Digital Audiometry Apparatus And Method

Abstract

This disclosure describes audiometry techniques for producing a first sound pressure wave having a first frequency and a first period and for producing a second sound pressure wave having a second frequency and a second period by means of a transducer. A sine table digital memory stores digital number signals representing a sine wave. A frequency selector determines the rate at which the sine table memory is addressed in order to produce a sinusoidal step function signal of the desired frequency. A threshold selector regulates the nominal magnitude of the step function signal. A digital storage circuit stores correction signals that correct the magnitude of the step function signal to accomodate the variable sensitivity of the human ear to different frequencies. The values of a correction signal and the threshold signal are combined to operate an attenuator network that alters the magnitude of the step-function signal so that a standardized pressure wave signal is produced by the transducer.

Description

BACKGROUND OF THE INVENTION

This invention relates to audiometry techniques and more particularly relates to a digital audiometry apparatus and method used for testing human hearing.

DURING THE LAST SEVERAL DECADES, THE AUDIOMETRY PROFESSION HAS DEVELOPED STANDARDS FOR TESTING HUMAN HEARING. In order to meet the standards, an audiometer must produce sound pressure waves that are carefully standardized in terms of frequency and magnitude.

The frequency of the sound pressure wave must be carefully controlled so that a patient hears only a single frequency of a known, standard value. If the frequency actually heard by the patient differs from the standard value, the test results cannot be accurately compared with standardized norms of hearing for the population as a whole.

The magnitude of the sound pressure wave also must be carefully controlled because the human ear is more sensitive to certain frequencies of sound than to others. In order to accurately test hearing over a wide range of frequencies, the magnitude of the sound pressure wave produced at various frequencies must be corrected according to a formula or set of curves developed by Messrs. Fletcher and Munson. Any deviation from these curves results in inaccurate test data which inhibits an accurate diagnosis of the patient's hearing ability.

In the past, audiometers have employed analog techniques for producing various frequencies of sound that require a substantial amount of production adjustment, thereby increasing the cost of production. Experience has shown that this circuitry is expensive to maintain, and also exhibits certain inherent inaccuracies. In order to control the magnitude of a sound pressure wave, the prior art audiometers have employed attenuators which consist of potentiometers. With these potentiometers, it is difficult to maintain accuracy and linearity over a large range of attenuation. In addition, the potentiometers tend to become noisy.

SUMMARY OF THE INVENTION

In order to overcome the deficiencies of the prior art audiometers, the applicants have invented a unique digital approach for controlling the frequency of a sound pressure wave. According to this technique, a plurality of digital number signals representing digital numbers are stored in a digital memory. Each number represents a value of a predetermined periodic waveform, such as a sine wave, at a discrete angular position. An addressing circuit is arranged to address the digital memory at a selectable, predetermined set of rates so that the number signals are read out of the memory at rates corresponding to a predetermined, discrete set of desired frequencies. A frequency selector controllable by an operator causes the addressing circuit to address the memory means at a particular rate in order to produce signals representing digital numbers at a desired frequency. The numbers read out of the memory are converted into an analog signal, such as a step function sine wave signal, having the desired frequency.

This apparatus can utilize a high frequency oscillator which is inherently more stable than the low frequency R-C oscillators generally employed in prior art audiometers. Since the digital numbers are stored as discrete values, the resulting analog signals are produced at precisely accurate frequencies by the stable high frequency oscillator. The applicants have found that such apparatus is capable of producing a sine wave signal having a precisely determined frequency over long periods of time with little or no maintenance.

According to another feature of the applicants' invention, a storage circuit stores a plurality of digital correction signals having values representing a series of corrections in the magnitude of the analog signal at the frequencies selectable by the frequency selector. By using this technique, correction factors corresponding to the Fletcher-Munson curves can be stored in a precise and reliable manner which enables the magnitude of the resulting sound pressure wave to be adjusted precisely over a wide range of frequencies and over a wide dynamic range.

According to another feature of the invention, a threshold selector generates a digital threshold signal having a value proportional to the magnitude of the sound pressure level desired. In order to control the magnitude of the analog signal, the digital threshold signal and a selected one of the digital correction signals are combined and transmitted to a variable gain transfer network. The combined value of these signals enables the network to vary the magnitude of the analog signal by a discrete increment so that a sound pressure wave having a precise and predetermined magnitude is produced.

The advantages of using the foregoing techniques are at once apparent. By using the digital approach described above, the applicants are able to generate a sine wave with a degree of accuracy and reliability previously unknown in audiometers. By storing digital correction numbers corresponding to the Fletcher-Munson curves and by combining the value of these numbers with a digital threshold signal, the magnitude of the resulting sound pressure wave can be controlled in discrete steps with a degree of accuracy and reliability previously unattainable.

DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the present invention will hereafter appear in conjunction with the accompanying drawings which illustrate a preferred form of the invention and in which:

FIG. 1 is a pictorial view of a preferred form of the present invention used to test the hearing of a patient;

FIG. 2 is a block diagram schematic drawing of the electronic components of the apparatus shown in FIG. 1;

FIG. 3 is an electrical schematic drawing showing a preferred form of frequency selector and divider circuitry;

FIG. 4 is an electrical schematic drawing showing a preferred form of an addressing divider circuit, a sine table memory, and a converter circuit;

FIG. 5 is an electrical schematic drawing showing a preferred form of correction storage circuitry;

FIG. 6 is an electrical schematic drawing showing a preferred form of a transfer network;

FIG. 7 is an electrical schematic drawing showing a preferred form of a threshold selector;

FIG. 8 is an electrical drawing illustrating a preferred form of an output amplifier and a filter;

FIG. 9 is a chart illustrating the manner in which the frequency of a primary oscillator used in the preferred embodiment is divided in order to produce multiple frequency signals;

FIG. 10A illustrates the manner in which the sine-table memory stores data;

FIG. 10B is a schematic drawing showing an idealized voltage waveform produced by converter 106;

FIG. 11 illustrates contour lines of equal loudness for normal ears in which the numbers on the curves indicate loudness level in phons;

FIG. 12 illustrates the manner in which FIGS. 3-5 should be arranged; and

FIG. 13 illustrates the manner in which FIGS. 6-8 should be arranged.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a preferred form of audiometer 1 made in accordance with the invention comprises a case 2 in which is mounted a VU meter 3. RT and LT transducers convert the output signal into a corresponding sound pressure wave in a well-known manner. In order to use the transducers, a heatset H is fitted on a patient's head so that transducers RT and LT cover his right and left ears, respectively. Transducer BT can be clipped over one ear so that the transducer presses against the patient's mastoid bone. Transducer BT converts the output signal into corresonding mechanical vibrations in a well-known manner.

An output selector knob 4 selects the transducer which receives the output signal. A frequency selector knob 5 selects the frequency of the sound pressure wave to be produced, and a threshold selector knob 6 selects the magnitude of the sound pressure wave to be produced. A keying bar 7 is depressed by an operator in order to conduct the output signal to one of the transducers so that the patient's hearing can be tested.

Referring to FIG. 2, a preferred form of apparatus for controlling the frequency and magnitude of the output signal transmitted to the transducers comprises a digital sine table memory 12, an addressing circuit 20, a frequency selector 100, a digital-to-analog converter 106, a variable gain transfer circuit 120, a digital correction storage circuit 302, an output selector 338, a threshold selector 350 and an output circuit 404. As shown in FIG. 2, address circuit 20 comprises an oscillator 21 and divider circuitry 22. Transfer circuit 120 comprises an attenuator 152, an adder 276, and a decoder 282.

Referring to FIGS. 3 and 4, sine table memory 12 comprises digital memory chips 14, 15, 16 and 17, each capable of storing 32, 8-bit digital numbers. One memory chip usable in audiometer 1 is model IM 5600 manufactured by Intersil, Inc. Each of the memory chips has addressing inputs A0-A4 and a chip enable input CE. According to a preferred practice of the invention, the chips collectively are loaded with 128, 8-bit digital numbers. Each of the numbers corresponds to the value of a sine wave displaced from an adjacent number by an angle of 2.815.degree.. By sequentially reading the digital numbers out of the memory chips, a sine table representing 128 discrete points on a complete sine wave cycle of 360.degree. can be generated. By converting the sine table into a corresponding analog signal, a "step-function" signal closely approximating a pure sine wave can be produced. Of course, if more digital numbers are used to approximate the sine wave, the resulting step function signal more nearly approximates a pure sine wave. It has been found that 128 digital numbers is adequate to produce a step function signal which approximates a pure sine wave with sufficient accuracy for use in an audiometer.

In order to illustrate the principle involved, FIG. 10A depicts a pure sine wave S divided by 16 segment lines, such as lines L, into 16 equal parts each displaced by 22.5.degree.. The X axis of FIG. 10A represents magnitude and the Y axis represents angular displacement in degrees. In order to produce a step function signal of the type shown in FIG. 10B, the value of the sine wave at its intersection point (e.g., points P1-P4) with each segment line would be stored as digital numbers at sequential addresses in memory 12. By sequentially reading out the digital numbers and converting them to an analog signal, a step function sine wave signal SF (FIG. 10B) would be produced. Of course, if 128 digital numbers, instead of only 16, were used, signal SF would closely approximate the shape of signal S.

Referring to FIG. 3, oscillator 21 comprises a crystal-controlled oscillator to oscillate at 3.072 megahertz (MHz.) This frequency is selected because audiometer 21 is designed to produce sound pressure waves having frequencies of 125, 250, 500, 750, 1,000, 1,500, 2,000, 3,000, 4,000, 6,000 and 8,000 Hz. The sine wave table produced by memory 12 is synthesized from 128 (2.sup.7) discrete steps. Since 128 times the lowest audiometric frequency (125 Hz) is greater than the highest audiometric frequency (8,000 Hz), the least common multiple of the audiometric frequencies is 24,000 Hz. Thus, the frequency of the oscillator is 128 times 24,000 Hz, or 3.072 MHz.

Divider circuitry 22 comprises a frequency divider circuit 24 (FIG. 3) and an address divider circuit 80 (FIG. 4):

Frequency divider circuit 24 (FIG. 3) comprises JK flip-flops 26, 28, 30 and 32 connected as shown. Each of the flip-flops has a J input, a K input and a trigger (T) input that flips the flip-flop to its opposite state whenever a clock pulse is received. Each flip-flop is biased by a B+ positive supply voltage. In addition, flip-flop 26 has a Q1 output and an S or set input, and flip-flop 28 has a Q2 output and a Q2 output. The Q2 output of flip-flop 28 produces a series of clock pulses CP1 that are transmitted to an input 36 of a divide-by-16 circuit 34 which comprises 4 additional flip-flop circuits (not shown). The outputs of the 4 flip-flop circuits are marked Q3-Q6. As a result, the repetition rate of the CP1 clock pulses is divided by 2, 4, 8 and 16 at the Q3, Q4, Q5 and Q6 outputs, respectively. Frequency divider circuit 24 also comprises NAND gates 44-55, inverters 58-61 and resistors 64-74, all connected as shown. A supplly conductor 76 supplies a positive 5 volt DC signal from a power supply (not shown), and conductors 77 and 78 interconnect various circuit components.

Address divider circuit 80 (FIG. 4) comprises divide-by-16 circuits 82 and 84 having outputs Q9-Q12 and Q13-Q15, respectively. Each of the circuits comprises 4 flip-flops (not shown) arranged so that clock pulses received at input 85 are divided by 2, 4, 8, 16, 32, 64 and 128 at outputs Q9, Q10, Q11, Q12, Q13, Q14 and Q15, respectively. Address divider circuit 80 also comprises NAND gates 86-89 and inverters 92-93 that provide chip-enable pulses to memory chips 14-17.

Frequency selector circuit 100 (FIG. 3) comprises terminals F1-F11 that may be selectively connected to ground potential through a grounded terminal 102 and a movable switch blade 104. Switch blade 104 is connected to frequency selector knob 5 of audiometer 1. Terminals F1-F11 correspond to frequencies of 125, 250, 500, 750, 1,000, 1,500, 2,000, 3,000, 4,000, 6,000, and 8,000 Hz, respectively.

Digital-to-analog converter 106 (FIG. 4) comprises a conventional converter such as model DAC-371-8 manufactured by Hybrid Systems, Inc. The input circuity to the converter is biased by resistors 108-115 and the output of the converter is transmitted over a conductor 117. The converter converts the digital numbers transmitted to its inputs into a corresponding step function sine wave signal, such as signal SF (FIG. 10B), which is transmitted over conductor 117.

Referring to FIGS. 6 and 7, variable gain transfer circuit 120 comprises a keying circuit 122, a half sine wave circuit 144, an attenuator network 152, an adder 276 and a decoder circuit 282:

Referring to FIG. 6, keying circuit 122 comprises a switching circuit 124 such as model MFC-6,040 manufactured by Motorola Corp. Circuit 124 receives a 15 volt DC signal from a supply conductor 125 connected to a power supply (not shown). The keying circuit also comprises an isolating operational amplifier 126, capacitors 128-131, resistors 134-141, and a manually operable keying switch 142 that is connected to keying bar 7 (FIG. 1). The depression of bar 7 closes keying swwitch 142 and transmits the step function signal on conductor 117 to amplifier 126 in such a way that the signal has a predetermined rise time. Half sine wave circuit 144 comprises a diode 146 and a non-inverting, impedance-matching amplifier 148 having an input grounded through a resistor 150. Circuit 144 applies the negative half of the step function sine wave signal appearing at the output of amplifier 126 to a switching circuit described hereafter.

Attenuator network 152 comprises non-isolated, L-type attenuator sections 154-157 that comprise resistors 160-167 having the values indicated on the drawings in ohms. Sections 154-157 are capable of attenuating the analog signal transmitted on conductor 117 by 1db, 2db, 4 db and 8db, respectively, when energized. Attenuator sections 154-157 are operated by a switching circuit 170 comprising NPN transistors 172-175 and PNP transistors 178-181. The transistors are biased by resistors 184-195 connected as shown.

An isolating amplifier 200 is controlled by capacitors 202-205 and resistors 206-208, and separates attenuator sections 154-157 from non-isolated, L-type attenuator sections 210-216. Attenuator sections 210-216 comprises resistors 220-233 connected as shown, and each section is capable of attenuating a signal by 16 db when energized. In order to achieve this result, the sections must be energized in descending numerical order. Attenuator sections 210-216 are controlled by a switching circuit 236 comprising NPN transistors 238-244 and PNP transistors 248-254. The transistors are biased by resistors 256-276R. The voltage applied to the emitters of transistors 178-181 and 248-254 is controlled by a 3.9 volt Zener diode 274D that is biased by a resistor 272R from a +15 volt source.

Referring to FIG. 7, adder 276 comprises adder chips 278 and 280 such as type 7483, manufactured by Motorola Corp. Outputs D1, D2, D4 and D8 of adder chip 280 are connected to resistors 192, 193, 194 and 195, respectively. Output terminals D16, D32 and D64 of adder chip 278 are connected to a decoder circuit 282 which comprises NAND gates 284-291 and inverters 294-299. The adder receives input signals from outputs O1-O7 of memory chip 308 (FIG. 5). Conductors 300A-300G interconnect the decoder circuit with switching circuit 236.

Referring to FIG. 5, transfer circuit 120 is controlled by a correction storage circuit 302 that includes a memory circuit 304 and an addressing circuit 320:

Memory circuit 304 comprises memory chips 306 and 308, each of which are identical to memory chips 14-17. Each of memory chips 306 and 308 has addressing inputs A0-A4 and a chip enable input CE. In addition, chip 308 has outputs O0-O7 that are connected to resistors 310-317 in the manner shown.

Addressing circuit 320 comprises NAND gate 322-328 and resistors 332-335. The input conductors to NAND gates 322-325 are connected to the like-lettered terminals of frequency selector 100 (FIG. 3).

Output selector switch 338 comprises a grounded terminal 340 and a switch blade 342 that can be selectively connected to terminals 344-346. Referring to FIGS. 1 and 5, terminal 344 corresponds to left transducer LT, terminal 345 corresponds to right transducer RT, and terminal 346 corresponds to bone transducer BT. Switch blade 342 is connected to output selector knob 4.

Referring to FIG. 7, threshold selector circuit 350 comprises a potentiometer 352 and a tracking analog-to-digital converter 360:

Potentiometer 352 includes a resistor 354 that is connected between a + 5 volt supply and ground potential. A slide 356 is movable on resistor 354 under the control of threshold selector knob 6 (FIG. 1).

Tracking analog-to-digital converter 360 comprises comparator circuits 362 and 364 that receive two of their input signals from an amplifier 366. The signals produced by the comparators are gated by NAND gates 368 and 370 to an up-down counter 372. Counter 372 comprises counter chips 374 and 376 having up, down, and clear inputs and a borrow output as shown. Counter chips comprise type 74193 manufactured by National Semiconductor. The outputs of the counter chips are connected to a digital-to-analog converter 380 that is connected to the input of amplifier 366. Converter 380 can be identical to converter 106. A one-shot multivibrator 382 receives a clocking input from the Q14 output of divide-by-16 circuit 82 (FIG. 4). The input is received at the zero crossing of the step function sine wave (Point P5 in FIG. 10B). This technique helps prevent clicks by allowing the attenuator to change from one section to another at the minimum signal magnitude of the step function sine wave. Converter 360 also comprises a NAND gate 384 having one input connected to each of outputs D1-D64 of adder 276. Resistors 386-389 and a capacitor 391 control the operation of amplifier 366.

Referring to FIG. 8, output circuit 404 comprises an amplifier 406, a portion of output selector 340, and an output filter 450:

Amplifier 406 comprises NPN transistors 408 and 410 that are controlled by resistors 412-419 and capacitors 422-425. The amplifier is connected to attenuator 152 by a conductor 407. The output of transistor 140 is connected to an output transformer comprising a primary winding 428 and a secondary winding 430. The amplifier is connected to a -15 volt supply conductor 431 and to a + 15 volt supply conductor 125.

The second section of output selector switch 340 comprises terminals 434-436 that are connected to phone jacks 438-440. The phone jacks receive complementary plugs that are connected to left transducer LT, right transducer RT, and bone transducer BT, respectively. The terminals may be selectively connected to secondary winding 430 through a switch blade 442 that is ganged to switch blade 342 and output selector knob 4.

Output filter 450 comprises NPN transistors 452-457, resistors 460-465 and capacitors 468-470 connected as shown. Resistors 461, 463 and 464 are connected to terminals F1, F2 and F3, respectively of frequency selector 100. Filter 450 has a low pass transfer characteristic which smooths the analog step function signal produced by converter 106. The frequencies corresponding to terminals F1-F3 (i.e., 125, 250 and 500 Hz.) require filtering in addition to that provided by the transducer and other circuit components.

The manner in which the audiometer produces a signal having a predetermined frequency will now be described. As previously noted, sine table memory 12 is loaded with digital numbers corresponding to a sine table for a complete 360.degree. cycle of a sine wave. By addressing the sine table memory at an appropriate rate, an analog step-function, sine wave signal having a precisely determined frequency is provided by converter 106. In order to achieve this result, the device is turned on so that oscillator 21 is operating and frequency selector switch blade 104 is moved to the frequency setting desired. For example, if a frequency of 125 Hz is desired, switch blade 104 is moved into contact with terminal F1.

In response to this connection, the input to inverter 59 is grounded or switched to its logical zero state and the output of inverter 59 is switched to its logical 1 state. This switching action conditions NAND gate 55 so that its output state is controlled by the operation of output terminal Q8 of flip-flop 32. The remaining NAND gates 49-54 have outputs switched to their 0 states due to the operation of inverters 60, 61 and NAND gates 45-48. The output of inverter 58 is switched to its 1 state so that NAND gate 44 functions as an inverter to assure that the J and K inputs of flip-flop 28 are in opposite states. In this mode of operation, the repetition rate of oscillator 21 is divided by three at output Q2 in a well known manner. If the output of inverter 58 is switched to its 0 state, the repetition rate of oscillator 21 is divided by two at output Q2.

As shown in the chart of FIG. 9, in order to produce a 125 Hz signal, the output of oscillator 21 is divided by 3 and then is divided successively by 2. As a rsult of this division, the repetition rate of the clock pulses produced by oscillator 21 is reduced to 16,000 Hz at input 85 of divide-by-16 circuit 84. As each clock pulse is received at input 85, divide-by-16 circuits 82 and 84 address a different one of the 128 address locations in memory chips 14-17 so that 128 different digital numbers are transmitted to converter 106 for each 16,000 clock pulses received at input 85. As a result of this operation, an analog, step-function sine wave signal having a predetermined frequency is transmitted over conductor 117. The step-function signal has its magnitude altered by transfer circuit 120 in accordance with the setting of threshold selector 350 and the data stored in correction storage circuit 302.

In order to produce a sound pressure wave having the proper magnitude, the proper correction numbers must be stored in correction storage circuit 302 before the audiometer is operated. The values of these numbers are determined empirically by testing the audiometer with a particular set of right, left and bone transducers.

In order to determine the correct number, frequency selector knob 4 is set at 1,000 Hz and threshold selector knob 6 is set at 0 db so that a 1,000 Hz sound pressure wave is produced by the transudcer. The magnitude of the sound pressure wave is measured and compared with the Fletcher-Munson curve shown in FIG. 11 to determine the amount of error, if any, in decibels (db). If an error is detected, a number is loaded into storage circuit 302 which will result in the energizing of appropriate sections of attenuator 152 so that the correspondence with the Fletcher-Munson curve is achieved (FIG. 11). In other words, the 1,000 Hz sound pressure wave should intersect the 0 db base line BL of FIG. 11. This result can also be achieved by adjusting the initial count loaded into counter 372 and by adding the correction number from the storage circuit. The binary number stored in counter 372 always corresonds to the decimal number shown on the threshold selector.

The magnitude of the sound pressure wave produced at each of the frequencies provided on frequency selector knob 5 is also measured. After the magnitude of each sound pressure wave had been measured, at the 0 db setting on knob 6, a number is stored in correction storage circuit 302 so that the magnitude of the resulting sound pressure wave is greater than or less than the 0 db level established at 1,000 Hz by the number of db indicated on the Fletcher-Munson curves (FIG. 11). For example, assuming the transducer has a flat frequency response, about 35 db must be added to the 125 Hz signal in order to produce a sound pressure wave that sounds as loud as the 0 db wave at 1,000 Hz. By following this technique, sound pressure waves having frequency characteristics corresponding to the 0 db curve DB of FIG. 11 can be accurately produced. If desired, additional memory can be provided to store additional correction numbers corresponding to settings of the threshold selector knob 6 other than the 0 db setting. In this way, sound pressure waves having frequency characteristics corresonding to each of the 10-120 phon curves of FIG. 11 can be provided. Of course, different correction numbers must generally be stored for different transducers because of their different frequency characteristics. In practice, these additional steps are not normally required since all hearing measurements are made relative to normal hearing.

Assuming correction storage circuit 302 has been loaded with appropriate correction numbers, the manner in which the audiometer controls the magnitude of the analog signal on conductor 117 will now be described assuming that threshold selector knob 6 is set at 0 db and output selector knob 4 is set in the LT position. Since switch blade 104 is connected to input terminal F1 as shown in FIG. 3, and switch blade 342 is connected to terminal 344, as shown in FIG. 5, NAND gates 322-324 (FIG. 5) are switched to their 0 states and only NAND gate 325 is switched to its 1 state so that the digital correction number stored in memory circuit 304 for use in connection with a 125 Hz signal produced by the left transducer LT is read out to output terminals O0-O7. The 7 least significant bits of the correction number are transmitted to adder 276 (FIG. 7).

While the correction digital number is being read out of memory circuit 304, a threshold digital signal is being generated by threshold selector 350. An analog voltage corresponding to the 0 db position of knob 6 is transmitted over slide 356 to the inputs of comparators 362 and 364. If this analog voltage is greater than the voltage produced on the output of amplifier 366, the output of comparator 364 is switched to its logical 1 state so that NAND gate 370 transmits clock pulses to the down input of counter 372. In response to the clock pulses, the total in the counter 372 is reduced as the clock pulses are transmitted. The total in the counter is monitored by D-A converter 380 which converts the total into a corresponding analog signal that is amplified by amplifier 366. As the total in the counter decreases, the magnitude of the signal produced by amplifier 366 also decreases until it equals the voltage on wiper 356. At this point in time, the output of comparator 364 returns to its 0 state so that clock pulses no longer are transmitted to counter 372. The total in the counter then remains constant until wiper 356 (or knob 6) is moved to a new location. The threshold digital signal produced by counter 372 is transmitted to corresponding inputs of adder 276. The value of the threshold digital signal is added to the value of the correction digital number from memory 304 to produce a sum at the output terminals D1-D64. The four least significant bits of the sum are used to operate switching circuit 170 so that any combination of attenuator sections 154-157 may be energized to attenuate the signal on conductor 117 from 0 to 15 db. in 1 db. increments. The three most significant bits produced on output terminals D16, D32 and D64 are decoded by decoder circuit 282 in order to energize various combinations of attenuator sections 210-216 to attenuate a signal from 0 to 112 db. in 16 db. increments. As a result, attenuator 120 can attenuate an input signal from 0 db. to 127 db. in 1 db. increments by energizing proper combinations of the attenuator sections.

An attenuator section is energized when an output of decoder 282 is switched to its 0 state. For example, if the empirical study indicates that the 125 Hz signal transmitted to left transducer LT requires 32 db of attenuation in order to conform with the Fletcher-Munson curves and if threshold selector 350 is set at a level requiring 8 db of attenuation, a correction digital number is transmitted to adder 276 which results in outputs 300A and 300B being switched to their 0 states so that attenuator sections 215 and 216 are enabled. In addition, a threshold digital number is transmitted to adder 276 which results in output D8 being switched to its 0 state so that attenuator section 157 is energized. The combined effect of attenuator sections 215, 216 and 157 is to reduce the magnitude of the analog step function signal by 32 plus 8 or 40 db. The circuitry operates on an analogous manner for other frequency selector and threshold selector settings.

Those skilled in the art will recognize that only a single preferred embodiment has been disclosed herein and that the embodiment may be altered by those skilled in the art without departing from the true spirit and scope of the appended claims.

Claims

What is claimed is:

1. In an audiometer including a transducer for producing a first sound pressure wave having a first frequency and a first period and for producing a second sound pressure wave having a second frequency and a second period, improved apparatus for generating and controlling electrical signals that energize the transducer to produce the first and second sound pressure waves comprising in combination:

memory means for storing a plurality of digital number signals representing digital numbers, each number signal representing a value of a predetermined waveform at a discrete angular position;

addressing means for cyclically addressing the memory means at a first rate such that a first set of the number signals representing a complete cycle of the waveform are read out of the memory means during a first time interval equal to the first period and for cyclically addressing the memory means at a second rate such that a second set of the number signals representing a complete cycle of the waveform are read out of the memory means during a second time interval equal to the second period;

converter means for converting the first set of number signals into a first analog signal having the first frequency and for converting the second set of number signals into a second analog signal having the second frequency;

output means for coupling the first and second analog signals to the transducer;

frequency selector means operable in a first condition for causing the addressing means to address the memory means at the firt rate so that the first analog signal is produced and operable in a second condition for causing the address means to address the memory means at the second rate so that the second analog signal is produced;

threshold selector means for generating a threshold signal having a value proportional to the magnitude of the sound pressure level desired;

storage means for storing a first digital correction signal having a value representing a correction in the magnitude of the first analog signal and for storing a second digital correction signal having a value representing a correction in the magnitude of the second analog signal; and

a variable gain transfer means connected between the converter means and the output means and responsive to the threshold signal and the first digital correction signal during operation of the frequency selector means in the first condition for altering the magnitude of the first analog signal so that a sound pressure wave having the first frequency and a first predetermined magnitude is produced by the transducer, said transfer means being responsive to the threshold signal and the second digital correction signal during the operation of the frequency selector means in the second condition for altering the magnitude of the second analog signal so that a sound pressure wave having the second frequency and a second predetermined magnitude is produced by the transducer.

2. Apparatus, as claimed in claim 1, wherein the memory means comprises a digital read-only memory.

3. Apparatus, as claimed in claim 1, wherein the predetermined waveform is a sine wave.

4. Apparatus, as claimed in claim 1, wherein the addressing means comprises:

an oscillator for producing a first set of clock pulses having a prime repetition rate;

first divider means for dividing the repetition rate of the first set of clock pulses by a factor dependent on the operating state of the frequency selector means to produce a second set of clock pulses; and

second divider means having an input for receiving the second set of clock pulses and having a plurality of outputs connected to the memory means, whereby the second divider means addresses a different one of the number signals in response to each clock pulse in the second set of clock pulses.

5. Apparatus, as claimed in claim 1, wherein the output means comprises:

an amplifier; and

an output terminal adapted to comate with the transducer.

6. Apparatus, as claimed in claim 1, wherein the frequency selector means comprises a single pole, multiple throw switch connected to the addressing means and the storage means.

7. Apparatus, as claimed in claim 1, where the threshold selector means comprises means for generating a digital threshold signal.

8. Apparatus, as claimed in claim 7, wherein the threshold selector means comprises:

a potentiometer for producing an analog voltage;

and

an analog to digital converter for converting the analog voltage to the digital threshold signal.

9. Apparatus, as claimed in claim 1, wherein the storage means comprises:

a second digital memory; and

second addressing means connected between the second digital memory and the frequency selecting means, whereby the first digital correction signal is read out of the second digital memory in response to the operation of the frequency selector means in the first condition and the second digital correction signal is read out of the second digital memory in response to the operation of the frequency selector means in the second condition.

10. Apparatus, as claimed in claim 1, wherein the variable gain transfer means comprises an attenuator.

11. Apparatus, as claimed in claim 10, wherein the attenuator comprises:

a first attenuator section operable during the first condition of the frequency selector means;

a second attenuator section operable during the second condition of the frequency selector means; and

electronic logic means responsive to said digital correction signals and said threshold signal for enabling the first and second attenuator sections.

12. Apparatus, as claimed in claim 11, wherein the electronic logic means comprises an electronic digital adder for adding the value of the threshold signal to the value of one of the first and second digital correction signals.

13. A method for producing a sound pressure wave having a predetermined frequency and a predetermined period by means of a transducer comprising the steps of:

storing at one time a plurality of digital number signals representing digital numbers, each number signal representing the value of a predetermined waveform at a discrete angular position;

cyclically transmitting the number signals at a first rate such that a set of the number signals representing a complete cycle of the waveform is transmitted during a time interval equal to the predetermined period;

converting the set of number signals into an analog signal having the predetermined frequency;

generating a threshold signal having a value proportional to the magnitude of the sound pressure level desired;

storing a digital correction signal having a value representing a correction in the magnitude of the analog signal;

altering the magnitude of the analog signal as a function of the values of the threshold signal and the correction signal; and

transmitting the corrected analog signal to the transducer so that a sound pressure wave having the predetermined frequency and a predetermined magnitude is produced.

14. A method, as claimed in claim 13, wherein the step of storing the number signals comprises the step of storing each number signal at a predetermined address location in a multi-address digital memory.

15. A method, as claimed in claim 14, wherein the step of transmitting comprises the steps of:

addressing the address locations in a predetermined sequence; and

transmitting the number signal stored at each address location as the address location is addressed.

16. A method, as claimed in claim 13, wherein the step of generating the threshold signal comprises the steps of:

generating a threshold analog signal having a value proportional to the magnitude of the sound pressure level desired; and

converting the threshold analog signal to a corresponding digital threshold signal.

17. A method, as claimed in claim 13, wherein the step of altering the magnitude comprises the step of:

adding the value of the threshold signal and the correction signal; and

altering the magnitude of the analog signal in proportion to the sum of the threshold signal and the correction signal.

18. A method, as claimed in claim 13, wherein the predetermined waveform is a sine wave.

19. In an audiometer including a transducer for producing a first sound pressure wave having a first frequency and a first period and for producing a second sound pressure wave having a second frequency and a second period, improved apparatus for generating and controlling electrical signals that energize the transducer to produce the first and second sound pressure waves comprising in combination:

generating means located within the audiometer for cyclically generating at a first rate a first set of digital number signals representing a complete cycle of a predetermined waveform during a first time interval equal to the first period and for cyclically generating at a second rate a second set of the number signals representing a complete cycle of the predetermined waveform during a second time interval equal to the second period, each number signal representing a value of the predetermined waveform at a discrete angular position;

converter means for converting the first set of number signals into a first analog signal having the first frequency and for converting the second set of number signals into a second analog signal having the second frequency;

output means for coupling the first and second analog signals to the transducer;

frequency selector means operable in a first condition for causing the generating means to generate the number signals at the first rate so that the first analog signal is produced and operable in a second condition for causing the generating means to generate the number signals at the second rate so that the second analog signal is produced;

threshold selector means for generating a threshold signal having a value proportional to the magnitude of the sound pressure level desired;

storage means for storing a first digital correction signal having a value representing a correction in the magnitude of the first analog signal and for storing a second digital correction signal having a value representing a correction in the magnitude of the second analog signal; and

a variable gain transfer means connected between the converter means and the output means and responsive to the threshold signal and the first digital correction signal during operation of the frequency selector means in the first condition for altering the magnitude of the first analog signal so that a sound pressure wave having the first frequency and a first predetermined magnitude is produced by the transducer, said transfer means being responsive to the threshold signal and the second digital correction signal during the operation of the frequency selector means in the second condition for altering the magnitude of the second analog signal so that a sound pressure wave having the second frequency and a second predetermined magnitude is produced by the transducer.

20. Apparatus, as claimed in claim 19, wherein the predetermined waveform is a sine wave.

21. Apparatus, as claimed in claim 19, wherein the output means comprises:

an amplifier; and

an output terminal adapted to comate with the transducer.

22. Apparatus, as claimed in claim 19, where the threshold selector means comprises means for generating a digital threshold signal.

23. Apparatus, as claimed in claim 22, wherein the threshold selector means comprises:

a potentiometer for producing an analog voltage; and

an analog to digital converter for converting the analog voltage to the digital threshold signal.

24. Apparatus, as claimed in claim 19, wherein the storage means comprises:

a digital memory; and

addressing means connected between the digital memory and the frequency selecting means, whereby the first digital correction signal is read out of the digital memory in response to the operation of the frequency selector means in the first condition and the second digital correction signal is read out of the second digital memory in response to the operation of the frequency selector means in the second condition.

25. Apparatus, as claimed in claim 19, wherein the variable gain transfer means comprises an attenuator.

26. Apparatus, as claimed in claim 25, wherein the attenuator comprises:

a first attenuator section operable during the first condition of the frequency selector means;

a second attenuator section operable during the second condition of the frequency selector means; and

electronic logic means responsive to said digital correction signals and said threshold signal for enabling the first and second attenuator sections.

27. Apparatus, as claimed in claim 26, wherein the electronic logic means comprises an electronic digital adder for adding the value of the threshold signal to the value of one of the first and second digital correction signals.