US1623561A - Acoustic horn - Google Patents

Description

April 5 1927.

WITNESSES:

J. SLEPIAN ET AL ACOUSTIC HORN Filed Feb. 4. 1924 wmxmm' v |NVENTORS Jose 1b Sleep/an 8r g l/nzon R. Han/7d ATTORNEY riod shall be materially lessened.

Patented Apr. 5, 1927.v

UNITED STATES PATENT OFFICE.

ASSIGNORS T0 WESTINGHOUSE ELECTRIC .ANiD MAIN'UFACTURING COMPANY, A.

CORPORATION OF PENNSYLVANIA.

ncous'rro HORN.

Application filed February 4, 1924. Serial No. 690,405.

This invention relates to horns of the type used with sound-reproducing instruments such as phonographs, loud-speaking telephones and the like.

It is an object of this invention to improve the volume of tone and the quality of the sound, delivered through such horn, from a vibrating diaphragm. Y

The energy radiated from the horn in the form of sound-waves is the quantity which determines-how loud the sound seems to the listener. This depends not only upon the shape and size of the horn, the size of the diaphragm and the volume of the space above the diaphragm, but also upon the velocity of the movement of the diaphragm. The relation between the energy radiated during a given short period and the average velocity of the diaphragm during said period, may be expressed as a ratio, which is referred to herein as the intensification.

It is a further object of this invent-ion to produce a horn which shall give an intensification that is substantially constant over a great range of pitch.

In all instruments including a horn and a diaphragm, a function of the horn which has frequently been overlooked is to load the diaphragm by compelling it to do work in moving the air within the horn and thus damping the motion of the diaphragm. It -is a further object of this invention to so proportion this acoustic damping of the diaphragm to the other dampingfactors that the sound distorting effect ofthe resonance of the diaphragm at its natural pe- By making the acoustic damping large, we are enabled not only to diminish the radiated power at diaphragm resonance, but to increase it at frequencies other than resonance.

In this way, we produce a more uniform output throu hout arrange of frequencies without diminishing the average output.

It is a further object of this invention. to provide acoustic damping to such degree as to prevent the diaphragm from vibrating beyond the amplitude within which the restoring forces are linear functions of the displacement. If the diaphragm be permitted to vibrate more than this, the characteristic frequencies of the diaphragm itself begin to influence the output and distortion of the sound to be reproduced results. This object of the invention is, therefore, to provide suflicient acoustic damping to prevent the amplitude of movement of the diaphragm from ever becoming great enough to introduce such distortions as can result from the characteristics which greater movements of the diaphragm possess.

It is a further object of this invention to so correlate the size of the opening at the large end of the horn to the length of the (horn and the size of the small end that suflicient radiation shall be obtained without requiring a horn of impractical length.

We have discovered, by mathematical analysis and confirmed by experiment, that the shape of the horn should be such that the area of the cross-section is an exponential function of the distance from the small end of the horn.

The rate of increase of the cross-section of a horn having an exponential form plays an important part, not heretofore-Linden.

stood, in determining alower limit to the range of pitch over which the intensification will be uniform. Said rate also fixes" the The size of i will cease to be equal to the volume displaced by the movement of the diaphragm. When these two volumes begin to depart from equality, further diminutions in the size of the throat does not result in a corresponding.;,increase in acoustic'damping. If the size of" the throat relative to the volume above the diaphragm be too small, the veloeity of air in the throat exceeds a certain critical value. Above this critical value, turbulence occurs, with corresponding increase in friction losses. The lower limit of the size of the throat is fixed by consideration, both of the necessity of equality between the two volumes explained above and of the necessity for avoiding turbulence.

- the volume of the chamberbetween the dia- 1phragm and the throat, than have heretofore een usual. In the sound-reproducing instruments of the prior art, it has been usual to have a cylindrical space between the diaphragm and the cover which opens into the horn. A oone contains only one-third the volume resent in a cylinder of the same base and eight and we reduce the volume of the space abovethe -diaphragm still more by making it approximately conical. The height of the cone in the instrument we have devised is much less than the height of the space (of whatever shape) above the diaphra m in instruments used hitherto.

0t er features of our invention and details of the construction will be readily understood from'the following description and the accompanying drawing in which Fig. 1 is a view in side elevation of an instrumenteembodying this invention, and

Fig. 2 is a view on a larger scale partially V in section and partially in elevation of the operating instrument and a portion of the horn. V

A casing 1 contains mechanism for vibratinga diaphragm 2. This may be the electromagnetic mechanism of a loud speaker, a phonograph or any other means for actuating the diaphragm. A stem 3 is shown by .way of example to indicate means by which the mechanism within the case 1 actuates the dia hragm 2. l

e diaphragm is held in place by a cap 4 I which is secured to the casing 1 in any desired manner. The edges of the diaphragm are clamped between the cap and the casing.

The clamping action is cushioned by means of soft rubber washers 5 and 6. A recess is provided in the upper part of the cap in order to accommodate the upper Washer. Preferably, the wall of this recess extends down over the inner surface of the washer, almost into engagement with the diaphragm. From the-lower edge of this wall, the in side of the cap slopes upwardly and inwardly, forming a cone 7. The drawing shows the height of this cone as small as can be illustrated with clearness. Actually, the height of the cone is relatively less. Preferably, it is only a few times the amplitude of the maximum movement of the d aphragm.

much smaller in this instrument than in instruments having horns of the form used is at the apex of the cone. This opening is much smaller than those in the horns used heretofore.

The horn 10-fits in the opening 8. The shape of this horn is given by the equation:

AzA e it, we select-a value for B which will prevent the length of the horn from being too inconvenient, when the areas of the ends are fixed, but this selection is based largely on considerations stated below. The selection must be a compromise between the range of pitch over' which the horn will give uniform results and a convenient length.

We have found that for horns of exponential shape, B, the rate of increase of the cross-section, plays an important part in determining the lower limit of the range of pitch over which the intensification is sufliciently constant to prevent noticeable distortion. This range is from very high pitches down to a pitch fixed by 2; I 4 B 2.5 X 10 where m is 211' times the number of cycles per second for that pitch, and B is computed for a length measured in centimeters. The power radiated begins to diminish before this point is reached and falls off rapidly as the pitch extends below this limit; but even so, the power radiated at low pitch is greater than for horns of other shapes. For example, it reatly exceeds that radiated by a conical liorn, having the same length and ends of the same respective areas at all low pitches except those very near the lower limit of audition.

The smaller the value of B, the lower the pitch at which efifective radiation begins to fall ofi that is, the better the quality of the reproduction. On the other hand, a smaller value of B requires a longer horn for given sizes of the ends.

The size of the small end of the horn plays an important part in determining the acoustic damping. Its area is. given by the foran A Jn 1 B0 in which p is the density of air, a the velocity of sound in air, n the permissible ratio of distortion, A, the diameter of the diaphragm, 00 the period of the lowest frequency which the horn is expected to radiate, and B5, is the stiffness of the diaphragm. The stiffness of a diaphragm is the force tending to return it to normal position when it has been displaced. I

The ideal instrument would radiate the same power at all pitches for the same force on the diaphragm. Such an instrument has not. yet been discovered. With all known instruments there is more power radiated at certain frequencies than at others for the same force on the diaphragm. The ratio between the power radiated at the frequency A practical horn which gives very good results has been constructed in which 13:.07 for c. m. units'of length, which corresponds to approximately twenty per cent increase in area of cross-section per inch of length. This gave a falling-soft point at w:1250, which corresponds to a pitch somewhat above middle 0. The diameter of the small end is 0.508 cm. or 0.20 inches. The length is 132 cm. or about 4 feet and the diameter at the large end 50.8 cm. or about inches.

If this volume be too large in proportion to the area of the small end of the horn, the motion of the diaphragm, instead of being completely transformed into velocity of the air in the throat, may be partly expended in compressing the air within said volume.

To obtain the desired uniformity of response over a desired range, the chamber volume must be not greater than that given by the formula in which 1, corresponds to the pitch at which the diaphragm is resonant, m correvolume of thechamber above the diaphragm must theoretically be made smaller than the physical diff culties of manufacture permit. w may, however, be as large as 40,000 without the difficulties due to this circumstance appearing.

The area of the large end-of the horn must be fixed by a compromise between the physical difficulties resulting from excessive length and the acoustic advantages which result from having this end very large. If the diameter of the final opening exceeds the wave length of the lowest pitch for which the horn is expected to correctly intensify, the reflections'which take place at the lar e end will be negligible; It is found that this result is obtained when the radius of the large end is This is the size which the .The lower limit of pitch for which the reflections are negligible is, with such a horn,

' the same as the pitch at which the intensification begins to fall off rapidly.

A horn of the dimensions stated above was used with a diaphragm having an area of 15.5 sq. cm. and a stiffness of 20 X 10 dynes per cm. over a range from somewhat l 1 to: (01 L02 below middle 0 to l-line 0 (0). The conical chamber above the diaphragm had a height of .05 cm. l/Vith a constant current input, the greatest intensity throughout the pitch-range, was not more than ten times the smallest intensity. Such a horn, used with a loudspeaking telephone, gave a reproduction of music which was not onlypleasing, but very close to the original. It'gave a reproduction of voice which was not only intelligible, but natural.

When the diaphragm. is actuated by a phonograph record, through a needle and lever system designed for use with an ordinary horn, greater power will be delivered from the record to the needle and greater volume of sound emitted by the horn. If desired, a different mechanical ratio may be employed in the needle-lever system, so that the displacement of the diaphragm is diminished. This would enable heavier memto use, with the ordinary lever system, records prepared especially for use with such a horn. The undulations or sinuosities in such records would be of smaller amplitude than heretofore, which-would be an advantage in the manufacture of such records. In all. of these ways of applying the horn to a phonograph, the advantages obtained with the ater acoustic damping causes greater free om from tendency to radiate disproportionately large power at those pitches which correspond to the natural period of free vibration of the moving parts.

Although we have described and illustrated but one particular embodiment of our invention, it is understood. that changes within the spirit of the invention can be made by those skilled in the art. We, therefore, do not purpose limiting the invention except as necessitated by the prior art or indicated in the claims.

We claim as our invention:

1. An acoustic horn, having an exponential variation of cross-section with the length and an internal diameter at the small end less than a third of an inch.

2. In combination, a diaphragm, a horn, an enclosure housing said diaphragm and opening into said horn, said opening being as small, relative to the volume of said enclosure, as may be without causing thevolumetric flow of air into said opening to be appreciably less than the volumetric rate of displacement of the diaphragm.

3. In combination, a diaphragm, means for vibrating said diaphragm, and acoustic means for so damping said vibration that the displacement of the diaphragm does not exceed an amount at which the restorative forces thereof cease to have a linear relation walls making a maximum angle of fortyfive degrees with the axis, and a throat, the minimum diameter of which is so small that the acoustic damping is greater than the sum of all other damping.

5. In combination, a diaphragm, a born, a chamber closed at one side by said diaphragm and opening into the horn, and means for vibrating said diaphragm, said chamber closely conforming to one extreme position of said diaphragm and the area of said opening being so small that the acoustic damping of the diaphragm prevents excursion thereof beyond the point of linear coefiicient of stiffness.

6. In a sound-reproducing instrument, a horn, a diaphragm, an enclosure providing a chamber between said diaphragm and horn, said chamber opening into the throat of said horn, the area of said opening being large enough relative to the area of said diaphragm and volume of said chamber, to ensure the volumetric flow of air into the opening shall be equal to the volumetric rate of displacement of the diaphragm, and small enough to damp the motion of said diaphragm more than the sum of all other damping. I

7. An acoustic horn, having an exponential variation of cross-section with-the length and an internal diameter at the small end less than two per cent of the internal diameter at the large end.

8. An acoustic horn, the area of the crossscction of which is an exponential function of the distance from the small end, the diameter of the small end being less than onehalf of one per cent of the length of the horn.

9. An acoustic horn, having an exponential variation of cross-section with the length, the rate of increase of cross-section per unit length being sufliciently small to bring the pitch limit of accurate reproduction within an octave of middle 0.

10. An acoustic horn, the area of whose cross-section is an exponential function of the distance from the small end, the rate of increase of the cross-section being less than twenty-five per cent per inch and the diameter oi the opening at thesmall end being less than two per cent of the diameter of the opening at the large end.

11. In a sound-reproducing instrument, a diaphragm, a conical housing covering said diaphragm, the altitude of the cone being less than one per cent of its base-diameter.

12. In a sound-reproducing instrument, a diaphragm, a horn, a conical housing covering said diaphragm and opening into said horn, the altitude of the cone being less than fifty times the maximum amplitude of the motion of the diaphragm and the said opening being so small that the diaphragm does not move beyond the point at which the. restorative forces thereof become a non-linear function of the displacement.

13. In a sound-reproducing diaphragm, a conical housing covering said diaphragm and having an opening. at its apex, the altitude of the cone being less than fifty times the maximum amplitude of the motion of the diaphragm.

14. In a sound-reproducing instrument, a diaphragm, a horn,a housing over said diaphragm opening into said horn, the height of said housing being less than fifty times the maximum amplitude of the motion of the diaphragm when loaded bysaid horn.

15. In a sound reproducing instrument, a chamber having an opening, and means cooperatingwith said chamber to produce an air movement in said opening corresponding to the sound to be reproduced, the area of said opening being such that the pressure within the chamber is maintained substantially constant.

16. In combination, a diaphra m, a horn, an enclosure housing said diap ragm and opening into said horn, said opening being instrument, a

tion, and the cross sectional area of the small end being such that the ratio of the damping of the diaphragm by .said horn to the mass of the diaphragm is greater than 10,000 c. g. s. units. 7

18. In an acoustic instrument, a diaphragm chamber, a diaphragi'n therein, a

born, the cross section of which is an exponential function of the distance from the small end, the small end of said horn cominunicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn,.said area being so small that the acoustic damping per unit mass of the diaphragm is sufficient to prevent the ratio of sound energy radiated at the natural resonance period of the diaphragm to the sound energy radiated at the owest frequency at which the horn can effectively radiate to be less than a normal car can distinguish when listening to music.

15). In an acoustic instrument, a diahragm chamber, a diaphragm therein, a 10m, the cross section'of which is an exponential function of the distance from the small end, the small end of said horn communicatin with said diaphragm chamber, whereby t 1e acoustic damping upon said diaphragm Will be a function of the area of said small end of the horn, said area being so small that the acoustic damping is suflicient to prevent perceptible distortion over the range of frequency for which the horn is inten ed. i

20. In an acoustic instrument a diaphragm chamber, a diaphragm therein, a horn, the cross section of which is an exponential function of the distance from the small end, the small end of said horn communicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn, aid area being so small that the acoustic damping is sufiicient to cause the radiated sound to be substantially independent of the frequenc from the highest frequencies for which'the instrument is intended to the cut-off frequency determined by the rate of increase of said horn.

21. In an acoustic instrument a diaphragnrchamber, a diaphragm therein, a horn, the cross section of which is an exponential function of the distance from the small end, the small end of said horn communicating with said diaphragm chamber, whereby the acoustic damping upon said diaphragm will be a function of the area of said small end of the horn, said area being so small that the acoustig damping per unit gt: where w,- isthe natural resonance frequency of the diaphragm and w, is the lowest frequency which the instrument is expected to radiate.

mass is greater than 22. In an acoustic instrument, a horn having a cross section which is an exponential 7 function of the distance from the small end, the rate of increase of said cross section being less than twent -five percent per inch and the area of the arge end being at least equal to that of a eirele having for its radius, where B is the constant in-the ex-i uary,1924.

1 JOSEPH SLEPIAN.

CLINTON R. HANNA.

Images