Speech Intelligibility Rating Explained

Author: Gina Cunsolo

Here’s an interesting topic:

Have you ever thought about the science of speech intelligibility - or, what makes certain speech easier to understand than others? It’s an important topic, particularly when considering emergency announcements in public areas. 

The ability to clearly understand emergency announcements in public places like shopping centers, stadiums, train stations and airports can make a life or death difference for the listener. Concertgoers and shoppers need to clearly hear exit instructions in an emergency - and the sound must be loud and clear enough to carry in spite of any potential panic noise from people that may ensue.

In fact, there’s an actual objective measurement process involved in determining the intelligibility of speech. It’s referred to as the STI, or Speech Transmission Index. Measuring the numeric value is a complicated process, comprising multiple components. For simplicity’s sake, it’s helpful to understand three components crucial to speech intelligibility: Signal-to-Noise Ratio, Decay, and Distortion. Let’s break down these three.


Signal-to-Noise Ratio:

  • This is the degree at which noise obstructs or masks the main voice signal. If you’ve ever had to work from home while your kids are in online classes on their laptops behind you, you understand that humans can successfully fight through the background noise to listen to the main information. Generally, we can tune out background noise. However, once the background noise becomes louder than the actual main sound, - or, if it sounds too similar to the main sound - it becomes much harder to focus and clearly hear what’s being said. A fancier way to say this? The speech becomes less intelligibile.


  • Decay refers to sound reflections that can obscure speech and confound the listener. Reverberations or echoes have this effect. What does this sound like? Well, we’ve all been in a restaurant with high ceilings, metal chairs, and glass walls - usually, these kinds of environments are rich in reverberation and low in speech intelligibility.


  • Distortion occurs because of faults in the speech transmission system. While not relevant for person-to-person conversation, this is important when considering speech intelligibility on a loudspeaker or any kind of sound projection. This typically occurs at public speaking events, where the technology isn’t cooperating and the speaker’s voice becomes unclear.


Because speech intelligibility is usually measured in public places using a loudspeaker or speech amplification system, not in person-to-person conversation, eliminating distortion is crucial to improving speech intelligibility. In order for a business or organization to meet speech intelligibility requirements of NFPA 72, or, the National Fire Alarm Code, the communication system must clearly amplify the speaker’s voice. 


Now, you might be thinking: Is it possible to intentionally make speech less intelligible?




This is exactly how Yamaha’s VSP-2 functions. The Speech Privacy Solution utilizes a custom combination of human speech and natural sounds to reduce the speech intelligibility of the external, accidental listener. But before we get to that, let’s first review how to improve speech intelligibility.


How to Improve Speech Intelligibility


Voice Alarm System

When trying to improve speech intelligibility, the first step is to check your voice alarm system. Most often, whoever set it up may not have wired the speakers correctly - or, maybe there’s a specific component of the system that stopped working. Any of these can create distortion, and ultimately, lessen speech intelligibility. Signal generators and measuring devices can help identify these types of issues.

Furthermore, many organizations have not installed their voice alarm systems correctly. Spaces often have installed too-few speakers, leaving audio gaps in sections where people may be standing or walking, and in order to compensate, some speakers are set to volumes that are way too loud for intelligibility. However, louder does not always mean clearer. The most effective layout consists of multiple speakers evenly distributed, all playing at the same volume. 


Room Acoustics

Question: How do you make sure someone hears you clearly? Answer: The direct sound of speech must be significantly louder than any reflections of that sound. When that’s not the case, it’s most likely due to an issue of room acoustics. We mentioned earlier that environments containing a lot of hard surfaces like glass, concrete, and metal will create reverberation and reduce speech intelligibility. Conversely, softer surfaces like carpets, upholstered furniture, curtains, or acoustic panels will absorb sound and reverse the effects of reverberation, making speech more intelligible.


Background Noise

Lastly, it may be obvious, but background noise obscures speech and decreases speech intelligibility. Outside noises such as traffic or construction can become less noticeable by updating windows or adding structures that shield the inside room from noise, improving speech intelligibility. 


How to Decrease Speech Intelligibility 


Though speech intelligibility is crucial in emergencies, there are certain situations in professional environments where speech should be harder to understand. Many modern shared offices are either open or built with partitions that do not successfully protect sound from leaking into other rooms. This can be particularly problematic during a sensitive conversation. How do you protect speech privacy without making a large-scale architectural overhaul?


The answer is by installing Yamaha’s VSP-2 Speech Privacy System. The VSP-2 uniquely disrupts speech intelligibility for those outside of the conversation. However, rather than by increasing reverberation or decay - or, projecting white noise across the entire space like traditional sound masking systems, the VSP-2 emits a unique combination of natural sounds that sound like speech. The combination of sounds includes Speech Sound Masker, Environmental Sound, and Musical Sound Effects.


When & Where to Decrease Speech Intelligibility 


What are some specific use cases when a business would want to decrease speech intelligibility? Think about anytime you wouldn’t want someone listening in on your conversation: this could be the doctor’s office, the bank, the huddle room, the boss’s office, or even the pharmacy. By simply placing the 2x2 micro speakers in these environments and directing the sound toward outsiders, you make it more difficult for them to listen to what you’re saying. To be clear: the VSP-2 is not noise canceling technology, nor does it muffle sounds. It simply adds more speech-like sounds to the atmosphere, so the person on the other end is less likely to accidentally hear the conversation.


Furthermore, because it is such a unique conversation of sounds, the VSP-2 is able to achieve the same level of sound masking at up to 8db lower than traditional sound masking systems. And, it perfectly compliments other Yamaha products such as ADECIA and personal Bluetooth Speakerphones YVC-200, YVC-330, and YVC-1000, because our products cancel out the speech masking noise! In other words, if my office is participating in a hybrid work model and I am working from home while my colleagues are in the huddle room speaking to me via ADECIA microphone system or YVC-100 Personal Bluetooth Speakerphone, I will not hear the speech masking sound from the VSP-2 on my end - the noise canceling microphone on the ADECIA or YVC-1000 will cancel it out!


If you need a way to incorporate speech privacy into your day-to-day, Yamaha Unified Communications is ready to assist with the equipment you need. Click here to learn more!