Noise cancelling a furnace with speakers ? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: January 31, 2024, 08:30:45 pm
content: There's an oil furnace in my house, and it's fairly loud. An electret microphone picks it up no problem in the next room. My scopes's FFT can't give a great picture of the frequencies, but it's mainly a low and high part, there's a blower fan, and it makes a lot of the highest pitches. Overall it's a very predictable sound, that changes little from day to day.So I have a few little speakers here, the best 3 would have 30W total. So with some op-amp circuits, isn't it possible to drive some speakers out of phase, by some amount that I can adjust for where I normally sit ? Or is that still sci-fi or done digitally, to be worth while ? Getting rid of the lowest frequencies would be hard with old PC speakers, but iirc a lot of it was up around 300-400Hz.I have a 5.1CH system for PC, and a lot of times I have to turn up the volume when the furnace kicks in. So if I make some analog circuits, and drive some of those other speakers, any sound cancelling would be a bonus. If common op-amp circuit's can do this, then I should already have all he needed parts.The green trace is the audio signal of the furnace, that I was picking up with a homebrew AM TX/RX

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 31, 2024, 10:39:35 pm
content: Quote from: MathWizard on January 31, 2024, 08:30:45 pmNoise cancelling headphones work, because the physical locations of mic and ear are very controlled, so the DSP has a chance to work.Hoping to use speakers, (which add more noise for others in the house), to magically create a Maxwell Smart Cone of Silence seems optimistic.It would seem smarter to use the time to sound proof the source. That works for everyone in the house, everywhere.Let us know how it works out

author: ajb
author_rank: Super Contributor
timestamp: January 31, 2024, 10:44:25 pm
content: Quote from: MathWizard on January 31, 2024, 08:30:45 pmIn theory, yes, but in practice this is very hard to do effectively on anything that isn't headphones. With headphones (or earbuds), the point where the outside noise is cancelled is well defined and fixed relative to the speakers and the microphones doing the cancelling, and the acoustic structure that couples them is also fixed and reasonably well-defined. With speakers in a room, it's a much messier situation. There are many paths between the sources of sound and the listener with different lengths, meaning different phases by the time they hit your ears (and they will hit one ear with different phasing than the other ear!). Even if you were able to get some useful amount of cancellation at one particular point in the room, remember that waves can interfere constructively as well as destructively -- so you will have areas in the room where the sound level isincreasedas well as areas where it is decreased. The speed of sound is about 342m/s, so at 350Hz there's about 1/2 meter between 0° and 180° phase relationships. That's a lot easier to deal with than the other end of the audio spectrum, but still challenging at the scale of a room.Fixing whatever is making the furnace noisy, or doing some sort of acoustic treatment, or just wearing noise cancelling headphones would be much simpler!

author: calzap
author_rank: Frequent Contributor
timestamp: January 31, 2024, 10:45:09 pm
content: Although it might be fun to try noise cancelling, I’d go the mechanical route first. Lubrication, vibration damping pads and sheaths, and sound absorbing foam. I once had an incredibly loud oil-less compressor. Put it in a plywood box with thick sound-absorbing foam, and it became a scarcely noticeable hum. Provided the box with a couple of long, skinny, foam-lined ducts for cooling.Mike

author: donlisms
author_rank: Frequent Contributor
timestamp: February 01, 2024, 03:14:23 am
content: Yeah - all that!The phase relationships as you turn your head or reach for a cracker will make it impossible to do effective cancellation.Here's an experiment you might like: generate a sine wave of some representative frequency, kinda loud but not painful, and just move around a little in the room. Listen for the cancellation and reinforcement if the level as the various reflections interact in various phase relationships. It should be pretty obvious.Now... imagine trying to generate the perfect waveform to cancel some other waveform. BOTH of them are essentially getting randomized by bouncing around the room; there's nothing perfect about them! There just won't be a way to get them to line up and behave as you wish. It's not "a waveform" you're dealing with, it's a conglomeration. A monster.

author: MathWizard
author_rank: Super Contributor
timestamp: February 01, 2024, 04:22:00 am
content: Yeah I'd be happy if I could reduce it even 10%, even if only the high pitches. I watched a few video's of people getting much much greater cancellation between 2 speakers, but that's all with modern DSP.I will have to look up some analog circuits, and give it a try, as a learning project if nothing else. As for soundproofing the room more, well maybe down the road, but another thing is I like to leave the furnace room door open, to let the heat out quicker.I should try and make some better measurements of the sound

author: Peabody
author_rank: Super Contributor
timestamp: February 01, 2024, 04:26:43 am
content: I have a similar problem with a furnace near my bedroom. There are three stages of noise. The first is the vent evacuation fan motor, the second is the gas burner, and the third is the main fan. Of course they are additive. What has worked for me is to have a competing noise going all the time. I use a small electric space heater in fan mode, and just leave it on all night. But they make all kinds of noise generators. Some are mechanical fans, and others are sampled recordings played back in a loop - white noise, pink noise, or my favorite brown noise. I think this is a pretty common solution.

author: MathWizard
author_rank: Super Contributor
timestamp: February 01, 2024, 07:10:02 am
content: I don't play video games much anymore, but I should start using my headphones in that. My PC is in the corner of a room, so I have poorly placed speakers anyways.But the furnace is just so much louder than normal listening volume, and worse it makes it hard to hear past my room. That's another reason I don't want to use HP all the time, or add more non-cancelling noise.At least I don't mind the sound of it, there's no part I consider annoying (besides volume)

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 01, 2024, 04:12:19 pm
content: There may be a nugget in figuring out how the sound gets from the furnace to your listening area and doing the cancelation at or near the source. Like in the furnace output duct. Still not easy but potentially possible.

author: jmelson
author_rank: Super Contributor
timestamp: February 01, 2024, 05:10:06 pm
content: I really don't think cancelling is going to work. The spread of frequencies you are fighting is very wide. 95 Hz to 11 KHz! Maybe you can find out how the noise is being carried to the living areas. Is it coming through the ducts? Is it transmitted via the duct metal? Is it transmitted through the building structure? Some kinds of isolation between these parts might solve the issue. Putting the burner assembly on a resilient material might help. Putting flexible joints in the ductwork might stop vibrations from carrying through to the walls, which then act as sounding boards.(We have a VERY quiet heating system, of the "hydronic" type. A pump circulates water through baseboard finned pipes. There's a clunk when the relay pulls in to start the pump, then a "whumph" when the burner lights, then maybe some faint clinking as the finned pipes expand. Really very quiet.The air conditioning is up in the attic, typical blowers and ductwork. makes a fair bit of noise.)Just a few ideas on your issues.Jon

author: Bud
author_rank: Super Contributor
timestamp: February 01, 2024, 06:08:30 pm
content: Furnace exhaust blowers are ridiculously loud and vibrating. I do not know why are made that way. Perhaps the assumption is they are installed in inhabitable locations. Or they are just shitty manufactured. I am having same problem with vibration from the exhaust blower transmitted through evacuation pipe to walls all the way to the roof. And cannot find a vibration dumping connector adapter- has to be a hot temperature material.

author: ajb
author_rank: Super Contributor
timestamp: February 01, 2024, 07:10:53 pm
content: Quote from: MathWizard on February 01, 2024, 04:22:00 amOne thing that video specifically points out is the need to model the transfer function between where the noise is measured and the cancellation point, and use that model in constructing the cancellation waveform. That will be more challenging in an analog circuit versus a DSP solution where you can take an impulse response measurement and crunch some numbers to plug into a set of digitally defined filters. Also note that the demonstration uses two microphones in specific locations and a simple dual tone noise source in a room covered in acoustic panels (that reduce reflections and reduce some of the problems with multiple acoustic paths). So it's kind of an ideal scenario for a demo, and even with equivalent equipment it would be much harder to get those kinds of results in your situation.QuoteAs others have mentioned, there are potentially many possible ways to remediate the noise that don't necessarily involve heavy room treatments once you identify the source of the noise. Tightening up loose sheet metal, replacing faulty parts, or covering up hard flat surfaces can be helpful with little cost/effort (well, depending on what you need to replace!). If the system is really that noisy, it may well benefit from a tune-up anyway.Quote from: Bud on February 01, 2024, 06:08:30 pmUsually (at least in the US) it's "Type B" double wall vent, and you can get flexible segments of it. If that's not enough, adding some mechanical support to the duct and/or some automotive acoustic mat (you can get materials designed for use in the engine bay that should be up to the job) may help.

author: Randy222
author_rank: Frequent Contributor
timestamp: February 01, 2024, 09:25:06 pm
content: Automotive has been doing active noise "cancelling" (aka attenuation) for many years now.To cancel the furnace noise, you don't put the speakers in the room you are in, you design and place the system in the room the furnace is in, the closer the better.Common harmonics in the noise spectrum can be phased out with 180deg speaker, but as you know it takes time for any dsp to process, so to get good 180ph the ckt needs to wait small time to begin the 180.Obviously when the harmonics shift the dsp needs a small time to adjust.