To translate electrical voltage values (mV) at the input of the Audio Precision APx555 into true physical sound pressure levels (dB SPL or Pascals), every precision measurement chain requires an absolute acoustic level reference.
The Brüel & Kjær Type 4231 (IEC 60942 Class 1) generates an ultra-stable reference sound pressure of exactly 94.0 dB SPL (equal to 1.0 Pascal) at 1,000 Hz. We use it to verify the absolute sensitivity of our measurement chain prior to every calibration batch.
Original DAkkS Certificate B&K 4231 (SN 2402714)
Our primary reference standard was calibrated on 2026-11-04 by the ISO/IEC 17025:2018 accredited calibration laboratory SPEKTRA Schwingungstechnik und Akustik GmbH Dresden (Calibration mark 18122 / D-K-15183-01-00 / 2026-11) per IEC 60942:2017 using the substitution method: