US 2115129 A
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Filed Aug. 29, 1956 INVENTOR ERICH TH IEN HAUS ATTORNEY Patented Apr. 26, 1938 UNITED STATES LOUD SPEAKER Erich Thienhaus, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b. IL, Berlin, Germany, a corporation of Germany Application August 29, 1936, Serial No. 98,457 In Germany June 8, 1935 6 Claims.
The present invention is concerned with a special design of casing for acoustic apparatus, especially for loudspeakers and microphones.
H In the operation of a piston-type diaphragm which vibrates freely in space and which represents a projector or radiator of the first order, an acoustic short-circuit is produced upon both sides of the diaphragm for low frequencies for reasons of pressure equalization. The result of 10 this action is that these particular frequencies are not radiated to any appreciable extent and do not therefore contribute to the faithful reproduction of the sound to be translated. It is known from the prior art that the said acoustic short-circuit is avoidable either by having the diaphragm built into an infinitely large acoustic baffle or else by having the posterior face of the diaphragm completely shut or enclosed so that equalization of pressure is precluded between the pressure waves radiated or projected by the anterior face of the diaphragm and the pressure waves so projected from the posterior face thereof. What thus results is a radiator or projector of order. A projector of the 0 order is a sound source at which the energy is at all sides radiated from the interior to the exterior. Its fundamental shape is represented by a sphere the surface of which borders the medium in question (air, water), said surface performing in all its points co-phasal and radial oscillation movements of equal amplitude due to the active, periodical forces.
However, inasmuch as the size of the acoustic bafile, in most instances, is limited or finite, and since, moreover, a closed box or case would tend to greatly raise the restoring force and thus the natural frequency of the diaphragm, a fact that would make itself felt in a very disagreeable manner in high-fidelity loud-speakers which inherently possess a very low natural period, the above mentioned two methods of remedying the situation may be used in practice only conditionally. The compromise method which has been suggested for relatively small-sized loudspeaker models, namely, to fit the loudspeaker in an open casing, however, fails to reduce completely the undesirable pressure equalization between the front and the back of the diaphragm. In addition, it has the drawback that reproduction is seriously impaired because of dissimilarities in box resonances.
In order to avoid the latter, it has been also suggested to mount within the casing, when the loudspeaker is built into a wall, another diaphragm which is capable of freely vibrating in this wall. This second diaphragm is only set in motion by the volume of air confined inside the casing and which is set in oscillatory motion by the driven diaphragm. With this form of construction, it is almost possible to suppress the acoustic short-circuit, but this form of construction involves the drawback that the very large dimensions of the casing must be chosen when the diaphragms are disposed in superposition. But if the second diaphragm is fitted in the rear of the casing, then the said second diaphragm must be afforded a chance to radiate freely. In other words, the casing should not be mounted directly on a wall, and the result is that the space requirement becomes rather considerable also for this type of loudspeaker.
Now, in order that such drawbacks as exist in the designs known in the earlier art may be avoided, pressure equalization or compensation, by complete closureof the posterior face of the diaphragm, is insured according to this invention, without an increase in restoring force. This is brought about not by having the enclosure or chamber situated posteriorly of the diaphragm filled with air (which, at normal temperatures is in normal gaseous state so that, for acoustic actions, the law of adiabatic change of state is obeyed), but by having the enclosure filled with a saturated vapor so that with each increase or reduction in the enclosure volume there results an evaporation or condensation in the absence of pressure changes. No matter what the circumstances, there prevail within the chamber the saturation pressures of the vapor in question at the prevailing temperature. For instance, the chamber could be filled with vapor to the 100- percent limit and could operate exactly at boiling point, or else a small residual volume of air or the like neutral gas is provided and one works somewhat below the boiling temperature. In this latter instance, the state of saturation which corresponds to the temperature which happens to prevail will be assumed automatically or spontaneously. The pressure inside the chamber is advantageously chosen always equal to the barometric pressure of the air. This automatic pressure equalization may be insured, for example, by the aid of a U-shaped pipe or other device filled with water or some other liquid. The pressure equalization is accomplishable also, for instance, by the aid of small bellows which follows or responds to the pressure of the outside atmosphere.
What should primarily be considered for the purpose of filling the chamber are gases or vapors of liquids which will liquefy in the neighborhood of normal room temperature. Particularly suited for the object have been found vapors of organic gases such as acetaldehyde, pentane, ethyl bromide, and the like, the boiling points of which, at atmospheric pressure, are in the neighborhood of to 40 degrees C. With a view to insuring more reliably a certain temperature interval or range, it may occasionally be advisable to resort to the heating of the chamber, for instance, by
the aid of a glow-lamp, a glow-tube or gaseousdischarge tube, a heater resistance, etc. Where the magnet systems of loudspeakers are energized from an outside or separate source of energy, it may turn out to be advantageous to use the rectifier or the magnet system to act as a heater element.
For low temperatures, evaporation and condensation occur in each period in accordance with this formula:
m: mmax.SlI1 wt where m is the amount or volume of medium in vaporous state existing at each instant contrasted with the normal or quiescent state; in other words, if merely the state of change occurring inside the chamber is taken into consideration, the evaporation amplitude or, with opposite sign, the condensation amplitude. Inasmuch as the temperature, in the evaporation and condensation process, stays stable or constant, it follows that also the pressure prevailing inside the chamber stays constant during the motion of the diaphragm, that is, equal to the saturation pressure of the vapor.
Hence, the process or action which takes place during the motion of the diaphragm inside the chamber is as follows: If the diaphragm is moving in the direction towards the interior of the chamber, in other words, if the volume of the chamber is reduced, a volume of gas corresponding to the displaced volume will be precipitated or condensed in liquid form, in other words, this quantity of gas is condensed. But when the diaphragm moves in the opposite direction, the volume of the chamber increases. As a result of this growth in volume, a corresponding quantity of liquid gets a chance to evaporate again and to fill this volume.
Inasmuch as during the inward motion of the diaphragm, part of the gas becomes condensed, no compression of the gas happens, for the volume of the condensate is negligibly small compared with the Vapor volume. In the operation of the acoustic apparatus, this, as will be seen, has the effect that no increase in restoring force is caused. In other words, what is thus obtained is a sound radiation characteristic of a radiator or sound projector of the order which, as is well known, is particularly favorable for the low frequencies, without the drawback of an undesirable increase in restoring force being occasioned, a condition which is inseparable from closed chambers filled with air.
In the practical construction of such loudspeakers, the chamber located posteriorly of the diaphragm may be chosen extremely small. In fact, the closure need be spaced from the diaphragm only to such an extent that when the diaphragm experiences a maximum. deflection, hitting of the closure wall by the diaphragm will be. safely avoided. In other words, it is thus feasible to use extremely reduced casing dimensions in acoustic apparatus such as loudspeakers or microphones.
It will be seen that the time available for the occurrence of evaporation and condensation becomes proportionately less as the frequency rises, with the result that a slight lag becomes noticeable in both the condensing and evaporating actions, with the further result that eventually a pressure variation of p=pn.sin (wt+) becomes added, where inn is the pressure amplitude and the phase shift. Under practical conditions this has as its physical efiect that in the presence of the low frequencies no restoring power is present, for the reason that for low frequencies adequate time is available for the proper production of the condensation and evaporation processes. However, restoring power makes itself manifest in proportion as the frequency grows as a consequence of the lag or delay produced in the condensing and evaporating actions, and this restoring force will grow gradually. This particular phenomenon could be used for the purpose of ininfluencing or governing the frequency response.
The annexed drawing shows an exemplified embodiment of the invention.
Confined inside a casing I is the drive system 2 and diaphragm 3. The casing, in this particular form of construction, embraces the posterior part of the loudspeaker diaphragm so that there is but a small distance between them. Placed inside the casing or chamber I is the cup 4 containing the liquid to be evaporated. Between the interior of the casing I and the outside air, pressure equalization is insured by Way of the valve 5. Case I is mounted upon a foot or base 6.
What I claim is:
1. Acoustic apparatus provided with a diaphragm and means for actuating said diaphragm, an enclosure completely enclosing the back of the diaphragm and forming therewith an air-tight chamber, and a saturated vapor within said chamber maintained at atmospheric pressure whereby a negligibly low restoring force is imparted to the diaphragm.
2. Acoustic apparatus according to the invention defined in claim I wherein the saturated vapor is produced from a liquid that is capable of vaporizing at room temperature.
3. Acoustic apparatus according to the invention defined in claim 1 wherein the chamber contains a small volume of a neutral gas in addition to the saturated vapor.
4. Acoustic apparatus provided with a diaphragm and means for actuating said diaphragm, an enclosure completely enclosing the back of the diaphragm and forming therewith an air-tight chamber, and means within the chamber for imparting a negligibly low restoring force to the diaphragm, said means including a vapor of an organic gas of the class including acetalydehyde, pentane, and ethyl bromide, which vapor is maintained at atmospheric pressure.
5. Acoustic apparatus provided with a diaphragm and means for actuating said diaphragm, an enclosure completely enclosing the back of the diaphragm and forming therewith an air-tight chamber, a saturated vapor within said chamber maintained at atmospheric pressure whereby a negligibly low restoring force is imparted to the diaphragm, and means including a U-shaped tube having a liquid therein communicating with the chamber whereby the pressure therein is automatically equalized with respect to atmospheric pressure.
6. Acoustic apparatus provided with a diaphragm and means for actuating said diaphragm, an enclosure completely enclosing the back of the diaphragm and forming therewith an air-tight chamber, a saturated vapor within said chamber maintained at atmospheric pressure whereby a negligibly low restoring force is imparted to the diaphragm, and heating means within the chamber whereby the vapor is maintained in a state of saturation.