Speaker system

Jan. 2, 1962

3,015,366

G. M. BISHOP

SPEAKER SYSTEM

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Jan. 2, 1962

3,015,366

G. M. BISHOP

SPEAKER SYSTEM

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3,015,366

Patented Jan. 2, 1962

United States Patent Office

3,015,366 SPEAKER SYSTEM

George M. Bishop, Box 347, Mebane, N.C.' Filed Dec. 14,1956, Ser. No. 628,401

Claims. (Cl. 181—31)

This invention relates to loud speaker systems. More particularly, the invention relates to speaker systems for providing true fidelity sound reproduction throughout the audible range.

It is an object of the present invention to provide an improved loud speaker system.

Another object of the invention is to provide a loud speaker system capable of reproducing sounds with true fidelity throughout the audible range.

A further object of the invention is to provide a speaker system embodying a plurality of sound propagating mechanisms which coact to achieve balanced relative intensity of sound throughout the audible range.

Still another object of the invention is to provide a speaker system embodying improved mechanical crossover means.

A still further object of the invention is to provide a speaker system having sound propagating elements capable of vibrating at the same frequency as that of a vibratory driver but at substantially increased amplitude.

An important object of the invention is to provide an efficient speaker system capable of precisely reproducing the characteristic quality of all sounds.

Other objects, features and advantages will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIGURE 1 is a longitudinal, generally schematic, sectional view of a typical speaker system according to the present invention;

FIGURE 2 is an enlarged fragmentary sectional view of a modified construction of one of the sound propagating mechanisms shown in FIGURE 1; and

FIGURE 3 is a generalized graphic illustration of certain operational characteristics of the speaker system of this invention.

The speaker system illustrated in FIGURE 1 is generally designated by the reference numeral 19 and includes a vibratory driver 11 and a plurality of sound propagating mechanisms 12α, 126, 12c and 12d arranged for being driven or vibrated by the driver 11. While four sound propagating mechanisms are shown, the invention is not intended to be thus limited since any number can be used depending upon the desired results.

The vibratory driver 11 may be of any suitable construction for producing linear vibrations corresponding to acoustical vibrations from a sound source. In the particular embodiment illustrated the vibratory driver is of the electromagnetic type capable of producing linear vibrations corresponding to current fluctuations received from an electronic audio reproduction system (not shown), such as an audio amplifier connected to a phonograph, tape recorder, radio tuner, or the like. Such electromagnetic driver may include a permanent magnet 14, formed of Alnico V or the like, and a shiftable electric voice coil 15 disposed in the magnetic field of the permanent magnet. Electric current corresponding to acoustical vibrations in a sound source or sound sources is fed through the coil 15 by means of a pair of leads 16, 16 connected to a conventional electronic audio amplifier (not shown). In the conventional manner, the electrically induced field of the voice coil 15 interacts with the field of permanent magnet 14 to oscillate the voice coil in a manner corresponding to the acoustical vibrations of the sound source, that is to say, the frequency of oscillation of the voice coil is the same as that of the sound source and the amplitude of oscillation varies directly with that of the sound source.

For transmitting the vibrations or oscillations of the voice coil 15 to the sound propagating mechanism 12a, 5 12Z>, 12c and 12d, a drive element is provided in the form of an elongated rigid rod 17. The rod 17 is fixedly connected to the voice coil by any suitable means such as a rigid bridge 18 fixedly connected to both the voice coil and the rod and retaining the rod in axially centered 10 alignment with respect to the voice coil. For convenience, the rod may be mounted vertically and will be so considered for purposes of this description, although it will be understood that the rod may be in any position as long as it is axially aligned with the voice coil. The rod 15 17 may be formed of any suitable rigid material such as wood, rigid plastic, non-magnetic metal or the like, such that the entire rod 17 vibrates at the same amplitude and frequency as the voice coil 15 and in phase with the voice coil. It is desirable that the rod 17 be as light as 20 possible, commensurate with the necessary strength requirements, in order that its inertia be as low as possible.

According to the present invention, the sound propagating mechanisms 12α, 12b, 12c and 12d include re25 spective sounding elements or disks 19α, 19b, 19c and 19d shiftably mounted on the drive rod 17 by resilient balancing means 20α, 206, 20c and 20d. The sounding disks may be identical in construction except for size and herein comprise respective pairs of thin shallow conical 30 members 21α, 21b, 21c and 21d, having their circular peripheral portions fixedly secured and their apexes oppositely disposed and axially aligned. The apexes are fixedly secured to respective axially disposed sleeves 22α, 22b, 22c and 22d each having opposite annular end 35 flanges 24, 24 and a central bore 25. The sleeves of the various disks are slidably disposed on the rod 17 in spaced positions as shown. The bores 25 are slightly larger in diameter than the rod in order to reduce friction as much as possible while still maintaining proper axial stability 40 of the disks. Ideally, of course, the sleeves should be free floating on the rod so that friction is zero.

The cones 21α, etc. and the sleeves 22α, etc. are formed of any suitable materials capable of maintaining the shape of the disks during all conditions encountered in opera45 tion of the speaker system. For example, the cones are preferably formed of paper or chemical pulp, although thin plastic or light metal might be used as long as the weight is not excessive. The sleeves may be formed of heavier paper, plastic or light metal or any suitable ma50 terial of light weight but sufficient rigidity to prevent collapse.

FIGURE 2 shows, in detail, an alternative construction of the sounding disks. In this embodiment the cones 21α are formed of paper, the sleeve 22α is formed of two 55 wooden spool portions 26, 26 held in axially aligned position by means of a paper cylinder 27 which is fixedly secured about the spools. In order to shiftably support the sleeve 22α on the rod 17 with minimum friction, a pair of felt bushings 28, 28 are disposed about the rod 60 at each end of the bore 25. The bushings may be connected to either the rod or the spools 26.

It should be understood that the disks can be of any construction or configuration as long as they are of proper weight and capable of vibrating sufficient air to accu65 rately reproduce sound with adequate volume.

The resilient balancing means 20α, 20δ, 20c and 2(W each comprises a pair of helical compression springs 29α, 29b, 29c and 29d resiliently urged against the opposite ends of the respective sleeves 22α, 22b, 22c and 22d. The 70 outer ends of the springs are bottomed on collars 30 which are secured in axially spaced positions along the drive rod 17. The collars 30 may be formed of any suit3,015,366 ³ able material such as rubber, metal, plastic or the like, and are arranged to maintain fixed positions on the rod during operation of the speaker system. However, the relative positions of the collars 30 may be adjusted manually in order to allow variation of the initial compressive forces exerted by the various springs. It will be noted that the intermediate collars 30 support springs of adjacent disks.

The springs 29a, 29b, 29c and 29d are of sufficient strength to support the respective disks 19a, 19b, 19c and 19d so that they are generally centrally located between the collars 30 when the speaker system is not in use. Of course, heavier disks are supported by heavier springs.

While the resilient balancing means 2θα, 2®b, 29c and 29d are herein shown to be helical compression springs, it is contemplated that any satisfactory means for providing, the necessary resilience could be used.

Each of the sounding disks 19 a, 19b, 19c and 19d is capable of independent axial oscillation with respect to the rod 17. Each of the sound propagating mechanisms has a particular resonant frequency of vibration depending upon the mass of its disk and the “resilience” (elastic restoring factor) of its springs. According to this invention, the disk weights and spring resiliences are such that the resonant frequencies of the various sound propagating mechanisms are spaced over the range of audible frequencies, in a manner to be described in more detail.

The audible range is generally considered as extending between 20 c.p.s. (cycles per second) and 20,000 c.p.s.

In operation of the speaker system, vibrations of the voice coil IS are transmitted directly to the drive rod 17 so that the drive rod vibrates longitudinally at the same frequency and amplitude as the voice coil and exactly in phase with the voice coil. The sound propagating mechanism 12a is. constructed so that its resonant frequency is at or near the low end of the audible frequency range, for example 30 c.p.s. The area of the disk 19a must be sufficient to vibrate a sufficient quantity of air to produce audible sound of the frequency being transmitted within a wide volume range. For example, the disk 19α may be approximately 15 inches in diameter.

When the rod 17 is vibrated at 30 c.p.s., the upper and lower springs 29a will be alternately compressed, arid the resultant, alternately acting force will be imposed against the opposite ends of the sleeve 22. Since the resonant frequency .of the sound propagating mechanism 12a is 30 c.p.s., according to the laws of physics the disk 19α will also vibrate at this frequency but exactly 180° out of phase with the vibration of the rod 17. In other words, as the rod reaches its top node in each vibratory cycle, the disk will reach its bottom node, and vice-versa. This is because of the relationship between the inertia of the disk and the resilience of the springs, which are exactly balanced at resonant frequency, so that the inertia of the disk on each vibration is completely overcome by the spring being compressed at the exact instant that the rod reaches its amplitude peak in the opposite direction.

At resonant frequency the maximum amount of force is transmitted from the rod through the collars 38 and the springs 29a to oppose the inertia force of the moving disk ⁷19α, and the disk vibrates at maximum amplitude for any given amplitude of vibration of the rod 17. However, the amplitude of vibration of the disk 19α will be many times greater than that of the rod 17. For example, the maximum amplitude of vibration of the rod 17 may be only a few thousandths of an inch while the maximum amplitude of vibration of the disk 19 a may be as much as Vs of an inch or even greater.

While the rod is vibrating at or near 30 c.p.s., the smaller disks 19b, 19c and 19d, being somewhat lighter, are carried along with the rod since the inertia forces on these disks are not sufficient to overcome the spring forces to any reasonable extent. Thus, the amplitudes of the smaller disks are very much smaller and the sound produced by these smaller disks is negligible.

If the frequency of vibration of the rod 17 is reduced slightly below 30 c.p.s., the resonant frequency, the vibration of the disk 19α will be slightly less than 180° outof-phase, and if the rod is vibrated at slightly greater than 30 c.p.s. the disk will vibrate at slightly more than 180° out-of-phase (approaching in-phase relationship with the next cycle). The amplitude of vibration of the disk will be slightly reduced in either case. If the frequency of vibration of rod 17 is reduced drastically below 30 c.p.s. to, say 5 c.p.s., the vibratory phase of the disk will approach that of the rod and for all practical purposes the two will vibrate together, or in phase. As the frequency of vibration of the rod 17 is increased drastically, for example to 90 c.p.s., the inertia of the disk 19α will no longer allow it to follow the vibrations of the rod and for all practical purposes the disk will stand still, or will “idle.”

The vibrating sound propagating mechanism 12α is subjected to a relatively large dissipative resistance, referred to as the “damping factor,” due almost entirely to air resistance of the large area disk. Of course, the purpose of the device is to overcome air resistance and to create air vibration or sound waves. The large air resistance or damping factor has an important influence on the usable range of vibration of the disk 19α for purposes of sound reproduction. If. the damping factor were small, the response of the vibrating system at resonant frequency would be great but the “tuning” would be sharp; that is, the vibratory response would drop rapidly as the frequency of vibration of the rod is altered from the resonant frequency. However, because of the large damping factor due to air resistance, the resonant response or amplitude of vibration is smaller but the peak is broader and the tuning is more diffuse. Thus, the usable or “resonant range extending both ways from the resonant frequency can .be varied by varying the area of the disk while maintaining substantially the same weight and spring resilience. Of course, increased range is obtained at the expense of decreased amplitude, and these factors and others must be balanced by trial and error to obtain the optimum area, weight and resilience.

The speaker system is arranged so that as the frequency of vibration of the voice coil 15 is increased, the next largest disk 19b will begin to vibrate in the same manner as the larger disk 19α before the largest disk ceases vibrating in its usable resonant range. The resonant frequency of the sound propagating mechanism 12Z> may be 75 c.p.s., for example. At 75 c.p.s. the amplitude of vibration of the larger disk 19α will be much reduced or the vibrations may have ceased for all practical purposes so that the disk 19α is “idling,” but the amplitude of vibration of the disk T9& will be at a maximum and, of course, will be many times greater than the amplitude of vibration of the rod 17. As the frequency of vibration of the voice coil 15 increases even further, the disk 19b will eventually stand substantially still or “idle” with respect to the rod, but before this happens the next smaller sound propagating mechanism 12c will begin vibrating in its resonant range. This process continues as the frequency of vibration of the voice coil increases and successively smaller disks vibrate at or near their resonant frequencies. In other words, the resonant ranges of the disks overlap so that the entire audible frequency band is covered.

It will be recognized that the resilient balancing means provide extremely efficient mechanical cross-over mechanisms. Among their other functions, they insure that the disks will absorb power and propagate sound only in their most efficient ranges as determined by their areas and weights when considered with the resilience of their associated springs. In other words, in the higher ranges the springs allow the larger disks to stand still and the function of sound propagation is automatically crossed over to the smaller disks.

3,015,366

The resonant range of each mechanism cannot be accurately determined or defined except by trial and error, inasmuch as many variables are present, including the taste Of the individual listener. However, the resonant range might be loosely defined as that within which the amplitude of vibration is close enough to the amplitude at the resonant frequency that the loudness and sound quality remain generally the same as at resonant frequency. The resonant ranges of the successive sound propagating mechanisms are arranged to be close enough that the relative intensities of sounds of different frequency are balanced. In other words, the ranges are close enough that the relative intensity of the reproduced sound is for all practical purposes the same as that of the original sound.

A rough illustration of the overlapping of the resonant ranges is presented in FIGURE 3 which is a plot of amplitude of vibration of the disks with respect to frequency of vibration for a particular output pattern of the voice coil. The first curve on the left illustrates the action of the sound propagating mechanism 12α, the next illustrates 20 126, etc. The lower curve illustrates the action of the rod 17, showing that the amplitude of vibration of the rod is very much smaller than the amplitudes of vibration of the disks when vibrating in their respective resonant ranges. Of course, the amplitude of the rod drops off as 25 the frequency increases.

The curves illustrate that maximum amplitude of vibration of the disks is reached at resonant frequency and that the amplitude drops off with increase or decrease of frequency. The curves also illustrate that the range covered 30 by the higher frequency disks is much greater than that covered by low frequency disks, a well known phenomenon in the field of sound reproduction. At substantial distances from the resonant frequencies of the respective disks the amplitude drops off, to that of the rod 17 on 35 the low side where the disks and the rod vibrate together, and to the rod amplitude or less on the high side where the disks stand substantially still with respect to the rod. The curves clearly illustrate the overlap of the resonant ranges of the various disks showing that much of the 40 time the sound is obtained from two or more disks. It is emphasized that the curves of FIGURE 3 do not purport to be accurate as to ranges or values, but are merely intended to illustrate the trend and the overlap of ranges.

Of course, the loudness level can be varied to suit by varying the power output of the audio amplifier and consequently the amplitude of vibration of the voice coil for the various frequencies being transmitted.

It will be understood that within reasonable limits the larger the number of sound propagating mechanisms, the truer the fidelity of reproduction will be.

Within narrow limits the resonant frequencies of the various disks may be varied by varying the relative positions of the spring seat collars 30. It will be understood that if the collars are moved closer together, the springs will exert more initial force against the disks and the resonant frequencies will be raised.

It is important to note that maximum power for vibrating each of the individual disks is required when the disks are vibrating at their resonant frequencies. If the drive rod is vibrating at a relatively high frequency so that a larger disk or disks idle, the only energy absorbed by these disks is that resulting from friction between the disks and the rod and in the springs themselves. This frictional loss is very small relative to the energy required for vibration of a disk at its resonant frequency. Since the disks having relatively high resonant ranges are considerably lighter and since these disks are vibrated at or near the very small amplitude of vibration of the rod when being vibrated below their resonant ranges, negligible energy is expended in vibrating these disks. Most of the energy transmitted from the voice coil to the rod 17 is absorbed by the disk which is vibrating in its resonant range, with a lesser amount being absorbed by disks on each side of the resonant range if these disks are vibrating effectively. Practically no energy is absorbed by disks not being vibrated effectively. Consequently the system is very efficient in its utilization of energy from the voice coil to reproduce sound.

Each of the sounding disks vibrates independently of all 5 of the other disks. The energy which each disk derives is from the rod only and the inertia force of each disk is always opposed by the rod and never opposed by any of the other disks. Consequently, there is never any interference between sounds of different frequency level and each tone whether high, low or intermediate will be reproduced precisely.

For clarity of description the operation has been described as though notes of only one frequency were reproduced at a time. In practice many different frequencies may be reproduced simultaneously, and at the same time there may be many overtones created as beat frequencies. The result is that some or all of the sounding disks may be responding simultaneously to complicated frequency patterns. Each and every fundamental tone or beat frequency is reproduced by the speaker system of this invention with each of the sounding disks accurately responding to each frequency within its resonant range. The result is that all of the brilliance and character of the original sound source or sources is reproduced in a highly efficient manner.

It will be understood that the present invention provides a new concept of sound reproduction embodying efficient, resiliently mounted vibrating disks which, within their resonant ranges, greatly magnify the amplitude of vibration of the vibratory driver with the resilient mounting means automatically acting as highly efficient mechanical frequency cross-over means. The result is true fidelity reproduction of all sounds.

Variations and modifications may be effected without departing from the scope of the novel concepts of the present invention.