SPL Measurements, very simple - New Plug-Ins - Audacity Forum
Decibel - Loudness Metering Plugin - 30 Days Trial
Youlean Loudness Meter - Free VST, AU and AAX plugin
Decibel Meter, Digital Sound Level Meter AP-882A Sound Monitor Tester Range from 30-130dB, Digital Decibel Meter with LCD Backlight/Max Hold/Sensitivity Adjustment and dBA/C Switch (Battery Included): Amazon.com: Industrial & Scientific