US3909531A

US3909531A - Acoustic transducer system

Info

Publication number: US3909531A
Application number: US454759A
Authority: US
Inventors: Jan P Plummer
Original assignee: Custom Electronics Inc
Current assignee: Custom Electronics Inc
Priority date: 1974-03-25
Filing date: 1974-03-25
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

Equal-sized speaker cones are housed in equal-sized closed compartments in a system of loudspeakers, a housing therefor and electrical crossover circuits which system results in substantially equal acoustic treatment for the midrange of acoustic frequencies and the lower or bass range of acoustic frequencies. The effective resonant frequency for both the bass and midrange speaker cones is primarily determined by the compliance of entrapped air within the respectively corresponding speaker enclosures, thus insuring that both the midrange and bass frequencies are produced with minimum distortion. Other combined features of the overall acoustic system assist in providing substantially equal acoustic treatment for the midrange and bass frequencies.

Description

United States Patent Plummer Sept. 30, 1975 ACOUSTIC TRANSDUCER SYSTEM Primary ExaminerThomas A. Robinson [75] Inventor: Jan P. Plummer, Atlanta, Ga. Ammmy Agent or Firmcushman Darby &

Cushman [73] Assignee: Custom Electronics Incorporated,

Atlanta, Ga. ABSTRACT [22] Filed: Man 2 974 Equal-sized speaker cones are housed in equal-sized closed compartments in a system of loudspeakers, a [2]] Appl. No.: 454,759 v housing therefor and electrical crossover circuits which system results in substantially equal acoustic [52] US. Cl. 179/1 E; 181/145 treatment for the midrange of acoustic frequencies [51] Int. Cl..... H04R 1/28 and the lower or bass range of acoustic frequencies- [58] Field f Search 179 E, 1 181/31 13 The effective resonant frequency for both the bass and 31 144 145 19 199; 325 352; 3 2 7 T midrange speaker cones is primarily determined by 24 /27 the compliance of entrapped air within the respectively corresponding speaker enclosures, thus insuring [56] References Cited that both the midrange and bass frequencies are pro- UNlTED STATES PATENTS duced with minimum distortion. Other combined features of the overall acoustic system assist in providing 93 3 substantially equal acoustic treatment for the mid- 33271004 4 1973 Bose 179/1 E range and bass frequenc'es' t's, ex 2 P 8 Claims, 4 Drawing Figures ACOUSTIC TRANSDUCER SYSTEM This invention relates generally to acoustic transducer systems and, more particularly, to a combination of speakers, a housing structure therefor and interconnecting electrical circuits for converting electrical signals from a conventional audio amplifier into acoustic vibrations perceptible to the human ear. The acoustic transducer system of this invention is especially useful for reproducing music at high volume levels and high efficiency and with a minimum of distortion.

There are, of course, many types of available acoustic transducer systems employing various speaker combinations, housing structures and various crossover electrical networks interconnecting such speakers. However, most such prior speaker systems have concentrated individually on various separate frequency ranges (i.e. bass, midrange and high frequency) with sharp cut-offs between the various frequency bands and their respectively associated speakers. The result has been effectively unequal treatment of the bass and midrange frequency ranges with a consequent undesirable and noticeable amount of distortion at crossover frequencies or other critical frequencies for such systerns or a great loss of efficiency and the inclusion of more complex crossover networks in an attempt to alleviate such distortion problems.

However, it has now been discovered that the acoustic transducer system of this invention provides a relatively simple and straightforward solution to the problem of optimizing speaker system performance characteristics. Broadly stated, the improved and desirable aspects of the invention flow from the combination of several speakers designed to produce substantially equal acoustic treatment of the bass and midrange frequencies while, at the same time, lowering distortion effects (especially intermodulation distortion) and increasing efficiency. There is no need for lossy electrical padding to match the efficiency of the bass and midrange speakers.

To begin with, the speaker cones for the bass and midrange drivers in the system of the invention are of equal diameter. Furthermore, they are housed in substantially equalsized, enclosed chambers. The closed speaker housing chambers thus cause the bass and midrange drivers to take on the characteristics of air suspension speakers wherein the resonant frequency characteristics of the speakers are primarily determined by the compliance of entrapped air behind the speaker cone within the speaker housing chamber. Since the entrapped air compliance in the cabinet is substantially more linear in its compressibility than the speaker suspension itself, this insures a greater linearity for both the bass and midrange speakers. The stiffness of the air in the enclosure is added to the stiffness of the speaker which in turn, makes cone movement more difficult. This is the controlling factor in power to the mid-range speaker in this range is limited by a 3 db/octave capacitive crossover. This aids in cut off, in minimizing cone excursions, and eliminates unnecessary load on the audio amplifier.

Furthermore, a gradual midrange crossover point (approximately 125 Hz in the preferred embodiment) is chosen to be much lower than normal such that the operating frequency ranges for the bass and midrange drivers actually overlap. The net result is a minimizaacoustical output, below resonance. The electrical' tion of phase distortion that generally occurs at the higher frequencies where a sharp midrange crossover point is more conventionallylocated. Furthermore, the rate of midrange crossover in the preferred embodi ment is only 3 db/octave as opposed to a more conventional sharp 12 db/octave crossover and is achieved by a simple efficient capacitive crossover connection. The electrical power delivered to the midrange speaker in the low range is thus limited to minimize excessive cone movement.

Compliance is controlled and standing waves within the speaker chambers are minimized by introducing porous, resilient materials, such as conventional fiberglass insulation material (normally used to insulate homes) within the speaker housing chambers. More fiberglass is introduced in the bass speaker housing chamber than in the midrange speaker housing chamber (more air is thus displaced by the fiberglass in the bass chamber thus making the resultant shift from a free air resonant frequency to the air suspension resonant frequency less than otherwise for the bass speaker). Although this might otherwise tend to cause an imbalance in efficiency (thermal conversion of acoustic energy from entrapped air to heat in fiberglass) between the midrange and bass speakers, this relative loss of bass efficiency is compensated for by placing the bass speaker in the lower cavity of the overall speaker housing structure such that the floor itself acts as a reflector.

The higher frequency range may also be included in the acoustic transducer system of this invention by including a small tweeter speaker in the upper or midrange speaker housing chamber. A simple capacitive 3 db/octave crossover connection is also used for driving the tweeter. Further, where the speakers are to be used in a stereo system, the tweeter is mounted towards the outside sides of a pair of stereo speaker enclosures. For instance, the tweeter would be located on the righthand side of the right-hand speaker and vice versa. The tweeter should preferably also have the same type of magnet structure as the bass and midrange speakers.

All these plus other improvements and advantages of the invention will be more clearly appreciated from the following detailed description taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a front view of an exemplary acoustic transducer system according to this invention with the front grillwork of the speaker housing partially cut away to reveal the speaker placement therebehind;

' FIG. 2 is a partially cut away side view of the exemplary embodiment shown in FIG. 1 with the cutaway portion taken along line 2-2 in FIG. 1;

FIG. 3 is a partially cut away top view of the exemplary embodiment shown in FIG. 1; and

FIG. 4 is an electrical schematic diagram showing the capacitive crossover connections between the various speakers of the exemplary system shown in FIGS. 1-3.

Referring to FIG. 1, the overall cabinet structure for the exemplary embodiment shown in FIG. 1 is indicated at 10. It comprises an upper speaker housing chamber 12 and a lower speaker housing chamber 14 of substantially equal volume. A bass loudspeaker 16 is housed in the lower chamber 14, while a midrange loudspeaker 18 is housed in the upper chamber 12. Both the bass speaker 16 and the midrange speaker 18 have speaker cones of equal, predetermined diameters. Furthermore, the basic magnet structure for the

speakers

16 and 18 is also the same. There are, of course, some differences in the voice coil windings and cone mass, as will be appreciated, to adapt the bass loudspeaker 16 to transduce electrical signals into acoustic signals within a lower frequency range while the voice coil of the midrange speaker 18 is adapted to transduce electrical signals into acoustical signals within a middle frequency range.

The upper and lower chambers are defined by solid, nonapertured walls on all but the front sides, as shown in FIGS. 1 and 2, which are also solid except for apertures directly in front of the speakers housed within these chambers, as also shown in FIGS. 1 and 2. The lower speaker chamber is raised by a pedestal 20 from any underlying surface, such as a floor, upon which the cabinet structure might be placed. The bass response is optimized by the spacing distance from the underlying surface, such as a floor, which acts as a reflector for the acoustic signals produced by the bass speaker 16, thus slightly increasing its efficiency relative to that of the midrange loudspeaker 18 and compensating for an extra amount of fiberglass insulation within the bass or lower chamber 14.

The fiberglass insulation 22 in the upper chamber 12 and the fiberglass insulation 24 in the lower chamber 14 is better shown in FIG. 2. Here it can be seen that the fiberglass insulation 22 comprises a layer around all of the inside surfaces of chamber 12 while the chamber 14 is substantially filled with fiberglass insulation 24.

If a tweeter speaker 26 is to be included within a system of this invention, it is preferably mounted in the upper or midrange chamber 12. It is mounted generally on the right-hand side of chamber 12 as shown in FIG. 1 if the cabinet structure 10 is to be used as the righthand speaker in a stereo reproduction system. The lefthand speaker cabinet structure would have the tweeter speaker 26 located in a symmetrical position generally on the left-hand side of the upper speaker chamber 12 as should now be apparent.

The top view shown in FIG. 3 is substantially selfexplanatory in view of the description already given with respect to FIGS. 1 and 2.

FIG. 4 shows the electrical circuit used in the exemplary embodiment for electrically connecting the electrical input connections 28 with the bass, midrange and/or

tweeter speakers

16, 18 and 26, respectively. As shown, the bass speaker 16 is directly connected to the input connections and is thus driven at all frequencies up to the mechanical cut-off frequency (approximately 600 Hz in the exemplary embodiment) of the bass speaker 16 itself. The midrange speaker 18 is connected through a high-pass filter or simple capacitive crossover network comprising a capacitor C, which effectively prevents electrical signals below a predetermined crossover frequency (approximately 125 Hz in the exemplary embodiment) from being applied to the midrange loudspeaker 18. Of course, frequencies above the crossover frequency are transduced by the midrange speaker 18 up to its mechanical cut-off frequency (approximately 17,000 Hz in the exemplary embodiment).

The simple capacitive crossover connection is not only simple and inexpensive, it also provides a substantially more gradual crossover rate of 3 db/octave as opposed to the more usual and conventional 12 db/octave. It is also essentially lossless thus improving the efficiency of the overall system.

Similarly, the tweeter speaker 26 is connected through another simple capacitive crossover connection C thus effectively preventing electrical signals below a second predetermined crossover frequency (approximately 17,000 Hz in the exemplary embodiment) from being applied to the high frequency range transducer or tweeter 26. The tweeter 26 will, of course, then continue to reproduce signals higher in frequency than the crossover point until its mechanical cut-off frequency (approximately 20,000 Hz in the exemplary embodiment).

In the preferred exemplary embodiment, the cabinet structure 10 has an overall height of 36 inches (thus making each speaker chamber approximately 18 inches in height minus allowances for the thickness of the chamber walls), a width of 20 inches, and a depth of 15 inches. The pedestal 20 is 12 X 16 inches and 3 /2 inches high. Since 12 inch bass and

midrange speakers

16 and 18 are utilized in the preferred embodiment, the circular apertures in the front side of the speaker housing chambers are approximately 11 inches in diameter and centered within the front face of each respectively corresponding speaker housing chamber. The tweeter aperture is approximately 1 7/8 inches in diameter.

The preferred exemplary embodiment comprises a Norelco l2-inch woofer, Model No. AD12100W8, for the bass speaker 16 (free air resonance of 15 Hz); a Norelco 12-inch full range speaker, Model No. AD121000M8, for the midrange speaker 18 (free air resonance of 65 Hz shifted to Hz by air suspension mounting); and a Norelco Model No. AD0160/T8 for the 2-inch diameter tweeter speaker 26. The first midrange crossover capacitor C is preferably a 250 microfarad, 30-volt capacitor, while the higher range capacitive crossover C is a 2 microfarad, 30-volt capacitor.

The preferred exemplary embodiment utilizing the components just mentioned has been found to provide a very efficient system wherein 1 watt of electrical input power or even less will produce 100 db sound pressure level (SPL) at a distance of 3 feet from the speaker enclosure at a frequency of 100 Hz. No more than 60 watts/channel are required to drive the entire system and the system will handle up to watts of electrical energy distributed normally within a music frequency spectrum.

The fiberglass insulation used in the preferred exemplary embodiment is simply conventional 3-inch fiber-,

glass insulation normally used for insulating houses but with the paper backing, etc. removed, thus leaving only the 3-inch layer fiberglass material.

Although only one specific embodiment of the invention has been described in detail, those in the art will recognize that obvious modifications may be made in specific features of the exemplary system without in any way departing from the unique and improved features of the system. Accordingly, all such modifications areintended to be within the scope of this invention as defined by the appended claims.

What is claimed is:

1. An acoustic transducer system for use with an audio frequency electrical signal amplifier, said system comprising:

a cabinet structure having separate closed upper and lower speaker housing chambers of substantially equal volume,

electrical input connections adapted for connection to the electrical output of an audio frequency electrical signal amplifier,

bass loudspeaker housed in said lower chamber and having a speaker cone of a first predetermined diameter, said bass loudspeaker being adapted for transducing electrical signals into acoustic signals within a first lower frequency range,

a midrange loudspeaker housed in said upper chamber and having a speaker cone of a second predetermined diameter equal to said first predetermined diameter, said midrange loudspeaker being adapted for transducing electrical signals into acoustic signals within a second middle frequency range which, at its lower frequency portion, overlaps the upper fre quency portion of the first lower frequency range,

each of said chambers being defined by solid, nonapertured walls on all but a front side, which front side is also solid except for an aperture therein directly behind which the respectively associated loudspeaker is mounted,

electrical circuit means for electrically connecting said electrical input connections to said bass and midrange loudspeakers including an electrical crossover network which effectively prevents electrical signals below a predetermined crossover frequency from being applied to said midrange loudspeaker.

2. An acoustic transducer system as in claim 1 further comprising: 4 a pedestal structure of predetermined dimensions attached to the underside of said cabinet structure for maintaining the lower chamber at a predetermined height above an underlying floor surface.

3. An acoustic transducer system as in claim ll wherein the internal surfaces of both of said chambers are lined with a predetermined thickness of porous, resilient material for displacing a predetermined volume of air within the chambers and wherein the lower chamber also contains a further quantity of the porous, resilient material for displacing an additional volume of air within the lower chamber.

4. An acoustic transducer system as in claim 3 wherein said porous, resilient material comprises a layer of fiberglass insulation material.

5. An acoustic tranducer system as in claim 1 wherein said electrical crossover network comprises a capacitor for producing a 3 db per octave crossover rate.

6. An acoustic transducer system as in claim 1 wherein the crossover network has a 3 db crossover frequency at approximately Hz and wherein the bass loudspeaker continues to effectively operate up to approximately 600 Hz thus producing an overlapping range from approximately 125 Hz to 600 Hz.

7. An acoustic transducer system as in claim 1 further comprising a high frequency range transducer housed directly behind a second aperture in said upper chamber and another electrical crossover network which effectively prevents electrical signals below a second predetermined crossover frequency from being applied to said high frequency range transducer.

8. A stereo acoustic transducer system comprising a pair of systems as defined in claim 7 wherein one such system is intended to be placed on the left side of a listener for use with signals representing left-side sounds and the other such system is intended to be placed on the right side of a listener for use with signals representing rightside sounds, wherein: the second aperture is disposed at the upper right-hand corner of the front side of the upper chamber in the system intended for the right side of a listener, and the second aperture is disposed at the upper left-hand corner of the front side of the upper chamber in the system intended for the left side of a listener.

* l= l l

Claims

1. An acoustic transducer system for use with an audio frequency electrical signal amplifier, said system comprising: a cabinet structure having separate closed upper and lower speaker housing chambers of substantially equal volume, electrical input connections adapted for connection to the electrical output of an audio frequency electrical signal amplifier, a bass loudspeaker housed in said lower chamber and having a speaker cone of a first predetermined diameter, said bass loudspeaker being adapted for transducing electrical signals into acoustic signals within a first lower frequency range, a midrange loudspeaker housed in said upper chamber and having a speaker cone of a second predetermined diameter equal to said first predetermined diameter, said midrange loudspeaker being adapted for transducing electrical signals into acoustic signals within a second middle frequency range which, at its lower frequency portion, overlaps the upper frequency portion of the first lower frequency range, each of said chambers beiNg defined by solid, nonapertured walls on all but a front side, which front side is also solid except for an aperture therein directly behind which the respectively associated loudspeaker is mounted, electrical circuit means for electrically connecting said electrical input connections to said bass and midrange loudspeakers including an electrical crossover network which effectively prevents electrical signals below a predetermined crossover frequency from being applied to said midrange loudspeaker.

2. An acoustic transducer system as in claim 1 further comprising: a pedestal structure of predetermined dimensions attached to the underside of said cabinet structure for maintaining the lower chamber at a predetermined height above an underlying floor surface.

3. An acoustic transducer system as in claim 1 wherein the internal surfaces of both of said chambers are lined with a predetermined thickness of porous, resilient material for displacing a predetermined volume of air within the chambers and wherein the lower chamber also contains a further quantity of the porous, resilient material for displacing an additional volume of air within the lower chamber.

6. An acoustic transducer system as in claim 1 wherein the crossover network has a 3 db crossover frequency at approximately 125 Hz and wherein the bass loudspeaker continues to effectively operate up to approximately 600 Hz thus producing an overlapping range from approximately 125 Hz to 600 Hz.

8. A stereo acoustic transducer system comprising a pair of systems as defined in claim 7 wherein one such system is intended to be placed on the left side of a listener for use with signals representing left-side sounds and the other such system is intended to be placed on the right side of a listener for use with signals representing right-side sounds, wherein: the second aperture is disposed at the upper right-hand corner of the front side of the upper chamber in the system intended for the right side of a listener, and the second aperture is disposed at the upper left-hand corner of the front side of the upper chamber in the system intended for the left side of a listener.