What’s all that racket?

On the subjective side, the challenge for anyone dealing with the problem of noise is that the tolerance of noise varies (often substantially) from person to person. This explains (at least in part) how teenagers can tolerate rock music at ear-splitting decibel ranges while their parents cannot.

Noise physics includes how to define, measure, and control noise. This can be a much more complex issue than you might think, with variables such as frequency and intensity of harmonics. Complicating matters further is the challenge of dealing with the interaction of the noise-producing device with the physical environment.

All noise is mechanical in origin. For noise to occur, some source must create waves of pressure that are transmitted through either air, liquids, or solid materials and have components within the frequency range discernible to the human ear (generally between 30 cycles and 20,000 cycles for a young person). For the purposes of this article, I’ll concentrate on motor noise (after all, I am the Motor Doctor).

There are numerous sources of noise in an electric motor. A motor produces so-called "electrical noise," which is generally at line frequency or a multiple of line frequency, when the magnetized parts of the motor have room to physically move. This movement occurs as these parts are alternately attracted and repelled from one another and is often referred to as "60-cycle hum." This tends to be a manufacturing problem, and there is little you can do to alleviate this problem on the job site.

Other noise sources in the motor result from air disturbances caused by moving parts and particularly by vibrations derived from imbalances in spinning parts. Once again, this is a manufacturing issue, and as an installer, you don’t have much control over this problem.

You can have an impact on the third area of noise—the interaction of the motor with the equipment in which it is mounted. Not only is this a potential source of noise generation, but also of noise amplification. Typically, when you are called to the job site, the customer will simply complain of a noisy motor. He or she won’t necessarily describe the 60-cycle hum or excess vibration. In situations like these, the first thing you’ll need to do is determine if the motor is performing sonically to its designed specifications. Most of the time, the best instrument to test for this doesn’t come in your toolbox, it’s your well-trained ear. The more time you spend in the field, the better you will become at determining if the problem lies with the motor or the application. Here are some creative remedies to the problem of noise on the job.

Mechanical isolation is usually the most straightforward action you can take. This is because any noise inherent in the motor transfers very efficiently through metal mounting assemblies. You can break this sound path in one of three ways:

1. Separate the motor base from the surface on which it is mounted with a resilient pad;
2. Couple the motor output shaft to the driven equipment with a "soft coupling" (one that incorporates a rubber bushing);
3. Make a substitution from a rigid-based fractional horsepower motor to an equivalently rated one with a resilient base.

But what if these three measures fail to produce the desired outcome? Don’t despair, resolution is still possible, but it will be somewhat more complicated. Your corrective action is based on the concept of harmonics. First a quick definition of the concept. All mechanical objects tend to vibrate at a number of specific frequencies. (For example, when you rub a wet finger over the rim of a glass, you tend to get a specific sound each time from that glass). These favored frequencies are called harmonic frequencies.

Now, if by circumstance, design, location, weight, mounting configuration, or any of an endless list of mechanical parameters, a piece of equipment (door operator, pump, fan) has a harmonic frequency that matches the output frequency of a motor, the result will be excessive noise. The good news is that you can attack this problem—and make a dramatic improvement in noise levels--by changing any one or several of that long list of parameters I just gave you.

While it may be difficult to calculate the exact effect, virtually any change to mass, speed, or separation distance will have some impact on harmonic noise. The trick is to find the most effective course of action. Here are some suggestions:

1. For a belt-driven device, try making a slight change in drive speed by varying the pulley sizes;
2. If the application uses rubber isolation (like a soft coupling) but appears ineffective, try changing the density (hardness) of the isolation devices. This may be enough to move the assembly off harmonic frequency;
3. Where space and the application permit, you can try changing the length and configuration of the drive train by adding or subtracting a belt length or chain link;

Once you understand the role of harmonics in creating noise, you can direct your efforts in the field to solving the problem by tackling those harmonics. I just wish it were that easy to deal with the teenagers and their loud music.

by Neil Simon, aka the Motor Doctor - regional sales manager for A. O. Smith Electrical Products Company.