James E. West

Noise levels in Johns Hopkins Hospital

This article presents the results of a noise survey at Johns Hopkins Hospital in Baltimore, MD. Results include equivalent sound pressure levels (L(eq)) as a function of location, frequency, and time of day. At all locations and all times of day, the L(eq) indicate that a serious problem exists. No location is in compliance with current World Health Organization Guidelines, and a review of objective data indicates that this is true of hospitals throughout the world. Average equivalent sound levels are in the 50-60 dB(A) range for 1 min, 1/2, and 24 h averaging time periods. The spectra are generally flat over the 63-2000 Hz octave bands, with higher sound levels at lower frequencies, and a gradual roll off above 2000 Hz. Many units exhibit little if any reduction of sound levels in the nighttime. Data gathered at various hospitals over the last 45 years indicate a trend of increasing noise levels during daytime and nighttime hours. The implications of these results are significant for patients, visitors, and hospital staff.

A beam tracing approach to acoustic modeling for interactive virtual environments

Virtual environment research has focused on interactive image generation and has largely ignored acoustic modeling for spatialization of sound. Yet, realistic auditory cues can complement and enhance visual cues to aid navigation, comprehension, and sense of presence in virtual environments. A primary challenge in acoustic modeling is computation of reverberation paths from sound sources fast enough for real-time auralization. We have developed a system that uses precomputed spatial subdivision and “beam tree” data structures to enable real-time acoustic modeling and auralization in interactive virtual environments. The spatial subdivision is a partition of 3D space into convex polyhedral regions (cells) represented as a cell adjacency graph. A beam tracing algorithm recursively traces pyramidal beams through the spatial subdivision to construct a beam tree data structure representing the regions of space reachable by each potential sequence of transmission and specular reflection events at cell boundaries. From these precomputed data structures, we can generate high-order specular reflection and transmission paths at interactive rates to spatialize fixed sound sources in real-time as the user moves through a virtual environment. Unlike previous acoustic modeling work, our beam tracing method: 1) supports evaluation of reverberation paths at interactive rates, 2) scales to compute highorder reflections and large environments, and 3) extends naturally to compute paths of diffraction and diffuse reflection efficiently. We are using this system to develop interactive applications in which a user experiences a virtual environment immersively via simultaneous auralization and visualization.

Charge dynamics for electron‐irradiated polymer‐foil electrets

Charge buildup, storage, and decay in 25‐μm polyfluoroethylenepropylene (Teflon type A) foils due to irradiation with partially penetrating monoenergetic electron beams in the 10–50‐keV range are investigated. Currents from the irradiated front surface and the nonirradiated rear surface are measured separately by means of 1000‐A evaporated aluminum electrodes. Charge storage due to irradiation and a charge recall effect caused by irradiation of previously charged samples are observed. Discharge currents measured after termination of the irradiation indicate the importance of the delayed component of radiation‐induced conductivity. Measurements of the spatial depth of the charge are also reported. A mathematical model is developed which takes into account the radiation‐induced conductivity and the electrical and geometrical parameters of the sample. It allows for the calculation of current transients during and after irradiation.

Self‐Biased Condenser Microphone with High Capacitance

A new kind of condenser microphone with solid dielectric between the electrodes is described. The system employs metallized dielectric foil electret and requires no external dc bias. The lower impedance eliminates the need of a cathode follower and allows the use of a long connecting cable. The frequency response of this system is flat within ±3 dB between 50 and 15 000 cps. The sensitivity of the system is −59 dBV for a sound pressure of 1 dyn/cm2 (74 db SPL).

Noise reduction processing arrangement for microphone arrays

A sound processing arrangement couples sound from a prescribed environment through a fixed microphone array to a signal processing arrangement having a specifiable preferred sound source location. The microphone pickup signals are combined with a set of weighting signals to adjust the directional response pattern in successive analysis time intervals. The weighting signals are modified in each analysis time interval so that the total acoustic signal power of the signal processing arrangement output signal is decreased toward a minimum while substantially unity power transfer of sound signals from said preferred location is maintained at all frequencies over a prescribed frequency range. In this way, the preferred source location is in the main beam while unwanted sound source locations are at the null points of the adjusted directional response pattern.

Nondestructive Laser Method for Measuring Charge Profiles in Irradiated Polymer Films

Relative charge-density distributions in the thickness direction of thin (>10¿m) polymer films are determined directly and accurately with a new laser-induced pressure-pulse (LIPP) method: A pressure pulse is generated in the sample by applying a short (70ps) and energetic (1-11mJ) light pulse from a Nd:YAG laser to a specially coated surface of the sample. Stress effects within this surface layer and possibly the recoil caused by ablation of some target material generate the desired pressure pulse of less than 1ns duration which propagates through the sample. From the electrode currents, the charge distribution can be evaluated. The new method is described in detail and some results for 20 to 50¿m thick films of Teflon FEP, Mylar PETP, and Kynar PVDF are given.

Foil‐Electret Microphones

Electrostaticmicrophones with foil electrets are described. Foil electrets are solid dielectric materials, such as fluorocarbon, prepolarized in an electrostatic field at an elevated temperature. The use of electrets eliminates the need for an external dc bias. The sensitivity of these microphones depends largely on the surface charge of the electret and is typically between −50 and −60 dBV for a sound‐pressure level of 1 μbar. The sensitivity of fluorocarbon electretmicrophones does not decay over periods of the order of years at room temperature. Extrapolation tn room temperature of results obtained at elevated temperatures indicates the time constant of the sensitivity of fluorocarbon microphones to be between 30 and 103 years, depending on the preconditioning (temperature cycling) of the systems. The relative humidity is of little influence on the time constant. Even at 80°C and high relative humidity, the time constant is still of the order of 100 days. The capacitance per unit area of electretmicrophones (about 60 pF/cm2) is three times greater than that of air‐gap systems. Because resonance frequency and sensitivity do not depend on the diameter of the system, a capacitance much higher than that of air‐gap systems can be achieved for a given bandwidth. The free‐field frequency response is within ±1.5 dB from 50 to 14 000 Hz when the microphone is terminated by a few megohms.

Investigation of piezoelectricity distributions in poly(vinylidene fluoride) by means of quartz‐ or laser‐generated pressure pulses

The direct determination of charge, polarization, or piezoelectricity profiles in thin dielectrics is now possible if piezoelectrically generated pressure steps or pulses and laser‐induced pressure pulses are employed. These recently developed high‐resolution methods were applied to the same piezoelectric poly(vinylidene fluoride) (PVDF) samples. Comparison of the respective results demonstrates the feasibility, the advantages, and the limitations of the new techniques for the study of piezoelectricity distributions. It is confirmed that, for relatively low poling fields, the piezoelectric activity of thermopoled PVDF foils is often confined to a layer near the positively biased surface. The same effect is found for poling with a positive corona discharge. For high‐field corona poling, the piezoelectric activity extends throughout the PVDF film.

Noise in the operating rooms of Johns Hopkins Hospital

Very little reliable information exists on the sound levels present in an operating room environment. To remedy this situation, sound pressure levels of the operating rooms in Johns Hopkins Hospital were monitored before, during, and after operations. The data were analyzed to determine background sound levels, average equivalent sound levels L(eq), frequency distribution, and peak sound pressure levels L(peak). Each surgery was matched to the period of noise it produced permitting the association of sound levels with particular types of surgeries and the determination of various sound measures for classes of surgery (e.g., orthopedic, neurological, etc.). Averaging over many surgeries, orthopedic surgery was found to have the highest L(eq) at approximately 66 dB(A). Neurosurgery, urology, cardiology, and gastrointestinal surgery followed closely, ranging from 62 to 65 dB(A). By considering the L(peak) along with the L(eq) values, a pattern emerges for the various surgical divisions. Gastrointestinal and thoracic surgery are relatively quiet among the surgical divisions. Neurosurgery and orthopedics have sustained high sound levels. Cardiology surgery has a more moderate average sound level but includes brief periods of extremely high peak sound levels. For neurosurgery and orthopedic surgery, peak levels exceeded 100 dB over 40% of the time. The highest peak levels routinely seen during surgery were well in excess of 120 dB.

CHARGING OF POLYMER FOILS WITH MONOENERGETIC LOW‐ENERGY ELECTRON BEAMS

Results of charge deposition on 12.7 and 25.4‐μm polymer foils with 10‐ to 40‐keV monoenergetic electron beams are reported. The trapped charges have densities of about 10−7 C/cm2, with a uniformity of ± 5% over the bombarded area. The time constants of the charge decay are typically of the order of tens of years. Thermally stimulated currents show a more densely populated high‐temperature portion than for foils charged with other methods.