This is about the 8930B calibrator for 1/2 inch microphones. I got mine from Thoman but I see it is now also available from Aliexpress.
https://images.static-thomann.de/pi...U5NjNka1UydndISEZVYWxFNlBiSUE1SEtFZUElM0QlM0Q.
The manual says this:
Carefully insert microphone up to the stop within the calibrator cavity. The O-ring will offer some resistance.
I suppose they want to tell the user to stick the microphone in all the way to the stop. And it makes sense in that the initially cylindrical entrance hole ends in a ridge that is about 1 mm smaller in diameter. Behind that is a hollow half dome chamber. The insertion depth is about 12 mm.
I was sceptical about this because on all B&K and Norsonic capsules I have, the protection grid is somewhat wider than the capsule itself. So the O-ring seals tightly during the first 7 or so mm and feels kind of loose the moment it sits on the capsule rather than the grid. Of course, the reading gets higher when the microphone is fully inserted.
However, I just bought my first new in box sound level meter, and it displays 93.8 dB out of the box only when fully inserted.
https://images.static-thomann.de/pi...U5NjNka1UydndISEZVYWxFNlBiSUE1SEtFZUElM0QlM0Q.
The manual says this:
Carefully insert microphone up to the stop within the calibrator cavity. The O-ring will offer some resistance.
I suppose they want to tell the user to stick the microphone in all the way to the stop. And it makes sense in that the initially cylindrical entrance hole ends in a ridge that is about 1 mm smaller in diameter. Behind that is a hollow half dome chamber. The insertion depth is about 12 mm.
I was sceptical about this because on all B&K and Norsonic capsules I have, the protection grid is somewhat wider than the capsule itself. So the O-ring seals tightly during the first 7 or so mm and feels kind of loose the moment it sits on the capsule rather than the grid. Of course, the reading gets higher when the microphone is fully inserted.
However, I just bought my first new in box sound level meter, and it displays 93.8 dB out of the box only when fully inserted.
You do need to insert to the stop. A small change can translate into a significant change is level. Level is proportional to volume. The B&K mike grids have openings on the side which require the o-ring spaced back about 3-5 mm. The really expensive B&K 4226 and 4231 calibrators have internal mikes that helps compensate for volume changes. I don't know of any others that do. There are other tricks to reduce the sensitivity to difference in capsule volume. However the change from this will be usually les than 1 dB.