Loudspeaker system, method and apparatus for absorbing acoustic resonances of loudspeakers

Abstract

A loudspeaker system (700, 800) and method for tuning a ported loudspeaker and reducing undesirable acoustic resonances reduces port noise, eliminates undesirable port resonances, and improves accuracy and fidelity of sound reproduction in a relatively high-efficiency loudspeaker system with an enclosure containing an Eigen Tone Filter (ETF) structure. The EFT structure includes a tube or set of tubes 720, 820 that are positioned within the loudspeaker vent to absorb the "open tube" acoustic resonances of the vent. The open tube acoustic resonances are undesirable and, if uncorrected, will interfere with the midrange performance of the loudspeaker in use.

Description

[Technical Field]

[0001] The present invention relates to sound reproduction, and in particular to the application of certain acoustic principles in the design of loudspeaker systems.

[0002] REFERENCE TO RELATED APPLICATIONS This application claims priority from commonly owned, related U.S. Provisional Patent Application No. 62 / 837,561, filed April 23, 2019, which is incorporated by reference in its entirety. This application is also related to the following commonly owned patent applications: (a) U.S. Patent Application No. 08 / 294,412, filed August 23, 1994 (now U.S. Patent No. 5,517,573); (b) U.S. Patent Application No. 10 / 660,727, filed September 12, 2003 (now U.S. Patent No. 7,039,212); and (c) U.S. Patent Application No. 10 / 337,347, filed January 7, 2003 (now U.S. Patent No. 7,162,049), which are also incorporated by reference in their entireties. [Background technology]

[0003] Vented box loudspeaker systems have been popular for at least 70 years as a means of obtaining high low-frequency efficiency from a given cabinet volume. Significant technological advances in understanding and analyzing vented loudspeaker systems were made possible with the work of Thiele and Small in the 1970s. Since then, readily available computer programs have made it possible to easily optimize vented loudspeaker designs. However, practical considerations often prevent these theoretically optimized designs from being realized in practice or from performing as intended.

[0004] There are two basic approaches in common use with vented loudspeaker systems: the ducted port (e.g., as shown in Figure 1A) and the passive radiator. While the passive radiator approach has some advantages, the ducted port has generally been more popular due to its lower cost, ease of implementation, and the generally smaller space required.

[0005] However, the ducted port approach has several drawbacks, primarily related to the undesirable noise and associated losses that may be generated by the port when moving the large air volumes necessary to control high low frequency sound pressure levels. For example, as is well known to those skilled in the art, vented loudspeaker systems have a specific tuning frequency f that is determined by the volume of air in the enclosure (e.g., 100), the acoustic mass provided by the port, and the compliance of the air within the enclosure. P Generally, the tuning frequency f P A low f is desirable for a high performance loudspeaker. Prior art (described in commonly owned U.S. Pat. No. 7,162,049) has shown that either a large acoustic mass in the port or a large compliance resulting from a large enclosure volume can cause the low tuning frequency f P The acoustic mass of a port is directly proportional to the mass of the air contained within the port, but inversely proportional to the cross-sectional area of ​​the port. This is because the low tuning frequency f PThis suggests that long ports with small cross-sectional areas should be used to achieve this. However, small cross-sectional areas are incompatible with the large air volumes required to reproduce high sound pressure levels at low frequencies. For example, if the port diameter is too small or is otherwise improperly designed, audible frequency distortion and reduced efficiency at low frequencies may occur, especially at high operating levels, as a result of nonlinear behavior, such as chuffing or port noise due to air turbulence. In addition, viscous drag due to air movement within the port may result in even greater low-frequency efficiency. Increasing the port cross-sectional area can reduce turbulence and losses, but for a given tuning frequency, the port length must be increased proportionately to maintain the appropriate acoustic mass. However, the required increase in length is often impractical.

[0006] Increasing the length and cross-sectional area of ​​a port can also introduce other problems. Organ pipe resonances occur in open ducts at frequencies inversely proportional to the duct's length. These organ pipe resonances can easily cause audible frequency distortion when they occur within a certain frequency range. For example, a 9-inch (22.86 cm) long duct will exhibit a high audible frequency primary resonance at approximately 700 Hz, while a 3-inch (7.62 cm) long duct will exhibit a very low audible frequency primary resonance at approximately 2,100 Hz. In fact, a typical approach used in designing vented loudspeaker systems is to use short ports such that the organ pipe resonance occurs at a high frequency where it is less audible and is unlikely to be within the range of transducers mounted within the enclosure. Additionally, large cross-sectional areas can result in undesirable transmission of midrange frequencies generated within the enclosure to the exterior of the enclosure. This may also result in audible frequency distortion in the form of frequency response variations due to interference with the direct sound produced by the loudspeaker system.

[0007] Thus, there are competing requirements for the design of ports in vented loudspeaker systems. A large cross-sectional area is necessary to avoid audible noise and losses due to nonlinear turbulence, but this makes it difficult to achieve the acoustic mass required for low tuning frequencies within practical size constraints. As those skilled in the art are familiar with, various approaches have been adopted to construct ports with reduced turbulence and loss. Referring to the example shown in FIG. 1A , a loudspeaker enclosure 100, viewed in cross section, includes a transducer 102 and a port 104 that is flared at one or both ends of the port to reduce turbulence. The flared port 104 acts to reduce turbulence by increasing the port cross-sectional area at one or both ends, thereby slowing the air particle velocity at the exit. This allows for a small cross-sectional area and high acoustic mass in the middle section of the port for a given length. However, the necessary flaring ends 106, 108 may need to be quite large to be effective, and may themselves significantly increase the overall port length without contributing significantly to acoustic mass. Increasing the cross-sectional area of ​​the flaring may increase the transmission of undesirable midrange frequencies from inside the loudspeaker cabinet, and an improperly selected flaring degree may actually increase turbulence.

[0008] Another conventional method used to reduce turbulence and loss is shown in FIG. 1B, which is a cross-sectional view of a loudspeaker enclosure 200 with a transducer 102 and multiple ports 204, 206. Using multiple ports 204, 206 reduces turbulence and loss by utilizing the sum of the cross-sectional areas of several ports. However, as with a single port, the length of each of the multiple ports must be increased to account for the increased overall cross-section. For example, if two identical ports are used, both ports would need to be approximately twice as long as a single port of the same cross-section to achieve the same acoustic mass and tuning frequency. As discussed above, this can result in unrealistic length requirements and significant audio-frequency organ-pipe resonances.

[0009] Other techniques are also used to reduce turbulence and losses and other problems associated with the port designs mentioned above, including ports with rounded or flanged ends, geometries to reduce organ pipe resonance, and many ways to achieve long ports through folding or other convolutions.

[0010] Commonly owned U.S. Pat. Nos. 5,517,573 (hereinafter "the '573 patent") and 5,809,154 disclose improved port designs for achieving desired acoustic mass in a compact space with reduced turbulence and losses, and are incorporated herein by reference in their entireties. FIG. 1C is a reproduction of FIG. 7 from the '573 patent. The approaches described in these patents use disks at one or both ends of a single duct to effectively create a gradually increasing cross-sectional area at both ends of the port. In some preferred embodiments, flow guides are also used to further enhance the efficiency of the port design. While this approach has the advantage of suppressing transmission of mid-range frequencies from inside the cabinet and providing the required acoustic mass in a more compact form that also reduces losses, such a compact form can also, in certain configurations, create problems associated with audio frequency organ pipe resonances; these challenges have been addressed in other ported cabinet configurations shown in Figures 1C, 1D, and 1F, taken from commonly owned U.S. Pat. No. 7,162,049, also incorporated by reference herein. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 7,162,049 [Patent Document 2] U.S. Patent No. 5,517,573 [Patent Document 3] U.S. Patent No. 5,809,154 [Patent Document 4] U.S. Patent No. 7,162,049 Summary of the Invention [Problem to be solved by the invention]

[0012] The vented loudspeakers of Figures 1A-1F were developed to provide increased output above these low-frequency tuning frequencies. One disadvantage of these vented designs is that the vents exhibit acoustic resonances well above the desired first-order Helmholtz resonance associated with low-frequency systems. These resonances are often audible and affect the frequency- and time-related resonances of the system in the midrange. Eliminating or reducing the amplitude of these resonances improves the system's midrange performance. There is also a desire to reduce the audibility of port noise. Such prior art methods, as well as structures and methods to alleviate these problems, for example, by reducing the cross-sectional area of ​​the port, result in secondary effects, such as increased air velocity, which increases the turbulence of the airflow (and port noise). Electrical correction for port noise or chafing is not possible because the vents are not driven directly by the associated electronics but rather by transducers in the system.

[0013] Therefore, what is needed is an effective system and method for tuning ported loudspeakers and reducing undesirable acoustic resonances, while reducing port noise, eliminating undesirable port resonances, and improving the accuracy and fidelity of reproduced sound in a relatively efficient loudspeaker system. [Means for solving the problem]

[0014] In accordance with the present invention, an effective system and method for tuning ported loudspeakers and reducing undesirable acoustic resonances reduces port noise, eliminates undesirable port resonances, and improves the accuracy and fidelity of reproduced sound in relatively efficient loudspeaker systems.

[0015] The loudspeaker system and enclosure of the present invention includes a vent with a lumen that fluidly connects the interior volume of the enclosure with the external ambient environment, the vent lumen containing an inherent tone filter ("ETF") tube or set of ETF tubes disposed within the vent to absorb the vent's "open tube" acoustic resonances, which are generally undesirable and can interfere with or degrade the midrange performance of the loudspeaker.

[0016] The ETF-equipped loudspeaker systems and enclosures of the present invention have several advantages, including: (a) the ETF system (“ETF”) is passive and therefore does not require electrical or data signal processing (“DSP”) to run it; (b) the ETF is relatively inexpensive and can be made with a few simple components; (c) the ETF system absorbers can be tuned to absorb vent resonances, cabinet resonances, or both; (d) with a dual-tube ETF system, the individual absorbers can be tuned separately to address different resonances; (e) the ETF is visible from the exterior of the loudspeaker enclosure and therefore has marketing advantages over internal solutions; and (f) the ETF-equipped loudspeaker systems and enclosures reduce audible frequency port noise when in use.

[0017] The ETF-equipped loudspeaker system and enclosure was developed after observing that an air column open at both ends exhibits an acoustic resonance whose wavelength is twice the length of the air column plus some amount to allow for end correction. Similarly, an air column closed at one end exhibits a resonance whose wavelength is four times the length of the air column plus the end correction. It was observed that by placing the open end of a closed air column approximately half its length near the center of the open air column, the closed air column acts as an absorber at the resonant frequency of the open air column.

[0018] During the development of our prototype, it was noted that two of these open-ended air columns could be placed face-to-face, with the openings located near the center of the open-ended air columns. The advantages of this configuration were observed to be numerous. First, two air columns have a larger surface area than a single air column, allowing for greater absorption. Second, the air columns can be placed concentrically, so that the flow in the primary air column is less perturbed by changes in cross-sectional area. Third, absorbing air columns can be easily placed within the primary air column, since they can then be attached to the ends or to features outside the main air column. Additionally, tapering the ends of the closed air columns reduces the absorber quality (Q), allowing the ETF absorber to be tuned to better match the quality of the resonance in the main air column. The prototype's taper was also observed to reduce turbulence in the main air column at the ends because of their aerodynamic nature. In other prototypes, foam, fiber, and other acoustic resistive elements were inserted into the absorber in configurations that also modified and influenced the quality (Q). These acoustic resistive elements were observed to work well at the closed (i.e., bottom) end, but good overall performance was obtained with the absorber placed at the open end, which configuration offered the easiest tuning method to provide good performance with a low amount of undesirable side effects.

[0019] Although the ETF-equipped loudspeaker system and enclosure of the present invention have been prototyped with a round vent for the loudspeaker, the principles and methods of the present invention may be adapted to work with vents of other shapes, and the absorber need not be round either.

[0020] Two preferred embodiments were developed during prototyping: one is a typical bookshelf loudspeaker embodiment, and the other is a loudspeaker embodiment with a Power Port™ floor-standing (tower) vent configuration. In the Power Port™ vent configuration, the ETF absorber may be located within the diffuser portion of the base to provide an attractive, efficient, and economical embodiment.

[0021] The end correction for the ETF absorber tends to be smaller than that of the main air column, so there must be a gap between the two absorbers, and in the case of the Power Port™ vent design, the main air column extends beyond a simple cylindrical section, and the absorber assembly tends to be longer than the assembly for the main air column. This allows the ETF absorber assembly to be conveniently mounted at the end of the main air column or in a flare section external to the main air column.

[0022] The opening between the two ETF absorbers affects the efficiency of the absorber. If the opening is too small, the efficiency of the absorber will decrease. A diameter to length ratio of 1:1.25 is preferred (i.e., the ratio between the diameter ID of the ETF absorber tubes and the length of the ETF tube gap between them, e.g., absorber diameter = 25 mm, gap between absorbers = 20-25 mm).

[0023] The size of the absorber affects its effectiveness. Larger cross-sectional area equates to better absorption. Since the absorber subtracts from the cross-sectional area of ​​the main air column, it is usually best to keep the absorber as small as necessary to achieve the desired absorption rate. A 0.15:0.2 ratio of absorber cross-sectional area to main air column cross-sectional area seems to work best.

[0024] Helmholtz tuning of the main air column (vent f P) changes with the insertion length of the absorber assembly because the cross-sectional area of ​​the main air column is reduced by the cross-sectional area of ​​the absorber assembly. It is sufficient to increase the size of the main air column to compensate.

[0025] It is possible to tune the absorber to absorb frequencies that are not necessarily caused by the main air column. For example, resonances (modes) present in loudspeaker cabinets often exit through vents and can be absorbed by the ETF absorber if properly tuned. This has been demonstrated in the prototype.

[0026] These and other features and advantages of the present invention will become apparent from the following detailed description of specific embodiments of the invention, particularly when considered in conjunction with the accompanying drawings, in which like reference numerals in the various figures are used to designate like components. [Brief explanation of the drawings]

[0027] [Figure 1A] 1 illustrates a prior art ported loudspeaker system and method; [Figure 1B] 1 illustrates a prior art ported loudspeaker system and method; [Figure 1C] 1 illustrates a prior art ported loudspeaker system and method; [Figure 1D] 1 illustrates a prior art ported loudspeaker system and method; [Figure 1E] 1 illustrates a prior art ported loudspeaker system and method; [Figure 1F] 1 illustrates a prior art ported loudspeaker system and method; [Figure 2]FIG. 1 is a cross-sectional side view of a non-power port segment of a bookshelf loudspeaker system showing an ETF-equipped loudspeaker system vent or port within the enclosure in accordance with the structure and method of the present invention. [Figure 3] FIG. 1 is a cross-sectional side view of a port-mounted segment of a tower-type loudspeaker system showing an ETF-mounted loudspeaker system vent or port within the tower enclosure in accordance with the structure and method of the present invention. [Figure 4A] 3 is a perspective view of the ETF-equipped bookshelf system vent or port of FIG. 2 in accordance with the structure and method of the present invention. [Figure 4B] 3 is a perspective view of the ETF-equipped bookshelf system vent or port of FIG. 2 in accordance with the structure and method of the present invention. [Figure 5A] FIG. 4 is a perspective view of the ETF-equipped tower system vent or port of FIG. 3 in accordance with the structure and method of the present invention. [Figure 5B] FIG. 4 is a perspective view of the ETF-equipped tower system vent or port of FIG. 3 in accordance with the structure and method of the present invention. [Figure 6] FIG. 1 is a cross-sectional perspective view of a bookshelf loudspeaker system incorporating the ETF configurations of FIGS. 2, 4A and 4B within an enclosure in accordance with the structures and methods of the present invention. [Figure 7] 7 is a frequency response plot illustrating the frequency response and performance of the conventional (non-ETF) and improved (port with ETF) bookshelf loudspeaker systems of FIG. 6 in accordance with the method of the present invention. [Figure 8A] FIG. 5C is a cross-sectional side view of the ETF-equipped tower system vent or port of FIGS. 3, 5A, and 5B in accordance with the structures and methods of the present invention. [Figure 8B] FIG. 5C is a cross-sectional side view of the ETF-equipped tower system vent or port of FIGS. 3, 5A, and 5B in accordance with the structures and methods of the present invention. [Figure 8C] FIG. 5C is a cross-sectional side view of the ETF-equipped tower system vent or port of FIGS. 3, 5A, and 5B in accordance with the structures and methods of the present invention. [Figure 9] 8A-8C are frequency response plots illustrating the frequency response and performance of conventional (non-ETF) and improved (port with ETF) tower loudspeaker systems in accordance with the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] Referring to FIGS. 2-9, in accordance with the present invention, a highly efficient system and method for tuning a ported loudspeaker and reducing undesirable acoustic resonances eliminates undesirable port resonances and improves the accuracy and fidelity of sound reproduction in a relatively efficient loudspeaker system.

[0029] The ETF-equipped loudspeaker system (e.g., 700 or 800) and enclosure of the present invention includes a vent with a lumen that allows fluid communication between the interior volume of the enclosure and the external ambient environment, and the open interior lumen of the vent has one or a set of Eigen Tone Filter (ETF) tubes positioned within the vent to absorb the vent's "open tube" acoustic resonances. These open tube acoustic resonances are typically undesirable and can interfere with or degrade the midrange performance of the loudspeaker. As discussed above, the ETF-equipped loudspeaker system and enclosure have several advantages over the prior art of Figures 1A-1F, including: 1) The ETF system (eg, 720A, 720B, or 820) is passive and therefore does not require electrical or digital signal processing ("DPS") to operate. 2) ETF systems (eg, 720A, 720B, or 820) are relatively inexpensive and made with a few simple components. 3) The absorber tube of the ETF system can be sized (eg, "tuned") to absorb vent resonances, cabinet resonances, or both. 4) In the case of a dual-tube ETF system, the individual absorbers can be tuned separately to handle different resonances. 5) The ETF system is visible from the outside of the loudspeaker enclosure and therefore has a marketing advantage compared to internal solutions. 6) ETF-equipped loudspeaker systems (e.g., 700, 800) and enclosures reduce audible frequency port noise.

[0030] ETF-equipped loudspeaker systems (e.g., 700, 800) were developed after observing that an air column open at both ends exhibits an acoustic resonance whose wavelength is twice the length of the air column plus some amount to allow for end correction. Similarly, an air column closed at one end exhibits a resonance whose wavelength is four times the length of the air column plus the end correction. It was observed that by placing the open end of a closed air column approximately half its length near the center of the open air column, the closed air column acts as an absorber at the resonant frequency of the open air column.

[0031] During the development of our prototype, it was noted that two of these open-ended air columns could be placed face-to-face, with the openings located near the center of the open-ended air columns. The advantages of this configuration were observed to be numerous. First, two air columns have a larger surface area than a single air column, allowing for greater absorption. Second, the air columns can be placed concentrically, so that the flow in the primary air column is less perturbed by changes in cross-sectional area. Third, absorbing air columns can be easily placed within the primary air column, since they can then be attached to the ends or to features outside the main air column. Additionally, tapering the ends of the closed air columns reduces the absorber quality (Q), allowing the ETF absorber to be tuned to better match the quality of the resonance in the main air column. The prototype's taper was also observed to reduce turbulence in the main air column at the ends because of their aerodynamic nature. In other prototypes, foam, fiber, and other acoustic resistive elements were inserted into the absorber in configurations that also modified and influenced quality (Q). These acoustic resistive elements were observed to work well at the closed (i.e., bottom) end, but good overall performance was obtained with the absorber placed at the open end, a configuration that offered the easiest tuning method to provide good performance with a low amount of undesirable side effects. Alternatively, the acoustic resistive elements could be placed elsewhere within the absorber.

[0032] Although the ETF-equipped loudspeaker system and enclosure of the present invention have been prototyped with a round vent for the loudspeaker, the principles and methods of the present invention may be adapted to work with vents of other shapes, and the absorber need not be round either.

[0033] Two examples are shown in Figures 2-9. One is an ETF-equipped bookshelf loudspeaker system 700 (best shown in Figures 2 and 6). The other embodiment is an ETF-equipped Power Port™ floor-standing (tower) loudspeaker system 800 (best shown in Figures 3 and 8A-8C). In the case of an ETF-equipped Power Port™, the ETF absorber assembly 820 may be located within the diffuser portion of the base. This is convenient and saves cost.

[0034] 2 and 6 and further referring to Figures 4A and 4B, a bookshelf-sized embodiment 700 of an ETF-equipped loudspeaker system of the present invention includes a ported loudspeaker enclosure 710 having a front baffle that supports and directs at least one loudspeaker driver (e.g., a mid-woofer and tweeter) and a rear baffle that supports an ETF assembly (e.g., 720A). The ported loudspeaker enclosure 710 has an interior volume that is ported to the ambient environment by a vent or port 730, which has a cylindrical interior vent lumen 740 with a central vent lumen axis. An ETF assembly 720A is supported within vent lumen 740 in coaxial alignment with the vent lumen axis, and the ETF assembly includes one or a pair of barrels or absorbers (750, 760) positioned within the loudspeaker vent lumen to absorb "open tube" acoustic resonances of vent lumen 740 when the loudspeaker is in use. ETF assembly 720A (visible in FIG. 6 ) includes a proximal closed end cap and an opposite distal rearward-projecting end cap with a circumferential slot or sidewall gap at an intermediate point that allows fluid communication between the interior volumes of the first and second axially aligned ETF barrel segments or absorbers (750, 760) and vent lumen 740. Because vent or port 730 comprises a tuned port that allows fluid communication between the interior of enclosure 710 and the ambient environment, vent or port 730 also allows fluid communication between the enclosure and the ambient environment, respectively, and the interior volume of the ETF absorber for ETF assembly 720A. Figures 2, 4A, and 4B provide a slightly different embodiment of an ETF assembly (e.g., 720B) for use in bookshelf system 700 in that both ends of the ETF barrel preferably support rounded or "bullet-nose" end caps that house absorber elements.2, ETF assembly 720B includes a proximal closed end cap and an opposite distal rearward-projecting end cap with a circumferential slot or sidewall gap at an intermediate location that allows fluid communication between the interior volumes of the first and second axially-aligned ETF barrel segments or absorbers and vent lumen 740. Each of the first and second axially-aligned ETF barrel segments or absorbers (750, 760) has an axial length that is approximately a quarter wavelength for the frequency of interest (e.g., 155 mm for 562 Hz and 122 mm for 789 Hz, resulting in the variations shown in FIG. 7).

[0035] In the method developed in accordance with the present invention, selecting (i.e., "tuning") the dimensions for the ETF tube was an iterative process. In the example bookshelf loudspeaker system 700, the "Stockport" data plotted in FIG. 7 shows the amount of unwanted energy in the range from 550 Hz to 800 Hz. To reduce or "notch out" this unwanted energy with the bookshelf loudspeaker system ETF 700A, the ETF tube segments must be properly sized and shaped (or "tuned"). A detailed example is provided below (for tower system 800).

[0036] 3, 8A-8C, and with further reference to Figures 5A and 5B, a floor-standing or tower-sized embodiment of an ETF-equipped loudspeaker system of the present invention similarly includes a ported loudspeaker enclosure 810 having a front baffle that supports and directs at least one loudspeaker driver (e.g., woofer, mid-woofer, and tweeter) and a bottom baffle that supports an ETF assembly (e.g., 820). Ported tower loudspeaker enclosure 810 includes an interior volume that is ported to the ambient environment via vent or port 830, which includes a cylindrical inner vent lumen 840 with a central vent lumen axis. ETF assembly 820 is supported within vent lumen 840 in coaxial alignment with the vent lumen axis, and includes a tube or set of tubes or absorbers (850, 860) positioned within the loudspeaker vent lumen to absorb "open tube" acoustic resonances of vent lumen 840 when the loudspeaker is in use. ETF assembly 820 (visible in FIGS. 3 and 8B) includes a proximal closed end cap and an opposite distal downwardly projecting end cap fitted within a Power Port™-style diffuser, with a circumferential slot or sidewall gap at its intermediate point that allows fluid communication between the interior volumes of the first and second axially aligned ETF tube segments or absorbers (850, 860) and vent lumen 840.

[0037] Because vent or port 830 comprises a tuned port that allows fluid communication between the interior of enclosure 810 and the ambient environment, vent or port 830 also allows fluid communication between the enclosure and the ambient environment, respectively, and the interior volume of the ETF pipe for ETF assembly 820. FIGS. 5A and 5B provide a slightly different embodiment of ETF assembly 820 for use in tower system 800, showing the proximal, interior, or upper end of the ETF barrel supporting a rounded or “bullet-nose” end cap 870 that preferably houses an absorber element (not shown). Referring back to FIG. 3 , ETF assembly 820 includes a proximal closed end cap and an opposite, distal, downwardly projecting end cap, and includes a circumferential slot or sidewall gap at an intermediate point that allows fluid communication between the interior volumes of first and second axially aligned ETF barrel segments and vent lumen 840. Each of the first and second axially aligned ETF tube segments or absorbers (850, 860) preferably has an axial length approximately equal to a quarter wavelength for the frequency of interest (e.g., 150 mm for 494 Hz, 100 mm for 756 Hz) for a 38 mm ID.

[0038] In the method developed in accordance with the present invention, selecting (i.e., "tuning") the dimensions for the ETF tube was an iterative process. For the example tower loudspeaker system 800, the "Stockport" data plotted in FIG. 9 shows the amount of unwanted energy in the range from 500 Hz to 750 Hz. To reduce or "notch out" this unwanted energy with the tower loudspeaker system ETF 820, the ETF tube segments must be properly sized and shaped (or "tuned"). Initially assuming the speed of sound at 20°C to be 343 m / s and using 100 mm, yields a quarter-wave frequency of f = 343 / (0.1 × 4) = 857.5 Hz. A 38 mm ETF tube adds 0.3 × 38 = 11.4 mm for end correction (as per several references), thereby changing the above to f = 343 / (0.1114 × 4) = 769.7 Hz. Because this is not a completely open tube, applicants believe this initial frequency tuning estimate may not be 100% accurate. It is believed that adding approximately 38 mm of foam into the 100 mm ETF somewhat reduced the Q of the tube and slowed the air velocity within the ETF, thereby accounting for the change to 756 Hz in the measured minimum difference curve as shown in the "Port with ETF" data plotted in Figure 9. Similarly, the 150 mm tuning is f = 343 / (0.15 × 4) = 571.7 Hz without end correction and f = 343 / (0.1614 × 4) = 531.3 Hz with end correction, so after adding approximately 76 mm of foam to the 150 mm ETF, the frequency is f = 494 Hz. As those skilled in the art will appreciate, this tuning does not appear to allow for preliminary and accurate calculations, since there is not a direct 1:1 relationship between length and quarter-wave frequency.

[0039] Because the ETF can assembly (e.g., 820) tends to have smaller end corrections than the main air column, a gap must exist between the two absorbers (e.g., 850, 860), and in the case of the Power Port™ embodiment shown in Figure 3, i.e., for a primary air column that extends beyond a simple can section, the absorber assembly 820 tends to be longer than the assembly for the main air column (e.g., as shown in Figures 5A and 5B). This allows the absorber assembly 820 to be conveniently mounted at the end of the main air column or in a flared section external to the main air column.

[0040] The circumferential slots or sidewall gap openings (e.g., 755, 855) between two axially aligned ETF tubes or tube-shaped absorbers (e.g., 850, 860) affect the efficiency of the absorber comprising the ETF assembly 820. If the slots or sidewall gap openings (e.g., 755, 855) are too small, the resonance absorption efficiency of the ETF absorber tubes will decrease. Preferably, the gap length between the absorbers and the diameter of the absorbers are selected such that the tube diameter is 1 to 1.25 times the gap length between the absorbers (so, for example, if the absorber diameter is 25 mm, the axial gap length between the absorbers is 20 to 25 mm).

[0041] The size of the absorber tube affects the efficiency of the absorber. Larger cross-sectional area equates to better absorption. Because the absorber subtracts from the cross-sectional area of ​​the main air column (e.g., vent or port 830), it is currently considered optimal to keep the absorber as small as necessary to achieve the desired absorption. A 0.15:0.2 ratio of absorber cross-sectional area to main air column (or vent lumen) cross-sectional area was determined to work best in prototype modifications. The Helmholtz tuning of the main air column (e.g., vent lumen 730 or 830) changes with the insertion length of the absorber assembly because the cross-sectional area of ​​the main air column decreases by the cross-sectional area of ​​the absorber assembly. To compensate for this, simply increase the size of the main air column (e.g., vent lumen 740 or 840).

[0042] It is possible to tune the ETF assembly absorber to absorb frequencies not necessarily produced by the main air column (or vent lumen 740 or 840). For example, resonances (modes) present in loudspeaker cabinets often exit through the vent and, if properly tuned, can be absorbed by the absorber. This has been demonstrated in prototypes.

[0043] While preferred embodiments of the new and improved system and method have been described, it is anticipated that other modifications, variations and changes will occur to those skilled in the art in light of the teachings set forth herein, and it is therefore to be understood that all such modifications, variations and changes are intended to be included within the scope of the invention.

Claims

1. A loudspeaker system (700 or 800), a ported loudspeaker enclosure having a first baffle supporting at least a midrange or mid-bass driver; the enclosure having an interior volume ported to an ambient environment by a first vent lumen, the first vent lumen including an inherent sound filter structure ("ETF") 720, 820, the ETF having at least a first barrel segment disposed within the loudspeaker vent lumen; the ETF having first and second coaxially aligned ETF cylindrical segments, an ETF intra-cylindrical volume in fluid communication with the opening; the ETF has a first ETF barrel segment having a first segment length, the first ETF barrel segment being in substantially coaxial alignment with a second ETF barrel segment having a second segment length, the first ETF barrel segment length being selected to have a value that is approximately a quarter wavelength at a first selected ETF port signal notch frequency that is within a frequency band that includes an open tube resonance of the vent lumen; the second ETF tube segment length is selected to have a value that is approximately a quarter wavelength at a second selected ETF port signal notch frequency that is also within a frequency band that includes an open tube resonance of the vent lumen; The ETF-equipped loudspeaker system 800 includes a ported loudspeaker enclosure 810 having a front baffle that supports and directs at least one loudspeaker driver and a bottom baffle that supports an ETF assembly (820); the ported tower loudspeaker enclosure 810 comprises an interior volume ported to the ambient environment by a vent or port 830, the vent or port comprising a cylindrical inner vent lumen 840 with a central vent lumen axis, the ETF assembly 820 being supported within the vent lumen 840 in coaxial alignment with the vent lumen axis, the ETF assembly 820 including a tube or set of tubes or absorbers (850, 860) positioned within the loudspeaker vent lumen to absorb "open tube" acoustic resonances of the vent lumen 840 when the loudspeaker is in use; the ETF assembly (820) includes a proximal closed end cap and an opposite distal downwardly projecting end cap fitted within a diffuser, and the opening at an intermediate point of the ETF assembly (820) is a circumferential slot or sidewall gap that allows fluid communication between the interior volumes of the first and second axially aligned ETF barrel segments or absorbers (850, 860) and the vent lumen (840).

2. the vent or port 830 constitutes a port that allows fluid communication between the interior of the enclosure 810 and the ambient environment, allowing fluid communication between each of the interior of the enclosure 810 and the ambient environment and the internal volume of the ETF cylinder for the ETF assembly 820; 10. The loudspeaker system of claim 1, wherein a proximal, inner, or upper end of said ETF barrel assembly supports a rounded or "bullet-nose" shaped end cap 870 that houses an absorber element, and said ETF barrel assembly includes a circumferential slot or sidewall gap at an intermediate point thereof that allows fluid communication between said interior volumes of said first and second axially-aligned ETF barrel segments and said vent lumen 840.

3. 3. The loudspeaker system of claim 2, wherein the first and second axially aligned ETF cylinder segments or absorbers (850, 860) preferably have an axial length substantially equal to 150 mm at 494 Hz when ID is 38 mm.

4. 3. The loudspeaker system of claim 2, wherein the first and second axially aligned ETF cylinder segments or absorbers (850, 860) preferably have an axial length substantially equal to 100 mm at 756 Hz for an ID of 38 mm.

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