Ohmic Audio

Audio Levels, Metering, and Headroom

This page is the house reference for the terms that get mixed up most often in DSP, measurement, and product specs: dBFS, dBu, dBV, sample peak, true peak, LUFS, headroom, and the reporting rules that keep SNR, dynamic range, and THD+N comparable.

State the Reference Before the Number

Decibels are relative units. A number in dB is incomplete until the baseline is clear.

House rule: all digital levels are in dBFS unless noted; all analog levels are in dBu unless otherwise specified.

Common Reference Values

Reference Meaning Voltage
0 dBu professional analog reference 0.775 Vrms
+4 dBu common pro line nominal about 1.23 Vrms
0 dBV consumer voltage reference 1.0 Vrms
-10 dBV common consumer line nominal about 0.316 Vrms

Conversion shortcut: dBV = dBu - 2.21.

dBFS: RMS, Sample Peak, and True Peak

dBFS needs a convention, not just a label. This reference treats 0 dBFS as full-scale sine RMS unless explicitly stated otherwise. Sample peak and true peak are separate measurements and should be reported separately when they matter.

True peak can exceed sample peak because the reconstructed analog waveform can overshoot between samples. That is why a system can avoid sample clipping and still clip a DAC or encoder downstream.

Headroom Must Reference a Ceiling

Headroom is the remaining margin between the operating level and a named limit. Do not say "1 dB of headroom" by itself.

Preferred phrasing: 1 dB headroom relative to -1 dBTP.

Use true-peak headroom for delivery and encoding chains. If a spec uses sample peak instead, say sample-peak explicitly.

True Peak for Real Delivery Safety

dBTP is the meter that matters when reconstruction or encoding can overshoot. A good default ceiling for general music delivery is -1 dBTP.

LUFS and Program Loudness

Use LUFS or LKFS for perceived program loudness, not RMS. LUFS is built from the ITU loudness model and uses gating so silence and very low-level passages do not bias the long-term number the same way a simple RMS meter would.

Meter Use it for Do not confuse it with
LUFS / LKFS program loudness sample peak or RMS power
dBTP delivery ceiling safety integrated loudness target
dBFS RMS signal power reference perceptual loudness

For practical work, loudness and true peak should be paired. One answers "how loud does the program average out?" and the other answers "will this overshoot the route or codec?"

SNR, Dynamic Range, and THD+N Are Not Interchangeable

Metric What it means What must be stated
SNR signal relative to noise floor reference level, weighting, bandwidth
Dynamic Range usable level span between floor and ceiling or criterion method, weighting, bandwidth, criterion
THD+N harmonic distortion plus noise under a specific test condition test level, bandwidth, weighting, frequency, method

Do not compare these numbers casually across products unless the measurement method matches. A-weighted and unweighted results are not interchangeable, and numbers collected under different standards or different bandwidth limits often look comparable when they are not.

Measurement Procedure Rule

Every published performance number should state the standard or clearly named custom method behind it. In pro and semipro digital audio measurement work, that often means AES17 or IEC 61606-3 style language. The key is not memorizing every clause. The key is naming the method so comparisons stay honest.

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