US3920904A - Method and apparatus for imparting to headphones the sound-reproducing characteristics of loudspeakers - Google Patents

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

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US3920904A
US3920904A US395371A US39537173A US3920904A US 3920904 A US3920904 A US 3920904A US 395371 A US395371 A US 395371A US 39537173 A US39537173 A US 39537173A US 3920904 A US3920904 A US 3920904A
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listener
input
eardrums
transfer function
loudspeaker
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Jens Blauert
Peter Laws
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EUGEN BEYER ELEKTROTECHNISCHE FABRIK
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S1/00—Two-channel systems
    • H04S1/002—Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution
    • H04S1/005—For headphones
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04S—STEREOPHONIC SYSTEMS 
    • H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
    • H04S2420/01—Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD]

Definitions

  • 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.
  • 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 loud speaker, 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 rej speaker) has'associated therewith a certain acoustical cordings.
  • earphones are used for technical
  • 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 listen- 1 ing 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.
  • earphone listening sets as opposed to ordinary loudspeakers, exhibit sound-reproducing characteristics markedly different from those of loudspeakers.
  • sound-reproducing characteristics include not only differences in frequency response, but equally important, differences in the sense of acoustical spaciousness experienced by the listener.
  • 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 parametersas 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 theroom 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 superim position upon the source waves of manytimes reflected sound waves.
  • a specific acoustical event s here the electroacoustically converted signal emanating from the loudperception h
  • This acoustical perception h 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 can only be determined by receiving verbal descriptions from a human hearer concerning his subjective experience of the acoustical event.
  • the acoustical perception h accordingly cannot be measured directly,as can the acoustical event s although clearly the acoustical perception h, is of far greater importance than the abstract acoustical event's
  • 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.
  • 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 listeners eardrums, etc.; instead, the
  • 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 trans mission time.
  • This arrangement operates on the same principle as disclosed in US. 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.
  • 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 listeners 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.
  • the sound signals impinging upon the listeners eardrums correspond as exactly as possible to the sound signals which would impinge upon the listeners eardrums if the same audio information were transmitted to him from a loudspeaker arrangement.
  • the actual acoustical perception of a listener is a highly subjective matter and differs markedly form one individual to another.
  • 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.
  • a distortionless time shift a distortionless time shift
  • 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 rever-' beration. v
  • 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 listeners head, are to be imparted to an earphone arrangement;
  • FIGS. 6-8 depict a second embodiment, similar to that of FIGS. 25, but involving a pair of loudspeakers symmetrically disposed with respect to the listeners head;
  • FIG. 9 depicts a third embodiment, similar to that of FIGS. 2-5, but involving four loudspeakers symmetrically disposed with respect to the listeners 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.
  • FIG. 18 is a circuit diagram of a compensating stage I tion 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 U) describes the relationship exist- -ing between the electrical voltage U 0) applied to the input of'the network and the signal pressures p U) and p ,(f) impinging upon the eardrums of the listener, when the sound source is the single loudspeaker oriented as shown in FIG. 2.
  • ear- 7 phones are to be used to produce pressure signals impinging upon the listeners eardrums and corresponding to p Q) and p U).
  • the electro-acoustical transfer function of the earphones must first be.determined,as depicted schematically in FIG. 4.
  • This further electroacoustical transfer function is l(fl IPrmLfl/UKDI [pTRrm/ rm]
  • 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.
  • transfer functions A U) and A 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 phasemeasuring equipment. If one divides the transfer function A U') by th transfer function A the circuit scheme depicted in FIG. 5 results, which can simultaneously serve to realize an (n+2k) terminal device.
  • the signalpressures p ff) and 11 m in FIG. 5 are the pressures impinging upon the listeners eardrums, when use is made of a pair of earphones having respective electroacoustical transfer functions A' (j) connected to the circuit stage X at junctions'Ban'd B.
  • the transfer function [A (f)]- /[A (f)] can be realized by an electrical network if one has determined the transfer function A Q) which depends upon the earphones and the general configuration of the outer ear of a human head.
  • a Q the transfer function which depends upon the earphones and the general configuration of the outer ear of a human head.
  • 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.
  • .four transfer functions areinvolved, although because" of the symmetrical set-up the four transfer functionsshould 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 0) and A 0) and by the two summing 7 sures when using the loudspeaker arrangement of FIG.
  • 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 (f) again identifies the electroacoustic transfer function of the earphone set employed in FIG. 4.
  • the transfer functions A (f) and A are again determined by means of a probe microphone. With the exception of A' U), the
  • the set of earphones with which one is to work, after the electroacoustical transfer function A U') thereof has been plotted, will be connected to the compensating network at connection points B, and B' as shown in FIG. 8.
  • More than one pair of earphones can be connected to the outputs B, B ifthe 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 and 8., 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 U) of the loudspeaker whose sound-reproducing characteristics are to be simulated, and likewise the determination of the transfer function A Q) 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 arriving at that location from the loudspeaker or from the earphones.
  • the special form of the probe tube guarantees that p can be measured even when the earphones are in place on the head of the listener. 1
  • FIG. 1 1 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 ,(j) of the eardrum Tr, and the selected reference plane BE.
  • the earphones are designated K, the loudspeaker designated L, the center of the listeners head M, the input signal to the loudspeaker designated U,. Also indicated area number of transfer functions. These are as follows:
  • 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 BEKU The above two transfer functions are of interest in determining the effective transfer functions for the earphones. Also indicated is 'rrsnm lPrrml [Pasml 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- U') of the eardrum, the ear canal transfer function A U) can readily be determined empirically.
  • 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.
  • 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.
  • the ear canal transfer function is computed, by empirical testing, it can be thereafter used incomputations involving a variety of different loudspeaker and earphone transfer functions.
  • the separate determination of the ear canal transfer function A 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.
  • the loudspeaker transfer function A Q) will evidently be I um asum rraslfl
  • the earphones transfer function A U') will evidently be M) ABEM rrsslf)
  • the frequency dependence of the magnitude of the transfer function A (1') and of the transfer function A U) 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 generates a fixed-amplitude waveform, the frequency ofwhich 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 frequencydependence of the magnitude of the measured transfer v function A (f) or A Q). Examples of curves which have resulted from experiments are shownin FIGS. 13
  • FIG. I3 depicts in broken lines the frequency variation of the magnitude of A U), the meaning of this transfer function having already been explained.
  • the loudspeaker in question was spaced three meters from the center of the listeners head, and happened to be of the type marketed under the designation ISOPHON KSB 12/8.
  • the experiment was performed in a nonreverberating 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 Q), 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 U), and the solidlind curve designated Tr represents the 'magni-.
  • the earphones employed for the test happened to be of the type marketed under the designation BEYER DT 48.
  • 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 I determine ,the frequency dependenceof the phase sidered advantageous to determine the phaseshift indi rectly by instead measuring the group delay time.
  • G m and G U can be calculated, if for a particular reference frequency fl, one measures the phase angles 9,,(fo) and G UZ) and then performs frequencydependency measurements of 1, ,0) and 7 0).
  • T,, (f) and 'r,, (f) can be calculated using the equations therefor given above.
  • the function r flfl 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 tothe eardrum.
  • v I g v FIG. 18,.finally depicts a circuit designed to have a transfer function [A (f)]/[A (f)] a and to, have the group delay time,r u,(f)/ xff)l ariff) mm nearly as possible.
  • the average delaytime difference amountsto about 9 milliseconds,,inasmuch asno attempt wasrnade to take into account the time required for the sound to travel the 3 meters frornthe. loudspeaker to the listenefis eardrum, because such distance and time delay can be ignored with the inventive approach.
  • 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
  • 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 Q) and A 0) 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 0) divided by A 0) and a first and second output stage having a transfer function substantially equal to A U') divided by A U'); 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
  • 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.

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  • Acoustics & Sound (AREA)
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US395371A 1972-09-08 1973-09-07 Method and apparatus for imparting to headphones the sound-reproducing characteristics of loudspeakers Expired - Lifetime US3920904A (en)

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US10225657B2 (en) 2016-01-18 2019-03-05 Boomcloud 360, Inc. Subband spatial and crosstalk cancellation for audio reproduction
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US10524078B2 (en) * 2017-11-29 2019-12-31 Boomcloud 360, Inc. Crosstalk cancellation b-chain
US10764704B2 (en) 2018-03-22 2020-09-01 Boomcloud 360, Inc. Multi-channel subband spatial processing for loudspeakers
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FR2199240B1 (es) 1978-01-13
IT998449B (it) 1976-01-20
JPS53283B2 (es) 1978-01-07
GB1437129A (en) 1976-05-26
CH571798A5 (es) 1976-01-15
JPS49133002A (es) 1974-12-20
FR2199240A1 (es) 1974-04-05
DE2244162C3 (de) 1981-02-26
DE2244162B2 (de) 1974-08-01
NL7312345A (es) 1974-03-12
DE2244162A1 (de) 1974-03-28

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