G.R.A.S. Sound & Vibration
Single-channel Low-noise Measuring System consisting of: Type 40HT and Type 12HF - Page 6
The choice of which frequency response to use is made via a two-position switch marked
Pres-
sure
/
Free Field
on the front panel of the chosen power module (see Fig. 2.1).
Fig. 1.4 shows the responses of the two compensation filters as well as the free-field response
for 0 ° incidence. Note: free-field corrections are added to the lowest curve.
Fig. 1.5 shows what these free-field corrections are for various angles of incidence.
Fig. 1.6 shows, for a complete low-noise measuring system, a typical noise floor in ⅓-octave
bands for both the linear and A-weighted cases.
1.3
Preamplifier Type 26HG
Similar to the standard G.R.A.S. Type 26AC but with 40 GΩ input impedance to enable low-level
and low-frequency noise measurements.
The preamplifier has an integrated 3 metre lightweight cable terminating in a 7-pin LEMO con
-
nector (which plugs in to the Gain and Filter Unit Type 26HT).
A ¼” to ½” adapter, GR0010, is included, so that it can also be used with G.R.A.S. ½” micro
-
phones. See separate data sheet.
Fig. 1.4 Typical frequency response curves of Type 40HT
