I heard this somewhere and was wondering if it's true that women's voices are one octave higher than men's?
Yes, roughly. The standard ranges are often listed as:
- Bass: E2-E4
- Baritone: A2-A4
- Tenor: C3-C5
- Contralto: F3-F5
- Mezzo-soprano: A3-A5
- Soprano: C4-C6
You can see that the three women's ranges are one octave above the corresponding men's, with one slight exception.
Of course, everyone's voice is unique and few people fit cleanly into these categories. Trained men frequently have a little more than the two octaves listed, and women frequently have at least half an octave more. But in general, you can fairly safely categorize voices as high, middle, or low, and think of women as being one octave above men. A great deal of vocal music is written gender-agnostic, assuming that men and women will simply pick the appropriate octave.
Well, if you compare the ranges of male and female voice types you will see that the female types are one octave higher than the male types. This means that the highest female voice is in general one octave higher than the highest male voice, and the same for the lowest. In classical music this might be a good generalization, because composers often try to explore the singer's range. So the soprano voice can be really one octave higher than the tenor voice, for example.
In popular music I don't believe it's true. You hardly see women singing above F5 or G5, while tenors, like Bruno Mars for example, easily reach A4 or C5. At the same time, you can find women (not only contraltos) singing around F3 and G3, but I can't think of any popular song these days where a male singer reaches one octave lower than this.
In terms of speech I don't believe it is true either. I don't have references for this, but try to talk one octave higher than you do and think of a woman who talks at this pitch (or do the opposite if you are a woman). I bet it will sound too high (or too low). It could only sound OK if you have a very low (or very high) voice, but these are extreme cases, not the average.
So, in terms of voice types it's indeed true, but I don't think that's the case most of the times.
Yes, that's broadly true. A soprano can squeeze out a top C, above the treble clef, a tenor can just about manage the C an octave lower. Basses can approach C below the bass stave (Mahler asks for a Bb!), the deeper female voices have a similar lower limit an octave higher.
This all gets a bit complicated in popular music where some women don't use their head register at all, and men often use their falsetto rather than a full-bodied tenor sound. So if you're publishing song copies for generic male and female voices, it might be sensible to print the female version a 5th or 6th higher than the male.
This is all very general rule-of-thumb stuff though. Choose the right key for the individual singer, not their gender.