A concert is a common event at which people gather to share a musical experience, 22 particularly in Western society. Although brain imaging techniques can implicitly measure 23 naturalistic musical perception without behavioural ratings 1-5 , these methods lack 24 applicability in more typical listening situations, particularly those where listening happens in 25 a group, such as in a concert. Not only does a concert provide a naturalistic setting, but live 26 performances can be more immersive 6 , evoke stronger emotional responses 7-9 , while co-27 presence of audience members can increase physiological and emotional experience 10 . 28 Portable methods such as motion capture 11 or mobile measurement of the autonomic 29 nervous system (ANS, e.g., cardiology) 10,12 seem promising in such a setting. As our interest 30 lay in the experience of a Western art music concert, in which listeners are typically still 13 , we 31 focused on ANS measurements during listening of full-length musical works in a naturalistic 32 concert context. 33 Responses of the sympathetic division of the ANS such as an increased (phasic) skin 34 conductance response (SCR, i.e., sweat secretion), an increase of heart rate (HR), and 35 respiration rate (RR) as well as responses of zygomaticus major (smiling) and the corrugator 36 supercilii (frowning) muscles (electromyography [EMG] activity) typically reflect stress, 37 attention 14,15 , or affective processing [16] [17] [18] [19] . In terms of auditory responses, changes in SCR, HR, 38 RR, and EMG activity -reflecting a startle 20-22 or orienting response 23 -have been associated 39 with pitch changes 24,25 and tone loudness (the louder the sound, the greater the SCR features, such as brighter tones and higher spectral centroid, are associated with higher 48 arousal 32,48,49 , which correlates somewhat to SCR 35,50 . Importantly, these physiological 49 responses are modulated by musical style: previous studies found that HR increases with 50 faster tempo in Classical music, but decreases with faster tempo in rock music 38 , whereas HR 51 is lower in atonal, compared to tonal music even with both styles controlled for emotion 51 .
Although this research generally supports the idea that physiology is associated with 53 musical features and style, there are many inconsistencies within findings such studies 54 (see 52, 53 ). This could partly be due to most studies having (too) carefully chosen or constructed 55 stimuli to have little variability in acoustic features (e.g., they use a constant tempo and 56 normalise loudness). While there is value in tightly controlling individual musical features, 57 more research into naturalistic stimuli -which typically has a rich dynamic variation of 58 interdependent features -is required to extend ecological generalisability 54 .
Although previous work on naturalistic music has correlated neural and physiological 60 responses to dynamically changing acoustic features 1-3,50 , or extracted epochs based on 61 information content in the music 12 , perhaps a more robust way to identify systematic 62 responses to naturalistic stimuli is through analysing synchrony of responses 10,55-57 , in 63 particular via inter-subject correlation (ISC, see review 58 ). This method -in which (neural) 64 responses are correlated across participants exposed to naturalistic stimuli 59 -is based on the 65 assumption that signals not related to processing stimuli would not be correlated. ISC research 66 has demonstrated that highly similar responses occur across subjects when exposed to 67 naturalistic films 59-62 , spoken dialogue 63-65 and text 66,67 , dance 68 , and music 1,5,69,70 , strongly 68 suggesting that highly reliable and time-locked responses can be evoked by (seemingly 69 uncontrolled) complex stimuli (for a review see 71 ). Although ISC in fMRI studies can identify 70 region of interests (ROIs) for further analysis (e.g. 59 ), ISC can also assess which kind of 71 feature(s) within dynamically evolving stimuli evoke highly correlated responses.
In response to auditory stimuli, higher synchronisation (operationalized via ISC) of 73 participants' responses is associated with structural coherence, familiarity, and emotional 74 context of stimuli. For example, ISC is higher when listening to original, compared to phase-75 scrambled, versions of music 69,70 and spoken text 67 . ISC may also reflect familiarity and 76 engagement: ISC is higher when listening to familiar music, compared to unfamiliar music; 77 however, upon repeated presentation, ISC drops with repetitions of familiar, but not 78 unfamiliar music 5 . Moments of collective synchrony additionally seem to be linked to 79 emotional arousal, where higher correlation coefficients of fMRI 59 , EEG 61 , SCR and 80 respiratation 72 coincided with moments of high arousal in films, such as a close-up of a 81 revolver 61 , gun-shots, or explosions 59 as well as close-ups of faces and emotional shakiness in 82 voice 72 . However, our understanding of music and ISC is still in its infancy, and it is unclear 83 which musical features -and at which level or time frame -can evoke synchronised 84 physiological responses, particularly in more naturalistic listening situations outside the 85 laboratory.
In addressing these questions, the current study is -the best of our knowledge -the 87 first of its kind to assess which musical features evoke shared physiological responses during 88 full-length, naturalistic music stimuli in a typical group-listening concert context. To our 89 knowledge, this is additionally the first study to compare how different groups react to the 90 same musical stimuli in such a setting. In light of the replicability crisis 73 , we conducted three 91 identical concerts with different participant groups to test the replicability of the induced 92 physiological responses to music, and thereby, the stability of using a concert hall as an 93 experimental setting. We invited participants to attend one of three instrumental chamber 94 music concerts in an 'ArtLab' performance hall (purpose-built for empirical investigations). physiological responses to music would be consistent across repeated concert performances.
Comparison of performances. As the music was performed by professional musicians, we 119 expected all concert performances to be acoustically similar. We found no significant 120 differences between performances for loudness, tempo, timbre, and duration (see 121 Supplementary Table S1 ). Correlations of instantaneous tempo, timbre, and loudness 122 between all performances reached r > .6, p < .001, confirming that they were comparable 
RMS energy. Figure 3a shows that lower RMS energy increased probability of HR 175 synchrony, but this was not consistent across concerts or pieces and only significant in concert three times with different audiences. Importantly, we found no significant differences 257 of length, loudness, tempo, or timbre across the concert performances, allowing us to assume 258 that varying patterns of audience responses were due to the weakness of an effect or 259 individual differences in audiences rather than a difference in acoustics. When responses were 260 highly correlated (within a 5-bar window), there was an overall higher SCR and EMG (smiling 261 muscle) activity, and increasing HR and RR, suggesting that synchronised physiology was 262 associated with increased arousal.
Synchronised RR and SCR responses were consistently predicted by tempo alone. However, we wish to note that faster tempi (centred around 120 bpm or 2 Hz in the current 276 study) might also be a more physiologically optimal and/or perceptually familiar range for 277 entrainment to music (see 80 for review), compared to slower tempi (centred around 50 bpm 278 .83 Hz in the current study). Entrainment, or perhaps just adaptation of a physiological 279 measure towards the musical tempo, might be one mechanism through which faster (or 280 optimally resonant) tempi induce more similar audience responses.
It is of further interest that SCR-RR synchrony was more probable at faster tempi only 282 in the Classical and Romantic styles, but not in the Contemporary style. We suggest, therefore, reported engagement did not differed across styles (see 81 ). This suggests that familiarity of 288 style, rather than engagement, may increase the probability of synchrony (though this 289 difference could be due to our stimuli being presented live rather than through a recording 5 ).
Beethoven and Brahms have a relatively structured and stable meter with very few 291 instantaneous tempo changes within movements, whereas many Dean passages contain 292 unstable meter (e.g., the first movement has alternating bars of four, five, or six beats) and 293 frequent tempo changes within movements (as it typically for each style). In view of this, the 294 fact that synchrony probability changes between styles could also be due to stimuli coherence. arousing noises (e.g., a ringing telephone or storm), but not with music 82 . As loudness in the 311 current study was embedded in naturalistic music (rather than in a tone sequence), this 312 highlights the generalisability limitations of reductionist stimuli to real-world contexts 54 .
However, this points to another limitation: despite employing such complex stimuli, we 314 computationally extracted only the most common acoustic parameters associated with 315 physiological responses, possibly leading to underfitting of our models 83 . It was important, 316 therefore, to explore additional higher-level parameters in the music that are not so easily 317 extracted computationally. 318 We observed that transitional passages, clear boundaries or immediate phrase 319 repetitions in the music -identified using music theoretical analysis -coincided with highly 320 synchronised physiological responses across concerts. This observation corroborates previous 321 findings that synchrony may occur in response to long-term, structural features in music 4,70 .
The fact that synchronised responses were evoked by arousing transitional passages 323 (characterised by changes in loudness, pitch register, and musical texture) supports the idea 324 that highly similar responses in all audiences are related to arousal 59,61,72 as well as supporting 325 the idea that audience members collectively 'grip on' to loudness and texture changes 6 . As 
Physiology pre-processing. Data were cut per movement. Missing data (gaps of less 415 than 50ms) were interpolated at the original sampling rate. Fieldtrip 102 was used to pre-416 process BVP, respiration, and EMG data. BVP data were band-pass filtered between 0.8 and 417 20 Hz (4 th order, Butterworth) and demeaned per movement. Adjacent systolic peaks were 418 detected to obtain inter-beat intervals (IBIs) and an additional filter was added to remove any 419 IBIs that were shorter than 300 ms, longer than 2 seconds, or had a change of more than 20% 420 between adjacent IBIs (typical features of incorrectly identified IBIs 103 ). After visual inspection 421 and artefact removal, IBIs were converted to continuous heart rate (HR) by interpolation.
Respiration data were low-pass filtered (.6Hz, 6th order, Butterworth) and demeaned. As in 423 the BVP data, maximum peaks were located and respiration rate (RR) was inferred by the peak 424 intervals. EMG activity was bandpass filtered (between 90 and 130 Hz, 4 th order, Butterworth), 425 demeaned and Hilbert transformed. Skin conductance data were pre-processed using 426 Ledalab 14 and decomposed into phasic and tonic activity. As we were interested in event-427 related responses, only (phasic) skin conductance responses (SCR) were used in further 428 analyses. All pre-processed physiological data (SCR, HR, RR, EMG) were resampled at 20Hz 19 , 429 z-scored within participant and movement, and averaged into bins per bar. 