BRUEL & KJAER 4212 Instructions And Applications page 6

- - - - - - - - - - - P.a - - - - - - Hearing A i d - - - -
Po
Artificial Ear
158!05
Fig. 1. Illustration of the sound pressure increase on the surface of a hearing
aid when placed in a free sound field.
The free-field response of a hearing aid is the ratio between the sound
pressure pa in an associated artificial ear connected to the earphone of the
hearing aid and the sound pressure Pl in the undisturbed free sound field
as a function of frequency, and
the pressure response of a hearing aid is the ratio between the sound
pressure pa and the sound pressure P2 measured directly on the surface of
the hearing aid facing the sound source as a function of frequency.
In Fig.
2
is seen a typical pressure characteristic and a typical free-field
characteristic of the same hearing aid. As can be seen from the figure, the
two curves coincide in the lower a.f. range but show a considerable difference
in the higher end.
-
ooooooooooo ooooooooooooooooooo ooooo oooooooooooooooo
BrO:!!!,Ia~r . ~
25
~;;Q~:I!III i l l l l ! l l l l l l l i l l l ! l l ! l l l l l l l l l l l i l l l ! l l l l l l ! l ! l ! ! l l l l l l l l l l
Free field
fir~ 0 s7~r~
F.
response p p
z-....,:-~S..:-
Ll.A.fl'.: _20_t.c.Na.: _
~ _50_o-,
_ _ .. '
w •.
s,..: _ S~gr..:
_ _
'-"- -'-"'"•- oo,. 50:
100) - , . , . . . . . . .
I C ....
QP 0123 10 100 1000 10000 - , •• ;" .. ,.
I
c ....
161092
Fig. 2. Pressure characteristic (curve P) and free-field characteristic (curve F)
of a hearing aid.
4