Patent Document

PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/807,053 filed Jul. 11, 2006. 
     
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to sound amplification systems and, more specifically, to long range hailing systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    The voice intelligibility of an audio public address (PA) system is highly dependent on its frequency response. For example, a deficiency in high frequency response above about 2 kHz will result in the listener having difficulty discerning consonants, such as the difference between the sounds of “f” and “s”; or between the sounds of “c”, “Z”, and “v.” A lack of smooth frequency response throughout the mid-range region, around 400-2000 Hz, can cause difficulties in discerning vowels and certain words. 
         [0004]    In the case of a system designed to be used at very long ranges, such as more than 500 yards, another effect becomes important. That is, that the propagation of sound through air is highly affected by temperature and by relative humidity (RH), and the propagation loss (called “loss by absorption”) is frequency dependent at any given combination of temperature and RH. 
         [0005]    Higher frequencies have a higher rate of loss per unit of distance than do lower frequencies, and the loss further depends on temperature and RH. There is not a linear function for describing loss versus frequency, temperature and RH. These effects must be considered holistically in a system in order to provide maximum intelligibility at long distances, such as in police, fire, military and other safety-related applications. For example, the user of a system in an emergency cannot take the time to optimize the system&#39;s frequency response for his environment and the range at which he intends a loud-hailing or P.A. system to transmit. 
         [0006]    Accordingly, it would be advancement in the art to provide a system for readily compensating for frequency dependent attenuation losses in long range hailing systems. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention includes a speaker coupled to a processor and one or more sensors suitable for sensing environmental conditions such as temperature and humidity. The processor reads the output of the sensors and selects one of a plurality of equalization tables suitable for compensating for frequency dependent attenuation likely to occur at the sensed environmental condition. In one embodiment, the user specifies a range that the sound is to travel and an equalization table compensating for attenuation at the desired range is selected according to the user input. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0009]      FIG. 1  is a schematic block diagram of a sound system compensating for environmental conditions; 
           [0010]      FIG. 2  is a graph illustrating frequency dependent attenuation in air for various temperatures; and 
           [0011]      FIG. 3  is a process flow diagram of a method for using the sound system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    Referring to  FIG. 1 , a system  10  includes a transducer  12 , such as a loudspeaker, that converts electrical signals into audible sound waves. An original source  14  of an audio signal is coupled to the transducer  12 . The source  14  is typically a microphone or a device storing audio information, such as a CD, MP3 or tape player. 
         [0013]    Signals from the source  14  are processed by a digital signal processor (DSP)  16 . The DSP  16  modifies the frequency profile of the signal from the source  14  and inputs the modified signal to an amplifier  18 , which generates an amplified signal input to the transducer  12 . 
         [0014]    The DSP  16  modifies the signal to compensate for environmental conditions and the distance the sound waves emitted by the transducer  12  will travel. In one embodiment, the DSP  16  uses a plurality of equalization tables (EQ 1 , EQ 2  . . . EQi) stored in a database  20 . The equalization tables store multipliers corresponding to a frequency or band of frequencies within the audible range of sound waves. The multipliers describe how much the intensity of a sound wave must be amplified at a given frequency in order to compensate for frequency dependent attenuation as the sound wave travels through air. In one embodiment, equalization tables have a form similar to Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Equalization Table 
               
             
          
           
               
                   
                 Frequency 
                 Multiplier 
               
               
                   
                   
               
               
                   
                 f 1 -f 2  Hz 
                 M 12   
               
               
                   
                 f 2 -f 3  Hz 
                 M 23   
               
               
                   
                 . 
                 . 
               
               
                   
                 . 
                 . 
               
               
                   
                 . 
                 . 
               
               
                   
                 f i -f j  Hz 
                 M ij   
               
               
                   
                   
               
             
          
         
       
     
         [0015]    The values for the multipliers are calculated according to known principles of sound propagation in air. The attenuation of sound in air due to viscous, thermal and rotational loss mechanisms is proportional to f 2 . However, losses due to vibrational relaxation of oxygen molecules are generally much greater than those due to the classical processes, and the attenuation of sound varies significantly with temperature, water-vapor content and frequency. A method for calculating the absorption at a given temperature, humidity and pressure can be found in ISO 9613-1 (1993). The table gives values of attenuation in dB km −1  for a temperature of 20° C. and a pressure of 101.325 kPa. The uncertainty is estimated to be±10%. 
         [0016]    Values used to calculate the attenuation of sound waves in air include: 
         [0000]                                    Pa   Ambient atmospheric pressure in kPa       Pr   Reference ambient atmospheric pressure: 101.325 kPa       Psat   Saturation vapor pressure ca equal:           International Meteorological Tables WMO-No. 188 TP94           World Meteorological Organization - Geneva Switzerland       T   Ambient atmospheric temperature in K (Kelvin):           K = 273.15 + Temperature in ° C. (by US known as centigrade,           Europe as Celsius)       To   Reference temperature in K: 293.15 K (20° C.)       Tol   Triple-point isotherm temp: 273.16 K = 273.15 + 0.01 K           (0.01° C.)       H   Molar concentration of water vapor, as a percentage       HR   Relative humidity as a percentage       f   Frequency       frO   Oxygen relaxation frequency       frN   Nitrogen relaxation frequency                    
The equalization tables each correspond to multipliers substantially compensating for attenuation that occurs at a value or range of values of one or more sensed environmental condition such as temperature, humidity or other environmental conditions such as ambient atmospheric pressure. In embodiments where equalization tables compensate for more than one environmental condition, each equalization table corresponds to a unique combination of environmental conditions or a unique combination of ranges or values for each environmental condition.
 
         [0017]    For example, the range of likely temperature may be divided into a plurality of subranges represented as values T 1 , T 2 , . . . T i , . . . T n  and the range of possible humidity may be divided into subranges represented as H 1 , H 2 , . . . H j , . . . H n . An equalization table may be provided for each of a plurality of unique combinations T i  and H j . In a similar fashion, the range of likely ambient pressure may be represented by a series of subranges P 1 , P 2 , . . . P k , . . . P n . Where ambient pressure is considered, an equalization table may be provided for each of a plurality of unique combinations T i , H j  and P k . 
         [0018]    In an alternative embodiment, the equalization tables are replaced by an equation describing the desired frequency profile as a function of frequency (f). Accordingly, an equation g ijk (f) may be provided for each of a plurality of unique combinations of subranges T i , H j  and P k  of one or more environmental conditions. 
         [0019]    In an additional alternative embodiment, the equalization tables are replaced by a multivariable equation, function or algorithm: g T,H,P (f) describing the desired frequency profile as a function of frequency (f) and one or more environmental variables of temperature (T), humidity (H) and pressure (P). The function g T,H,P (f) may evaluate to a real or imaginary number that may have a continuous or a discrete number of values. The variables f, T, H, and P may be a real number and have a continuous or a discrete number of values. 
         [0020]    In certain embodiments, the equalization tables also compensate for the distance that sound will travel. The further sound travels, the greater the impact of frequency dependent attenuation. Accordingly, equalization tables for each combination of subranges of the environmental conditions may be provided for a plurality of distances D 1 , D 2 , . . . D j , . . . D n . In certain embodiments, simple range divisions may be used, for example, near and far ranges. In such embodiments, only two sets of equalization tables for each combination of subranges of the environmental conditions need be provided. For example, the near range may be defined as a distance of less than 400 yards and the far range as a distance of 400 yards or more. 
         [0021]    In the preferred embodiment, a user provides an input indicating the desired range. Various types of user input devices may be incorporated into the present system. For example, the system may provide a dial, discrete buttons each corresponding to a range of distances, a number pad, touch screen, or the like, enabling a user to input the range. In some embodiments, a range finder using a laser, radar, or like means, is used to determine the range. 
         [0022]    The use of any one parameter including temperature, humidity, pressure and range is optional. Alternative embodiments of the invention may use less than all of these parameters. In systems not mapping equalization tables to all of these parameters, a typical or known value for the unused parameter may be considered to calculate the equalization tables. For example, where the expected distance is known, the equalization tables compensates for attenuation that is likely to occur for the known distance across a range of environmental conditions such as temperature, humidity and/or pressure. 
         [0023]    With reference again to  FIG. 1 , in the preferred embodiment, the equalization table used by the DSP  16  is selected by a processor  22  that receives inputs from a range finder/controller  24 , a temperature sensor  26  and a humidity sensor  28 . The range finder/controller may permit a user to input the value for the range. The range finder may also automatically determine a range. In the preferred embodiment, the range finder is omitted and a user manually indicates a range. The humidity sensor may include any humidity sensor known in the art, such as a resistive, capacitive, thermal conduction or infrared humidity sensor. The outputs of the temperature sensor  26  and humidity sensor  28  may be conditioned by a temperature circuit  30  and a humidity circuit  32 , respectively. The temperature and humidity circuits  30 ,  32  convert a signal from the sensors  26 ,  28  into a form readable by the processor  22 . The circuits  30 ,  32  may therefore scale the output, remove noise, or convert the output to a digital signal. In embodiments using ambient pressure to select an equalization table, a pressure sensor and a corresponding signal conditioning circuit may provide an input to the processor  22 . 
         [0024]    In the preferred embodiment, the processor  22  receives the inputs from the sensors  26 ,  28  and determines which of the equalization tables in a database  20  corresponds thereto. The processor  22  and DSP  16  may be modules of the same program or processor chip. Alternatively, the processor  22  and DSP  16  may be separate software applications or distinct processor chips. 
         [0025]    The components of the system  10  illustrated in  FIG. 1  may be discrete components. Alternatively, the functionality of two or more of the illustrated components may be combined in a single device providing equivalent functionality. For example, the functionality of the processor  22 , database  20  and DSP  16  may be incorporated in a single processing chip or a single application executed by a general purpose computer chip. In embodiments where less than all of the distance, temperature and humidity factors are used, the structure used to input these parameters to the processor  22  may be omitted. For example, in embodiments where the distance is known or assumed, the range finder/controller may be omitted. In embodiments where ambient pressure is used to select the equalization table, the system of  FIG. 1  may further include additional components necessary to determine ambient pressure, preferably controlled by the processor  22 . 
         [0026]    In other embodiments, different configurations may be used, such as systems that implement analog, digital or a hybrid of analog and digital components (e.g. processor controlled digital potentiometers that control an analog equalizer). Also, the processor  22  and the DSP  16  may be the same device. 
         [0027]    Referring to  FIG. 2 , the phenomenon for which the system  10  compensates is evident in the plot lines corresponding to different temperatures and humidities. Particularly at high frequencies, the amount of attenuation over a one kilometer distance is extremely temperature and humidity dependent. Equalization curves corresponding to the temperatures and humidities corresponding to the plot lines would, therefore, boost higher frequencies according to the anticipated attenuation. 
         [0028]    Referring to  FIG. 3 , a method for using the system  10  may include sensing the temperature at block  34  and sensing the humidity at block  36 . Sensing the temperature at block  24  may include use of one or more thermistors in an analog tone control circuit to change the frequency response of the system  10  in response to a temperature change to compensate for temperature dependent attenuation in air. In the preferred embodiment, the temperature is sensed at block  24  and the sensed value is used to select an equalization table. 
         [0029]    The anticipated or desired distance that the sound will travel is input at block  38 . At block  40  the equalization table corresponding to the conditions determined at blocks  34 ,  36  and  38  is selected. At block  42 , audio signals from the audio source  14  are equalized according to the compensation information obtained from the equalization table selected at block  40 . In embodiments where ambient pressure is used to select the equalization table the method of  FIG. 3  may further include sensing the pressure. 
         [0030]    In an alternate embodiment, the processor  22 /DSP  16  analyzes the frequency spectrum of the output from the audio source  14  and adjusts the equalizer settings (power supplied to frequencies in the spectrum) based on the analysis. For example, if the output from the audio source  14  is below a predefined threshold in a certain frequency range, the system reduces or does not increase power to that frequency range in the amplifier even if analysis of the environmental conditions indicates an increase is warranted. 
         [0031]    In another embodiment, the capabilities of the amplifier are taken into consideration before the audio signal is altered. The degree of frequency response modification is varied according the amplifier power that is available. For example, the solution determined at a particular RH, T, and range may call for a 41 dB boost at 4 kHz. If only 25 dB of amplifier headroom is available at that time, the system will limit the amount of boost to 25 dB to avoid distortion and/or amplifier overload. 
         [0032]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment.

Technology Category: h