Robert D. Collier

Hybrid feedforward-feedback active noise reduction for hearing protection and communication.

A hybrid active noise reduction (ANR) architecture is presented and validated for a circumaural earcup and a communication earplug. The hybrid system combines source-independent feedback ANR with a Lyapunov-tuned leaky LMS filter (LyLMS) improving gain stability margins over feedforward ANR alone. In flat plate testing, the earcup demonstrates an overall C-weighted total noise reduction of 40 dB and 30-32 dB, respectively, for 50-800 Hz sum-of-tones noise and for aircraft or helicopter cockpit noise, improving low frequency (<100 Hz) performance by up to 15 dB over either control component acting individually. For the earplug, a filtered-X implementation of the LyLMS accommodates its nonconstant cancellation path gain. A fast time-domain identification method provides a high-fidelity, computationally efficient, infinite impulse response cancellation path model, which is used for both the filtered-X implementation and communication feedthrough. Insertion loss measurements made with a manikin show overall C-weighted total noise reduction provided by the ANR earplug of 46-48 dB for sum-of-tones 80-2000 Hz and 40-41 dB from 63 to 3000 Hz for UH-60 helicopter noise, with negligible degradation in attenuation during speech communication. For both hearing protectors, a stability metric improves by a factor of 2 to several orders of magnitude through hybrid ANR.

Lyapunov tuning of the leaky LMS algorithm for single-source, single-point noise cancellation

LMS algorithms have performance issues related to insufficient excitation, nonstationary reference inputs, finite-precision arithmetic, quantization noise, and measurement noise. Such factors cause weight drift and potential instability in the conventional LMS algorithm. Here, we analyze the stability and performance of the leaky LMS algorithm, which is widely used to correct weight drift. A Lyapunov tuning method is developed to find an adaptive leakage parameter and step size that provide optimum performance and retain stability in the presence of measurement noise on the reference input. The method accounts for nonpersistent excitation conditions and nonstationary reference inputs and requires no a priori knowledge of the reference input signal other than a lower bound on its magnitude or a minimum signal-to-noise ratio. The tuning method is demonstrated for three candidate adaptive leaky LMS algorithms. Stability and performance tradeoffs of each candidate Lyapunov tuned algorithm are evaluated experimentally in a single source, single-point acoustic noise cancellation system.

A review of the vibration and sounds of the crack of the bat and player auditory clues

The purpose of this paper is to review the state‐of‐the‐art in the acoustics of baseball. As is well known, the crack of the bat is an important phenomenon of solid wood bats and metal bats. Each has a very different sound signature. At the 148th meeting of the ASA in 2004, the author and coauthors Ken Kaliski and James Sherwood presented the results of laboratory and field tests, which showed that the spectral characteristics of radiated sound are dependent on the ball‐bat impact location and resultant bat vibration of both solid wood and tubular metal bats. These results will be reviewed together with those of other investigators in the context of player auditory clues and the player’s response in game situations.

Vibration and sound radiation of solid wood and tubular metal baseball bats as a function of ball–bat location

The ‘‘crack of the bat’’ is an important part of the game of baseball played with solid wood bats. The spectral characteristics of the radiated sound depend on the location of the ball–bat impact location along the length of the barrel and the resulting bat vibration. Balls hit on the sweet spot generally are hit harder and result in a distinct and recognizable ‘‘crack’’ sound. On the other hand, balls hit in on the handle or off the end of the barrel result in different modes of vibration and low‐frequency radiation, i.e., a ‘‘thunk‐like’’ sound. Analytical predictions and modal analyses, supported by both laboratory and field measurements, provide a more comprehensive picture of bat vibration and radiated sound relationships. Comparisons are made with tubular metal bats which exhibit the narrow‐band signature due to excitation of the cylindrical breathing (bell) modes which are essentially independent of hit location. The differences in bat vibration and sound radiation can provide important clues for b...

Experimental evaluation of leaky LMS algorithms for active noise reduction in communication headsets

An adaptive leaky LMS algorithm has been developed to optimize stability and performance of active noise cancellation systems. The research addresses performance issues related to insufficient excitation, nonstationary noise fields, and signal‐to‐noise ratio. The algorithm is based on a Lyapunov tuning approach in which three candidate algorithms, each of which is a function of the instantaneous measured reference input, measurement noise variance, and filter length, provide varying degrees of trade‐off between stability and performance. Each algorithm is evaluated experimentally for reduction of low‐frequency noise in communication headsets and compared with that of traditional LMS algorithms. Acoustic measurements are made in a specially designed acoustic test cell which is based on the original work of Shaw, Brammer and co‐workers and which provides a highly controlled and uniform acoustic environment. The stability and performance of the ANR system, including prototype communication headsets, are inve...

A low‐complexity environmental noise monitoring system for unattended operation in remote locations

There is a need to measure environmental noise in remote locations such as national parks over extended periods of time with a simple, readily deployed, robust, unattended data acquisition system. The original Dartmouth College prototype system was demonstrated in 2001 [Kaliski et al., Proceedings NOISE‐CON 2001, October 2001]. The improved Dartmouth system is described in a patent awarded to Dartmouth College in August 2006. The system conforms to ANSI/IEC Type I and Type II standards. It samples, processes, and stores 1‐s A‐ and C‐weighted equivalent sound level (Leq) values over a period of about 2 weeks. In addition, maximum, minimum, and statistical percentile values are stored in nonvolatile memory. The instrument is powered by a rechargeable battery backed up by a solar power converter. A communication interface provides for remote control. Separate software is provided to display statistical information and detection and classification clues based on user‐selected time period events. The capabilit...

Active noise reduction communication earplug for helicopter crew

A custom designed shallow insert communication earplug for helicopter crews consists of a system based on a patented hybrid ANR feedback‐feedforward algorithm [Ray et al., J. Acoust. Soc. Am. 120(4), 2026–2038 (2006)]. The hybrid system provides the benefits of feedback ANR while extending the bandwidth and magnitude of total ANR performance with feedforward ANR, which improves attenuation of tonals. The development includes optimized algorithms, a custom earplug, and a miniaturized battery powered ANR module, weighing approximately 250 g. The hybrid ANR earplug requires two miniature microphones, one inside and one external to the earplug and an internal speaker to deliver the cancellation signal. The software automatically adapts the ANR algorithm to the transfer function characteristics of the system to both improve performance and accommodate individual human fitting differences. The testing protocol and results are based primarily on measurements with a HEAD Acoustics manikin in the Sound Innovations sound room [Duncan et al., Proceedings of Internoise 2006, December 2006]. MIRE testing is planned for the Spring of 2007. Results to‐date demonstrate comparable passive attenuation to a commercially available communication earplug and an added average active attenuation of approximately 6 dB for UH‐60 helicopter noise. [Work supported by U.S. Army.]

Modal analysis and computer aided design studies of acoustic guitar structural designs

This study seeks to systematically identify modern Computer Aided Design (CAD) techniques to optimize acoustic guitar design. The study uses traditional design as a starting point for analysis of variations in materials and bracing structures of the front sound plate. CAD techniques are used to model the guitar and provide a broadband input that allows analysis of its modal response. Design optimization occurs in creating the most favorable modal response of the instrument while still maintaining structural integrity through effective support of the forces created by the strings’ tension in the bridge and neck. The measures chosen to help identify the most favorable response are modal density, magnitude of the response at each mode, and the locations of the largest magnitudes. Increased modal density provides a richer timbre while the amplitude of the response determines the relative intensity of the sound projected. Increased modal response in the upper bout of the instrument can improve the timbre of pitches in the upper frequency range of the instrument. After analyzing these measures for multiple variations and iterations of a selected set of structural parameters, an optimal design is suggested. The results show good agreement between experimental modal measurements and computer‐aided design modeling.

Compact test method for the evaluation of acoustical transmission loss and insertion loss of new helmet material samples

There is a need to establish a simple and accurate measurement technique for determining the transmission loss of sample materials for helmets over a frequency range of 300–3 kHz. Standard methods, e.g., ASTM E 90‐02, for measuring transmission loss of building materials and structures, based on adjacent reverberation chambers, are too costly and impractical. A 1.22‐m‐long double‐wall tube, packed with Owens Corning R13 insulation, has been fabricated using QUIK‐TUBETM cardboard concrete forms of 200 and 300 mm diameters. A circular sample of material, also 300 mm in diameter, is placed on the end of the tube and subjected to an external sound field. Transmission loss is established by external and internal microphones. This paper describes the measurement and analysis procedures and examines the associated variables and error terms. Results are presented for 16 material samples with surface weights covering a range from 0.3 to 14.7 kg/m2 and compared with analytical predictions including mass law models. The acoustical characteristics of commercial helmet materials and liners are evaluated in the context of hearing protection systems. The transmission loss measurement procedure has the potential for meeting standardization objectives.

Hybrid feedforward‐feedback active noise control for circumaural headsets

Traditional stability‐performance tradeoffs pose limitations on active noise reduction (ANR) using feedback control, which are evident in circumaural communication headsets. In these systems, the cavity resonant behavior necessitates low feedback gains, redcing performance. Feedforward ANR using a Lyapunov‐tuned least‐mean‐square filter dramatically enhances noise reduction performance compared with feedback ANR [Cartes et al., J. Acoust. Soc. Am. 111, 1758–1771 (2002); Collier et al., NOISECON (2003)]. However, feedforward performance is sensitive to the noise source stationarity, and the frequency‐dependent forward path gain reduces stability margins. This paper presents experimental results for a hybrid feedforward‐feedback ANR system, which enhances performance and gain margins for both stationary and nonstationary noise. Algorithms are optimized and measurements are performed with Thayer’s rapid prototyping system and associated low‐frequency acoustic test cell using a circumaural hearing protector. In the frequency range 50–800 Hz, the hybrid system provides an average of 27 dB active noise reduction (40 dB total) for tonal noise and 17 dB reduction (32 dB total) for nonstationary noise. Performance below 100 Hz improves by as much as 15 dB over that of individual control components, and gain margin of the hybrid system improves substantially over individual feedforward or feedback components.