Microphone And The Artificial Ear - BRUEL & KJAER 4212 Instructions And Applications

under free field conditions in the anechoic chamber, and the characteristic
("B"
) measured in the Hearing Aid Test Box. It is seen that the pressure
increase starts asserting itself about looo Hz ( c/ s). However, in the Test Box it
appears a little later (at about 1500Hz (c/ s) . This is because the sound field in
the Test Box is not a true free sound field , but rather a pressure field at
1ooo Hz ( c/s) . Being a function of the mechanical dimensions of the hearing
aid this deviation can be compensated for , if desired, only by developing a
correction curve for every type of hearing aids to be measured. In Fig. 17
is seen a frequency characteristic measured on a hearing aid with the
dimensions 4.0 X 5.0 em ( 1 % X 2 inches) in a free sound field and in the
Test Box. The difference between the two curves is seen to be almost
negligible. (The conditions may, however, be improved by carrying out the
measurements with the lid of the Test Box opened).
Another factor which may distort the picture of the correct frequency
response curve considerably is the condition of the rubber seal in the
coupler of the artificial ear (see page 8). If this is worn so that the
mechanical connection b etween the earphone and the coupler is not air tight,
a r es ult as shown in Fig. 18 may b e obtained. The curves shown are the
frequency characteristics of a hearing aid measured with tight and leaking
coupler. As a rule of thumb may be used that when the rubber seal can
no more lock the nub in position, the seal should be replaced.
-
DDODDDDDOOO OOO OD DDDD DO DOODDDD DDDDDD DDDOODD DO DOD DD DD
8rii:!.!_!Jaor . S 0
25
-""'""- -
_.___ _.___
Hearing A jd
db •
Ereq~ "'
?d~6:o£l:gs~tBo: "
coupler
z- l-~_ rop.s.,.: _
LlJM.Fr.: 2.Q_il:ec.No.: _ .
, .... __5_0_o..., _ _ ...
'W•.Sto.--Sio<o.:- -
... , _ _ f ,.q. Scolo o_ oo)()
OP 0123 10
100
-'-+'---+---' - --
'-l- '
Leaking coupler
tight coupler
.0000
20000 cb .tOOOO A. I CU...
1000
1QQQQ
U~tl/21111
A I
C
Lin.
Fig. 18. Frequency characteristic of a h earing aid m easured with tigllt and
leaking rubber seal.
Mutual Influence of th· e Sound Waves Reflected from the Regulating
Microphone and the Artificial Ear.
A factor which must b e consider ed whenever fr ee fi eld m easurements are
carried out according to the comparison m ethod is the inevitable mutua]
influence of the sound r efected from the r egulating microphone and the
hearing aid, du e to the sound waves which tra vel p erp endicular to the main
direction of the sound propagation. To decrease the effect of this disturbance
the distance be tween the r egulating microphone and the h earing aid should
be as gr eat as possible. In a fr ee spherical sound fi eld , the sound pressure
decrea ses with 6 db for a doubling of the distance from th e s ound source.
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