This invention relates to a sound-field-modifying structure and more particularly to a sound-field-modifying structure that makes use of air-coupled surface waves to provide noise reduction, spectral shaping, or sound amplification.
The modification of sound fields using passive, physical structures is useful in many application areas. These include applications where noise reduction or attenuation is the main goal, as with highway noise barriers or sound-absorbing ceiling tiles. In other applications, sound amplification is desired, as with parabolic dish microphones. And others involve attenuation in some frequency bands resulting in relative amplification in others, i.e., spectral shaping of sounds, as with the design of concert halls. Typical strategies include the use of porous damping materials, the incorporation of Helmholtz resonators, and the use of barriers, shaped reflectors and diffusers.
It is also possible to make use of an entirely different physical mechanisms, such as air-coupled surface waves, to achieve improvements in performance in all of these areas. Air-coupled surface waves form and propagate over porous surfaces that have been designed to have appropriate acoustic impedance. Acoustical energy collects into the surface wave and is localised close to the surface as it propagates over the surface. These structures are useful for sound attenuation through the introduction of acoustically absorbing materials into sections of the surface wave structure, so that acoustical energy is trapped into a surface wave and then dissipated by the absorbing materials. Thus, improved noise reduction is achieved.
For example, in U.S. Pat. No. 4,244,439 entitled xe2x80x9cSound-absorbing structurexe2x80x9d, issued to Wested, a structure for use to reduce traffic noise is proposed. The mechanism used to reduce the noise, although not explicitly noted as such, is air-coupled surface waves.
Different frequency ranges are addressable in different fashions, so spectral shaping of different signal types such as speech, music, and noise are achievable. Optionally, a surface wave structure is designed so that it behaves differently for sound arriving from different directions: there is a directivity potential that is optionally exploited. Also, surface waves propagate with a phase speed that is different than the free field sound speed.
Efforts are often made to reduce noise in boardrooms and conference rooms using absorptive panels and carpets. However, such noise control efforts also reduce the intensity level of speech signals resulting in difficulties hearing individuals at opposing ends of a room, particularly for long rooms. This reduced audibility is even more of a problem when a microphone is being used to pick up the speech signals because the visual cues are not present at the remote listening end. Two procedures in current use to reduce the above noted problem are (i) reinforcing the speech signals along the length of a boardroom by installing an overhead, ceiling-mounted reflective panel and (ii) use of electronic amplification with microphones at each talker position. However, the installation of an overhead reflector can involve considerable structural, aesthetic and lighting considerations and, moreover, the effects of the original noise control efforts are offset by such an approach. Electronic amplification requires electronic hardware, such as microphones, amplifiers, loudspeakers and mixers, and a technician to ensure that equipment is running properly and levels are appropriately set.
It would be advantageous to provide a method and structure for improving acoustic communication.
According to an embodiment of the invention there is provided a surface wave apparatus having reduced sound attenuation across a surface along a known path having a path distance when compared to sound attenuation along a same path distance through air comprising:
a plurality of cells defining a first surface for supporting acoustical communication between a sound field incident on the first surface and the plurality of cells, each cell including:
an end that is approximately acoustically sealed such that most acoustic energy does not pass therethrough and spaced from the first surface for providing an effective acoustic surface impedance for which air-coupled surface waves form and propagate at selected sound frequencies,
at least a bounding sidewall having 2 opposing bounding sides, between the first surface and the end, that are approximately acoustically sealed such that most acoustic energy does not pass therethrough, the 2 opposing bounding sides of adjacent cells approximately defining boundaries of the known path,
the at least a bounding sidewall having further sides between the first surface and the end spaced apart by a distance less than a wavelength of sound at a known frequency and each disposed across the known path on the surface wave apparatus.