The 31.5 Hz to 8 KHz is the standardised 1/3 octave division range of frequency analysers, noise . Noise regulation standards and rules are based on this range.
Proper analyzers are sensitive to the entire spectrum, the 31.5-8000 range is just a setting.
Here at my work we regularly use Bruel & Kjaer meters and analyzers, and they are linear across the human hearing range, even beyond that. For example, the Bruel & Kjaer 2260, which can measure from 6.3 Hz to 20 KHz.
The disadvantage of these devices is the price, which is in the five figure range. I'm afraid a decent, calibrated SPL meter (no spectral analysis or other fancy stuff) will set you back several hundred dollars.