Encoding a source at a small radius adds proximity bass boost but distorts dangerously near zero, so clamp the radius and high-pass first

When encoding with radial distance (e.g. HoaEncodeDirection’s radius argument or HoaNFProx), the ATK simulates near-field proximity filtering: smaller radii give a noticeable bass boost as the modelled source approaches the listener. But as the radius approaches zero the system is synthesizing a ‘perfect’ microphone whose response extends down to 0 Hz, so the signal distorts dramatically and dangerously (large low-frequency overflow). Two safeguards: keep the radius no smaller than about 0.3 metres, and pre-process the signal with a high-pass filter (cutoff at least 20 Hz, higher if there is heavy low-frequency content). Signals with DC offset — common in FM synthesis — are especially dangerous and must be handled carefully. This is both a sound-design tool (proximity bass) and an ear-safety concern.

Examples

HPF.ar(sig, 20) before HoaNFProx; clamp modulated radius with .max(0.3) so a swept distance never drops into the danger zone. Reducing radius on a bass-heavy pad audibly boosts its low end (proximity effect) until it overflows near zero.

Assessment

Explain why a very small encoding radius causes the signal to blow up, and name the two mitigations. Why is an FM signal with DC offset particularly risky here?