Glenn E. Warnaka

Noise reducing system for voice microphones

A conventional voice microphone placed in non-critical spaced relation to a source of intelligible speech sound while exposed to an acoustical field of ambient noise, electrically transmits output signals attenuated under control of a signal processing controller to which a sampled input of noise signals is fed by a reference microphone exposed to the same acoustical noise field as the voice microphone for audio reproduction of the speech sound without background noise by programming of the controller.

Structure for absorbing acoustic and other wave energy

A structure for absorbing wave energy, particularly acoustic wave energy, which includes a first essentially planar surface against which wave energy is directed and a second essentially planar surface generally parallel to and spaced from the first surface. Wedge-shaped elements of sound-absorbing material are disposed between the planar surfaces. All of the wedge-shaped elements, which may be of triangular cross section or pyramidal, have generally flat base portions which form said first planar surface against which wave energy is directed. The apex portions of the wedge-shaped elements are in substantial abutment with the second planar surface. The invention has utility in anechoic applications; however, in contrast to prior art structures of this type, the apex portions of the sound-absorbing material face away from incident wave energy rather than facing toward it.

A different look at the anechoic wedge

Anechoic wedges are conventionally installed with the tapered point of the wedge pointing toward the test space and with the flat base of the wedge either against a rigid wall or separated from the wall by an air gap. A comparative investigation has been carried out in which the wedges were first conventionally oriented and then turned 180° so that the points of the wedges faced the wall. These comparative tests were performed in reverberation rooms both here and in Europe and also in two different impedance tubes. The results are extremely interesting. They tend to indicate that a ‘‘reversed’’ anechoic wedge can give a lower cutoff frequency than when conventionally oriented. Some possible explanations are offered for the measured performance.

Geometric sound absorbers

Acoustic absorption is a commonly used technique for noise reduction. Most acoustically absorbent treatments use fibrous or porous materials. In general, such treatments are not rugged and have limitations in many applications. For example, fibrous and porous materials can retain pollutants present in the environment. This leads to unsanitary and unhealthy conditions and to fire and explosion hazards. This paper describes novel geometric acoustic absorbers, based on closely coupled quarter‐wave resonators that provide a high level of acoustic absorption over a broad range of frequency. Because the absorbers depend on their geometry for absorption, the absorbers may be made of any material and, hence, the material may be chosen on the basis of cost, environmental resistance, ruggedness, etc. Further, the absorbers can be cleaned when required to eliminate the presence of contaminants. Cleaning can be accomplished by conventional means such as washing, vacuuming, high‐pressure air, etc. Two absorbers are de...

Improving the intelligibility of aircraft PA systems

The speech intelligibility of commercial aircraft audio systems is of great importance. The authors have noted however, that subjectively, the intelligibility of such systems can vary considerably. Measurements made on a number of commercial flights indicate that signal to noise ratios are often around 0 dBA and frequently negative. As aircraft loudspeakers are located relatively close to the passenger, the collapsing narrower dispersion and beaming of the high frequencies from conventional cone loudspeakers can cause loss of intelligibility for passengers not on the axis of the loudspeaker and produce uneven sound distribution within the cabin. Distributed mode loudspeakers (DML) have demonstrated intelligibility superior to conventional loudspeakers in other applications such as public address systems and the opportunity was taken to evaluate their performance within an aircraft cabin environment. DMLs are very light in weight and can even be built into the internal trim, resulting in a near zero weight...

Intelligibility of aircraft loudspeakers

Commercial aircraft audio systems provide audible messages including greetings, details of the flight, comments on entertainment, and important safety information. Hence, the speech intelligibility of aircraft audio systems is of great importance. Distributed‐mode loudspeakers (DMLs) are known to have wider sound dispersion, especially at higher frequencies, than conventional cone loudspeakers. As aircraft loudspeakers are located relatively close to the passengers, the narrower dispersion and beaming of the high frequencies from conventional cone loudspeakers can cause loss of intelligibility for passengers not on the axis of the loudspeaker and produce uneven sound distribution within the cabin. DMLs have demonstrated intelligibility superior to conventional loudspeakers in other applications such as public address systems. DMLs have other advantages in aircraft installations. They are very lightweight and can even be built into the internal trim, resulting in a nearly zero‐weight loudspeaker. Their ess...

New geometric sound absorbers

In a previous paper of the same name, two new geometric sound absorbers were discussed. The geometric absorbers use closely coupled quarter‐wave resonators to absorb sound. Because the structures absorb sound by their configuration, they do not require the use of conventional porous or fibrous sound‐absorbing materials and can be made of nearly any substance such as plastic, wood, metal, etc. The absorbers can be made very rugged and long‐lasting for industrial, military, and outdoor use. The previous paper discussed two basic sound absorbers, configuration delta and flat absorber, but the paper concentrated on the former sound‐absorber configuration. This paper will review configuration delta but will concentrate on describing the structure and performance of the flat absorber. This absorber can be made in a thin treament, less than 25 mm thick, that provides high acoustic absorption. A number of different designs will be shown, and the results of standing wave tube and reverberation room tests will be g...

Prospects for active noise control in power plants

Active noise control has been applied in power plants to suppress noise from a variety of sources. Applications have included combustion fans, large exhaust stacks, emergency diesel generators, dust collectors, high temperature exhausts, etc. Active noise control systems have been used to control the noise from fan blade passage frequencies, turbine blade frequencies, and other similar noises. In addition, tests have been successful in suppressing random noise, especially at low frequencies. Active noise control has also been used to extend high‐frequency attenuation beyond the usual frequency range by using the active noise control systems to spoil the symmetry of the duct and so prevent cross‐mode formation. In other cases, active noise control has been coupled with known elements of passive noise control to achieve synergistic results. Active noise control sources can be protected from difficult environments such as high‐flow velocities, weather, and high temperatures by using waveguides or Helmholtz r...

The current status of the development of an active noise control microphone

This paper describes the current status of the development of a simple unitary active microphone that provides superior noise cancellation. The microphone, based on active noise control principles, is small and does not require an independent sample of the ambient noise. At the same time, there is no requirement for locating the microphone near the speakers mouth, as required with conventional passive noise canceling microphones. Instead, the microphone under development can be placed several feet, or more, from the speaker, since superior performance of the new microphone can still be realized. The microphone element can discriminate between the voice and noise even at a distance without voice recognition algorithms, and all kinds of noises, including random noise, can be canceled. A simple electronic control unit can be used to produce commercial systems.

An active noise canceling microphone

This paper describes a noise canceling microphone that uses the principles of active noise control. Both physical (acoustic) cancellation, by means of a loudspeaker, and electronic cancellation have been applied to the problem. The results show that a high level of performance may be obtained while removing the requirement to have the microphone extremely close to the lips. Excellent cancellation was achieved at a distance of 24 in. from the signal source using tones, harmonic noise, pink noise, and a variety of cockpit noises from a helicopter, a turbo‐prop, and a jet fighter. Conventional microphones may be used to implement the active noise canceling microphone, or a conventional noise canceling microphone may also be used as a basis for implementation of this concept. The active noise control microphone is superior to conventional noise‐canceling microphones in rejecting background noise.