Overall Loudness Rating

Description

The Overall Loudness Rating (OLR) is a key parameter in the 3GPP telephony transmission planning model, defined in the ITU-T G.107 E-model and adopted by 3GPP. It represents the total loudness loss experienced by a speech signal traversing the complete end-to-end circuit from the talker's mouth at the sending terminal to the listener's ear at the receiving terminal. OLR is expressed in decibels (dB) and is derived from the algebraic sum of three primary component ratings: the Send Loudness Rating (SLR), the Receive Loudness Rating (RLR), and a term representing the effect of sidetone masking (STMR - Sidetone Masking Rating and LSTR - Listener Sidetone Rating).

The calculation is defined as OLR = SLR + RLR + 10*log10(1+10^((STMR - LSTR)/10)). SLR characterizes the loss from the microphone input to the network interface, encompassing the acoustic-to-electric conversion and send-side digital processing. RLR characterizes the loss from the network interface to the earphone/speaker, covering receive-side digital processing and electric-to-acoustic conversion. The sidetone term accounts for the psychological effect where a talker's own voice, heard through the terminal's sidetone path, can mask the perceived loudness of the far-end speech.

In network planning and terminal design, OLR targets are specified to ensure loudness consistency. 3GPP defines default OLR values and acceptable ranges for different service types (e.g., narrowband, wideband, fullband voice). Network operators and device manufacturers design their systems to meet these OLR objectives, which involves carefully balancing the SLR and RLR of terminals and the transmission loss of the core network. Conformance testing for terminals includes verifying that their OLR (calculated from measured SLR, RLR, and STMR) falls within the standardized limits, ensuring interoperability and a predictable user experience. The specifications covering OLR include TS 26.131 (terminal acoustic characteristics), TS 26.132 (codec specific audio processing), and TS 29.809 (study on voice quality monitoring).

Purpose & Motivation

The OLR metric was created to solve the fundamental problem of inconsistent and unpredictable loudness in telephone calls. Historically, without standardized loudness planning, calls could be too quiet (causing listener strain) or too loud (causing distortion or discomfort) depending on the specific combination of handsets and network equipment used. This degraded user experience and made network interoperability challenging.

Its purpose is to provide an objective, calculable target for end-to-end loudness performance, enabling engineers to design networks and terminals that deliver a consistent and comfortable listening level. It addresses the limitations of subjective testing alone by offering a reproducible engineering model. The OLR, as part of the broader E-model used for transmission planning, allows the loudness contribution of each network element (terminal, codec, transmission line) to be quantified and controlled.

The adoption of OLR in 3GPP standards, starting from 3G systems, was motivated by the need to maintain voice quality in evolving mobile networks with diverse codecs, wideband audio, and VoIP technologies. It ensures that new services like VoLTE and VoNR provide loudness stability comparable to or better than traditional circuit-switched voice. By defining OLR requirements, 3GPP ensures that a voice call from a 5G smartphone to a legacy 2G handset, traversing multiple network generations, still has predictable and acceptable loudness, which is a cornerstone of basic telephony service quality.