Acoustics and Noise Control Research at the School of Mechanical Engineering, Purdue University

Acoustic (ăkau∙stik, ăkű∙-). a. 1605. 1. Pertaining to the sense of hearing, used in hearing; adapted to aid hearing, used in hearing; pertaining to the science of audible sounds. 2. as sb.  a medicine or appliance which aids hearing. In pl. : see ACOUSTICS.

Acoustics (ăkau∙stiks, ăkű∙-). sb. pl. 1683. [See Acoustic, -ICS] The science of the phenomena of hearing.

Noise (noiz). sb. ME (Middle English not Mechanical Engineering) … 3. A loud or harsh sound of any kind (defined by the context).

[Adapted from The Shorter Oxford English Dictionary.]

The Acoustics and Noise Control Research Group of the School of Mechanical Engineering are based at the Ray W. Herrick Laboratories. The Herrick Labs was established in 1958 as an interdisciplinary laboratory with the aid of a large grant from Ray W. Herrick, who was then President of Tecumseh Product Company. The labs were funded with a mission to instill cooperative enterprise between Purdue University and industry. In the past 50 years, Herrick Labs has emerged as one of the world’s renowned centers for their research and education program in Acoustics and Noise Control.

The research activities for acoustic and noise control at the Herrick Laboratories include the study of the wave propagation, voice production, vibration, human perception of sounds, control and abatement of environmental noise, and fluid dynamics as well as signal processing, numerical techniques, measurements, controls, and design theory applied to making our acoustical environment safer and more pleasant. Research applications include automobiles, water craft, aircraft, domestic products and appliances, heavy equipment, design of advanced acoustical materials for noise control, health care industries, trucks, compressors, heating/ventilating equipment, computers, and fans.

Research Focus

Both experimental and analytical research is performed within the Research Group. Much of the experimental research is focused on finding better ways to identify noise sources and paths. Investigators within the Group continue to work on improvement of near-field acoustical holography and other array processing techniques that can be used to visualize noise sources. Others are investigating better methods of using modern, high capacity data acquisition systems to understand complicated machinery and other noise generators. For experimental aeroacoustics, the Group has built a quiet wind tunnel where aerodynamic sources of noise, such as air flow over automobiles and fans can be studied.

Other experimental research is focused on development of methods to diagnose and predict potential problems using sound and vibration signatures.  These techniques are integrated into intelligent maintenance scheduling systems, and into control systems designed to manage and optimize safe operation of impaired systems and thus avoid catastrophic failure. These methods utilize advanced signal processing and data reduction techniques to identify fault-sensitive parameters.  Models are then developed for observation of fault propagation dynamics, and these are used to diagnose and predict fault behavior.

Research, in collaboration with faculty in Psychological Sciences and in Hearing Sciences, is aimed at understanding and quantifying human reaction to noise: to understand the characteristics of sounds that people respond to and how these characteristics combine to give a person an overall impression of sound. These psychoacoustic models are combined with acoustical source-path models to have people-focused optimization of acoustic environments.  Recent applications include quantifying annoyance due to environmental noise, the sound quality of refrigerators, HVAC systems, diesel engines, and chillers, and the impact of vehicle interior noise on speech intelligibility for older drivers.

Another collaborative research effort is in the design of compressors that use sound to control cooling.  This combination of thermal science and acoustics research is called thermoacoustics.  Recent effort has focused on the translation of successful small-scale experiments into a practical system that could be employed in applications such as refrigeration.  An important part of much of the research at the Laboratories is accepting and solving the scientific challenges that arise when developing practical engineered systems.  Another group of acoustics and cooling research projects are focused on lowering noise while maintaining electronics cooling  performance; these are part of the Cooling Technologies Research Center based in Mechanical Engineering .

Hybrid analytical/experimental methods are also pursued in order to find the fundamental  mechanisms that control the acoustic and vibration behavior of machines, systems and materials. Currently, these methods are used to understand noise generation in tires, window-seal assemblies in automobiles, information technology equipment, bearings, brakes, cooling fans, and in the human vocal tract and in artificial vocal fold systems.

Many of the analytical studies are intended to develop models that can be used to design machinery or acoustical treatments in order to avoid noise problems. A significant body of research work is ongoing to develop general purpose numerical procedures to complement popular finite element and boundary element methods. These efforts include new finite element procedures to predict the behavior of porous acoustical materials and new finite element and boundary element methods to predict structural/acoustic behavior at high frequency. Advanced numerical models are also developed for the prediction of sound propagation outdoors in a complex urban and sub-urban environment.

An important outcome of much of the acoustics group’s research is the development of new noise control treatment methods. This includes the design and optimization of the acoustical characteristics of both acoustical materials and composite systems of materials.  Applications include fuselage-trim assemblies for aircraft, car door treatments and low-noise wall construction for buildings.  In all research at the Laboratories, there is a strong interest in system performance in practical environments, and hence techniques are being developed for in-situ determination of the acoustical properties of sound absorption materials. . The Group is also involved in the investigation of active noise and vibration control approaches and “intelligent” noise control treatments: devices that adapt themselves to changes in operating conditions or the environment in order to maintain optimal performance. Examples include smart foams, adaptive engine mounts, and adaptive mufflers.

The Acoustics and Noise Control Research Program is also involved in a University Transportation Center, entitled the Institute for Safe, Quiet and Durable Highways (SQDH), sponsored by the U.S. Department of Transportation. This is a multi-disciplinary center, primarily with the Purdue University School of Civil Engineering, focused on finding techniques to make tires and pavement quiet while maintaining or improving current standards of safety, durability, and cost. As a result of the Institute, world class facilities have been built at Herrick Labs for testing tires and realistic pavement sections. It is expected that the research of the Institute will result in less dependence on noise barriers along highways and an improved environment particularly for people living  in heavily populated areas.

The Ray W. Herrick Laboratories faculty are also active in the Center of Excellence for the Partnership for Air Transportation and Noise Emission Reduction (PARTNER) program, supported by the Federal Aviation Administration (FAA). The main thrusts of these efforts are aimed at understanding and mitigating the effects of low frequency aircraft noise, examining existing and new metrics to quantify the effect of airport noise on communities, land-use around airports and website design for informing the public about airport noise.   A sponsored project by NASA in conjunction with FAA PARTNER program is the study of human annoyance to sonic booms. The perceptual attributes of transient events caused by shaped boom of supersonic aircraft are being examined. Further studies are concentrated on how these perceptual attributes lead to human annoyance in reaction to these events and to understand the response of the human auditory system to shaped booms.

Within Mechanical Engineering faculty in the Acoustics and Noise Control group interact very closely with the Mechanics and Vibrations program since noise radiation and transmission often result from the vibration of machine elements (e.g. the shells of compressors). The Ray W. Herrick Laboratories acoustics research is part of a broad range of acoustics research activity on campus.  Examples of research in this campus-wide group include: study of ultrasonic sound to promote thermonuclear fusion reactions (Nuclear Engineering);  speech quality and speech to text translation (electrical and computer engineering); hearing aid design and modeling of the auditory system including effects of hearing damage (Speech, Language and Hearing Sciences); impact of environmental noise on wildlife (Forestry); effects of noise on cognitive performance (Psychological Sciences); acoustics of pianos and guitars (Physics); biomedical acoustics and acoustics biosensors (Biomedical Engineering).