CHAPTER 3
Operation
25
3.11 Using an External Generator
A signal within the range of 0.5 to 1.5 V from an external signal generator may be used with
Type 4226. The connection is via the
Ext. Gen.
socket and must be completed by selecting Ext.
Gen. using the
Frequency Step
push buttons. This is a useful feature in that alternative test signals
may be used with Type 4226, for example, discrete tones not generated by Type 4226 or tones
sweeping continuously in frequency.
💡 Hint: If the external signal exceeds 1.5 V, the
Ext. Gen.
LED will start flashing.
The compression loop is still active when the
Function
switch is set to Calibration. Therefore, it is
essential that any external frequency sweeps are slow enough to avoid generating unstable
sound levels in the coupler. Instability is indicated when the
Ext. Gen.
LED flashes.
It is important to note that the compression loop works from the signal coming directly from the
reference microphone embodied within the coupler. Therefore, the accuracy of the simulated
sound levels generated in the coupler will be as good as the accuracy of this microphone’s
frequency response. Frequency response corrections are made for this automatically at each
discrete frequency when Type 4226 uses its own internal signal generator, but not when an
external generator is used, therefore they must be applied manually.
These frequency response corrections can be determined quite easily for their respective discrete
frequencies using the external signal generator and a sound level meter acoustically coupled to
Type 4226, see section A.2 in the Appendix for details.
Finally, the
Microphone
and
Sound Field
switches do not influence the sound levels in the coupler
when an external signal generator is used. This is because the adjustments for free fields are
applied at discrete frequencies, and only when these are generated by Type 4226’s own signal
generator. This is also true of the inverted A-weighting correction, therefore the
Ext. Gen.
LED will
flash when 94 Inv. A is selected with the
Sound Level
switch.
The corrections applied for inverted A-weighting are shown in Table A.1 and the adjustments for
free-field responses are shown in Table A.2. These, added to the frequency response corrections
mentioned above, constitute all adjustments necessary when using an external generator. Make
a smooth plot of each to arrive at adjustments for intermediate frequencies.
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