Patent Application
20110243362
This specification describes radiation patterns of loudspeakers and for increasing the frequency range in which the loudspeaker has an omnidirectional radiation pattern.
In one aspect a loudspeaker, includes an acoustic device including an acoustic enclosure; an acoustic driver, mounted in the acoustic enclosure; and a sheet of compliant material with a aperture therethrough, mounted in the enclosure, between the acoustic driver and the environment. The compliant material may be neoprene foam. The sheet may be neoprene foam 0.32 cm (0.125 inches) to 2.54 cm (1 inch) thick. The aperture may be circular and have a diameter of between 1.27 cm and 2.54 cm. The aperture may be non-circular. The aperture may be rectangular. The aperture may be square. The aperture may be a rectangle elongated vertically. The loudspeaker may further include a second acoustic device positioned above the first loudspeaker. The second acoustic device include a second acoustic enclosure; a second acoustic driver, mounted in the second acoustic enclosure; and a second sheet of compliant material with a second aperture therethrough, mounted in the enclosure between the second acoustic driver and the environment, wherein the second aperture is a rectangle elongated vertically. The sheet of compliant material may be dimensioned and configured so that there are open slots between the edge of the sheet and the enclosure through which low frequency acoustic energy can be radiated. The sheet may cause the acoustic driver to radiate proportionately less acoustic energy in the axial direction and proportionately more acoustic energy in off-axis directions than without the sheet. There may be a plurality of apertures through the sheet. The acoustic driver may include a linear motor and a diaphragm, coupled to the linear motor so that the diaphragm vibrates along an axis passing through the aperture. The acoustic driver may include a dust cover having a diameter, wherein the diameter of the aperture is less than the diameter of the dust cover. The sheet may be formed so that the distances from all points on the radiating surface to the sheet measured in a direction parallel to the axis of the acoustic driver are substantially the same. The sheet may have a thickness of less than half of the diameter of the aperture. The loudspeaker may further include an acoustic lens between the sheet and the environment.
In another aspect, an array loudspeaker includes: a plurality of acoustic drivers; a sheet of compliant material having a vertical slot shaped aperture therethrough, between the plurality of acoustic drivers and the environment.
Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which:
One method of characterizing the directionality of a loudspeaker is to specify the angle in a plane (typically a horizontal plane) within which the radiation is within some range, for example, −10 dB of the maximum radiation in any direction in the plane. A smaller angle indicates a more directional radiation pattern. A wider angle indicates a less directional radiation pattern. A radiation pattern in which the radiation in all directions is within some range, for example −10 dB, is said to be “omnidirectional”. In this specification, the angle within which the radiation is within −10 dB of the maximum radiation in any direction will be referred to as “the −10 dB angle”. Other ranges, for example, −6 dB or −12 dB can be used in place of the −10 dB figure.
A characteristic of acoustic drivers is that at wavelengths that are close to or less than the diameter of the radiating surface, the acoustic driver tends to have a more directional radiation pattern than at longer wavelengths. For example,
This increase in directivity is typically undesirable. For example, if the acoustic driver of
One way of adjusting for the increased directivity at higher frequencies is to provide a second acoustic driver with the radiating surface facing a slightly different direction. However, this is disadvantageous because the radiation pattern of the combined two acoustic drivers may have prominent lobes and dips due, for example, to interaction between radiation of the acoustic drivers. Another approach is to use smaller diameter acoustic drivers for the high frequencies (“tweeters”), but this also requires additional acoustic drivers and crossover networks.
Another way of adjusting for the increase directivity at higher frequencies is to alter the radiation pattern of the loudspeaker so that proportionately less acoustic energy is radiated on-axis and proportionately more acoustic energy is radiated off-axis. One way of altering the radiation pattern of a loudspeaker is to use a device such as a diverging acoustic lens, for example as described in Olson, Acoustical Engineering published in 1991 by Professional Audio Journals, Inc., Philadelphia Pa., pages 19-20.
In one implementation, the acoustic driver 12 is a 5 cm cone type driver. The enclosure 14 is 66 mm from side to side, 83 mm from front to back, and 56 mm from top to bottom. The sheet 18 is made of closed cell neoprene foam, 3 mm thick, with a round aperture with a diameter of about 1.2 cm, positioned approximately at the center of the sheet so that the axis 22 of the acoustic driver is approximately lined up with the center of the aperture 20. The sheet 18 is positioned 3 mm from any moving portion of the acoustic driver (which may be the surround, not shown in this view) of the acoustic driver. In other implementations, the sheets may be made of closed cell foams of other materials; felts or fabrics; or other compliant, non-acoustically transparent materials; generally, closed-cell foam provide the best results.
The sheet should be sufficiently compliant to dampen any standing waves that may form in the space between the radiating surface and the sheet and so that pressure does not build up in the volume between the radiating surface and the sheet. The sheet should have sufficient acoustic opacity to prevent substantial high frequency energy from radiating through the material; the sheet should absorb as little low frequency radiation as possible; and the sheet should be formable and should retain its shape and geometry. The desirable characteristics may be obtained by material selection, by geometry of the sheet, or by both. Pressure buildup between the acoustic driver and the sheet is undesirable. If there is a pressure buildup in the volume between the radiating surface and the sheet may be alleviated by using a more compliant material, by using a thinner sheet of material, or both. The sheet should be sufficiently compliant to dampen any standing waves that may form in the space between the radiating surface and the sheet and so that pressure does not build up in the volume between the radiating surface and the sheet. The sheet should have sufficient acoustic opacity to prevent substantial high frequency energy from radiating through the material; the sheet should absorb as little low frequency radiation as possible; and the sheet should be formable and should retain its shape and geometry. The desirable characteristics may be obtained by material selection, by geometry of the sheet, or by both. Pressure buildup between the acoustic driver and the sheet is undesirable. If there is a pressure buildup in the volume between the radiating surface and the sheet, it may be alleviated by using a more compliant material, by using a thinner sheet of material, or both. Sheets of material having compliances of 3.348×10−4 m/N and 3.723×10−4 m/N (when 13 mm disks 3 mm to 6 mm thick are subjected to a simple stress/strain test) have been found to be suitable. By contrast, hard plastics have substantially lower compliances, for example around 0.01490×10−4 m/N under similar conditions, more than two orders of magnitude less than the compliance of the compliant sheets. Also, sheets of material having compliances significantly greater than 3×10−4 m/N m, for example, conventional open cell foams may not have sufficient acoustic opacity to prevent substantial high frequency energy from radiating through the material or to retain its shape and geometry.
For best results, the thickness of the sheet should be equal to or less than the diameter of the hole. Omnidirectionality to higher frequencies can be obtained with circular aperture diameters from 1.27 cm (0.5 inches) to 2.54 cm (1 inch) and sheet thicknesses of 0.32 cm (0.125 inches) to 2.54 cm (1 inch) of neoprene foam. Generally, larger aperture diameters result in greater sensitivity, while smaller aperture diameters result in omnidirectionality to higher frequencies.
In the embodiment of
The distance between the radiating surface 13 of the acoustic driver 12 to the sheet 18 should be less than the diameter of the aperture 20. Preferably, sheet 18 should be as close to the radiating surface 13 as possible, without contacting the radiating surface at maximum excursion. One result of placing the sheet as close as possible to the radiating surface is that standing waves that may occur are likely to have a wavelength and corresponding frequency that is out of the range of operation of the acoustic driver or are easily damped.
In one implementation, the aperture 20 has a diameter of about 1.2 cm, which is approximately equal to the wavelength corresponding to a wavelength of 20 kHz, which is above the range of operation of most acoustic drivers and above the range of frequencies to which most human ears respond. One consequence of an aperture is that for directionality purposes, the diameter of the effective radiating surface may approach the diameter of the aperture. Therefore, if the arrangement of
The geometry of the aperture may be other than circular. For example,
Numerous uses of and departures from the specific apparatus and techniques disclosed herein may be made without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features disclosed herein and limited only by the spirit and scope of the appended claims.
