Sound advice

Extend the range of possibilities through ultrasonic technology

Have you ever seen an individual who, through experience, can walk through a plant and just by listening, tell if there is a problem with a specific piece of equipment? In fact, there are many machine operators who are so accustomed to the operational sounds of their equipment, they can tell if something is wrong by noting a change in the sound pattern.

There are many types of sounds around a facility that provide valuable information about the health of equipment and of the plant in general. Changes in sound pattern, pitch--a sudden bang, pop, hiss, or buzz--help us learn to recognize warning signals. In essence, every employee who learns to listen becomes a contributor to the health and, ultimately, to the bottom line of the company. This does not mean they can diagnose effectively. It does mean that they help alert trained inspectors to potential problems. Just as when a strange sound is heard in a car, the driver usually brings the car in for inspection. While the sound was the warning, the trained mechanic provides the diagnosis.

Diagnostic sound related tools are being brought into many standard proactive and predictive maintenance programs. They range from the naked ear to variations of a doctor's stethoscope to vibration and ultrasound instruments. They rely on the ability of the sensing element to detect and of the observer to recognize and interpret. As the need to interpret becomes more sophisticated, so too will the tools involved in the process.

How do we hear?
Our sense of hearing is an extremely sophisticated system. This marvelously complex organ was one of the reasons our ancestors were able to survive. We can hear and recognize our infant's cry, detect the subtle changes in voice, alert us to potential danger and, in the course of our jobs, determine normal versus abnormal conditions.

In order to comprehend how the sense of hearing works, it's important to understand the basics of sound. Sound is produced by a vibrating source. This source causes surrounding molecules to vibrate also by transference of energy. This effect continues through an elastic medium until it is perceived by a listener as sound. The medium in which sound vibrations are carried is called an elastic medium since it contains particles of matter. If there is no particle--as in a total vacuum--sound cannot exist. The more densely packed the medium the easier it is for sound to travel. As an example, sound travels faster in steel than in air.

The human hearing system is both a receptive and interpretive device. In fact, it is a very sophisticated transducer that converts mechanical energy into electrical energy. The basic system consists of a collector called the pinna or external ear. The curves in the outer ear don't just collect sound vibrations; they channel them into the ear canal--external auditory canal. At the end of the ear canal is the eardrum or tympanic membrane. Made of thin, flexible tissue, the eardrum vibrates when excited by sound. These vibrations set off compatible vibrations on the other side or inner ear. Attached to the eardrum are three very small bones that transfer the sound energy with a minimum of loss. These resemble an hammer, anvil, and stirrup--malleus, incus, and stapes. As these bones vibrate, they conduct the sound into a very sensitive organ that resembles a seashell and is therefore called the cochlea. At the very tip of the receiving end of the cochlea are tiny nerve fibers that are the nerve endings of the 8th cranial nerve. Once the sound vibrations reach these nerve endings, they are converted into electro-chemical impulses. These impulses are carried to the temporal lobe of the brain and transferred to various nerve clusters around the brain where they eventually help us recognize that a sound has been sensed and give us the ability to then store and interpret the received signals.

The frequency range of human hearing is best remembered as 20 to 20, or 20 Hertz to 20 kilohertz. The average high-frequency hearing range in adults is 16.5 kilohertz. The bulk of human speech ranges between 300 Hertz and 5,000 Hertz.

Since the nerve endings in the cochlea are fragile, they are subject to damage when exposed to frequent or prolonged high intensity sound levels or to sudden extremely loud sounds such as from an explosion. This is the reason OSHA set regulations for sound level exposure. It is advisable to use hearing protection devices when entering loud environments. If you are not sure, consult with OSHA or with your local industrial audiologist for advice.

Plant warning sounds
While it would be impossible to define every warning sound equipment makes, we can make a generalization of sound groups. A walk through of a plant reveals many opportunities for sound anomaly recognition. The easiest way to start is to walk around listening for the obvious. The key is understanding the equipment and becoming familiar with the normal sound to determine the abnormal.

Create a sound awareness exercise by listening to known equipment as you walk around your facility. To make things simple, choose one small, safe area that does not require hearing protection. Close your eyes and listen. Next, try to focus on specific sounds. Try to determine the location and the equipment. Open your eyes and see if you were correct. Then try this exercise with your eyes open and walk around listening to the various sounds in that area. A loud, constant hissing sound can reveal gross air leakage or steam leaking from a valve stem or steam trap. Most steam traps produce an intermittent rushing sound while others such as a thermodynamic trap have a recognizable click and rush. Bearings produce a constant smooth rushing sound; reciprocating compressors generate a consistent, rhythmical clicking sound as the valves open and close. A frying or crackling sound accompanied by a buzzing noise around high voltage electrical equipment warns of potential breakdown and possible explosion.

The human hearing system is both a receptive and interpretive device.

Encourage employees to learn to listen as well. They can become sound detectives who help your maintenance staff keep on top of things. The object is not to overwhelm the maintenance department, but to help them become more aware of potential trouble in order to plan around it.

Clearly, the human ear has its limitations. The walk-around is restricted by safety considerations and by the range of human hearing. As explained, overexposure to loud sounds contributes to hearing loss. OSHA set standards for hearing protection. In addition, there are many signals produced by operating equipment that are not within the range of human hearing. They can be difficult for the unaided human ear to locate and interpret. For this reason, advancements in technology produced extremely sophisticated listening devices that can be considered an extension of the sense of hearing.

The meter on these instruments can be used to define the degree of amplitude, the potential for failure, as well as to trend mechanical condition.

These instruments help detect potential problems by converting the high frequency range down into the audible range. The sounds are then heard through headphones and given an added dimension of intensity recognition with a meter.

With the added protection of the headphones, these instruments make detection and location of leaks, mechanical failure, and electrical problems simple and easy. Since the ultrasounds are heterodyned, the translated audible signals are easily recognized and understood. As an example, the hiss of an air leak or the rushing sound of a bearing in normal condition can be identified even in a noisy environment. The added advantage is that the short wave nature of ultrasound is localized to the source making identification and location of the signal relatively easy. Therefore, the hissing sound that might be heard with the ear but difficult to find or to interpret as either an air leak or a steam leak, can be located quickly and identified.

The meter on these instruments defines the degree of amplitude, the potential for failure, as well as to trend mechanical condition. As an example, ultrasonic bearing monitoring can determine the degree of bearing failure. A rise of 8 dB over a baseline accompanied by a uniform increase in sound pattern indicates a pre-failure condition often due to lack of lubrication. A 12 to 16 dB gain with rushing, cracking, or clicking noise is the initial stage of failure while a 35 to 50 dB gain accompanied by loud rough or crackling sounds indicates catastrophic failure.

Electric sound emissions also lead to a quick diagnosis. Scanning switchgear, insulators, transformers, or busses detects problems indicated by arcing, tracking, or corona discharge. The underlying difference among these emissions is the sound pattern. Corona produces a steady buzzing sound while tracking has a gradual build-up of sound intensity with a sudden drop off. Arcing produces extreme crackling or buzzing sounds similar to that of frying something in oil.

An added extension to the ability to hear with the listening aid of an ultrasonic instrument is the development of spectral analysis software. This software enables users to not only hear and see but to analyze captured sounds. Sound samples may be recorded with a tape recorder or directly onto a computer--typically a notebook computer. The sounds are taken directly from the heterodyned output of the ultrasonic translator and provide accurate waveforms for analysis.

An added extension to the ability to hear with the listening aid of an ultrasonic instrument is the development of spectral analysis software.

The first is a bearing. The sound of this bearing was relatively rougher than that of a bearing in good condition. The operator was concerned that the bearing was in the failure mode due to an increase in amplitude of greater than 16 dB over baseline. When analyzed carefully considering the speed and the number of ball bearings, a technician determined that the harmonic frequency of the peaks at 90 Hertz apart demonstrated the fault frequency of this particular bearing. This enabled the viewer to confirm the original diagnosis.

The next sample is a spectral view of arcing. Note the 60-cycle harmonics of the peaks at 120 Hz and 240 Hz. This is in contrast with the other spectrum of corona, that indicates a steady noise with little distinction among the peaks.

While just listening alone in a facility is a very valuable exercise, adding more sophistication to aid the listening process accomplishes much more. Using our ears alone detects the more advanced or gross status of leakage and equipment disorder. Adding to the acoustic evidence in stages--naked ear to ultrasonic translator to spectral analysis--establishes a more detailed account of the status of the equipment providing for a more accurate diagnosis and analysis of potential cause. Incorporating every opportunity to listen to noises in a facility improves productivity, helps meet production schedules, and reduces wasted energy. Trained and motivated personnel make the art of listening a truly profitable experience for everyone.