Cirrus Logic AN168 - Manual (Page 15)

AN168
AN168REV2 15
1.5.1 Acoustic Coupling
The term ‘Acoustic Coupling’ refers to the gain
between the AO and the APO pins on the CS6422.
It includes the speaker driver gain, the efficiency of
the speaker, the loss in the air path between the
speaker and the microphone, the sensitivity of the
microphone, and the gain of the microphone
preamp.
Here we present two methods of measuring the
acoustic coupling, the loop gain method and the
frequency response measurement method. The
loop gain method is quick, easy, and requires no
additional test equipment to perform, yet provides
only one worst-case coupling number. The
frequency response method is more thorough and
yields more information, but requires automated
test equipment.
1.5.1.1 Loop Gain Method
When a system contains a closed loop whose gain
is greater than 1, howling, or regenerative
feedback, results. See Figure 10.
This howling phenomenon can be used to make a
quick and accurate measurement of the acoustic
coupling between a speaker and a microphone.
In essence, we create a known gain between APO
and AO inside the CS6422 using the TVol, NSdt,
and RVol controls, and increase this gain until
howling results. At the point that howling occurs,
the loop gain of the system is 0dB, or a factor of ‘1’.
In this particular example, we set TVol to +12dB,
NSdt to -12dB, and increase RVol until howling
occurs. The speaker driver gain and mic preamp
gain are known, and the remaining unknown, the
air coupling between the speaker and the mic, is
determined. See Figure 11.
‘a’ dB ‘b’ dB
‘c’ dB
‘d’ dB ‘e’ dB
‘f’ dB
Loop Gain (dB) = a + b + c + d + e + f
When Loop Gain is very slightly greater than 0dB (a factor of ‘1’), howling begins
Loop Gain is the gain of a ‘closed’ loop.
A closed loop has a path that returns to its starting point.
Figure 10. Loop Gain Diagram