# Will a 3-wall acoustic barrier work or will sound go around?

I have two heat pumps directly outside my house and they are noisy so I am considering the viability of using a sound barrier to block the sound. The acoustic barrier itself will definitely absorb and eliminate the sound in three directions, but the question is whether the sound will be able to go around from the open side and get into the house. Here is the basic set up:

The heat pumps are shown in blue. The proposed acoustic barrier is shown in red.

Will sound be able to go around the edges of the barrier and get back into the house or will it radiate outwards and not come back into the house?

It will help to shield you from direct high pitch noise waves. because these waves short wavelengths will be blocked by objects in the range of your partition size.

Say a noise of frequency of 500-hertz has a wave length (Lambda) of 340/500 = 68cm,

but at 170-hertz lambda = 200cm.

And much of the noise generated by the heat pumps will go around or be transmitted through the bases of equipment to the ground and your building walls.

It would help if you could mount the heat pumps on rubber brackets on the concrete slab, or use noise attenuating neoprene seals . Then enclose the pumps, as you've shown.

Or, ideally, sound proof your home from outside with acoustic foam panels, but it could be too expensive.

If you can moving the pumps back a little bit can help too.

• The noise is not coming in through the ground, it is coming in through the windows. You say the noise will go around the panels, but do not give any explanation. Do you having any formal training in acoustics or are you just guessing? Your bio says you are a structural engineer. Dec 1 '18 at 3:49
• I did give explanation, I said objects that are in the same order of size as the wave length will act as obstacles and impede the propagation of sound waves, but base sound will pass through them. You may hear the sound coming through the windows and much of it does, but the walls vibrate and sound is reproduced by them. I did have courses in acoustics back in the day. Dec 1 '18 at 4:55
• Ok, maybe I should rephrase the question. Assuming that sound is emitted in a single direction, as the waves refract around a corner, there will be some attenuation. The greater the angle of bending, the greater the attenuation obviously. So, given the diagram as shown in the diagram, what would be the attenuation for the sound that bends around the corner of the barrier and returns to the house? Dec 1 '18 at 5:02
• There are some lengthy complex contour integrations for a single harmonic source sound pressure attenuation around a wedge with an angle wider than pi, not very useful in this case because the sound is composite and the edge is a sheet, not a solid wedge. in practice the insulation you've shown can resonate and even amplify the noise. Dec 1 '18 at 5:39

For fans used to move air, their primary noise radiation frequency will be (number of blades) x (revs per second of the fan shaft) and will be emitted in all directions if the fan shaft is vertical, as in most heatpump units. A 3-blade fan running at 1750 RPM will produce a hum at 88Hz. This will have a wavelength of about 11 feet and will diffract strongly around a wall less than 11 feet in scale length (height or width).

You can still get away with a "small wall" if it is folded into a box that completely encloses the fan unit except for an inlet baffle which must be positioned facing away from the dwelling's windows.

In this case, the combination of (enclosure + inlet baffle) will have a tendency to resonate (see "helmholtz resonator") and if the resonance matches the fan noise, the enclosure will be ineffective in blocking the noise unless you line it with absorptive material like fiberglass.

• Sand is another effective medium to consider for in-fill. Not terribly practical for an indoor setting, but in this particular case it might work well.
– user16
Dec 1 '18 at 12:54