Acoustic measurements in an open room (free field) are highly susceptible to room reflections, comb filtering, temperature fluctuations, and ambient noise. Unless you own a full anechoic chamber, open-room testing mostly measures the room rather than tenth-of-a-decibel capsule variations.
That is why we use the active electro-acoustic coupler Spectra SQ-4.2 for secondary calibration according to IEC 61094-5 and IEC 61672 (operating range 31.5 Hz to 20 kHz):
| Feature & Frequency Range | Physical Reality Inside the Pressure Chamber |
|---|---|
| Simultaneous 2-Channel Port (1/2" Body) | The ECM8000 houses a 1/4" electret capsule inside a standard 1/2-inch body (12.7 mm). This allows it to sit directly opposite our 1/2" reference microphone (G.R.A.S. 40AE) inside the same cavity for simultaneous capture. |
| 31.5 Hz to 12 kHz: Maximum Repeatability | Up to approx. 12,000 Hz, the acoustic wavelength is sufficiently large relative to the cavity dimensions. The pressure field remains homogeneous and yields highly reproducible measurements with over 60 dB SNR (94 to 134 dB SPL test level). |
| 12 kHz to 20 kHz: Chamber Symmetry & Homogeneity Limits | While the coupler measures up to 20 kHz, above 12 kHz the acoustic wavelength becomes so short (< 28 mm) that geometric differences between the reference face (flush 1/2" metal diaphragm) and the DUT head (recessed 1/4" capsule inside a 1/2" tip), along with tiny mounting tolerances, disturb acoustic symmetry and field homogeneity. We plot the full curve to 20 kHz to show the capsule's HF peak trend, while transparently marking the 12 kHz precision boundary. |