Can You Hear Me Now?
Jun 1, 2005 3:58 PM, By Mei Wu and James Black
Effects of Physical Environment on Speech Intelligibility in Teleconferencing
Figure 1. Speech intelligibility in noise for different types of test materials. Speech intelligibility is shown as function of speech-to-noise ratio for sentences, monosyllabic words, and nonsense syllables. These curves are approximate and depend on the test conditions, vocabulary size, and how the speech level is specified (See note 5).
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We all probably have experienced poor speech intelligibility during teleconferencing. It happens even when high-quality sound systems are used in both the talker's and listener's rooms, and even when both rooms have good speech intelligibility during local conferences. These observations reveal two facts: (1) The physical environment in a conference room is as important as a sound system, and (2) Physical environment requirements for teleconferencing are more stringent than for local conferencing. This article shows how physical environment affects speech intelligibility in teleconferencing. A bad environment can spoil speech intelligibility despite the merit of a sound system, and a good environment can enable a sound system to achieve its design potential.
Assessing Speech Intelligibility with STI
Speech intelligibility is determined by measuring the proportion of test items, such as words or syllables, that are heard correctly. In a typical speech intelligibility test, a specified set of syllables, words, phrases, or sentences is presented to a listener. The listener responds by writing down what was heard. Test results collected over the years show that speech intelligibility is reduced by the increase of background noise (or decrease of signal-to-noise ratio) and by increase of reverberation time. Typical curves showing the relationship between speech intelligibility and signal-to-noise ratio or reverberation time are shown in Figures 1 and 2.
Figure 2. Speech intelligibility as a function of reverberation time and speech-to-noise ratio (See note 6).
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Instead of going through speech intelligibility tests, indices have been developed to assess speech intelligibility based on measurements of physical environment—i.e. background noise and reverberation time. The commonly used indices include Speech Interference Level (SIL), Articulation Index (AI), Speech Intelligibility Index (SII) and Speech Transmission Index (STI). The authors prefer to use STI because the first three indices calculate speech intelligibility only from signal-to-noise ratio (with modifications for reverberation time and other factors). STI is the only method which takes into account both signal-to-noise ratio and reverberation time.
Figure 3. A typical speech waveform showing instantaneous sound pressure as a function of time. The resolution of the time scale is such that the waveform itself cannot be seen in any detail, but the amplitudes of the instantaneous pressure variations are clearly visible as a function of time at a distance of 1 meter (See note 7).
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Speech Transmission Index (STI) was developed in the Netherlands by Tammo Houtgast and Herman Steeneken (See note 1). It determines speech intelligibility based on the modulation depth of speech waveform. Figure 3 is a typical speech waveform showing instantaneous sound pressure as a function of time. It has a high frequency carrying wave with its amplitude modulated by a low frequency modulation wave. The resolution of the time scale in Figure 3 is such that the carrying wave itself cannot be seen in any detail, but the amplitude modulation is clearly evident. The amplitude-modulated speech waveform in Figure 3 shows major peaks at roughly 100, 300, 600, 850, and 1100 milliseconds. The difference in level between a peak and an adjoining valley is referred to as the depth of modulation. If no noise or reverberation alters speech, there is very little energy in the valleys between peaks, modulation depth is 100 percent, STI value is 1 and speech intelligibility is excellent. Background noise and/or reverberation add energy in the valleys, reduce the depth of modulation, and reduce STI value and speech intelligibility. When STI value goes down to 0, a speech is totally unintelligible. The table below shows STI values and corresponding speech intelligibility assessment.
STI and Speech Intelligibility
In the complete version of STI testing, the modulation depth is measured in 98 tests over 14 modulation frequencies (0.63Hz to 12.5Hz in 1/3 octave bands) for seven octave bands (from 125Hz to 8,000Hz) of carrying wave. Simplified Speech Transmission Index measurement methods, such as RASTI (Rapid Analysis—or Room Acoustics as known by some—Speech Transmission Index) and STIPA (Speech Transmission Index—Public Address), are defined in Standard IEC 60268-16.
STI can also be determined from calculated modulation transfer function, when the impulse response of a room can be regarded as a well-behaved room response with an exponential decaying envelope. A simplified formula for modulation transfer function at frequency F can be expressed as a function of the reverberation time T and effective signal-to-noise ratio S/N:
Case Studies
During teleconferencing, when speech in a talker's room is transmitted to a listener's room, background noise in the talker's room is also transmitted and amplified in the listener's room. While direct sound is transmitted to the listener's room, reflected sound is also transmitted to the listener's room. This reduces the signal-to-noise ratio and extends the reverberation time, and consequently reduces speech intelligibility. As a result, even if the sound system is perfect, speech intelligibility during teleconferencing is lower than during local conferencing. The authors' experience is that conference rooms with good to fair speech intelligibility for local conferencing may have fair to poor speech intelligibility in teleconferencing. Therefore, before installing a teleconferencing system, it is sensible to consult a professional to ensure that the physical environment in a conference room will sustain a good to fair speech intelligibility.
The following case studies demonstrate how the physical environment, such as background noise, reverberation time, microphone distance, and orientation, affects speech intelligibility. To concentrate on the effects of the physical environment, we assume the sound systems are perfect. The physical conditions of each case are listed in tables. Column 1 lists the case numbers. Column 2 is the tested or predicted STI values. Column 3 gives a description of speech intelligibility. Column 4 shows either the STI is in the talker's room or the listener's room. Column 5 shows the distance from the talker to the receiver (a microphone or listener in the talker's room). Column 9 gives a brief description of the rooms. The last column is the distance between a loudspeaker and listeners in the listener's room.
Cases 1 and 2 are STI values measured in a 12x24 square feet conference room with acoustical ceiling, gypsum board walls, and carpeted floor. We can see that for local conferencing, speech intelligibility is good for listeners at 45 degrees 3ft. from a talker with a normal voice (Case 1) and at 60 degrees 6ft. from a talker with a normal voice (Case 2). When the speech is transmitted through a perfect sound system to another room, with the talker using a raised voice, speech intelligibility reduces to fair and poor (See note 2) (Cases 3 and 4).
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Comparing Case 4 with Case 3, we can see that when the microphone is moved closer to the talker (from 6ft. to 3ft.), speech intelligibility improves from poor to fair.
Cases 5 through 9 are telemedicine cases. The talker's room is a 20'x30'x10' operating room with gypsum board ceiling and walls, tile floor, and eight people. The microphone is located 3ft. above the surgeon's head at about 60 degrees. The background noise level in the operating room is about NC-45 (See note 3). The noise is mostly generated by the air handling system and surgery equipment. A group of 10 people is observing the surgery in an observation room (listener's room). The room is 20'x15'x10' with gypsum and glass walls with acoustical ceiling and carpeted floor. The voices are transmitted from the operating room to the observation room through a perfect sound system. The loudspeaker in the observation room is located in the front of the observers, facing them. The background noise in the observation room is about NC-40.
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Case 5 shows that in the operating room, when the surgeon speaks, the speech intelligibility is poor (STI 0.40) to a person at 2ft. in front of him/her. Case 6 shows that speech intelligibility is bad (STI 0.24) to the person standing at 90 degrees from the surgeon's face at the end of the operating table. Case 7 shows that speech intelligibility to the people in the observation room is also poor (STI 0.31). Case 8 shows that if the surgeon raises his/her voice, speech intelligibility improves some but is still poor (STI 0.36). Case 9 shows that moving the loudspeaker closer to the observers does not help much (STI 0.37).
These STI values are expected considering the fact that the operating room has no sound absorptive material and has a reverberation time of 2.4 seconds. In a room with such a long reverberation time, people may be able to understand simple orders, but cannot carry a conversation. Traditional speech intelligibility vs. reverberation time curves shown in Figure 2 also indicate a poor speech intelligibility (Percent Intelligibility below 35 percent) in a room with a 2.4 second reverberation time.
Case 10 shows how adding sound absorptive materials on the ceiling of the operating room improves speech intelligibility from poor (STI 0.36 in Case 8) to fair (STI 0.52). Here we assume the sound-absorbing materials meets the cleanability, bacteria resistance, and low particle shedding requirements for operating rooms.
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Case 11 shows how adding 70 square feet of sound absorptive materials (See note 4) to the walls of the conference rooms in Case 3 improves speech intelligibility from fair (STI 0.55) to good (STI 0.62).
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Cases 12 and 13 show test results of how reducing the background noise from 45dBA to 35dBA in a 25'x25' conference room improves speech intelligibility from fair to good, although we realize that in some cases reducing background noise may not always be a valid option, if the noise is mostly audience noise.
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Final Notes
The case studies presented in the article demonstrate how physical environment, such as acoustical treatment, background noise, and microphone location and orientation, affects speech intelligibility during teleconferencing. By improving the physical environment, without changing the sound system, speech intelligibility can be improved from poor to good. It should be noted, however, that the calculations listed in the tables should not be used as general rules to predict the performance of any conference rooms because STI values vary with acoustical conditions. It is recommended that professionals be consulted and STI calculations be conducted for important teleconferencing rooms to ensure that the physical environment will sustain the performance of the sound system.
Mei Wu and James Black are acoustical consultants at Mei Wu Acoustics. Their resumes can be found at www.mei-wu.com. For questions or further information on speech intelligibility please contact Mei Wu at meiwu@mei-wu.com.
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