US3143183A - Direct radiator high fidelity loudspeaker - Google Patents

Description

1954 s. HERBSTMAN I 3,143,183

- DIRECT RADIATOR HIGH FIDELITY LOUDSPEAKER Filed Feb. 25, 1963 I 2 sheets-sneez'l SIDNEY HERBSTMAN INVENTOR.

ATTORNEY 1954 s. HERBSTMAN 3,143,183

DIRECT RADIATOR HIGH FIDELITY LOUDSPEAKER Filed Feb. 25, 1965 2 Sheets-Sheet 2 FREQUENCY IN CYCLES PER SECOND FIG 2 SIDNEY HERBSTMAN INVENTOR.

ATTORNEY United States Patent 3,143,183 DIRECT RADIATOR HIGH FIDELITY LOUDSPEAKER Sidney Herbstman, 412 Tenafly Road, Englewood, NJ.

Filed Feb. 25, 1963, Ser. No. 260,624

5 Claims. (Cl. 181-31) The present invention relates to loudspeakers, and more particularly to loudspeakers used in high fidelity reproduction of sound. The term high fidelity generally relates to the tone spectrum audibleto' normal human ears.

In most loudspeaker high fidelity arrangements a low frequency device or woofer and a high frequency device or tweeter are used together rather than a single device. This is done to reduced problems associated with the design and production of a single wide range electroacoustic reproducer (loudspeaker), e.g. efficiency disparity between the upper and lower frequencies, power handling capabilities, the various types of distortion (frequency,

.amplitude, intermodulation, and transient) and cone breakup. Advantages of using separate low and high frequency devices include ease of output level balance, better control of high and low frequency efiiciency design factors, minimization of intermodulation and frequency distortion, and improved response to transients.

The present invention is a variant type of extended range loudspeaker Woofer, and is directed to the minimization of four major types of distortion, amplitude, frequency, intermodulation and transient, which are typically evident in current extended woofer designs, particularly during the reproduction of low to very low frequencies (20 c.p.s. to 100 c.p.s.).

Compared to the rest of the links in the high fidelity cha.inpickup, disc, amplifier, tuner, the distortion percentages of woofers are extremely high. In the region between 20 and 40 c.p.s., most woofers exhibit harmonic distortion levels ranging from 5 to 100%; a 30 c.p.s. input to a woofer distorting 100% at this frequency would result in an ascoustic output of almost pure 90 c.p.s. High distortion in the low frequency ranges can be considered characteristic of modern commercial woofers (see Distortion in Loudspeakers, E. Villchur, Electronics World, p 28, June 1962).

Distortions in woofers are due to two major causes: non-linearity of their cone suspensions and non-uniformity of the magnetic field over the path of voice coil excursion.

Non-linearity shown by the woofers mechanical suspensions is a characteristic of elastic material; as the suspensions are stretched by the movement of the attached voice coil the elastic stiffness increases until the disphragm cannot move further, although the input signal continues in its cycle. Consequently this increased mechanical impedance in the face of an applied force must result in a decreased mechanical response. The acoustic waveform is then distorted, usually at each side of the diaphragm excursion, to result in a symmetrical or odd-harmonic wavefront. Commonly, the most prominent harmonic component in low frequency loudspeaker distortion is the third; hence the tripling of 30 to 90 c.p.s. is a manifestation of non-linearity in the particular woofers suspen- S1011.

Non-uniformity of the magnetic field over the voice coil path also operates to distort signal peaks symmetrically. At extreme positions of the voice coil impelled diaphragm, the voice coil is usually partially present in the magnetic field. The force on the voice coil is consequently weakened, even though the input signal has not altered, and disphragm travel is reduced at the extremities of its travel. Third harmonic distortion will here tend to predominate.

The effect of transient distortion is more complex, and will be discussed meaningfully with regard to the present invention later in this application. The technological and theoretical aspects of distortion and design considerations relating to loudspeakers are covered fully in the listing of over 200 authoritative references in Radiotron Designers Handbook, edited by F. Langford-Smith, 4th Edition, 876-879, 1953.

It is an object of the present invention to provide a speaker device of the direct radiator high fidelity type providing a high fidelity response in the range of between about 20 c.p.s. and about 300 c.p.s. with excellent low frequency acoustic pressure response aswell as very little distortion as compared to the desired sound.

With the foregoing and other objects in view, the invention resides in the novel arrangement and combination of parts and in the details of construction hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.

The accompanying drawings, illustrative of one embodiment of the invention, together with the description of the construction and method of operation and utilization thereof will serve to clarify further objects of my invention. Other advantages will become aparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of a speaker device constructed in accordance with my inventive concept having certain portions cut away to illustrate constructional features; and,

FIGURE 2 graphically depicts the sound pressure level curve and the transient distortion curve for the loud speaker of FIGURE 1.

The contemplated loudspeaker 8 generally comprises a large square-shaped flat piece of thermoplastic, e.g., polystyrene radiator diaphragm 9 having a central aperture 10 to which is coupled a standard voice coil (not shown). Surrounding the central voice coil aperture 10 is a circular inertial ring 11 made of wires, aflat steel strip or of other high density and not readily flexible material. This inertial ring, its position and its effect on the system will be discussed later in the present specification.

The outer edge of flat diaphragm 9 is edge damped in that there is an inner square portion 12 forming part of diaphragm 9 and a corresponding outer portion 13 commagnet and voice coil, result in a woofer mechanism capable of fiat pressure level response from subsonic to upper midrange frequencies of about 20 c.p.s. to 300 c.p.s. Distortion due to excessive voice coil excursion at low frequencies is minimized by utilizing a very largesize square diaphragm. Efiiciency at low frequencies is improved by virtue of the very low Youngs Modulus of Elasticity-to-density ratio characteristic of the polystyrene. The diaphragm acts as a pure piston to frequencies up to about 300 c.p.s.

The frequency range is extended above about 300 c.p.s. by adding the inertial ring to the diaphragm. The ring, which consists of a circle of wire or flat strip, or a circle of any other nonflexible material, considerably denser than polystyrene is bonded to the polystyrene at a location well to woofer operation.

.more bearable.

,the aural effect.

3 varying from the center to the outside edge; the position of this 'clamp'thereby affecting the frequency response of the diaphragm. In effect, above about 300 c.p.s. suspension of the diaphragm appears at the wire circle and propagation of sound above about 300 c.p.s. occurs within the wire circle. The mass of the moving system decreases at frequencies above'about 800 c.p.s.; hence efiiciency of midrange frequencies (to about 3000 c.p.s.) improves. a

Motion of the diaphragm at its edge, together with the determination of free-air resonance, is due to flexible suspension tape 17-. The self-damping characteristic of polystyrene is utilized to reduce-distortion due to internal material resonances. Transverse or unwanted waves of prominent amplitude are further damped by setting the outer polystyrene diaphragm portion 14 between strips of polyurethane foam or any suitable damping material as hold clamping piece 15. At frequencies below about 1000 c.p.s. the effect of reduced transient distortion due to this method of edge damping can be observed through suitable transient distortion test equipment.

In minimizing non-linear distortion, use is made of the principle of radiation resistance. sistance of a loudspeaker diaphragm determines its acoustic ,power transmitting capability. The specific acoustic jimpedanceof air is 42 ohms; therefore the optimum diaphragm size should be such that a radiation resistance of 42 ohms/sq. cm. is realized. In the present invention this is achieved in the following manner:

.The ratio of diaphragm diameter to wavelength is fixed by the diaphragm size. Below a ratio of 1:1, the radiation resistance falls from 42 ohms/ sq. cm. to lower values. As the frequency is lowered, the wavelength increases. To maintain a ratio of 1:1, as the frequency is lowered, the diaphragm size must be enlarged. In the present invention the diaphragm area is perhaps five times or more that of the average 15-inch i.e. Side (1rr 5 commercial cone woofer. The most efficient coupling to air, where the radiation resistance matches that of the acoustic impedance, is thus more easily realized. For

a given power output, diaphragm excursion is lessened .as the diaphragm area is increased. For the same diaphragm excursion, a loudspeaker woofer will handle more power as its areais increased. Therefore, designing minimal diaphragm excursion also minimizes distortion effects due to non-linearity of diaphragm suspension, and reduced voice coil travel lessens distortion due to nonuniformity of the magnetic field over the voice coil path. The stiffness of polystyrene, as Well as its lightness, lends These two physical characteristics together with large diaphragm size permit design of the edge compliance to yield a very low system free air resonance. By making the rear wave stiffnesscontrolled (utilizing basic reflex enclosure design principles) a very even pressure level response is obtained to below c.p.s. The minimal voice coil excursion at this very low frequency results in low harmonic distortion. Thus, any subsonic thump or pulse transmitted from audio source to loudspeaker will be faithfully reproduced at its subsonic frequency, but the minimization of its harmonic content will make aural listening It should be realized, from the earlier discussion of high commercial loudspeaker distortion values, that in the average woofer the same subsonic thump or pulse would result in the generation of very objectionable harmonics that would modulate or color But the present invention puts high fidelity woofer reproduction on a more realistic basis.

While sound pressure level curves are used to measure loudspeaker performance, transient tests are now also increasingly used. Transient distortion is particularly reduced in the area of very low frequencies in the present invention. a

In the reproduction system for speech and music, it is important that the reproduced soundstart and build The radiation re- 7 A up as fast as the original signal and when the signal ends the woofer diaphragm should also stop. Thus, when the hammer hits the string of a piano, the sound starts suddenly as the energy of the impact causes the string to vibrate. The loudspeaker must be able to start sud denly also, and when the piano damper comes down on the string at the end of the note, the loudspeaker must stop suddenly.

One test which is used to observe this performance is to apply a sine wave train suddenly to the voice coil of the loudspeaker and to observe the sound pressure with a microphone in front of the speaker on the axis. After a short time the applied sine wave train is suddenly reduced to zero. The build up and decay of the sound pressure is then observed on an oscilloscope. The trains of suddenly applied sine waves are termed tone bursts.

In general, due to poor electromechanical damping of the diaphragm, particularly at the very low frequencies, low frequency bursts will be detected by the microphone as distortion after the burst is reduced to zero. This effect is sometimes called hangover, and introduces a muddiness into the sound, because the hung-over sound continues while the next sound is initiated. The hanggver, or sound decay, is a true distortion of the original urst.

Transient distortion test equipment of the Acoustics Laboratory, Delco Radio Division, General Motors Corp., Kokomo, Indiana, has been used-to determine the transient distortion characteristics of the herein described woofer loudspeaker. The equipment operates as follows:

When the transient distortion of a loudspeaker is measured by means of an automatic frequency response plotter, the sound pressure curve is first drawn as a reference. The transient distortion analyzer is then switched into the tone burst mode of operation and, to obtain the best representation of the transient distortion of the loudspeaker under test, a curve is drawn to cover the frequency spectrum from 20 c.p.s. to 20,000 c.p.s. The equipment is calibrated and adjusted to remove all data except the decay characteristic from the detected tone burst. It is generally accepted in most loudspeakers testing laboratories that a 'sprea of 30 db between the decay and the sound pressure curves is the minimum standard of excellent transient response for the tested system. 7

Unlike cone-type transducers, where frequency discrimination due to on-axis beaming occurs, the space distributionpattern of the speaker is essentially that of a Wide figure-8. Consequently, for full fidelity reproduction, the speaker will offer a pleasing, non-fatiguing, sound-complex of unusual transparency.

Unlike the standard loudspeaker cone, where beaming of high frequencies is due to the small apex of the diaphragm, the fiat polystyrene direct radiator can be considered to consist of infinite apexes Within the ring clamp, each beaming the frequency separately. Therefore, the sound Will be propagated on a planar wavefront, rather than a conicalbeam especially discriminatory to higher frequencies.

For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative example is given:

The output of the loudspeaker was recorded by a Graphic Level Recorder manufactured by the General Radio Company of West Concord, Massachusetts, on a "chart CTP-501'supplied by said company. This chart ference of sound pressure level over transient distortion as follows:

. Furthermore, the sound pressure level is fairly constant between 20 db and 3000 db.

The extended high frequency response of the woofer is to a large measure determined by the placement of the inertial ring. The closer to the center the ring is placed, the greater is the efliciency towards the upper midrange.

The closer to the edge that the ring is placed, the greater is the efiiciency towards the lower midrange. The ring supplies the mechanical crossover from the very low frequencies to the low and midrange frequencies. The physical location of the inertial ring to a large extent depends on the density, size and thickness of the thermoplastic or polystyrene material. In order to properly place the inertial ring, for a particular speaker, the frequency pressure response curve is first taken without the ring. This will show where the pressure response starts to drop off and will enable the placing of the ring so as to get a better response. Generally, the position of the ring, depending of course on the density, size, and thickness of the flat radiator diaphragm, will be between about a position relatively close to the central aperture to a position somewhat past midway between the central aperture and the flexible suspension tape. A rough rule of the thumb approximation would be about one-third the distance out from the theoretical center. Although a square shaped radiator diaphragm has been described herein, other perfectly symmetrical shapes are possible between the square and a perfect circle. But in practice, the perfectly square shape is preferable both from the high fidelity results obtained and the ease of manufacture. It would appear that in the present invention, the square acts more like a circle than a true circle. In size the square can vary from around two square feet to well over five square feet i.e., well over two feet square and in thickness the polystyrene material may be under one inch to over three inches thick.

The present invention is not to be confused with certain systems described by Dr. Harry F. Olson in his excellent book entitled Acoustical Engineering published by D. Van Nostrand Co., Inc., 1957 Edition, on page 137 and 139. These devices of the prior art have a high frequency tweeter unit at the center and a separate low frequency woofer unit surrounding the high frequency unit.

It may be here observed that the inertial ring utilized in this invention may have the same effects as the corrugated rings evident in many current cone-type loudspeaker extended range designs. This effect is the reduction in intermodulation distortion by utilizing the corrugations as mechanical crossovers. However, since frequency (Doppler) modulation distortion cannot be reduced, it is in fact aggravated by using smaller diaphragms as direct radiators. This type of distortion will be prominent in current loudspeaker extended range designs.

In the present invention, the above-mentioned distortions, intermodulation and frequency modulation, are drastically reduced by the expedient of using a large direct radiator area. How this is done can be explained as follows:

Intermodulation Distortion is the result of non-linearity in either electrical or mechanical systems. By designing the extended range woofer so that non-linearity of the flat diaphragm direct radiator suspension system is reduced radically, and the non-uniformity of the flux gap is rendered completely uniform, intermodulation distortion 6 products are also radically reduced. This is one of the keys of the novelty of the device, the very low mechanical displacement of the diaphragm at very low frequencies due to the large diaphragm size, high force to low voice coil displacement factor.

Frequency modulation distortion is similarly reduced due to the high force to low voice coil displacement factor. This type of distortion is directly proportional to low frequency diapragm excursions, which modulate any high frequencies present. It is clear that a minimum displacement at low frequencies will yield a minimum frequency modulation distortion.

Early 32-inch woofers, manufactured by the Cinaudigraph Company for the 1939 Worlds Fair had no mechanical crossovers, corrugations or similar devices fitted on their cone diaphragms. The only 36-inch woofer on the market today, manufactured by the Mitsubishi Company of Japan does not have mechanical crossovers,but relies on a separate midrange loudspeaker to extend the low to midfrequencies. In the designs of current British and American polystyrene woofers, as well as in the designs of standard paper-cone woofers, mechanical crossover means are non-existent. As a panacea for the effect of intermodulation and frequency modulation distortion,

separate midrange squawkers and tweeter combinations are found in the industry, and electrical crossovers are utilized instead.

Because of the great design and manufacturing cost factors associated with manufacture of large size full range loudspeakers, the major state of the art has involved standard size (8-15 inches) conical full range units to keep cost to a competitive value. In the present invention, ease of manufacturing and low material costs will render the complete device available at costs far below the cost of a quality competitive woofer system, and provides an extended frequency response with very low distortion as well.

It is to be observed therefore that the present invention provides for an improved loudspeaker or loudspeaker woofer section so as to produce a fairly even sound pressure level well above transient decay distortion at frequencies between about 20 c.p.s. and 300 c.p.s. and generally includes, an outer frame, a large area symmetrically shaped, e.g., square-shaped thermoplastic or polystyrene radiator diaphragm in said frame, said diaphragm having a central aperture therein so that a voice coil can be coupled thereto; inner and outer edge portions on said diaphragm, said outer edge portion being rigidly fixed to said outer frame, said inner and outer portions being coupled by flexible, e.g., tape suspension means; and, a ring of steel wire, flat steel or other not readily flexible material between said central aperture and said inner edge portion.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

I. A woofer loudspeaker section having a fairly even sound pressure level well above transient distortion at frequencies between about 20 c.p.s. and 3000 c.p.s., comprising in combination,

an outer frame defining a plane;

a flat, large area square-shaped polystyrene radiator diaphragm in said frame and in said defined plane, said diaphragm having a central aperture therein so that a voice coil can be coupled thereto;

inner and outer edge portions on said diaphragm, said outer edge portion being rigidly fixed to said outer frame, said inner and outer portions being coupled by flexible suspension means; and,

a steel wire ring of high density material between said central aperture and said inner edge portion.

'2. A Woofer loudspeaker section having a fairlyeven sound pressure level well above transient distortion at frequencies between about 20 e.p.s. and 300 c.p;s., com prising incombination,

an outer frame defining a hase I a flat, large area square-shaped polystyrene radiator diaphragm in said frame and in said defined plane, said diaphragm having a central aperture therein so 7 that a voice coil can be coupled thereto; 7

inner and outer edge portions on said diaphragm, said outer edge portion being rigidlyfixed to saidouter prising in combination,

frame, said inner and outer portions being coupledby flexible suspension means; and,

a flat steel ring of high density material between said central aperture and said inner edge portion. I v

3. A woofer loudspeaker section having a fairly even sound pressure level well above transient distortion at frequencies between about c'.p.s. and 3000 c.p.s., comprising in combination,

an outer frame defining a plane;

a flat, large area square shaped polystyrene radiator diaphragm in said frame and in said defined plane, said diaphragm having a central aperture therein so that a voice coil can be coupled thereto; 7

inner and outer edge portions on's'aid diaphragm, said outer edge portionbeing rigidly fixedto said outer frame, said inner and outer portions being coupled by flexible tape suspension'means; and,

a steel wire ring of high density material between said central aperture and said inner edge portion. 4. A Woofer loudspeaker section having a fairly even sound pressure level well above transient distortionat frequencies between about 20 -c.p,s. and 3000 c.p.s., coman outer frame defining aplane; a,.flat, large area square-shaped polystyrene radiator diaphragm in said frame and in saidndefine'd plane, said diaphragm having a central aperture therein so that a voice coil can be coupled thereto; inner and outer edge portions on said diaphragm', said outer edge portion being "rigidly fixed to saidont'er frame, said inner and outer ,porti'onsfbein'g "coupled by flexible .tape suspension means; and, v, a flat steel ring of highdenSity material between said central aperture and said inner edge'portion. I 5. A device as claimed in elaim 4, said large area being between about two square fe'et'to over two feet squared.

References Cited in the file of this patent UNITED STATES PATENTS 2,003,908 Smith et a1. frune 4, 1935 2,685,935 Lenz Au 10, 2,834,424 Badmaieif May 13', 1958 2,905,260 Williams Sept. 22, 1 959 3,093,207 Bozak June 11, 1'96'3

Claims (1)

1. A WOOFER LOUDSPEAKER SECTION HAVING A FAIRLY EVEN SOUND PRESSURE LEVEL WELL ABOVE TRANSIENT DISTORTION AT FREQUENCIES BETWEEN ABOUT 20 C.P.S. AND 3000 C.P.S., COMPRISING IN COMBINATION, AN OUTER FRAME DEFINING A PLANE; A FLAT, LARGE AREA SQUARE-SHAPED POLYSTYRENE RADIATOR DIAPHRAGM HAVING A CENTRAL APERTURE THEREIN SO THAT A VOICE COIL CAN BE COUPLED THERETO; INNER AND OUTER EDGE PORTIONS ON SAID DIAPHRAGM, SAID OUTER EDGE PORTION BEING RIGIDLY FIXED TO SAID OUTER FRAME, SAID INNER AND OUTER PORTIONS BEING COUPLED BY FLEXIBLE SUSPENSION MEANS; AND, A STEEL WIRE RING OF HIGH DENSITY MATERIAL BETWEEN SAID CENTRAL APERTURE AND SAID INNER EDGE PORTION.

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