Short definition: the range is the gap between the lowest and the highest note
When we speak of a hymn's range (its ambitus), we simply mean the span of its notes. A hymn that drops to a low D and climbs to a high F has a range of about a tenth: that's a lot. A hymn that stays between A and C has a range of a third: that's very little.
Tessitura, on the other hand, is the zone where the hymn usually sits. A hymn can have a wide range but a comfortable tessitura, provided the extreme notes appear only once or twice and are "prepared" by the writing.
What a congregation can hold, in practice
A mixed parish congregation, with no preparation, can sing an octave at most, ideally between a low C and a high C (C3 → C4 in choral convention). Beyond that, you lose voices: the men struggle in the high register, the women in the low one.
The hymns most widely sung in French parishes (Église du Seigneur, Devenez ce que vous recevez, Tu es là présent…) respect this rule: a range of an octave or less, with a tessitura centred around A-B.
Conversely, the hymns that "don't work" with a congregation, often very fine compositions, fail through too wide a range or too high a peak. A high F held on a stressed syllable means a congregation that drops out.
How I go about it when I compose
For hymns meant for the congregation, I set myself two constraints:
- A range of an octave at most, most often a seventh.
- The highest note reached twice at most, and prepared by the melodic line: never on an attack, never on a closed vowel.
For hymns meant for the choir (refrain taken up by the congregation, verses by the choir), I widen the verse's range to a tenth, sometimes more. The verse is carried by trained singers. But the refrain, always, respects both rules.
The "choir pitch" trap
Many choir directors conduct at the choir's pitch, that is, at the height where the singers are comfortable. That is legitimate for pieces the congregation does not sing. But for hymns meant to be taken up, you must transpose downwards: by a tone, sometimes a third. It's uncomfortable for the sopranos, who then sing in their middle register. But it's the congregation that sings, not the sopranos.
This simple act of transposition already solves half the problems of a congregation's engagement.
A matter of respect, in the end
Writing at the right range is not a technical concession. It is an act of respect towards the voices that will sing. The beauty of a melody that sits right, that is caught without effort, that is taken up year after year, it is worth all the harmonic pyrotechnics. And it is, I believe, the first thing expected of a composer of sacred music today.