WO1996039754A1

WO1996039754A1 - Ultrasonic sound system and method for producing virtual sound

Info

Publication number: WO1996039754A1
Application number: PCT/US1996/009565
Authority: WO
Inventors: Christian Constantinov
Original assignee: Christian Constantinov
Priority date: 1995-06-05
Filing date: 1996-06-05
Publication date: 1996-12-12
Also published as: AU6162596A; EP0857378A1

Abstract

A sound system (fig. 2) is provided which utilizes finite amplitude ultrasonic sources. These sources may be used alone or in combination with finite amplitude sonic sources. A controller (90) manipulates the phase, frequency and amplitude parameters of the sources so that they interact with each other and create combinatorial and differential frequencies in particular locations of the acoustic field. These frequencies further interact with their by-products, as well as with sonic frequencies to create a complex multi-dimensional acoustic field (fig. 3). The audible portion of this complex multi-dimensional acoustic field (fig. 3) is what the human auditory system detects and perceives as sound.

Description

ULTRASONIC SOUND SYSTEM AND METHOD FOR PRODUCING VIRTUAL SOUND

Background of the Invention

This invention relates to a new and useful sound system used to create multi-dimensional acoustic fields. Generally, sound is produced by vibrating surfaces and bodies of air. This is seen in the examples of sound produced by musical instruments. One example is a violin from which sound is produced by the vibrations caused by the bow rubbing against the string and activating vibrations in the string, which in turn causes the body of the violin to resonate. The frequency of the sound primarily depends on the length of the string. The tonal spectrum of the sound results from the structure of the body of the violin.

Another example is a trumpet. The air flow is produced by the player, hitting the edge of the mouthpiece, which destabilizes the air enclosed by the trumpet's body and activates vibrations. The frequency of the vibrations depends on the length of the enclosed air column (position of openings) and the force of the blowing.

In both examples, there is air vibration. This vibration is caused directly or indirectly by some generating force. The fluctuations caused by the contraction and expansion of the enclosed air is the sound. Air transmits these fluctuations in the same way as coupled springs transmit vibrations from one to the other.

Sound travels through the air and is perceived by a human through the human auditory system. The human auditory system consists primarily of two sensory systems, namely, the ears and the brain. Each ear is divided into an outer, middle and inner ear. The outer ear is composed of the pinna and auditory canal. Sound travels down the canal and causes the eardrum to vibrate. These vibrations are transmitted through the middle ear to the inner ear by three small bones.

The distance between the two ears aids the listener in determining the location of the sound. For example, if the sound arrives first at the left ear, it obviously is coming from the left side. If the sound is louder in the left ear, it is probably coming from the left side. Also, if more high frequencies are heard at the left ear, or if faster transitions are heard at the left ear then the sound is probably coming from the left. Each of these characteristics of sound, time of arrival, loudness and transitions, either alone or in combination, provide information to the listener to enable the listener to determine sound direction. The determination of sound direction is also based on the ability of the human auditory system to recognize complex auditory patterns.

The listener is constantly surrounded by complex auditory patterns and the resultant multi¬ dimensional acoustic fields generated by the natural environment of live sound. This is so because sound, even if generated by a single source, travels through the air and interacts with the air and signals already existing in the air. This interaction creates reflected and refracted images of the original signal which in turn interact with each other as well as with the original signal. In this way, very complex and constantly changing multi-dimensional acoustic fields are created. The same is true with ultrasound and ultrasonic signals.

The human auditory system learns to recognize and understand these complex auditory patterns and multi-dimensional acoustic fields of live sound and can distinguish between two sound environments which technically measure the same. Also, the complex auditory patterns and multi-dimensional acoustic fields generated by live sound communicate to the listener spatial and temporal information about sound events which surround the listener and reach the listener from every conceivable direction. Relying on the sense of hearing alone, the listener knows if someone approaches him from behind, if a vehicle passes by, and if so, in which direction the vehicle is moving. The sound waves which reach the listener also allow him to determine information about his location, i.e., is he in the outdoors, is he in an empty room, or is he in a crowded concert hall?

One goal of the present invention is to recreate for the listener the complex auditory patterns and multi-dimensional acoustic fields of live sound. No existing sound system accomplishes this goal.

All currently known sound systems generate audible sound using transducers operating within the audible range of human hearing, i.e. from about 16 Hz to about 20,000 Hz. In these systems the transducer itself is the audible source of the sound. For instance, if a 1000 Hz sound is to be reproduced, the transducer is fed with alternating current (AC) with a frequency of about 1000 Hz. The transducer then converts the current into mechanical vibrations so as to vibrate the diaphragm of the transducer with the frequency of 1000 Hz, which in turn causes the air to vibrate. These vibrations are detected and perceived as sound by the human auditory system.

Unlike the natural environment of live sound, in which the complex auditory patterns and multi¬ dimensional acoustic fields of sound surround the listener from every conceivable direction, the sound produced from sound systems comes from the transducer, or speaker, and thus approaches the listener from limited directions dictated by speaker position and acoustics of a room in a way inconsistent with the original sound field. Thus, the listener's ability to hear produced sound is largely dependent on the positioning of the speaker or speakers. Also, much of the spatial and temporal information provided by the complex patterns of live sound is lost since the sound produced from sound systems does not carry the complex patterns. Therefore, a listener of reproduced sound cannot place the sound events in their proper location in space and time.

Thus, reproduced sound is ineffective in relaying all of the information of a live sound event to the listener. This diminishes the listener's enjoyment of the produced sound event.

A need exists to enhance the listener's enjoyment of produced sound. Moreover, with the advances of technology and virtual reality, a need exists for a sound system which produces virtual sound, i.e., a system which produces acoustic fields with some or all of the characteristics the original, physical or artificially created sources without the original, physical or artificially created sources being present. The only way presently known to create for the listener something remotely resembling the natural environment of live sound, or virtual sound, is to have a large number of speakers positioned evenly on the walls, floor and ceiling, and have the listener sit in a very narrow spot, known as the "golden spot", which is determined by the position of the speakers. However even in this case, the sound will be distorted if the listener moves his head or body a small distance from the golden spot.

It is extremely difficult using the existing technology to create a stable sound source which is not coming from the direction of the speakers. Thus, a need exists for a sound system which creates for the listener the natural environment of live sound.

Summary of the Invention

It is an object of the present invention to provide a sound system which utilizes ultrasonic signals. It is also an object of the present invention to provide a sound system which uses biased transducers.

It is an object of the present invention to provide a method of biasing transducers. It is an object of the present invention to provide a sound system which creates for the listener the environment of live sound.

It is a further object of this invention to create a sound source that can fill an area with sound without the sound being perceived as coming from a particular location in the area.

It is also an object of this invention to create a sound source which emanates from a particular location in an area which is perceived equally well from any other location in the area.

It is also an object of the present invention to create a sound source which may emanate sound from variable locations in a particular area.

It is an object of this invention to modify the resultant sound from existing acoustic fields.

It is also an object of this invention to create a sound source which is contained within a particular area.

The disadvantages and limitations of previous sound systems are overcome by the present invention, which is a system for creating the natural environment of live sound, or virtual sound. The present invention is a sound system which utilizes ultrasonic sources. This sound system creates sound events in the audible range. These sound events occur at a location other than the location of the ultrasonic source. Thus, the sound produced does not come from a speaker. Rather, the produced sound event occurs at a particular place in space and time much like live sound.

The invention utilizes finite amplitude ultrasonic sources. These sources may be used alone or in combination with finite amplitude sonic sources. A controller manipulates the phase, frequency and amplitude parameters of the sources so that they interact with each other and create combinatorial and differential frequencies in particular locations of the acoustic field. These frequencies further interact with their byproducts, as well as with sonic frequencies to create a complex multi-dimensional acoustic field. The audible portion of this complex multi-dimensional acoustic field is what the human auditory system detects and perceives as sound. Thus, audible acoustic fields are achieved by changing the phase, frequency and amplitude parameters of the sources. The sound events which occur in these complex multi-dimensional acoustic fields are perceived by the listener as live sound.

Brief Description of the Drawings

The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

FIG. 1 is a simplified drawing of the pattern of interaction resulting from two transducers; FIG. 2 is an illustration of a sound system according to the present invention;

FIG. 3 is a simplified drawing of the pattern of interaction resulting from three transducers of a sound system according to the present invention; and FIG. 4 is a simplified drawing of the pattern of interaction resulting from six transducers of a sound system according to the present invention.

Detailed Description of the Invention

This invention is a sound system utilizing ultrasonic transducers either alone or in combination with sonic transducers. Furthermore, the invention is a method of creating complex multidimensional acoustic fields through the use of an array of two or more transducers. The greater the number of transducers, the more complex is the acoustic field, which is desired when creating virtual sound.

The transducers of the system are biased with an ultrasonic frequency, i.e. an ultrasonic frequency is fed into the transducer in addition to any other signal may be fed into the transducer. Transducers biased with an ultrasonic frequency in accordance with this invention may also be used in prior art sound systems to make them more linear and improve the quality and performance of the transducer.

Nonlinearities which naturally occur when audible acoustical signals interact are considered undesirable, and thus eliminated through the use of various methods. Instead of eliminating these nonlinearities, the present invention utilizes these nonlinearities to create greater interactions among the signals and in turn, more complex multidimensional acoustic fields. Furthermore,, the transducers used in the system of the invention may be more powerful than those generally used in prior art systems. The more powerful transducers cause the signals to have greater nonlinearities so as to create a more complex multidimensional acoustic field.

The interaction of ultrasonic signals in the complex multidimensional acoustic field result in combinatorial and differential frequencies which occur in various points in the acoustic field. Sound events occur at these various points when these combinatorial or differential frequencies fall within the audible range. Where these sound events occur in the acoustic field can be predetermined by manipulating the number of active transducers in the array, as well as the amplitude, frequency or phase of the signals transmitted by the each of the transducers. This manipulation may be accomplished with use of a controller alone or in combination with a computer.

Thus, in the system of the invention, sound is not emitted from a number of transducers, as in done in prior art systems. Rather, as with live sound, sound events occur in space. The listener perceives these sound events as live sound, or virtual sound. Moreover, manipulation of the system allows for the creation of various types of sound events in the acoustic field. For example, the system may produce a single sound event in a single point in space in the acoustic field, or may produce many sound events throughout the acoustic field. The system may then move the produced sound events to different locations in the acoustic field. The system may also be used with sonic sources to manipulate the acoustic field created by the sonic sources.

The occurrence of sound events at a point in space away from the transducer is better understood with reference to FIG. 1, where a simplified drawing of the acoustic field resulting from the interaction of ultrasonic signals emitted from two transducers is shown. In this figure, a first ultrasonic sinusoidal signal is emitted from transducer 100 and a second ultrasonic sinusoidal signal is emitted from the transducer 200 which is of equal amplitude and 180° out of phase with the signal from transducer 100. The signals combine at each of their minimum amplitudes along areas illustrated by lines 104, 108 and 112. These lines are called minima. The signals from the transducers combine at each of their maximum amplitudes along areas illustrated by lines 102, 106, 110, and 114. These lines are called maxima. If the transducers were in phase, lines 104, 108 and 112 would be maxima and lines 102, 106, 110, and 114 would be minima. Modifying the phase of the either sinusoidal signal either alone or in combination will alter the position of the maxima and minima. One with skill in the art will readily understand that the maxima and minima described are useful terms to indicate changes in the wave shape due to wave interactions and are non- stationary. Audible sound events are created by emitting signals from the transducers such that either the resultant combinatorial and/or differential frequencies are in the audible range.

If transducer 100 emits a signal of about 20 kHz and transducer 200 emits a signal of about 21 kHz, the resultant differential frequency of approximately 1 kHz is in the audible range. If the transducers are in phase, then this frequency can be heard along lines 102, 106, 110 and 114 — the minima. If the transducers are about 180° out of phase then this frequency can be heard along lines 104, 108 and 112 — the maxima. The placement of the lines can be moved by altering the phase of the signal from transducers 100 or 200 either alone or in combination. In this way, sound events of approximately 1 kHz occur in the acoustic field without the transducers emitting audible frequencies. The addition of transducers to the system, and the manipulation of the amplitude, frequency and/or phase of the signals emitted from the transducers allows for the production of various sound events throughout the acoustic field.

Also shown on FIG. 1 are transducers 1100 and 1200. Whereas transducers 100 and 200 function as speakers, transducers 1100 and 1200 function as microphones. If transducers 100 and 200 both transmit ultrasonic signals which are of equal amplitude and frequency, and are approximately 180° out of phase, then the signals cancel each other such that a resultant frequency of about 0 Hz occurs along minima 104, 108 and 112. Transducers 1100 and 1200 are located at the center of minima 104 and 108 respectively. If these are traditional unbiased transducers, i.e., not having an ultrasonic input frequency, then these transducers will not detect any signals along the minima. This is so because the unbiased transducer will not detect a signal unless the signal encounters the unbiased speaker. Since there are basically no signals occurring at the minima, nothing encounters the unbiased speaker, and thus no signals are detected. However, if these transducers are biased with an ultrasonic frequency, the they will be able to encounter and detect irregularities in the sound field occurring about minima 104 and 108. This is so because the biasing frequency causes the diaphragm of transducer to be in motion constantly. This constant motion enables the transducer to move about the minimum areas and encounter and detect irregularities in the acoustic field which would not have encountered the unbiased transducer. The system of the invention is better understood with reference to FIG. 2, an illustration of a sound system according to the present invention. Transducers 100, 200, 300, 400, 500 and 600, which may act as speakers, and transducers 1100 and 1200 which may act as microphones, are connected by wires 10, 20, 30, 40, 50, 60, 70 and 80, respectively, to controller 90. Circuitry within controller 90 controls the amplitude, frequency and phase of transducers 100, 200, 300, 400, 500, 600, 1100 and 1200. The transducers may be turned on and off with controls such as switches 92 on the front of the controller. Likewise, the amplitude, frequency, and/or phase of the signals transmitted by transducers 100, 200, 300, 400, 500, 600, 1100 and 1200 may be manipulated through the use of controls such as dials 94 on the front of the controller. A computer 96 may be connected to the controller 90 with a cable 98. The computer turns transducers 100, 200, 300, 400, 500, 600, 1100 and 1200 on and off and manipulates the amplitude, frequency and/or phase of the signals transmitted by transducers 100, 200, 300, 400, 500, 600, 1100 and 1200 automatically in accordance with a pre-programmed algorithm. The computer 96 of the present invention may be a personal computer (such as one based on the 80X86 series of microprocessors originally developed by Intel Corporation, of Santa Clara, California) .

Example 1 This example demonstrates how an array of three ultrasonic transducers may be used to fill the acoustic field with sound events. Referring to FIG. 3, transducers 100, 200, and 300 are emitting sinusoidal ultrasonic signals with the approximately the same amplitude and with frequencies of about 20 kHz, 21 kHz and 20 kHz, respectively. Lines 102-114 represent the interaction between the signals emitted from transducers 100 and 200. Lines 202-214 represent the interaction between the signals emitted from transducers 200 and 300. Lines 302-314 represent the interaction between the signals emitted from transducers 100 and 300.

The solid lines, 102, 106, 110, 114, 202, 206, 210, 214, 302, 306, 310 and 314 represent the minimum zones. In this example, an audible frequency of approximately 1 kHz can be heard along zones illustrated by lines 102, 106, 110, 114, 202, 206, 210 and 214. A signal of approximately 0 kHz exists along zones illustrated by lines 302, 306, 310 and 314.

The dotted lines, 104, 108, 112, 204, 208, 212, 304, 308 and 312 represent the zones of the maxima. In this example, an ultrasonic signal of about 41 kHz exists along zones illustrated by lines 104, 108, 112, 204, 208 and 212. An ultrasonic frequency of about 40 kHz exists along zones illustrated by lines 304, 308 and 312. The maxima and minima interact with each other and with themselves in points in space in the acoustic field. These points are illustrated as intersections in FIG. 3. The resultant signal at these points of interaction are audible if the combinatorial or differential frequency is within the audible range. For example, when the minima of approximately l kHz occur along the zones illustrated by lines 102, 106, 110 and 114 interact with the minima of approximately 1 kHz which occur along the zones illustrated by lines 202, 206, 210 and 214, then the resultant frequency at each of these points of interaction will be approximately 0 kHz or 2 kHz in addition to the 1 kHz, depending on the phase of each signal. In this way, the resultant frequencies at the intersections shown in FIG. 3 range from approximately 0 kHz to 122 kHz depending upon the frequency or phase of the maximum and/or minimum signals which interact. The locations of these interactions and resultant frequency at these interactions can be predetermined by manipulating the phase of the ultrasonic signals emitted by transducers 100, 200 and 300 with the controller 90, either with or without the aid of the computer 96. The overall result is a complex multidimensional acoustic field in which audible frequencies ranging from approximately 0 to 2 kHz exist throughout the entire acoustic field.

Example 2 This exampl - demonstrates how an array of six ultrasonic transducers may be used to create a sound event which exists in a single point in space. Referring to FIG. 4, transducers 100, 200, 300, 400, 500 and 600 are emitting sinusoidal ultrasonic signals with the approximately the same amplitude and frequencies of about 280 kHz, 220 kHz, 140 kHz, 100 kHz, 61 kHz, and 21 kHz, respectively. For simplification, FIG. 4 only illustrates the zones of maxima and minima resulting from the interaction of the signals emitted from transducer pairs 100 and 200, 300 and 400, and 500 and 600 respectively. Although not illustrated, the signals emitted from each transducer interact with the signals emitted from every other transducer in the array to create a multidimensional complex acoustic field.

In FIG. 4, tha differential frequency resulting from the interaction of the signals emitted from the transducers is as follows: between transducers 100 and 200 the differential frequency is about 60 kHz; between transducers 300 and 400 the differential frequency is about 40 kHz; and between transducers 500 and 600 the differential frequency is about 21 kHz. Each of these frequencies are in the inaudible range. These differential frequencies interact with each other. The differential frequency resulting from the interaction of the 60 kHz signals with the 40 kHz signal is about 20 kHz. This signal is also inaudible. However, this signal also interacts with the 21 kHz signal with a resultant differential frequency of about 1 kHz. This 1 kHz signal, which is in the audible range, results in a sound event which is heard in an isolated area, illustrated in FIG. 4 as point 1000, which is where the differential frequencies from the different transducers meet. This sound event may be moved about the acoustic field by using the controller 90, either alone or with the aid of the computer 96 to alter the frequency of one or more of the various signals which contribute to the signal. Example 3 This example describes how an array of two or more ultrasonic transducers may be used to create the natural environment of live sound, or virtual sound. It is obvious to one of skill in the art that two or more of the transducers of the array describe_d in Example 2 could be used to fill the acoustic fiel_d with sound events as was described in Example 1. Thereafter, the number of transducers and the frequency emitted from each of the transducers may be modified _by the controller 90, either alone or with the aid of the computer 96, to create an isolated sound event as is described in Example 2. The frequency of the transducers may also be modified by the controller 90, either alone or with he aid of the computer 96, so as to move the isolated sound event to different locations in the acoustic field as described in Example 2.

In a similar way, the controller, eithor alone or with the aid of a computer may alter the number of transducers as well as the amplitude, frequency and/or phase of the signals emitted from the transducers to create a cluster of sound events in a location in the acoustic field which may be moved about the acoustic field. Thus, the system creates the natural environment of live sound by constantly altering the number of transducers emitting ultrasonic signals as well as the frequency, amplitude and/or phase of the signals so as to create a constantly changing complex acoustic field to in which differing sound events occur as required by the natural environment of live sound which is being recreated.

Example 4 This example illustrates how an array of two or more transducers may be used to modify the resulting sound from existing acoustic fields. Referring to FIG. 4, the system illustrated and described in Example 2 may be used to eliminate a signal emitted from a sound source existing at point 1000. As discussed in Example 2, if transducers 100- 600 emit sinusoidal ultrasonic signals with approximately the same amplitude and frequencies of about 280 kHz, 220 kHz, 140 kHz, 100 kHz, 61 kHz, 61 kHz and 40 kHz, the result is a sound event of about l kHz which occurs at an area illustrated as point 1000 in FIG. 4. This 1 kHz sound event will eliminate a signal of approximately 1 kHz emitted from an existing sound source at point 1000 if the sound event is approximately 180° out of phase with the existing sound source. The system of the present invention may be used to alter an existing sound system. Referring again to FIG. 4, transducers 100-400 emit ultrasonic signals with the approximately the same amplitude and frequencies of about 81.1 kHz, 80.6 kHz, 21 kHz and 20.4 kHz respectively, and transducers 500 and 600 emit sonic signals of the approximately the same amplitude and frequencies of 500 Hz, and 600 Hz, respectively. The resultant differential frequency between the signals emitted from transducers 100 and 200 is approximately 500 Hz, and the differential frequency between the signals emitted from transducers 200 and 300 is approximately 600 Hz. These differential frequencies interact with the signals emitted from transducers 500 and 600. These interactions are illustrated on FIG. 4 as intersections. The signals cancel each other out at those intersections where the signals have approximately the same frequency and are about 180° out of phase with each other. If the signals are not completely out of phase with each other, then the interaction with the differential frequencies serves to expand the acoustic field resulting from the signals emitted from transducers 500 and 600.

Thus, a system for creating the natural environment of live sound, or virtual sound is provided. The system may also modify existing sound fields. Although various embodiments have been disclosed, persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration, and not of limitation, and the present invention is limited only by the claims that follow.

Claims

WHAT IS CLAIMED IS:

1. A sound system comprising an array of two or more transducers operating at ultrasonic frequencies.

2. The system of claim 1 wherein the transducers are biased with an ultras. nic frequency.

3. The system of claim 1 further comprising a controller for turning each of the transducers in the array on or off.

4. The system of claim l further comprising a controller for altering the amplitude, frequency or phase of the signals of each of the transducers in the array.

5. The system of claim 3 further comprising a computer in connection with the controller.

6. The system of claim 4 further comprising a computer in connection with the controller.

7. A sound system comprising an array of two or more transducers emitting ultrasonic frequencies.

8. The system of claim 7 wherein the transducers are biased with an ultrasonic frequency.

9. The system of claim 7 further comprising a controller for turning each of transducers in the array on or off.

10. The system of claim 7 further comprising a controller for altering the amplitude, frequency or phase of the signals emitted from each of the transducers in the array.

11. The system of claim 9 further comprising a computer in connection with the controller.

12. The system of claim 10 further comprising a computer in connection with the controller.

13. A sound system comprising an array of two or more transducers detecting sonic or ultrasonic frequencies.

14. The system of claim 13 wherein the transducers are biased with an ultrasonic frequency.

15. The system of claim 13 further comprising a controller for turning each of the transducers in the array on or off.

16. The system of claim 13 further comprising a controller for altering the amplitude, frequency or phase of the signals of each of the transducers in the array.

17. The system of claim 15 further comprising a computer in connection with the controller.

18. The system of claim 16 further comprising a computer in connection with the controller.

19. A method of creating a complex multidimensional acoustic field comprising emitting ultrasonic signals from an array of two or more transducers.

20. The method of claim 19 wherein the transducers are biased with an ultrasonic frequency.

21. The method of claim 19 further comprising altering the number of transducers in the array.

22. The method of claim 19 further comprising altering the amplitude, frequency or phase of the signals emitted from each of the transducers.

23. A method of creating multiple sound events in a complex multidimensional acoustic field comprising emitting ultrasonic signals from an array of two or more transducers.

24. The method of claim 23 wherein the transducers are biased with an ultrasonic frequency.

25. The method of claim 23 further comprising altering the amplitude, frequency or phase of the signals emitted from each of the transducers.

26. A method of creating a single sound event in a complex multidimensional acoustic field comprising emitting ultrasonic signals from an array of two or more transducers.

27. The method of claim 26 wherein the transducers are biased with an ultrasonic frequency.

28. The method of claim 26 further comprising altering the amplitude, frequency or phase of the signals emitted from each of the transducers.

29. A method of modifying an acoustic field with a complex multidimensional acoustic field comprising emitting ultrasonic signals from an array of two or more transducers.

30. The method of claim 29 wherein the transducers are biased with an ultrasonic frequency.

31. The method of claim 29 further comprising altering the amplitude, frequency or phase of the signals emitted from each of the transducers.

32. A method of creating virtual sound with a complex multidimensional acoustic field comprising emitting ultrasonic signals from an array of two or more transducers.

33. The method of claim 32 wherein the transducers are biased with an ultrasonic frequency.

34. The method of claim 32 further comprising altering the amplitude, frequency or phase of the signals emitted from each of the transducers.

35. A method of biasing a transducer comprising inputting an ultrasonic frequency to the transducer in addition to the input of any other signal to the transducer.