The painstaking process of "voicing" the Grand Organ at York Minster has begun as a once-in-a-century £2 million refurbishment project enters its final phase.
Voicing is a process to ensure all the organ's 5,000-plus pipes are playing the correct pitch, tone and volume.
The work is being carried out by specialists from Durham-based organ builders Harrison and Harrison and will take place over several weeks between November and March.
It is hoped that the fully refurbished instrument will be back in use by spring 2021, the Minster said.
The process to voice the organ is the final stage the refurbishment project and is done entirely by ear.
Each pipe in the organ - which range from the size of a pencil to 10m long - plays an individual note, and the voicer's job is to ensure all the pipes in each stop are playing the right pitch, tone and volume.
Andrew Scott, head voicer and director at Harrison and Harrison Organ Builders, said: "Voicing is the name given to the process that happens once in a generation when an organ is given its musical personality.
"In many ways, it is a similar sounding process to regular tuning, but whereas tuning is the correction of pitch, voicing alters the physical parameters of each pipe, such as the tone and volume.
"Rather like a choir director moulds the ensemble from the individual voices assembled in the choir, the voicer's art is to ensure all of the pipes in each stop are speaking harmoniously to create the tonal architecture of the organ's ensemble."
The organ, which is one of the largest in the country - weighing around 20,000kg and containing 5,403 pipes - was removed from the minster in October 2018 for cleaning, repair and replacement of its parts in a £2 million project.
It is the first time a project of this scale has been undertaken on the instrument since the last major refurbishment in 1903.
Another part of the final stage of the project is the cleaning of the newly revealed Pulpitum, known as the Kings' Screen - the 15th century stone screen which separates the cathedral's Quire from its Nave.
The Pulpitum features 15 stone statues of medieval monarchs, and conservation experts are using museum grade vacuum cleaners and brushes to clean years of dirt and dust from the detailed carvings.
The screen was revealed again last month after it was surrounded by scaffolding for two years.
Alex McCallion, York Minster's director of works and precinct, said: "We're thrilled the work on this once-a-century project is entering its final stages, allowing us to see the beautiful detail of the Pulpitum unveiled again and start to hear the pipes play again for the first time in more than two years."