ACOUSTIC FILTER FOR A COAXIAL ELECTRO-ACOUSTIC TRANSDUCER

Abstract

An acoustic filter suitable for an electro-acoustic transducer is provided. The acoustic filter has a relatively high frequency driver and a relatively low frequency driver situated on a common axis. The acoustic filter includes a baffle body having an outer side and an inner side, such that said outer side serves as a baffle for said high frequency driver and said inner side forms a first wall of at least one Helmholtz resonator including a chamber and a vent duct communicating with said chamber.

Claims

1. An acoustic filter suitable for an electro-acoustic transducer having a relatively high frequency driver and a relatively low frequency driver situated on a common axis, said acoustic filter comprising: a baffle body having an outer side and an inner side, such that said outer side serves as a baffle for said high frequency driver and said inner side forms a first wall of at least one Helmholtz resonator including a chamber and a vent duct communicating with said chamber wherein the baffle body is arranged to convert low end response of the high frequency driver to half space radiation (2 pi steradian); wherein said Helmholtz resonator acts with said baffle body to provide an acoustic crossover between said drivers; and wherein said low frequency driver includes a cone and wherein said cone forms a second wall of said Helmholtz resonator.

2. An acoustic filter according to claim 1 wherein said Helmholtz resonator is tuned to a crossover frequency above which it acoustically rolls off.

3. An acoustic filter according to claim 1 wherein said baffle body in combination with said high frequency driver is adapted to cover a piston area associated with said low frequency driver defined by a circular section with a radius about the main axes of at least 80% of a piston radius associated with said low frequency driver.

4. An acoustic filter according to claim 1 wherein said baffle body is adjusted to contribute to vent dimensions and/or to contribute to tuning said Helmholtz resonator to a crossover frequency.

5. An acoustic filter according to claim 1 wherein said Helmholtz resonator is adapted to boost output of said low frequency driver above piston range both on-axis and off-axis to provide a response perceived by a listener to be substantially flat over a wide range of listening angles

6. An acoustic filter according to claim 1 wherein said high frequency driver includes a diaphragm and said baffle body provides separation between said diaphragm of said high frequency driver and the cone of said low frequency driver to reduce cross-talk between said high and low frequency drivers.

7. An electro-acoustic transducer including an acoustic filter according to claim 1.

8. A method of acoustically filtering an electro-acoustic transducer having a relatively high frequency driver and a relatively low frequency driver situated on a common axis to form an acoustic crossover between said drivers, said method comprising: forming a baffle body having an outer side and an inner side, such that said outer side acts as a baffle for said high frequency driver and said inner side forms a first wall of at least one Helmholtz resonator including a chamber and a vent duct communicating with said chamber wherein the baffle body is arranged to convert low end response of the high frequency driver to half space radiation (2 pi steradian); wherein said Helmholtz resonator acts with said baffle body to provide an acoustic crossover between said drivers; and wherein said low frequency driver includes a cone and wherein said cone forms a second wall of said Helmholtz resonator.

9. A method according to claim 8 including tuning said Helmholtz resonator to a crossover frequency above which it acoustically rolls off.

10. A method according to claim 8 including adapting said baffle body in combination with said high frequency driver to cover a piston area associated with said low frequency driver defined by a circular section with a radius about the main axes of at least 80% of a piston radius associated with said low frequency driver.

11. A method according to claim 8 including adjusting said baffle body such that it contributes to vent dimensions and/or contributes to tuning said Helmholtz resonator to a crossover frequency.

12. A method according to claim 8 including adapting said Helmholtz resonator to boost output of said low frequency driver above piston range both on-axis and off-axis to provide a response perceived by a listener to be substantially flat over a wide range of listening angles.

13. A method according to claim 10 wherein said high frequency driver includes a diaphragm and including arranging said baffle body to provide separation between said diaphragm of said high frequency driver and the cone of said low frequency driver to reduce cross-talk between said high and low frequency drivers.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0026] FIGS. 1a and 1b shows an acoustic crossover filter for a low frequency driver crossing to a high frequency driver according to the present invention.

[0027] FIGS. 2a and 2b shows a practical example of a coaxial transducer fitted with an acoustic crossover filter.

[0028] FIG. 3 shows off axis frequency response of a mid-range driver before and after adding an acoustic filter according to the present invention.

[0029] FIG. 4 shows off axis frequency response of a tweeter before and after adding an acoustic filter according to the present invention.

[0030] FIG. 5 shows a typical off axis frequency response for a coaxial driver without an acoustic filter.

[0031] FIG. 6 shows a typical off axis frequency response of a coaxial driver including an acoustic filter according to the present invention.

DETAILED DESCRIPTION

[0032] Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings. The attached drawings are intended to show the breadth of scope of the present invention. In particular FIGS. 1a and 1b show a pedestal mounted tweeter as is common in the art and FIGS. 2a and 2b show an independently mounted tweeter.

[0033] FIGS. 1a and 1b show coaxial transducer 10 comprising a relatively low frequency driver such as a mid-range driver 11 and a relatively high frequency driver such as a tweeter 12. The cone 13 of mid-range driver 11 is shown together with its surround 14. The remaining parts of mid-range driver 11 are not shown as they do not form part of the acoustic crossover filter. A person skilled in the art may readily identify mid-range driver 11 from the parts shown in FIGS. 1a and 1b.

[0034] Tweeter 12 is shown mounted on pedestal 15 which passes through cone 13 of mid-range driver 11. Helmholtz resonator chamber 16 is formed between baffle body or plate 17 and cone 13 of mid-range driver 11. Baffle body 17 substantially covers cone 13 except for vent 18 for air to escape. Baffle body 17 acts as a baffle for tweeter 12 while also minimizing undesirable interaction between mid-range driver 11 and tweeter 12.

[0035] Helmholtz resonator 16 may be tuned to provide an acoustic roll-off at an appropriate crossover frequency. The crossover frequency may be in a range above piston range of mid-range driver 11 and below what may be a limit of acceptable output capability of tweeter 12, if tweeter 12 did not have baffle body 17. Interaction of tweeter 12 with Helmholtz resonator 16 may assist with alignment of drivers 11, 12 by adjusting the crossover frequency, baffle size, and/or parameters associated with tweeter 12.

[0036] FIGS. 2a and 2b show coaxial transducer 20 comprising a relatively low frequency driver such as mid-range driver 21 and a relatively high frequency driver such as tweeter 22. FIG. 2b shows a cross-sectional view. Cone 23 of mid-range driver 21 is shown together with its surround 24. The remaining parts of mid-range driver 21 are not shown as they do not form part of the acoustic crossover filter. A person skilled in the art may readily identify mid-range driver 21 from the parts shown in FIGS. 2a and 2b.

[0037] Tweeter 22 is shown mounted in circular body 25 which in turn may be mounted to a frame (not shown) associated with mid-range driver 21. Circular body 25 may form a baffle plate. The internal wall of body/baffle plate 25 in combination with tweeter 22 may form a first or outer wall of chamber 26. Cone 23 of mid-range driver 21 may form a second or inner wall of chamber 26. Chamber 26 may serve as a Helmholtz resonator chamber with air trapped therein. Annular gap 27 between body/baffle plate 25 and cone 23 may serve as a vent duct for the Helmholtz resonator. Mounting pillars 28 may be adjustable in height to control the size of gap 27.

[0038] The volume of air trapped in chamber 26 may be minimised as shown in FIG. 2b, to produce acceptable tuning for the acoustic crossover filter. The Helmholtz resonator generated high frequency extension of mid-range driver 21 may boost frequency response of transducer 20 up to a frequency chosen for the crossover.

[0039] The boost in frequency response provided by an acoustic crossover filter according to the present invention has been shown to have an audible effect of compensating for lack of off-axis response above a piston range. Listening tests have confirmed that a transducer incorporating such an acoustic crossover filter is perceived to have flat frequency response over a wide range of listening angles and the result is almost indistinguishable from a continuously omni-directional flat response. One advantage of using a Helmholtz resonator to boost frequency response is that it may maintain output capability, which may otherwise be lost if instead electrical equalization was used to provide extension and/or boost.

[0040] The outer wall of body/baffle plate 25 serves as a baffle for tweeter 22 and theoretically boosts its low frequency output capability by 6 dB. Low end response of tweeter 22 may be adjusted by adjusting its baffle size (diameter if circular) such that all tweeter radiation is into a half space (2 pi steradians). For obvious reasons this may be more effective if body/baffle plate 25 is substantially circular. At this size mutual coupling of tweeter 22 to the Helmholtz resonator may be substantially optimized and minor adjustments may be made to the size (diameter if circular) of body/baffle plate 25 to complete an optimisation. This may be done by trial and error as is known in the art without undue experimentation. As a guide the baffle plate 25 may substantially cover the cone 23 as shown in FIGS. 1a, 1b, 2a and 2b. Listening tests at various angles should show a uniformity of response.

[0041] FIG. 3 shows off axis frequency response of a mid-range driver before adding an acoustic filter (shown in dotted line) and after adding an acoustic filter (shown in solid line) according to the present invention. The off axis mid-range driver roll off is caused by tuning the Helmholtz resonator up to an octave above piston range to minimize peaking and to maximize steepness of roll off.

[0042] The dotted response curve shows a substantial loss of output above A which coincides with the upper limit of piston range for the mid-range driver. It is the frequency at which parts of the acoustic waves interact with each other causing off axis cancellations in the radiation pattern. The curve is seen to undergo a roll off B and then a rebound C at higher frequencies. The rebound may be quite varied for different drivers and at different angles off axis. However any rebound may cause a problem because it contributes to sudden changes in the polar pattern and cannot be equalized electrically.

[0043] The solid curve shows how an acoustic filter according to the present invention may boost the response in the region A to D and then cause a sharp roll off at E, followed by substantial attenuation F at higher frequencies. The amount of boost may be controlled by adjusting volume of the Helmholtz chamber and/or dimensions of the vent duct. The response of this example may be suitable for a car door application and shows how an extreme amount of boost is possible.

[0044] FIG. 4 shows off axis frequency response of a tweeter before adding an acoustic filter (shown in dotted line) and after adding acoustic filter 9 (shown in solid line) according to the present invention.

[0045] The dotted curve shows loss of output capability at the lower end of the response X such that it cannot match up with the mid-range driver. It also shows relatively severe deviation W in the response caused by interaction between the tweeter and the mid-range driver.

[0046] The solid curve shows how an extended baffle may boost response at the low end Z and further shows how an acoustic filter may attenuate deviation Y in the response.

[0047] FIG. 5 shows a typical off axis frequency response curve for a coaxial driver wherein output capability of a high frequency driver does not reach down to piston range of a low frequency driver.

[0048] FIG. 6 shows a typical off axis frequency response curve for a coaxial driver including an acoustic filter according to the present invention which provides a seamless crossover even though output capability of the high frequency driver may not reach down to piston range of a low frequency driver.

[0049] The components of the acoustic crossover filter of the present invention should not be confused with a phase plug or a secondary cone.

[0050] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.