A spec for a mixed-use space in a large office building in Houston called for 70 dB. No weighting specified, no measurement position, no ambient conditions. The system handled music, paging, and overflow for a nearby conference room.
After the install, the customer said the system was not loud enough and that it didn’t meet the spec. I went out to the site with my SPL meter. The customer showed me his smartwatch, which gave a reading in the low 60s on A-weighting. My meter at the same position read the same. Switching to C-weighting without moving, the reading jumped to nearly 70. Depending on the weighting, the system was either falling short of the spec or meeting it. The spec never said which one counted.
The room had significant low-frequency energy from HVAC, other mechanical systems, and the occupied spaces around it. A-weighting is the correct curve for hearing-damage risk, and every major occupational standard is built on it for good reason. It most closely resembles the response of human hearing at moderate levels. But the complaint in that room had nothing to do with hearing safety. The customer needed to know whether the system was doing enough work, and the measurement was discounting the exact energy behind the complaint.
The larger issue was the ambient floor. During commissioning, the HVAC was off and the building was empty. Ambient measured about 50 dBC. On a busy office day with the HVAC running, it was closer to 65. The system had been tuned in a room that was very different from the one the customer experienced during business hours.
The fix took 3 changes. The MEP team returned to rebalance the HVAC, we added acoustic treatment, and we increased output from the PA. None of those solutions could have come from a spec that read “70 dB.”
When a loudness spec omits the weighting, the measurement position, and the conditions, everyone in the room holds a different number and no one can determine which one answers the question.
Complex systems. Seamless experiences.