Do I have to notate "eighth equal quarter" when the time signature changes?

Question

I cannot recall which tempo marking I saw but it is something like "♩=♪" or "♩=♩."on the top of the staff when time signature changes. Does that mean the overall speed doesn't change or becomes two times faster than previous phrase? Is it necessary to write markings like that when it comes to time signature changes?

If I do not indicate "♩=♪" or "♩=♩." on the score, does performer know that only the time signature changes instead of the speed? (That is, the duration of the eighth and quarter note remain unchanged)

Answer 1

It's not strictly necessary to include notation-specific tempo markings, but the meaning and relative speeds change depending on what is or is not indicated.

Example: switching from 4/4 to 6/8

quarter-note = dotted quarter-note

For example, in a switch from 4/4 time to 6/8 time, one might see <quarter note> = <dotted quarter note>, which means "the pace of quarter notes in the 4/4 section will now be the pace of dotted quarter notes in 6/8 time." The tempo is considered to be unchanged, because the basic pulse — the BPM — hasn't changed; only the notation of the pulse has changed.

eighth-note = eighth-note

On the other hand, in a switch from 4/4 time to 6/8 time, one might see <eighth note> = <eighth note>, which means "keep the pace of the eighth note constant across the time-signature change." Here, the tempo will change, because what used to be 1/2 beat in 4/4 now become 1/3 beat in 6/8 time. If the original tempo was, say, 120 BPM (i.e., quarter-note = 120), the new tempo will be 80 BPM (i.e., dotted-quarter note = 80).

unrelated tempi

Another possibility is that there isn't a direct, proportional relationship during a change from 4/4 to 6/8. In that case, an explicit tempo indication is needed, such as <dotted quarter note> = 80, or <eighth note> = 96.

no indication

In the absence of a tempo indication, the presumption is that the basic pulse remains at the same tempo. That is, it would be equivalent to <quarter note> = <dotted quarter note>.

Answer 2

When the time signature changes but the beat remains the same speed, 'l'istesso tempo' was often the instruction.

That's fine, if context makes it absolutely clear what is intended. A march that moves from 2/4 to 6/8 is probably not going to stop being a march. There will still be two beats in a bar, a 'q = dotted q' notation is not needed.

Much of today's music (heck - much of the music of the last 100 years!) is less predictable. 2/4 into 6/8 MIGHT be 'l'istesso tempo'. Or it might be '8th = 8th'. So it's become common to clarify the point with what you may have learnt from your notation program to call a 'metric modulation' (though that term actually has a rather more precise meaning).

The note values quoted need not be the beat values. 2/4 and 6/8 are still normally both 2-in-a-bar, even if they're connected by an '8th = 8th' indication.

Nowadays 'q = dotted q' invariably means 'old = new'. A quarter in the preceding section is equal to a dotted quarter in the following one. But beware! Some older editions did it the other way round, 'new = old'. It will USUALLY be clear from the context which is intended...

And please do NOT write this when you want swing. Swing is not triplets. All good notation programs respond to the word 'Swing' in playback, and any human players who don't know what it means need to be told, not given an inaccurate mechanical instruction.