Method and apparatus for imparting to headphones the sound-reproducing characteristics of loudspeakers

Abstract

Sound signals are created at the eardrums of a listener to correspond to sound signals which would be created at the eardrums of such listener in a predetermined acoustical environment in response to first electrical signals applied to a loudspeaker having known sound-reproducing characteristics. The soundwaves at the eardrums of the listener are measured as a function of frequency to determine the pressure signals at the eardrums of the listener when the listener is listening to acoustical information transmitted by the loudspeaker. An electroacoustical transfer function, relating the electrical input signal of the loudspeaker to the pressure actually impinging upon the eardrums is determined. The same is done for a set of earphones, so as to determine a second transfer function associated with the earphones. A compensating network is connected to the earphones. The compensating network has a transfer function corresponding to the quotient of the first transfer function divided by the second transfer function.

Description

The invention relates to a method for imparting to an earphone set at least some of the sound-reproducing characteristics normally exhibited only by a loudspeaker, or by a plurality of spaced loudspeakers in the case of stereo reproduction. The invention relates not only to the frequency response characteristics of earphone sets versus loudspeakers, but is furthermore concerned with the possibility of simulating the spaciousness of the sound reproduced by a loudspeaker, and particularly that produced by a plurality of speakers spaced apart from each other.

Earphones are being used nowadays in ever greater numbers, especially in the entertainment industry, wherein they are used now very commonly for listening to radio broadcasts, phonograph records and tape recordings. In addition, earphones are used for technical purposes, e.g., for monitoring purposes during recording sessions and during broadcasting.

One of the principal reasons for the growing popularity of earphone sets for home use, as opposed to the more familiar loudspeakers, is that they permit listening to broadcast and recorded material without disturbing other persons not wishing to listen, and likewise prevent the listener from being distracted by other sound sources which may be present.

A significant disadvantage associated with the use of earphone listening sets, as opposed to ordinary loudspeakers, is that earphones, even those of high quality, exhibit sound-reproducing characteristics markedly different from those of loudspeakers. These different sound-reproducing characteristics include not only differences in frequency response, but equally important, differences in the sense of acoustical spaciousness experienced by the listener.

In order to make clearer the concepts in question, reference should be had to FIG. 1. An acoustical event has objective temporal, spatial and spectral characteristics. The objectively determinable parameters of an acoustical event include the nature of the sound source, the distance of the sound source from the hearer, the orientation of the sound source with respect to the hearer, the particular sound field or sound signals impinging upon the eardrums of the hearer, i.e., such parameters as can be measured for purposes of an objective description of the physical phenomena associated with the perception of an acoustical event.

FIG. 1 depicts, by way of example, a human hearer VP and a loudspeaker L spaced some distance from the human hearer in a room characterized by very low reverberation, i.e., the walls, floor and ceiling of the room tend to absorb rather than reflect the sound waves impinging upon them, so that the sound perceived by the hearer VP is not markedly complicated by the superimposition upon the source waves of many-times reflected sound waves.

If an electrical signal is applied to the electrical input of the loudspeaker, the human hearer VP will experience an acoustical perception h.sub.1, located, so far as the hearer can determine, at a distance r and oriented in the indicated direction with respect to the center of the listener's head, the acoustical perception appearing to be more or less sharply localized in the graphically depicted position of FIG. 1. In other words, and this is of importance, under certain definite experimental conditions a specific acoustical event s.sub.1 (here the electro-acoustically converted signal emanating from the loudspeaker) has associated therewith a certain acoustical perception h.sub.1. This acoustical perception h.sub.1 has its existence in the nervous system and brain of the listener, and does not necessarily correspond to the actual location and orientation of the acoustical event, as is clear from FIG. 1. The characteristics of the acoustical perception h.sub.1 can only be determined by receiving verbal descriptions from a human hearer concerning his subjective experience of the acoustical event. The acoustical perception h.sub.1 accordingly cannot be measured directly, as can the acoustical event s.sub.1, although clearly the acoustical perception h.sub.1 is of far greater importance than the abstract acoustical event s.sub.1.

When an earphone set is plugged into the same electrical outputs into which are plugged the inputs of a loudspeaker or loudspeaker system, in order to run a comparison test using the same musical composition, for example, very marked differences are observed in the acoustical perceptions of the listener, when using the earphones instead of the loudspeaker. Aside from the differences in frequency response, there are other differences of a psychological nature relating to the spatial characteristics of the perceived sound, the most important of which is the well-known "orchestra in the head" effect. The orchestra, in the case of an orchestral composition, is perceived as being located within the head of the listener or at a distance from the listener's head on the order of magnitude of the distance between the listener's ears. This is especially the case when listening to loud music, which is frequently done when listening with high-quality stereophonic equipment.

It has been extremely difficult to deal in a systematic and scientific manner with these psychological phenomena. The causes of these phenomena have always been assumed to include such factors as unavoidable differences in the sound-reproducing characteristics of the two transducers of an earphone set, the exact positioning of the earpieces with respect to the listener's ears, the pressure with which the earpieces press against the listener's ears, the sound transmissivity of the skull bone of the particular listener, the effect of the listener moving his or her head while listening, and many other such psychological and physiological factors. Also to be considered is the fact that when a listener employs an earphone set the signals actually impinging upon his eardrums are produced in a manner different from the manner in which signals are produced by a loudspeaker, especially a large loudspeaker. In particular, when listening with earphones, the total electroacoustical transduction phenomenon does not include the factors of substantial transmission distance, the sound distribution within the room between speaker and listener, the diffusion of sound before the sound reaches the listener's eardrums, etc.; instead, the total electroacoustical transduction depends more directly upon the transducer characteristics of the earphones.

There is great practical interest in the problem of eliminating both the spatial and spectral distortion associated with the use of earphone sets, i.e., as specifically compared to the spatial and spectral phenomena associated with high-quality loudspeakers employed to listen to the same material.

German Offenlegungsschrift 1,927,401 discloses an attempt to deal with this problem. According to the approach in question, experiments are conducted on an artificially constructed human head provided with two microphones in the regions of the ears of the artificial head. The acoustical characteristics of an actual human head are simulated to the extent possible, and measurements are taken of the sound reception in the eardrum locations of such head. As a result of the measurements taken, recording engineers can modify their recording technique in such a manner as to produce recordings or broadcasts which, when listened to with earphones, will have the desired improved qualities. However, this approach is of little value for practical reasons. Firstly, it would establish an entirely new category of recording and broadcasting techniques, namely those which would be used with either earphone or loudspeaker listening specifically in mind. This is evidently undesirable because the result would be the manufacture of records and tapes, and the transmission of broadcasts falling into one or the other of the two categories, with the listener being compelled to listen in the selected way, or else settle for a very considerable amount of distortion.

The use of this "artificial head" method for program consumers is really out of the question right from the start, since for the program conversion use must be made of a loudspeaker arrangement in order to apply to the "artificial head" the corresponding loudspeaker program. However, it is exactly this which should be avoided in many instances, because of the resulting disturbing noise and because of the considerably increased cost of the required equipment.

German Offenlegungsschrift 2,007,623 discloses an arrangement, not making use of an artificial human head, having the special purpose of converting electroacoustical stereophonic intensity information into information modified to take into account sound transmission time. This arrangement operates on the same principle as disclosed in U.S. Pat. No. 3,088,977. It attempts to avoid the distortion in spatial characteristics associated with headphones, as compared to loudspeakers, through the use of delay circuitry and frequency-dependent damping circuitry for both earpieces of the earphone set and cupled to each other. The coupling together is such as to deliberately introduce cross-talk into the two channels, in an attempt to simulate the "cross-talk" in the acoustical perception of a listener listening to spaced stereo loudspeakers.

However, the electroacoustical transducer characteristics of conventionally employed earphones vary markedly from one earphone type to another. For this reason and others, such an arrangement is not very effective, even within the limits of the specific context and purpose for which it is intended, because the transducer characteristics of the earphones are in no sense taken into account. As a result, the known arrangements do not make it possible to significantly avoid the spatial and spectral distortions in the subjective acoustical experience of the earphone user described above. Likewise, it is not possible with such arrangements to simulate the specific spatial and spectral transducer characteristics of loudspeakers, or of the particular loudspeaker to which the listener is accustomed and which usually forms the basis of what the listener subjectively judges to be an undistorted reproduction of sound. The creation of acoustical perceptions, which take into account the characteristics of employed loudspeakers, can however for example be necessary for a sound engineer, who in certain circumstances may be forced to monitor a broadcast or recording session using earphones and will require and expect an exact correspondence between the spectral and spatial characteristics of the sound he hears using earphones and the sound he would hear if he employed the studio loudspeaker arrangement.

The general purpose of the invention is to overcome the disadvantages of the known methods and arrangements, and to provide a method which, with program material normally edited through the use of loudspeakers, such as is the case with radio broadcasting, phonograph record recording, and tape recording, etc., can be listened to using earphones which produce a subjective acoustic experience corresponding as exactly as possible, both in spatial and spectral respects, to the acoustic experience of a listener listening to loudspeakers.

The invention exploits the surprising realization that it is possible for a person listening with earphones to have the subjective experience of listening to loudspeakers, if only the physically measurable sound signals impinging upon the listener's eardrums when listening with earphones are made to correspond as exactly as possible to the sound signals impinging upon the eardrums when listening to loudspeakers. This realization and basic approach are new. Hitherto it has always been taken for granted that the problem in question could be solved only by taking separately into account a sizable number of complicated and usually rather nebulous psychological and physiological considerations.

It is essential to the inventive concept that the sound signals impinging upon the listener's eardrums correspond as exactly as possible to the sound signals which would impinge upon the listener's eardrums if the same audio information were transmitted to him from a loudspeaker arrangement. As explained before, the actual acoustical perception of a listener is a highly subjective matter and differs markedly form one individual to another. We have determined, after many series of experiments, that it is in fact possible to give the listener using earphones the impression that he is listening to a loudspeaker arrangement simply through the duplication of the sound signals impinging upon the listener's eardrums when he listens to a loudspeaker.

In fact, experimentation has indicated that the basic principle of the invention results in more than a mere improvement in the correspondence between the listening experience using earphones and that had when listening to loudspeakers; the resemblance between the two listening experiences has been made so extreme as to be astounding. In addition, the inventive expedient has proved realizable without any great increase in the usual costs of production.

According to the invention, it has been found advantageous to establish a close correspondence between the Fourier transform of the signals impinging upon the eardrums when the sound source is an earphone set and the Fourier transform of the signals impinging upon the eardrums when the sound source is a loudspeaker arrangement. The Fourier transforms are advantageously made to correspond both with respect to magnitude and phase, and the result is a distortionless time shift of the sound signals. However, a distortionless time shift, i.e., a shift relative to the longer sound travel time when using loudspeakers, of the sound signals is of no consequence to the listener, provided that there is a sufficiently close correspondence between the Fourier transforms of the signals with respect to magnitude and phase. In other words, the distance between the sound source and listener when a loudspeaker arrangement is employed can be ignored when attempting to duplicate that listening experience using only an earphone set.

The drawing depicts several embodiments of the method and arrangement of the invention. These exemplary embodiments deal with the problem of imparting to earphones the sound-reproducing characteristics of symmetrical arrangements of one, two or four loudspeakers, in an environment substantially free of reverberation.

FIG. 1 depicts in a very simplified and highly schematic manner the difference between an objective acoustical event and a subjective acoustical perception;

FIGS. 2-5 depict a first exemplary embodiment of the invention in which the sound-reproducing characteristics of a single loudspeaker, disposed symmetrically with respect to a listener's head, are to be imparted to an earphone arrangement;

FIGS. 6-8 depict a second embodiment, similar to that of FIGS. 2-5, but involving a pair of loudspeakers symmetrically disposed with respect to the listener's head;

FIG. 9 depicts a third embodiment, similar to that of FIGS. 2-5, but involving four loudspeakers symmetrically disposed with respect to the listener's head;

FIG. 10 depicts a test probe for insertion into the ear of a listener;

FIG. 11 is a diagram indicating the meaning of a number of different transfer functions, and showing the set-up for a particular test;

FIG. 12 depicts schematically an arrangement for measuring the frequency dependence of the magnitude of several transfer functions;

FIGS. 13 and 14 depict graphically the measurements made using the arrangement of FIG. 12;

FIG. 15 depicts schematically an arrangement for measuring the frequency dependence of the group delay time associated with several transfer functions discussed with regard to the invention;

FIGS. 16 and 17 depict graphically the results of tests made using the arrangement of FIG. 15; and

FIG. 18 is a circuit diagram of a compensating stage according to the invention.

In FIG. 2, a loudspeaker L.sub.1 is disposed symmetrically with respect to a listener's head. The electrical signal driving the loudspeaker is the voltage U.sub.1 (f). The sound signals actually impinging upon the left and right eardrums of the listener are sound pressures respectively designated p.sub.TR1 (f) and p.sub.TRr (f).

FIG. 3 depicts the effective input-output network existing in the situation of FIG. 2. The transfer function of this network is

and constitutes a first approximation for the determination of the (n+2k) terminal device required for the simulation, n being the number of loudspeakers and k the number of pairs of earphones. The electroacoustical transfer function A.sub.M (f) describes the relationship existing between the electrical voltage U.sub.1 (f) applied to the input of the network and the signal pressures p.sub.TR1 (f) and p.sub.TRr (f) impinging upon the eardrums of the listener, when the sound source is the single loudspeaker oriented as shown in FIG. 2.

According to the basic object of the invention, earphones are to be used to produce pressure signals impinging upon the listener's eardrums and corresponding to p.sub.TR1 (f) and p.sub.TRr (f). To accomplish this, the electro-acoustical transfer function of the earphones must first be determined, as depicted schematically in FIG. 4. This further electroacoustical transfer function is

This transfer function is that of the employed earphone pieces, and is equal to the ratio of the Fourier transforms of the driving voltage applied to the earphone pieces and the output signal pressures impinging upon the left or right eardrum of the listener. The earphone is assumed to be a linear system. These transfer functions A.sub.M (f) and A.sub.K (f) can be determined, both with respect to magnitude and phase, through the use of probe-shaped microphones inserted into the ear of a listener and through the use of attenuation and phase-measuring equipment.

If one divides the transfer function A.sub.M (f) by the transfer function A.sub.K (f), the circuit scheme depicted in FIG. 5 results, which can simultaneously serve to realize an (n+2k) terminal device. The signal pressures p.sub.TR1 (f) and p.sub.TRr (f) in FIG. 5 are the pressures impinging upon the listener's eardrums, when use is made of a pair of earphones having respective electroacoustical transfer functions A.sub.K (f) connected to the circuit stage X at junctions B and B'. The transfer function [A.sub.M (f)]/[A.sub.K (f)] can be realized by an electrical network if one has determined the transfer function A.sub.K (f) which depends upon the earphones and the general configuration of the outer ear of a human head. In other words, when use is made of an arbitrarily selected set of earphones, having a known electroacoustical transfer function, such earphones must be connected in the manner depicted in FIG. 5 to a network X having the transfer function indicated in FIG. 5. The network X takes into account both the transfer function of the earphones employed and the transfer function of the loudspeaker arrangement to be simulated. Such a selection and connection of networks constitutes the essential step for the achievement of a completely satisfactory listening experience with earphones. In this way, and only in this way, will the subjective acoustical peprceptions of the listener using the earphone set correspond to those he would have when listening to the loudspeaker arrangement.

FIGS. 6-8 depict the embodiment wherein n=2, i.e., where the sound-reproducing characteristics to be simulated are those of a pair of loudspeakers L.sub.1 and L.sub.2 disposed symmetrically with respect to the listener's head. In this situation, four transfer functions are involved, although because of the symmetrical set-up the four transfer functions should be composed of a first pair of identical transfer functions and a second pair of identical transfer functions. The entire transducer system can in the end be satisfactorily described by the two transfer functions A.sub.C (f) and A.sub.S (f) and by the two summing circuits S.sub.1 and S.sub.2 (see FIGS. 7, 8). The two summing circuits take into account the cross-talk effect of the two loudspeakers--i.e., they take into account the fact that sound waves transmitted from the left speaker L.sub.1 reach not only the left eardrum but also reach the right ear drum, and vice versa with respect to the right loudspeaker L.sub.2. It is to be understood that the "circuit" of FIG. 7 is a physical representation of the relationships between the electrical input voltages and eardrum pressures when using the loudspeaker arrangement of FIG. 6.

FIG. 8 depicts, in block diagram form, a circuit arrangement for imparting to an earphone set-up the overall electroacoustical transducer transfer functions of the loudspeaker arrangement of FIG. 6. A.sub.K (f) again identifies the electroacoustic transfer function of the earphone set employed in FIG. 4. The transfer functions A.sub.C (f) and A.sub.S (f) are again determined by means of a probe microphone. With the exception of A.sub.K (f), the various transfer functions indicated in FIG. 5 and indicated in FIG. 8 can readily be realized through the synthesis of appropriate input-output networks using conventional techniques of transfer function synthesis.

The set of earphones with which one is to work, after the electroacoustical transfer function A.sub.K (f) thereof has been plotted, will be connected to the compensating network at connection points B, and B' as shown in FIG. 8. Self-evidently, more than one pair of earphones can be connected to the outputs B, B' if the number of pairs of earphones k is greater than one.

FIG. 9 depicts a third embodiment, similar to the two just discussed, but intended to impart to a pair of earphones the sound-reproducing characteristics of a set of four loudspeakers disposed symmetrically with respect to the head of a listener. Accordingly, n = 4. The analysis of this circuit will be self-evident from the description of the two previous embodiments, except that two additional summing circuits S.sub.3 and S.sub.4 are provided, in order to take into account the fact that each ear receives signals from all four of the loudspeakers.

The determination of the transfer function A.sub.M (f) of the loudspeaker whose sound-reproducing characteristics are to be simulated, and likewise the determination of the transfer function A.sub.K (f) of the earphones to be used in the simulation, can be achieved by employing a specially designed microphone probe SM. This probe is depicted in FIG. 10. It is comprised of a probe tube so configurated that when inserted into the ear of a test listener the open end of the probe tube will be located 4 mm behind the entrance into the lateral cartilaginous ear canal (as seen in the direction of the eardrum) in the reference plane BE, and it will detect the pressure signals p.sub.BE arriving at that location from the loudspeaker or from the earphones. The special form of the probe tube guarantees that p.sub.BE can be measured even when the earphones are in place on the head of the listener.

FIG. 11 depicts schematically the head of the test listener VP (as seen from above), the outer ears OM of the listener, the ear canal GG which terminates in the acoustical impedance Z.sub.Tr (f) of the eardrum Tr, and the selected reference plane BE. The earphones are designated K, the loudspeaker designated L, the center of the listener's head M, the input signal to the loudspeaker designated U.sub.1. Also indicated are a number of transfer functions. These are as follows: ##EQU1## and ##EQU2## The above two transfer functions are of interest with respect to the determination of the effective transfer function for the loudspeaker L. Also indicated are ##EQU3## and ##EQU4## The above two transfer functions are of interest in determining the effective transfer functions for the earphones. Also indicated is

The above transfer function is associated with the short length of the ear canal extending from the selected reference plane BE at which the pressure measurement is actually performed to the eardrum Tr itself. Given the impedance Z.sub.Tr (f) of the eardrum, the ear canal transfer function A.sub.TrBE (f) can readily be determined empirically. Clearly, the advantage of separately computing the ear canal transfer function is that this procedure, which involves an actual determination of the pressure variations at the eardrum itself, is a sensitive and potentially dangerous one. By separately determining the transfer function A.sub.TrBE (f), the pressure in the ear canal need only be determined at the reference plane BE, at a distance of 4 mm from the ear canal entrance, avoiding the need to contact or extremely closely approach the eardrum. When the ear canal transfer function is computed, by empirical testing, it can be thereafter used in computations involving a variety of different loudspeaker and earphone transfer functions. It should be noted that the separate determination of the ear canal transfer function A.sub.TrBE is not absolutely necessary. It would be possible to insert the probe so deeply into the ear as to be spaced the smallest possible distance from the eardrum itself. Evidently, however that would be a less convenient and much more delicate procedure.

With these transfer functions computed, the transfer functions actually of interest can be easily computed. The loudspeaker transfer function A.sub.M (f) will evidently be

The earphones transfer function A.sub.K (f) will evidently be

The frequency dependence of the magnitude of the transfer function A.sub.BEM (f) and of the transfer function A.sub.BEK (f) can be determined, for example, by employing an attenuation measurement unit such as depicted in FIG. 12. The probe SM is inserted into the ear canal, both for the loudspeaker test and the earphone test. A conventional attenuation recorder 100, 101 is employed to determine the shape of the curve of the magnitude of the transfer function versus frequency. The generator 100 generates a fixed-amplitude waveform, the frequency of which rises from zero linearly with time. This voltage is applied to the input of either the earphone set or to the input of he loudspeaker L, depending upon which transfer function is to be determined. The pressure-responsive microphone SM responds to the pressure variations corresponding to the generated audio signals, and applies a corresponding electrical signal to the input of an equalizing amplifier 102 which flattens the response curve of SM. The recorder device 100, 101 is provided with a chartering device 101 comprised of a roll of graph paper which is advanced in synchronism with the linear rise of frequency of the output voltage of the voltage generator 100. The device 101 includes a non-illustrated moving scribe whose position varies in dependence upon the output signal of amplifier 102. The resulting curve inscribed upon the chart is a graph of the frequency-dependence of the magnitude of the measured transfer function A.sub.K (f) or A.sub.M (f). Examples of curves which have resulted from experiments are shown in FIGS. 13 and 14.

FIG. 13 depicts in broken lines the frequency variation of the magnitude of A.sub.BEM (f), the meaning of this transfer function having already been explained. The loudspeaker in question was spaced three meters from the center of the listener's head, and happened to be of the type marketed under the designation ISOPHON KSB 12/8. The experiment was performed in a non-reverberating acoustical environment. The actual curve of interest is shown in solid lines and represents the frequency dependence of the magnitude of the transfer function A.sub.M (f), defined above. The equation permitting derivation of the solid-line curve from the broken-line curve has already been given.

FIG. 14 is similar to FIG. 13 but represents the test results when the characteristics of the earphone set were determined. The broken-line curve BE represents the magnitude of the transfer function A.sub.BEK (f), and the solid-lind curve designated Tr represents the magnitude of the transfer function A.sub.K (f). The earphones employed for the test happened to be of the type marketed under the designation BEYER DT 48.

As will be understood by persons skilled in the art, the transfer functions in equation will exhibit frequency dependence not only with respect to magnitude but also with respect to phase. It is accordingly necessary to determine the frequency dependence of the phase shifts associated with the transfer functions A.sub.M (f) and A.sub.K (f).

However, it is not necessary to measure the phase shifts .THETA..sub.M (f) and .THETA..sub.K (f) directly. In particular, it is considered advantageous to determine the phase shift indirectly by instead measuring the group delay time. ##EQU5## and ##EQU6## Inasmuch as ##EQU7## .THETA..sub.M (f) and .THETA..sub.K (f) can be calculated, if for a particular reference frequency f.sub.o one measures the phase angles .THETA..sub.M (fo) and .THETA..sub.K (f.sub.o) and then performs frequency-dependency measurements of .tau..sub.gM (f) and .tau..sub.gK (f).

The determination of .tau..sub.gM (f) and of .tau..sub.gK (f) is per se conventional, and can, by way of example, be performed with the arrangement depicted in FIG. 15 (the measurement technique in question being known as the Nyquist-Brand technique). The equations are as follows: ##EQU8## and ##EQU9## After these determinations, .tau..sub.gM (f) and .tau..sub.gK (f) can be calculated using the equations therefor given above. The function .tau..sub.gTrBE (f) associated with the path from reference plane BE to the eardrum can be determined empirically, and is advantageously determined separately, for the same reason that the magnitude of the transfer function associated with the terminal portion of the ear canal is determined separately, namely, to reduce the number of times that a probe is brought extremely near to the eardrum.

It is to be noted that instead of employing the equipment shown in FIG. 15 to determine the frequency dependence of the phase-shifts associated with the several transfer functions, it is possible to employ the conventional method of measuring the phase-shift directly at a plurality of different frequencies and then interpolating to form a phase-shift versus frequency curve. An ordinary phase detector can be used for this purpose.

FIG. 16 depicts the frequency dependence of group delay time when determining the transfer function A.sub.M (f). The curve designated BE represents the group delay time .tau..sub.gBEM (f) associated with the transfer function A.sub.BEM (f), whereas curve Tr represents the group delay time .tau..sub.gM (f) associated with the transfer function A.sub.M (f).

The curves depicted in FIG. 17 correspond to those depicted in FIG. 16, except are taken with respect to the earphones, instead of the loudspeaker.

It is to be noted that the specific curves depicted represent averages of the results of tests employing twelve different listeners.

FIG. 18, finally, depicts a circuit designed to have a transfer function [A.sub.M (f)]/[A.sub.K (f)] and to have the group delay time .tau..sub.g [A.sub.M (f)/A.sub.K (f)] = .tau..sub.gM (f) - .tau..sub.gK (f) as nearly as possible. The average delay time difference amounts to about 9 milliseconds, inasmuch as no attempt was made to take into account the time required for the sound to travel the 3 meters from the loudspeaker to the listener's eardrum, because such distance and time delay can be ignored with the inventive approach.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such modifications and adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits and constructions different from the types described above.

While the invention has been illustrated and described as embodied in a method and apparatus for imparting to earphones some of the sound-reproducing characteristics of loudspeakers, it is not to be considered limited to the details shown, since various modifications and structural and circuit changes could be made without departing in any way from the spirit of the invention.

Claims

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. Method for creating sound signals at the eardrums of a listener by means of earphones, to correspond to sound signals which would be created at said eardrums of said listener in a predetermined acoustical environment in response to first electrical signals applied to a loudspeaker, comprising, in combination, the steps of measuring the amplitude and delay time of soundwaves at said eardrums of said listener as a function of frequency in response to electrical signals applied to said loudspeaker when said listener is in said acoustical environment in such a manner as to determine a corresponding desired transfer function; measuring the amplitude and delay time of soundwaves at said eardrums of said listener as a function of frequency in response to electrical signals applied to said earphones in such a manner as to determine a corresponding earphone transfer function; and furnishing an electrical network having a network transfer function corresponding to the ratio of said desired transfer function to said earphone transfer function, said electrical network having first input for receiving said first electrical signals and a pair of outputs; and connecting said earphones to said pair of outputs.

2. A method as set forth in claim 1, wherein a predetermined time shift exists between soundwaves produces at said eardrums of said listener by said earphones and the soundwaves which would exist at said eardrums of said listener in said predetermined acoustical environment in response to said first electrical signals.

3. Arrangement for creating sound signals at the eardrums of a listener to correspond to sound signals which would be created at said eardrums of said listener in response to electrical signals applied to a loudspeaker positioned symmetrically to said eardrums of said listener, and wherein a transfer function A.sub.M (f) defines the sound pressure at said eardrums of said listener in response to predetermined first electrical signals applied to said loudspeaker, comprising, in combination, earphone means for creating an earphone transfer characteristic A.sub.K, said earphone transfer characteristic constituting a measure of the variation with respect to frequency of the sound pressure at said eardrums in response to earphone test signals applied to said earphones; network means having at least one input for receiving electrical signals, a pair of outputs, and a network transfer function corresponding to the ratio of A.sub.M (f) divided by A.sub.K (f); and connecting means for connecting said earphones to said pair of outputs of said network means and said first electrical signals to said input of said network means.

4. An arrangement as set forth in claim 3, wherein said predetermined acoustical environment comprises a first and second loudspeaker positioned symmetrically to said eardrums of said listener and responsive, respectively, to first and second electrical signals; wherein a first and second desired transfer function A.sub.C (f) and A.sub.S (f) define, respectively, the transfer function from the input of said first loudspeaker to said first and second eardrum of said listener respectively; wherein said network means have a first and second input for, respectively, receiving said first and second electrical signals; wherein said electrical network means comprise a first and second summing stage, each of said summing stages having a first and second input and a summing output; wherein said electrical network means further comprise a first and second input stage having a transfer function substantially equal to A.sub.C (f) divided by A.sub.S (f) and a first and second output stage having a transfer function substantially equal to A.sub.S (f) divided by A.sub.K (f); wherein said network means further comprise first connecting means connecting said first input of said network means directly to said first input of said first summing stage, and said second input of said network means directly to said first input of said second summing stage, second connecting means for interconnecting said first input stage between said first input of said network means and said second input of said second summing stage; third connecting means for connecting said second input stage between said second input of said network means and said second input of said first summing stage; and fourth connecting means connecting said first output stage between said summing output of said first summing amplifier and said first output of said network means and for connecting said second output stage between said output of said second summing stage and said second output of said network means.

5. An arrangement as set forth in claim 4, wherein said predetermined acoustical environment further comprises a third and fourth loudspeaker, arranged symmetrically with respect to said eardrums of said listener; further comprising additional network means, corresponding to said network means set forth in claim 4, and connected to said third and fourth loudspeaker, said additional network means also having a pair of outputs; further comprising a first and second additional summing stage each having a first and second input and an output; further comprising means for connecting one of said pairs of outputs of said first and additional network means respectively to said first and second input of said first additional summing stage, and the other of said outputs of said first and additional network means to said first and second inputs of said second additional summing stage; and means for connecting said first and second earphones respectively to said output of said first and second additional summing stages.