building where each unit's HVAC system/closet is located directly across a party-wall from the neighboring unit's bedroom, and adjacent to a shared external curtain wall. This is a bad recipe that leads to significant low-frequency structure-borne noise transmission through the curtain wall as well as airborne noise transmission across the party wall. The result is noise complaints and sleepless nights. Numerous HVAC servicing efforts and traditional soundproofing techniques have proven ineffective to address the low frequency noise (more background details are in this post). One option that we have not yet tested however is active noise cancellation (ANC), and that is the basis of this question.
Why Active Noise Cancellation May Have Potential
My understanding is that ANC is not something to consider casually. Outside of very controlled situations (e.g. headphones) it seems like the number of variables that must be controlled are overwhelming. Despite this, I think the concept may still have potential as some variables are mitigated a bit in this case:
- The troubling noise is consistent and predictable. It's all stems from cyclical mechanical noise, and much of it approximates harmonics of the electrical line voltage (e.g. tonal noise from motors).
- The majority of problematic sound energy is very low frequency (120Hz or lower) as soundproofing efforts already attenuate the mid-to-high ranges effectively. This should also mean that the wavelengths to be cancelled are generally equal-to or greater-than the dimensions of the room where ANC would be applied (in these conditions 120Hz should have a wavelength of about 10' or 3m).
- It is possible to implement sampling and/or cancellation very near to the source, even from a neighboring unit that is isolated from the actual HVAC equipment itself. This is because the HVAC blower and compressor motors are only approx 2' (0.6m) away from the party wall.
For further clarification on the quality of the noise, see the spectrum graph below. This helps demonstrate the concentration of problematic sound energy around 120Hz.
To do ANC right is seems like complex digital signal processing and predictive phase-control is needed (e.g. microphone arrays and super-low-latency sampling/processing on very expensive hardware). As we do not currently have the time or resources to go this route, at least without some basic confirmation on the concept, I am wondering if it's worth experimenting with a purely analog proof-of-concept.
My thought was to start with a balanced unidirectional mic with low frequency response (like this one), send that signal to a preamp (it seems like a cheap mixer is the best option, like this one), and then run the line-level out to a self-powered sub which includes an integrated low-pass-filter (I have one already). With that I suppose I could experiment with mic and sub placement (e.g. place the sub 1/2 wavelength away) or even flip pins 2 and 3 on the mic to invert the input and place the mic and sub close together with some attenuation material between them.
The big assumptions I am making include:
- There would be almost no latency in this whole system. I assume that would be the case, as the whole pathway is analog, but I may be mistaken.
- Physical space and/or attenuation material between the mic and sub would be sufficient to avoid a negative feedback loop that would negate the whole ANC effect.
- Mid-to-high frequencies could be reduced reasonably via the preamp equalizer plus low pass filter on sub. Without that I suppose the added noise and distortion would be worse than the original noise.
- Room harmonics will not drastically interfere with the ANC effect, at least not in some target areas (like the head of the bed). This is probably the most risky assumption.
So what I'm wondering is if there is any chance ANC could be feasible in a case like this. I have a background is systems engineering, and am not afraid to experiment, but I'd also like to be told if I'm suffering from the Dunning–Kruger effect by even considering this.
If there is any thread of possibly benefit here, is my proposed proof-of-concept a solid way to begin experimentation?