Figures 5.9(a-e) through 5.11(a-e) illustrate the target room response curves and the listener curves at each position. The target room response was added to the measured in-room response, the in situ loudspeaker flattening curve in each cinema and position, in addition to the flattened headphone response. These curves depict what the listeners actually heard during the tests given their choice of curve and their listening position in the cinema. The curves for each listener position are shown offset for clarity and in 1/6 octave resolution.
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Figures 5.9 a-d: Target room response curves and the listener curves at each position in cinema A
Figure 5.9(a): Cinema A: Green - Curve A, Blue - Front Row, Black - 2/3L, Red - Back Row. SMPTE X-curve and tolerances
in grey
Figure 5.9(b): Cinema A: Green - Curve B, Blue - Front Row, Black -
2/3L, Red - Back Row
Figure 5.9(c): Cinema A: Green - Curve C, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.9(d): Cinema A: Green - Curve D, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.9(e): Cinema A: Green - Curve E, Blue - Front Row, Black -
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Figures 5.10 a-d: Target room response curves and the listener curves at each position in cinema B
Figure 5.10(a): Cinema B: Green - Curve A, Blue - Front Row, Black - 2/3L, Red - Back Row. SMPTE X-curve tolerances in
grey
Figure 5.10(b): Cinema B: Green - Curve B, Blue - Front Row, Black -
2/3L, Red - Back Row
Figure 5.10(c): Cinema B: Green - Curve C, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.10(d): Cinema B: Green - Curve D, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.10(e): Cinema B: Green - Curve E, Blue - Front Row, Black - 2/3L, Red -
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Figures 5.11a-d: Target room response curves and the listener curves at each position in cinema C
Figure 5.11(a): Cinema C: Green - Curve A, Blue - Front Row, Black - 2/3L, Red -
Back Row. SMPTE X-curve and tolerances in grey
Figure 5.11(b): Cinema C: Green - Curve B, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.11(c): Cinema C: Green - Curve C, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.11(d): Cinema C: Green - Curve D, Blue - Front Row, Black - 2/3L, Red -
Back Row
Figure 5.11(e): Cinema C: Green - Curve E, Blue - Front Row, Black - 2/3L, Red -
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Five different mono music programs were selected, level matched and edited into short 20-30 second loops in ProTools. They included a piece of orchestral score mixed in a room that was not calibrated to the X-curve (Sc), the same piece of score mixed for a film trailer in an X-curve calibrated room (Sc-T), a piece of score that was mixed for a theatrically released film in an X- curve calibrated room (Sc-F), Steely Dan's "Cousin Dupree" (SD), and Toy Matinee's "Last Plane Out" (TM). All five programs have broad and smooth long term spectra, and have been found to be revealing of spectral features based on previous tests and listener training exercises. The mono program from the original 5.1 channel music selections were down mixed according to recommended ITU practices as shown in table 5.2.
Mono 1/0 Format
Left Right Centre Left
Surround
Right Surround
C' 0.7071 0.7071 1.0000 0.5000 0.5000
Table 5.2: Downward mixing equations for 3/2 source materials, from (ITU, 2012a)
This process was accomplished using Waves M360° Surround Manager software, which has the ITU standard mix-down levels as a selectable option, shown in figure 5.12 (Waves, 2005).
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Figure 5.12: Screen capture of Waves M360° Surround Manager software showing mono ITU mix-down option
Critical listening assessments conducted by five professional sound designers revealed no audible artefacts from the mix-down process.
The ITU standard does not state a specific method for the down-mix of the low frequency enhancement (LFE) channel. However, it does state that the low frequency content of a mix, from 20 to 120Hz, should not be carried solely by the LFE channel. Therefore, the channels listed in the above table all carry signals in this frequency range. However, the LFE channels of the audio tracks were part of the final mono mix and were added at a ratio of approximately 0.30. The exact amount of LFE added to the mono mix varied by program. However, the levels were verified against the original 5.1 audio tracks in ProTools, through informal listening tests and via spectral analysis as shown in figure 5.13.
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Figure 5.13: Long-term spectral power density of the five pieces of program material as calculated using the Welch method (Welch, 1967). Levels normalized at 500Hz 5.3.8 Selection of Listeners
A total of 14 trained listeners were used for these tests, 10 of whom are Harman employees paid for their participation. The other 4 listeners were audio professionals working in cinema sound design and editing with a range of experience from 4 to 30 years in the industry. These listeners were remunerated for their participation. The age range of the listeners was 24 to 56 years, with a media age of 32 years. The listeners were tested to confirm that they have audiometric normal hearing. In addition, all listeners needed to have passed listener training tasks that required them to reliably identify and discriminate among different spectral distortions added to different music programs, achieving a skill level of eight out of ten or higher in all tasks. This training was carried out using Harman's "How to Listen" software (Harman, 2011). The listeners' experience with formal listening tests ranged from 1 to 23 years.
5.3.9 Test Design
The listening test was administered using a MUSHRA-style test where subjects were asked to rate their preference amongst five different in-room target response curves applied to three different seating locations within three different cinemas.
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This listening test utilized a 5 x 5 x 3 x 3 x 2 repeated measures ANOVA design, with the following independent variables: five room response curves (A through E), five programs (three film score and two popular music pieces), three rooms, three listening locations and two observations. The dependent variable was preference rating. For the listening tests, the target curves were comparatively rated on an 11-point preference scale (0-10) that had semantic differentials on every second interval labelled: Strong Dislike (1), Dislike (3), Neither Like/Dislike (5), Like (7), and Strong Like (9). A strong preference between target curves was indicated by a separation in preference ratings of ≥ 2 points, a moderate preference ≥1 points, and a slight preference ≤ 0.5 rating. The graphical user interface (GUI) for the preference test is shown in figure 4.14 in chapter 4.
All presentations of the stimuli were performed using a double-blind, multiple comparison method (five target curves at a time) that utilized a randomized presentation order for both the target curves and programs. The order in which the various cinema / position tests were completed was equally distributed among the subjects to minimize possible order biases. For all tests the stimuli were presented the same number of times and at the same playback level. Each listener participated in a listening session lasting approximately 20-30 minutes each. In each test session, the listener completed 10 trials (5 programs x 2 observations). 1,260 trials were carried out in total (10 trials x 3 cinemas x 3 listening positions x 14 test subjects). In each trial, the test subject made comparative judgments between the five in-room target response curves until their final preference ratings were recorded. The listening tests were administered in random order across all of the listeners over a 6 month period with no test subject taking more than 2 listening test within a given day. In some cases, listening tests were administered weeks apart.
5.3.10 Test Setup and Hardware
The hardware for the listening test setup consisted of the Sennheiser HD518 headphones, two Windows laptops (both Lenovo Thinkpad T420s), two digital sound cards (RME Fireface UC and MOTU) a programmable digital signal processor (BSS Audio BLU-800) and magnetic field head-tracker (Razer Hydra). The Razer Hydra head-tracker utilized has a latency of 10ms. The latency of the entire BRS playback system is less than 50ms which, as previously referenced in chapter 4, does not cause audible effects under normal listening conditions.
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The relative playback levels of the different target response curves and program material were adjusted for equal loudness based on the ITU-R1770.3 loudness standard (ITU, 2012b) and verified by informal listening to assure that there were no discernible loudness differences between test curves. The average playback level of the music was adjusted to a comfortable level of 80dBA. The experiments were conducted with the listeners sitting in one of the loudspeaker labs at Harman, where the background noise level was measured to be between 30 and 35dBA.
5.4 Results
The following sections report on the statistical analysis utilized and the results of the listening tests.
5.4.1 Statistical Analysis
The data was analyzed using SAS Stat View software running a one-way repeated measures analysis of variance (ANOVA). Additional analyses were completing using Matlab and custom designed Excel spreadsheets. Since the same participants experienced all conditions that were assessed in the tests, a one-way repeated measures ANOVAs was deemed appropriate in order to identify whether there were any significant differences of the mean ratings for preference between the five presented target curves. A complete factorial analysis was used in the ANOVA model with a significance level of 0.05 utilized for all statistical tests. Since an audible anchor was included in the target curves, normalizing the data prior to statistical analysis was deemed unnecessary. In addition, noting that outliers can provide important information during analyses, no outliers were removed from the data sets.
A Mauchly’s test of sphericity with a significance level of 0.05 was utilized on various groups of data (grouped by curve, room, program, listening position and cinema) to confirm local sphericity. The grouped data from the listening tests were found to be non-significant (p>0.05) are therefore were considered locally spherical.
After the ANOVA analyses were completed, a repeated measures Scheffe post hoc analysis with a significance level of 0.05 was completed in order to define which means were significantly
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different from each other. Following the Scheffe analysis, several effect size analyses were complete in an effort to identify the degree of the differences between and within factors. 5.4.1. Results
The intention of these tests was to study for preferences among cinema target curves based on cinema size and listening position within a cinema. Data from the listening tests were checked for Gaussian distribution and subjected to a repeated measures ANOVA test. The following were observed:
• Observation was not significant: F (1,13) = 0.186, p = .6731 • Listening Position was not significant: F (2,26) = 1.04, p = .3676 • Target Curve was significant: F (4,52) = 547.123, p < .0001 • Cinema was significant: F (2,26) = 15.639, p = .0001 • Program was significant: F (4,52) = 12.607, p = .0001
• Target Curve*Program was significant: F (16,208) = 34.868, p = .0001 • Cinema*Target Curve was significant: F (8,104) = 7.873, p = .0001 • Cinema*Program was significant: F (8,104) = 5.608, p = .0001
• Cinema*Listening Position was significant: F (4,52) = 2.744, p = .038
The full ANOVA table for this experiment can be found in appendix K.
Only those higher-order factors or combination of factors found to be significant were analyzed separately and are elaborated upon in the following sections.