Stopping an 80 Hz wave

Summary:

Help me solve a way to not hear 80 hz waves inside my home.
My neighbor has two AC condensers are driving me insane.
I took measurements of the units, and they were between 33.5db - 47.9db at 80HZ.
About 3 feet away is a cinderblock privacy fence between us. It is about 10 feet tall. His home is on an elevation about 4 feet higher than mine (so the fence on his side is 6 feet). And then about 8 feet away is my home.
Inside my bedroom, I was finding 31.5db at 80 HZ when ambient is 11.7db, and this spike is causing a lot of disturbance whenever their ACs kick in and is on.

After complaining, they put noise absorption blankets on the units as well as filled in part of the cinderblock wall with concrete.
Sadly, this made things worse everywhere except a decrease right above the wall (49.6dB to 35.1dB) and right next to the wall (35.5dB to 13.3dB) - both of which are outside and I don't care about. Inside the bedroom, things went from up by 2-4dB at the 80 HZ. What's worse are the things that wasn't such a big deal before such as my bathroom (18.1dB to 24.6dB) and my closet which was no noise before to where I didn't measure to now being 24dB at 80HZ as well as other parts of the house where there was no issue before. I also took a measure from the far corner of my backyard and since adding the concrete, the measurement went from 25.4 to 29.1dB at the 80 HZ.

So now we know that filling in part of the cinderblock wall made it worse.
What can we do to make it better and eliminate the noise?
I've been told that soundproofing my own home won't work due to the low frequency of the noise so I'm stuck.

Related Mechanical Engineering News on Phys.org
Shoot, I'm new here. Should this be under "Other Physics Topics"?
I figured the closest type of engineering to acoustical engineering would be MechE.
Thanks!

Baluncore
2019 Award
Welcome to PF.
The question is engineering, so close enough.

Baluncore
2019 Award
There are a few possibilities. You could;

1. Install a passive reflector that directs the energy upwards, or back the way it came.

2. Install an active cancellation system that transmits a synchronous anti-phase signal.

3. Investigate the resonant frequency of the wall. Hit the wall and identify the note.
If the wall is resonant at 80Hz, then load it or raise it to reflect at the 80 Hz frequency. Place sandbags on top of the wall to see what happens.

sysprog, phinds and Lnewqban
Tom.G
From the numbers on the drawing, the wall and blanket changes at the source were quite effective at reducing the actual sound, meaning air vibrations, in your side yard.

Can you get some sound readings outside the windows at the top of the drawing? Get readings both near the windows and several feet away.

A few suspicions I have:
• The room could be resonant at 80Hz if one of its dimensions is a multiple of 7ft (that's a rough calculation of half a wavelength of 80Hz. in air)
• Your walls (or something else in your residence) are resonant near the 80Hz frequency
• The transmission is via ground vibration
• There may be a solid structure underground that carries the vibration (water or sewer pipe, old foundation from a previous building that was on the site, etc.)
• Is your side yard between your residence and the wall paved, concrete, blacktop, etc.? That could transmit vibrations.

Cheers,
Tom

sysprog, phinds and russ_watters
...filled in part of the cinderblock wall with concrete.
I think that was not really a good idea. Simple concrete is really god at transmitting noise, so I think now you have a big-big 'membrane' there guiding the noise along.

With some more tweaking and you can easily turn this configuration into a drum.
Maybe time to call an expert before that happens. Especially since the solution might include cutting or replacing that newly filled wall.

For noise management you might check this topic for some ideas and keywords.

jrmichler
Mentor
Assuming that the problem is an 80 Hz resonance in your house, it might be possible to change the driving frequency. 80 Hz is 4800 cycles per minute. That sounds like a four blade fan running 1200 RPM. If the fan was changed to five blades, the frequency would change to 100 Hz. Or possibly put a variable frequency drive on the fan and change the speed slightly.

You would need to work with the supplier of the condenser on this.

sysprog
1. I cannot change the fan blade because it's my neighbors and they refused.
2. I did not authorize the filling in of the wall, the builders did! So I had no choice :(

3.
There are a few possibilities. You could;

1. Install a passive reflector that directs the energy upwards, or back the way it came.

Can you give me an example of this and would it work with such a low frequency sound?

2. Install an active cancellation system that transmits a synchronous anti-phase signal.

Can you give me an example of this product? I keep looking for something like this and can't find it.
4.
Can you get some sound readings outside the windows at the top of the drawing? Get readings both near the windows and several feet away.

Ok, this might have to take a bit... I have to get someone to come take the measurements.

A few suspicions I have:
• The room could be resonant at 80Hz if one of its dimensions is a multiple of 7ft (that's a rough calculation of half a wavelength of 80Hz. in air)
Yes, my dimensions are 16 x 14 x 12 high... so the side to side measurement is 14 feet... Oh no. What can I do?
• Your walls (or something else in your residence) are resonant near the 80Hz frequency
How can I check this?
• The transmission is via ground vibration
• There may be a solid structure underground that carries the vibration (water or sewer pipe, old foundation from a previous building that was on the site, etc.)
• Is your side yard between your residence and the wall paved, concrete, blacktop, etc.? That could transmit vibrations.
No, it's just rocks.

sysprog
I think that was not really a good idea. Simple concrete is really god at transmitting noise, so I think now you have a big-big 'membrane' there guiding the noise along.

With some more tweaking and you can easily turn this configuration into a drum.
Maybe time to call an expert before that happens. Especially since the solution might include cutting or replacing that newly filled wall.

For noise management you might check this topic for some ideas and keywords.
Also, what type of expert should I be looking for? I've been looking... and don't know where else to go. Builders obviously have no clue on physics. My sound engineer doesn't know much about building things. If you have a suggestion, please let me know!

jrmichler
Mentor
Is there room to plant trees between you and your neighbor? If there is, search trees sound damping for some ideas.

Joe591
Main source of noise is the interaction of the fan's blades with the upwards flow of air they generate.
I would shield the line formed by the [fan-soffit of neighbor-windows of your house] path with as many layers of soft material (including vegetation) as possible.

For noise management you might check this topic for some ideas and keywords.
There are some hits here:
I have no idea if any are good.

I can sympathize, I used to live near a noisy place.

Baluncore
2019 Award
"Install a passive reflector that directs the energy upwards, or back the way it came."
Can you give me an example of this and would it work with such a low frequency sound?
You need to identify the direction of the energy arrival. Does it come from a point source or a resonant wall? Acoustic energy is reflected at each acoustic impedance mismatch. Hanging a heavy curtain will reflect energy. The curtain could be a sheet of lead, steel or concrete. The dimensions of a hard sheet may be critical to prevent resonances.

"Install an active cancellation system that transmits a synchronous anti-phase signal."
Can you give me an example of this product? I keep looking for something like this and can't find it.
They are used in heavy truck and tractor cabs.
Start here; https://en.wikipedia.org/wiki/Active_noise_control

sysprog and Lnewqban
I live further north where AC is not a problem. I have 2x6 stud walls with fiberglass insulation and double glazed windows. The nearby highway is annoying with the windows open but hardly noticeable when closed (but can hear neighbors' lawnmowers).
What are your walls and windows like, and were the measurements taken with the windows open?

Spinnor
Gold Member
The noise is driven by one of the four motors but where does the noise originate? Do both units come on at the same time? Is the noise a factor when only one is on? Is it a metal against metal sound? I would want to physically touch the unit or units in operation to try and determine from where cometh the sound. Touching it in the right location may deaden the sound. Maybe something as simple as a piece of rubber sheet shoved into the right location would significantly quite the unit or the application of sound deadening mat which is not that expensive and would improve their sleep and peace as well. Would you please record the sound and put it on youtube for us.

We have a noisy window AC unit, touched the right spot and noise reduced significantly, shoved in some cardboard, much more quiet.

tech99 and Lnewqban
Spinnor
Gold Member
tech99
Gold Member
I appreciate the problem you are in. I hope that touching the unit will allow the cause to be found. However, these low frequencies tend to be carried by "conduction" through building walls etc and even the ground. The best approach, if the owner will agree, might be to mount the offending unit on a sprung or soft platform so that the resonant frequency of the mounting is lower than 80Hz. Any solid mechanical connection to the unit, just a single screw, will transmit the 80Hz. I reduced the noise of a shower pump by mounting it on a heavy board which was then foam mounted on a concrete slab.

sysprog and hmmm27
Tom.G
I finally figured out how to record Audio from a youtube video, so here are the results of the investigation.

This is the waveform of the air conditioner noise, and the lower image below is the spectrum.
For the waveform the horizontal is time with grid marks at 1 millisecond intervals. Vertical is amplitude in arbitrary units from Zero to One. As you can see, the major peaks are spaced about 4ms apart.

The Spectrum is displayed in two forms.
The upper display is a Spectrogram representation with horizontal axis being time and vertical is frequency, with the amplitude at various frequencies shown as color.
The lower display shows the result of an FFT (Fast Fourier Transform) of the signal. Horizontal is frequency and vertical is amplitude

As you can see, the noise is rather wideband and covers the whole audio range. The frequency band with maximum amplitude is centered around 256Hz. (the band includes 180Hz thru 360Hz.)

You stated that survey you had done showed a peak around 80Hz as opposed the 250Hz in this recording. The discrepancy is likely due to different microphones and the fact my analysis passed thru a computer audio system twice. Home computer audio system are known to have very poor low frequency response and this one has not been modified or calibrated.

So consider this as just another data point so far. We perhaps could extract more information if you could make a few recordings, using the same equipment as on this one, inside your house in some of the spots shown in your sound map, and also a few outside the window shown at the top of your sound map.

Cheers,
Tom

Lnewqban, Spinnor and sysprog
Spinnor
Gold Member
I finally figured out how to record Audio from a youtube video, so here are the results of the investigation.
...

Cheers,
Tom
Can you post the sound recording?

jrmichler
Mentor
If the source is wideband, and the noise measured inside the house is mostly 80 Hz, that points to a resonance of the house itself. Look for a window, wall, or floor that is resonating at 80 Hz. You might even be able to feel it with your fingers. If so, it might be possible to add vibration damping, or even a tuned mass damper.

Tom.G
Tom.G