On the importance of early reflections for speech in rooms.
Summary (4 min read)
On the Importance of Early Reflections for Speech in Rooms
- J.S. Bradley and H. Sato National Research Council, Ottawa, Canada, K1A 0R6 M. Picard Ecole d’orthophonie et d’audiologie, Université de Montréal, Montréal, Canada, H3C 3J7 Shortened title: “Early reflections for speech” PACS numbers: 43.55.
- Early reflections for speech - 1 ABSTRACT.
- They provide little information on the expected improvements to speech intelligibility scores in actual rooms due to early reflection energy.
- In addition, the importance of early reflections in rooms is apparently not well appreciated in conventional room acoustics design which is usually based primarily on obtaining a particular preferred reverberation time [11].
- There is often a misconception that reverberation time must be minimized rather than optimised.
II SPEECH INTELLIGIBILITY TESTS IN SIMULATED SOUND FIELDS
- The goal of the speech intelligibility tests in simulated sound fields was first to confirm directly that increased speech energy in early reflections has a similar effect to increased direct speech energy.
- The second goal was to demonstrate that both hearing-impaired and non-impaired listeners benefit from early arriving reflections of speech sounds.
- The benefits of increased early reflections are then shown to be present in more realistic sound fields that also included reverberant sound typical of many rooms for speech communication.
- Finally, in situations where the direct sound is reduced in level such as when the talker’s head is turned, early reflections are shown to be essential to achieving adequate speech intelligibility.
1. Sound field simulation procedures
- All simulated sound fields were produced using an 8-channel electroacoustic system with loudspeakers arranged around the listener in an anechoic room.
- Each of the 8 channels of electronics included programmable digital equalizers that included time delays and reverberators that could all be changed under computer control via a MIDI interface.
- The other seven loudspeakers each produced one early reflection and in some Early reflections for speech - 6 experiments reverberant sound.
- Figure 1 illustrates a measured impulse response for a condition that included a direct sound, early reflections and reverberation.
- Each loudspeaker also reproduced simulated ambient noise with a spectrum shape corresponding to that of an NC 40 contour and with a measured overall level at the listener of 47.6 dBA.
2. Subjects and speech intelligibility tests
- Speech intelligibility scores were obtained using a Fairbanks rhyme test as modified by Latham [5] and as used in previous tests [6-8].
- The test words were embedded in the sentence “Word number ___ is ___ write that down” and were spoken by a male talker.
- No subjects were included who used hearing aids or who had known serious hearing impairment.
- In some analyses the results of the subjects were divided into two groups according to their measured hearing loss (HL).
- This group had a mean age of 60 years and again their mean hearing loss was similar to the expected median for this age [12].
1. Comparison of effects of varied direct sound and varied early reflection levels on
- The first comparisons were based on the results of tests in which subjects performed speech intelligibility tests for sound fields with varied speech signal-to-noise ratio (S/N) and for two types of reflection conditions.
- In one series of tests the sound fields consisted of only a direct sound and varied S/N was obtained by varying the amplitude of the direct sound with constant noise level.
- Results were first considered for all 21 subjects and were not divided according to their level of hearing threshold shift.
- For both series of reflection conditions, speech intelligibility scores increased in a statistically significant manner with increasing S/N(A) as was expected.
- There was no identifiable difference between cases where the same increased S/N(A) was due only to varied direct sound and those where it was due to varied added early reflections.
2. Comparison of the effects of varied early reflection levels for impaired and non-
- In a second analysis of the same data, subjects were divided into two almost equal-sized groups according to their measured hearing loss.
- That is, the trends of the results are in agreement with expectation from previous studies that have considered hearing-impaired listeners [13].
- Again the statistical significance of the results was tested using ANOVA on the conditions with similar S/N(A).
- Therefore these results show that both the impaired and non-impaired listeners benefit in a similar manner from added reflections.
- The results in Figure 4 suggest that these particular impaired listeners would require approximately 5 dB higher S/N(A) values to have the same intelligibility scores as the non-impaired listeners.
3. Evaluation of the effect of varied early reflection levels in the presence of later
- It was thought that the benefits of early reflections might be masked or diminished by the presence of reverberant sound which would normally be present in typical rooms intended for speech communication.
- Thus these ‘effective’ speech levels and S/N(A) values are exactly the same as in the test series where early reflection level was varied without the inclusion of reverberation (given in Table II).
- There were 10 ‘non-impaired’ subjects and 6 ‘impaired’ subjects.
- The mean intelligibility scores are plotted versus effective S/N(A) in Figure 5 along with the results for the previous tests repeated from Figure 4.
- Thus the reverberant energy is relatively small compared to the ambient noise energy and therefore its detrimental effect is negligible for these cases.
4. Demonstration of the benefits of early reflections when the talker’s head is
- In most situations in rooms the presence of early reflections increases intelligibility by effectively enhancing the direct sound component and hence increasing the signal-tonoise ratio.
- One particular example is when the talker is not pointing directly toward the listener but is directing their speech in some other direction.
- Figure 6 plots the mean speech intelligibility scores versus talker angle for cases with a direct sound only and also for cases with added constant early reflections.
- The results in Figure 6 indicate that if there were no early reflections and the talker’s head turned 90 degrees, listeners would find it very difficult to understand speech.
III THE RELATIVE IMPORTANCE OF EARLY REFLECTION ENERGY IN
- The results presented in the previous section have confirmed the benefit of early reflections to obtaining adequate speech intelligibility in rooms for both impaired and non-impaired listeners.
- The experiments in the previous section confirm that the speech energy in early reflections is equally beneficial to intelligibility as similar speech energy in the direct sound.
- Thus the early reflection benefit (ERB) is proposed as a simple measure of the effectiveness of a room’s acoustical design obtained by measuring the relative benefit of the early reflection energy.
- ERB = 10 log{E50/E10}, dB Values of ERB were determined for several rooms used for unamplified speech communication.
- Measurements were made in the unoccupied condition.
2. Four rooms for speech
- The generality of the results from the boardroom were further explored by considering measurements in 4 other rooms that represent a range of conditions intended for unamplified speech communication.
- They varied from a round table meeting room with a considerable amount of added sound absorption (used for teleconferencing) to a small auditorium and a small theatre.
- The maximum ERB values at the largest source-receiver distances are close to +9 dB.
- A brief examination of data from larger auditoria indicated even lower slopes.
- Thus the variation of benefits of early reflections may depend on the overall room size as well as the source-receiver distance.
IV DESIGN EXAMPLE CALCULATIONS
- This section explores the use of a modern room acoustics computer model to evaluate the effects of room design on the relative strengths of early reflections in rooms for speech.
- In the first calculation example, the magnitude of early reflection energy was varied by varying the height of this room.
- The audience seating area was absorptive and the other surfaces were highly reflective.
- For the ‘live’ and ‘optimized’ cases G50 values decrease by only 3 dB with increasing source-receiver distance because there are larger amounts of beneficial early reflection energy which lead to larger ERB values.
- Early reflections for speech - 20 However, the complete room acoustics design process must also include consideration of late-arriving sound levels.
V DISCUSSION
- These new results have confirmed that early reflections of speech sounds are important for achieving adequate speech intelligibility in rooms and have approximately the same effect as increased direct sound energy for both non-impaired and impaired listeners.
- There is therefore no reason to expect that subjects with mild to moderate peripheral hearing loss will not benefit fully from the precedence effect.
- The common practice of focussing on reverberation time as the primary acoustical design parameter can distract us from the more important details of the acoustical design of rooms for speech.
- Obtaining an optimum reverberation time should not be thought of as a primary design goal but as something that is a consequence of the need to maximize early reflections without including excessive later arriving reflection energy.
- There is not a precise optimum value for some particular condition but a relatively broad range of acceptable values [18].
VI CONCLUSIONS
- The results of the new studies presented in this paper show that increased early reflection energy has the same effect on speech intelligibility scores as an equal increase in the direct sound energy.
- This would lead to very important increases in speech intelligibility scores in typical rooms for speech.
- That is, early reflections are important for good speech communication and in many situations where the direct sound is reduced they are Early reflections for speech - 23 essential to satisfactory speech communication.
- Room acoustics design for speech should focus first on maximizing early reflection energy.
- While the hearing impaired listeners in this study benefited from added early reflections, there is a need to verify that this is also true for some other special groups.
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Citations
213 citations
204 citations
Cites background from "On the importance of early reflecti..."
...impulse response, he(t), extends 50 ms after the end of the direct sound impulse response [3], and the late reflection impulse response extends from the end of he(t) to the end of h(t)....
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201 citations
152 citations
Cites background or methods or result from "On the importance of early reflecti..."
...Yang & Bradley Speech intelligibility in classrooms - 29 distant from the talker, adequate speech intelligibility depends on reflected sound and in such cases early-arriving reflections can increase S/N by 7 dB or more [8]....
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...Sound Field Simulation and Headphone Playback Procedure Conditions simulating those in classrooms were created using an 8-channel electroacoustic sound field simulation system located in an anechoic room and quite similar to a previously described system [8]....
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...Increased levels of the direct speech and early reflections of the speech arriving within about 50 ms after the direct sound lead to increased intelligibility, but later-arriving reflections reduce the intelligibility of the speech [8]....
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...Of course there are also many particular situations where early-arriving reflections are critical to understanding speech, such as when the talker is not facing the listener, or is at a more distant position in the classroom from the talker where the level of early-arriving speech energy can be as much as 7 dB or more greater than the direct sound [8]....
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...The non-significant effect of adding later arriving sound is also similar to previous results for adult listeners [8]....
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112 citations
Cites background or result from "On the importance of early reflecti..."
...The addition of excessive noise on reduced task performance compared with excessive reverberation is consistent with results presented by others (e.g., Bradley, 1986; Bradley et al., 2003; Yang and Hodgson, 2006)....
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...These results highlight the need for diligent work during classroom design to minimize the amount of masking background noise and employ reverberation optimization techniques such those described by Hodgson and Nosal (2002), Yang and Hodgson (2006), and Bradley et al. (2003) for example....
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References
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323 citations
"On the importance of early reflecti..." refers background in this paper
...However, the precedence effect is thought to function at a more central cognitive level [16]....
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203 citations
"On the importance of early reflecti..." refers background or methods in this paper
...Speech intelligibility scores were obtained using a Fairbanks rhyme test as modified by Latham [5] and as used in previous tests [6-8]....
[...]
...This concept has been shown to be well correlated with speech intelligibility test scores in a wide range of rooms [5-7] and to be strongly correlated [8] with the newer speech transmission index (STI) concept....
[...]
183 citations
"On the importance of early reflecti..." refers background or methods in this paper
...Speech intelligibility scores were obtained using a Fairbanks rhyme test as modified by Latham [5] and as used in previous tests [6-8]....
[...]
...U50 is a useful to detrimental sound ratio [6] where ‘useful’ is the sum of the direct and early-reflection energy arriving in the first 50 ms and ‘detrimental’ is the sum of later-arriving speech sounds and the ambient noise....
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...This concept has been shown to be well correlated with speech intelligibility test scores in a wide range of rooms [5-7] and to be strongly correlated [8] with the newer speech transmission index (STI) concept....
[...]