Who do I get peak values of over 90dB when measuring in my office?

The answer to this question has two parts.

Firstly, the Peak is usually C-weighted (shown as ‘PeakC’). The C frequency weighting includes the low frequencies that are filtered out by A-weighting.

In office conditions, there are usually fairly high levels of low-frequency noise – from sources such as air conditioning, fans and motorised equipment – but they aren’t picked up by the human ear, so they go unnoticed.

Secondly, the Peak detector logs the highest single instantaneous sound pressure during the measurement period, unlike Leq and Sound Level which are averaged over time.

Such a peak can be very short-lasting (down to roughly 100 microseconds). So a single event, such as a door closing or a file dropped onto a desk, will be reported as the Peak value for the entire measurement.