**PWL** = **L**_{w} =
Acoustic Watts = Sound Power Level; Re: **10**^{-12} Watts |

**SPL** = **L**_{p} =
Sound Pressure Level % Pr; Re:20 micropascals (20-µPa) |

**dB** = Decibel = 10
log10 (P_{x} / P_{r})^{2} = 20 log10 (P_{x} /
P_{r}) |

**Hz**
= Hertz = Frequency = cycles per second |

**1/1**-Octave
or **1/3**-Octave = most common frequency groupings |

**dBA**
= Decibel, A-weighted = Units of EPA & OSHA audible criteria |

**TWA**
= Time weighted average of dBA (exchange rate dependent: “Q”) |

**dBG**
= Decibel, G-weighted = Units of Infrasound criteria (low freq.) |

**LEQ**
= Equivalent continuous SPL or PWL (time integrated; Q=3) |

**TL**
= Transmission Loss = static figure of merit by 1/1 or 1/3-bands: dB |

**IL**
= Insertion Loss = dynamic figure of merit by 1/1 or 1/3-bands: dB |

**STC**
= Sound Transmission Class = single figure of merit for TL in
speech frequencies (should not be used for mechanical equipment) |

**NR** = Noise Reduction =
figure of merit for SPL reduction = L_{p1} - L_{p2} |

**NC**
= Noise Criteria = architectural criteria for occupied spaces |

**NRC**
= Noise Reduction Coefficient = arithmetic mean of absorption coefficients
from 250-Hz to 2.0-kHz, rounded to nearest +/- 0.05 |