Frequency response testing apparatus

Frequency response testing apparatus, developed for use with hearing aids, comprises a generator providing a predetermined varying-frequency input for the device under test, and a discriminator which applies incremental output level signals from the device to a display matrix of LED's under the control of timing signals derived from the generator, the overall arrangement providing a display in the form of a linear graphical plot.

This invention concerns frequency response testing apparatus and more 
particularly, but not exclusively, such apparatus for testing the 
frequency response of acoustic devices. 
In practice the invention has been developed for testing hearing aids, but 
it will be appreciated that the invention is equally applicable to the 
testing of other electroacoustic devices, such as microphones and 
loudspeakers. Indeed the invention is more generally applicable to the 
testing of an electrical device required to exhibit a predetermined 
frequency response in its operating characteristics. Examples of such 
devices which are non-acoustic include amplifiers and filters. 
Currently available apparatus such as used to test the frequency response 
of a hearing aid commonly takes one of two general forms. In one of these 
forms the apparatus is relatively simple and involves successive testing 
at progressively varied discrete frequencies to provide data from which a 
graphical plot of the relevant response can be prepared manually. This is 
clearly disadvantageous in terms of the time taken to test a device. The 
other form of apparatus avoids this disadvantage only by greater 
complexity which is itself disadvantageous in terms of the consequent cost 
and a need for skilled operators. 
An object of the present invention is to reduce these disadvantages and to 
this end, there is provided frequency response testing apparatus 
comprising a generator for providing a first electrical signal of 
predetermined varying-frequency form representing an input for application 
to a device to be tested; a receiver for response to a second electrical 
signal representing the output of said device when subjected to said 
input, said receiver including timing means connected to said generator to 
provide a plurality of sequentially occurring third electrical signals 
representing correspondingly occurring increments of said first signal, 
discriminating means responsive to said second signal to provide a 
plurality of fourth electrical signals respectively representing 
successively increasing amplitude levels therein; and a matrix of 
electrically-operable light-emitting elements, successive columns and rows 
of said elements being respectively connected for response to 
corresponding ones of said third and fourth signals, and each of said 
elements being operable only in response to the simultaneous occurrence of 
the respective ones of said third and fourth signals. 
It will be appreciated that the proposed apparatus operates to provide 
automatically, by way of the matrix, a visual representation of the second 
signal in graphical form and this facilitates the testing procedure. In 
practice, it will usually be desirable that the first signal be provided 
in a repetitive sequence to give rise to a correspondingly repeated 
display at the matrix, or the first signal be provided singly and the 
matrix be adapted to hold its display. 
Also, it will be appreciated that, when the device to be tested is of 
electroacoustic form, the apparatus comprises an electroacoustic coupling. 
It is, in any case, preferred that the apparatus comprises a feedback 
circuit and attenuator whereby the first signal is controlled to provide 
an input to the device under test, of constant amplitude in terms of 
voltage, current, or sound pressure, and this feedback can also include an 
electroacoustic coupling.

The illustrated embodiment serves for testing hearing aids and FIG. 1 
illustrates the overall apparatus very generally, while FIGS. 2 and 3 
respectively illustrate electrical and coupling parts of the apparatus in 
more detail. 
In FIG. 1 a generator for generating a first electrical signal of 
predetermined frequency form is denoted at 1, and this signal is applied 
to a loudspeaker 2 to provide an audio input for a hearing aid 3. The 
loudspeaker 2 and hearing aid 3 are located in an acoustic test box 4 and 
represent a first acoustic coupling. 
The test box 4 also houses a microphone 5 which responds to the loudspeaker 
2 to form a second acoustic coupling which provides a feedback signal 
applied to the generator 1. This feedback signal controls the output of 
the generator 1 so that the loudspeaker 2 provides a corresponding output 
at constant sound pressure. 
The output from the generator 1 is additionally applied to a timer 6 which 
provides a plurality of sequentially occurring electrical signals 
representing correspondingly occurring increments of the generator output. 
The hearing aid output is applied by way of a 2cm.sup.3 acoustic coupler 7, 
or an artificial mastoid in the case of an aid of bone conduction type, to 
discriminator 8 which operates to provide a plurality of electrical signal 
outputs respectively representing successively increasing amplitude levels 
of the input thereto. 
The remaining part of FIG. 1 is a matrix 9 of electrically-operable 
light-emitting elements of which the successive columns and rows are 
respectively connected for response to corresponding outputs of the timer 
and discriminator. The elements of the matrix are operable only in 
response to simultaneous occurrence of the respective ones of the timer 
and discriminator output signals so that, during the first increment of 
operation of the generator 1, the element which is disposed in the first 
column of the matrix and also represents the output amplitude of the 
hearing aid at that time as illuminated, and so on. Thus, the matrix is 
operated to provide a visual representation in graphical form of the 
response of the hearing aid to the generator signal. 
Turning to the additional detail of FIG. 2: the generator 1 is seen to 
comprise a ramp generator 11 which applies a D.C. sawtooth voltage to 
control the frequency of an oscillator 12 in a predetermined progressively 
increasing manner. The oscillator output is applied, by way of an 
attenuator 13 and amplifier 14, to the loudspeaker 2. The attenuator 13 is 
operated to control the loudspeaker output to a constant sound pressure, 
this being effected by feedback, from the microphone 5, through a 
pre-amplifier 15, variable gain monitoring amplifier 16, and rectifier 17. 
A switch 18 is connected between the rectifier and attenuator to allow 
disconnection of automatic feedback control and resort to manual control 
of the attenuator. 
The timer 6 comprises a plurality of similar sets of circuits 19 one set 
for each column of the matrix 9, of which only one set need be described 
in detail. Each set includes a trigger circuit 20 operable in response to 
a predetermined voltage threshold in the output of ramp generator 11, the 
trigger circuits associated with successive columns being operable at 
successively increasing threshold levels. The trigger circuit operates a 
timing circuit 21 to produce an output for a predetermined duration 
normally terminating no later than when attainment of the next trigger 
circuit threshold occurs in the ramp generator output. The timing circuit 
is connected to open, during its period of operation, a gate circuit 22. 
The discriminator 8 is connected to the coupler 7 by way of an amplifier 31 
and comprises a linear rectifier 32 to rectify signals of common 
amplitude, but varying frequencies, to corresponding D.C. levels. This 
rectifier is connected, through a logarithmic amplifier 33 and bias 
amplifier 34, to a voltage-to-frequency converter 35. The amplifier 33 
compresses the possibly large range of received input and facilitates 
representation of the final output in a decibel scale, the amplifier 34 
biases the compressed input into the input range of the converter 35, and 
the converter provides a pulse train output at a frequency related to the 
input amplitude. This pulse train is applied to the gate circuit 22 to be 
passed thereby, for the duration of the associated timing circuit input 
thereto, through a frequency divider 36, to a counter 37. The operation of 
the counter is additionally directly controlled by the ramp generator 11, 
and the counter outputs representing successive counts are respectively 
applied to the elements in the corresponding rows and the relevant column 
of the matrix 9. 
In practical development of the invention, an embodiment such as described 
so far has been successfully constructed and operated with: an output from 
the oscillator 12 which varies from 0 to 5 kHz; attenuation of the 
oscillator output to provide an output from the loudspeaker which is at 
any of a plurality of selector values within a 40dB range; and a matrix of 
light-emitting diodes, which matrix has eight columns and nine rows, the 
rows representing a 45dB range in 5dB intervals. In the embodiment in 
question, alternative operating modes are available whereby the ramp 
generator provides a single frequency-sweep output and the matrix display 
is held thereafter, or the ramp generator provides a cyclically swept 
output with repetitive display at the matrix. Also, the relevant 
embodiment allows for additional facilities, such as an X-Y plotter or pen 
recorder controlled from additional outputs 41 and 42 of the ramp 
generator and bias amplifier. 
The remaining FIG. 3 shows a presently preferred form for the coupler 7 of 
FIG. 1. This coupler should accord with the appropriate international 
standard, IEC 126, which requires, inter alia, that the hearing aid under 
test be coupled, by way of an ear mould substitute and then a cylindrical 
cavity of 2cm.sup.3 .+-. 1%, with a suitable calibrated microphone. 
Conventionally the ear mould substitute and the microphone are mounted 
directly in a housing to define therewith the cavity, and the microphone 
is of relatively expensive capacitor form. In the present case, a 
relatively low-cost miniature microphone 51 of the hearing aid type is 
used and this microphone is mounted in the centre of a rigid baffle 52 
connected with an integrated ear mould/housing component 53 so that the 
baffle 52 and the component 53 define the relevant cylindrical cavity 54 
without involvement of the microphone for this purpose.