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Active Noise Reduction

• Jun 05, 2007

DURING the past half century, technology has improved virtually every aspect of human life. However, during this period, not much has changed in the world of personal protective equipment. For hearing safety, workers may choose either ear plugs that are inserted into the ear or ear muffs that cover the ears. These are called passive ear defenders because they employ acoustic foam to block noise waves from entering the ear. Passive protection is very effective against mid- and high-frequency noise, but low-frequency noise generated by engines, motors, and fans is more challenging. Low-frequency noise waves, prevalent in many industrial environments, are longer, can travel great distances, and can penetrate passive barriers, even cement walls.

Active Noise Reduction (ANR) is the only effective method of attenuating low-requency noise. This technology is also commonly termed Electronic Noise Cancellation (ENC), Active Noise Cancellation (ANC), or “anti-noise.” ANR is achieved by electronically coupling a noise wave with its exact mirror image, thereby canceling the noise. Figure 1 shows the relationship in time of a noise signal, an anti-noise signal, and the residual noise that results when they meet.

Figure 1

The benefits of ANR have been well understood by the military and aviation markets for many years. Aircraft cabin noise, for example, is well within the effective range of ANR, and therefore most aviation headsets incorporate the technology. The price of communications headsets incorporating ANR can be as high as $1,200.

The consumer audio market has also embraced the technology during the past 10 years, with every major audio manufacturer offering noise-canceling headphones for use on airplanes and in other low-frequency noise environments where consumers are using portable audio devices. These noise-canceling headphones range in price from $39 to $299 and have varying levels of performance.

Although the technology is commonplace in the aviation, military, and consumer worlds, ANR has not been widely available to the industrial worker. This is now beginning to change with cost-effective ANR ear muffs, priced in the $149 range, that are self-contained, conveniently powered, and relatively lightweight.

The Technology’s Evolution
Electronic noise reduction technology is not a new idea. The concept of creating a copy of a sound and using that copy to cancel the original sound dates back to the early 1900s. These first electronic noise cancellation systems used a simple “delay and invert” approach that was limited in its effectiveness. In the mid- 1970s, rapid advancement of ANR systems was achieved with the use of adaptive filters to generate the antinoise. This allowed the systems to adapt continuously to changes in their external world and in their own components.

A second breakthrough in the mid- 1970s was the recognition that many noises, particularly those produced by man-made machines, are periodic or tonal. This tonal noise allowed for a more effective solution because each repetition created a predictable harmonic pattern, which enabled the electronic system to generate a more accurate anti-noise signal.

As was previously mentioned, by the early 1990s, the technology was commonly seen in aviation and the military; by the mid-1990s, consumer noise canceling audio headphones were a widely known and well-understood product category.

Low-frequency Noise
Low-frequency noise from engines, motors, and fans can dominate many industrial settings, yet this component of the noise spectrum is largely unaddressed by passive hearing protection. Some of the loudest low-frequency environments and equipment include airfields, forestry, payloaders, diesel locomotives, forges, factories, highways, ship engine rooms, and heavy tractors.

In addition to hearing loss, prolonged exposure to noise, including low-frequency noise, is known to cause many detrimental psychological and physiological effects, including fatigue, anxiety, depression, loss of concentration, reduced productivity, headaches, and high blood pressure. Low-frequency noise is also particularly detrimental to communication because it masks consonant sounds—the sounds that make speech intelligible.

In most environments, there is noise in a wide frequency range. As previously discussed, most currently available hearing protection products are passive devices intended to prevent noise from entering the ear, making them effective against the mid and high frequencies but ineffective against the low frequencies. Aside from the fact that passive hearing protection does not address the full range of noise frequencies, the measure of effectiveness for these devices, the Noise Reduction Rating (NRR), cannot be taken at face value. The hearing protection level is often degraded because of improper insertion of ear plugs and wear and tear on the clamping force and ear seals of ear muffs. The most complete hearing protection currently available to workers is a device that combines ANR and passive noise control methods. Figure 2 shows a cross section of an ANR ear muff.                                   Figure 2

This type of ear muff uses a microphone inside the ear cup (1) to listen to noise coming into the ear (2). Using electronics (3), the system takes that information and uses it to create a noise wave that is identical to, but directly opposite of, the one coming into the ear. The “anti-noise” wave is output through a speaker (4), also located in the ear cup. When the two waves (the noise wave and the anti-noise wave) meet, the noise is significantly reduced (5).

Figure 3 is a photograph of an ANR ear muff. As you can see, the outer appearance of an ANR ear muff is virtually identical to that of a standard passive ear muff; however, ANR ear muffs deliver 20dB of electronic noise cancellation within the 20-800 Hz range. For the most complete hearing protection, these ear muffs also deliver a passive NRR of as high as 26. Additionally, these ear muffs feature an audio input so workers who are allowed to listen to music on the job can listen at safe volume levels.

Figure 3

Conclusion
ANR is the next technological breakthrough in industrial hearing protection because it remedies the noise problems that previously had no solution. There have been no significant advances in passive hearing protection, because passive is well understood and has been fully exploited. The only opportunity to provide workers with the next generation in hearing protection will be through electronic means such as ANR.

Currently available ANR ear muffs provide a greater level of hearing protection at a reasonable cost. As hearing loss continues to be a problem and noise standards become more stringent, we expect advanced hearing protection will be more widely adopted and even required in certain noise environments.

This article originally appeared in the June 2007 issue of Occupational Health & Safety.