B&K 2250 Light Uživatelský manuál stáhnout pdf (Strana 13)

Hand-held Analyzer Type 2250 –

Instruction Manual

2.3 Measuring at Low Static Pressure

The frequency response of the microphone depends on the static pressure. Using a Sound Level Cali-

brator to adjust the sensitivity of a Sound Level Meter at the calibration check frequency provides no

information on the influence of static pressure on frequency response. Conformance to the specified

standards ensures that the system measures within the standards’ tolerances, in the range from 85 kPa

to 108 kPa. Data for the frequency response as a function of static pressure for Microphone Type 4189

can be found in the Microphone Handbook BA 5105.

At the calibration check frequency, both the microphones and the recommended Brüel & Kjær Sound

Level Calibrator Type 4231 are rather insensitive to variations in the static pressure, for the Sound

Level Calibrator Type 4231, below 0.001 dB/kPa.

2.4 Frequency Weightings

Both broadband and spectrum measurements can be frequency weighted with A-, C- or Z-weighting.

The A- and C-weightings conform to the requirements in IEC 60651 and IEC 61672 – 1.

The Z-weighting (Zero frequency weighting) is a linear, unweighted, frequency weighting. It confirms

to the Lin response defined in IEC 60651 and the Z-weighting defined in IEC 61672 – 1.

Table 2.1 states the design goal frequency responses for the frequency weightings according to

IEC 61672 – 1. They are stated for the complete instrument including the microphone. For the A- and

C-frequency-weightings, these design goals are equivalent to the ones specified in IEC 60651. The

corresponding tolerance limits can be found in the standards.

2.5 Measured Quantities

2.5.1 Introduction

All the measured quantities can be displayed on the screen (a transflective, back-lit, 240 × 320 dot

matrix, colour touch screen) as numerals, with a resolution of 0.1 dB over the measuring range. See

section 4.9 for further information on measurement ranges.

2.5.2 Instantaneous Broadband Measurements

These measurements are done continuously, independent of measurement Start, Pause and Stop. They

cannot be saved and are only displayed.

Overload

For Instantaneous Measurements, the Overload indication is displayed as long as the overload condi-

tion exists, or for 1 s, whichever is the greater.

Overload is indicated as ‘Overload’ on the screen and by a flashing red ‘traffic light’ indicator.

Overload is common to all results of Instantaneous Measurements.

Under-Range

No under-range indication is given because the instrument only has one range, which extends upwards

from the inherent noise level.

Time-weighted Sound Level, F and S Time-weighted

The time-weighted sound level, L

(t), is defined as twenty times the logarithm to the base ten of the

ratio of a given root-mean-square sound pressure to the reference sound pressure, root-mean-square

sound pressure being obtained with a frequency weighting, x, and standard time weighting, y, where:

BE171211.book Page 8 Thursday, May 27, 2004 1:01 PM

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 121 122

Žádné komentáře

B&K 2250 Light Uživatelský manuál Strana 13