Why does a machine that reads 80 dB outside show 86 dB in the shop? The environment changes the result. Outdoors approximates a free field—sound spreads and falls about 6 dB per distance doubling. Indoors, reflections, absorption, and geometry reshape the numbers.
Distance in free field
Imagine a point source outdoors. Double the distance, and the level typically drops ~6 dB. This “inverse square law” works best away from the ground and large surfaces. Close to the ground, interference and reflections change the pattern.
Rooms aren’t free fields
In rooms, sound hits walls, floors, and ceilings, then returns to the listener. This builds a reverberant field that raises the steady‑state level. Highly reflective rooms (tile, concrete, glass) sustain energy; absorptive rooms (carpet, curtains, acoustic panels) reduce it. That’s why a vacuum can feel harsher in a bathroom than in a carpeted living room at the same distance.
Measuring reliably indoors
- Use A‑weighting for general comparisons; C‑weighting to examine low‑frequency build‑up.
- Stand at least a meter from large surfaces when possible.
- Take several readings around the listening area; average them or use LAeq for a minute.
- Note room size and materials; a brief description helps interpret the number later.
Simple acoustic improvements
- Add absorption: Rugs, curtains, and acoustic panels reduce reverberation and perceived loudness.
- Break up reflections: Bookshelves and irregular surfaces scatter sound.
- Relocate sources: Moving a machine away from corners can reduce low‑frequency build‑up.
Worked example: shop vs driveway
You measure a planer at 92 dB(A) at 1 m in the driveway, then 96 dB(A) at 1 m in a small shop. Why higher? The shop adds a strong reverberant field and nearby boundaries that reflect energy into the mic. Adding soft panels and moving the tool away from walls can bring the indoor number closer to the outdoor baseline.
Takeaways
Distance and environment shape the number on your meter. Control what you can—placement and materials—and document the rest so you can compare fairly over time.