The importance of good Speech Intelligibility
The fundamental purpose of an airspace with or without a sound reinforcement system is to deliver clear speech intelligibility to the listener at a comfortable volume level. A surprising number of spaces fail to achieve this basic goal. There can be many reasons for this, ranging from inadequate signal to noise ratio to poor room acoustics or inappropriate choice or location of loudspeaker.
It is the job of the acoustic and sound system engineer to take these factors into account when designing a room layout / shape / sound system and selecting devices to provide the degree of intelligibility required. Although sound quality and speech intelligibility are inextricably linked, they are not the same thing.
It is quite possible for example to have a poor sounding system that is intelligible or alternately a high quality system that is virtually unintelligible such as a hi-fi speaker in an aircraft hanger. A common mistake is to confuse audibility with clarity. Just because a sound is audible does not mean to say it is intelligible.
Audibility relates to the ability of a listener to physically be able to hear a sound, whereas clarity describes the ability to detect the structure of the sound. In the case of speech, this means hearing the consonants and vowels correctly in order to identify the words and sentence structure and so gives the speech sounds intelligible meaning.
What Factors Affecting Speech Intelligibility?
Primary factors
- Room reverberation time (RT 60).
- Sound system bandwidth and frequency response.
- Loudness and signal to noise ratio.
- Volume and shape of airspace.
- Distance of the listener to the loudspeaker.
- The number of loudspeakers operating in the airspace.
- The direct to reverberant ratio.
- Talker annunciation/rate of delivery.
- Listener acuity.
Secondary factors
- System distortion.
- System equalization.
- Uniformity of coverage.
- Presence of early reflections.
- Sound focusing or late arrival high level reflections.
- Direction of sound arriving at the listener.
- Direction of interfering noise.
- Gender of talker.
- Vocabulary and context of speech information.
- Talker microphone technique.
How to Measure Speech Intelligibility?
Various measurements have been developed for measuring speech intelligibility in air spaces for both acoustic and electro acoustic sound sources.
1. Percentage of articulate loss of consonants.
2. C50, ratio of total energy occurring in the first 50ms to the total sound energy of the impulse response.
3. Speech transmission index. (STI)
4. Rasti.
The STI technique was also developed in Holland at about the same time as Puetz developed % Alcons. While the % Alcons method has become popular in the United States, the STI has become popular and far more widely used in Europe and has been adopted by a number of European Standards and codes of practice relating to sound system speech intelligibility performance as well as International Standards relating to audio performance. It is interesting to note that while % Alcons was developed primarily as predictive technique, STI was developed as a measurement method as is not straightforward to predict.
The technique considers the source/room (audio path)/listener as a transmission channel and measures the reduction in modulation depth of a special test signal as it traverses the channel. A unique and very important feature of STI is that it automatically takes account of both reverberation and noise effects when assessing potential intelligibility.
Schroeder later showed that it is also possible to measure the modulation reduction and hence STI via a system’s impulse response. Modern signal processing techniques now allow a variety of test signals to be used to obtain the impulse response and hence compute the STI – including speech or music.
A number of instruments and software programs are currently available that enable STI to be directly measured. However care needs to be taken when using some programs to ensure that any background or interfering noise is properly accounted for.
The full STI technique is a very elegant analysis method and is based on the amplitude modulations occurring in natural speech. Measurements are made using octave band carrier frequencies of 125 Hz to 8 Hz, thereby covering the majority of the normal speech frequency range. Fourteen individual low frequency (speech like) modulations are measured in each band over the range -.63 Hz to 12.5 Hz.
A total of 98 data points are measured for each STI value (7 octave bands each with 14 modulation frequencies). Because STI method operates over almost the entire speech band it is well suited to assessing audio performance. The STI scale ranges from 0 to 1. Zero represents total unintelligibility while 1 represents perfect sound transmission.
What is a speech intelligibility scale?
This is a test developed at the Hearing Aid Research Laboratory of the University of Memphis with support from other departments. When taking the test, listeners rate the intelligibility of certain test passages on an equal-appearing interval scale from 0 – 100. 0 indicating when none of the words is understood and 100 indicating when all the words were understood. The test is known as (SIR) Speech Intelligibility Rating.
Read more …… Influence of Reverberation on speech
0.5 seconds of reverberation.
2. seconds of reverberation.
Below is a table of Recommended Reverberation times in seconds for various venues. Where speech indelibility is important such as classrooms and court rooms the reverberation times are low.
Recommended Reverberation Times |
|
|
Venue |
Recommended Reverberation times (Sec) |
|
Bedrooms |
0.5 seconds |
|
Churches and Worship Centres |
>1.2 seconds or less |
|
Churches with classics music |
<1.2 seconds |
|
Class Room |
0.6 Seconds |
|
Conference rooms |
>0.7 seconds |
|
Courthouses – unamplified |
0.7 seconds |
|
Courthouses – amplified |
1.0 second |
|
Gymnasium |
1.4 seconds to 2.2 seconds |
|
Homes |
0.9 seconds |
|
Lecture Hall |
1.0 Second |
|
Library |
0.8 seconds |
|
Meeting Room |
0.5 seconds |
|
Multi Purpose Auditoriums |
>1.3 Seconds not greater than 1.5 seconds |
|
Music Rehearsal rooms |
0.9 seconds to 1.2 seconds |
|
Open Office |
0.8 seconds |
|
Performance Venues |
1.2 seconds |
|
Recording Studios |
0.3 seconds |
|
Restaurants – intimate |
0.8 seconds |
|
Restaurants – lively |
1.1 seconds |