WO1999004599A1

WO1999004599A1 - Integrated tri-flare wave guide and trim ring

Info

Publication number: WO1999004599A1
Application number: PCT/US1998/014915
Authority: WO
Inventors: Calvin C. Perkins; Terry L. Wetherbee; David D. Bie
Original assignee: Mackie Designs Inc
Current assignee: Mackie Designs Inc
Priority date: 1997-07-18
Filing date: 1998-07-17
Publication date: 1999-01-28
Anticipated expiration: 2000-01-18
Also published as: AU8497098A

Abstract

A contoured horn/wave guide controlling directivity, and increasing power handling capacity and sound pressure level (SPL) of a small diaphragm, direct radiating, dome high frequency speaker (77). The contour of the horn/wave guide combines three different flare rates, each of which extends approximately one-third of the length from a throat of the horn to its mouth. The first third disposed adjacent to the throat of the speaker is a conical contour (30). The second third of the horn length follows an exponential flare contour (34) to provide resistive loading. The final third of the horn length employs a modified tractrix contour (38) that prevents mid-band beaming and provides uniform pattern control of the sound waves emanating from the speaker. An integral trim ring (80) that mounts the guide/horn to loudspeaker enclosure is also included. The integral trim ring includes a pair of joined rings (82, 84) adapted to surround the wave guide and an adjacent low frequency speaker also mounted within the same speaker enclosure as the dome high frequency speaker.

Description

INTEGRATED TRI-FLARE WAVE GUIDE AND TRIM RING

Related Application

This application claims priority to U.S. provisional application Serial No. 60/053,068, filed July 18, 1997, and entitled "Integrated Tri-Flare Wave Guide and Trim Ring," of which is hereby incorporated by reference.

Technical Field The present invention generally relates to a high frequency loudspeaker, and more specifically, to a high frequency loudspeaker that has a horn that serves as a wave guide and is shaped to control the pattern of sound emitted by the loudspeaker.

Background of the Invention

High frequency dome-type speakers, or tweeters, typically cover a frequency range from approximately 2000 Hz to above 20 kHz. At the lower frequencies, the radiation of sound energy from the dome speaker into free space is hemispherical because the wavelength of the sound propagated by the radiator is considerably greater than the diameter of the radiating source. As the radiated frequency increases, the propagated wave front becomes less hemispherical and more elliptical in shape. When frequency of the radiated sound is sufficiently high that the wavelength of the radiated sound is about equal to the diameter of the radiating dome, the wave front becomes even more directional. The preponderance of high frequency energy is concentrated along the central axis of the dome with very little sonic energy radiated to the sides, which is undesirable. Referring to FIGS. 16A-16F, this characteristic is true for any radiating piston such as those shown in the different radiation patterns 2, 4, 6, 8, 10 and 12. FIG. 16A shows the hemispherical shape as a wavelength of the sound propagated by the radiator, which is considerably greater than the diameter of the radiating source (the dome). This is typical of low frequencies. FIG. 16B shows the wavelength not quite as hemispherical when the wavelength is approximately one-half the diameter of the propagating dome. FIG. 16C shows the diameter equal to the wavelength such that the wave front becomes more directional and slightly more elliptical-shaped. FIG. 16D shows the diameter twice as great as the wave length with a wave front decisively elliptically-shaped. FIG. 16E and 16F show even more directional wave fronts where the diameter is even greater than that of the wave length. For example, the diameter is four times greater than that of the wave length. 16F shows the dome diameter six times greater than that of the wave length.

The frequency at which the sound waves produced by a speaker begin to beam, or narrow, is a function of the diameter of the piston d_p that acts on the radiator. The larger the diameter of the piston, the lower the frequency. A six to eight inch diameter piston is already beaming, or not producing hemispherical radiation, well below the lowest crossover transition frequency that can be prudently used in a two-way speaker system where the high frequency information is handled by the typical dome tweeter. This crossover frequency is usually set above 1500 Hz.

In addition to the sound radiation pattern of a sound source, power handling capacity is also of great concern. The maximum sound pressure level (SPL) of conventional dome tweeter speakers is limited by: (1) the maximum excursion of the radiator at low frequencies, and (2) the thermal limit of the voice coil (electromagnetic driver coil) used in the speaker device. To reproduce high frequencies, a high frequency driver voice coil must be small and light, so that it has a sufficiently low moving mass. But these characteristics severely limit the maximum thermal limit of the voice coil.

The converse is true of a low frequency driver where the voice coil is large and heavy in order to enable it to handle low frequency power. In low frequency drivers, the moving mass plays a smaller part in the frequency response of the driver. Because of the physical differences in the size and mass of the voice coils used in high frequency speakers compared to low frequency speakers, there is also a commensurate difference in power handling capacity. To offset the difference in power handling, it is desirable to find a method whereby the efficiency of the high frequency unit is increased.

A considerable increase in efficiency can be gained at the lower frequency range of a dome radiator by the method known as "horn loading." This technique increases efficiency by focusing a hemispherical radiation pattern produced by the device so that it has more energy on the axis. This achieves a result more commonly referred to as "pattern control." The challenge is to maintain the same pattern control over the entire operating bandwidth or frequency range, namely 2 kHz to 20 kHz. A common practice is to use a conical or rectangular wave guide, which is also known as a conical horn. This conical horn is positioned in front of the dome to define and control the pattern of sound waves that the device produces. There are, however, certain limitations in pattern control and frequency response that arise in the prior art approach.

FIGS. 1 and 2 illustrate the prior art approach for mating a conical wave guide to a high frequency driver. The wave guide (horn) 14 has a side wall 16 of a "cone" (shown in cross-section), which is straight (and denoted at 17) for approximately 2/3 of its length L, as measured from dome 18 to the outer extent 20 of horn 14. Side wall 16 terminates at a mounting surface, which is a front panel of the speaker enclosure (not shown). The pattern control is established by the included angle A, which is between the side 16, horn 14, and the diameter of the mouth of horn D. Below a certain frequency, known as the intercept frequency, the radiation pattern is hemispherical, which is desirable. However, above the intercept frequency, the pattern is controlled by the included angle A and by the diameter of the throat d. This approach makes two compromises in pattern control and frequency response, which is undesirable as explained below. In an ideal system, where the driver exhibits perfect pistonic behavior, the frequency response of the system precisely follows the power response of the driver. Referring to FIG. 6, the frequency response characteristics of an on axis "zero degree incidence" how response of a tweeter in free space without a wave guide is shown. Line 24 represents the on axis frequency response at 0° and line 26 represents the power response at 60° off axis of a dome high frequency driver having a one-inch diameter. Unfortunately, neither the driver nor the wave guide that are used in such prior art devices exhibit ideal behavior. The frequency response and pattern control deviate considerably from the desired ideal theoretical response. Referring to FIG. 8, line 28, which represents existing state of the art circular wave guide, does not accurately follow the driver power response.

Referring to FIG. 3 there are five basic mathematical horn contours that have been used to determine frequency bandwidth and pattern control or directivity. The first one, conical, noted at 30, has already been discussed as well as its pitfalls. The others, parabolic at 32, exponential 34, hyperbolic 36 and tractrix at 38, all have failed to produce an ideal response system.

Exponential, hyperbolic, or tractrix horn/wave guide realizations will load the driver down to the desired lowest frequency of operation, thereby increasing the efficiency of the device with a subsequent increase in power handling capacity, as shown in FIG. 17. The graph of FIG. 17 shows the relationship of the acoustical resistance for a hyperbolic horn, line 52, exponential horn, line 54, parabolic horn, line 56, and a conical horn, line 58. However, these horn/wave guide designs have very poor pattern control characteristics in of themselves.

The tractrix flare rate is essentially an exponential flare for the first 50° of the horn length, but thereafter flares out at an increasing rate to its fully developed mouth, at an included angle of 180°. A true exponential expansion would continue on to infinity, whereas the conical contour flare rate abruptly terminates at its finite length L, as shown in FIG. 1.

Since neither the driver nor wave guide of the prior art accurately follow the theoretical ideal response as a system, a new approach is needed to improve the frequency response in the higher frequencies. The present invention is directed to this need.

Summary of the Invention The present invention is directed to an integrated tri-flare wave guide for controlling directivity, increasing power handling capacity and sound pressure level of a dome high frequency speaker. The wave guide includes a side wall flaring outwardly from a throat of the dome speaker to form a continuous variable flare rate from the throat to a mouth of the dome speaker. The wave guide shape is such that a substantially circular cross-sectional form is generated when a place is cut through the wave guide parallel to the throat.

The side wall flares a three distinct contour rates. The first flare rate is a conical contour adjacent the throat. The second flare rate is adjacent the conical contour and contains an exponential flare contour. The third flare rate is between the mouth and the second flare rate. The third flare rate is a tractrix contour.

In preferred form, each of the three flare rates are approximately equally spaced along the length of the side wall such that the conical contour is a first 1/3 of the side wall length, the exponential flare contour is the second 1/3 of the side wall length, and the tractrix contour is the last 1/3 of the side wall length. According to one embodiment of the present invention, a subtending angle formed by the conical contour of the wave guide and the throat ranges from 45° to 150°. Preferably, the angle is 120°.

According to another embodiment, the throat of the dome speaker has a diameter as larger then 0.65 inches. The subtending angle may be increased by further providing an outwardly flaring radius formed of the exponential flare contour.

In another preferred embodiment, the throat includes a phase compensation transition section, which is a small cylindrical transition section to equalize the pressure and time arrival difference caused by the suspension and driven piston. The present invention way further include a trim ring that is adapted to the mouth of the wave guide in order to facilitate mounting within speaker enclosure. The trim ring may further include an integral larger ring to adapt to an adjacent low frequency speaker, typically included in a powered speaker. These and other features and benefits will be further discussed in the

Description of the Preferred Embodiment.

Brief Description of the Drawings

Like reference numerals are used to denote like parts throughout the several figures of the drawing. The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawing, wherein:

FIG. 1 (PRIOR ART) is a cross-sectional view of a typical prior art conical wave guide; FIG. 2 (PRIOR ART) is a plan view of the circular wave guide shown in

FIG. 1;

FIG. 3 is a graph illustrating five different horn contours that have been used for speakers in the prior art;

FIG. 4 is a cross-sectional view of a horn in the present invention having a wave guide of multiple flare rates;

FIG. 5 is an enlarged partial cross-sectional view illustrating a phase compensation throat of the horn used in the present invention; FIG. 5 A is an enlarged view of that shown in FIG. 5;

FIG. 6 is a graph of an on axis amplitude vs. frequency of a one-inch diameter dome high frequency unit overlaid with a total radiated sound power response, both without the wave guide;

FIG. 7 is a graph comparing the on axis response of a one-inch diameter dome driver without a wave guide horn as a reference point, and the comparative effects of the phase compensation throat of FIGS. 5 and 5 A on the new wave guide horn; FIG. 8 is a graph comparing the frequency response of the prior art circular horn and the multi-flare unit with phase compensation throat of the present invention;

FIG. 9 is a polar plot comparing the prior art circular horn with the present invention at 1250 Hz and 20 kHz; FIG. 10 is a front elevational view of the present invention including a trim ring and additional clamp assembly for a trim rate over a low frequency speaker;

FIG. 11 is a perspective rear view of the tri-flare cone with phase compensation throats and trim ring shown in FIG. 10 and better disclosing a plurality of outwardly extending gussets from the rear of the tri-flare cone; FIG. 12 is a cross-sectional side view taken substantially along line 12-12 of

FIG. 11;

FIG. 13 is a detailed cross-sectional view of a portion of one of the gussets, taken substantially along line 13-- 13 of FIG. 10;

FIG. 14 is a graph illustrating the efficiency differences between a conical wave guide and an exponential horn at the same overall dimensions;

FIG. 15 is a graph of a beamwidth vs. frequency for an ideal horn with perfect pattern control;

FIGS. 16A-16F are six graphs of polar radiation patterns that illustrate the relationship between dome diameter and wavelength of radiated sound produced by a driver; and

FIG. 17 is a graph illustrating the acoustical resistance at the throat of four different types (shapes) of horns.

Description of the Preferred Embodiments The present invention relates to new multi-flare wave form or horn used for a high frequency dome typed speakers in order to achieve real theoretical frequency response and pattern control. The invention also includes a compensation throat of the wave form or horn and a trim ring that supports the wave form within a powered speaker enclosure and with its additional clamp assembly ring for low frequency speaker. The present invention wave form described below improves the low frequency loading over that of the prior art conical horn of FIGS. 1 and 2 by as much as 58 percent. The present invention increases the power handling in the same frequency band by 58 percent. The present invention also provides a smoother frequency response with smaller amplitude deviations, and provides better frequency pattern control than previously known wave forms or horns.

Referring to FIG. 4, the diameter D of the horn of the present invention 40 primarily determines the lowest usable frequency of the speaker device. Pattern control is achieved by selection of the included angle A. Thus, if angle A has a value of 120°, and D (mouth or horn diameter) is five inches, the polar pattern should be 120° for audible frequencies above 2000 Hz. For frequencies above those where the wave number, k, times the radius R equals one (i.e., kR = 1), the size of the effective piston 42 in throat d begins to take precedence over the polar patter control. This is because the wave front propagating down the horn is narrower than the included angle A between the sides 44 (side wall) of horn 40. In a preferred embodiment of the present invention, the throat diameter is about d is approximately 1.5 inches, which indicates that the sound wave beamwidth should collapse to about 90° at 9 kHz. If phase integrity is maintained across the 1.5-inch diameter of the throat, then the polar pattern will be that of a 1.5-inch piston. Polar radiation patterns 2, 4, 6, 8, 10 and 12 of FIG. 16A-16F, respectively, and already discussed in the Background of the Invention, show the radiated polar response of a planar piston as a function of the wavelength of radiated sound, and diameter of the piston. For equal pattern control from 2 kHz to 20 kHz, the physical geometry of the radiating piston would suggest that the diameter of the throat d should be no larger than 0.675 inches. This diameter is about half that of most existing prior art foreign designs. The primary reason for the difference between this ideal throat diameter and the actual throat diameter used by others and by the present invention is that the effective opening is not determined only by the diameter of the piston. Most one-inch diameter dome high frequency drivers have an effective opening of from 1.3 to 1.5 inches. Referring to FIGS. 4, 5 and 5 A, according to the preferred embodiment, the effective diameter of throat d (referred to as d') because of its size difference from that of the prior art, is controlled by manufacture's design of the particular high frequency driver utilized. For the high frequency driver used in the preferred embodiment in which the effective throat opening diameter is not solely determined by the diameter of the piston d_p but is also affected by an inside diameter d₃ and an outside diameter d₄ of a diaphragm suspension, which is not discussed further here, and the diameter of any protective screen 46. A smaller high frequency driver could be utilized, but power handling capacity would be seriously impaired using a smaller driver. To avoid the effect of acoustic reflections down the length L of the horn caused by an abrupt termination of the horn, it is a common practice to either create a radius in the final 1/3 of the horn's length, or to double the subtended angle A with a new straight-sided section (not shown). If the horn is not properly terminated, there will be a region in the frequency range over which it operates where the polar pattern narrows. This result can be shown by plotting the log of the beamwidth (coverage angle) as a function of frequency for the horn. Since their function is pattern control, most wave guides are terminated in the manner noted above. A conical wave guide, although providing good pattern control, does not optimally load a high frequency driver over its operating frequency range, as lines 48 (exponential wave form) and 50 (conical wave form) and graph FIG. 14 clearly show. This result occurs because the acoustic resistance presented to the driver's diaphragm (piston) begins to fall off at a frequency well within the operating frequency range of the unit.

Other wave forms as discussed in the Background of the Invention and illustrated in FIGS. 3 and 17, have proven to be successful. However, in a certain combination, a novel and successful result has been achieved.

The general mathematical expressions for these basic horn types is provided below, where is the mouth area, A, is the throat area, T is a coefficient that determines the type of horn, x₀ is the start of the expansion, and x is any length down the horn/wave guide. If T = 0, the expansion is hyperbolic. If T = 1, the expansion is exponential. If T = infinity, the expansion is conical. The expansion rates are defined as follows:

Hyperbolic T = 0 A_^ = A_t(coshx/x₀ + Tsinhx/x₀)²

Exponential T = 1 A_x = A,e^2x/x0 Conical T = infinity A_x = A,(l + x/x₀)²

It was determined that certain portions of the various known wave forms in a select combination cited superior results and is the subject of the present invention. Referring to FIG. 4, the present invention is a tri-flare horn/wave guide under the trademark TRI-FLARE CONPODENTIALTRAX™. The flare rate for the first third of the horn is conical and noted at 60. Conical portion 60 establishes the polar radiation angle (subtending) for sound waves. Preferably, polar radiation angle is 120°, but can be virtually any desired angle from approximately 45° to 150°, depending on the design operating frequency range of interest. The second third of the horn length is primarily exponential to provide resistive loading down to the cut-off (lowest usable) frequency of the device. This is noted as section 62. The last third of the wave guide/horn length is a modified tractrix portion 64 to prevent mid-band beaming and to provide uniform pattern control. Tests have shown that the three different flare rates can be approximated by using two different radii of curvature to implement the design.

FIG. 15 compares an ideal beam width curve 56 is compared to a curve 68 that exhibits the phenomena of a mid-band beamwidth narrowing. Above 9 kHz, the high frequency driver of the preferred embodiment of the present invention exhibits non- ideal polar patter characteristics, because the suspension area is almost equal to that of the diaphragm (piston). Thus, there are two radiating sources in the throat of the horn/wave guide. A small cylindrical transition section, referred to as phase compensation throat 70, and best shown in FIGS. 4-5A, is included to equalize the pressure and the time arrival differences caused by the ring (radiator) suspension and driven piston. Without phase compensation in the throat transition section, as shown in FIG. 5 A at 72, there is a two dB reduction in output from the horn/wave guide above 7 kHz as indicated in FIG. 16 and 17. Other attempts in the prior art to mate a conical wave guide to a dome high frequency unit have resulting in over 2.5 dB of ripple in the amplitude response of the resulting device. The frequency response graph of FIG. 8 compares the improved wave guide, shown at 22, with that of an existing competitive unit of similar dimensions, shown, and discussed earlier as 28. With a phase compensation throat 70 (FIGS. 4 and 5), the high frequency polar response of the present invention more closely approximates that of an ideal 1.5 -inch diameter radiating piston. FIG. 7 compares the effects of a tri-flare wave guide to a dome tweeter (26) with phase compensation throat (22) and without phase compensation throat (24). Referring to FIG. 9, a polar response plot compares the results for the curve of the present invention 76 with that of the results of the current state of the art device or prior art device at 20 kHz (line 78) and 1250 Hz (line 79).

Referring particularly FIGS. 10-13, the present invention also includes a trim ring that is adapted to surround the wave guide of the present invention and also to have a second integral trim ring that is adapted to surround an adjacent low frequency speaker that is mounted in the same speaker enclosure. This trim ring is shown at 80. The smaller ring 82 is adapted to fit over the mount diameter D of the horn of the present invention. The larger trim ring is denoted at 84. The trim ring is essentially a low frequency driver trim ring that reduces high frequency diffraction from the low frequency driver cone and speaker enclosure. As best shown in FIGS. 11, 12, and 13, a plurality of gussets 86 extend outwardly from the base 28 of the horn 40. FIG. 13 is a cross-sectional enlarged view of an individual gusset. The trim ring 80 would mount to a front panel of the speaker enclosure (not shown).

The illustrated and described embodiments are presented by way of example. The scope of protection is not to be limited by these examples. Rather, any patent protection is to be determined by the claims which follow, construed in accordance with established rules of patent claim construction, including the use of doctrine of equivalents and reversal of parts.

Claims

WHAT IS CLAIMED:

1. An integrated tri-flare wave guide for controlling directivity, increasing power handling capacity and sound pressure level of a dome high frequency speaker, the wave guide comprising: a side wall flaring outwardly from a throat of the dome speaker to form a continuous variable flare rate from the throat to a mouth of the dome speaker such that the cross-sectional wave guide cut by a plane parallel to the throat will generate a circular form; and said side wall flares along its length in three portions, wherein a first portion forms a conical contour adjacent the throat, a second portion forms an exponential flare contour adjacent the conical contour, and a third portion, adjacent the mouth, forms a tractrix contour.

2. The wave guide according to Claim 1 , wherein the first portion conical contour is approximately 1/3 of the length of the wave guide side wall, the second portion exponential flare contour is approximately 1/3 the length between the first portion conical contour and the third portion tractrix contour portion, and the third portion is the last third of the length of the side wall.

3. The wave guide according to Claim 1, wherein a subtending angle formed by the conical contour and the throat is in a range from 45° to 150°.

4. The wave guide according to Claim 3, wherein the subtending angle is 120°.

5. The wave guide according to Claim 1, wherein the throat has a diameter no larger than 0.65 inches.

6. The wave guide according to Claim 3, wherein the throat has a diameter no larger than 0.65 inches.

7. The wave guide according to Claim 4, wherein the throat has a diameter no larger than 0.65 inches.

8. The wave guide according to Claim 3, further comprising: an enlarged mouth being formed by an outwardly flaring radius formed of the exponential flare contour to increase the subtending angle at the mouth.

9. The wave guide according to Claim 1 , wherein the throat further includes: a phase compensation transition section.

10. The wave guide according to Claim 1 , further comprising: a trim ring that adapts to the mouth of the wave guide and into the speaker enclosure.

11. The wave guide according to Claim 10, wherein the trim ring further includes an adjacent and integral second trim ring of a size to adapt to an adjacent low frequency speaker also mounted within the speaker enclosure