SNR — Signal-to-Noise Ratio

Signal-to-Noise Ratio

For how SNR relates to gain structure and level setting, see glossary/gain-structure. For SNR in room acoustics and STI, see glossary/sti.

SNR (Signal-to-Noise Ratio) is the ratio of the desired signal level to the background noise level, expressed in decibels. A higher SNR means the signal is much louder than the noise — the audio is clean. A low SNR means noise is nearly as loud as the signal — the audio sounds hissy, grainy, or muddy. SNR is a fundamental specification for every audio component: microphones, preamps, ADCs, DSPs, amplifiers, and cables. In AV system design, SNR determines the quietest signal the system can handle before noise becomes audible, and it sets the practical limit on how quiet a room must be for intelligible speech.

SNR Formula

SNR (dB) = 20 × log₁₀(Signal Voltage / Noise Voltage)
         = Signal Level (dBu) − Noise Floor (dBu)

A microphone with a noise floor of −128 dBu and a maximum output of −24 dBu has an SNR of 104 dB. A DSP with an analog input noise floor of −95 dBu and a clip point of +24 dBu has an SNR of 119 dB.

Typical SNR Values in AV Components

A-weighted SNR measurements apply a frequency weighting that matches human hearing sensitivity (de-emphasizing very low and very high frequencies). This is the standard for audio equipment SNR specifications.

SNR and Room Noise Floor

Room background noise (NC rating, dBSPL) must be well below the signal level for intelligible speech. For conferencing:

• Target speech level at the microphone: 65–75 dBSPL
• Target room background noise: < 45 dBSPL (NC-35 to NC-40)
• Resulting SNR at the microphone: 20–30 dB

At SNR < 15 dB, speech intelligibility (STI) drops below acceptable levels even with perfect electronics. Room acoustic treatment and HVAC noise control are as important as microphone selection for achieving adequate SNR in a conferencing space.

A-Weighting and Measurement Standards

SNR specifications use several weighting curves:

• A-weighted (dBA): matches human hearing sensitivity; used for most audio equipment specs and acoustic noise measurements
• Unweighted (flat): used in some technical specifications and test standards
• IEC 61672: defines sound level meter classes and the A-weighting filter for measurements

When comparing SNR specifications between devices, verify both use A-weighting — comparing an A-weighted spec to an unweighted spec gives a meaningless result.

Common Pitfalls

• Insufficient SNR in the room acoustic environment. A microphone with 80 dB SNR is irrelevant in a room with 55 dBSPL HVAC noise and 65 dBSPL speech — the room SNR is only 10 dB. Fix: address room acoustics and HVAC noise before specifying high-specification microphones; the weakest link is almost always the room, not the electronics.

• Daisy-chaining multiple preamp stages, each adding noise. Each gain stage adds its own noise floor contribution. A signal that passes through four preamp stages accumulates noise from each stage. Fix: apply gain as close to the signal source as possible (at the microphone preamp); subsequent stages should amplify signal, not noise.

• Confusing SNR with dynamic range. Dynamic range is the ratio of the loudest signal the device can handle to its noise floor (clip point to noise floor). SNR is the ratio of a specific signal level to the noise floor. A device can have high dynamic range but poor SNR if the operating level is set close to the noise floor. Fix: distinguish between the two; set gains to ensure operating level is well above the noise floor (good SNR) while remaining well below clip (good headroom).