Loudspeaker system, method and apparatus for absorbing acoustic resonance of loudspeaker

Abstract

To provide a loudspeaker system and a method for tuning a ported loudspeaker and reducing an undesirable acoustic resonance.SOLUTION: A loudspeaker system 700 includes a ported loudspeaker enclosure 710 having a front baffle that supports and directs at least one loudspeaker driver and a rear baffle that supports an ETF assembly 720A. The ported loudspeaker enclosure 710 has an interior volume that is ported to the ambient environment by a vent or port 730 that has a cylindrical interior vent lumen 740 with a central vent lumen axis. The ETF assembly 720A is supported within vent lumen 740 in coaxial alignment with the vent lumen axis to absorb "open tube" acoustic resonances of vent lumen 740 when the loudspeaker is in use.SELECTED DRAWING: Figure 6

Description

Technical Field

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

[0002]

Reference to Related Applications

Background Art

[0003] Vent (ventilation) type 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. In the 1970s, there were significant technological advancements in understanding and analyzing vented loudspeaker systems due to the research results of Thiele and Small. Since then, easily available computer programs have made it possible to easily optimize vented loudspeaker designs. However, due to practical considerations, these designs optimized in theory are often hindered from being realized in reality or functioning as intended.

[0004] In connection with vented loudspeaker systems, there are two basic approaches in general use, which are duct-shaped ports (such as those shown in FIG. 1A) and passive radiators. While the passive radiator approach has several advantages, duct-shaped ports have been more prevalent generally because they are inexpensive, easy to implement, and generally require less space.

[0005] However, the duct-shaped port approach has numerous drawbacks. These are mainly associated with the undesirable noise that may be generated by the port and the various losses attendant thereto during the movement of the large air volume required for adjusting the high low-frequency sound pressure level. For example, as is well known to those skilled in the art, a vented loudspeaker system has a ratio resonance frequency f determined by the volume of air in the enclosure (e.g., reference numeral 100), the acoustic mass provided by the port, and the compliance of the air within the enclosure. P Generally, it is desirable for a high-performance loudspeaker to have a low resonance frequency f. P According to the prior art (described in the common assignee's U.S. Patent No. 7,162,049), either a large acoustic mass in the port or a large compliance resulting from a large enclosure volume is required to achieve the low resonance frequency f. P The acoustic mass of the port is directly proportional to the mass of the air within the port but inversely proportional to the cross-sectional area of the port. This means that the low resonance frequency f PIt is suggested that in order to achieve this, long ports with a small cross-sectional area should be used. However, a small cross-sectional area is not compatible with the large air volume required to reproduce high sound pressure levels at low frequencies. For example, if the port diameter is too small or if it is otherwise inappropriately designed, non-linear behavior, such as chuffing due to air turbulence or port noise, can result in audible frequency distortion and efficiency reduction at low frequencies, especially at high operating levels. Additionally, due to viscous resistance resulting from air movement within the port, efficiency reduction at low frequencies may occur further. By increasing the cross-sectional area of the port, turbulence and losses can be reduced, but for a given resonance frequency, the length of the port must be increased proportionally to maintain the proper acoustic mass. However, the necessary increase in length is not achievable.

[0006] Another problem may also occur when the length and cross-sectional area of the port increase. Organ pipe resonances occur within the open duct at frequencies inversely proportional to the length of the duct. These organ pipe resonances can easily cause audible frequency distortion when they occur within a certain frequency range. For example, a duct with a length of 9 inches (22.86 cm) exhibits a primary resonance at a relatively high audible frequency of approximately 700 Hz, and a duct with a length of only 3 inches (7.62 cm) will exhibit a primary resonance at a very low audible frequency of approximately 2,100 Hz. In fact, the typical approach used in the design of vented loudspeaker systems is to use short ports such that organ pipe resonances occur at high frequencies where the audibility of the organ pipe resonances is low and where they are unlikely to be within the range of the transducer mounted within the enclosure. Additionally, a large cross-sectional area may lead to undesirable transmission of midrange frequencies generated within the enclosure to the outside. This can 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] Accordingly, the design of the port of a vented loudspeaker system has conflicting requirements. A large cross-sectional area is necessary to avoid audible noise and losses due to non-linear turbulence, but this makes it difficult to achieve the acoustic mass required for a low resonance frequency within the constraints of a practical size. As those skilled in the art are well aware, various methods have been employed to construct ports with reduced turbulence and losses. Referring to the example shown in FIG. 1A, in cross-section, the loudspeaker enclosure 100 has a transducer 102 and a port 104, and the port 104 has its ends flared in a horn shape at one or both ends to reduce turbulence. The flared port 104 increases the port cross-sectional area at one or both ends, thereby acting to reduce turbulence by decelerating the particle velocity of the air at the outlet. This takes into account the small cross-sectional area and high acoustic mass of the middle section of the port for a given length. However, the ends 106, 108 of the required horn flare are extremely large in diameter in order to be effective, and they may themselves significantly increase the overall length of the port without contributing much to the acoustic mass. An increase in the cross-sectional area of the horn flare may increase the transmission of unwanted mid-range frequencies from inside the loudspeaker cabinet, and an inappropriately selected degree of horn flare 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 a number of ports 204, 206. Using a number of ports 204, 206, turbulence and loss are reduced by utilizing the sum of the cross-sectional areas of several ports. However, as in the case of a single port, the length of each of the multiple ports has to be made longer taking into account the increase in the total cross-section. For example, if two identical ports are used, both of these 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 the same tuning frequency. As described above, this can result in unrealistic length requirements and large audible frequency organ pipe resonances.

[0009] Other techniques are also used to reduce turbulence and loss and other problems associated with the port designs described above. These techniques include ports with rounded or flanged ends, geometric shapes for reducing organ pipe resonance, and many ways of implementing long ports by folding or other turns.

[0010] U.S. Patent No. 5,517,573 (hereinafter referred to as the " '573 Patent Specification") and U.S. Patent No. 5,809,154, both of which are commonly owned, disclose an improved port design method for achieving the required acoustic mass in a compact space with reduced turbulence and loss. These U.S. patents are hereby incorporated by reference in their entirety and made a part of this specification. FIG. 1C reproduces FIG. 7 of the '573 Patent Specification. In the method described in these U.S. patent specifications, disks are used 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 structure. This method has the advantages of suppressing the transmission of mid-range frequencies from the inside of the cabinet and providing the required acoustic mass in a more compact form that also reduces loss. However, such a compact form may also, in certain configurations, give rise to problems associated with audible frequency organ pipe resonance. These difficulties are addressed in other ported cabinet configurations shown in FIGS. 1C, 1D, and 1F taken from U.S. Patent No. 7,162,049 of the common owner, which is also incorporated by reference and made a part of this specification.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0012] The vented loudspeakers of FIGS. 1A-1F were developed to provide an increased output above these low-frequency tuning frequencies. One disadvantage of these vented design examples is that the vent exhibits an acoustic resonance that significantly exceeds the desired primary Helmholtz resonance associated with the low-frequency system. These resonances are often audible and also affect the resonances associated with the frequency and time of the system in the midrange. Removing or reducing the amplitude of these resonances improves the midrange performance of the system. Also, it is desired to reduce the audibility of port noise. Such prior art methods and structures and methods for reducing these problems, such as reducing the cross-sectional area of the port, result in an increase in air velocity that increases secondary effects, such as turbulence (and port noise). Electrical correction of port noise or chuffing is not possible because the vent is not directly driven by the associated electronics but is driven by a transducer within the system.

[0013] Accordingly, there is a need for an effective system and method for tuning a ported loudspeaker to reduce unwanted acoustic resonances while reducing port noise, eliminating unwanted port resonances, and improving the accuracy and fidelity of the reproduced sound within a relatively efficient loudspeaker system.

Means for Solving the Problems

[0014] According to the present invention, an effective system and method for tuning a ported loudspeaker to reduce unwanted acoustic resonances reduces port noise, eliminates unwanted port resonances, and improves the accuracy and fidelity of the reproduced sound within a relatively efficient loudspeaker system.

[0015] The loudspeaker system and enclosure of the present invention have a vent with a lumen that fluidly communicates the internal volume of the enclosure with the external ambient environment, and the lumen of the vent contains one or a set of unique sound filters ("ETF") cylinders disposed within the vent that absorb the "open tube" acoustic resonance of the vent. This open tube acoustic resonance is generally undesirable and interferes with or degrades the midrange performance of the loudspeaker.

[0016] This ETF-equipped loudspeaker system and enclosure of the present invention have several advantages, such as (a) the ETF system ("ETF") is passive and thus does not require the operation of electrical or data signal processing ("DSP"), (b) the ETF is relatively inexpensive and made of several simple components, (c) tuning the ETF system absorber can absorb vent resonance, cabinet resonance, or both, (d) when using a double-cylinder ETF system, tuning the individual absorbers separately can handle different resonances, (e) the ETF is visible from the outside of the loudspeaker enclosure and thus has a marketing advantage compared to internal solutions, and (f) the ETF-equipped loudspeaker system and enclosure can reduce audible frequency port noise during use.

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

[0018] During the development of the applicant's prototype, it was noted that two of these open-ended air columns could be placed face-to-face, with these openings located near the center of the open-ended air columns. Many advantages of this configuration were observed. One is that since two air columns have a larger surface area than one air column, a large absorption rate is possible. Second, the air columns can be arranged concentrically, and as a result, the degree to which the flow in the primary air column is disrupted by changes in cross-sectional area is small. Thirdly, the absorption air columns can be easily arranged within the primary air column, because these absorption air columns can then be attached to features at the ends or outside the main air column. Also, by tapering the ends of the closed air columns, the quality (Q) of the absorber is reduced, thereby making it possible to tune the ETF absorber to better match the quality of resonance within the main air column. The tapering of the prototype was also observed to reduce the turbulence within the main air column at the ends, because these prototypes are aerodynamic. In other prototypes, forms, fibers and other acoustic resistance elements were inserted into the absorber in a configuration that was also configured to modify and affect the quality (Q). These acoustic resistance elements were observed to work well at the closed (i.e., bottom) ends, but good overall performance was obtained with the absorber placed at the opening, in which form the easiest tuning method was provided that provided good performance with a reduced amount of undesirable secondary results.

[0019] The ETF-equipped loudspeaker system and enclosure of the present invention were prototyped in the form of a round vent for a loudspeaker, but it is preferred that the principles and methods of the present invention can be modified to be useful with other shaped vents. The absorber also does not need to be round.

[0020] Two preferred embodiments have been developed during the prototype stage. One is a typical book-shelf type loudspeaker embodiment. The other is a loudspeaker embodiment with a Power Port (trademark) type floor-standing (tower) vent structure. In the case of the Power Port (trademark) type vent structure, the ETF absorber is preferably provided within the base diffuser portion to provide an aesthetically pleasing, high-efficiency and economical embodiment.

[0021] The end correction for the ETF absorber tends to be smaller than that of the main air column, and thus there should be a gap between the two absorbers. In the case of the Power Port (trademark) type vent structure, the main air column extends beyond a simple cylindrical portion, 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 attached at the end of the main air column or to the flare portion outside 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 decreases. A diameter-to-length ratio of 1:1.25 is preferred (i.e., the ratio of the diameter ID of the ETF absorber cylinder to the length of the ETF cylinder gap between them, for example, absorber diameter = 25 mm, gap between absorbers = 20 - 25 mm).

[0023] The dimensions of the absorber affect the effectiveness of the absorber. A large cross-sectional area is equivalent to good absorption rate. Since the absorber subtracts from the cross-sectional area of the main air column, it is usually optimal to keep the absorber as small as necessary to achieve the desired absorption rate. A ratio of 0.15:0.2 of the absorber cross-sectional area to the 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, which is because the cross-sectional area of the main air column only decreases by the cross-sectional area of the absorber assembly. It is sufficient to simply 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 within a loudspeaker cabinet often exit through the vent and can be absorbed by the ETF absorber if properly tuned. This has been demonstrated in prototypes.

[0026] The above features and advantages of the present invention, as well as other features and advantages, will become apparent upon consideration of the following detailed description of specific embodiments of the invention, particularly in connection with the accompanying drawings. The same reference numerals in the various figures are used to indicate the same components.

Brief Description of the Drawings

[0027]

Figure 1A

Figure 1B

Figure 1C

Figure 1D

Figure 1E

Figure 1F

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6

Figure 7

Figure 8A

Figure 8B

Figure 8C

Figure 9

[0028] Referring to FIGS. 2-9, according to the present invention, there is provided an efficient system and method for tuning a ported loudspeaker and reducing unwanted acoustic resonances, eliminating unwanted port resonances, and improving the accuracy and fidelity of reproduced sound in a relatively efficient loudspeaker system.

[0029] The ETF-equipped loudspeaker system (e.g., 700 or 800) and enclosure of the present invention include a vent having 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) cylinders disposed within the vent to absorb the "open tube" acoustic resonance of the vent. This open tube acoustic resonance is typically undesirable and interferes with or degrades the midrange performance of the loudspeaker. As described above, the ETF-equipped loudspeaker system and enclosure have several advantages compared to the prior art of FIGS. 1A-1F, and these advantages are as follows. 1) The ETF system (e.g., 720A, 720B or 820) is passive and thus does not require the operation of electrical or digital signal processing ("DPS"). 2) The ETF system (e.g., 720A, 720B or 820) is relatively inexpensive and is made of a few simple components. 3) The absorber cylinder of the ETF system can be dimensioned (e.g., "tuned") to absorb vent resonance, cabinet resonance, or both. 4) In the case of a dual-tube ETF system, the individual absorbers can be tuned separately to handle different resonances from each other. 5) The ETF system is visible from outside the loudspeaker enclosure and thus has a marketing advantage compared to internal solutions. 6) The ETF-equipped loudspeaker system (e.g., 700, 800) and the enclosure reduce audible frequency port noise.

[0030] The ETF-equipped loudspeaker system (e.g., 700, 800) was developed after observing that an air column with both ends open exhibits acoustic resonance, and its wavelength is twice the sum of the length of the air column plus some amount accounting for end correction. Similarly, an air column with one end closed exhibits resonance where the wavelength is four times the sum of the length of the air column plus the end correction. By placing the open end of a closed air column, whose length is approximately half, near the center of the open air column, it was observed that the closed air column acts as an absorber at the resonance frequency of the open air column.

[0031] During the development work of the applicant's prototype, it was noted that two of these open-ended air columns could be placed face-to-face, with these openings located near the center of the open-ended air columns. Many advantages of this configuration were observed. Firstly, since two air columns have a larger surface area than one air column, a greater absorption rate is possible. Secondly, the air columns can be arranged concentrically, and as a result, the flow in the primary air column is less disturbed by changes in cross-sectional area. Thirdly, the absorption air columns can be easily placed inside the primary air column, because these absorption air columns can then be attached to the ends or to features outside the main air column. Also, by tapering the ends of the closed air columns, the quality (Q) of the absorber is reduced, thereby making it possible to tune the ETF absorber to better match the quality of resonance in the main air column. It was also observed that the tapering of the prototype reduces the turbulence in the main air column at the ends, because these prototypes are aerodynamic. In other prototypes, foam, fiber, and other acoustic resistance elements were inserted into the absorber in a configuration that was also configured to modify and affect the quality (Q). These acoustic resistance elements were observed to work well at the closed (i.e., bottom) ends, but good overall performance was obtained with the absorber placed at the opening, and that configuration provided the easiest tuning method to provide good performance with a reduced amount of undesirable secondary results. As a variant, the acoustic resistance elements could be placed elsewhere in the absorber.

[0032] The ETF-equipped loudspeaker system and enclosure of the present invention were prototyped in the form of a round vent for the loudspeaker, but it is preferable that the principles and methods of the present invention can be modified to be useful with vents of other shapes. The absorber also does not need to be round.

[0033] Two embodiments are shown in FIGS. 2-9. One is an ETF-equipped bookshelf-shaped loudspeaker system 700 (shown best in FIGS. 2 and 6). The other embodiment is a floor-standing (tower) loudspeaker system 800 with an ETF-equipped Power Port™ (shown best in FIGS. 3 and 8A-8C). In the case of the ETF-equipped Power Port™, the ETF absorber assembly 820 is preferably provided within the base diffuser portion. This is convenient and cost-saving.

[0034] Referring again to FIGS. 2 and 6 and further to FIGS. 4A and 4B, a bookshelf-sized embodiment 700 of the 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 midwoofer and a tweeter) and a rear baffle that supports an ETF assembly (e.g., 720A). The ported loudspeaker enclosure 710 includes an internal volume portion that is port-connected to the surrounding environment by a vent or port 730, and the vent or port 730 includes a cylindrical internal vent lumen 740 having a vent lumen central axis. The ETF assembly 720A is supported within the vent lumen 740 in a coaxial alignment relationship with the vent lumen axis, and this ETF assembly has one or a set of cylinders or absorbers (750, 760) disposed within the loudspeaker vent lumen to absorb the "open tube" acoustic resonance of the vent lumen 740 during use of the loudspeaker. The ETF assembly 720A (visible in FIG. 6) includes a proximal closed-end cap and a distal rear-projecting end cap opposite thereto, and at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the internal volume portions of the first and second axially aligned ETF cylinder segments or absorbers (750, 760) and the vent lumen 740. Since the vent or port 730 includes a tuned port that enables fluid communication between the interior of the enclosure 710 and the surrounding environment, the vent or port 730 also enables fluid communication between each of the enclosure and the surrounding environment and the internal volume portion of the ETF absorber for the ETF assembly 720A. FIGS. 2, 4A, and 4B provide a slightly different embodiment of the ETF assembly (e.g., 720B) used in the bookshelf-shaped system 700 in that both ends of the ETF cylinder preferably support round or "bullet tip" end caps that house absorber elements.Referring back to FIG. 2 and referring to this, the ETF assembly 720B includes a proximal closed end cap and a distal end cap that protrudes rearwardly on the opposite side thereof, and at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the inner volume portions of the first and second axially aligned ETF tube segments or absorbers and the vent lumen 740. Each of the first and second axially aligned ETF tube segments or absorbers (750, 760) has an axial length that is approximately one-quarter wavelength for the frequencies of interest (e.g., 155 mm for 562 Hz and 122 mm for 789 Hz, which has brought about the changes shown in FIG. 7).

[0035] In the method developed based on the present invention, selecting (i.e., "tuning") the dimensions for the ETF tube was an iterative process. In an embodiment of the bookshelf loudspeaker system 700, the "stock port" data plotted in FIG. 7 shows an undesirable amount of energy in the range from 550 Hz to 800 Hz. In order to reduce or "notch out" this undesirable energy by the bookshelf speaker system ETF 700A, the ETF tube segments need to be properly dimensioned and shaped (or "tuned"). Detailed embodiments are provided below (for the tower system 800).

[0036] Next, referring to FIGS. 3, 8A-8C and further to FIGS. 5A and 5B, the floor-standing or tower-sized embodiment of the 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). The ported tower-type loudspeaker enclosure 810 includes an internal volume portion that is port-connected to the surrounding environment via a vent or port 830, and the vent or port 830 includes a cylindrical inner vent lumen 840 having a vent lumen central axis. The ETF assembly 820 is supported within the vent lumen 840 in a coaxial alignment relationship with the vent lumen axis, and the ETF assembly 820 includes one cylinder or absorber or a set of cylinders or absorbers (850, 860) disposed within the loudspeaker vent lumen to absorb the "open tube" acoustic resonance of the vent lumen 840 during use of the loudspeaker. The ETF assembly 820 (visible in FIGS. 3 and 8B) includes a proximal closed-end cap and a distal end cap that protrudes downwardly on the opposite side fitted within a Power Port™ diffuser, and at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the internal volume portions of the first and second axially aligned ETF cylinder segments or absorbers (850, 860) and the vent lumen 840.

[0037] The vent or port 830 comprises a tuned port that enables fluid communication between the interior of the enclosure 810 and the ambient environment, so the vent or port 830 also enables fluid communication between each of the enclosure and the ambient environment and the internal volume of the ETF pipe for the ETF assembly 820. FIGS. 5A and 5B provide slightly different embodiments of the ETF assembly 820 used in the tower-type system 800, showing a state where the proximal, internal or upper end of the ETF cylinder preferably supports a round or "bullet tip" end cap 870 that houses an absorber element (not shown). Referring back to FIG. 3, the ETF assembly 820 includes a proximal closed end cap and a distal end cap that protrudes downward on the opposite side, and at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the internal volumes of the first and second axially aligned ETF cylinder segments and the vent lumen 840. Each of the first and second axially aligned ETF cylinder 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) in the case of 38 mm ID.

[0038] In the method developed based on the present invention, selecting (i.e., "tuning") the dimensions for the ETF tube was an iterative process. In an embodiment of the tower - type loudspeaker system 800, the "stock port" data plotted in FIG. 9 shows an undesirable amount of energy in the range from 500 Hz to 750 Hz. To reduce or "notch out" this undesirable energy with the tower - type speaker system ETF820, the ETF tube segments need to be properly sized and shaped (or "tuned"). Assuming the speed of sound at 20°C is initially 343 m / s and using 100 mm, a quarter - wavelength frequency of f = 343 / (0.1×4)=857.5 Hz is obtained. For a 38 - mm ETF tube, adding 0.3×38 = 11.4 mm for end correction (according to some references), thereby changing the above to f = 343 / (0.1114×4)=769.7 Hz. Since this is not a completely open tube, the applicant believes that this initial frequency tuning estimate may not be 100% accurate. Adding approximately 38 mm of foam into the 100 - mm ETF reduces the Q of the tube and somewhat slows down the air velocity within the ETF, thereby taking into account the change to 756 Hz in the measured minimum difference curve as shown in the "ported with ETF" data plotted in FIG. 9. Similarly, for a 150 - mm tuning, without end correction f = 343 / (0.15×4)=571.7 Hz, and with end correction f = 343 / (0.1614×4)=531.3 Hz. Thus, after adding approximately 76 foam to the 150 - mm ETF, the frequency becomes f = 494 Hz. As will be understood by those skilled in the art, this tuning does not seem to have room for preliminary and accurate calculations. This is because there is no direct 1:1 relationship between the length and the quarter - wavelength frequency.

[0039] The ETF cylinder assembly (e.g., 820) tends to have an end correction that is smaller than the main air column. Therefore, there needs to be a gap between the two absorbers (e.g., 850, 860). In the case of the Power Port (trademark) embodiment shown in FIG. 3, that is, for a primary air column that extends beyond a simple cylinder portion, the absorber assembly 820 tends to be longer than the assembly for the main air column (e.g., as shown in FIGS. 5A and 5B). This allows the absorber assembly 820 to be conveniently attached at or outside the end of the main air column to the flare portion.

[0040] The circumferential slots or sidewall gap openings (e.g., 755, 855) between two axially aligned ETF cylinders or cylindrical absorbers (e.g., 850, 860) affect the efficiency of the absorber having 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 cylinder decreases. Preferably, the gap length between the absorbers and the diameter of the absorbers are selected such that the cylinder diameter is 1 to 1.25 times the length of the gap between these absorbers (therefore, for example, if the diameter of the absorber = 25 mm, the axial gap length between the absorbers = 20 - 25 mm).

[0041] The size of the absorber cylinder affects the efficiency of the absorber. A large cross-sectional area is equivalent to good absorption. Since the absorber subtracts from the cross-sectional area of the main air column (e.g., vent or port 830), it is considered optimal at present to keep the absorber as small as necessary to achieve the desired absorption. A ratio of 0.15:0.2 of the absorber cross-sectional area to the cross-sectional area of the main air column (or vent lumen) was determined to work best in the prototype retrofit example. The Helmholtz tuning of the main air column (e.g., vent lumen 730 or 830) changes with the insertion length of the absorber assembly. This is because the cross-sectional area of the main air column only decreases by the cross-sectional area of the absorber assembly. To compensate for this, it is sufficient to 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 that are not necessarily generated by the main air column (or vent lumen 740 or 840). For example, resonances (modes) present within a loudspeaker cabinet often exit through the vent and, if properly tuned, can be absorbed by the absorber. This has been demonstrated in the prototype.

[0043] Although the preferred embodiments of the novel and improved system and method have been described, other modifications, variations, and changes may be envisioned by those skilled in the art in view of the teachings described herein. Accordingly, it is to be understood that all such variations, modifications, and changes are considered to be within the scope of the present invention.

Claims

1. A method for tuning a ported loudspeaker (e.g., 700 or 800), reducing port noise while reducing unwanted acoustic resonances, eliminating unwanted port resonances, and improving the accuracy and fidelity of the reproduced sound in a relatively efficient loudspeaker system, comprising: providing a ported loudspeaker enclosure including a first baffle supporting at least a midrange or midbass driver, said enclosure having an internal volume portion port-connected to the ambient environment through a first vent lumen having a central axis; placing in said first vent lumen, in alignment with said lumen central axis, eigen-tone filter structures ("ETFs") 720, 820, said ETFs having one or more tubes with an ETF tube internal volume portion and an opening configured or tuned to absorb the "open tube" acoustic resonance of said vent lumen when said loudspeaker is operating; wherein said absorbed open tube resonance is substantially attenuated, thereby improving the midrange performance of said loudspeaker.

2. The step of placing said ETFs includes placing first and second axially aligned ETF tube segments, said ETF tube internal volume portion being in fluid communication with an opening configured or tuned to absorb the "open tube" acoustic resonance of said vent lumen. The method according to claim 1.

3. The step of placing said ETFs includes placing a first ETF tube segment having a first segment length in substantially coaxial alignment with a second ETF tube segment having a second segment length, said first ETF tube segment length being selected to have a value that is approximately one-quarter wavelength at a first selected ETF port signal notch frequency within a frequency band including the open tube resonance of said vent lumen. The method according to claim 2.

4. The second ETF tube segment length is also selected to have a value that is approximately one-quarter wavelength at a second selected ETF port signal notch frequency within a frequency band including the open tube resonance of said vent lumen. The method according to claim 3.

5. The method step of selecting (i.e., "tuning") the dimensions of the ETF tube is an iterative process, the method according to claim 4.

6. The "stock port" data of the loudspeaker system is plotted to identify a frequency range having unwanted open - tube resonance energy (e.g., in the range from 500 Hz to 750 Hz), The ETF pipe segment is sized and shaped (or "tuned") to reduce or "notch out" the unwanted open - tube resonance energy by the ETF820, the method according to claim 5.

7. The method further includes a method step of removing the quarter - wavelength frequency (e.g., for a 100 mm ETF, f = 343 / (0.1×4)=857.5 Hz) to obtain an initial frequency tuning estimate, the method according to claim 6.

8. The method further includes a method step of adding a foam material into the ETF material to determine the effect of slowing down the air velocity within the ETF, the method according to claim 7.

9. A loudspeaker system (e.g., 700 or 800), comprising a ported loudspeaker enclosure having a first baffle supporting at least a mid - range or mid - bass driver, the enclosure having an internal volume portion port - connected to the ambient environment by a first vent lumen, the first vent lumen including a tuned Helmholtz filter structure ("ETF") 720, 820, the ETF having at least a first tube segment disposed within the loudspeaker vent lumen to substantially absorb and reduce the "open - tube" acoustic resonance of the vent lumen, a loudspeaker system.

10. The ETF has first and second coaxially aligned ETF tube segments, and the ETF tube internal volume portion is in fluid communication with an opening configured or tuned to absorb the "open - tube" acoustic resonance of the vent lumen, the loudspeaker system according to claim 9.

11. The ETF has a first ETF barrel segment having a first segment length, and the first ETF barrel segment is in a substantially coaxial alignment relationship with a second ETF barrel segment having a second segment length. The first ETF barrel segment length is selected to have a value that is approximately one-quarter wavelength at a first selected ETF port signal notch frequency within a frequency band including the open tube resonance of the vent lumen. The loudspeaker system according to claim 10.

12. The second ETF barrel segment length is also selected to have a value that is approximately one-quarter wavelength at a second selected ETF port signal notch frequency within a frequency band including the open tube resonance of the vent lumen. The method according to claim 11.

13. The loudspeaker system 800 equipped with the ETF 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). The ported tower-type loudspeaker enclosure 810 includes an internal volume portion port-connected to the surrounding environment by a vent or port 830. The vent or port includes a cylindrical inner vent lumen 840 having a vent lumen central axis. The ETF assembly 820 is supported within the vent lumen 840 in a coaxial alignment relationship with the vent lumen axis. The ETF assembly 820 includes one tube or a set of tubes or absorbers (850, 860) disposed within the loudspeaker vent lumen to absorb the "open tube" acoustic resonance of the vent lumen 840 during use of the loudspeaker. The ETF assembly 820 (visible in FIGS. 3 and 8B) includes a proximal closed-end cap and a distal lower protruding end cap fitted within a Power Port (trademark) type diffuser, and at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the internal volume portion of the first and second axially aligned ETF barrel segments or absorbers (850, 860) and the vent lumen 840. The loudspeaker system according to claim 12.

14. The vent or port 830 constitutes a tuned port that enables fluid communication between the interior of the enclosure 810 and the ambient environment and between each of the interior and ambient environment of the enclosure 810 and the internal volume of the ETF tube for the ETF assembly 820. The proximal, inner or upper end of the ETF tube assembly preferably supports a round or "bullet tip" end cap 870 that houses an absorber element. The ETF tube assembly includes, at an intermediate location thereof, a circumferential slot or sidewall gap that enables fluid communication between the internal volume of the ETF tube segments aligned in the first and second axial directions and the vent lumen 840. The ETF tube segments or absorbers (850, 860) aligned in the first and second axial directions preferably have an axial length that is substantially equal to a quarter wavelength for a frequency of interest. The loudspeaker system according to claim 13.

15. The ETF tube segments or absorbers (850, 860) aligned in the first and second axial directions preferably have an axial length that is substantially equal to 150 mm for 494 Hz when the ID is 38 mm. The loudspeaker system according to claim 14.

16. The ETF tube segments or absorbers (850, 860) aligned in the first and second axial directions preferably have an axial length that is substantially equal to 100 mm for 756 Hz when the ID is 38 mm. The loudspeaker system according to claim 14.

17. The natural sound filter structures ("ETF") 720, 820 are aligned substantially coaxially with the vent port lumen and have an inner diameter selected to be in the range from 25 mm to 38 mm. The loudspeaker system according to claim 9.

18. The intrinsic sound filter structure ("ETF") has ETF cylinder segments or absorbers (850, 860) aligned in first and second axial directions, and circumferential slots or sidewall gaps (e.g., 855) are provided between the ETF cylinder segments or absorbers. The sidewall gap length between the ETF cylinder segments is selected to be in the range of 1 to 1.25 times the diameter of the absorber (thus, for example, in the case of a 25 mm diameter ETF cylinder segment, the axial length of the gap or slot between the ETF cylinder segments is selected to be 20 - 25 mm), the loudspeaker system according to claim 18.

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