It's being able to sing a piece of music at first sight.
Normally speaking, by "music", I mean "vocal sheet music", but it could mean almost any notated music.
If I learn to sight-sing, will I be able to sing any piece of music that I haven't seen before?
No. Sight-singing is a skill that consists of many levels of knowledge and techniques that can be learnt and practiced. So your level of ability to sight-sing can always be improved, but there is no such thing as perfection.
There are a number of basic skills and techniques that are applicable to the sight-singing of most styles of music, ranging from Gregorian chant to modern pop.
However, there are some techniques and skills that will help you sight-sing some types of music, but would not apply to others.
For example, with learning and practise, you might become good at sight-singing choral music of the 17th century, but this does not mean you can sight-sing a piece of modern music.
How long will it take me to learn to sight-sing?
If you don't know anything about music or singing, then - let's be honest - it could take years to become a good sight-singer, but :
You can learn the basic skills in a few days;
You could be sight-singing simple phrases within a few hours.
If you know some music (for example, if you play another instrument), then you will find it very easy to pick up the basics.
If you have done some singing before, you have a big advantage because you will already have an understanding of many concepts of pitch and rhythm (even if you don't realise it!).
No matter what your current skill level, you should be able to significantly improve your skills in a matter of weeks:
Sight-singing is a progressive skill - everyone can always improve and no-one is ever perfect.
There are a number of levels you may attain:
With a very basic level of knowledge and skill you may be able to sight-sing a simple piece of music, and this may give you enough to decide whether you know it or not, or possibly whether you like it or not.
With a reasonable level of knowledge and skill you should be able to sight-sing simple music, and make a good attempt at more complicated music, perhaps making some mistakes.
With a good level of knowledge and skill you should be able to sight-sing most styles of music, although you will still make mistakes.
Why should I learn to sight-sing?
to be able to recognise music from a printed score;
to be able to join a choir;
to be able to sing more confidently and to enjoy singing more;
to be more involved in music and to get more from it;
to have a better understanding of music;
to be able to sing solos without learning music "by heart";
to spend less time on "note bashing";
to spend more time on finer performance points;
your own other reasons....?
Are there any drawbacks of improving my sight-singing?
There are perhaps one or two possible disadvantages:
A good sight-singer might get bored when "note bashing" while singing in a choir or group which contains less able members. A choir leader or conductor has to work near the speed of the slowest learner, or people will inevitably be left behind feeling lost. However, it is up to the good sight-singer to lead the others with the correct notes, and therefore make it easier for the others to learn more quickly. It only requires a small number of good sight-singers within a choir to vastly increase the speed at which a piece of music can be learnt by the whole choir.
A good sight-singer singing a solo piece or in a group where all the members are good sight-singers may not get the chance to commit a piece to memory. There is also a possibility that a person who is a good sight-singer may never develop the ability to learn a piece of music by heart. This can be a danger, but I think the advantages outweigh this possible disadvantage.
What is sight-reading then? Is it different from sight-singing?
Sight-reading is the ability to play a piece of music on any specific instrument at first sight. The voice is an instrument, so sight-singing is a specialisation of sight-reading.
Yes, sight-singing is different in a number of ways from sight-reading on most other instruments. The same knowledge of music theory is required, but a number of different techniques are required.
Learning to sight-read on a musical instrument (other than the voice) could be argued to be easier in one respect than learning to sight-sing. The reason is that the ability to 'hear' the tune (or only the next note) in advance of attempting to make the sound is absolutely required in singing, but is only required to a limited (but variable) degree in learning to sight-read on other instruments.
To take the furthest extreme, sight-reading a part for the bass drum or triangle simply requires the player to hit the instrument in the right place at the right times, with the required strength. This means (theoretically) that anyone who can count and swing a stick can do it. There is no absolute necessity to be able to 'hear' the result in advance of swinging the stick.
Moving up a level, sight reading on a keyboard instrument requires (among other things) the manual dexterity to be able to hit the right keys at the right time. If you can do this, the right notes will sound. Of course, in learning a piece of music, a performer gets to know what it sounds like, and this is a prerequisite for any reasonable interpretation of a piece of music. A good sight-reader on the piano will in fact anticipate the sounds and structures of a piece before and while playing, and will therefore be able to give more than simply a wooden rendition of the notes of a piece even when sight-reading.
Brass instruments are a further step up again; the embouchure (the positioning of the lips) is so crucial to the production of the sound that it is very difficult (although not impossible) to play a tune without knowing what the tune should sound like. This might be a surprise to non-brass players, but it is the case. The brain has to 'hear' the correct pitch, and then interpret that into the correct tightness of the lips to produce the correct note.
Even with brass instruments, however, there is still some leeway, and in singing there is no leeway. The vocal chords and related muscles are entirely responsible for producing a note, and therefore must be absolutely correctly positioned to produce the correct note.
All this has some very important consequences. It means that to be able to sight-sing requires some ability to look at a line of music and to know what it sounds like without using a piano or other instrument. This in turn requires some ability to properly read music, by which I mean more than just an understanding of the meaning of the dots and squiggles on the stave. It requires some translation of those dots and squiggles into a mental realisation of the sound that they will make when sung.