The changing adolescent male voice can be a huge problem for choirs. One month boys can be singing Faure's "Panis Angelicus", and the next they might be more suited to a gravelly rendition of "Ol' Man River".
Dealing with the changes was one of the issues addressed at the International Boys' Voices conference at the Jordanhill campus of Strathclyde University in Glasgow last weekend. But there are other, perhaps more fundamental, issues concerning boys' voices: those of recruiting and retaining boys in choirs.
In England, where cathedral choirs are the foundation of the male choral tradition, there has been a gradual decline over the past 20 years in the number of boy choristers. That has led great cathedrals, such as those in Salisbury and Wells, to controversially admit girls into their ranks.
Scotland has never had a strong tradition of boys' choirs, but the formation of a National Boys' Choir two years ago, under the umbrella of the National Youth Choir of Scotland, was an attempt to buck that trend.
"Apart from the general decline in the opportunities for young people of both genders to sing, there is a particular concern about the lack of boys who are singing in a structured way," says Ian Mills, general manager of NYCoS, who helped to organise the conference.
"This whole initiative is to boost boys' singing and raise its profile," he says. "We are well aware of the boys' voice tradition in England and in a sense this conference is giving our boys an idea of what to aspire to."
The Cantemus Boys' Choir from Hungary and the Manchester Boys' Choir held workshop demonstrations that gave a feel for how they achieve their very different sounds. The National Boys' Choir is relatively new in comparison but has gone a step further than those groups by forming a choir specifically for those with changing voices. It currently has 12 members.
"In many organisations, once the boys' voices change they're out," says Mr Mills. "It's our philosophy to retain the boys through the changed voice period."
Christopher Bell, artistic director of NYCoS, believes it is possible to sing through four of the five stages of the changing voice . He showed how to test the adolescent male voice and how to identify problem areas. He argued that it is important to keep boys singing throughout this period and to retain them within the social life of the choir so they don't drop out and never return.
Compiling a suitable repertoire is a problem. Marilyn de Blieck, who runs the Keynote Trust Ayrshire Voices, says there was plenty of music for changing voices available more than 30 years ago when a lot more teachers specialised in this area. "It is temporarily forgotten about," she says, "and then thrown out when it becomes unfashionable."
She urges people to hold on to old singing books and there was a suggestion that a database should be set up of available works.
Rehearsing the music can be another problem. There is a tendency to learn lyrics rather than pitch first, but Professor Graham Welch, of the Institute of Education at London University, says there is scientific evidence to indicate that learning a new piece should start with the melody, adding the rhythm next and words last.
He also showed the conference audience data on the development of musical ability, which could support having separate boys' and girls' choirs at late primary age. At age seven, almost 40 per cent of boys sing out of tune, compared to 20 per cent of girls. By age 11, there are eight times as many boys singing out of tune as girls. It is believed that boys sense this underachievement when singing in a mixed choir and that can drive them away.
Mr Bell sees the effects of separating the sexes at puberty on a more practical level. "What's interesting is that when you have a chorus of just boys they rub along a whole lot better than if you have a mixed chorus at this age," he says.
"The crisis in boys' voices is very much the topic of the moment because it is considered that there is a crisis in boys' education as well, with the idea that girls are attaining higher standards than boys."
With the National Boys' Choir at least there is a chance that the pendulum could begin to swing the other way.