In my last 2 posts, we have discussed what is sound and what the SRN of an air conditioner is . Now I’ll try to approach how this is applied to air conditioning “noise”.
Remembering that sound is a pulsation of air pressure capable of being heard and that “noise” is unwanted sound, it is no surprise that the moving components of an air conditioning system will generate noise. Fans can create air turbulence and air turbulence generates sound. Units have vibrating surfaces and vibrating surfaces generate sound. The problem is to keep the sound generated to a minimum (the manufacturer’s problem) and to keep such sound as is generated from being an annoyance (the dealer’s problem).
Manufacturers are the ones who are responsible for the SRN of their equipment. The marketing department wants the quietest unit possible with the lowest SRN so they can tout that number. So what does a manufacturer do? They look at compressor design along with the piping design in the unit to reduce vibration. They look at the compressor and fan mounts to reduce vibrating surfaces. They address air noise with fan blade design and motor RPM (using swept wing fan blades with low RPM motors reduces a lot of air noise). Some use ECM or X-13 condenser fan motors. They put compressor blankets or sound enclosures over the compressors. They try to design the base pan in the unit to minimize vibration from the compressor on the base pan. Some manufacturers even have their own sound rooms where they can test designs to assure the lowest possible SRN for their equipment. Unfortunately, all of this is for nothing if the unit is installed in places where reverberation of noise can occur or the unit just is not installed properly. The manufacture has done their part to keep the unit quiet, now it is up to the dealer to do his part.
Most air cooled condensers are located outdoors and are placed on a base or pad at ground level and adjacent to one of the walls of the house. All other things being equal, the condensing unit can be placed anywhere outside the home.
Generally, the dealer will want to install the unit as close as possible to the cooling coil so the refrigerant lines will be as short as possible. The home owner, on the other hand, will wish the condensing unit to be installed where it is as inconspicuous as it can possibly be. He may prefer that it be located on the side of the house, and thus, close to the home of a neighbor. Sometimes he may want it by an inside corner or “L”. Either of these may be good locations from the standpoint of appearance and proximity of the cooling coil but usually are quite bad from the standpoint of nuisance noise.
Direct and reflected sound energy work together to create annoying sound levels. Placing the unit between houses causes the sounds to “bounce back and forth” between the homes. Locating the unit in an “L” may be even worse, the reflection of the sound from both walls of the “L” may head right towards a neighbor’s open windows. It is usually wise to locate the unit near the rear wall of the home where direct and reflected sound waves can be dissipated as they travel across the back yard and before they cross the property line.
You also need to be aware of local code when it comes to noise at the property line. Most local governments require less than 55 dBA at the property line closest to the unit with a sound meter held approximately belt high at the property line. You need to check with your local agency to see what their requirement is for your area.
Although much of the air conditioner’s noise is generated by moving parts — the fan and compressor –the turbulent air passing through the unit is also a noise source. This air can create noise after it leaves the unit. For example, air from a vertical discharge condenser strikes the overhanging eave, noise pulsations will often result. Of course, one spot that should be obvious to avoid, but I see it all the time, is having the unit under a bedroom window.
The following recommendations are taken directly for an A.C.C.A. Technical Topics paper titled “Noise in the Neighborhood:
The above information makes a lot of sense when you think about it. Nevertheless, the air conditioning industry must always be looking for ways to “contribute” as little as possible to the sound level in any city or neighborhood. The development of quieter equipment by the manufacturers will help with this. But when all is said and done, the primary responsibility for keeping sound levels down lies with the installing dealer; you select the equipment, you determine where it will be located, and you set it in place. If you do your job well, if you avoid locations that aggravate the noise problem, that air conditioner will run at acceptable levels of sound that is not an annoyance to the home owner or the neighbors.
By the way — shrubbery does not make a good sound barrier:
(thanks to the A.C.C.A. publication and to Don Branch for information used and presented in the last 3 posts and to Bruce Porter for the picture above.)