Noise cancellation

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Engineering Silence: Active Noise Cancellation

Michael Benoit Christopher Camastra Kimberly Li Richard Romanowski

Melissa Kenny Kevin Shannon

In order to create active noise canceling headphones, a pair of Sennheiser HD 202 headphones was modified withmicrophones and a series of op-amp circuits. Attenuation of low frequency ambient noise was successfully observed.

Noise cancellation technology is aimed at reducing unwanted ambient sound, and is implemented through two different methods. The first of these is passive noise cancellation: an approach that focuses on preventing sound waves from reaching the eardrum, and includesdevices such as circumaural headphones or earbuds [3]. The other technique used to achieve the same – and often better – result is active noise cancellation, which uses aural overlap and destructive interference to target and attenuate background noise. While passive and active noise cancellation may be applied separately, they are often combined to attain maximum effectiveness in noisecancellation.

Although active noise canceling devices are still being integrated commercially, the concept has existed since the beginning of the 20th century. In 1933, a German patent was issued to a Paul Lueg for the concept of active noise cancellation; he was the first to realize the possibility of attenuating background noise by superimposing a phase flipped wave [5]. In the 1950s, Olsen successfullydemonstrated Lueg’s concept in rooms, ducts, and headsets [5]. Research and development of active noise reduction (ANR) headphones truly began in 1978 after Dr. Amar Bose felt the need to develop headphones that masked the low rumbling of plane engines and other cabin noises [6]. With the invention of integrated circuits – opamp circuits – and miniature microphones, the existence of ANR headsetsbecame increasingly probable. ANR headsets were first used in the Armed Forces, and constantly develop in versatility. However, they are not used in all military organizations and are only slowly being commercially released [2]. Current applications include noise propagation in industrial air handling systems, reduction of propeller noise in aircrafts and tonal noise from electric power, as wellas isolation of vibration from noise

radiating structures. Even with these applications, the transition of active noise control from the laboratory to the market is far from complete [4]. A hands-on implementation of noise canceling headphones provides for a strong understanding of the conceptual framework of both passive and active noise canceling. In addition, it allows an introduction to thebasic components and operations of analog circuits. In particular, attention is given to the theory behind resistor networks, op-amp circuits, and filtering circuits. Secondary goals of the project include learning about the importance of circuit design, using oscilloscopes to understand and test circuits, and mastering the skills involved in building circuits.

semi-circumaural ear pads [10].In order to achieve active noise cancellation, a circuit mainly comprised of a series of op-amps and microphones was added to the headphones for active noise cancellation. Circuit Laws Before constructing the noise canceling circuit, a general comprehension of circuitry was required to fully understand how the specific noise canceling circuit operates. Every circuit contains various components,each of which have correlative quantities; these quantities are often measured during circuit construction and may be altered in order to produce the proper results. The most common of such quantities include the voltage, current, and resistance of the circuit components. The voltage represents the potential difference needed to move a unit of charge across any circuit component. The current...