Eardrum Reference Point

Description

The Eardrum Reference Point (DRP) is a critical, standardized concept in 3GPP specifications for the acoustic testing of User Equipment (UE), such as mobile phones and headsets. It is not a physical point on a device, but a precisely defined theoretical location within an acoustic test fixture, specifically an artificial ear or head and torso simulator (HATS). The DRP is defined as a point located 25 mm from the reference plane on a specific ear simulator (like the IEC 60318-4 or ITU-T P.57 compliant simulators), which models the position of the human eardrum. This allows for reproducible and comparable acoustic measurements across different laboratories and device manufacturers.

In practice, during testing, the mobile device's speaker (receiver) is acoustically coupled to an artificial ear. The microphone inside this artificial ear is positioned and calibrated so that its measurement point corresponds to the defined DRP. When testing sending (transmit) characteristics, an artificial mouth (loudspeaker) is used to generate sound from a defined position relative to the device's microphone, and the resulting electrical signal is analyzed. The DRP serves as the origin for defining the acoustic path. Key performance metrics are evaluated relative to this point. For example, the loudness rating of the receiver (RLR) measures how much the acoustic signal is attenuated from the device's electrical input to the sound pressure level at the DRP. Similarly, sidetone measurements evaluate the path from the artificial mouth to the DRP and back to the device's electrical output.

The definition and use of the DRP involve complex calibration of the entire test setup, including the artificial ear, the test chamber, and the audio analyzer. Specifications like 3GPP TS 26.131 and 26.132 provide detailed procedures for aligning the acoustic test fixture to ensure the DRP is correctly realized. This includes using calibrated sound sources to verify the frequency response and sensitivity of the measurement microphone at the DRP location. By anchoring all acoustic requirements to this standardized reference point, 3GPP ensures that a device passing the tests in a certified lab will provide a consistent and acceptable audio quality experience to the end-user, regardless of the device's internal design or the test lab's location.

Purpose & Motivation

The DRP was formally introduced in 3GPP Release 8 to solve the long-standing problem of inconsistent and non-reproducible acoustic testing for mobile terminals. Prior to its standardization, manufacturers and network operators often used different reference points and test methods, leading to significant variation in measured audio performance. A device deemed acceptable in one lab might fail in another, causing compatibility issues, poor user experience, and increased costs for re-testing and re-design. The lack of a unified reference made it difficult to enforce minimum audio quality standards across the industry.

The creation of the DRP was motivated by the need for objective, repeatable, and globally harmonized testing criteria. It provides a common 'ruler' for acoustic measurements. By defining a single, precise reference point linked to international standards (like those from IEC and ITU-T), 3GPP enabled the development of test specifications that yield consistent results. This benefits all stakeholders: manufacturers can design to a clear target, operators can verify device quality uniformly, and end-users experience predictable and adequate voice call quality. The DRP is fundamental to specifications defining terminal acoustic characteristics (TS 26.131), background noise transmission (TS 26.132), and related audio performance metrics, forming the cornerstone of voice quality assurance in mobile networks.

Key Features

• Standardized theoretical point for acoustic measurement, simulating the human eardrum position
• Located 25 mm from the reference plane on specified IEC/ITU-T compliant ear simulators
• Enables reproducible measurement of key audio metrics like Send Loudness Rating (SLR) and Receive Loudness Rating (RLR)
• Serves as the reference for sidetone masking rating (STMR) and other voice quality parameters
• Fundamental to the calibration and alignment of acoustic test fixtures (artificial ears)
• Ensures consistent terminal audio performance testing across different laboratories and manufacturers

Evolution Across Releases

Defining Specifications