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Friday 29 June 2018

It’s been another busy summer for the Media Technology programme with various research activities and equipment upgrades to complete. Following the installation of Dolby Atmos in 2015, we made some further changes to the loudspeakers in our cinema and recalibrated the equipment to improve the sound quality even further.

The screen loudspeakers were upgraded from two-way to three-way JBL 3730 drivers. The high and mid frequency drivers have been bi-amped with the low frequency driver and then time-aligned. Technical Instructor, Matt Hickling, oversaw the installations and commented:

“The addition of a mid-range driver gives us more control and therefore better clarity of speech.  Separate horns for the high and mid ranges can be optimised to produce a more even dispersion of sound throughout the cinema. Overall this will result in less distortion and less variance.”

The original Atmos install saw us install new front sub-woofers for low frequency effects but we repurposed the existing drivers to use as rear subs. However, the cinema gets a lot of use and one of the coils was starting to rub a little so we replaced the rear subs with new QSC SB5118 units to provide renewed “trouser flapping bass”1.

Top-left: Out with the old screen channels  Top-right: in with the new  Bottom: an old rear sub on the left and a new driver on the right

Having changed the drivers in the room the system then needed to be recalibrated to ensure that the level, frequency response and phase is correct throughout the auditorium. We did this with the help of cinema systems integrator Sound Associates before the final tweaks and sign-off were completed by Dolby. Sound Associates ‘voiced’ the loudspeakers by creating the active crossover points and any other subtle adjustments based on measurement data and their previous experience. The system was then equalised based on a detailed 101-point Fast Fourier Transform (FFT), which could then be adjusted using a graphical interface to zoom into the spectrum and make higher resolution changes to the Grossman filter. After this process, the CP850 cinema processor rotated and panned pink noise around the room so the engineers could listen to adjacent loudspeaker interactions and check for any audible errors.

For objective measurements, a spatially averaged array of measurement microphones was placed in a single plane around the critical mixing area. This may seem limited but experience has shown this method to be most effective at providing good results throughout the auditorium. During playback a course ‘bulk’ EQ is applied on the amplifiers to bring any variance within the range of the detailed equalisers of the cinema processor.

It’s good practice to regularly calibrate cinemas and we look forward to hearing the results during performances over the next few weeks and beyond. As usual second year students who choose to take the Sound Design module will also use the cinema this year to mix and render in Dolby Atmos.

1 a signature phrase of Media Technology stalwart and founder of our cinema Roger Lownsborough

New loudspeakers? I'm on it!