Abstract:
This invention relates to a system for amplifying a sound in an auditory environment. The system comprises a microphone for transforming said sound to an electric sound signal; a band-pass filtering means connecting to said microphone and outputting a filtered sound signal; and an amplifier amplifying said filtered sound signal and outputting a filtered and amplified sound signal to a loudspeaker. The band-pass filtering means comprises a passive first filter having a first bandwidth and first gain, an active second filter having a second bandwidth and a second gain larger than said first gain, and an active third filter having a third bandwidth and a third gain larger than said second gain.

Description:
FIELD OF INVENTION 
     This invention relates to a method and system for amplifying auditory sounds. In particular, the present invention relates to an amplifier method and system for use in a classroom, a conference room or an office room. 
     BACKGROUND OF INVENTION 
     During a teaching session in a classroom the background noise may, firstly, prevent some of the participating students in hearing the voice of the teacher, since the signal to noise ratio in the classroom is low, and thereby reducing the possibility of learning. Consequently, the background noise may, secondly, cause the teacher to increase vocal amplitude thereby straining the teacher&#39;s vocal chord. 
     U.S. Pat. No. 5,818,328; discloses an area audio amplification system, which provides an improved signal to noise ratio and which is not susceptible to interference from frequency modulated signals. The system comprises a speech processor processing the audio signal to adjust signal level and response. In order to achieve this, the system comprises a low pass filter and de-emphasis network for limiting audio response to speech frequencies. The corner frequency of this network is 5 kHz. However, the system provides a time constant amplification of the audio signal, and therefore the background noise as such is not considered relative to the auditory sound, which primarily was attempted to communicate. 
     Further, American patent application no.: US 2003 0144847; discloses a sound masking system for masking out noise generated in large working environments. The system comprises a plurality of flat panel sound generators emitting highly effective and spatially uniform masking sounds within various zones. Hence the system provides masking of distracting noises in the working environment. Thus the system, as such, does not provide amplification of speeches, rather, oppositely reduces the noise relating to vocal communication. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and system for amplifying auditory sounds as a function of background noise. In particular, an object of the present invention is to augment speech in noisy conditions. 
     A further object of the present invention is to provide a method and system for filtering selected parts of selected sound and compensate the consequence of the filtering by increasing gain of amplification, thus preserving the overall sound pressure level (SPL). 
     A particular advantage of the present invention is the provision of a controllable amplifier ensuring that desired sounds are amplified to increase speech intelligibility. 
     The above objects and advantage together with numerous other objects, advantages and features, which will become evident from below detailed description, are obtained according to a first aspect of the present invention by a system for amplifying a sound in an auditory environment and comprising a microphone for transforming said sound to an electric sound signal; a band-pass filtering means connecting to said microphone, receiving said sound signal, and outputting a filtered sound signal; and an amplifier receiving said filtered sound signal, amplifying said filtered sound signal, and outputting a filtered and amplified sound signal to a loudspeaker; and wherein said amplifier has an amplification gain being a function of bandwidth of said band-pass filtering means. 
     The term “auditory environment” is in this context to be construed as a definition of an environment defined by reverberation, background noise i.e. noise floor, and sounds to be amplified. That is, the auditory environment is determined by the physical properties of, for example, a classroom; by number of sound generating elements in said classroom; and finally by the sound to be amplified. 
     The system according to the first aspect of the present invention may provide amplification of an auditory signal in accordance with the lower and upper cutoff frequencies of the filtering means. Hence when the bandwidth of the auditory signal is limited, the noise disturbing frequencies are eliminated and consequently not amplified. In addition, the amplifier gain is advantageously controlled so as to increase gain as bandwidth is narrowed. This ensures that even though the auditory sound may loose speech power in the filtered auditory signal this is compensated by increasing gain of the filtered auditory signal. 
     The term “cutoff frequency” is in this context to be construed as a frequency at which the amplification of the amplifier is reduced by 3 dB. Further, the term bandwidth is in this context to be construed as a frequency span defined between a lower cutoff frequency and an upper cutoff frequency. 
     The above objects, advantages and features together with numerous other objects, advantages and features, which will become evident from below detailed description, are obtained according to a second aspect of the present invention by a system for amplifying a sound in an auditory environment and comprising a microphone for transforming said sound to an electric sound signal; a band-pass filtering means connecting to said microphone, receiving said sound signal, and outputting a filtered sound signal; and an amplifier receiving said filtered sound signal, amplifying said filtered sound signal, and outputting a filtered and amplified sound signal to a loudspeaker; and wherein said band-pass filtering means comprises a passive first filter having a first bandwidth and first gain, an active second filter having a second bandwidth and a second gain larger than said first gain, and an active third filter having a third bandwidth and a third gain larger than said second gain. 
     The system according to the second aspect of the present invention provides means for advantageously controlling amplification and bandwidth of the overall system. The utilization of active filters establishes a simple approach. 
     The above objects, advantages and features together with numerous other objects, advantages and features, which will become evident from below detailed description, are obtained according to a third aspect of the present invention by a system for amplifying a sound in an auditory environment and comprising a microphone for transforming said sound to an electric sound signal; an band-pass filtering means connecting to said microphone, receiving said sound signal, amplifying said filtered sound signal, and outputting a filtered and amplified sound signal to a loudspeaker; and wherein said band-pass filtering means comprising a plurality of active filters each having an amplification gain associated with frequency bandwidth. 
     The band-pass filtering means according to the first, second and third aspect of the present invention may comprise switching means for switching between a plurality of filters having associated lower and upper cutoff frequencies. Thus by operating the switching means a user may select a filter bandwidth and amplification gain required for obtaining a sufficient signal to noise ratio. Alternatively, the system may comprise a switch controller, which based on the noise floor of the auditory signal switches between the plurality of filters. 
     The switching means according to the first, second and third aspect of the present invention may further simultaneously switch between a plurality of gains of said amplifier having each gain associated with a specific filter of said plurality of filters. Hence by switching between the plurality of filters, or rather switching between different lower and upper cutoff frequencies, the gain of the amplifier is appropriately simultaneous changed. 
     The plurality of filters according to the first, second and third aspect of the present invention may comprise a first filter having a lower cutoff frequency in the range between 20 and 100 Hz, such as 70 Hz, and an upper cutoff frequency in the range between 9 and 20 kHz, such as 12 kHz. This reproduces the auditory sound of a talker, such as a teacher, including relevant harmonics below 100 Hz and above 8 kHz. The first filter may comprise an associated first gain of the amplifier. The first filter may be established by the amplifiers gain frequency response, or may be established by passive elements inserted before and/or after the amplifier. 
     The plurality of filters may further comprise a second filter having a lower cutoff frequency in the range between 100 and 400 Hz, such as 300 Hz, and an upper cutoff frequency in the range between 3 and 9 kHz, such as 5 kHz. This bandwidth represents 95% of speech intelligibility with upward masking and with a minimum of boundary reflections. The second filter may comprise an associated second gain being larger than the first gain. Hence the gain is increased thereby compensating for cutting away the lower frequency part of the speech so as to preserve speech intelligibility. 
     The plurality of filters may further comprise a third filter having a lower cutoff frequency in the range between of 400 and 800 Hz, such as 600 Hz, and an upper cutoff frequency in the range between 3 and 9 kHz, such as 5 kHz. This bandwidth further focuses the amplification to frequencies essential to speech, which is centered at 2 kHz and reduces amplification for unwanted background noise. The third filter may comprise an associated third gain larger than the second gain. Hence the gain is further increased relative to the second gain thereby compensating for the loss of speech power due to the narrowing of the bandwidth. In fact, the system converts speech power from the lower frequencies to higher frequencies. 
     The system according to the first aspect of the present invention may further comprise an active filtering means comprising said first, second and third filters, and said amplifier. By incorporating the amplifier into the bandwidth filtering means a simple construction is achieved. 
     The above object and advantage together with numerous other objects, advantages and features, which will become evident from below detailed description, are obtained according to a fourth aspect of the present invention by a method for amplifying auditory sounds, and comprising receiving a sound signal from a microphone; band pass filtering said sound signal and outputting a filtered sound signal by means of a band-pass filter; amplifying said filtered sound signal as a function of bandwidth of said band pass filter and outputting a filtered and amplified sound signal to a loudspeaker. 
     The method according to the second aspect of the present invention may further comprise switching between a plurality of filters and between a plurality gains associated with each of said filters. 
     The method according to the second aspect of the present invention may incorporate any features of the system according to the first aspect of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawing, wherein: 
         FIG. 1 , shows a overview of the system according to the preferred embodiment of the present invention; 
         FIG. 2 , shows a graph of the frequency responses of the system according to the preferred embodiment of the present invention; and 
         FIGS. 3   a  through  3   c , show a first, second and third embodiment of the system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description of the various embodiments, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. 
       FIG. 1 , shows a classroom designated in entirety by reference numeral  100 . In the classroom  100  a teacher  102  speaks to an audience of students  104 . The teacher  102  carries a microphone around the neck or attached on a collar of a coat or shirt. The microphone converts the sound from the teacher  102  to an electric auditory signal. The classroom  100  comprises signal processing element receiving the electric auditory signal and performing a filtering and amplification of the auditory signal. In one embodiment of the present invention the signal processing system is implemented a wireless transmitter transmitting the auditory signal to a wireless receiver  106 , and in a second embodiment of the present invention the signal processing system is implemented in the receiver  106 . 
     Following filtering and amplification of the auditory signal, which will be described in detail with reference to  FIGS. 3   a  through  3   c , the receiver  106  communicates the filtered and amplified auditory signal to a plurality of loudspeakers  108 ,  110 ,  112  and  114 . The filtered and amplified auditory signal may be communicated to the loudspeakers through a wired or wireless connection known to a person skilled in the art. 
     The signal processing system may be controlled by the teacher  102  by operating a switch or may be controlled by a switch controller, shown in  FIGS. 3   a  through  3   c  as reference numeral  316 , so as to adjust the bandwidth of the auditory signal to be amplified.  FIG. 2  shows the asymptotic frequency responses  202 ,  204 ,  206  of the signal processing system in three different switching positions. 
     The signal processing system is designed to augment speech signals in noisy conditions. Depending on the noise floor, i.e. noise base in the frequency spectrum, the teacher  102  may select a ‘low’ position providing the frequency response  202 . This selection yields a full speech bandwidth from 70 Hz up to 12 kHz covering all necessary harmonics. By including the lower frequencies in the amplification the reproduced signal has more fidelity components. 
     If the noise floor is moderate the teacher  102  may select a ‘medium’ position providing frequency response  204 . This selection yields a lower cutoff frequency of approximately 300 Hz (0 dB at 200 Hz) and an upper cutoff frequency of approximately 5 kHz (−6 dB at 7 kHz). Since the frequencies below 300 Hz and above 5 kHz only contribute little to the overall speech intelligibility, and furthermore since the lower frequencies may mask higher frequencies within the bandwidth of the speech, the speech is in fact more understandable. 
     If the noise floor is very high the teacher  102  may select a ‘high’ position providing frequency response  206 . This selection yields a lower cutoff frequency of approximately 600 Hz (0 dB at approximately 300 Hz) and an upper cutoff frequency of approximately 5 kHz (−6 dB at 7 kHz). Hence the frequency response  206  of the ‘high’ position is shifted upward in frequencies relative to the frequency response  204  of the ‘low’ position. 
     Similarly, the switch controller operates the switch between the ‘low’, ‘medium’ and ‘high’ positions. The switch controller receives a part of the auditory signal and evaluates whether the auditory signal contains ambient noise beyond predetermined thresholds. Hence if the switch controller evaluates a higher ambient noise level the switch controller switches to a higher level, namely ‘medium’ or ‘high’. 
       FIG. 3   a  shows a first embodiment of the signal processing system according to the present invention designated in entirety by reference numeral  300 . The system  300  comprises an input  302  connecting to a microphone worn by a person making a presentation, such as the teacher  102 . The microphone converts the sound of the person to an electric signal. The electric signal is communicated from the input  302  to a switching unit  304  enabling switching between a range of filtering and amplification modes of the system  300 . 
     The system  300  further comprises a filter block  306  comprising a plurality of individual filters, such as filter  308 , each having a specific bandwidth. Each filter of the filter block  306  is selected through the switching unit  304  and provides a filtration of the electric signal thereby generating a filtered electric signal. The filtered electric signal is forwarded from the filter block  306  to an amplifier block  310  comprising a plurality of individual amplifiers, such as 312, for each filter in the filter block  306 . 
     The amplifier in the amplifier block  310  amplifies the filtered electric signal according to the bandwidth of the filter so as to compensate for the losses of in the speech power caused by the removal of lower frequencies in the electric signal. Following amplification of the filtered electric signal the amplified electric signal is forwarded to a loudspeaker unit  314  converting the amplified electric signal to sound. 
     The switching operation may be performed manually by the person using the microphone or sound technician, or may be performed by a switch controller  316 . The switch controller  316  identifies a noise floor in the electric signal, and when the noise floor exceeds a predetermined threshold the switch controller  316  switches the switch  304  accordingly. The switch controller  316  may receive an estimate of the noise floor from a separate noise detector or may integrate a noise detector. 
       FIG. 3   b  shows a second embodiment of a system according to the present invention, which system is designated in entirety by reference numeral  320 . Like components in the first and second embodiment of the present invention are referred to by like reference numerals. 
     The system  320  comprises an input  302  forwarding an electric signal from the microphone (not shown) of the system  320 . The electric signal is forwarded from the input  302  to a switching unit  322  for switching between a range of filtering and amplification modes. The switching unit  322  is directly coupled to simultaneously switch triggering a number generator  326 , by for example switching a ‘high level’ to different inputs. The number generator  326  thus generates a number ‘X’ in accordance with the switching operation of the switching unit  322 , which number matches the selection of the bandwidth, or rather the selection of the filter. 
     The number ‘X’ generated by the number generator  326  is forwarded to an amplifier block  328 , which accordingly selects an appropriate gain function for amplifying the filtered electric signal received from the filter block  306 . The amplified signal is as before forwarded to a loudspeaker unit  314 . 
       FIG. 3   c  shows a third embodiment of a system according to the present invention, which system is designated in entirety by reference numeral  350 . Like components in the first, second and third embodiments of the present invention are referred to by like reference numerals. 
     The system  350  comprises an input  302  receiving an electric signal from a microphone. The electric signal is forwarded from the input  302  to a switch  352  operable to switch between a plurality filtering and amplification modes. The switch  352  connects an active filter block  354  comprising one or more active filters  356 ,  358   360 . The number of active filters in the active filter block  354  determines the number of gain frequencies responses. 
     The filtered and amplified electric signal is forwarded from the operating active filter  356 ,  358 , or  360  to a loudspeaker unit  314 . 
     Common for the first, second and third embodiments of the systems  300 ,  320 ,  350  of the present invention is the frequency response of the gain function is determined so as to compensate for the removal of the lower frequency ranges by increasing the gain in the remaining frequency gain bandwidth. The compensation is determined in accordance with amount of speech power removed from the gain frequency response by increasing the lower cutoff frequency, and is effected by providing an increase in speech power in the remaining gain frequency response by increasing the gain substantially corresponding to the lost speech power. Further, the compensation additionally may advantageously incorporate the human frequency response curve to ensure that the lost speech power is compensated appropriately according to human hearing perception. 
     Further, common to the first and second embodiments of the systems  300  and  320  one of the filters in the filter blocks  306  may, in an alternative embodiment, be bypassed thus providing a frequency response determined by the amplifier blocks  310  and  328 . Similarly, in the third embodiment of the system  350  one of the active filters  356 ,  358 ,  360  may, in an alternative embodiment, be substituted by a single amplifier  362 , which consequently determines the overall frequency response in one of the switch positions. 
     Furthermore, common to the first, second and third embodiments of the systems  300 ,  320 ,  350  the preferred frequency response is a first frequency response having a lower cutoff frequency of 70 Hz and an upper cutoff frequency of 12 kHz together with a base level gain, a second frequency response having a lower cutoff frequency of approximately 300 Hz and an upper cutoff frequency of approximately 5 kHz together with a maximum gain 2.5 dB above the base level gain, and a third frequency response having a lower cutoff frequency of approximately 600 Hz and an upper cutoff frequency of approximately 5 kHz together with a maximum gain 6 dB above the base level gain. Obviously, the cutoff frequencies and gain may be adjusted accordingly to accomplish any desired effect. 
     Finally, common to the first, second and third embodiments of the systems  300 ,  320 ,  350  the systems may be implemented in analogue or digital circuit technology as will be known to persons skilled in the art.