Volume 4, Issue 3, July 2008
Index of content:
4(2008); http://dx.doi.org/10.1121/1.2994726View Description Hide Description
Signal processing in acoustics is based on one fundamental concept—extracting critical information from noisy, uncertain measurement data. Acoustical processing problems can lead to some complex and intricate paradigms to perform this extraction especially from noisy, sometimes inadequate, measurements. Whether the data are created using a seismic geophone sensor from a monitoring network or an array of hydrophone transducers located on the hull of an ocean‐going vessel, the basic processing problem remains the same—extract the useful information. Techniques in signal processing (e.g., filtering, Fourier transforms, time‐frequency and wavelet transforms) are effective; however, as the underlying acoustical process generating the measurements becomes more complex, the resulting processor may require more and more information about the process phenomenology to extract the desired information. The challenge is to formulate a meaningful strategy that is aimed at performing the processing required, even in the face of these high uncertainties. In this article, we briefly discuss this underlying signal processing philosophy from a “bottoms‐up” perspective enabling the problem to dictate the solution rather than visa‐versa. Once accomplished, we ask ourselves the final and telling question, “Did it work or are we kidding ourselves?” Are the results science or are they science fiction?
4(2008); http://dx.doi.org/10.1121/1.2994724View Description Hide Description
The late eighteenth century witnessed many revolutions in both science and society, but one of the former remains relatively unsung—the revolution in binaural hearing. The experimental endeavors of Ernst Florens Friedrich Chladni (1756–1827) are well known. Indeed, Chladni has been called the father of acoustics. Chladni investigated the characteristics of vibrating strings and plates and advanced the physical analysis of sound. However, he paid relatively little attention to the then contemporary deliberations on binaural hearing. In his Akustik of 1802 he did cite the experiments of Giovanni Battista Venturi (1746–1822), and repeated Venturi's suggestion that localization of sounds was dependent upon inequalities at the two ears. Despite the fact that between 1796 and 1802, Venturi described essentially the same four experiments in French, German and Italian, few (other than Chladni) took note of them. Even earlier, in 1792, William Charles Wells (1757–1817) examined some theoretical aspects of binaural hearing. We here describe the work of Wells and Venturi on binaural hearing, as well as that of others in the early nineteenth century, particularly Alison and his stethophone. The stethophone was invented in 1859 to listen to different sounds separately with each ear. It was the auditory equivalent of the stereoscope. In much the same way that binocular vision was studied theoretically and experimentally before the invention of the stereoscope, binaural hearing was examined before suitably selected sounds could be delivered to each ear by means of the stethophone. The early studies on binaural hearing were informed by comparisons with binocular vision, and so we introduce this history with a contrast between these two aspects of integrated perception.
Managing Acoustic Feedback: Micro Electro Mechanical Systems (MEMS) Contact Microphones for Musical Instruments4(2008); http://dx.doi.org/10.1121/1.2994725View Description Hide Description
The integrated circuit wafer fabrication process has matured and scaled rapidly since the invention of the transistor. Semiconductor technology has the major advantage of scale and reproducibility; many thousands of identical chips can be produced on one wafer and the repeatability from wafer to wafer is very well controlled. Silicon integrated circuit technology now has the capability of creating device geometries of less than 100 nanometers. Micro Electro Mechanical Systems (MEMS) is a technology that builds on the core silicon fabrication infrastructure that has been developed for the integrated circuit industry. Micromechanical structures are created on a silicon wafer by etching defined patterns on a silicon substrate to form core sensor elements or mechanical actuators that can move fractions of a micron. Pressure sensors were one of the first high‐volume applications and hundreds of millions of MEMS pressure sensors are now in use in applications such as pressure‐sensing in engine manifolds and tire pressure monitoring systems. MEMS accelerometers have been used in automotive applications for over 15 years as crash sensors for airbag deployment, for rollover detection and car alarm systems. More recently MEMS accelerometers are used for motion sensing in consumer applications such as video games and cell phones. MEMS micro‐mirror optical actuators have found use in overhead projectors and projection televisions. In recent years MEMS microphones have begun to proliferate in the broad consumer market including cell phones, Bluetooth headsets, personal computers and digital cameras. In university and industry‐based research departments today there is significant investment in bio‐MEMS which is focusing on the use of micro‐ or nano‐technologies to execute diverse applications from DNA testing to disposable medical diagnostic kits. This article summarizes some of the key technologies deployed in MEMS accelerometers and then discusses how this technology may bring a new dimension to acoustic transducers for musical instruments.