Scott Anthony Morgan

Investigation of the mechanisms of low‐frequency wind noise generation outdoors

Simultaneous measurement of wind noise and the instantaneous wind speed were performed for bare and screened microphones outdoors. Analysis of these measurements demonstrates that the dominant source of pressure fluctuations at the microphone outdoors is the intrinsic turbulence in the flow. This is in contrast to the results of measurements performed in low‐turbulence environments by Hosier and Donavan [Natl. Bur. Stand. Rpt. NBSIR79‐1599 (Jan. 1979)] and by Strasberg [J. Acoust. Soc. Am. 83, 544–548 (1988)]. For low‐turbulence conditions the fluctuating wake of the screen is the dominant noise source. This finding has important implications for windscreen design for outdoor measurements since the principles described by Hosier and Donavan apply only to low‐turbulence conditions.

Speech command input recognition system for interactive computer display with speech controlled display of recognized commands

The present invention provides a solution for users of voice recognition systems who still need visual feedback in order to confirm the accuracy of spoken commands but need to operate in a “hands-off” mode with respect to computer input. In an interactive computer controlled display system with speech command input recognition, the present invention provides a system for confirming the recognition of a command by first predetermining a plurality of speech commands for respectively designating each of a corresponding plurality of system actions and providing means for detecting such speech commands. There also are means responsive to a detected speech command for displaying said command for a predetermined time period, during which time the user may give a spoken command to stop the system action designated by said displayed command. In the event that said system action is not stopped during said predetermined time period, the system action designated by said displayed command will be executed. The user need not wait for the expiration of the time period if he notes that the displayed command is the right one; he has speech command means for executing the system action designated by said displayed command prior to the expiration of said time period. This may be as simple as just repeating the displayed command.

Low‐frequency wind noise for a microphone inside a spherical foam windscreen

Windscreens are commonly used to reduce wind noise in outdoor measurements by shielding the microphone from the incoming flow. In this paper a theoretical model for wind noise reduction of a spherical foam windscreen and experimental evidence supporting this model are presented. Results show that wind noise reduction is approximately independent of windscreen diameter for turbulence scale sizes that are much larger than the windscreen and that wind noise reduction scales well with the screen number (defined as the ratio of the windscreen diameter to the scale size of the turbulent eddies) for screen number values less than one. Calculations using average windscreen surface pressure measurements yield wind noise reductions that are of the same order of magnitude as those measured.

Noise production by turbulent flow over an unscreened measurement microphone

In previous work, it has been shown that the wind noise in an unscreened microphone placed in a low‐speed turbulent flow is caused by the turbulence intrinsic in the flow [S. Morgan and R. Raspet, J. Acoust. Soc. Am. 92, 1180–1183 (1992)]. The theory of pressure fluctuations in a turbulent flow as related to wind noise in an unscreened measurement microphone is discussed in this paper. A Poisson’s equation for fluctuation pressure in terms of mean and fluctuation velocity derivatives with respect to spatial dimensions results from the theory. This implies that the mean and fluctuation velocities must be known at every point in space in order to solve for the fluctuation pressure at a given point; however, through comparison of theory with experimental data, it is shown that the fluctuations in the boundary layer near the microphone where velocity gradients are very large provide the major source of wind noise. An approximation method for estimating the contribution of this ‘‘local interaction’’ is also di...

On the placement of microphones for outdoor measurements

Acoustic measurements outdoors typically involve microphones located either on the surface or on stands which hold the microphone approximately 1.0 m above the ground. Arguments given for placing microphones on the surface include lower wind noise and ease of installation. Quite often microphones on the surface are mounted above a large solid sheet. In this talk, the effects of wind noise and ground impedance on the optimum design of measurement systems for different frequency and source position are considered.

An analysis of low‐frequency wind noise in turbulent environments.

Simultaneous measurement of wind noise and the instantaneous wind speed were performed for bare and screened microphones outdoors. Analysis of these measurements demonstrates that the dominant source of pressure fluctuations at the microphone outdoors is the intrinsic turbulence in the flow. This is in contrast to the results of measurements performed in low turbulence environments by Hosier and Donavan and by Strasberg. For low turbulence conditions the fluctuating wake of the screen is the dominant noise source. This finding has important implications for windscreen design for outdoor measurements since the principles described by Hosier and Donavan apply only to low turbulence conditions.

Analysis of wind noise measurements

Wind noise in outdoor microphone measurements is composed of both the noise due to intrinsic turbulence in the flow and that due to the flow interacting with the microphone. This paper presents the analysis of wind noise and turbulence data taken in an outdoor environment in an effort to determine the primary source of the wind noise. Bare microphone data are compared with data taken during the same time period from microphones covered with a nose cone, a Bruel and Kjaer spherical windscreen, and a variety of experimental windscreens developed at the University of Mississippi. The theory behind these experimental screens is also presented and is compared to the experimental data. Conclusions concerning the dominant source of wind noise are drawn by examining microphone data and turbulence data and comparing them with published microphone data. The effectiveness of different windscreens is also examined. [Work supported by MIT Lincoln Laboratory.]