A tenor’s vocal range typically extends at least one and a half octaves, from the D below middle C on a piano keyboard to the A above it. A baritone might be expected to sing at least seven semi-tones below this to hit the key “money notes” in their register.

Like any muscles, those involved in singing can atrophy or weaken with age. But opera singers are like athletes in that they train their bodies to be able to perform. Their throats are typically very open when they sing, with the larynx physically held lower in the throat to produce a fuller tone.

They will work on developing the weakest parts of their range by using different muscle groups to achieve the same quality of sound, and Placido Domingo has probably worked on this to achieve and project lower notes than usual.

Physically, four elements are crucial to singing. The first is breathing, which is largely natural, but considerable skill in using lung-power is needed to sustain and support long, loud notes.

Next is the vibration of the vocal cords, or “folds”, which largely control the voice’s natural pitch. Complicated structures that lie behind the Adam’s apple, they can be stretched and vibrated to produce sound. The vocal folds are composed of a muscle, a layered area of varying thickness of tissue, a jelly-like fluid and an outer layer just four cells thick, so they can be very vulnerable to damage or swelling.

Swelling on the vocal cords caused by smoking or other damage can lower the pitch of the register — an effect that some rock singers may try to cultivate, but opera singers must try to avoid if they want to keep hitting the high notes.

How air is resonated in the vocal tract is also important for the quality of richness of the sound. The last part is articulation, which is easy once you know the words.

Sue Jones is head of Speech, Voice and Swallowing Services at University Hospital South Manchester NHS Trust

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