Transformer Hum Research Articles

Extracting Electrical Network Frequency From Digital Recordings Using Frequency Demodulation

Electric power system instantaneous frequency, also referred to as electrical network frequency (ENF), pollutes digital recordings due to a variety of factors (e.g., transformer hum). This unwanted interference has been used as a forensic tool in conjunction with a database of ground truth ENF measurements to determine the time at which a recording was made, as well as to detect tampering. Recent work has suggested that that ENF could be used to geolocate a digital recording under the assumption that the low-energy location-unique power system dynamics are observable, which is not always the case in the presence of noise and interference. This letter aims to take advantage of the intrinsic nature of power system instantaneous frequency in order to frame the ENF extraction problem as one of FM demodulation. By considering the ENF as a sinusoid of nominal power system frequency that is frequency modulated by a weak stochastic signal, a 0-Hz Intermediate Frequency (IF) signal can be created and analyzed rather than direct measurements. In doing so, significant data reduction can be achieved through extensive downsampling, FM demodulation algorithms can be applied to extract the ENF, and the spectrum of the IF signal provides a straightforward way of estimating the maximum observable frequency content of the ENF buried in a digital recording. These findings are supported through the analysis of audio recordings made in the presence of ENF interference in the U.S. Eastern Interconnection power system.

Multi-Domain Processing for Micropower Sensors

Multi-Domain Processing for Micropower Sensors

Cooperating with Miguel on improvements of the acoustical product - SOUNDBLOX

Some 15 years after joining Miguel's consulting firm Cambridge Acoustical Associates around 1960 he asked me to help him with improving the sound absorbing quality of a patented slotted concrete block. The original concept of the block, sold under the trademark, SOUNDBLOX, was create a Helmholtz resonator by providing a slot in one face of a standard concrete block whose natural frequency would match the fundamental frequency of a common noise source, transformer hum. A broader absorption spectrum was desired to expand the rather limited applications of the original design to compete with acoustical materials widely used in schools, gymnasiums, auditoriums, and swimming pools. This presentation will describe the nature of our cooperation and the successes and failures of various concepts. The fact that the product, first introduced in the late 1950's is still on the market, reflects well on our collaboration.

Determination of solar cycle variations of midlatitude ELF/VLF chorus and hiss via automated signal detection

[1] An automated algorithm for detecting chorus and hiss emissions in ground-based extremely low frequency/very low frequency (ELF/VLF) wave receiver data is developed and applied to 10 years of data collected at Palmer Station, Antarctica (L = 2.4, 50°S invariant latitude). The algorithm consists of three major processing steps. First, sferics and power line hum are removed from the broadband data. Second, individual events are detected and a set of 19 scalar event parameters are determined. Finally, on the basis of the parameters, detected events are categorized by a sequential pair of neural networks as either chorus, hiss, or noise. The detector runs on a modern 8-core computer at a speed of 350x real time. Results of training indicate that the neural networks are capable of differentiating between noise and emissions with a 92% success rate and between chorus and hiss with an 84% success rate. Data collected at Palmer from May 2000 to May 2010 were processed, and yearly and seasonal trends of chorus and hiss are analyzed. Yearly occurrence rates of chorus and hiss are strongly dependent on the geomagnetic disturbance level, as measured by Kp and AE, whereas seasonal occurrence rates are more strongly dependent on variations of the day/night terminator and associated variations in ionospheric absorption.

Discriminator design considerations for time-interval measurement circuits in collider detector systems

An analysis of timing jitter from leading-edge discriminators (LED's) and constant-fraction discriminators (CFD's) is presented. The jitter is calculated for the cases of random white noise, coherent noise (e.g., digital switching noise or ac power-line hum), and mixed (random and coherent) noise for both discriminator architectures. A general jitter equation valid for all of these conditions is derived. It is shown that the discriminator bandwidth for minimum jitter is strongly dependent on the amount of coherent noise. This effect is shown to be more pronounced for the LED. Even though off-line walk adjustments are possible for many timing measurements, the CFD is shown to provide a major advantage by acting as a correlated double sampler that removes much of the coherent noise expected in large, multichannel collider detector systems.

A noise-rejecting current amplifier for surface atmospheric ion flux measurements

Large area electrodes are required to determine atmospheric charge fluxes at the earth’s surface. These also act as antennas for power line hum, swamping the small atmospheric electric signals. A battery-powered head-amplifier resolving 0.025 pA is described here, which uses active compensation to substantially cancel the interference, allowing the atmospheric charge fluxes to be resolved in the presence of the interference. The overall response is nominally −100 mV/pA, with 30 dB of 50 Hz rejection. The head amplifier can drive a low impedance line.

IBM PC Music Software: LTA Productions FORTE II Sequencer, FWAP!, and TrackGenie

with the unit a visual pleasure. A pair of MIDI jacks (IN and THRU) is provided with the DRP-20, but the MIDI implementation on the test unit we received was minimal. We understand that the MIDI implementation on new units is significantly enhanced. Still, there is, as yet, no available Macintosh, Atari, or PC-based editor program for the DRP-20, in contrast to established effects units like the Lexicon PCM-70, for example. The DRP-20 is packaged as a 2 um rackmount box weighing a mere 5.5 kg (12 lb). There is no fan-and so no fan noise-and we heard no transformer hum. Contact: Dynacord, Hirschberger Ring 45, D-8440 Straubing, West Germany, telephone 09421 / 706-0, telex 65520. In the USA, contact Drum Workshop, 2697 Lavery Court, 16 Newbury Park, California 92320 USA, telephone (805) 499-6863.

Active systems for global attenuation of noise

Theoretical approaches to active noise absorption [G. A. Mangiante, J. Acoust. Soc. Am. 61, 1516–1523 (1977)] provide criteria that may prove useful for the design of any system for active noise attenuation, especially one which must produce attenuation in all directions around a complex sound source. (1) Topological fitness: a proper net of “anti‐sources” ought to surround completely either the primary noise source(s), or the space to be silenced. (2) Directivity: each anti‐source ought to radiate only towards the space to be silenced. (3) Concentration: practical system design requires substituting the theoretical continuous distributions of anti‐sources by discrete ones. This approach has been successfully applied to reducing the audible hum of an actual electric substation transformer. The active abatement method is easier for discrete frequencies, such as the spectrum of transformer hum, and is most effective at the relatively low frequencies—120, 240, 360, and 480 Hz—most prominent in the transformer hum spectrum. A relation has been found experimentally for the attenuation achievable versus the number of sources for each of these hum frequencies. The active attenuator interferes less with the normal ventilation and cooling of a transformer than do passive methods. Active abatement also allows attenuation differing in different directions.

Control of transformer noise at Ontario Hydro

The general philosophy is reviewed for controlling transformer hum in the community to acceptable levels. The various considerations in the design of acoustic treatments e.g., vibration isolation, walls combining high transmission loss and absorption, short ventilation ducts with high insertion losses, and the isolation of oil lines are discussed. Treatments for large units up to 750 mVA 500 kV working, having insertion losses up to 30 dBA are described.

Power Transformer Noise Its Characteristics and Reduction

Audibility characteristics of the ear and the units used for measuring noise are briefly discussed. A theoretical investigation was made to determine the different force components and other causes of transformer hum. Surface vibration measurements were made on transformer parts to determine the sources which cause noise in transformers and how these noise sources are afected by variations in operating conditions. Noise surveys were made to measure the range of intensities, the frequency characteristics and distribution of sound from transformers in service. The effects of substation acoustic conditions on noise distribution were investigated.