Perceived Urgency of Voice Alarms in Emergency EvacuationsHuiyang Li1, Xianghong Sun2, Kan Zhang2, Nadine Sarter11 University of Michigan2 Institute of Psychology, Chinese Academy of SciencesEffective and compelling warnings are critically needed in emergency evacuations, especially in large buildings or in facilities where occupants may be unfamiliar with the environment. Voice alarms have the potential to capture attention and direct occupants efficiently and effectively in an emergency. To inform the design of voice alarms, most previous studies have examined the perceived urgency of individual words in a warning signal. However, very few studies have been conducted on voice alarms that present entire sentences and are broadcast to a large group of people during emergency evacuations. The present study investigated how to optimize the acoustic (speech rate, noise level, SNR, gender of speaker) and semantic parameters of Chinese synthesized speech warnings to ensure that occupants react to an emergency promptly and appropriately.INTRODUCTIONEffective and compelling warnings are critically needed in emergency evacuations, especially in large buildings or in facilities where occupants may be unfamiliar with the environment. V oice alarm systems are a promising candidate for supporting this need. They have the potential to capture attention and direct occupants in an emergency efficiently and effectively. V oice alarms have been used for fire alarms and evacuation instructions since the 1970s. One of the first “vocal alarm systems” was designed by Keating and Luftus (1975) for the Seattle Federal Building. This alarm system provided three essential pieces of information: (1) tell the occupants exactly what has happened; (2) instruct the occupants what they are to do; (3) explain to the occupants why they should do it. Bryan (2001) summarized both observational and experimental data that demonstrated the effectiveness of a wide range of voice alarm systems. These data show that the alerting power and urgency are among the most important factors that the designer of a voice alarm should consider (see McCormick (1964)). For example, one of the early voice-based fire alarm systems, which was presented at the “International Conference on Fire Safety in High-Rise Buildings” in 1971, listed as one of its important features that it “can capture attention of the people and alert them to emergencies at hand by preconditioning”.To inform the design of voice alarms, most previous studies have examined the perceived urgency (e.g. Hellier, Edworthy, Weedon, Walters and Adams, 2002), intended carefulness (e.g. Barzegar and Wogalter, 1998) and believability, and the appropriateness and importance (e.g. Edworthy, Hellier, Walters, Clift-Matthews, and Crowther, 2003) of individual words in a warning signal. These characteristics of voice alarms is influenced by both their semantics and acoustics (e.g. their physical structure as described by Fourier analysis and other objective measurements of acoustic variables such as pitch, pitch range and level). For example, Hellier, Edworthy, Weedon, Walters and Adams (2002) tested 10 words representing warning signals which were presented in urgent and non-urgent acoustic styles. The three words ranked as most urgent were: Deadly, Danger, and Warning. An acoustic analysis showed that the words were not spoken differently; thus, the urgency ratings had to be due to intrinsic and probably semantic differences between the words. However, the impact of spoken words is determined also by acoustic properties. Barzegar and Wogalter (1998), for example, presented voice alarms in 3 different styles: emotional, monotone and whisper. Their findings show that emotional presentation produced significantly higher ratings of intended carefulness than monotone style, with ‘whisper’ falling between the two. Hellier, Edworthy, Weedon, Walters and Adams (2002) asked actors to speak a set of words in three different styles: urgent, non-urgent and monotone. Again, the results showed significant effects of presentation style on ratings of perceived urgency, with an increase in perceived urgency from monotone, through non-urgent, to urgent. Acoustic analysis of the words revealed that the urgent versions of the words were louder, higher in pitch, and had a larger pitch range than thenon-urgent version. With regard to the interaction between semantic and acoustic effects, results showed that semantic and acoustic effects interact for some words more than others. For example, for a word such as ‘no’, acoustics have a big effect on perceived urgency relative to semantics. However,for a word such as ‘deadly’, semantics have a big effect on perceived urgency relative to acoustics.Research has also been conducted on the intelligibility of voice alarms. For example, messages for fire information display system for firefighter commander in Chinese language were studied (Zhang, Sun, and Zhang, 2006). The effect of speech rate and tune on the intelligibility of fire-related information – both words and short sentences - was investigated under different noise levels. Also, synthesized voice alarms to warn operators of dangerous events in special working environments was investigated (Zhang, Zheng, Zhu, and Jin, 1997) to determine optimal speech rate.In the above studies, both synthetic and natural voice was used. The advantages of synthetic signals are the accuracy of parameter control, flexibility of selecting parameters and easeof creating signals. In the present study, all alarm signals were therefore generated using an on-line voice synthesizer. The rating methods used in previous study include magnitude estimation and rating scales (for an example employing magnitude estimation, see Li, Xu, Sun, and Zhang, 2008). In the present experiment, a rating scale was used to obtain absolute judgments of the perceived urgency of warnings and to compare these data to findings from earlier magnitude estimation studies.Overall, the above research has yielded important quantitative data on the effectiveness of alarms but still involved a number of limitations. It focused on either the perceived urgency of English words, or the intelligibility of warnings in Chinese language in a variety of domains. But no studies have been conducted on the perceived urgency of alarms in Chinese language in the forms of sentences that are broadcast to a large group of people during emergency evacuations. To fill this gap, the present study investigated how to optimize the acoustic and semantic parameters of Chinese synthesized speech warnings to ensure that occupants react to an emergency promptly and appropriately.METHODSParticipantsEleven participants (age range: 23.5 2.5 yrs; 5 male and 6 female) completed the experiment. None reported any hearing problem. Participants were paid 10 RMB for completing the experiment.±DesignThe study employed a 2 (gender of speaker) × 3 (speech rate: 4 Chinese characters per second, 5 chars/sec, and 6 chars/sec) × 3 (content: nature of emergency; same, plus required response; same, plus reason for required response) × 2 (noise level: 50dB (A) and 70dB (A)) × 3 (signal/noise ratio (SNR): 10 dB, 15dB, and 20dB) within-subjects design. Noise levelsof 50dB and 70dB were chosen to represent a quiet public place (e.g. the National Library) and a noisy environment (e.g. the subway station), respectively. The dependent variable was the participants’ rating of the perceived urgency of each signal on a 9-point scale (ranging from ‘extremely leisurely” (1) to‘neither leisurely nor urgent’ (5) to ‘extremely urgent’ (9)). Stimuli and MaterialsAll signals were generated using a KDVoice V2.0 Speech Synthesizer (on-line version), developed by Anhui USTC iFLYTEK Co., Ltd.. The voices of two young adults (one male, one female) were used to create the warnings in Mandarin. Three sentences with the same number of Chinese characters were spoken in 36 different ways, according to their acoustic chracteristics (see Design). The three sentences were: 1. “Attention please, all occupants should evacuate immediately, take the stairwell, please evacuate building #1 immediately” tells occupants what to do); 2. “Attention please, this is the control room, evacuate the office building immediately, evacuate the office building immediately” (tells the occupants what to do and source of information); 3. “Attention please, the third floor is on fire, evacuate the third floor immediately, repeat, evacuate the third floor immediately” (tells the occupants what to do and what happened).Noises were generated with Goldwave V 5.20. The underlying equation is:rand(2)-1 (1) The noises were presented simultaneously in E-prime slides with the signals when necessary. The volume of noise and signal were measured and adjusted at the ear before experiment started.ApparatusE-prime 1.1 was used to run the experimental procedure. The signals were presented to each participant in a laboratory using Dell Optiplex 745 computer connected to a pair of Shock Wave SC – 1 II speakers. Participants sat 1.2 meters from the speakers. A Lutron SL-4001 sound level meter was used to measure and calibrate the volume of the messages.ProcedureEach participant rated the urgency of each of the 108 signals once. All signals were presented in a random order. After each signal, the 9-point scale appeared on the computer screen, and the participant entered the rating using a computer keyboard. After the experiment, participants were debriefed and asked about their perception of signal urgency.RESULTSA repeated-measures ANOV A showed a main effect of speech rate, F (2, 20)=34.549, p<0.001, with significant differences between all three levels of this factor (p<0.001). As can be seen in Table 1, the faster the alarm message is spoken, the higher the participants’ sense of urgency is.Table 1. Mean urgency ratings under different speaking rates Speechrate4 chars/s5 chars/s6 chars/sM 4.25 5.82 6.78SE.436 .252 .266A main effect of signal/noise ratio (SNR) was observed, F (2,20) = 13.525, p<0.001. Table 2 indicates a positive correlation between SNR and perceived urgency. Significant differences were found between the 10dB and 15 dB conditions (p=0.049) and between the 10dB and 20dB conditions (p=0.001). This result indicates that the requirement of 15 dB for voice alarms proposed in the ‘Standard for automatic fire alarm system design’ (Standard for fire technology in buildings, 2006) should be applied.Table 2. Mean urgency ratings under different SNRsSNR10dB15dB20dBM 5.43 5.59 5.82SE.293.281.266Noise level had a significant effect on perceived urgency as well, F (1, 10) = 21.671, p =0.001. The higher the noise level, the higher the participants’ sense of urgency was (see Table 3). Table 3. Mean urgency ratings under different noise levelsNoise50dB 70dBM 5.17 6.06SE .319 .265Finally, there was a main effect of message content, F (2, 20) = 10.551, p=0.001. Table 4 shows the mean urgency ratings (plus standard error) for the three different contents. Post-hoc tests showed significant differences between contents 1 and 3 (p=0.015) and between contents 2 and 3 (p=0.003). Table 4. Mean urgency ratings under different contents (Content 1: what to do; Content 2: what to do, plus information source; Content 3: what to do and what happened)ContentContent 1 Content 2 Content 3M5.39 5.266.21SE.309 .293 .314A significant interaction was observed between gender and speech rate, F (2, 20) = 4.459, p=0.025. As shown in Table 5 and Figure 1, at the lowest rate, signals spoken by males are rated more urgent while at higher speaking rates, signals spoken by females are rated more urgent. The increase in perceived urgency was bigger for the female voice than for the male voice.Table 5. Mean urgency ratings under different speech rate by speakers with different gender.Speechrate4 chars/s5 chars/s6 chars/sGender M SE M SE M SEM 7.38 .407 5.81 .249 6.65 .254F 4.12 .469 5.83 .260 6.91 .300Figure 1. Interaction between speaker gender and speech rate.A significant interaction was also observed between speech rate and content, F (4, 40) = 6.014, p=0.001. As shown in Table 6 and Figure 2, at the speech rate of 4 chars/s, content 1 was rated more urgent than content 2; at 5 chars/s, the two types of content were rated as equally urgent; at 6 chars/s, content 2 was rated more urgent than content 1. Content 3 was rated as the most urgent under three speech rates.Table 6. Mean urgency ratings under different speech rate and content.Speechrate4 chars/s5 chars/s 6chars/sContent M SE M SE M SEContent1 4.14.411 5.54 .325 6.48.313Content2 3.59.509 5.54 .239 6.64.310Content3 5.01.466 6.39 .287 7.21.275Figure 2. Interaction between contents and speech rate.Figure 3 shows a significant interaction between noise level and content, F (2, 20) = 8.405，p=0.002. As shown in Table 7 and Figure 3, at the lower noise level, contents 1 and 2 were rated as equally or less urgent, respectively, than content 3; at the higher noise level, content 2 was perceived as the least urgent.Table 7. Mean urgency ratings at different noise levels and different contentsNoise50dB 70dBContent M SE M SE1 4.93 .343 5.84 .3172 4.93 .320 5.59 .2823 5.65 .361 6.76 .302Figure 3. Interaction between noise level and content.DISCUSSION AND CONCLUSIONSThe findings from this experiment show that speech rate, SNR, noise level and content of the signal all have a significant effect on the perceived urgency of alarm messages. The only independent variable for which no main effect was found is the gender of the speaker. The largest proportion of variance was accounted for by speech rate which showed a positive correlation with perceived urgency.These findings were confirmed by the debriefings during which participants were asked which signals they perceived as most urgent. The most frequently mentioned message acoustic characteristics were ‘loud’, ‘fast’ and ‘in a noisy environment’, which are the equivalents of the main effects of SNR, speech rate, and noise level.Results obtained in this study are consistent with findings from the authors’ previous study (Li, H., Xu, M., Sun, X. and Zhang, K., 2008) which used the method of magnitude estimation. Main effects of speech rate, SNR, and noise level were found in the previous study as well. However, the main effect of content and interactions between content and speech rate and content and noise level were not observed. This difference may be attributed to the way in which magnitude estimation was used. Although participants were trained to use a ratio-scale rather than distance-scale, the range of ratings was limited (20-500 with a reference of 100). Thus, differences between the perceived urgencies for the various signals may have been compressed and were further reduced by transforming the original ratings to a logarithmic scale.In contrast to the method of magnitude estimation, rating scales make it possible to obtain absolute judgments from participants. These ratings can indicate which signals should be avoided for alarms (i.e., when their perceived urgency is even lower than neutral). Since participants do not need to compare signals with a reference or preceding signal, rating scales also lead to higher accuracy. The disadvantage of rating scales is that the scale is defined arbitrarily by an experimenter. Too low a resolution can make it impossible to distinguish between high and extremely high perceived urgency, for example.Acoustic analyses was carried out in previous studies (e. g. Hellier, Edworthy, Weedon, Walters and Adams, 2002) to better understand why words spoken in different styles were perceived to convey different levels of urgency. Parameters such as average fundamental frequency, pitch range, speed of the word and amplitude were examined. These parameters prevail in articles dealing with the relationship between speech and emotion, they are key parameters affecting urgency innon-verbal auditory warnings, and they are easy to analyze and synthesize. The present study manipulated these parameters as well and confirmed that louder, faster spoken signals are rated more urgent. Signals at higher speech rate may be more closely associated with the notion of ‘pushing’, and those with higher SNR may be perceived as more dangerous and important. This likely contributed to the high urgency ratings. Signals presented at higher noise levels were rated more urgent also. This may be due to the fact that people have learned to associate noise with dangerous, emergency situations. Signals informing participants about what happened were rated more urgent than those telling them only what to do and, in some cases, the source of the information also. One possible explanation for this finding was mentioned my participants in the debriefing. The word ‘fire’, which is always related to danger, was mentioned in the former condition only. Although the specific word “fire” was not used in previous studies, related words such as deadly, danger, warning and hazard have also been reported to be considered more urgent than others (Hellier, Edworthy, Weedon, Walters and Adams, 2002). The interaction between speech rate and speaker gender might be attributed to the increase in pitch of the female voice with speech rate. Further acoustic analysis and manipulation of more parameters is needed to explain the difference between male and female voice, among different speech rate and different SNR and their impact on perceived urgency.The present study has some limitations. First, only synthetic speech alarms were tested. In future research, alarms using natural speech should be tested and compared with synthetic speech. Design guidelines for both cases should be derived. Also, only few levels of each factor or independent variable were employed. A wider range of values should be tested in a future experiment. In addition, the experiments were run on a desktop computer. No actual or simulated emergency was present. More research is needed to examine the perception of warnings and alarms under more realistic conditions to determine how ratings and responses may be affected. Finally, the design of voice alarms involves not only the urgency of the signals but also the intelligibility of the system. Fast speech rate may induce lower intelligibility. Thus to provide guideline for designers, interaction with other variables should be take into account.Still, in summary, the findings from this study can inform the design of effective auditory warnings that will save livesby being perceived and responded to in a proper and reliablefashion. To this end, a voice alarm should be spoken at a sufficiently fast rate (no less than 5 Chinese characters per second), and the signal-to-noise ratio should be no less than 15dB). At higher speech rates, urgent messages are better broadcast using a female voice. Finally, alarm messages should indicate what happened and the potential danger.ACKNOWLEDGMENTThe authors gratefully acknowledge financial support for this project from Beijing Municipal Natural Science Foundation (Grant number: 9062004), Fund for Graduate Student in Science and Social Practice-Creative Research Project, Chinese Academy of Sciences, and Emergency Manage Office of the State Council (Grant number:GYJ2007A19).REFERENCESBarzegar, R. S. & Wogalter, M. S. (1998). Effects of auditorily - presented warning signal words on intended carefulness. Contemporary ergonomics 1998. 311-315.Bruck, D & Ball, M. (2007). Optimizing Emergency Awakening to Audible Smoke Alarms: An Update. Human Factors. 49 (4), 585-601.Bryan J. L. (2001) Psychological Variables That May Affect Fire Alarm Design. Fire Protection Engineering. 11. 42-46.Edworthy, J, Hellier, E J, Walters, K, Clift-Matthews, W & Crowther, M.(2003). Acoustic, semantic and phonetic influences in spoken warningsignal words. 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