the invention is not limited to the following described embodiments , which provide readily adaptable implementations . one skilled in the art will see enhancements that keep within the scope described by the embodiments , and other devices in which the present invention may be disposed . as background , the teachings of u . s . pat . no . 6 , 809 , 635 are briefly reviewed . u . s . pat . no . 6 , 809 , 635 describes a mobile station that uses a conventional vibration motor to perform the additional function of reproducing audio communications received by the user of the mobile terminal . the use of the vibration motor either eliminates altogether the requirement for a separate loudspeaker or enhances the reproduction of lower frequency components of the received audio communications which permits the loudspeaker to function as a tweeter to reproduce only the higher frequency components of the received audio communications to produce an overall better reproduction of the received audio communication . in one embodiment reproduced here as fig1 a , u . s . pat . no . 6 , 809 , 635 describes applying a demodulated output signal 104 , in the form of audio communications , to an input of an audio amplifier 106 that amplifies the input audio communications to a sufficient signal level to drive the vibration motor 108 to reproduce an audible sound 110 . the gain level is chosen to provide the required output sound level , and the audio amplifier 106 may utilize amplification , which need not be a linear function of frequency so as to better reproduce the natural sound of the caller . in another embodiment reproduced here as fig1 b , u . s . pat . no . 6 , 809 , 635 describes disposing a capacitor 202 between the audio amplifier 106 and a speaker 204 . in this embodiment , the amplifier 106 drives both the speaker 204 and the vibration motor 108 , but the capacitor 202 acts as a high pass filter , substantially attenuating the lower frequencies so the speaker 204 acts as a tweeter that reproduces 212 only the higher frequency components of the audio signal 104 , leaving only the vibration motor 108 to reproduce 210 the lower frequency components . a third embodiment reproduced here as fig1 c uses a cross - over network ( passive or active ) to selectively split the signal output from the audio amplifier 106 into two frequency - distinguished channels 304 , 306 . the lower - frequency channel 304 feeds an input of the vibration motor 108 , and the higher - frequency channel 306 feeds an input of the speaker 204 . a fourth embodiment of u . s . pat . no . 6 , 809 , 635 , reproduced here as fig1 d , describes splitting the input signal among two branches , the signal on each branch then being frequency distinguished by high and low pass filters 310 , 308 respectively . the output of these filters 310 , 308 is then separately amplified by amplifiers 314 , 312 of comparatively different gains . one amplifier 314 drives the speaker 204 and the other amplifier 308 drives the vibration motor 108 . the different gains reflect the difference in efficiency between the speaker / tweeter 204 and the vibration motor 108 . in each of the above embodiments of u . s . pat . no . 6 , 809 , 635 , the vibration motor 108 may be driven separately in its traditional silent - alert mode by a signal indicative of an incoming page / phone call rather than by an audio signal 104 . now embodiments of this invention are described , wherein the audio signal is mapped to a driving signal for the vibration mechanism . preferably only the filtered low frequency components thereof are mapped , the high frequency components being filtered previous to the mapping , such as by the low pass filter 308 ( fig1 d ) of the cross - over network ( fig1 c ). an embodiment adapting fig1 d is shown in fig2 . in fig2 , an audio signal 20 is split into two parallel paths 20 a , 20 b . the audio signal may derive from a locally stored digital music file , a demodulated voice signal or internet - based streaming audio ( e . g ., a webcast or a “ podcast ”) received over a wireless link , or any of a variety of other audio signal sources wired or wireless . preferably , only musical audio signals are mapped as described herein , where the device recognizes signals as musical audio signals by , for example , the compression format of those signals . consider the audio signal 20 to be composed of both high and low frequency components , wherein frequency refers to frequency as transduced by a conventional speaker into an audible form between about 15 - 20 , 000 hz , the normal range of human hearing for a young adult . low frequency components may be those ranging up to about 2 , 000 hz , 5 , 000 hz , or 10 , 000 hz , though any cutoff between low and high frequency components may be chosen based on specific response of hardware ( e . g ., the vibration mechanism ) chosen for a particular embodiment . the high frequency components are then the remaining higher frequencies of the overall audible band . of course , a mid - range speaker ( not shown ) may be used for fuller sound quality by transducing only those frequencies between the higher frequency components and the lower frequency components . for simplicity , the following description assumes no mid - range speaker . processing of the signal 20 along the first path 20 a that transduces the high frequency components is as previously described : a high pass filter 22 attenuates low frequency components and allows only high frequency components to pass , a first amplifier 24 amplifies with a first gain and drives a tweeter 26 with its output . the first gain is matched to the audio response of the tweeter 26 as is well known in the acoustic arts . generally , the first gain is relatively small as the actual transducing components ( e . g ., the former and cone ) of the tweeter 26 are not massive and their vibration requires little power . processing of the signal 20 along the second path 20 b that transduces the low frequency components is now described . the second path 20 b is of course parallel to the first 20 a , as both high and low components of the input signal 20 are re - combined in air once transduced . a low pass filter 28 attenuates high frequency components and allows only low frequency components to pass . a vibration mechanism , such as the electric vibration motor 30 previously described , is coupled to the output of the low pass filter 28 through a signal - converter block 32 . the signal - converter block 32 converts intensity of the low frequency component signal that is input into the block 32 into a driving signal length for the vibration motor 30 that is output from the block 32 . in some embodiments , the vibration motor may vibrate within the range of human hearing to act as a low frequency speaker or sub - woofer . in other embodiments , the motor does not vibrate in place of a woofer / sub - woofer but in conjunction with it to provide a bass effect or ‘ kick ’ stimulus . for example , the hpf 22 may removed so that the speaker 26 outputs both high and low human hearing frequencies , or another low - pass filtered output of the signal 20 may feed a traditional low - frequency speaker 27 through another amplifier 25 in addition to those components shown in fig2 . as shown in fig2 , a third parallel signal path 20 c passes the signal 20 through a separate low - pass filter whose pass frequency may differ from the first lpf 28 given the differences between speaker 27 and vibration motor 30 . alternatively , the output of a single low pass filter 28 may feed both the low frequency speaker 27 and the vibration motor 30 , with the mapper 32 clipping the signal that it outputs to the vibration motor 30 so that it provides a ‘ kick ’ stimulus simultaneous with low frequency tones output from the low frequency speaker 27 . the kick stimulus is felt by a user through the touch sensation as vibrations of the device body , not necessarily as audible tones . fig3 shows a further detail of an embodiment of the signal - converter block 32 , with representations of the signal present above the signal converter block 32 at various nodes between components of the block 32 . the low frequency component signal present at input node 40 has a sinusoidal waveform as illustrated above the input node 40 . a rectifier diode 42 , forward biased to allow current only from left to right as illustrated in fig3 , rectifies that sinusoidal waveform to produce at a first intermediate node 44 a rectified waveform as illustrated above that first intermediate node 44 . there are various forms a rectified waveform can take ; that illustrated is merely a common one . a voltage block 46 then quantifies an intensity ( voltage ) of the rectified signal , which is output on a second intermediate node 48 . a mapper 50 , which may be for example a monostable multivibrator , a general purpose processor , or a digital signal processor , converts the signal intensity from the second intermediate node 48 to a pulse length , which is output to a third intermediate node 52 and illustrated above that third intermediate node 52 as a square pulse with a determinate length in milliseconds . a driver 54 then amplifies the signal on the third intermediate node 52 to drive the vibration motor 30 ( fig2 ) with , a pulse length representative of the low frequency component ( at node 40 ) of the original audible signal 20 ( fig2 ). the driver 54 may be a part of the signal - converter block 32 or separate from it , but is preferably between the mapper 50 and the vibration motor 30 . other implementations are also viable to embody the inventive concept . for example , rather than a rectifier diode and voltage block , the entire input signal at the input node 40 may be processed in a digital signal processor by a series of transistors to convert input signal amplitude ( which may be determined in any number of manners , rms , squared , etc .) to a pulse length used to set a driving frequency of the vibration motor 30 . various different embodiments and implementations will be recognizable to those skilled in the art ; the embodiment of fig3 is seen as advantageous for implementation in a mobile station due to its minimal use of processing power , relatively small size , and cost effectiveness . the various embodiments such as that of fig3 may also be imposed between the cross - over network and the vibration motor 108 of fig1 c , or between a low - pass filter and the vibration motor of fig1 b . the above description generally contemplates direct mapping , where each pulse of the input signal 20 that passes a low pass filter 28 is realized as an excitation of the vibration motor 30 . in some instances , this may result in excitation pulses to the motor that are too frequent , leaving a bass sound that is nearly continuous . in some instances , this will give a sub - woofer sound that is irritating , or at least which is not perceived by a user as enhancing the listening experience . where end users experience an irritating sub - woofer for certain music , they may tend not to prefer the feature of a vibration motor 30 as sub - woofer at all . to this problem , embodiments of the present invention employ the concept of adaptive pulse control . adaptive pulse control uses appropriate dynamic control of the filtered low frequency sound , in order to adapt the amount of vibration to be at a minimum but still perceivable in a wider range of music content , either as humanly audible or vibrations to be felt . in this arrangement , compression ( limiter ) and automatic gain control ( agc ) algorithms are used . these enable an implementation , where basically short pulses of vibration are output and spaced from one another , instead of more continuous vibrations following the low frequency notes in some music . these are illustrated in fig4 a through 6 . fig4 a illustrates a graph of low frequency signal intensity over time . the solid bass - intensity line 51 represents an analog version of the lower frequency portions of the input audio signal 20 . consider this an analog version of a signal output from the low pass filter 28 that is input into the signal converter block 32 . the vibration pulses 53 result from pulse - width mapping the intensity of the bass - intensity line 51 in the signal converter block 32 . direct mapping is shown in fig4 a , where each pulse - width modulated pulse mapped from the input signal ( represented as line 51 ) is an excitation pulse to the vibration motor 30 . direct mapping a first bass movement a yields six excitation pulses 53 that are output to the vibration motor 30 . similarly , a second bass movement b yields two excitation pulses 53 to the vibration motor 30 , and a third bass movement c yields four excitation pulses 53 . as illustrated in fig4 a , each set of pulses 53 associated with a single bass movement are closely spaced in time . since the pulse length derives from the bass intensity , these excitation pulses can 53 blend together when experienced by a user , especially considering the mechanical / inertial constraints of the physical vibration motor and its unbalanced rotor . fig4 b illustrates an adaptive pulse control solution , where each bass movement a , b , c , results in only a single pulse 53 output to the vibration motor 30 . this may be considered a ‘ kick ’ effect , wherein each bass movement begins with a sub - woofer imitating pulse to the vibration motor 30 . using the ‘ kick ’ effect , the adaptive pulse control may be used in addition to a traditional woofer or other speaker , rather than in place of a woofer / low - frequency response speaker . fig5 a - b illustrate one implementation to achieve the result of fig4 b . fig5 a is identical to fig4 a , and fig5 b is identical to fig4 b except for the addition of an adaptive threshold 55 and an adaptive time delay 57 in fig5 b . the adaptive threshold 55 is a threshold intensity at which the signal converter block 32 is allowed to provide an output that drives the vibration motor 30 . any input signal with intensity below that threshold 55 yields no excitation pulse 53 to the vibration motor 30 . whether the pulses corresponding to signal intensity below the threshold 55 are generated at the signal converter block 32 and inhibited from driving the motor 30 , or not generated in the signal converter block 32 at all , are variances in implementation . upon the bass intensity line 51 positively crossing the threshold level 55 , shown in fig5 b by reference numbers 59 , the signal converter block 32 becomes active and maps the input signal to an output pulse 53 ( or any inhibiting of generated pulses is suspended in an alternate implementation ). in one embodiment , an adaptive time delay 57 is imposed from the start of the output pulse 53 , or from the instant 59 at which the bass intensity positively crossed the adaptive threshold 55 . during that adaptive time delay 57 , the signal converter block 52 is prevented from providing an output excitation signal to the vibration motor 30 , resulting in a ‘ dead time ’ for which the actual bass intensity line 51 is irrelevant to stimulation of the vibration motor 30 , by inhibiting generation of additional pulses or blocking all pulses except the first pulse during that ‘ dead time ’. the inventors have concluded that a span of about 100 - 200 milliseconds is an appropriate adaptive time delay 57 for enhancing the user sub - woofer / vibration experience , though other time periods may be used in keeping with this invention . in another embodiment , the signal converter block 32 is prevented from providing an output ( or the output is blocked from the vibration motor 30 ) until after the bass intensity line 55 crosses the threshold 55 in the negative direction , shown as reference numbers 59 ′ in fig5 b . of course , the first excitation pulse 53 sent after the bass intensity line 51 first crossed the threshold 55 in the positive direction is not inhibited from being sent to the motor 30 . in order to avoid short - spaced excitation pulses 53 when the bass intensity line 51 varies frequently across the threshold 55 , one threshold may be used for the positive crossing 59 to generate one pulse and a different , preferably lower threshold may be used for the negative crossing 59 ′ to enable the signal converter block 32 to again provide an output to the motor 30 . only one threshold 55 is needed for the adaptive time delay 57 embodiment . fig6 a - b illustrate the advantages of adaptive pulse control as compared to the direct mapping . in the direct mapping shown in fig6 a , lower intensity of bass notes yield minimal pulse strength / duration , which the user experiences as minimal or no vibration at all from the vibration motor 30 . conversely , at high bass note intensity , direct mapping yields very intense pulse strengths ( very long pulses ), which to a user can overwhelm other sounds of the audio signal 20 produced by conventional speakers 26 , or be experienced as continuous vibration without clear relation to the underlying music . with adaptive pulse mapping as illustrated in fig6 b , the vibration pulse intensity ( duration ) is limited by the gain control and signal compression that are typically already applied to the signal 20 . as was discussed above , gain control applied to different speakers ( e . g ., tweeter and woofer ) varies based on the signal , as well known in the art . also known in the art is to vary compression of the signal 20 based on the frequency response , yielding a compression curve . this is inherent in the compression format used to digitize the musical audio signal , and common compression formats for music include mp3 and aac . the portable device already uses gain control and compression in order to process the audio signal . gain applied to a portion of the signal rises with signal intensity in order to drive the traditional speaker properly . that same gain control curve , and the corresponding compression curve , are used to adaptively control the vibration pulse output from a bass signal input . both those curves depend in part from signal intensity , especially agc . in fig6 b , the slope of the adaptive curve ( in solid lines ) results from adapting to signal compression , while the position of the solid line curve varies along the horizontal axis based on gain control . implementation of fig6 b is shown in simplified block diagram form at fig7 , which is imposed between the low pass filter 28 and the signal converter block 32 of fig2 . the output from the low pass filter 28 ( fig2 ) is input into an automatic gain control agc block 61 which applies an agc algorithm . many such algorithms are known in the digital audio signal processing arts for driving a speaker . a compression block 63 then applies a compression algorithm to the signal , shifting the slope of the intensity / pulse length curve from the dotted line ( direct mapping ) to the solid line ( adaptive mapping ) as shown in fig5 b . the output of the compression block 63 then becomes the input at node 40 of fig3 for the previously described signal converter block 32 . for implementation within a mobile station , the voltage of the vibration pulse may be pulse - width modulated pwm at 100 % for optimum fidelity where the vibration motor is used as an audible subwoofer , though in some embodiments this may be changed to some value greater than or less than 100 % so that the pulse width ( node 52 ) is proportionally scaled to voltage amplitude ( node 48 ). it is also preferable that the pulse length ( node 52 ) is constrained to be less than about 50 milliseconds . the inventors have determined that pulse lengths above about 50 milliseconds tend to exceed the nominal rpm , and the subwoofer starts to feel too intense , leading to an uncomfortable user experience . in addition , if played longer than about 50 milliseconds , the experience to the user is no longer a ‘ kick ’ but more or less a continuous stimulation leading to an uncomfortable user experience . in general , the ideal operating range for embodiments of the present invention , when used strictly as an audio device rather than providing a sensory ‘ kick ’ accompanying traditional speaker output , is seen to be between about 70 hz and about 200 hz . commonly available vibration motors 30 tend not to be sufficiently responsive to reproduce sound below 70 hz , and above 200 hz the result is seen as overstimulating and uncomfortable . note that pulse width modulation , pulse duration modulation , and pulse length modulation refer to the same general concept of modulating an output pulse to an instantaneous sample of an input wave by varying a leading , trailing , or both edges of that pulse to achieve a particular spacing between those edges . given the mass of certain vibration motor embodiments , some implementations may need to actively synchronize outputs from the speaker ( s ) 26 , 27 and the vibration motor 30 . while each may receive their input simultaneously or nearly so , the signal response of the speaker 26 , 27 far exceeds that of more massive vibration motors 30 . one embodiment to ensure that the output , rather than just the input , of both transducers are synchronized is to impose a delay buffer 29 along the first path 20 a and third path 20 c of fig2 . the delay buffer 29 delays the signal feed to the speakers by a fixed time interval that corresponds to the response differential between the speakers 26 , 27 and the vibration motor 30 . as the response times of a traditional woofer 27 and tweeter 26 typically vary only a negligible amount , especially in portable devices of the handheld size that include two speakers 26 , 27 , a single delay buffer 29 may be used from which the first 20 a and third 20 c signal paths diverge . in this variation , the second signal path 20 c that feeds the vibration motor 30 branches from the source 20 prior to the delay buffer 29 . a method according to an exemplary embodiment is then rectifying an audio signal or at least its low frequency component , measuring intensity of the rectified signal , mapping that intensity to a pulse length , and driving a vibration mechanism with the pulse length . preferably , the high frequency components of the audio signal are filtered out prior to rectifying , though they may be filtered between rectifying and measuring intensity with some minimal loss of efficiency . fig8 is a block diagram of an exemplary mobile station 60 in which the present invention may be disposed . these blocks are functional and the functions described below may or may not be performed by a single physical entity as described with reference to fig8 . a display user interface 62 , such as a circuit board for driving a visual display screen , and an input user interface 64 , such as a unit for receiving inputs from an array of user actuated buttons , are provided for interfacing with a user . the user may select between the traditional silent alert mode at the input user interface 64 , so that the vibration mechanism is actuated , silently , upon receipt of a page or incoming call . the ms 60 further includes a power source 66 such as a self - contained battery that provides electrical power to a motherboard 68 that controls functions within the ms 60 . the motherboard represents one or more circuit boards on which a general processor and / or a digital signal processor are disposed , as well as the diodes , amplifiers , and mappers described with reference to fig2 - 3 . within the processor ( general or dsp ) of the motherboard 68 are functions such as digital sampling , decimation , interpolation , encoding and decoding , modulating and demodulating , encrypting and decrypting , spreading and despreading ( for a cdma compatible ms 60 ), and additional signal processing functions known in the art . voice or other aural inputs are received at a microphone 70 that may be coupled to the processor of the motherboard 68 through a buffer memory 72 . computer programs such as drivers for the display 62 , algorithms to modulate , encode and decode , data arrays such as look - up tables , and the like are stored in a main memory storage media 74 which may be an electronic , optical , or magnetic memory storage media as is known in the art for storing computer readable instructions and programs and data . the main memory 74 is typically partitioned into volatile and non - volatile portions , and is commonly dispersed among different storage units , some of which may be removable . the memory 74 may also store music or other audio files that may serve as an audio signal source for the invention as detailed above . the ms 60 communicates over a network link such as a mobile telephony link via one or more antennas 76 that may be selectively coupled via a t / r switch 78 or diplex filter , to a transmitter 80 and a receiver 82 . the ms 60 may additionally have secondary transmitters and receivers for communicating over additional networks , such as a wlan , wifi , bluetooth ®, or to receive digital video broadcasts . any of these links may serve as a source for the said audio signal processed by the present invention . known antenna types include monopole , di - pole , planar inverted folded antenna pifa , and others . the various antennas may be mounted primarily externally ( e . g ., whip ) or completely internally of the ms 20 housing . audible output from the ms 60 is transduced at a speaker 84 and at a vibration mechanism 88 , as detailed above . while described above as incorporated within a mobile station , the invention is not so limited . for example , the invention may be embodied as a portable electronic device that is physically separated from a mobile station , radio , mp3 player , or other external source device . that external source device need not be portable itself . the portable electronic device of such an embodiment has disposed in it the vibration motor for vibrating its own housing , and a plug - in input port and / or an antenna / receiver arrangement as the source for receiving an electronic signal from the external source device . a wired connection between the portable electronic device and the external source device couples at the input port . a wireless connection , such as over a bluetooth or other personal area network , a low power fm band ( or other broadcast radio ), or other wireless protocol is established with the antenna / receiver , which serves as the source of the signal in this embodiment . various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description , when read in conjunction with the accompanying drawings . as but some examples , the use of other similar or equivalent components or combinations of components may be attempted by those skilled in the art . however , all such and similar modifications of the teachings of this invention will still fall within the scope of the non - limiting embodiments of this invention . furthermore , some of the features of the various non - limiting embodiments of this invention may be used to advantage without the corresponding use of other features . for example , advantages of the present invention may be gained without employing the tweeter to transduce high frequency audio components . as such , the foregoing description should be considered as merely illustrative of the principles , teachings and exemplary embodiments of this invention , and not in limitation thereof .