US3796839A

US3796839A - Loud speaker system

Info

Publication number: US3796839A
Application number: US00285012A
Authority: US
Inventors: W Torn
Original assignee: Dukane Corp
Current assignee: Dukane Corp
Priority date: 1972-08-30
Filing date: 1972-08-30
Publication date: 1974-03-12
Anticipated expiration: 1991-03-12

Abstract

This invention provides a loudspeaker system having two coaxial cone diaphragm type nested speakers (woofer and tweeter) for handling an extended range of audio frequencies. The speaker system embodying the invention is characterized by a phasing baffle plate extending part way from a region spaced near the rim of the tweeter toward the woofer cone diaphragm. By controlling the dimensions, spacing and material of the phasing baffle plate, the overall frequency response for the two speakers may be rendered substantially uniform over the extended range.

Description

United StatesiPatent [1 1 Torn [ LOUD SPEAKER SYSTEM [75] lnventor: William R. Torn, St. Charles, 111.

[73] Assignee: Dukane Corporation, St. Charles, 111.

[22] Filed: Aug. 30, 1972 [21] Appl. No.: 285,012

[52] US. Cl. 179/116, 179/115.5 R

[51] Int. Cl H041 9/06 [58] Field of Search l79/115.5 R, 115.5 PS, '179/116; 181/31 R, 31B

[56] References Cited UNITED STATES PATENTS 2,757,751 8/1956 Tavares 181/31 B 2,496,589 2/1950 Marquis l79/115.5 PS

2,047,290 7/1936 Ringel t r 181/31 B Engholm 179/116 1 Mar. 12, 1974 Primary Examinerl(athleen' H. Claffy Assistant Examiner-Thomas L. Kundert Attorney, Agent, or Firm--Robert L. Kahn 57 ABSTRACT This invention provides a loudspeaker system having two coaxial cone diaphragm type nested speakers (woofer and tweeter) for handling an extended range of audio frequencies. The speaker system embodying the invention is characterized by a phasing baffle plate extending part way from a region spaced near the rim of the tweeter toward the woofer cone diaphragm. By controlling the dimensions, spacing and material of the phasing baffle plate, the overall frequency response for the two speakers may be rendered substantially uniform over the extended range.

4 Claims, 10 Drawing Figures LOUD SPEAKER SYSTEM This invention relates to a loudspeaker system of tht type having nested coaxial cone diaphragm type speakers for extended frequency response. The larger speaker (woofer) is relied upon for handling the lower frequency range, which may extend from the lowest frequency to be handled, generally something less than about 50 Hz., to somewhere in the region of about 1,200 Hz. Nesting within and coaxial with the conical region defined by the woofer diaphragm is a smaller speaker (tweeter) which is suitably supported on the woofer. The tweeter is relied upon to handle frequencies from the order of about 800 Hz. to the maximum value to be handled, such maximum value being about 8,000 Hz. to 16,000 or more, depending upon the top frequency to be handled by th speaker system.

The tweeter cone diaphragm may have a diameter in the range of about 6 inches to about 3 inches and the woofer cone may have a diameter in the range of about 18 inches down to about 8 inches. The tweeter has its pot or permanent magnet field structure usually carried by the permanent magnet structure of the woofer so that the voice coils and diaphragms of the two speakers may vibrate independently. Voice currents over the entire frequency range to be handled are fed to both speakers in which case the mechanical properties of the woofer and tweeter may be relied upon for separating the low and higher frequency ranges. Or electrical networks may be provided for frequency separation of voice currents to the woofer and tweeter, there being a cross-over range of frequencies from about 800 Hz. to about 3,000 Hz.

A speaker system embodying the present invention is used as conventional woofer and tweeter speaker combinations, with conventional baffle plate for the woofer, all housed in a conventional cabinet. A cabinet for housing a tweeter andwoofer combination, whether embodying the present invention or not, may have an open back or may utilize a substantially sealed cabinet having acoustic absorbing material within the cabinet for loading. In such a speaker system, it must be borne in mind that the frequency cross-over network, if provided, or the mechanical response of the woofer and tweeter is such that from about 800 Hz. and higher, the response of the woofer becomes weaker with frequency rise while the tweeter response increases. This gradual shifting of the speaker response is thus generally operative over a substantial frequency range from the order of about 800 Hz. to the order of. about 3,000 or more Hz.

PRIOR ART AND ITS DRAWBACKS In designing a high quality audio frequency amplifier and speaker system, the frequency response characteristics of an amplifier as one means and that of a speaker system as a transducer are separately designed. This is due to the general practice of engineering and production of audio frequency amplifiers as one field of commercial activity and speakers or speaker systems as another field of commercial activity. Even if one source provides amplifiers and speakers, the above practice of separately engineering the two is still generally true. The technical qualifications for the two fields are quite different.

In theory, an audio frequency amplifier having a uniform frequency characteristic when feeding a high quality speaker system also having a uniform frequency response characteristic should provide an overall uniform frequency response over the frequency range being handled. In practice, this ideal is rarely if ever realized.

As a practical matter, the frequency response characteristic of a speaker or speaker system usually repre sents an idealized characteristic derived from smoothing out the actual frequency response curve. This may be accomplished by rapid frequency changes over the spectrum. Generally, it is not possible to produce a speaker system and its component parts free from some sharp resonance peaks or anti-resonance holes (dips) in the desired working range. The difficutly is due primarily to the multiplicity of separate physical parts in a speaker system, each having individual resonances an being physically or acoustically coupled to each other. Consequently over the entire frequency spectrum to be handled by a speaker system, it becomes well nigh impossible to avoid some resonant peaks or anti-resonant holes within the normal listening range. The factors which may be involved in having undesired peaks and holes in the speaker response may relate to such physical elements as the speaker baskets, the cone diaphragm and suspension for either or both speakers, the nature of parts and airchambers providing physical or acoustic coupling between the two speakers, the voice coils and mountings.

When a speaker system exhibits resonance peaks or holes of substantial amplitude, a painstaking analysis of possible sources of trouble as well as some physical changes in the speaker system may be required to improve the frequency response characteristics. Such a program for critical analysis and possible physical changes requires not only a substantial amount of time and expenses for engineering but may in addition require changes in tooling for the manufacture of such speakers. The testing requirements involve actual production samples with production tolerances. Thus unlike most production procedures, where engineering or production is condemned because of costs in tooling or production changes, speakers require production units for testing.

The depth of the tweeter ,within the woofer cone may be varied and effect some changes (good or bad) in the frequency response characteristics of this speaker combination. For the most part however, such a procedure does not alter the basic frequency characteristics. The generally rough frequency response characteristic of such a speaker combination usually remains.

THE INVENTION GENERALLY AND ITS ADVANTAGES The invention generally contemplates the addition of an annular member of suitable material attached to one of the speakers, preferably the tweeter, at the anchorage of the large tweeter cone diaphragm end and within the space encompassed by the woofer cone diaphragm. The annular member is preferably flat, extending beyond the rim of the tweeter diaphragm in a plane generally normal to the coincident cone axes. The nature of the material may vary widely and should be self supporting or have reenforcements to provide a selfsupporting structure and such material should have good acoustic properties. Such materials as natural or synthetic woods, wall-board, plastic sheet, particle board material may be used. Any material that is useful as a sounding board may be used. The thickness is one of the dimensional parameters which must be determined by testing. Generally, material from about oneeighth inch to three-fourths inches may be used. While metal may be used, it should be much heavier than conventional sheet metal. As a rule, metal should be about one-sixteenth inch or thicker. Lighter materials like Styrofoam, having a smooth outer molded skin, are satisfactory in which case a thickness of about threeeighths inch to about three-fourths inch may be good, depending upon desired frequency characteristics.

Considerable latitude in the shape of inner and outer plate edges, plate thickness, spacing between inner plate edge and tweeter cone outer edge, spacing between plate outer edge and woofer cone wall is permissible, all such variables affecting the frequency response of the speaker system. The space between the outer edge of the phasing baffle plate and adjacent woofer cone wall should be at least about an inch for a woofer, 12 inch cone and more for a 15 inch cone. Too narrow a space between the woofer cone wall and plate outer edge tends to throttle low frequency response.

The invention directs engineering efforts to improve speaker response from a production speaker system (that has been reasonably well designed) to a phasing baffle plate by controlling the above identified parameters. The relative spacing between woofer and tweeter within the woofer cone and relative spacing of phasing baffle plate along the cone axis are also parameters which can be operated on with engineering and testing. Once a phasing baffle plate for a particular speaker combination has been satisfactorily determined, then quantity production of the entire speaker system can proceed. The selection of relative woofer and tweeter sizes will be governed for the most part by conventional engineering technique. There are so many variables in adapting a phasing baffle plate to a speaker combination that an improved frequency response characteristic can be obtained by concentrating engineering changes on the phasing baffle plate and, for the most part, leaving the woofer and tweeter speakers as is.

Commercially, the invention makes possible the quantity production of speaker combinations having exceptionally desirable frequency response characteristics at a cost substantially less than conventional high quality speaker systems.

In a simple embodiment of the invention, a phasing baffle plate is flat with a suitably shaped window therethrough providing an inner edge which will normally be partly or wholly spaced from the mounting rim of the tweeter to provide an air path. The extent of the air path is an important parameter and is experimentally determined. The plate has an outer edge portion extending toward the woofer diaphragm, the plate being disposed in acoustically satisfactory relation with respect to the concentric rims of the two speakers. It is possible to have the outer edge of the plate noncircular or non-symmetrical with respect to the axis of the tweeter. The latter modification may provide different effects on the frequency response characteristic along the axis of the speaker combination or off the axis.

Similarly, while the phasing baffle plate is preferably flat, a dished plate shape may be used. In such case, having the phasing baffle plate concave when viewed from the voice coil end of the cone may be more desirable.

A combination of low and high frequency range speakers embodying the invention can be marketed like present day speaker combinations, where the woofer and tweeter are rigidly secured to each other, the speakers being adapted to have the woofer mounting rim bolted to a conventional baffle board forming part of the housing or cabinet in which the speaker combination operates. It is understood that the woofer and tweeter combination must be operated in an appropriate mounting for developing its frequency response characteristics. Usually the manufacturer of the speaker combination prescribes or recommends a desired mounting for the speaker in a housing or cabinet.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be disclosed in connection with drawings wherein:

FIG. 1 is a plan view of a speaker combination provided with a phasing baffle plate embodying the present invention, the broken lines suggesting a mounting for the speaker combination.

FIG. 1A is a plan view of a modified embodiment of the invention.

FIG. 1B is an enlarged detail on line 1B-1B of FIG. 1A.

FIG. 2 is a transverse sectional view along line 22 of FIG. 1, the section line going around the center bolt for ease of illustration.

FIG. 3 is a frequency response curve of a conentional speaker combination having a 12 inch woofer and a 5 inch tweeter without the new phasing baffle plate, the X axis showing, logarithmically, frequencies ranging from 20 Hz. up to about 20,000 I-Iz., the Y axis showing decibel (dB) values from about to over 100.

FIG. 4 shows a corresponding frequency response characteristic for the same speaker combination of FIG. 3, the combination in this instance being provided with a phasing baffle plate of 45 pound density particle board having a thickness of substantially three-eights of an inch with concentric circular inner and outer edges 8-% inches and 4-% inches diameters respectively, washers 51 for spacing the phasing baffle plate being one-sixteenth inch thick, the frequency scale and dB scale being the same as FIG. 3.

FIG. 5 shows a frequency response curve of the same speaker combination as set forth in connection with FIG. 3 wherein voice currents were fed only to the tweeter, the speaker combination not having the phasing baffle plate.

FIG. 6 shows the frequency response curve of the tweeter in the same combination of speakers as before, with the phasing baffle plate, the woofer being inoperative.

FIG. 7 is a frequency response curve of the same speaker combination provided with the phasing baffle plate identified in connection with FIG. 3 with the woofer only operating and the tweeter or highfrequency speaker being inoperative.

FIG. 8 is a simple circuit diagram illustrating a network for separating and feeding high-frequency voice currents to the tweeter.

DESCRIPTION OF A SIMPLE WOOFER AND TWEETER SPEAKER COMBINATION MINUS INVENTIVE FEATURE Permanent magnet 10, generally of toroidal shape, has pole faces 11 and 12. Pole faces 11 and 12 have cemented or otherwise joined thereto bottom and

top pole plates

13 and 14 respectively. The pole plates are of soft iron or soft steel, with the outer portion of permanent magnet projecting beyond

plates

13 and 14. The inner edges of permanent magnet 10 are laterally and outwardly offset from inner edge of face 15 of top plate 14. Bottom plate 13 is a solid disc having a countersunk central aperture 17 through which extends bolt 18. Cylindrical pole piece 20 is disposed about the body of bolt 18. In order to minimize magnetic reluctance, the opposing surfaces between the magnet and the pole plates as well as pole piece 20 are ground to a smooth finish and the opposing surfaces are cemented together with a thin layer of cement.

Resting against the top face of top pole plate 14 is annular flat portion 21 of a steel (usually soft) basket 22 which extends upwardly (as seen in FIG. 2) from flat portion 21 and outwardly to provide mounting flange or rim 24. Basket 22 as a rule has windows punched out therefrom to reduce weight and improve the speaker acoustic properties.

The outer cylindrical surface of pole piece 20 cooperates with inner cylindrical pole face 15 of top plate 14 to provide an annular air gap within which voice coil 26 operates. As is well known, woofer voice coil 26 consists of a number of turns of wire wound over a fabric or synthetic bobbin of suitable stiffness. During woofer operation, the voice coil moves axially in response to voice currents, the amplitude of excursion being a function of the amplitude of voice currents in the voice coil. The voice coil operates in a narrow air gap about the outer surface of pole piece 20, the gap defining close spacing between opposed pole surfaces.

The bobbin of the voice coil is securely attached, as by cementing, to annularly corrugated spider 28 whose outer edge 29 is securely attached to adjacent basket portion 21. The inner edge of spider 28 is also attached to small end 30 of woofer cone diaphragm 31. Cone diaphragm 31 has its large cone edge 32 attached to or extending into annularly corrugated cone diaphragm hinge portion 33, the outer edge of which is securely clamped at rim portion 24 of the basket. Bolts 35 extend through openings in rim portion 24 of the basket and cooperate with cardboard ring 36 for tightly clamping the various parts together. Bolts 35 are used to secure the rim portion of the speaker basket to a baffle board of housing for mounting a speaker in a suitable manner.

To protect the woofer voice coil and the air gap in which it operates from dirt and dust, cover 37 of foam rubber or plastic is provided. Foam dust cover 37 is cemented to thecone diaphragm 31 near the small end thereof. Disposed about bolt 18 above the top of pole piece 20 is rigid mouting spacer 39. This spacer is preferably of non-magnetic metal and extends through the central portion of dust cap 37. Boll 18 has its threaded end portion 40 screwed into the bottom of a tweeter cone-type speaker unit to support the same in coaxial relationship with the woofer unit which has just been described. In general, the tweeter speaker unit is a small version of the structure so far described and requires no detailed description. In this particular tweeter unit, basket 42 does not have windows as is true of the woofer basket. However some tweeters have apertures through the basket to change acoustic characteristics. The tweeter basket has mounting rim 43. The voice coil in each of the two speakers is connected by wires to an outer terminal board, s'uch wires being led along a path to prevent excessive vibration and crystallization of the wire metal.

Referring to FIG. 8, a cross-over network for a speaker combination is illustrated. Audio frequency voice currents are fed to wires 61 from a suitable source, usually an amplifier. Wires 61 are connected to voice coil 26 of a cone diaphragm type of loudspeaker 62, as for example the low-freqency speaker illustrated in FIG. 2. Branching from wires 61 are

wires

64 and 65. Wire 64 is connected through series connected capacitors 66 to 67 to one terminal of the voice coil of highfrequency or tweeter speaker 69.

Capacitors

66 and 67 have suitable values for passing audio frequency currents. Connected between the common junction of

capacitors

66 and 67 is audio frequency inductor 70, the bottom terminal of which is connected to line 65. The voice coil for high-frequency tweeter 69 is connected between the free terrninal'of capacitor 67 and supply wire 65.

The combination of

capacitors

66 and 67 and inductor 70 cooperates to pass voice currents having frequencies above about 600 Hz. Generally a sharp cutoff for high frequencies is not desirable. However networks simplicity or complexity may be used. No great precision in establishing a frequency cross-over region is necessary since the large and small speakers themselves function to effect frequency separation.

The speaker arrangement so far described is well known and widely used. In the case of a 12 inch speaker, the large cone diameter (usually measured at the outer rim edge of the basket) may be any one of 8 inches, 10 inches, 12 inches or I5 inches (these four sizes being quite common) and have tweeters which may have large end cone diameters of about 3 inches or less for an 8 inch woofer, about 3 or 5 inches for 10 and 12 inch woofer or 4 or 6 inches for the tweeter for use with 15 inch woofers.

As has been previously indicated, for testing a speaker combination unit, as illustrated in FIG. 2, must be a production unit and must be suitably mounted in a baffle board for testing acoustic response. As shown by the curve in FIG. 3, a coaxial speaker combination utilizing a l2 inch woofer and a 5 inch tweeter tested out with holes at about 1,100 Hz. and 2,000 I-Iz., a high resonance peak at about 4,500 Hz., a bad drop at about 7,000 Hz. followed by another bad drop at about 8,000 Hz. and bad drops at about 11,000 and about 13,000 Hz.

To correct these bad drops and objectionable peak at around 4,500 l-Iz., various expendients had been relied upon. For example the tweeter axial position can be adjusted without too much trouble to move the tweeter diaphragm axially along the space within the large diameter cone. Such a change or adjustment however has limited effects and will not smooth out the usually rough frequency response curve. It had been the practice for an experienced speaker engineer to visually analyze the response curve as FIG. 3 for example) and quess where the trouble is, By playing with the sizes and nature of various parts making up the speaker and the gauge of metal subject to possible vibration, some changes in the characteristic curve may be effected. However such changes are usually in the realm of production changes, are expensive, time consuming and may involve changes in tooling.

Even then, the fundamentally rough frequency response of the two way speaker combination still remains. Additionally, makers of equipment having speakers as parts thereof are usually unable to persuade the speaker manufacturer to improve the frequency response characteristic unless leverage in the form of large orders or general customer demand is present. There are good guality speakers available but the prices frequently cannot be justified by the economics of the situation.

Modification of the Speaker Combination in Accordance with the Invention In order to incorporate the invention in a conventional speaker combination construction as described previously, apertured phasing baffle plate 50 is provided. This plate is bolted or otherwise secured to rim 43 of tweeter basket 42.

In accordance with the invention, an air path is provided between the tweeter rim and inner edge 54 of phasing baffle plate 50. The size of the air path is one of the parameters which must be experimentally determined by a skilled speaker engineer to provide a desired frequency response characteristic. A simple means for creating such an air path is illustrated in FIGS. 1 and 2. Spacer washers 51 of suitable material as iron, aluminum, etc. are disposed about bolts 52 between tweeter rim 43 and the inner edge portion of plate 50 to secure plate 50 in desired spaced relation thereto. In the speaker system tested, for which frequency response characteristics are given, washers 51 were one-sixteeth inch thick, for three-sixteenth inch bolts.

No attempt has been made to show parts or air spaces in proper proportion in the drawings. The air spaces created by washers 51 may range from that created by a thin washer at each bolt 52 to thick washers, depending upon the circumferential length of all the air spaces due to washers 51. Large thickness of washers 51 at each bolt 52 may be avoided by removing plate material at the inner edge or adopting a different plate support means.

Referring to FIG. 1A, plate 50' has inner edge 54' generated by a four sided window or aperture, the sides being curved outwardly. The aperture is larger than the tweeter rim. Fingers 55 of suitable material (the material may be the same as plate material) are securely attached, as by cementing, to one face of plate 50 and extend radially inward to the cone axis overlie tweeter rim 43. Bolts or screws 52' (with or without spacer washers) extend through tweeter rim 43 and anchor plate 50 to the tweeter rim. By controlling the shape and dimensions of the window or aperture of phasing baffle plate 50', any desired size for the air path around inner edge 54' of plate 50' may be provided.

As an example, each finger 55 may be of particle board, three-eighths inch thick, where it is secured to the bottom face of plate 50', and one-sixteenth inch thick where it extends over tweeter rim 43 and about seven-sixteenths inches wide. The length may be about I-iinches. The outer circular edge of plate 50' may have a diameter of about 8-%inches and thickness of about one-half inch. The radius of the circle upon which the four arcuate sides of the window lay can be substantially 4- /2inches. The window is symmetrical with respect to the circular outer edge, arcuate window sides being similar. The entire plate is useful with a 5 inch tweeter having a 12 inch woofer. In this instance, plate 50' is bevelled at the outer edge. This is not essential. By reversing fingers 55 prior to cementing or by reversing the entire plate, after the fingers are secured, to change plate position, some changes in the response curve may be obtained.

In case the tweeter mounting rim is unsuitable for attaching a phasing baffle plate, it is possible to attach mounting fingers to the plate, such fingers extending outwardly toward the woofer mounting rim and be secured thereto.

It is generally desirable to have the phasing It is generally desirable to have the phasing baffle plate positioned so that it lies within th woofer cone. If it projects forwardly beyond the woofer, it may be diff cult to dispose a fabric covering for decoration. On the other hand, having the tweeter too deep in the cone is not desirable for reasons of sound dispersion. In general, conventional testing can be used to determine the most desirable location.

It has been found that the annular space between the outer edge of plate 50 (or 50' and mounting rim 24 of a 12 inch woofer should be at least 1 inch and preferably greater so that the low frequency response of the entire speaker combination will not be adversely effected. For an 8 inch woofer, the annular space may go to one-half inch while a 15 inch woofer may require more than 1 inch space. It has already been pointed out that plate 50 (or 50 may be made of various materials having the properties set forth and that for the most part, depending upon the material, a thickness of the order of about one-tenth inch is a minimum. As the test curve indicate, a plate of 45 pound density particle board having a thickness of three-eights of an inch was satisfactory. The thickness of the plate is generally not critical.

Referring now to FIG. 4 this curve shows the frequency response characteristic of the speaker combination used in connection with the frequency response characteristic illustrated in FIG. 3 except that, a threeeights inch thick phasing baffle plate of above identifled particle board having cconcentric inner and outer circular edges whose diameters respectively were 8- %and 4-%inches was used, the plate being attached to the tweeter rim by screws as shown in FIGS. I and 2 of the drawings, spacing washers 51 being one-sixteenth thick. The form of the curve shown in FIG. 4 is generally similar to that of FIG. 3 up to about 1,000 Hz. The drop or hole in FIG. 3 at about 1,100 Hz. has disappeared. The curve of FIG. 4 has been smoothed out somewhat between 1,000 Hz. and about 2,000 Hz. The hole or drop at about 2,000 Hz. in FIG. 3 has disappeared and the high amplitudes of speaker response in the range of from about 2,000 to about 6,000 Hz. has been considerably reduced in FIG. 4. The sharp drops or holes in FIG. 3 for 7,000 Hz. and for about 11,000 Hz. have disappeared in FIG. 4, a sharp hole or drop in FIG. 4 now appearing at about 13,000 or 14,000 Hz. in FIG. 4.

At this high frequency of well over 10,000 Hz. such a drop or hole may not be objectionable. This drop in FIG. 4 at about 12,000 Hz. can be moved toward the higher frequency end of the curve if desired by making some changes in the phasing baffle plate, such as for example slightly reducing the diameter of the outer edge. It is also possible to increase the amplitude of speaker response in the range of'between about 5,000 and about 9,000 or 10,000 by suitable operation on the phasing baffle plate. The frequency response curve illustrated in FIG. 4 does show great improvement overthe response curve illustrated in FIG. 3. Insofar as quality of speaker response is concerned, the curve illustrated in FIG. 4, as distinguished from the curve illustrated in FIG. 3, reflects a smoother and substantially better sounding speaker system due to the presence of the phasing baffle plate. It should be understood that the 45 lb. density particle board is one example of a readily available particle board. Denser or less dense materials maybe used.

Referring now to the frequency response curve illustrated in FIG. 5, this shows the speaker combination used in connection with obtaining the response curve illustrated in FIG. 3, the speaker being a combination of 12 inch and 5 inch speakers with no phasing baffle plate and wherein the woofer is not connected to receive voice currents. It will be noted from FIG. 5, tha there is little or no response below about 500 Hz. At about 1,200 Hz. there is a deep hole while at about something over 2,000 Hz. there is a substantial reduction in amplitude. Beyond that frequency, the response curve illustrated in FIG. 5 is generally similar to the corresponding frequency range for the curve illustrated in FIG. 3. 1

The frequency response curve illustrated in FIG. 6 shows the operation of the same speaker combination with the tweeter only being operated, the woofer not receiving any voice currents and the tweeter now having the phasing baffle plate previously described. It will be noted that the response curve from about 800 Hz. to about 12,000 Hz. has been smoothed out substantially.

FIG. 7 shows the frequency response curve of the speaker combination previously identified wherein the phasing baffle plate is provided and the woofer only operating. the tweeter being inoperative. The frequency responseup to about 800 Hz. is generally similar to the frequency response curve of FIG. '4 wherein both speakers are operating with FIG. 7, the response characteristic falls off quite rapidly. It is evident throughout the characteristic curve illustrated in FIG. 4 that the tweeter response, when operating in conjunction with the woofer, in the combination having the phasing baffle plate, shows that the contribution of the tweeter in this combination begins to be significant from about 600 Hz. and up and that the tweeter response or contribution to the speaker response begins to be the major factor in frequencies beyond about 2,000 Hz.

Adapting The Phasing Baffle Plate To aSpeaker Combination For Obtaining The Benefits of the Invention To adapt a phasing baffle plate to a coaxial speaker combination and obtain the benefits of the invention, it is necessary first to obtain with a conventional speaker combination having large and small'production speakers disposed in fixed relation to each other as illustrated in FIG. 2 of the drawings. Thereafter a phasing baffle plate must be adapted to the speaker. It is understood that the speaker combination minus phasing baffle plate will be tested to show frequency response characteristics over the entire frequency range to be covered by the speaker combination. The test should be both along and off the cone axis. Then a suitable material for the phasing baffle plate, such as particle board, must be selected. Assuming that concentric outer and inner circular edges are to be used, select the plate material having the proper thickness, (for 45 lb. density particle board, a thickness of three-eighths inch will serve as a starter) cut the window through the plate. This may be a circular opening or window whose diameter is large enough to permit mounting the phasing baffle plate upon the tweeter rim as shown in FIGS. l and 2. Then arbitrarily trim plate material so that the outer edge will have a diameter 2 inches greater than that of the window. The entire speaker combination to gether with the experimental phasing baffle plate must be suitably mounted in a large baffle board either in a housing or cabinet or if the baftle board is large enough, the entire construction can be tested in open air. A conventional frequency response test is now run and the response curve is plotted. This response curve should be compared to the response curve of the same speaker combination with similar baffle board but without the phasing bafile plate. Different thicknesses of washers 51 should be tried. Thereafter other baffle plates having increasing diameters in steps of onefourth inch for the outer edge should be substituted in the speaker combination. Beyond a certain point in the various phasing baffle plate sizes and spacings there will usually be an optimum response curve, the curve beginning to deteriorate beyond a certain outer edge diameter of the phasing baffle plate and/or spacer washer thickness. At or close to the best dimension for the phasing baffle plate, some small variations in diameter of both inner and outer edges of phasing baffle plate may be tried to determine the most desirable frequency characteristic curve. Due to the number of variables, it will generally be desirable to vary only one or two parameters at a time while holding others constant. It is possible to obtain satisfactory frequency response characteristics for a speaker combination for a number of parameter values. Some parameters have greater response effects than others. In general, the air space at tweeter rim-43 and space between the plate edge and woofer cone are important. Trying different tweeter locations along the cone axis may also be investigated. Once a desirable phasing baffle plate is obtained, no changes in the speakers should be made to avoid making changes in the plate.

Once the parameters of the phasing baffle plate are determined experimentally, production of phasing baffie plates determined by the experiments can proceed. It will be found that a substantial free annular region between the outer edge of the phasing baffle plate and the nearest portion of the woofer cone will be indicated, such annular space as a rule having at least-a width radially of more than one-half inch between the edge of the phasing baffle plate and adjacent woofer cone. Inasmuch as phasing baffle plates are easy to make and the plate material and their dimensons are easily changed, testing of various phasing baffle plates may be accomplished economically and quickly. Frequency response tests for speakers are well known.

In testing a speaker combination embodying the present invention, variations in the shape of the outer edge of the phasing and baffle plate, such as indentations, will produce different effects on the frequency response characteristic. In making such changes in the phasing baffle plate it may be desirable to test the speaker response not only along the cone axis of the speakers but laterally away from the axis from a region about 4 or more feet along the axis from the speaker.

The modified phasing baffle plate shown in FIG. 1A may be used with equal convenience. Desirable plate shapes, dimensions, and air spaces may be determined by tests. A skilled speaker engineer will be able to determine in which direction to change plate dimensions, plate density, air spacing, etc. to improve speaker response.

What is claimed is:

l. A self-contained loudspeaker combination comprising a cone-diaphragm type woofer and a conediaphragm type tweeter, each cone-diaphragm carrying a voice coil operating in an annular air gap of a magnetic circuit and having a stationary mounting rim for supporting an annular hinge portion at the large cone end, the woofer being for low-frequency operation and having its large end cone diameter of at least about 8 inches, the tweeter being for higher frequency operation and having its large end cone diameter substantially less, means for supporting the tweeter in coaxial nested relation within the volume defined by the woofer cone, and a pushing self-supporting flat baffle plate of material, whose thickness is between about one-tenth and about three-fourths inch, said plate having a curved outer edge and a window therethrough to provide an inner edge, means for supporting said plate in symmetrical relation to the coincident diaphragm axes and perpendicular thereto, said plate being small enough to be supported within the large diaphragm end portion with its outer edge stopping short of the large cone rim portion by between about one-half inch to the order of about 1 inch, depending upon the woofer speaker size, the plate inner edge being dimensioned and located with reference to the tweeter diaphragm mounting rim so that an acoustically suitable peripherally continuous extended air gap is provided, said plate material having suitable acoustic properties and the plate and spacing dimensions, shape and axial location being so selected that said speaker combination, with phasing baffle plate, when mounted for normal speaker operation with a conventional baffleboard will have a desired uniform frequency response over the desired audio-frequency range and be substantially free of resonance peaks or holes having abnormal amplitudes in such range, even though the same speaker combination minus the phasing baffle plate, when tested, will have at least one resonance or anti-resonance peak of abnormally large amplitude within the audio-frequency range, said phasing baffle plate smoothing out the frequency response characteristic and improving speaker performance, the addition and mounting of said phasing baffle plate permitting engineering efforts to be directed thereon to improve frequency response whereby tooling changes for manufacturing the speaker combination proper may be substantially eliminated.

2. The construction according to claim 1 wherein the woofer cone diameter ranges from about 8 inches to about 15 inches, said phasing baffle plate having its outer edge circular with the center being on the cone axis.

3. The construction according to claim 1 wherein the plate is of particle board.

4. The construction according to claim 1 wherein the plate is of styrofoam having a smooth, moulded outer surface.

Claims

1. A self-contained loudspeaker combination comprising a conediaphragm type woofer and a cone-diaphragm type tweeter, each cone-diaphragm carrying a voice coil operating in an annular air gap of a magnetic circuit and having a stationary mounting rim for supporting an annular hinge portion at the large cone end, the woofer being for low-frequency operation and having its large end cone diameter of at least about 8 inches, the tweeter being for higher frequency operation and having its large end cone diameter substantially less, means for supporting the tweeter in coaxial nested relation within the volume defined by the woofer cone, and a pushing self-supporting flat baffle plate of material, whose thickness is between about one-tenth and about three-fourths inch, said plate having a curved outer edge and a window therethrough to provide an inner edge, means for supporting said plate in symmetrical relation to the coincident diaphragm axes and perpendicular thereto, said plate being small enough to be supported within the large diaphragm end portion with its outer edge stopping short of the large cone rim portion by between about one-half inch to the order of about 1 inch, depending upon the woofer speaker size, the plate inner edge being dimensioned and located with reference to the tweeter diaphragm mounting rim so that an acoustically suitable peripherally continuous extended air gap is provided, said plate material having suitable acoustic properties and the plate and spacing dimensions, shape and axial location being so selected that said speaker combination, with phasing baffle plate, when mounted for normal speaker operation with a conventional baffleboard will have a desired uniform frequency response over the desired audio-frequency range and be substantially free of resonance peaks or holes having abnormal amplitudes in such range, even though the same speaker combination minus the phasing baffle plate, when tested, will have at least one resonance or anti-resonance peak of abnormally large amplitude within the audio-frequency range, said phasing baffle plate smoothing out the frequency response characteristic and improving speaker performance, the addition and mounting of said phasing baffle plate permitting engineering efforts to be directed thereon to improve frequency response whereby tooling changes for manufacturing the speaker combination proper may be substantially eliminated.