Noise: We clamor for it at a ballgame and shun it in the library, yet it's always present. In fact, we only notice it when it's gone.

Most people are unaware of the effect noise has on their ability to concentrate. And they don't realize that the technology is available to remove it - at least, some of it.

Noise is the result of undesired sound traveling through the air as waves of differences in air pressure.

Improper selection, application, and installation of hvac systems are among the primary causes of acoustic problems indoors. Building and mechanical system renovations are another cause.

Currently, acoustic specifications are forcing engineers to model building designs to predict acoustic levels and guarantee that the design will meet specified levels. ASHRAE and the Air-Conditioning and Refrigeration Institute (ARI) are enacting standards to set acoustic guidelines and standardize equipment acoustic rating methods.

Traditional Attenuation

Until recently, only passive techniques controlled undesired sound and vibration.

Passive techniques, which don't use a power source, have long been thought of as a viable means of controlling unwanted sound and vibration coming from an hvac system. However, while passive techniques are effective at high frequencies, they become increasingly ineffective as the frequency is decreased. This is because passive technology for the control of sound and vibration depends upon a passive structure or device covering a relatively large fraction of an acoustic wavelength in length or thickness.

Traditionally, passive techniques called "duct silencers" have included various types of fibrous, porous packing materials to absorb noise energy levels.

A major drawback to passive techniques is that they are often applied too close to the fans or duct elbows, thus restricting airflow and causing additional pressure drops.

Active Silencing

The basic concept behind active noise control is to create an acoustic field equal in amplitude and frequency to the undesired noise, but opposite in phase. This opposing field cancels the noise field.

Active noise control is a proven solution to low-frequency noise (LFN) problems. The major advantage of active silencing (over passive) is its superior low-frequency attenuation with negligible airflow restriction. Decreased flow restriction equates to significant energy savings over passive techniques.

Additional benefits of active noise control are its light weight, its ability to be placed into existing duct arrangements and, significantly, its ability to shape the noise entering a room by selective attenuation1. The shape of the noise spectrum in a room determines the quality of the noise and, ultimately, the acoustical comfort in the room.

Here's how active attenuation works.

Say the operation of a fan generates sound pressure waves that travel down a duct or pipe. As sound passes the upstream input microphone, an electrical analog signal of the acoustic energy is transmitted to a controller.

Electronics within the controller convert the signal to a digital format. The controller calculates a signal that will be the mirror image of the noise when it reaches the loudspeaker, converts it back to analog, and sends the signal to the internal amplifier. The power amplifier output is sent to the loudspeaker, which converts the electrical signal to acoustic energy to cancel the duct-borne noise.

The error microphone, located downstream of the loudspeaker, detects any residual noise. The error signal is fed back to the controller and by continuous adaptation, cancellation of duct-borne noise is optimized.

The system is adaptive. The controller compensates for source changes, such as fan speed, and environmental changes. The controller maintains optimal cancellation by constantly adjusting for changes in the speed of sound and input and output transducer characteristics.

The controller model processes the digital signal to compensate for delay and amplitude changes from the input microphone to the loudspeaker. The signal is further processed to compensate for the frequency response of the loudspeaker.

The controller uses a recursive filter to compensate for acoustic feedback from the canceling loudspeaker to the input microphones2. With this filter, the controller creates an electronic model of the duct acoustics.

Noise detected by the input microphone is processed by the controller, which responds by playing the appropriate canceling sound just as the noise reaches the speaker downstream. This feed-forward approach is critical for cancellation of random or broadband rumble, such as that typically generated by hvac fans.

Active noise control systems can also continuously update the electronic model of the duct acoustics by playing low-level white noise through the cancellation speaker. This on-line calibration system uses a known signal to allow the controller to determine the characteristics of the loudspeakers, microphones, and the environment. The information is included in the transfer functions the controller applies to the input microphone signal, so that it can achieve the correct canceling sound wave3.

A Case Study

A pilot study was recently conducted by the University of Gothenburg, Sweden, Department of Environmental Medicine, to assess methods of evaluating the effects of LFN on performance. One goal was to look at objective and subjective effects on performance involving cognitive aspects.

Fourteen subjects with an average age of 26 years were exposed to two types of ventilation noises, midfrequency noise (MFN) and LFN.

The subjects were asked to perform three computerized cognitive tests. Test I was a rotation figure test, Test II a short-term memory test, and Test III a verbal reasoning test.

Tests I and II were performed together with a secondary task, which consisted of a set of four colored lamps, placed at four different positions at an arch in the periphery of the subjects' visual fields. Each of the four lamps was lighted at random intervals and at random positions. The subjects were to respond only to one of the four lamps (the yellow lamp). The object was to push the button with the color that corresponded to the lamp that had been lit before the yellow lamp.

The first session was always a learning encounter performed with background ventilation noise less than noise criteria ( The results showed that LFN was estimated to interfere more strongly with performance having an effect on cognitive elements. This was especially pronounced in the last parts of the tests, suggesting the effects appear over time. The reason for this may be a lower learning ability in the LFN, which may be due to a lower ability to habituate to the LFN condition.

The relationship between the reduced activity and reaction time, which was especially pronounced in the LFN condition, may also indicate that increased fatigue was a factor in the results. The consequences on mood in a work situation are difficult to postulate, but they may be of importance in work situations requiring teamwork and cooperation.

Although these results came from a small sample of subjects, those subjects were young, healthy students tested during a limited period of time, and at a level usually considered to be acceptable. It is therefore justifiable to continue studying the consequences of LFN over a full working day.

In many cases, optimum control of sound and vibration is most effectively and efficiently accomplished through the combined use of active and passive control techniques4.

Summary and Trends

In addition to excellent effectiveness at low frequencies, active control systems can offer low-airflow-restriction silencing, improved process or product quality, and increased productivity in the workplace.

The performance of active sound and vibration control systems is continuing to improve as new microprocessors, transducers, and algorithms are developed and application experience is gained.

The costs of active systems are continuing to come down, driven by design improvements and increased production volumes. The resulting improvement in the cost-benefit ratio for active control systems should ensure increasingly widespread application.

Any site that is sensitive to LFN is a prime candidate for active noise cancellation. This technology has been used successfully in cleanrooms, hotels, hospitals, recording studios, and meeting rooms. ES