US1788103A - Loud-speaker - Google Patents

Description

Patented Jan. 6, 1931 UNITED .STATES PATENT OFFICE ULINTON B. HANNA, OF EDGEWOOD, PENNSYLVANIA, ASSIGNOB T WFSTINGHOUBI me k IANUFACTUBING COMPANY, A CURPOBATION 0l' PENNSYLVANIA LOUD-EP Application illed June 22,

' This invention relates to sound-reproducing devices and particularly to such devices intended for handling `large amounts of power.

Sounds of low pitch are frequently sounds of lar e power. They are frequently produced y instruments in which the vibrating parts have large amplitude. For this reason, sounds from drums from low-pitch organ pipes from tubes or ass horns and from similar sources have heretofore been unsatisfactorily represented by sound-reproducing devices of the moving-diaphragm type because a diaphragm cannot make an adequate excursion without striking some stationary art.

In the patent to Joseph Slepian and myself, No. 1,623,561, granted April 5, 1927, for an improvement in acoustic horn, rules for the 2o design of loud speakers which will result in equalizing the response over a large range of frequenc are given.

A horn esigned according to the disclosure of the above-named patent and adapted for the radiation of very low notes is of great length. For the range of pitches usually covered, which does not extend much below 250 cycles per second the directions to make the diaphragm-chamher of a height only a few a0 times the amplitude of the maximum movement o f the diaphragm were equivalent to requiring the chamberv to be as small as it could oonveniently'be made.

When the formula for the volume of the diaphragm chamber given in said patent is applied to a loud speaker intended for sounds o low pitch and great power, either the up,

' per frequency limit must be taken far below what has heretofore been the practice or there will not be good clearance for the diaphragm r movement.

` An instrument comprising a vibratingdiaphragm anda horn usually requires a greater excursion of the4 diaphragm at-low notes for the same power in the radiated sound; If the horn be of the exponential form, the

"radiated sound is the same for the same velocity of the dia hragm at all frequencies, from the cut-olf requency ofthe horn up to 1987. Serial No. 900,540.

chamber causes the radiatedsound to fallof perceptibly.

With instruments of this form, therefore, at low frequency,the amplitude will be large because, during the whole of a long halfperiod, the velocity is in one direction. At

igh frequency, the period is short and the amplitude, therefore, must be small. With other forms of horns, the excessive amplitude :tl low frequencies is -even greater than As pointed out in the above-mentioned patent, the 'consequence of making the diaphragm chamber larger than the formula there given indicates is a failure to efficiently' re roduce sound'of high fre uency.

oreover, a loud speaker designed with a small dia hragm chamber and intended to accommo ate only small excursions of the dlaphragm, ordinarily is adapted only for small power and, if large power is impressed on the input of such a loud speaker, the instrument will be damaged thereby.

If the production of sufficient power at low notes, wlth instruments as heretofore de 'signed be attempted, a too great amplitude w1ll be produced and, even i the diaphragm does not actually strike a ainst the walls of the diaphragm chamber, its movement will exceed that at which correct quality is obtained. l

Heretofore, when it has been necessary t0 produce sound enough to fill a ball room or a large auditorium, it has been usual either to group a number o f loud speakers together or to connect a number of separately-drlven similar dia hragms to one horn by separate channels. he number 4 of diaphragms for 'the latter method must be impractically large in order to afford suflicient power at low notes without excessive amplitude of "diaphragm movement.

If, following `the first of said two methods a plurality of .instruments are operated together, the disadvantage ust explained is still present. Moreover, for t e low notes, the cutoff point of each horn must be low` and that necessitates bulkyhorns. The plurality of large horns requires a larger space -thanis or- `not be effective for notes a It is an object of my invention to so modify the loud speaker described in the above-mentioned atent that it shall be particularly adaptedp to the reproduction of low notes. In order to accomplish this object it is ro osed herein to constructa loud spea er w ic will ove some 200 to 500 cycles er second.

It is a urther object .of my invention to devise an effective system for the reproduction of sound inwhich the loud speaker just mentioned shall be sufpplemented by a loud speaker that will satis actorily reproduce the h1 hnotes.

It is a further object of my invention to so combine the loud speakers that the loud speaker adapted for the higher notes shall be protected from the heavy currents of low frequenc which are necessary for the proper lrepro uction of the low notes.

It is a further object of m invention to provide a combination of lou speakers in a circuit, in which the several parts of the circuit shall be so designed that each loud speaker therein will be protected from all frequencies lower than those for which it is intended.

Itis a further object of my invention to prevent any loudfspeaker in such a combination from diverting current of a frequency to which it is no`t responsive away from the loud speaker which is responsive thereto.

It is a further object of my invention to provide such a circuit in which the impedance shall be independent of the frequency.

Other-objects of this invention and details of the construction Will be apparent to those skilled in the art from the following speciiication and the accompanying drawing, in which Figure 1 is a sectional view of a portion of a loud speaker particularly adapted for low notes,

l Fig. 2 is a diagram of circuits and apparatus representing a system in which a pluralityof loud speakers are used, and

Fig. 3 is a diagram illustrating a modification of the circuits which may be used.

In Fig. l, the ma net 1 may be a permanent magnet or may e supplied with a winding 2 to constitute an electroma et.V Preferably, the ma et l is to pro uce a very powerful flux t rough an annular gap, as is indicated by the letters N and S on the two sides of the gap 3.

Upon the magnetic structure, a ca 4 is secured which has an o enin 5 inten ed to be connected to the smal en of a horn. The horn, as explained in the above-mentioned patent, should have the area of its cross-section increase with the distance from the throat 5, according -to an exponential law. Below the orifice 5, the 4cap 4 provides a diaphragm chamber 6.

The diaphragm 7 has the shape of a cone.

At the edge of the cone, the diaphra m is integral with a cylindrical skirt 8, including a heavy portion provided with a groove 9 and a thin portion l1 which is in the air ga At the bottom of the portion 11, a iange l may be provided to cooperate with the heavy part 8 to form an annular channel, in which is located a coil 13.

The groove 9 receives the inner edge of a collar or annular member 14 of heavy rubber or other fie'xible material. The outer edge of the collar 14 is clam ed between the cap 4 and the upper part o the magnet. The cap may be caused toy compress the collar by any desired means.

In my invention, I associate with the throat 5 in Fig. 1, a horn having so small a rate of fiare that the cut-off frequency is lower than cycles per second. Such a horn is very bulky and it is, therefore, highly desirable that the results be obtained with only one such horn.

In order to radiate the power corresponding to a big organ, a band of some fifty pieces, or

a chorus of several hundred voices, the sounddelivered by the loud speaker should have a power of several watts. I have found, for example, that it is necessary to radiate 21/2 watts in the form of sound in order to satisfactorily reproduce such sounds.

In order to exert as much power as this, it is necessary that the orifice 5 be small. Practical considerations of convenience in mounting render it inadvisable to make the diameter of the diaphragm 7 more than about four inches. Also, the practical limit of movement for the diaphragm, when mounted by the Hexible strip 14, as shown, is about nl; of an inch. From these considerations, I find that the diameter of the throat 5 should be about 1/2 inch.

From these dimensions, the volume of air which must flow into or out of the throat foi` each movement of the diaphragm is easily computed. At low frequencies the movement of the diaphragm is large and the Volumetric displacement produced by said movement is a large fraction ofthe volume of the diaphragm chamber. Consequently, the pressure change in the chamber is large enou h to eifectually drive the air through the throat of the horn. The volumetric flow through the throat will, therefore, at low frequencies be substantially equal to the volumetric displacement by the diaphragm.

At high frequencies, the amplitude of the diaphragm movement is small and the volumetric displacement is a small fraction of the volume of the diaphragm chamber. The pressure in the chamber, therefore, changes y only a small amount and, consequently, will be more. or less ineffective in driving the air through the throat. The volumetric How through the throat thus becomes less than the volumetric displacement by the dia- A disadvantage.

phragm at high frequencies and this causes the instrument to give out less response atvcham er, even at maximum .displacement of the diaphragm, themovement of the diaphragm makes a smaller percentage change 1n the volume. GreaterI ycompression of the air will occur, therefore, when the clearance is smaller.

When the compression is more than very slight, the reaction against the diaphragm 7 by the compressed air follows a non-linear law, with the conse uence that the radiated sound, instead of faithfully representing the movements of the diaphragm, will have certain extraneous frequencies. I have found that this diliiculty may be avoided for frequencies below about 500 cycles per second bv making the clearance between the diaphragm 7 and the walls of the chamberv about of an inch, that is to say, about twice the maximum amplitude of the movement of the diaphragm.

A structure, as described above, will have such damping characteristics that the greatest excursion of the diaphragm will be less than one half of the clearance afforded by the diaphragm chamber when the radiated power in watts is as great as one two- -thousandth of the square of the frequency in cycles persecnd.

Because the damping is large, the mass of the vibrating system may be great without A large mass `means vineffectiveness at high frequencies, but, because the clearance of the diaphragm chamber has been so chosen that high frequencies will not be reproduced effectively in the instrument, the choice of a large mass will sacrifice hardly anything which has not already been sacr1 ced. `I have found it advantageous to l make the combined mass of the diaphragm 6 and the coil 13 about 1A? pound. Moreover, by making the diaphragm and skirt of aluminum, the proportion of themass-,of the movin system which is in the coil is increase This weight is sufficient to enable me to make the coil of a conductor s o long that the desired force may be developed by the interaction between sa'id coil and the magnetic flux, and,lat the same time, to make the conductor of ample cross-section, whereby the resistance of the device is readily brought within the limits indicated below.

The circumstance that the coil is attached to the diaphragm at its circumference, enables the coil to be made with a small radial thickness. The air gap may, therefore, be short. That the portion 11 of the skirt beside the coil 13 is thin not only is another reason why the-gap may be short but has also the advantage that eddy currents are minimized.

- In Fig. 2, threeloud speakers 20, 21 and 22 are shown in parallel. The line 23, across which each of these loud speakers is connected, is supplied "with signal-conve ing current. This is indicatedby the microp one 18 and amplifier 19. The loud speaker 20 is designed, as explained in connection with Fi 1, for. the production of low notes and wi l not readily produce notes above 25o-or, at most 500 cycles per. second. The cutoff frequency of the horn forming part of the loud speaker 20 is as low as the lowest frequency of the signal-conveying current. It may very well be below 30 c cles per second. In order to res ond with, proportionate power to this low requency, the loud speaker 20 must possess inherent limitations which prevent it from effectively radiating frequencies above, at most, `500 cycles per second.

The vloud speaker 21 is of an intermediate character. The horn associated with it should have a cut-off fre uency well below the frequency at which t e instrument 20 ceases to be effective. For example, the instrument 21 should be capable of effectively reproducing frequencies lower than 250 cycles per second. Because designed for suf' ficient power at those low frequencies, the instrument 21 must fail to gi've full power at very high frequencies.

The loud speaker 22 should be designed to reach frequencies of 5,000 or more cycles per second. The cut-ofi` frequency of the horn forming part of the instrument 22 should be below the upper limit of the yfrequencies at which the instrument 21 'is effective. The smaller horns used with the instruments 21 and 22 in Fig. 2 may be added to the large horn used with the instrument 20 without adding seriously to the bulkiness of the arrangement.

In the branch circuit includin the instrument 20, a resistor 25 and an in uctor 26 are shown. It is permissible to so design the instrument that the desired resistance and inductance are obtained wit-hout the addition of external inductance.

The branch circuit including the instrument 21 is provided with a resistor 27 and an inductor 28 which, likewise, may be provided by the electrical properties of the instrument itself. This branch of the circuit also includes a condenser 31 which is not a part of the instrument itself, but must be added to the branch circuit. The condenser 31 is of a suitable size to make the branch circuit including it of comparatively small impedance to the medium frequencies, but the branch circuit is not tuned in the sense of having a well defined maximum of response at a particular frequency.`

The resistor 27is of sufficient .magnitude to insure that the branch circuit will be nearlynntuned. Very low frequencies, however, will not pass through this circuit because, for them,l the condenser 31 acts as a blocking condenser.

In' the branch circuit including the instrument 22, no inductance is represented. The instrument possesses a winding, and, therefore, i4will have some inductance, but by expedients well known in the art, and particularly by providing conductive plating upon the surfaces of the gap, as explained in connection with my Patent No. 1,691,243, dated Nov. 13, 1928, the inductance of this instrument maybe made'very small. A condenser 32 is placed in -the branch of the circuit including the instrument 22 and acts as a means by which the higher frequencies are received through-"this circuit more readily than the lower frequencies.

The resistances R of the instruments 20 and 22 in Fig. 2 are preferably equal, and

equal to L` van where L is the inductance of the inductor 26 and C is the capacity of the condenser 32. Also, the resistance 27 of the instrument 21 is made twice this magnitude. If then, the inductance of the inductor 28 is made and the .capacity of the condenser 31 is made 2C, the impedance of the circuit 23 will be independent ofthe frequency.

If it is preferred to use two parallel branches in the `circuit instead of three, the resi-stance ofthe two instruments should be made equal to each other and equal to If kthis is done, the impedance of the twonal-conveying current in any desired way.

As in Fig. y2, this is indicated by means of a microphone 18 and an amplifier 19.

The instrument 40 is, inv design,.like the instrument 2O in Fig. 2. Its resistance is represented bv the resistor 45. and. over its intended range of frequencies, its inductive reactance may be neglected.

The instrument'41 is designed like the instrument 22 in Fig. 2, and the resistor 43 represents the resistance of this instrument.' The condenser 42, unlike the condenser 32 in Fig. 2, is in parallel with an instrument and the instrument is the low-frequency one.

A connection 47 and a'connection 48 establish a closed circuit including the instrument 40 andthe condenser 42. The connection 48 and a connection'49 establish a closed circuit including the instrument 41 and the inductor 46. 'lhe lcombination 4of `the two closed circuits constitutes a connection across 1 the line 33. The return conductor may be a part of the line itself, as illustrated, or it may be the usual ground connection.

1t is possible, by making as before, to produce a circuit including two instruments which will have a constant imwhich is modulated in accordance with sound is impressed upon the line 23. The microphone 18 and the apparatus represented by the rectangle at the right of the microphone are intended to indicate any kind of apparatus for producing such modulatedcurrent.

The low frequencies in the current in line 23 of Fig. 2 pass readily through the instrument 20. The high frequencies will not ass through the branch of the circuit including the instrument 20 because of the large inductance in ,this branch.

High frequencies pass more readily through the branch including the instrument 22 than lower frequencies. because of the presence 4of the condenser 32. p This condenser also serves to preventthis branch from acting as a shunt through which low-frequency currents may bev diverted from the instrument 20. In the same way, Ithe .indurctancevin the branch circuit .including'the instrument 2Q prevents this branch from shunting high-frequency energy from the. instrument 22.

In the same way, .but to a smaller degree, the branch'including theinstrument 21 con.- ducts intermediate frequencies andr is prevented from short. circuiting .ei-ther the low frequencies or .the high frequencies.

The instrument y20 in the inductive branch responds to the'very lowest frequenciesv and produces sound therefrom. At, frmnnnno lll above about 200 cycles per second, theresponse of the instrument begins to fall off. The falling off is not abrupt, but gradual, and a. considerable portion of the energy at frequencies up to 500 cycles is translated into sound bv the instrument 20.

The intermediate instrument 21,` or the high-frequency instrument, if there are but two. as a cut-ofi' frequency so far below 500 cycles that it effectively supplements the instrument 20, at those frequencies for which the response from the instrument 20 is small. The higher frequencies to which the instrument 21 Will respond extend well beyond the point of cut-off for the instrument 22.

The upper limit of frequencies to Which the instrument 22 will respond is as high as the highest frequencies to which the system is intended to respond. In practice, it is usually zo sufficient if this limit is about 5,000 cycles.

If only two instruments are used, the upper one must cover the range from well below 500 cycles to above said limit. y

In this way, a faithful reproduction, both z5 of the extremely low pitches and of the high pitches, is obtained without overloading any of the instruments. Moreover, since the combination of several instruments in parallel presents a constant impedance to the line 23,

the load upon said line is independent of the frequency.

In the circuit illustrated in Fig. 3,1ow-frequency currents pass' through the instrument 40 which is designed to produce sound effectively therefrom. High frequency currents are not substantially impeded'by the instrument 40 because the condenser 42 acts as a short-circuit about the instrument 40 for them. Moreover, the inductance of the instrument 40, although presenting a negligible impedance to frequencies within the range of that instrument, offers considerable impedance to frequencies Well above its intended range.

Similarly, the inductance 46 affords a short circuit about the instrument 41 for the low frequencies to which it is not responsive and a path of little impedance for the currents to which the instrument 40 responds. Thus, the current in each instrument to which that instrument is responsive follows mainly a path outside the other instrument.

In both Fig. 2 and Fig. 3, it is a consequence of the constant impedance of the circuit that, for those frequencies to which more than one instrument responds, the current so di-vides between the responsive instruments that no over emphasis of vsuch frequenciesl results.

Many variations of -this invention and many applications of it beside those specially` stated herein will beapparent to those skilled in the art. I, therefore, do not Wish the specific description to be construed as a limitation but I intend to be limited only by the prior art and by the claims.

I claim as my invention:

1. A loud speaker comprising a diaphragm chamber, a diaphragml constituting one wall thereof, means for moving kthe diaphragm and means for acoustically dam ing said diaphragm and radiating sound, t e magnitude of said damping being so great that the greatest excursion of the diaphragm is less than one half the clearance afforded by the diaphragm' chamber when the radiated power in watts is as reat as one two-thousandth of the square of t e frequency in cycles per second.

2. A loud speaker comprising a diaphragm chamber, a diaphragm constituting one wall thereof, means for moving the diaphragm and means including a horn communicating with said diaphragm chamber through an orifice for acoustically damping said diaphragm and radiating sound, the volume lof said diaphragm being so great in comparison to the area of said orifice that the power of the radiated sound in Watts at any frequency is greater than one two-thousandth of the square of said frequency when the excursion of said diaphragm is one-half the clearance afforded by the diaphragm chamber.

3. A high-power loud speaker comprising a diaphragm, means for acoustically damping said diaphragm and radiating the resulting sound, means for driving the diaphragm against said damping and a diaphragm chamber affording clearance between its walls and said diaphragm, the acoustic damping being sufficient to cause the power radiated as sound to exceed a half a watt when the driving means moves the diaphragm through one-half of said clearance at the lowest frequency for which the speaker is intended and the clearance being great enough to prevent eflicient radiation above 1,000 cycles per second.

4. In a sound-producing instrument, a diaphragm, means for vibrating said diaphragm, a diaphragm chamber provided with an' orifice through which air moves upon movement of the diaphragm and acoustic damping means associated with said orifice, the ratio of the area of the diaphragm to the area of said orifice being great enough to so limit the movement of the diaphragm at frequencies above 500 cycles that, at a given power, the volumetric displacement by diaphragm will be so small a fraction of the volume of the diaphragm chamber that the volumetric flow through said orifice will be substantially less than said volumetric displacement and said ratio being small enough to permit so great a movement of the diaphragm at frequencies below 250 cycles per second that, at the same power, the volumetric displacement of the diaphragm will be a sufficiently large fraction of the volume of the diaphragmchamber to cause the volumetric flow through said orifice to be substantially equal to said volumetric displacement.

6 memos 5. In a loud speaker, a conical diaphragm, acoil support 'integral' therewith, a magnetic circuit'having an air gap in which said coil support is located and a coil on said coil sups' port, the mass of said coil 'exceeding the mass of said diaphragm.

6.l In aloud speaker, a conical diaphragm, a coil support integral therewith, a magnetic circuit having an air gap in which sald 10 coil support is located, a coll on said support, and an annular lexiblemember seated in an annular channel formed in said diaphragm for guiding and supporting said diaphragm against permanent dis lacement.

15 In testimony whereo I have hereunto subscribed my name this 13th da of June, 1927.

CLINTON HANNA.

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