Transmission line loudspeaker

An electro-acoustic driver including an acoustic waveguide includes an enclosure, an acoustic transmission line formed within the enclosure, and a plurality of acoustic transducers contained within the enclosure and disposed along a length of the acoustic transmission line. Each acoustic transducer is configured to emit acoustic energy directly into the acoustic transmission line at two separated locations along the length of the acoustic transmission line.

BACKGROUND

This invention relates to an acoustic transmission line loudspeaker.

Many conventional loudspeakers utilize waveguides to guide sound pressure waves along a convoluted path within their enclosures. Depending on the characteristics of a given waveguide, a certain portion of the energy present in the sound pressure waves is absorbed while traveling through the waveguide and another portion of the energy passes through the waveguide and is radiated as sound into an external environment. It is often the case that the waveguide is configured such that sound radiated from the waveguide enhances the low frequency output of the loudspeaker.

Some complex conventional loudspeakers include a number of volumes, at least some of which are connected by ports and/or passive radiators. Such loudspeakers include an acoustic transducer which radiates directly into one or two of the volumes. The sound radiated from the transducer propagates through the volumes, through the ports and/or passive radiators, and is eventually radiated into an external environment. The number and size of volumes along with the number, size, and placement of the ports and/or passive radiators are chosen to achieve a desired characteristic in the sound radiated into the external environment.

SUMMARY

In a general aspect, a loudspeaker including an acoustic waveguide includes an enclosure, an acoustic transmission line formed within the enclosure, and a plurality of acoustic transducers contained within the enclosure and disposed along a length of the acoustic transmission line, each acoustic transducer configured to emit acoustic energy directly into the acoustic transmission line at two separated locations along the length of the acoustic transmission line.

Aspects may include one or more of the following features.

The acoustic transmission line may be a folded acoustic transmission line, the enclosure may include an internal wall with each side of the internal wall forming at least some of a boundary of the folded acoustic transmission line, and the plurality of acoustic transducers may be disposed along the internal wall. The internal wall may be corrugated. The internal wall may include a plurality of ridges separated by a plurality of grooves, at least some of the plurality of grooves having one or more of the plurality of acoustic transducers disposed therein.

Each acoustic transducer may be configured to emit a first acoustic energy from a first location of the two separated locations along the length of the acoustic transmission line and to emit a second, complementary acoustic energy from a second location of the two separated locations along the length of the acoustic transmission line. The acoustic transmission line may have a closed end and an open end, the acoustic transmission line tapering from the open end to the closed end. The closed end of the acoustic transmission line may taper to a point.

A cross-sectional diameter of the transmission line at its open end may be greater than a cross-sectional diameter of the transmission line at its closed end. Each acoustic transducer may be a speaker driver. Each speaker driver may include a diaphragm having a front side and a back side, both sides configured to emit acoustic energy into the acoustic transmission line. The enclosure, the acoustic transmission line, and the plurality of acoustic transducers may be configured to generate an acoustic output having a band-pass characteristic. The enclosure, the acoustic transmission line, and the plurality of acoustic transducers may be configured to have two or more impedance minima.

The enclosure, the acoustic transmission line, and the plurality of acoustic transducers are configured to have two or more motion nulls at frequencies in a pass-band of the acoustic output.

Embodiments may include one or more of the following advantages:

Among other advantages, the acoustic transmission line of the loudspeaker reduces high frequency harmonic peaks when compared to conventional loudspeakers due to the closed end of the acoustic transmission line terminating in a point.

The loudspeaker has acoustic transducers mounted on the internal wall such that both sides of the acoustic transducers emit energy into the acoustic transmission line. This reduces high frequency output and improves low frequency output when compared to conventional loudspeakers with acoustic transducers mounted on an external wall.

The loudspeaker has a single outlet and therefore requires no grilles allowing for the placement of objects onto the loudspeaker.

The acoustic transmission line has an inverted taper causing the outlet into the outside environment to be large, resulting in a decrease in the velocity of air leaving the loudspeaker as compared to conventional loudspeakers.

Due to the modifiable shape of the internal wall, the loudspeaker can be configured into a number of different application-specific form factors.

In other aspect, an acoustic waveguide system may comprise an enclosure having a closed end and an open end; an acoustic transmission line within the enclosure; and at least one electro-acoustic transducer disposed along a length of the acoustic transmission line to emit acoustic energy directly into the acoustic transmission line, and constructed and arranged to prohibit exciting at least one resonant mode above a fundamental resonant mode of the acoustic waveguide system.

Aspects may include one or more of the following features.

The at least one electro-acoustic transducer has a front side and a rear side. The acoustic energy output from the front side and the rear side may be out of phase, such that the at least one electro-acoustic transducer prohibits exciting the at least one resonant mode.

The acoustic transmission line may be a folded acoustic transmission line. The enclosure comprises an internal wall with each side of the internal wall forming at least some of a boundary of the folded acoustic transmission line. The at least one electro-acoustic transducer may be disposed along the internal wall.

The at least one electro-acoustic transducer may be coupled to the internal wall such that a front side and a rear side of the electro-acoustic transducer are symmetric about a point along the length of the acoustic transmission line.

The at least one electro-acoustic transducer may prohibit exciting the first resonant mode above the fundamental resonant mode of the acoustic waveguide system.

The point along the length of the acoustic transmission line may be at approximately one third of the length of the acoustic transmission line, measured from the open end of the enclosure.

The at least one electro-acoustic transducer may prohibit exciting the second resonant mode above the fundamental resonant mode of the acoustic waveguide system.

The at least one electro-acoustic transducer may be coupled to an internal wall of the enclosure such that a front side and a rear side of the electro-acoustic transducer are symmetric about a point on the acoustic transmission line. The point may be at approximately one fifth of the length of the acoustic transmission line, measured from the open end of the enclosure.

The at least one electro-acoustic transducer may comprise a plurality of electro-acoustic transducers, and wherein none of the electro-acoustic transducers excite the at least one resonant mode above the fundamental resonant mode.

The acoustic transmission line comprises at least two folds. The at least one electro-acoustic transducer may be arranged at a fold of the at least two folds nearest the open end of the acoustic transmission line.

The acoustic waveguide system may comprise a tapered acoustic transmission line that tapers from the closed end to the open end, and further comprises internal and external walls having a curved geometry.

The internal wall may comprise a fold of the waveguide system such that the internal wall includes locations along the internal wall such that distances on one side of the internal wall versus the other side of the internal wall maintains a match in pressure amplitude according to a mode function.

In other aspect, an acoustic waveguide system may comprise an enclosure having a closed end, an open end, and an internal wall; an acoustic transmission line within the enclosure; and at least one electro-acoustic transducer disposed along a length of the acoustic transmission line to emit acoustic energy directly into the acoustic transmission line. The at least one electro-acoustic transducer may be coupled to the internal wall such that a front side and a rear side of the electro-acoustic transducer are symmetric about a point along the length of the acoustic transmission line where the at least one electro-acoustic transducer prohibits exciting at least one resonant mode above a fundamental resonant mode of the acoustic waveguide system.

Aspects may include one or more of the following features.

The acoustic transmission line may be a folded acoustic transmission line.

The point along the length of the acoustic transmission line may be at approximately one third of the length of the acoustic transmission line, measured from the open end of the enclosure.

The at least one electro-acoustic transducer prohibits may excite a second resonant mode above the fundamental resonant mode of the acoustic waveguide system.

The at least one electro-acoustic transducer may be coupled to an internal wall of the enclosure such that a front side and a rear side of the electro-acoustic transducer are symmetric about a point along the length of the acoustic transmission line. The point may be at approximately one fifth of the length of the acoustic transmission line, measured from the open end of the enclosure.

The acoustic transmission line comprises at least two folds. The at least one electro-acoustic transducer may be arranged at a fold of the at least two folds nearest the open end of the acoustic transmission line.

The acoustic waveguide system may comprise a tapered acoustic transmission line that tapers from the closed end to the open end, and further comprises internal and external walls having a curved geometry.

In another aspect, an acoustic waveguide system may comprise an enclosure having a closed end and an open end; a tapered acoustic transmission line within the enclosure, the tapered acoustic transmission line comprising internal and external walls, each having a curved geometry; and at least one electro-acoustic transducer disposed along a length of the acoustic transmission line to emit acoustic energy directly into the acoustic transmission line. The at least one electro-acoustic transducer may be positioned along the acoustic transmission line in the enclosure to drive at least one resonant mode above the fundamental resonant mode at a same amplitude and phase on a front side and a back side of the at least one electro-acoustic transducer.

The curved geometry may comprise locations along which a distance on one side of the waveguide system versus the other side of the waveguide system maintains a match in pressure amplitude according to a mode function.

DESCRIPTION

Referring toFIG. 1, a loudspeaker100includes a substantially hollow enclosure102including an internal wall110and a number of acoustic transducers106(i.e., drivers) disposed within the enclosure102.

In some examples, the enclosure102includes an opening107at a first end122of the enclosure102, a substantially rounded u-shaped inner side surface108, an inner top surface118(shown transparently for the purpose of providing visibility into the enclosure102of the loudspeaker100), and an inner bottom surface120. The internal wall110extends from the inner side surface108at a point near or at the first end122of the enclosure102and partially along a length, L, of the enclosure102. The internal wall110also extends from the inner bottom surface120to the inner top surface118of the enclosure102.

2 Acoustic Transmission Line

The inner surface108of the enclosure102together with the internal wall110forms a boundary of an acoustic transmission line104. The term “acoustic transmission line,” as used herein refers to a rigid walled, tubular structure through which sound pressure waves propagate without encountering impediments such as ported walls. In general, an “acoustic transmission line” is long and thin as compared to the wavelength of sound pressure waves present therein. In some examples, a fundamental tuning frequency of the acoustic transmission line is defined by the length of the acoustic transmission line. For example, the modes of a straight waveguide are given by:

fn=2⁢⁢n-14⁢cL-,
where c is the speed of sound and L is the length of the waveguide. Normalizing the modes in terms of c/L gives the frequencies as 0.25, 0.75, 1.25, and so on.

Referring toFIG. 2, the first three modal distribution functions for a straight-walled waveguide of length L are illustrated with the open end on the left. For a waveguide with a length, L, of 2 meters, the frequencies of the modes are 42.4 Hz, 127.3 Hz, and 212.1 Hz.

In the loudspeaker100ofFIG. 1, the acoustic transmission line104is folded in that a first side115of the internal wall110forms a first part of the boundary of the acoustic transmission line104and a second side116of the internal wall110forms a second, different part of the boundary of the acoustic transmission line104. That is, the internal wall110serves as a shared boundary for at least some of the acoustic transmission line104.

The acoustic transmission line104has a first end112which is closed to an outside environment116and a second end114which opens to the outside environment116through the opening107in the enclosure102. In operation, acoustic energy present in the transmission line propagates from the first end112to the second end114and into the outside environment116through the opening107.

In some examples, the internal wall110extends in a direction along the length, L, of the enclosure102at an angle, θ relative to the inner side surface108. By extending at the angle, θ, the acoustic transmission line104is tapered such that a cross sectional area of the acoustic transmission line104at its first end112is smaller than a cross sectional area of the acoustic transmission line104at its second end114. In some examples, this type of taper is referred to as an “inverted taper.” In some examples, the taper of the acoustic transmission line104reduces a velocity and turbulence of the air exiting the acoustic transmission line104thereby reducing unwanted nose. In some examples it is desirable to maintain the velocity of air exiting the port at less than 15 m/s. Referring toFIG. 3, a plot of port velocity vs. frequency for a conventional waveguide (shown in green) and a band-pass waveguide (shown in red) illustrates a reduced port velocity for the band-pass waveguide at a number of frequencies.

In some examples, the angle, θ is adjusted to optimize the reduction in noise and to suppress the propagation of unwanted high frequency harmonic peaks. In some examples, the first end112of the acoustic transmission line104tapers to a point.

In some examples, a rounded (e.g., teardrop shaped) member124is disposed at a detached end126of the internal wall110for the purpose of facilitating the flow of air around the detached end126of the internal wall110. In some examples, the rounded member124reduces turbulence in the air as the air propagates past the detached end126of the internal wall110. In some examples a size of the teardrop shaped member124is made substantially large relative to the cross-section of the acoustic transmission line104in order to increase the path length of the acoustic transmission line104, thereby reducing the tuning frequency of the acoustic transmission line104.

In some examples, the output characteristic of the loudspeaker100can be varied by altering the physical characteristics of the acoustic transmission line104. For example, a loudspeaker designer may vary the length of the acoustic transmission line104, the angle, θ of taper of the acoustic transmission line104, the total volume of the acoustic transmission line104, the overall size of the enclosure102, the size of the opening107in the enclosure102, the length of the internal wall110, and so on.

In some examples, acoustically absorbent material (e.g., foam) is placed in the acoustic transmission line104(e.g., at the closed end112of the acoustic transmission line104) to attenuate harmonic peaks.

In some examples, the acoustic transducers106are conventional loudspeaker drivers, each having a diaphragm (e.g., a cone) which moves back and forth to generate pressure waves in the air in front of and behind the diaphragm. The acoustic transducers106are disposed through the internal wall110and therefore along a length of the acoustic transmission line104. Due to this arrangement, each transducer106is positioned and completely contained within the acoustic waveguide104such that the transducer emits acoustic pressure waves in a direction substantially perpendicular to the internal wall110and directly into the acoustic transmission line104at two separated locations along the length of the acoustic transmission line104.

For example, focusing on a single acoustic transducer106a, the acoustic transducer106ais disposed through the internal wall110such that a front side of the acoustic transducer's diaphragm faces into the acoustic transmission line104at a first location, L1, and a back side of the acoustic transducer's diaphragm faces into the acoustic transmission line104at a second location, L2, which is separated from L1along the length of the acoustic transmission line140.

When an electrical signal is applied to the acoustic transducer106a, the diaphragm of the acoustic transducer moves back and forth. Due to the movement of the diaphragm, the acoustic transducer106aemits acoustic pressure waves from the front of the diaphragm directly into the acoustic transmission line104at location L1. The acoustic transducer106aalso emits acoustic pressure waves from the back side of the diaphragm directly into the acoustic transmission line104at location L2.

In some examples, the acoustic transducers106are equally spaced. In other examples, the acoustic transducers106are unequally spaced to obtain a desired output characteristic (e.g., to reduce harmonic peaks at high frequencies).

In some examples, the number of acoustic transducers106can be increased or decreased, resulting in a corresponding increase or decrease in the total amount of diaphragm area present in the loudspeaker100. Increasing or decreasing the total amount of diaphragm area causes a corresponding increase or decrease in an output power of the loudspeaker100. In some examples, having a larger number of acoustic transducers106present in the loudspeaker100may result in better high frequency performance for the loudspeaker100due to an increased cone area which causes a spreading or randomization in the propagation of high frequency harmonic peaks as opposed to acting at a single narrow point. Alternately, a similar effect may be achieved by using fewer acoustic transducers, each with wider (e.g., oblong) cones that also spread out or randomize the propagation of high frequency harmonic peaks. In some examples, a single acoustic transducer with a cone spanning the internal wall110may be used.

In operation, an electrical signal is applied to one or more of the acoustic transducers causing the diaphragms of the one or more acoustic transducers to move back and forth. Due to the movement of the diaphragms, the acoustic transducers106emit acoustic pressure waves from both the front and back sides of their respective diaphragms directly into the acoustic transmission line104.

In some examples, the same electrical signal is provided to each of the acoustic transducers106, causing the acoustic transducers106to generate sound pressure waves in phase with one another.

In a simple example, when a sinusoidal electrical signal of sufficiently low frequency is provided in phase to each of the acoustic transducers106, the back sides of the diaphragms of the acoustic transducers106move toward the back sides of the acoustic transducers106causing an increase in acoustic pressure in the portion of the acoustic transmission104line behind the acoustic transducers106. Due to the shape of the acoustic transmission line104, the acoustic pressure generated behind the acoustic transducers106propagates through the acoustic transmission line104, in a direction from the first end112of the acoustic transmission line104to the second end114of the acoustic transmission line107.

As the acoustic pressure propagates into the portion of the acoustic transmission line104in front of the acoustic transducers106, the front sides of the diaphragms of the acoustic transducers106move toward the front of the acoustic transducers106, causing an additional increase in acoustic pressure (i.e., by constructive interference) in the portion of the acoustic transmission line104in front of the acoustic transducers106. In this way, the output of the loudspeaker100is boosted at certain frequencies by combining the acoustic pressure generated at the back sides of the acoustic transducers106with the acoustic pressure generated at the front sides of the acoustic transducers106. The combined acoustic pressure propagates to the outside environment116through the second end114of the acoustic transmission line104at the opening107in the enclosure102. Referring toFIG. 4, a plot of system output vs. frequency for a conventional acoustic transmission line (shown in red) and a band-pass waveguide (shown in green) illustrates a boost in output in the region 45 to 95 Hz. and at approximately 200 Hz.

In other examples, the phase of the electrical signal applied to the acoustic transducers106is varied to alter the characteristics of the sound pressure waves emitted into the outside environment116. In some examples, the phase of the electrical signal applied to the acoustic transducer106near the closed end112of the acoustic transmission line104is varied to alter frequency characteristics in a narrow frequency range around the fundamental tuning frequency of the acoustic transmission line104.

In yet other examples, different electrical signals are applied to each of the acoustic transducers106(or to subsets of the acoustic transducers106) to alter the characteristics of the sound pressure waves emitted into the outside environment116. For example, one or more acoustic transducers106near the closed end112of the acoustic transmission line104may be supplied with a higher voltage (causing a greater cone excursion) than the other acoustic transducers106successively spaced along wall110. In some examples, doing so has the same acoustic effect as if the inner wall110were pivoted at the teardrop shaped member124and the portion of the inner wall110near the closed end112of the acoustic transmission line104moved back and forth to generate pressure in the in the acoustic transmission line104.

Referring toFIG. 5, a simple example of an acoustic transmission line illustrates the effects of acoustic transducer placement and acoustic transmission line length. The acoustic transmission line includes two acoustic transducers #1, and #2. Transducer #1is disposed at the closed end of the acoustic transmission line and acoustic transducer #2is disposed at 1/10ththe length of the acoustic transmission line.

Referring toFIGS. 6 and 7, the system output vs. frequency as measured at 1 m from the opening of the acoustic transmission line ofFIG. 5and the acoustic transducer displacement vs. frequency of the two acoustic transducers ofFIG. 5are illustrated, respectively.

Referring toFIG. 8, the pressure load from the modes of the waveguide on the two acoustic transducers ofFIG. 5is illustrated along with the positions of the acoustic transducers in the modal distribution. InFIG. 8, the first acoustic transducer is sketched in blue with the front of the driver a solid line and the back a dashed line, similarly, the second acoustic transducer's position is shown in green.

It can be seen that at the first mode (shown in blue) the first acoustic transducer has high pressure on the front and little to no pressure on the back; the mode loads the acoustic transducer heavily at this frequency and reduces the displacement as seen at around 41 Hz in the acoustic transducer displacement plot ofFIG. 7. The second acoustic transducer is in a similar situation, with high pressure (but slightly lower than the first acoustic transducer) on the front and low pressure (but above zero) on the back, so, again, the mode loads the acoustic transducer and reduces displacement. The effect is smaller than on the first acoustic transducer because the pressure change is smaller—this can be seen in the displacement plot ofFIG. 7.

For the second mode (shown in green), the first acoustic transducer is again at high pressure on the front and low pressure on the back. The second acoustic transducer is at high pressure on the front and negative pressure on the back. The second mode very heavily loads the second acoustic transducer so the acoustic transducer displacement goes down significantly, as seen in the displacement plot ofFIG. 7.

Finally, for the third mode (shown in red), the first acoustic transducer is at high pressure on the front and zero pressure on the back. The second acoustic transducer, however, is at high pressure on the both the front and the back so this mode doesn't load this acoustic transducer and the displacement is unaffected in the displacement plot ofFIG. 7.

3 Experimental Results

Referring toFIG. 9, a graph of on-axis acoustic pressure vs. frequency is presented for one exemplary configuration of the loudspeaker100ofFIG. 1. The example loudspeaker100used to generate the data shown in the graph has an acoustic transmission line104with a length of 2 m, a 4° angle of taper, and an opening107with an area of 7E−3m2.

Due to the above-described physical characteristics of the loudspeaker100, the graph of on-axis pressure vs. frequency includes a first “fundamental” resonant peak228at approximately 52 Hz and a second resonant peak230at approximately 95 Hz. The second resonant peak230is the first harmonic of the fundamental resonant peak228occurring at 52 Hz. In some examples, internal turbulence and absorbent material can alter the frequency of the second resonant peak230.

Together, the two resonant peaks, which are closely grouped in frequency, result in a band-pass effect in the output of the loudspeaker100by boosting the output in the frequency range of 52 Hz-156 Hz and attenuating the output at frequencies above approximately 180 Hz.

Referring toFIG. 10, a graph of the magnitude of the output impedance of the example loudspeaker100described above includes a first impedance minimum234(indicating that a motion null near is nearby in frequency) at approximately 52 Hz and a second impedance minimum236at approximately 95 Hz.

When viewingFIG. 10in light ofFIG. 9, it becomes apparent that the two impedance minima234,236inFIG. 10are, as expected, approximately frequency aligned with the two resonant peaks228,230ofFIG. 9.

In some examples of closed ended acoustic transmission lines, a first motion null or impedance minimum occurs when the length of the waveguide is equal to ¼λ, where λ is the wavelength of the frequency being reproduced. A second motion null occurs when the length of the acoustic transmission line is equal to ¾λ, and a third motion null occurs at 5/4λ, and so on.

Referring toFIG. 11, another example of a loudspeaker400is similar to the loudspeaker100ofFIG. 1with the exception that the loudspeaker400has a corrugated internal wall410and a non-tapering acoustic transmission line404.

Owing to the corrugated shape of the internal wall410, acoustic transducers406can be installed in the internal wall410with an alternating direction of installation. That is, at least some of the acoustic transducers406are installed with their front sides facing outward from a first side415of the internal wall410and the remaining acoustic transducers406are installed with their front sides facing outward from a second, opposite side416of the internal wall410. In some examples, the alternating direction of installation of the transducer406reduces harmonic distortion due to a change in cone area that results from the cone travelling inward and outward in the acoustic transducer.

Furthermore, the corrugated wall allows for the acoustic transducers406to be disposed through the internal wall410such that they emit acoustic pressure waves in a direction substantially parallel to a direction of extension of the internal wall410and directly into an acoustic transmission line404at two separated locations along the length of the acoustic transmission line404.

The above-described arrangement of the acoustic transducers406in the corrugated internal wall410acts to reduce or cancel unwanted vibrations in the internal wall410. The corrugated internal wall410can also permit use of a reduced length acoustic transmission line404while maintaining the same number of acoustic transducers406(e.g., to reduce the overall size of the loudspeaker400) or to increase the number of acoustic transducers406while maintaining the length of the acoustic transmission line (e.g., to increase the output power of the loudspeaker400).

Referring toFIG. 12, another example of a loudspeaker500is similar to the loudspeaker100ofFIG. 1with the exception that internal wall510of the loudspeaker500is corrugated (having corrugation grooves540and corrugation ridges542) and is tapered.

Due to the corrugated shape of the internal wall510of the loudspeaker500, acoustic transducers506included in the loudspeaker500are disposed through the internal wall510such that they emit acoustic pressure waves in a direction substantially parallel to a direction of extension of the internal510and directly into an acoustic transmission line504at two separated locations along the length of the acoustic transmission line504.

Furthermore, the acoustic transducers506are installed in the internal wall510such that the front sides of the acoustic transducers506facing into a given corrugation groove540face one another and the back sides of the acoustic transducers506facing into another, different corrugation groove540face one another.

The above-described arrangement of the acoustic transducers506in the corrugated internal wall510acts to reduce or cancel unwanted vibrations in the internal wall510. The corrugated internal wall510can also permit use of a reduced length acoustic transmission line504while maintaining the same number of acoustic transducers506(e.g., to reduce the overall size of the loudspeaker500or to change the form factor of the loudspeaker500) or to increase the number of acoustic transducers506while maintaining the length of the acoustic transmission line (e.g., to increase the output power of the loudspeaker500).

In some examples, the corrugation grooves540of the corrugated internal wall510increase in depth as the corrugated internal wall510extends from a front side522of the enclosure502of the loudspeaker500to a back side544of the enclosure502. This increase in corrugation groove depth causes at least some of the acoustic transmission line504to taper at an angle, θ. The taper in the acoustic transmission line504provides the similar benefits as the taper in the acoustic transmission line104ofFIG. 1.

Referring toFIGS. 6-11, a number of alternative loudspeaker configurations include multiple drivers disposed in various configurations within acoustic transmission lines of various shapes and sizes.

Referring toFIG. 13, one alternative loudspeaker configuration600has an acoustic transmission line604extending past a first end622of an enclosure602. Referring toFIG. 14, another alternative loudspeaker configuration700has an acoustic transmission line704which does not extend all the way to a first end722of an enclosure702. Referring toFIG. 15, another alternative loudspeaker configuration800has a lengthened and substantially spiraling acoustic transmission line804. Referring toFIG. 16, another alternative loudspeaker configuration900has a bifurcated acoustic transmission line904. Referring toFIG. 17, another alternative loudspeaker configuration1000has two internal walls1010a,1010b, each having an acoustic transducer1006disposed therein. Referring toFIG. 18, another alternative “hybrid” loudspeaker configuration1100has one of its acoustic transducers1107emitting directly into an outside environment1116.

As described herein, an acoustic folded transmission line waveguide can be designed to include a compact enclosure and one or more electro-acoustic drivers or transducers. To provide bass reinforcement, waveguide systems provide any number of resonant modes, including a desirable fundamental mode that can reinforce an output at low frequencies. However, the higher frequency resonant modes of a waveguide system can lead to an uneven frequency response and be detrimental to the range of operation of the waveguide. Accordingly, it is desirable for a waveguide system to be configured to suppress the higher frequency waveguide modes.

One approach is to reduce the height of such peaks by positioning foam or other absorbent material in the waveguide. However, this approach undesirably lowers the waveguide output at the lowest frequencies, and accordingly, impacts the fundamental mode.

FIG. 19is another embodiment of a waveguide system1200that includes at least one electro-acoustic driver1206positioned within an acoustic folded transmission line pass-band waveguide1204. The at least one electro-acoustic driver1206has a front side1207and a back side1208, both of which emit acoustic energy directly into the acoustic transmission line.

The waveguide1204includes at least one electro-acoustic driver1206that drives the waveguide1204at two locations, i.e., at the back of the electro-acoustic driver1206(location A) and at the front of the electro-acoustic driver1206(location B). However, the waveguide1204is configured to have a single output at the opening of the waveguide1204(location C). The waveguide1204can have a uniform cross-sectional area, for example, a rectangular cross-section or a hollow tube of a uniform cross-sectional area. Alternatively, the waveguide1204can have a non-uniform cross-sectional area, for example, a hollow tube of a narrowing cross-sectional area such as a taper configuration shown and described herein. The internal and external walls of the waveguide1204may be substantially straight or curved. The one or more electro-acoustic drivers1206may be positioned in a number of locations along an internal wall of the waveguide1204.

As discussed above, waveguide systems produce a number of resonant modes.FIG. 20is a graph of pressure amplitude of three resonant modes of the waveguide system1200ofFIG. 19. The three resonant modes are plotted against x/L of the waveguide tube shown inFIG. 19(described below), with the open end (location C) at the left side of the graph and the closed end at the right side of the graph. Although three resonant modes are illustrated, the waveguide system1200can have any number of resonant modes. The illustrated resonant modes are the first three resonant modes of the waveguide system1200ofFIG. 19, i.e., Modes0,1, and2. Mode0is typically referred to as the fundamental mode. The modes can be numbered in the order of increasing frequency. The modes will vary depending on the geometry of the waveguide, and the modes shown inFIG. 20are just exemplary.

FIG. 20can be used to determine the position of one or more electro-acoustic drivers1206within the waveguide1204, such that the electro-acoustic drivers1206prohibit excitation of undesirable resonance frequencies above the fundamental mode, i.e., Mode0. As described above, the electro-acoustic driver1206drives the waveguide1204at two locations in the front and back of the electro-acoustic driver1206(locations A and B). The pressure produced by electro-acoustic driver1206in the front of the transducer is 180-degrees out of phase with that behind it. Accordingly, one or more electro-acoustic drivers1206can be positioned within the waveguide such that the front and rear of the drivers are driving the first mode equally, but in opposite directions, effectively preventing that mode from being excited.

As shown inFIG. 20, the first resonant mode (Mode1) above the fundamental mode (Mode0) has a peak when x/L is approximately ⅓, where x/L is the relative distance from the open end of the waveguide1204along the walls of the waveguide1204, where L is the length of the waveguide1204and x is the distance from the open end). At this peak, the pressure amplitude is highest. Thus, to prevent Mode1from being excited, one or more electro-acoustic drivers can be positioned such that a front side and a rear side of the drivers are symmetric about the peak of Mode1, i.e., where x/L is approximately ⅓. In this manner, the front1207and rear1208of the driver1206, respectively, cancel each other out at Mode1. There are several such positions symmetric about the peak, indicating that the front and back of the electro-acoustic driver1206load that particular mode, i.e., Mode1, equally and in opposite phase. The two stars1211,1212shown inFIG. 20identify two such positions along the Mode1plot. The location of the stars1211,1212indicate that the electro-acoustic driver1206is positioned within the waveguide1204, more specifically, placed symmetrically about the position on the acoustic transmission line where x/L is approximately ⅓, for prohibiting excitation of the first mode (Mode1) above the fundamental mode (Mode0).

Similarly, one or more electro-acoustic drivers can be positioned to prohibit excitation of the second resonant mode (Mode2) above the fundamental mode (Mode0). As shown inFIG. 20, the second resonant mode (Mode2) has a peak when x/L is approximately ⅕. At this peak, the pressure amplitude is highest. Thus, to prevent Mode2from being excited, one or more electro-acoustic drivers can be positioned such that a front side and a rear side of the drivers are symmetric about the peak of Mode2, i.e., where x/L is approximately ⅕. In this manner, the front1207and rear1208of the driver1206, respectively, prohibit excitation of Mode2. The two ovals1216,1217shown inFIG. 20identify two positions along the Mode2plot that are symmetric about the peak, indicating that the front and rear of the driver1206load the particular mode, i.e. Mode2, equally and in opposite phase. Other locations of symmetry can be found along the plot for Mode2. Although three resonance modes are shown inFIG. 20, any number of modes could be suppressed using the techniques described herein. Moreover, as described above, the modes will vary depending on the waveguide geometry, so in other examples, the locations of the peaks (and thus the locations of symmetry) will vary.

Applying principles from the graph illustrated atFIG. 20, i.e., that one or more electro-acoustic drivers should be placed within a waveguide symmetrically about a point on the acoustic transmission line where a resonant mode is at its peak, one or more electro-acoustic drivers can be positioned within the waveguide in a manner that prevents excitation of one or more modes above the fundamental mode.FIGS. 21 and 22illustrate an acoustic folded transmission line pass-band waveguide system with multiple electro-acoustic drivers positioned within the waveguide in a manner that prohibits excitation of the first mode (Mode1) above the fundamental mode (Mode0). More specifically, the electro-acoustic drivers are positioned symmetrically about the one-third point along the acoustic transmission line.FIG. 21is an embodiment of a geometry of a plurality of electro-acoustic drivers1306within an acoustic folded transmission line pass-band waveguide system1300.FIG. 22is a perspective view of the acoustic folded transmission line pass-band waveguide system1300ofFIG. 21. As described herein, waveguide system1300is constructed and arranged to prohibit excitation of only one resonant mode, e.g., Mode1or Mode2, but not both Mode1and Mode2.

As shown inFIGS. 21 and 22, the system1300includes a waveguide1304that is folded so that multiple drivers, transducers, or the like, for example, electro-acoustic drivers1306, are arranged about or approximately about the one-third point along the acoustic transmission line, or first fold, of the two-fold waveguide1304. The two folds of the waveguide1304are demarcated by internal walls1307and1308, respectively, to form multiple boundaries. In this example, the length of the folds are substantially the same, but in other examples, they could be different.

The waveguide1304includes a closed end1316and an opening1317at an open end. In operation, acoustic energy present in the transmission line propagates from the closed end1316and into an outside environment through the opening1317.

The electro-acoustic drivers1306are disposed through the internal wall1307so that the rear of each electro-acoustic driver1306faces internal wall1308. The electro-acoustic drivers1306are at positions approximately symmetrical about the one-third point (e.g., x/L=0.33 ofFIG. 20) of the acoustic transmission line. The front and rear of the electro-acoustic drivers1306are 180-degrees out of phase, so the drivers are positioned such that they will drive Mode1equally and out of phase. The net result is that Mode1is not excited by the drivers1306. Although three drivers are shown inFIGS. 21 and 22, any number of drivers could be used, as long as they are positioned symmetrically about approximately the one-third point along the acoustic transmission line.

Accordingly, referring again toFIG. 20, the Mode1line can illustrate a smoother response since the first resonance above the fundamental (Mode0) is removed from the output. For example, the dip shown inFIG. 23at 124 Hz is removed. In particular, the folded waveguide1304with the drivers positioned approximately symmetrically about the one-third (⅓s) point as shown inFIGS. 20-23permits drivers1306to be symmetrically positioned about the peak of Mode1. Since the front and back of electro-acoustic drivers1306are out of phase, the net effect is that Mode1of the waveguide is not excited.

In sum, the position of one or more electro-acoustic drivers1306symmetrically about the one-third point does not drive the first mode above the fundamental mode (Mode0), permitting any number of electro-acoustic drivers1306to be located in a space along the interior of the waveguide, and thereby permitting the system1300to produce a greater output while still providing a response as shown in the graph ofFIG. 23.

FIG. 24is an embodiment of a tapered waveguide1400, i.e., one in which the maximum width is at the closed end of the waveguide, and the edges of the waveguide decrease in width when moving away from the closed end.FIG. 25is a graph of pressure amplitudes of three resonant modes of the tapered waveguide1400ofFIG. 24.

As previously described with regard toFIG. 20, the peak amplitude of the first mode (Mode1) above the fundamental mode is at about x/L=0.33 for a straight waveguide1200. The symmetric shape of Mode1inFIG. 20allows multiple drivers to be positioned symmetrically, for example, as shown inFIGS. 21 and 22, in order to generate a greater output from the waveguide system, due in part by preventing the excitation of higher order modes.

The graph inFIG. 25illustrates modes of a waveguide1400having a closed end that is about twenty times larger than the open end (though in practice, other sized ends could be used, producing a different ratio between the closed end and the open end). In the graph ofFIG. 25, the shape of the modes of the tapered waveguide1400are shifted slightly towards the open end of the waveguide1400as compared to a straight waveguide, for example, as shown inFIG. 20. Moreover, Mode1shown inFIG. 25is not symmetric about its peak, i.e., x/L=0.30. Thus, electro-acoustic drivers positioned symmetrically about this point do not enjoy the same advantage as that shown and described inFIGS. 20-22, i.e., preventing the excitation of higher order modes.

FIG. 26is an embodiment of a tapered waveguide1600having internal and external walls having a curved geometry. The waveguide1600includes a curved wall1607at which one or more electro-acoustic drivers1606are positioned that drive a first mode (Mode1) at the same amplitude and phase on the front and back of electro-acoustic driver1606. For example, the curved geometry may include an interior wall comprising a fold of the waveguide such that the wall includes locations along the wall such that distances on one side of the wall versus the other side of the wall maintains a match in pressure amplitude according to a mode function. Accordingly, the waveguide1600can be curved to exploit the same or similar benefit as the straight waveguide ofFIGS. 20-22. Although two electro-acoustic drivers1606are shown inFIG. 27, any number of electro-acoustic drivers could be used.

To determine the curved shape for the tapered waveguide1600, the graph inFIG. 25can be referenced. Locations are determined along the Mode1curve inFIG. 25that match in amplitude about the peak on the left of the Mode1curve, even if these locations are not symmetric about the peak. The determined matching pairs can provide the path length difference between each side of the wall1607shown inFIG. 26such that the curvature of the waveguide1600at each discrete step away from the open end of the waveguide can be calculated, such that the distance on one side of the waveguide1600versus the other side of the waveguide1600maintains a match in pressure amplitude according to the mode function, for example, as shown. As the distance from the open end of the waveguide1600increases, the curvature at each step along the length of the waveguide can be determined to ensure that the acoustic path on each side of the wall1607of the waveguide is such that the pressure amplitudes match front to back across the wall according to the modal distribution function shown inFIG. 25. The process can continue along the full length of the wall1607until the distance reaches the position of the peak (Mode1), for example, about x/L=0.3 shown inFIG. 25. In particular, the overall shape of the waveguide1600can be calculated by varying the curvature accordingly until the full length of the wall1607has been calculated, and the rest of the waveguide length toward the closed end is added.

This position represents the center of the turned corner, similar to how the x/L=0.33 corresponds to the bend in the geometry illustrated inFIG. 22. By determining the wall curvature in this manner, the rest of the waveguide from the x/L=0.3 location to the closed end can be added without impacting the features described herein with respect to the waveguide1600. Accordingly, the waveguide1600can be folded beyond the x/L=0.3 location to reduce the waveguide footprint, for example, as shown inFIG. 26.