Patent Publication Number: US-9900705-B2

Title: Tone generation

Description:
BACKGROUND 
     Mechanical sirens, horns, buzzers, etc. provide distinctive sounds used in various applications such as signaling tones (lunch or break time indicators in factories), warning tones or sirens (severe weather warnings), indicators for sports events in stadiums and arenas (scoreboard buzzer), etc. 
     An example of an electromechanical device for producing such sounds includes a flexible diaphragm, typically made of metal, with a striker that is magnetically activated to move the striker against the diaphragm to generate a tone. Some electronic tone production devices reproduce the sound of mechanical horns and buzzers by simply playing an amplified analog or digital recording of the desired sound through a loud speaker system. Such electronic sound production systems typically include an input signal source, an amplifier circuit and a loudspeaker. 
     Improvements in sound generation systems are desired. 
     SUMMARY 
     In accordance with aspects of the present disclosure, a tone generation system includes a square wave signal generator configured to generate a first series of square wave signals to replicate a fundamental frequency of a desired mechanical tone, and a second series of square waves signals to replicate a second harmonic of the desired mechanical tone. An amplifier is configured to receive the first and second series of square waves, and a speaker is connected to receive an output signal from the amplifier. 
     In accordance with further aspects of the present disclosure, a tone generation method includes generating a first series of square wave signals to replicate a fundamental frequency of a desired mechanical tone, and generating a second series of square waves signals to replicate a second harmonic of the desired mechanical tone. In some implementations, a plurality of series of square wave signals are generated to replicate a respective plurality of harmonics of the desired mechanical tone. The square wave signals are sent to an amplifier, and the amplified signals are played through a speaker. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating an example of a tone generation system in accordance with aspects of the present disclosure. 
         FIG. 2  is an exploded view of the system shown in  FIG. 1 . 
         FIG. 3  is a perspective view illustrating an example of another tone generation system in accordance with aspects of the present disclosure. 
         FIG. 4  is an exploded view of the system shown in  FIG. 3 . 
         FIG. 5  is a plot illustrating sound pressure levels. 
         FIGS. 6A-6D  illustrate examples of the speaker and piezoelectric drivers shown in  FIG. 4 . 
         FIG. 7  is a first perspective view illustrating an example of another tone generation system in accordance with aspects of the present disclosure. 
         FIG. 8  is an exploded view of the system shown in  FIG. 7 . 
         FIG. 9  is a second perspective view illustrating the tone generation system shown in  FIGS. 7 and 8 . 
         FIG. 10  is another exploded view of the system shown in  FIG. 9 . 
         FIG. 11  is a block diagram illustrating an example of a tone generation system in accordance with aspects of the present disclosure. 
         FIG. 12  illustrates an analog waveform of a mechanically-produced tone to be replicated. 
         FIG. 13  is a scope screen shot illustrating an example of an analysis of the analog waveform shown in  FIG. 12 . 
         FIG. 14  illustrates an example of a square wave signal generated in accordance with aspects of the present disclosure. 
         FIG. 15  illustrates an analog waveform of the sound produced by a square wave signal generated in accordance with aspects of the present disclosure. 
         FIG. 16A  is a chart showing sinusoidal waveforms representing a fundamental frequency and a harmonic, and a generated square wave. 
         FIG. 16B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 16A . 
         FIG. 17A  is a chart showing the pulse width and gap of the square wave modified from the square wave shown in  FIG. 16A . 
         FIG. 17B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 17A . 
         FIG. 18A  is a chart showing the pulse width and gap of the square wave modified from the square wave shown in  FIGS. 16A and 17A . 
         FIG. 18B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 17B . 
         FIG. 19A  is a chart showing sinusoidal waveforms representing a fundamental frequency and a harmonic, and a generated square wave. 
         FIG. 19B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 19A . 
         FIG. 20A  is a chart showing the pulse width and gap of the square wave modified from the square wave shown in  FIG. 19A . 
         FIG. 20B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 20A . 
         FIG. 21A  is a chart showing sinusoidal waveforms representing a fundamental frequency and a harmonic, and a generated square wave. 
         FIG. 21B  is a chart illustrating resultant amplitude levels for the fundamental frequency and harmonics shown in  FIG. 19A . 
         FIG. 22  is a schematic diagram illustrating an example tone production circuit in accordance with aspects of the present disclosure.  FIGS. 22A-22G  are close-up views of respective sections of the circuit shown in  FIG. 22 . 
         FIG. 23  is a schematic diagram illustrating another example tone production circuit in accordance with aspects of the present disclosure.  FIGS. 23A-23J  are close-up views of respective sections of the circuit shown in  FIG. 23 . 
         FIG. 24  is a schematic diagram illustrating another example tone production circuit in accordance with aspects of the present disclosure.  FIGS. 24A-24S  are close-up views of respective sections of the circuit shown in  FIG. 24 . 
         FIGS. 25A-25D  illustrate examples of square wave signals for replicating sounds of mechanical devices. 
     
    
    
     DETAILED DESCRIPTION 
     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as top, bottom, front, back, etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense. 
     Some electronic tone production devices reproduce the sound of mechanical horns and buzzers by simply playing an amplified analog or digital recording of the desired sound through a loud speaker system. Such electronic sound production systems typically include an input signal source, an amplifier circuit and a loudspeaker. 
     With digital sound recording, digital audio is directly recorded to a storage device as a stream of discrete numbers. The analog sound signal is transmitted from an input device to an analog-to-digital converter (ADC), which converts the signal by repeatedly measuring the momentary level of the analog (audio) wave and then assigning a binary number with a given quantity of bits (word length) to each measuring point. The frequency at which the ADC measures the level of the analog wave is called the sample rate, and a digital audio sample with a given word length represents the audio level at one moment. To playback the sound, the binary numbers are transmitted from the storage device into a digital-to-analog converter (DAC), which converts the numbers back to an analog signal using the information stored in each digital sample, thus rebuilding the original analog waveform. This signal is then amplified and played through loudspeakers. 
     Some form of data storage is required for storing the recorded sounds, as well as complicated processing devices and associated circuitry. Further, reproducing the sounds in this manner requires a complicated and powerful amplifier, which requires bigger devices that generate undesirable heat and consume considerable power. 
     Various examples of sound generation systems are disclosed herein, where a small, portable system is provided that replicates mechanically produced sounds such as buzzers, horns, sirens, etc.  FIGS. 1 and 2  illustrate one example system  10  that includes a housing  100 , a speaker  102 , a baffle  104  and a circuit board assembly  106 . In some examples, the height h, width w and depth d dimensions of the system  10  are all less than 5 inches, and in one particular implementation the dimensions are 4.40×4.40×3.53 inches. 
       FIGS. 3 and 4  illustrate another example system  12  that also includes a housing  100 , a speaker  102  and a circuit board assembly  106 , as well as a grille  110  situated over the housing  100 . The system  12  further includes piezoelectric devices  112  in addition to the speaker  102  for playing the desired sounds. The piezoelectric devices  112  are received in a frame  114  with a gasket  116  positioned between the piezoelectric devices  112  and the frame  114 . In some examples, the height h, width w and depth d dimensions of the system  12  are all less than 5 inches, and in one particular implementation the dimensions of the system  12  are 4×4×2.8 inches. 
     The example shown in  FIGS. 3 and 4  includes a 2.5 inch, 25 ohm speaker  102  with a frequency response of 200 Hz to 5 kHz (−10 dB). Three piezoelectric horn drivers  112  are received in the frame  114  so as to position the piezoelectric devices  112  in front of the cone of the speaker  112  to allow the low frequency sound from the moving coil speaker  112  to pass between the piezoelectric devices  112  and obtain the desired frequency response and sound pressure level while minimizing the a small physical size of the package.  FIG. 5  compares sound pressure levels across a frequency range for a device such as that illustrated in  FIGS. 3 and 4  (plot  150 ), compared to a device using only piezoelectric drivers (plot  152 ). As shown in  FIG. 5 , the inclusion of the cone speaker  102  significantly increases the sound pressure level at lower frequencies (e.g., below about 1,700 Hz). 
       FIGS. 6A-D  illustrate the speaker  102  and the piezoelectric devices  112  of the example system  12  shown in  FIG. 3 . As noted above, the three piezoelectric devices  112  are positioned in front of the cone of the speaker  112  to allow the low frequency sound from the moving coil speaker  112  to pass between the piezoelectric devices  112 . More particularly, in the illustrated example, the speaker  102  is connected to receive an output signal from an amplifier to generate a desired tone as discussed further herein below. The speaker  102  includes a speaker cone  130  connected to a frame  132  such that the cone  130  can move (vibrate) relative to the frame  132  in response to the signal received from the amplifier. A magnet  134  is situated adjacent one side of the speaker cone  130  opposite the frame  132 . 
     The speaker  102  defines a speaker diameter SD, which in the illustrated example is about 2.6 inches. The piezoelectric drivers  112  are adjacent the speaker cone, opposite the frame  132  and magnet  134 . The piezoelectric drivers  112  each define a driver diameter PD, which is about 1.9 inches in the illustrated example. The piezoelectric drivers  112  are arranged such that a first portion of each driver diameter is within the speaker diameter and a second portion of each driver extends beyond the speaker diameter. In other words, if an imaginary cylinder were extended from the periphery of the speaker  102 , a portion of each piezoelectric driver  112  would be within the cylinder and a portion of each piezoelectric driver  112  would extend beyond the cylinder. In  FIG. 6C , a broken line  140  defines the speaker diameter and represents such a cylinder. Thus, each of the piezoelectric drivers  112  has a first portion  112   a  inside the speaker diameter  140  and a second portion  112   b  extending outside the diameter  140 . 
     As shown in  FIG. 6B , each of the piezoelectric drivers  112  includes a piezoelectric bimorph  136  with a cone  138  attached thereto. Each of the cones  138  of the piezoelectric drivers  112  defines a center axis  142 , and in some embodiments, at least one of the piezoelectric drivers  112  has its center axis  142  situated within the speaker diameter SD. In the illustrated example, two of the center axes  142  are completely within the speaker diameter SD and the lower piezoelectric driver  112  shown in  FIGS. 6C and 6D  has its axis on the periphery  140  of the speaker diameter SD. 
     As noted above, gaps between the piezoelectric drivers  112  are provided to allow sound from the speaker  112  to pass between the piezoelectric drivers  112 . In the illustrated example, the top two piezoelectric drivers  112  are positioned with a minimal gap G 1  therebetween, less than 0.1 inches in the illustrated example. A larger second gap G 2  is provided between the top piezoelectric drivers  112  and the bottom driver  112 , about 0.14 inches in the example of  FIG. 6C . The illustrated piezoelectric drivers  112  each have a diameter PD of about 1.9 inches, and the distance D 1  between the centers  142  of the top piezoelectric drivers  112  is thus also about 1.9 inches or slightly larger to achieve the small gap G 1  between the horizontally-aligned top devices  112 . The distance D 2  between the centers  142  of the top piezoelectric devices  112  and the bottom piezoelectric device  112  center  142  is about 1.8 inches. As shown in  FIG. 6B , the piezoelectric drivers  112  are situated such that there is a distance D 3  between the front of the piezoelectric driver cones  138  and the front of the speaker cone  132  so as to provide space between the piezoelectric drivers  112  and the speaker  102  in an axial direction. In the example of  FIG. 6B , the distance D 3  is about 0.8 inches. 
       FIGS. 7-10  illustrate yet another example system  14 . The system  14  includes a main housing  100 , as well as an inner housing  103 . The main housing  100  receives a driver speaker  102  and driver cap  120 . A rear housing  110  is positioned opposite the main housing  100 . A circuit board assembly  106  is situated between the rear housing  110  and the inner housing  103 . In one particular implementation, the height h, width w and depth d dimensions of the system  14  are about 7 inches, 9 inches and 8.7 inches, respectively. The example system  14  illustrated in  FIGS. 7-10  uses a compression driver loudspeaker, in which the housing components  100  and  103  form a horn that receives the driver speaker  102  and driver cap  120 . 
     The illustrated example systems  10 ,  12 ,  14  are each configured to produce tones that replicate the sounds of desired mechanical sound production devices. The disclosed systems produce tones with a frequency harmonic content and thus create sounds that are similar to mechanical sirens and horns, for example. In some embodiments, this is done through the use of a combination of square wave signals compiled into a train of varying pulse widths and spaces to generate the desired harmonics while controlling the polarity of the pulses at key points in the waveform to re-enforce or suppress other harmonics. 
       FIG. 11  is a block diagram conceptually illustrating further aspects of the systems  10 , 12 , 14 . Generally, each of the systems  10 , 12 , 14  include a power input section  200  that is configured to receive an AC or DC voltage, depending on the particular configuration. A signal generator  202  generates electrical signals to create a predetermined sound, and the signals output by the signal generator  202  are received by an amplifier  204  that provides amplified signals to the speaker  102 . 
     In some implementations, the signal generator  202  is a square wave generator that is configured to generate a first series of square wave signals that replicate a fundamental frequency of a desired mechanical tone, and also to generate further series of square waves signals to replicate respective harmonics of the desired mechanical tone. Because the signal output by the square wave generator  202  is comprised of square waves, it allows for the use of a simple digital amplifier  204  that then applies the modulated pulse train to a speaker. This avoids the inefficiency and heat produced by linear amplifiers and the complexity of class “D” PWM amplifiers. This simplicity improves reliability and durability over that of mechanical sirens, horns and buzzers and reduces size and cost. 
     In some implementations, the pulse width and spacing for the various series of square wave signals are determined by analyzing the sound to be replicated. For example, an inverse Fourier transform can be performed on the sound to be replicated to determine the fundamental frequency and harmonics and their associated levels.  FIG. 12  shows an analog waveform  210  of a mechanically-produced tone to be replicated. The mechanical device that produced the tone includes a metal diaphragm being struck 120 times per second. The diaphragm rings at a fundamental frequency and the metal forming the diaphragm adds harmonics.  FIG. 13  is a scope screen shot  212  illustrating an example of an analysis of the analog waveform  210 . The desired mechanical tone is thus analyzed to determine the fundamental frequency and level thereof, as well as frequency and level of the desired harmonics. 
       FIG. 14  illustrates an example of a square wave signal  214  output by the square wave generator  202  to replicate the sound represented by the waveform  210  shown in  FIG. 12 . The square wave signal  214  includes groups of pulses  216 . In one example, the pulses in the groups  216  produce a tone with a fundamental frequency of 1900 Hz, and the width of the pulses sets the strength of the fundamental and the second and third harmonics. Further, the fundamental is chosen to be the frequency where the cone of the speaker  102  will ring and add harmonics. The groups of pulses  216  are turned on and off 120 times per second to reproduce the sound of the metal diaphragm being rung 120 times per second. 
       FIG. 15  shows an analog waveform of the sound produced by the square wave signal  214  amplified and played through the speaker  201 . In some implementations, the process of determining the various aspects of the square wave signal  214  includes analyzing the analog waveform  218 , and adjusting the pulse width and/or polarity of the square wave signals based on the analysis. The waveform  218  created by the square wave signal  214  can be compared to the wave form  210  of the mechanically produced tone, and the pulse width and/or polarity of the square wave signals can then be adjusted until the waveforms  214  and  218  are satisfactorily similar. 
       FIGS. 16-21  conceptually illustrate aspects of a process for creating the square wave signal  214  output by the square wave generator  202 .  FIG. 16A  shows a first sinusoidal waveform representing a fundamental frequency  220  and a second sinusoidal waveform representing a second harmonic  222 . A first square wave  230  is shown that is out of phase or has no energy in phase with the second harmonic  222 . The first square wave  230  therefore will suppress the second harmonic  222  as shown in  FIG. 16B .  FIGS. 17A and 18A  show the pulse width and gap between pulses of the first square wave  230  changing to move energy into phase with the second harmonic  222 , resulting in increased levels of the second harmonic as shown in  FIGS. 17B and 18B . 
     In  FIG. 19A , a third sinusoidal waveform is shown representing a third harmonic  224 . When the square wave signal  230  is out of phase or has no energy in phase with the third harmonic  224 , its level is suppressed as shown in  FIG. 19B . In  FIG. 20A , the pulse width of the square wave signal  230  is changed to move energy into phase with the third harmonic  224 , resulting in the level of the third harmonic  224  rising as shown in  FIG. 20B . 
     Thus, pulses of various widths and/or spaces are generated to replicate the fundamental tone and harmonics of the desired mechanical tone. Adjusting the spacing, width and polarity of the square wave pulses  230  provides a way to selectively emphasize or deemphasize the fundamental and various harmonics match the original sound.  FIG. 21A  illustrates an example where the square wave pulses  230  are generated such that energy has been removed from the fundamental  220  and added to the second harmonic  222 . 
     Replicating the desired mechanical tone using square wave signals allows the use of a simple amplifier  204 , which reduces the number of components required, improves efficiency, eliminates or reduces the need for heat sinks, and lowers overall cost. One example implementation operates at an efficiency between 97% and 98%. Because the amplitude of the square wave pulses  230  is fixed, the energy in each harmonic is relative to the width and phase of the pulse. Since the square wave signal is very simple (composed of rectangular functions), the levels can be determined using a discrete Fourier transform. 
       FIG. 22  is a schematic diagram illustrating aspects of an example of a disclosed tone generation system.  FIGS. 22A-22G  provide detailed views of the respective portions of the circuit  301  shown in  FIG. 22 . In one implementation, the circuit  301  shown in  FIG. 22  is employed in the system  12  shown in  FIGS. 3 and 4 , though the circuit  301  and/or various aspects thereof could be used in other physical implementations. The circuit  301  operates on DC power, so the power input section  200  includes voltage input terminals  308  configured to receive a DC voltage input. Components such as a transistor  310  control the start-up voltage. The power input section  200  provides power for the tone generator  202  and amplifier  204 . 
     The tone generator  202  includes a timer  312 , which in the illustrated example is a LM556 dual timer available from Texas Instruments (www.ti.com). Using the timer  312  to generate the square wave signal simplifies the circuit, eliminating the need for a clock which, in turn, reduces heat generated so that heat sinks are not required. The timer  312  includes a voltage input  314  for receiving an input voltage from the power input section  200 . The timer  312  is configured to generate square wave signals to replicate a fundamental frequency and harmonics of a desired mechanical tone. The timer  312  further includes an output terminal  316  connected to a signal input  320  of the amplifier  204 . 
     As noted above, using square wave signals to replicate the desired mechanical tone allows for the use of a simple amplifier. In the example shown in  FIG. 22 , the amplifier  204  includes a single transistor  322 . A voltage input terminal  324  of the amplifier  204  (source of transistor  322 ) receives an input voltage from the power input section  200 , and an output terminal  326  of the amplifier (drain of transistor  322 ) is connected to terminals  330  of the speaker  102 . The signal input  320  (gate of transistor  322 ) receives the square wave signal directly from the timer  312 . 
     Since there is only the single transistor  322  in the amplifier  204 , only one portion of the square wave signal provided to the speaker  102  from the timer  312  is amplified. The speaker  102  (and piezoelectric devices  114  in the illustrated embodiment) include “+” and “−” speaker terminals  330   a ,  330   b . Only the + terminal  330   a  receives an amplified signal. A typical amplifier includes at least two devices to source voltage to a speaker in response to an input signal. Thus, the cone of the speaker is typically “pushed” and “pulled” in response to respective portions of the input signal. With the illustrated amplifier  204  including the single transistor  322 , only one portion of the input square wave signal is amplified such that movement of the speaker  102  is amplified in one direction only. During the non-amplified portion of the input signal, the speaker is allowed to ring naturally, which creates additional harmonics. 
       FIG. 23  is a schematic diagram illustrating aspects of another example of a disclosed tone generation system, with  FIGS. 23A-23J  providing detailed views of the respective portions of the circuit  302  shown in  FIG. 23 . The circuit  302  shown in  FIG. 23  is configured to receive an AC input voltage, and thus voltage input terminals  308  connect to an AC power source. The power input section  200  includes a transformer  340  and full wave rectifier  342 . As with the circuit  301  shown in  FIG. 22 , the power input section  200  includes a transistor  310  for suppressing an initial power surge upon start-up. The tone generator  202  in the circuit  302  also includes a timer  312 , which in the illustrated example is a Texas Instruments LM556 dual timer. The timer  312  includes a voltage input  314  for receiving an input voltage from the power input section  200 . The timer  312  is configured to generate square wave signals to replicate a fundamental frequency and harmonics of a desired mechanical tone. The output terminal  316  of the timer  312  is connected to the signal input  320  of the amplifier  204 . As with the example shown in  FIG. 22 , the amplifier  204  shown in  FIG. 23  includes a single transistor  322 . The voltage input terminal  324  of the amplifier  204  (source of transistor  322 ) receives the input voltage from the power input section  200 , and the output terminal  326  of the amplifier (drain of transistor  322 ) is connected to terminals  330  of the speaker  102 . The signal input  320  (gate of transistor  322 ) receives the square wave signal directly from the timer  312 . 
     The circuit  302  shown in  FIG. 23  includes jumper connections  332  configured to vary the volume of the tone output from the speaker  102 . In the illustrated example, the jumper connections  332  arranged to select between low and high power outputs. Selecting the desired jumper connection  332  varies the width of the square wave pulses to vary the volume of the tone. In one embodiment, the circuit  302  is used in the system  10  illustrated in  FIGS. 1 and 2 . 
       FIG. 24  illustrates aspects of another example of a disclosed tone generation system.  FIGS. 24A-24S  provide detailed views of the respective portions of the schematic diagram shown in  FIG. 24 . In one implementation, the circuit  303  shown in  FIG. 24  is employed in the system  14  shown in  FIGS. 7 and 8 , though the circuit  301  and/or various aspects thereof could be used in other physical implementations. Similarly to the circuit  301  shown in  FIG. 22 , the circuit  302  operates on DC power so the power input section  200  includes voltage input terminals  308  configured to receive a DC voltage input. AC versions are also possible. 
     The circuit  303  shown in  FIG. 24  is configured to selectively generate multiple sounds. The particular version shown generates four sounds. To make the different square wave signals to produce the respective sounds, the tone generator  202  includes a microcontroller  340 , which in the illustrated example is an ATtiny84A microcontroller available from Atmel (www.atmel.com). Push on jumper connections  342  are provided for selection of the desire sound. Using the microcontroller  340  to generate the square wave signals allows for more variation in the types of sounds produced and the number of different sounds the device can generate. 
     Output signals from the microcontroller  340  are received by a driver  344  that boosts the output square wave signals to levels appropriate for the amplifier  204 . In the illustrated circuit  303 , the drive  344  is an LM5110 driver available from Texas Instruments. The boosted signals are then output to the amplifier  204 . In the example shown in  FIG. 24 , the amplifier  204  includes first and second transistors  322   a ,  322   b  that receive respective outputs from the driver  344 . The first and second transistors  322   a , 322   b  provide the amplified square wave signal to a transformer  346  that drives the speaker  102 . 
       FIGS. 25A-25D  illustrate examples of square wave signals generated for producing the four respective sounds output by the circuit  303 . In  FIG. 25A , the wave  400  has a period of 1,210 μs with a 74.05% duty cycle. Two pulses  402 , 403  are each 448 μs long, separated by an off period of 157 μs. The width and shape of the pulses  402 ,  403  of the wave  400  generate the first through the fifth harmonics of the sound being replicated. The space  404  between the pulses  402 ,  403  suppresses the second and fourth harmonics. With the wave  400  shown in  FIG. 25A , the second harmonic is suppressed more than the wave form being replicated because the response of the speaker  102  boosts the level of the second harmonic.  FIG. 25B  illustrates a similar waveform  410  that has a period of 945 μs with a duty cycle of 60%. Two pulses  412 , 413  are each 333.6 μs long, separated by off periods  414  of 138 μs. The waveform  410  produces a siren sound that winds up to a higher pitch than the sound produced by the waveform  400  illustrated in  FIG. 25A . 
       FIGS. 25C and 25D  illustrate waveforms  420 ,  430  that replicate the sound produced by mechanical horns that have a vibrating metal diaphragm with a trumpet horn. The waveform  420  shown in  FIG. 25C  has a period of 3,168 μs with a 33% duty cycle. The waveform  420  includes two short pulses  422 , 423  that are each 132 μs, and two long pulses  424 , 425  that are each 396 μs long. Off periods  426  of 132 μs separate the pulses  422 ,  424 ,  425 ,  423 . The waveform  430  illustrated in  FIG. 25D  has two short pulses  432 ,  433  that are each 110 μs separated by an off period  434  of 13 μs. A long pulse  436  is 233 μs long, separated from the short pulse  433  by an off period  438  of 80 μs. 
     Various modifications and alterations of this disclosure may become apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that the scope of this disclosure is not to be unduly limited to the illustrative examples set forth herein.