Transportation hubs are self-contained worlds of controlled chaos. Travelers are inundated with a wide range of sights, sounds and smells as they pass through any busy transportation hub: arrival, security checkpoints, boarding and departure announcements, safety and security information, marketing and services messaging, and the droning murmur of fellow travelers moving from point A to point B. It is a flood of input for the senses at every step. Transportation managers are challenged with the need to cut through the clutter, capture attention and deliver actionable and perhaps critical safety information to a widespread, quickly-moving public.
Speech intelligibility is paramount in transportation hubs. The larger the room or space, the more critical speech intelligibility becomes. If travelers can’t hear or understand announcements and instructions, their ability to learn, retain and act on the information is compromised.
Intelligibility plays a significant role in information retention and audio fatigue. If someone is in a reverberant space like an airport, train or bus station and all they hear is muffled speech, they’ll quickly give up trying to understand what’s being communicated. The higher the speech intelligibility, the easier it is for listeners to retain information and understand what actions, if any, they need to take.
Speech Intelligibility and STI/STIPA
Simply put, speech intelligibility is the percentage of speech a listener can understand. Intelligibility is affected by the volume level and quality of the speech signal, the type and level of background noise, room reverberation and more. It’s an essential consideration for AV systems in transportation hubs.
The Speech Transmission Index (STI) is a numeric representation measure of communication channel characteristics whose value varies from 0 (bad) to 1 (excellent). STI predicts the likelihood of syllables, words and sentences being comprehended. The only way to provide an accurate speech intelligibility rating is to measure it in the space or room under typical ambient conditions (which is highly recommended). To measure STI, a test signal is played through the audio system, which is then measured with a measurement microphone. Distortion in the audio system, background noise and reverberation affect the transmissions, which the measurement mic will detect. Comparing the original signal with the received signal determines the STI rating. A rule of thumb for the threshold of 100% intelligibility is usually a signal-to-noise ratio (SNR) of 12 decibels (dB), meaning the signal should be roughly four times louder than the noise floor.
The Speech Transmission Index for Public Address Systems, or STIPA, is a modified form of the STI scale regarding intelligibility ratings. It’s considered a fast and accurate way to measure the STI of a given area, taking into account the quality of the loudspeakers, signal interference, room acoustics and more.
An important point to remember is that effective coverage in a space depends more upon loudspeaker placement, the type of loudspeaker and spacing between loudspeakers than on the loudspeaker's volume. Achieving sufficient intelligibility is more than just a matter of turning up the volume or increasing the wattage of the loudspeaker. Many poor communications result from trying to compensate for an insufficient number or poor placement of loudspeakers with too much volume.
For transportation causeways, concourses and gates, beam-steering technology allows loudspeakers to tightly control the sound and place it where needed — on the passengers — while keeping it away from other surfaces that may cause echoes, reflections and reverberation. Beam steering allows system designers and integrators to control sound dispersion in the vertical axis on a driver-by-driver basis (most loudspeakers have more than one driver), focusing the sound where they want rather than broadcasting it uselessly towards the ceiling or floor. Beam-steering loudspeakers are typically much taller than wide and mounted flat against the wall since the beams are digitally steered. For that reason, they’re much more cost-effective than flying a line array and much easier to blend into the building’s architecture. Properly installed and tuned beam-steered arrays can result in as little as a 2 dB drop in volume front to back (closer to the loudspeaker vs. further away).
Audibility and Intelligibility in Emergency Situations
Most transportation hubs have strict requirements for audibility and intelligibility per local or national fire codes such as NFPA 72. Think of the adage “loud and clear” — “loud” refers to audibility, while “clear” refers to intelligibility. Both are critical to providing proper instructions in an emergency to building occupants, but the terms are not the same.
Audibility is the state or quality of being perceptible by the human ear and is measured in A-weighted decibels (dBA). A-weighting gives more value to frequencies in the middle of human hearing and less importance to frequencies at the edges than a flat audio decibel (dB) measurement. According to NFPA 72, “public mode signaling” is required to provide and maintain a sound pressure level of 15 decibels (dBA) above ambient sound levels. The designer should design for and then measure the installation for proper SPLs. Every time the distance from the loudspeaker is doubled, the sound pressure drops by as much as 6 dB, so the designer must plan for this and install the loudspeakers accordingly.
An important term to understand when designing for intelligibility is an Acoustically Distinguishable Space (ADS). NFPA 72 defines an ADS as an emergency communications system zone, subdivision, physically defined space or section of a room that can be distinguished from other spaces due to different acoustic, environmental or use characteristics such as ambient noise floor. All parts of a building intended to have occupant notification are subdivided in ADS. If a room has different acoustic properties in different areas, the areas become multiple ADS. Each ADS must be identified as either requiring intelligibility or not requiring intelligibility.
NFPA 72 states that intelligibility is acceptable if at least 90 percent of the measurement locations within each ADS have a measured STI of not less than 0.45 and an average STI of not less than 0.5 STI. For native speakers, 0.5 STI equates to roughly 58% of syllables being understood, 83% of words understood and 93% of sentences understood.
Audio Improvements at LAX
The $1.6 billion West Gates addition at Los Angeles International Airport (LAX) features 15 aircraft gates and an interior organized into a series of “neighborhoods,” each including multiple aircraft gates, a restroom core and a wide selection of various retail and concession spaces. The terminal expansion includes characteristic high ceilings, reflective glass walls, hard reflective floors and ambient noise expected from a major transportation hub.
The audio portion of the 750,000-square-foot concourse construction was critical to LAX as crisp and intelligible sound helps the airport’s millions of annual travelers stay on the move and up to date with important announcements and communications. Thus, the loudspeakers had to provide precise beam control and high speech intelligibility while offering a discrete footprint to fit the architectural requirements and constraints.
The airport deployed ICONYX Gen5 arrays from Renkus-Heinz throughout the terminal to meet their acoustic needs. Designed explicitly for terminal-wide passenger announcements and gate announcements, the system covers all common areas within the terminal, including all gate seating areas, restrooms, retail and restaurants in the concourse. This digitally steerable capability produces crisp and intelligible audio, ensuring everyone hears announcements clearly.
Clarity Amongst the (Controlled) Chaos
Every listener deserves the clearest sound possible, and intelligibility is especially critical in transportation hubs. Designing for intelligibility can be complex since many factors, such as ambient noise, reverberation, distortion, frequency response and physical room characteristics, can affect how much of the message is understood. Designers significantly impact speech intelligibility through their choice of equipment, the number, distribution and placement of loudspeakers, and the power at which those loudspeakers are driven. When designing AV systems for transportation hubs, consulting an acoustician or acoustical engineer and utilizing acoustic modeling software are recommended to ensure adequate coverage, audibility and intelligibility. Using loudspeakers with high directivity — such as digitally steerable arrays — provides you granular control over where the audio is distributed, decreasing audio fatigue in the traveler and even reducing travel-related stress.
Garrison Parkin is the Western Regional Sales Manager for Renkus-Heinz.