Tuesday, January 5, 2010

Speech Intelligibility

Have you ever been to a conference, church service, or any event where a speech is made, and it is hard to understand what is being said? Not because the guest speaker is Charlie Browns school teacher, but because your in a tin building, the speakers are aimed at the ceiling, and the microphone is 5’ from the speakers mouth.

Many sound system designers and day to day engineers do not take into consideration the importance of speech intelligibility. Though, for most of us sound engineers, it is not a life threatening issue. But imagine an event where your potential clients, employees, or congregation did not receive a clear message. Misunderstanding simple words, leading to a false understanding of the message. Or imagine a situation where an air traffic controller is giving instructions to two different pilots and one of them mis-understands one word. Now this situation could be life threatening!

Speech intelligibility issues can come from various situations in the public address system, the interaction of the public address system with the room, placement of microphones, and many more.

In a perfect world, we would only hear the direct sound of the speakers, eliminating the sounds of rooms, and their reflections.

The interaction of a speaker system in a room is very complex to understand, model, or measure. But there are some great tools out there that engineers are using to analyze rooms and their acoustical responses to a public address system. One of these tools is a software called SMAART. "Smaart" stands for Sound Measurement Acoustical Analysis Real Time Tool. Smaart is used to measure PA's in specific venues or recording studios, mainly to initially tune and adjust the PA to operate at it’s maximum potential in a specific venue.

The proper use of SMAART can greatly improve intelligibility, increasing the impact of a speech by the CEO who has something really important to say.

Speech Intelligibility is measured by %ALcons (Percentage Articulation Loss of Consonants). It is computed from measurements of the Direct-to-Reverberant Ratio and the Early Decay Time using a set of correlations defined by SynAudCon, and is specified in percentages.

Consonants play a more significant role in speech intelligibility than vowels. If the consonants are heard clearly, the speech can be understood more easily.

Since %ALcons expresses loss of consonant definition, lower values are associated with greater intelligibility. It is generally assumed that the maximum allowable value for typical paging applications is 10%, assuming that the environment is relatively free of masking noise. For learning environments and voice warning systems, the desired value is 5% or less.

Alcons is the measured percentage of Articulation Loss of Consonants by a listener. % Alcons of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is heading toward bad intelligibility, and 15% typically is the maximum loss acceptable.

Capsicum Pro Audio & Visual uses SMAART to acoustically correct our sound systems to match the venues and address the issues of reverberation, time alignment, and phase problems that occur in most rooms and venues.

Line Arrays have allowed us to drastically reduce the amount of acoustical energy directed at reverberant surfaces, and focus the energy at the listeners.Greatly enhancing the quality of our services by allowing us to achieve lower %ALcons.

Feel Free to check out our Blog on Line Array technology.


Noah Waldron



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