← Back to T5’s FAQ, answered (item 15)

Sound

Understating the hum.

T5’s FAQ describes the Illinois noise limits as “expressed as A-weighted levels” and reports a single result, 44 dBA. The rule it cites, 35 Ill. Adm. Code 901.102(b), actually sets a separate, unweighted limit for each pitch, starting at 31.5 Hz. Here is what that difference means in practice.

It makes T5’s results look better

This happens whenever the sound includes a low hum. The state rule sets a separate limit for each pitch, including deep bass. A-weighting shrinks the bass on paper before the comparison, so a deep hum is counted at a small fraction of its actual sound energy: about 1/400th at 63 Hz (26 dB) and about 1/8,000th at 31.5 Hz (39 dB).

The other two consequences can be heard.

Listen: same dBA, different sound. Use headphones or a speaker with real bass. Phone and laptop speakers typically reproduce little bass, so the hum may be faint or missing on them. Set a comfortable volume on clip 1 and leave it there. These clips show the relative difference only: no recording can play back at a true 44 dBA, because the loudness depends on your device and volume setting.

It hides the hum

  1. Reference: steady broadband noise, like light rain.
  2. A transformer-style hum (120 Hz, with harmonics at 240 and 360 Hz) set to the same A-weighted level as clip 1. Unweighted, it carries about 10.6 dB more sound energy, roughly 11 times the power.

It sidesteps the tonal rule

  1. Clip 1 with the hum added at 6 dB below it in A-weighted terms. The A-weighted total rises by only about 1 dB, yet the hum is a distinct tone, about 15 dB above the noise around 120 Hz.

Levels are computed on the files themselves using the standard A-weighting curve (IEC 61672). Transformers on a 60 Hz grid hum at 120 Hz because their cores vibrate at twice the line frequency.