Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 62777: Quality Evaluation Method for Sound Field of Directional Loudspeaker Array Systems
Measuring Personal Audio Space — A New Benchmark for Focused Sound Technology
1. Introduction to Directional Loudspeaker Arrays
Directional loudspeaker array systems represent a paradigm shift in personal audio. Unlike conventional loudspeakers that radiate sound broadly in all directions, these systems use beamforming techniques — controlling the phase and amplitude of multiple transducers — to create focused sound beams that can be directed to a specific listener. This allows a person to enjoy audio content without disturbing others nearby, making earphones or headphones unnecessary.
IEC 62777, published in February 2016, establishes the first internationally standardized method for evaluating the sound field quality of directional loudspeaker array systems. It defines the concept of the Personal Acoustic Zone (PAZ) and introduces quantitative metrics — the Personal Acoustic Zone Index (PAZI) family — to objectively measure how well a system focuses sound for an individual listener.
The PAZ concept is the core innovation of IEC 62777. It defines a three-dimensional acoustic “bubble” around a listener’s head where the directional system should deliver sound, and where sound should be minimized outside this zone.
2. The Personal Acoustic Zone and PAZI Metrics
2.1 Defining the Personal Acoustic Zone (PAZ)
The PAZ is defined as the acoustic space occupied by a person — essentially a volume surrounding the listener’s head where the directional loudspeaker system is intended to deliver sound. IEC 62777 specifies precise measurement geometries with defined surface positions around the listener at ear height, enabling reproducible evaluation across different systems and test environments.
2.2 PAZI Measurement Indices
To quantify the focusing performance, IEC 62777 defines several index metrics. These ratios compare the sound pressure level within the target PAZ against levels at other listening positions, providing a clear figure of merit for directivity performance.
| Metric | Definition | What It Measures |
|---|---|---|
| PAZI | Ratio of target PAZ SPL to total SPL of all listeners | Overall focusing efficiency |
| PAZI-x | Front SPL / (Front + Front-Right + Front-Left) SPL | Lateral sound confinement |
| PAZI-y | Front SPL / (Front + Rear) SPL | Front-to-back sound isolation |
| PAZI-xy | Front SPL / (Front + Right + Left + Rear) SPL | Combined lateral + front-back isolation |
| PAZI-xyz | Front SPL / (All positions including upper) SPL | Full 3D sound confinement |
PAZI values range from 0 to 1. A value close to 1 means the system delivers sound almost exclusively to the target listener. In practice, well-designed directional arrays achieve PAZI values above 0.7 in anechoic conditions.
3. Measurement Setup and Methodology
IEC 62777 prescribes a detailed measurement framework to ensure reproducibility. The key test conditions include:
Test Environment: Measurements are conducted in an anechoic or semi-anechoic room to minimize reflections that would interfere with the directional sound field characterization. The standard also provides guidance for in-situ measurements for systems that cannot be moved to a laboratory.
Microphone Array: Sound pressure levels are measured at multiple positions arranged on a spherical surface centered on the listener’s head position. The standard specifies the precise angular coordinates for measurement microphones, with the number of measurement points determined by the required spatial resolution.
Personal Distance: A critical parameter is the “personal distance” — the intended listening distance between the user and the directional loudspeaker array. The standard requires measurements at multiple distances to characterize how the sound field changes as the listener moves relative to the array.
| Parameter | Specification |
|---|---|
| Test signal | Pink noise (per IEC 60268-1) |
| Measurement distance | 0.3 m to 2.0 m (personal distance dependent) |
| Microphone height | Ear height of seated listener (typically 1.2 m) |
| Frequency range | 100 Hz to 10 kHz (one-third octave bands) |
| Room condition | Anechoic or semi-anechoic (free-field above 200 Hz) |
| Sound level meter | Class 1 per IEC 61672-1 |
A common pitfall in PAZI measurement is improper microphone positioning. Even small angular deviations of 2-3 degrees can cause significant errors in PAZI-y and PAZI-z indices, particularly at higher frequencies where the directional beam is narrowest.
4. Engineering Design Insights and Applications
The PAZI metrics defined in IEC 62777 give audio system designers clear, quantifiable targets for array optimization. A key design trade-off emerges: increasing the number of array elements improves beam directivity (higher PAZI) but increases system cost, power consumption, and physical footprint. The standard provides the objective tools needed to find the optimal balance for each application.
Directional loudspeaker arrays are finding growing adoption in consumer electronics (smartphones, tablets, laptops, and gaming devices), digital signage (museum exhibits and retail displays), automotive audio (individual listening zones for each passenger), and public information systems (ticket counters and information kiosks). IEC 62777 enables manufacturers to specify and compare the performance of these diverse implementations on a common, internationally recognized scale.
For automotive applications, IEC 62777 is particularly relevant. Car cabins are acoustically challenging environments with strong reflections from windows and upholstery. PAZI measurements can help engineers evaluate how well a directional system maintains individual listening zones despite these challenging conditions.
5. Frequently Asked Questions
Q1: Does IEC 62777 apply to all types of directional loudspeaker arrays?
Yes, the standard is technology-neutral. It applies to any directional loudspeaker array system regardless of the underlying beamforming technique — delay-and-sum, phased array, parametric array (ultrasound-based), or acoustic metamaterial approaches. The measurement methodology is based solely on the resulting sound field.
Q2: How do PAZI measurements relate to subjective listening experience?
While PAZI is an objective physical measurement, higher PAZI values generally correlate with better subjective privacy — meaning a listener outside the target zone has greater difficulty understanding the audio content. However, the standard acknowledges that subjective evaluation may require additional psychoacoustic testing beyond the scope of PAZI metrics.
Q3: Can PAZI be measured in a normal room instead of an anechoic chamber?
IEC 62777 provides guidance for in-situ measurements, but results in non-anechoic environments will include room reflection artifacts. For comparable and reproducible specifications, anechoic measurements are recommended. In-situ measurements are best used for application-specific validation rather than absolute performance claims.
Q4: What is the recommended minimum number of measurement positions?
The standard recommends a minimum of 6 measurement positions around the PAZ for basic characterization, with more positions (12-24) needed for higher spatial resolution, particularly when evaluating PAZI-xyz for full 3D confinement analysis.
