US2978060A - Loudspeaker enclosure - Google Patents

Description

April 4, 1961 A. H. ROBERTS 2,978,060

LOUDSPEAKER ENcLosURE Filed VApril s, 1959 INVENTO /I fred/M Roger ex-(12 w a Freya/amy (6,06) 2 ATTORNEY United States Patent O 2,978,060 LOUDSPEAKER ENCLOSURE Alfred H. Roberts, 1615 Monk Road, Gladwyne, Pa. Filed Apr. 3, 1959, Ser. No. 803,856 3 Claims. (Cl. 181-31) This invention relates to acoustical devices and more particularly to loud speaker mountings and enclosures.

In seeking faithful reproduction of voice and music, acoustical designers have devoted much effort to the lowest three octaves'of the audible spectrum, encompassing 16 to 128 cycles per second.

The free-air resonant frequency of most loudspeakers of the familiar electromagnetic (dynamic) type lies within this bass portion of the audible spectrum.

Above this resonant frequency, as is known, the diaphragm of such a speaker is mass-controlled and means of coupling it to the air are relatively easy to devise. Below this resonant frequency, the diaphragm is stiliness-controlled and means of coupling it to the air are relatively diflicult to devise.

With a large room space available, the horn is acknowledged as the best acoustic coupler in the present state of the art. A horn is a structure having a throat, a

mouth, and an enclosed intervening air column so tapered as to present a resistive impedance to the driver diaphragm over a range of frequencies down to a lower limit, called the cutolf frequency. The horn is an acoustical transformer, matching a driver of high impedance at the throat (sound pressure divided by volume velocity) to a low impedance at the mouth.

At any cross section of a perfect horn, an acoustic impedance match exists; the impedance is resistive throughsoA out the length of the horn; and there are no reflec- Y tions (standing waves) from mouth or throat.

All finite horns are, however, imperfect. With each imperfection in the flare rate and each deficiency in size of the mouth, the impedance becomes reactive; standing waves arise in the air column; and the frequency response is irregular.

The organ pipe, or untapered air column, is another method of coupling a transducer to its acoustical environment. Longitudinal reflections (standing waves) occur in a straight pipe in a regular harmonic series. A pipe closed at one end and open at the other will speak or resonate at frequencies corresponding to odd multiples of standing quarter-waves within the pipe. That is, the pipe will have a fundamental pitch having a Wave length four times that of the pipe, and harmonics arise at 3f, 5f, 7 f, etc., where f is the fundamental frequency.

On the other hand, a pipe closed at both ends or open at both ends will speak at a fundamental pitch corresponding to a Wave length twice that of the pipe, and at the even harmonics thereof, viz., 2f, 4f, 6j, etc.

It is important to note that, 'to a transducer located in the closed end of a pipe which is open at `the other end, a standing wave is antiresonant; that is, 'at f, 3f, 5f, etc., the air column behaves like an electrical parallel resonant circuit and the impedance presented to the driver reaches a maximum.

Thus a wave of rarefaction traveling down the tube from the diaphragm is reflected back from the mouth as avwave of pressure (i.e., reversed in phase at the mouth) next pressure wave front down the tube. In other words, the vibrating diaphragm and the reflected wave move in antiphase and tend to buck each other out at f, 3f, 5f, etc.

The action of the quarter-wave tube or pipe as described above is closely analogous to a quarter-Wave radio transmission line short circuited at one end, often called a quarter-wave stub. Each acts as an impedance transformer, inverting the pressure-current ratios at the ends.

The'constants of a pipe, i.e., acoustic mass, compliance and resistance, are said to be distributed along its length. For example, in the quarter-wave stub pipe just cited, the impedance near the closed end, viewed toward the closed end, is a compliance; the compliant air is cornpressed as by a bicycle pump. Looking out the open end, the impedance is a mass; the air moves back and forth as a plug; it is accelerated but not compressed.

Equations for the tube given in acoustical textbooks (Acoustics, L. L. Beranek, McGraw-Hill Book Co., 1954, pp. 131 et seq.) are confined to tubes with not too great a diameter, in terms of Wavelength; otherwise transverse resonances must be taken into account. The radius in meters, it is stated, should be less than about At c.p.s. this radius would be 61/2 inches, the diameter 13 inches.

A third device for coupling a transducer to the air is the vented enclosure, known variously as bass-reflex enclosure, Helmholtz enclosure, acoustic filter, acoustic phase inverter, or distributed-port enclosure (see A. L. Thuras, Sound Translating Device, U.S. Patent No. 1,869,178, Frank Robbins and William Joseph, Acoustic System for Loudspeakers, U.S. Patent No. 2,694,463, Jordan J. Baruch and H. C. Lang, Some Vented Enclosures for Loudspeakers, Acoustics Lab., M.I.T., January 1953).

The Helmholtz resonator combines a nearly pure compliance, in the form of `a volume of air confined within a sphere or box which is small in terms of wave length, with a nearly pure mass, a plug of air within a duct or tube, very short compared to a wave length. The duct is attached to the enclosure. At resonance the inductive reactance of the mass cancels the capacitive reactance of the compliance, in a manner well known in acoustical and electrical alternating-current theory.

In the bass-reex enclosure, a dynamic loudspeaker is mounted on one face of the box, usually adjacent to the port or duct, and the area of the latter is made equal to or smaller than the effective area of the speaker diaphragrn. There are innumerable variations.

and it hits the diaphragm just as the -latter is pushing the 1.

It is interesting to note that the Thuras Patent No. 1,869,178 shows thirteen tubular ducts three inches in r diameter and six inches in length, ranged around a loudspeaker having a -resonant frequency of about 55 c.p.s., Iall contained in a box of approximately thirteen cubic feet. Among the enclosures of this basic type at present being manufactured in the United States, there are many having a single duct approximating three inches in diameter and six inches in length, but perhaps of different shape, a loudspeaker of approximately the same resonant frequency, and a volume of one to two cubic feet. With the reduction in size there has been a corresponding reduction in translation efficiency.

A brief comparison of the three aforementioned types of air couplers is pertinent. The horn is an acoustical transformer having an impedance match throughout its length. The transformation ratio, i.e., impedance ratio, is established by the relative cross-sectional areas of mouth and throat; the radiation resistance is determined by the area .of the mouth.V The straight cylindrical or aaraoeo rectangular pipe exhibits a varying complex impedance along its length, the impedance becoming resistive at resonance or antiresonance. The quarter-wave pipe closed at one end can act as an impedance transformer; but only near antiresonance, when the impedance is resistive, can it radiate substantial acoustic power from the open end. The bass-reflex enclosure, unlike the pipe, has lumped mass and lumped compliance. It has only one antiresonance (i.e., no harmonics) and like the quaterwave pipe, radiates usefully fromy tlieduct principally at and near antiresonance. These are broad generalities, but they are believed useful in connection with an understanding of the present invention.

In accordance with the present invention an, air coupler is provided which has certain properties (a), of a horn, (b) of a Helmholtz resonator or'bass-reiiex enclosure, and (c) of a pipe folded once, butwhich does not satisfy all the criteria for any one of the three types of' couplers referred to above.

It is the principal object of thepresen-t iiufentionr to provide a loudspeaker enclosure having an increasedl response at the lower audible frequencies, an improved response at the middle audible frequencies, and a diffusion of the output at the higher audible frequencies.

It is a further object of they present invention to p-rovide an acoustical device which is economical, in its embodiments, of labor, materialsand living space.

1It is a further object of the present invention to provide an acoustical device which is faithful in its reproduction of speech and musical sounds.

It is a further object of the present invention to provide an acoustical device which is efficient in the, reproduction of sound.

It is a further object ofV the present invention to provide an acoustical device'which will function well with loudspeakers of diverse operational properties.

`It is a further object of the present invention to provide an acoustical device which is adapted to unitary or modular construction and which is adaptable to medium and high power sound projection, when arrayed or stacked in multiple, without the use of auxiliary horns, reflectors or baffles, save the wall, ceiling or floor nearby.

Other objects and advantageous features of the invention will be apparent from the description and claims. The nature and characteristic featuresl of the invention will be more readily understood from the following description, taken in connection with the accompanying drawings forming part thereof, in which:

Figure l is a front elevational view of an acoustical device in accordance with the present invention and positioned against a vertical wall and opening towards the ceiling;

Fig. 2 is a vertical central sectional view taken approximately on the line 2-2 of Fig. 1;

Fig. 3 is a view of an acoustical device in accordance with the present invention positioned against a vertical wall and in spaced relation to a iloor;

Fig. 4 is a perspective view of an acoustical device in accordance with the present invention positioned in the corner of a room and at intersecting walls;

Fig. 5 is an enlarged side elevational view of a speaker having a damping ring carried thereby;

Fig. 6 is a front view of the structure shown in Fig. S;

Fig. 7 is a diagrammatic View showing the impedancefrequency curve of the structure in accordance with the present invention; and

Fig. 8 is a diagrammatic view of the acoustical circuit with approximate electrical analogs.

It should, of course, be understood that the description and drawings herein are illustrative merely, and that various modifications and changes can be made in the structure disclosed without departing from the spirit `of the invention.

Like numerals refer to like parts throughout thesev-l eral views.

Referring now more particularly to Figs. 1 and 2 of the drawings, in which a preferred embodiment is illustrated, the enclosure in accordance with the invention in a practical and useful embodiment can have an exterior width of the order of l0 to 16 inches, an exterior depth of the order of 16 inches, and a height in feet of the order of where fo of the speaker for a conventional S inch speaker is 60 c.p.s.

The enclosure includes a horizontal bottom wall panel 10, spaced parallel vertical side wall panels 11 and 12, -a vertical rear wall panel 13 which is adapted to be disposed against a vertical wall 14 of a room or other enclosure, and a vertical front wall panel 15. A top wall panel section 16 is provided horizontally disposed and connecting the side wall panels 11 and 12 and the front wall panel 15. A baie or mounting board 17 is provided extending downwardly frorn the panel section 16 and between the side wall panels 11 and 12. The mounting board 17 is rearwardly inclined, preferably at an angle of the order of l5 degrees.

The wall panels 10 to 13, inclusive, can be made of any desired material such as wood, plywood, chipboard, pressed cellulose board, laminated plastic and the like, and with an approximate panel thickness of three fourths of an inch. The construction is rigid and the joints between meeting wall panels are effectively air-tight. VAn interior covering of fibrous material 1S, such as glass bers, mineral Wool, textile fibers or the like, is provided, ofa thickness of the order of one inch on the interior of the front wall panel 15, side wall panels 11 and 12, rear wall panel 13 and bottom wall panel l1t).

The baille or' mounting board 17 preferably has an opening 19 therethrough for the diaphragm of a speaker 20 which is secured to the mounting board 17 in any desired manner. The speaker 20 has a damping ring 21 mounted thereon asl hereinafter more fully explained.

The speaker 2i) can be of vany desired type such as electro magnetic or dynamic, as desired.. Mid-range or treble speakers can also be employed, an auxiliary speaker 22 being shown as mounted to face the listener. The speaker 20 has one of the surfaces 26a of its diaphragm or cone facing in one direction at the opening 19, and the other surfacey 20!) thereof oppositely disposed.

The space or cavity Within the wall panels has a throat portion. 30, an energy sink 31 communicating therewith, and a mouth portion 32 extending froml the sink 31 with the terminus 31211 of the mouth portion 312. substantially in the plane of the wall panel section 16.

In Fig. 3, the enclosure structure is shown as turned so that the wall panel 10a provides a top wall, and short legs 23 are provided, which can be of a height of the order of five inches, to support the structure with the mouth facing the floor. Y

`In the form of the invention illustrated in Fig. 4 in place'of a fiat rear wall panel 143, a pair of rear wall panels 13a and 13b are providedv so that the structure can be placed in the corner of a room with` the mouth facedupwardly as in Figs. l and 2.

From the foregoing it will be seen that the enclosure structure may be employed in any desired arrangement with the rear wall panel 13 parallel and closely positioned with respect to aroom enclosing wall and with the mouth 32 up or down, or if desired in any other direction.

The enclosure structure as shown may be employed as a single unit, in pairs or in multiple for stereophonic sound reproduction, and separated at the desired distances to provide the desired effect in the particular room enclosure in which the same are employed.

As illustrated in Fig. 2, a hard surfaced rellector plate 33 can be provided along the top of the back or rear wall panel 13 for a dilfusion of higher frequencies in a manner known in the art.

Reference will now be made to the acoustical properties of the enclosure in accordance with the invention.

This enclosure in accordance with the invention de-v pends upon the quarter-wave action of a folded air column to establish an antiresonance at and near the freeair resonant frequency of a dynamic loudspeaker. In this respect this invention resembles a pipe, folded once.

To achieve a maximum area and consequent radiation resistance at the mouth, the baille board 17 for the speaker 20 is tilted backward at approximately 15 degrees, as previously stated and as shown in Figure 2. The throat 30 behind the speaker 20 is four to six times smaller in cross-sectional area than the mouth 32 of the coupler. This relationship establishes a transformationratio between mouth 32 and throat 30 for better resistive loading of the back of the speaker diaphragm. In thisy respect the enclosure in accordance with the present invention resembles a horn.

To realize the maximum mutual radiation impedance between the mouth 32 of the coupler and the front of the speaker diaphragm, the speaker 20 is placed within the mouth 32 in one embodiment shown in Figs. 1 and 2. Below the baille plate 17 and bounded by wall panels 10, 11, 1'2, 13 and 15 there exists a compliant volume of air in the cavity or sink 31 which functions as an energy sink at and near antiresonance. In these respects the enclosure structure in accordance with the invention resembles a Helmholtz resonator, or bass-reflex enclosure.

The enclosure in accordance with the invention does I not satisfy all the criteria for any one of the aforementioned three types of coupler. It is not a horn because of the large reactive impedance existing in the cavity or sink 31. It is also not a horn because of the shortness of the air column with respect to the wave lengths transmitted. It is not a pipe because of the large reactive impedance existing in the cavity or sink 31. IIt is not a Helmholtz resonator because the antiresonance frequency of the device is not controlled by the cross-sectional area at any point, but rather by the length of the air column.

The compliance referred to and other acoustical parameters of the acoustical device in accordance with the i11- vention are shown in Fig. 8 in which the speaker parameters are designated as follows:

C3-compliance of speaker suspension Rs--resistance of suspension plus radiation resistance of `front of diaphragm M3-mass of speaker voice coil, diaphragm and air in front of diaphragm Rd-dissipative resistance introduced at back of diaphragm by damping ring 21 (to be described later herein)l lActing through the network, under pressure P3, is volume velocity U3.

The coupler proper is represented electrically and acoustically by the shunt compliances C1 at the throat 30, C2 at the energy sink 31, and C3 in the mouth 32, together with the masses M1, M2 and M3. Each branch takes a branch current of volume velocity U1, U2 and U3. The useful load on the air column, i.e., radiation impedance, yis represented by Ra within a border. Rl and Rd vary with the second power of the frequency.

'1" he entire network, as shown in Fig. 8 is in effect a low-pass wave filter composed of T sections, employed to match the air load at the mouth 32 of the coupler` to that at the throat 30 at the back of the speaker 20.

At antiresonance, when the resistive impedance at the mouth 32 is falling rapidly, the driver diaphragm encounters an impedance peak which limits its excursion, reduces harmonic distortion arising from a non-linear speaker suspension and maximizes radiation at the mouth 32. This is similar to the action of the bass-reflex except that the mouth 32 here resembles a horn, rather than a port or duct.

-Since Rd is falling with frequency, along with R., a

rough impedance match is maintained over a broader band of frequencies than hitherto possible in a coupler having lumped constants.

Above antiresonance, the shunt compliances C2, C2 and C3 progressively shortcircuit the back radiation of the speaker 20 so that the air column becomes acoustically opaque. This effect is enhanced by the increase of R4 with frequency. Thus at middle and high sound frequencies the front of the speaker 20 must do all the work, and the best balance is achieved between the output of the first three octaves (16 to 128 c.p.s.) and the output above 128 c.p.s., with the dimensions previously mentioned (approximately 12 inches by 16 inches by 24 inches) the upper cutoff frequency of the low-pass filter is computed to be about c.p.s.

Due -to the end effect of the air column, especially when the mouth 32 radiates along a plane surface, the antiresonant frequency is lowered. `In a rectangle of side elevation 24 by 16 inches, the total folded air column is approximately 2 X 24 plus 6 inches=54 inches against a wall or oor. The antiresonance then occurs at approximately 63 c.p.s. This is well-related to many ldynamic loudspeakers of medium stiffness.

Just below the antiresonancetfrequency of the quarterwave air column and the speaker (series-resonant branch of the network), the impedance at the mouth 32 of the coupler becomes inductive. Likewise, the compliances within the coupler (C1, C2, C3) fade away. This leaves, in effect, an inductive air column, with vestigial radiation resistance atits outer end, to resonate with the compliance of the speaker surround. The last useful radiation at the low end, therefore, is accomplished by the rear of the speaker diaphragm, Iacting through the larger area of the mouth 32.

In this respect, this invention is an improvement over couplers heretofore available. In the bass-reex couplers a similar lower resonance occurs, but the radiation is limited by the relatively small size of the port.

In Figs. 5 and 6 there is shown in more detail a preferred form of damping ring 21 which is preferably employed in connection with the speaker 20 and consists of a ring 40 of glass wool, mineral wool, textile bers or the like, snugly fitted within a truncated cone or cylinder 41 which serves as a cowl. The cylinder 41 is composed of any suitable material for retaining the glass wool ring 40 in position around the dust cover 42 of the speaker 20.

Referring now more particularly to Fig. 7 in which frequencies are plotted as abscissas, and impedances expressed in decibles are plotted as ordinates, the curve designated at A is a typical curve for an eight inch permanent magnet speaker 20 having a free air cone resonance of 68 c.p.s. The curve designated at B shows the relative electrical impedance of the speaker 20 plotted against frequency with the cowl 41 attached, but without any padding ring 40 employed. The air mass within the cowl 41 has a loading effect which, as indicated, has effected a reduction of the resonance frequency to 60 c.p.s.

The curve designated at C shows the impedance effect with a one inch thick ring of glass wool 40 snugly tted within the cowl 41 and provides a considerable drop Yin the impedance peak. The curve designated as D shows the impedance with a glass wool ring 40 of ya thickness of two inches within the cowl 41 and with a further reduction in the impedance peak.

It will be noted that the Q of the speaker, which is the measure of vibratory energy stored against vibratory energy expended, has been lowered in a ratio of more than four to one, so that transient hangover in the low frequency range is repressed.

The curve shown at E gives the corresponding electrical impedance within the air coupler, and it will be noted that a dipoccurs at speaker resonance caused by the quarter wave air column antiresonance. The entire impedance variation is held to about two decibels through the tirst four octaves.

The damping ring 21 shown particularly in Figs. 5 and 6 has an additional purpose of maintaining an imp pedance match as the radiation Yresistance at. the mouth 32 falls -away with the frequency. The insertion Vof the damping ring 21, diagrammatfi'callyV illustratedl at Rd in Fig. 8, places at the driver end of the transmission network a frequency dependent resistance similar to that imposed on thel mouth 32 by limitation of. size*d ./TheV damping ring 21 has a still fuxtherzpurpose in assisting in discriminating against mid` and: high` range frequency response from the back of the speaker 20. n

1. An acoustical device comprising. azlouds'peaker and an enclosure for said loudspeaker; said enclosure having a plurality of spaced side wall panels connected along their meeting margins, an 'endf wall panel closing said side wall panels at one endthereof; a second end `wall panel closing part of the opposite-endet said side wall panels, a battle connected; to and extending inwardly from said second end wallv panel; to an* end margin beyond the loudspeaker and providing on onev side thereof with certain of said side wall panels a mouth portion and on the other side thereof with certain. of said side wall panels and -said second end wall panel anvinterior throat portion, said side wall panels at said'. oppositeY end. terminating in the same planeN as said second, end Wall panel and providing the terminus; of the mouth portion coplanar with said second end wall panel;Y said bale having van opening therethrough, andsaidiloudspeaker being mounted at said opening with one diaphragm surface facing into said throat portion and one diaphragm surface facing across said mouth portion;v saidsidewall panels, bale and end wall panels providing an air column extending from the throat portion to the terminus of the mouth portion Iand folded back uponitself at said first end wall panel, the length of said airl column -being not more than a quarter wave at the primary resonant frequency of the loudspeaker; said side Wall panels between the end margin of said baffle and said end wall panel providing an energy sinkv of larger volume than either the throat volume or the mouth volume.

2. An acoustical device as dened in claim 1 in which the rear of the loudspeaker is provided with a pad of fibrous material. Y

3. An acoustical device as dened in claim 1` in which one of the side walls in facing relation to the opening in the bale has a hard surface reilector plate mounted thereon.

References Cited in the tile of this patent UNITED STATES PATENTS 2,217,279 Karns Oct. 8, 1940 2,440,078 Devine Apr. 20, 1948 2,622,693 Leslie Dec. 23, 1952 2,765,864 Glenn Oct. 9, 1956 2,801,704 Martin Aug. 6, 1957' 2,871,971 Beecroft et al. Feb; 3, 1959' FOREIGN PATENTS 747,263 Great Britain Mar. 28, 1956

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