APPENDIX A – 1000 HZ TYPANOMETRY & MEATUS COMPENSATION
55
D-0115696-C (OM038)
– Otowave 302 Operating Manual
Contrast this with Figure 2 where the phase angles are very different and a scalar subtraction would
erroneously give a value close to zero, instead of the length of the vector in orange (
Y
ec
).
Figure 2:
1 kHz probe tone: Although the susceptance values B
n
and B
ec
are the same as in the 226 Hz diagram, the
distance between the n
th
sample Y
n
and the baseline sample
Y
ec
is nothing like the difference in lengths between Y
n
and
Y
ec
(which here would be close to 0), due to the conductance values and the possibility of mass dominated
measurements. Vector subtraction (
𝑌𝑛
̅̅̅̅ − 𝑌_𝑒𝑐
̅̅̅̅̅̅
) is necessary.
Even for 226 Hz probe tones, the subtraction strictly should be a complex subtraction, but the loss of accuracy
arising from using the scalar subtraction method described above is not large enough to be of clinical
importance (as shown in Figure 1), and this approach is taken by most if not all commercial tympanometers.
But for 1 kHz measurements, the Otowave 302 optionally can take the more advanced approach, employing
vector based subtraction. It is a mathematically more thorough and accurate way of performing compensation
and is made possible by the advanced electronics and software within the device.
Vector based baseline compensation always generates positive values; it calculates the length of a line joining
two points in 2-D space and can therefore never be negative. This can cause a tympanogram to rise up at the
end opposite to that used for the baseline reference. If that is the case, changing the baseline from
400 or +
200 daPa or vice versa can improve the display. This effect can be most clearly demonstrated by performing
a tympanometric sweep on a 2 ml or 5 ml hard walled cavity. When viewed in Scalar mode the baseline should
always rise from
400 or + 200 daPa and switching between
400 or + 200 daPa should simply raise or lower
the trace so that the selected end is at 0; but when the Vector mode is selected the baseline always rises from
the selected end, so the slope changes direction.
10.3.
SCALAR VS. VECTOR BASELINE
There are differences between the tympanograms obtained with
scalar
and
vector baseline
compensation:
1 kHz tympanograms may appear quite flat when viewed with scalar baseline compensation; they are typically
clearer with vector compensation. Moreover, vector baseline compensation leads to results that follow a more
easily interpretable pattern, which means that the middle-ear pressure can be defined with greater certainty.
Although vector subtraction is the only correct solution at 1 kHz, it may be unfamiliar to users and therefore
the Otowave 302 offers the option of selecting either scalar or vector baseline compensation for 1 kHz
tympanograms. Use of scalar baseline compensation will give results similar to those from some other
instruments and be comparable with publications that have used scalar baseline compensation.
