Do louder spells have more power then quieter ones?

Question

I don't mean by voice but by the loudness of the cast from the wand out, for example, a lightning spell makes a big loud noise, is that the case for all or most spells or not really?

Answer 1

No

In Half-Blood Prince, the non-verbal spells are mentioned. That is, almost any spell can be cast silently, but that requires more knowledge and skill from the caster.

_{Harry Potter and The Half-Blood Prince, Chapter 9, The Half-Blood Prince}

“... you are, I believe, complete novices in the use of nonverbal spells. What is the advantage of a nonverbal spell?”

Hermione’s hand shot into the air. Snape took his time looking around at everybody else, making sure he had no choice, before saying curtly, “Very well — Miss Granger?”

“Your adversary has no warning about what kind of magic you’re about to perform,” said Hermione, “which gives you a split-second advantage.”

“An answer copied almost word for word from The Standard Book of Spells, Grade Six, ” said Snape dismissively (over in the corner, Malfoy sniggered), “but correct in essentials. Yes, those who progress to using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some” — his gaze lingered maliciously upon Harry once more — “lack.”

Damage caused to the target is not always associated with noise, either. As mentioned in the comments to the question, the Death Spell (Avada Kedavra) is not loud.

_{From Harry Potter and The Goblet Of Fire, Chapter 14 - The Unforgivable Curses}

There was a flash of blinding green light and a rushing sound, as though a vast, invisible something was soaring through the air - instantaneously the spider rolled over onto its back, unmarked, but unmistakably dead.

This spell is one of the most powerful in Potterverse, as there is no counter curse, and the only known survivor is Harry Potter himself.

Voldemort managed to kill the entire Riddle family using this spell, without anyone hearing anything.

_{Harry Potter and The Half-Blood Prince, Chapter 23, Horcruxes}

“The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how the Riddles died, for the Avada Kedavra curse does not usually leave any sign of damage. ... The exception sits before me,” Dumbledore added, with a nod to Harry’s scar. “The Ministry, on the other hand, knew at once that this was a wizard’s murder.

Probably the idea of the question comes from the movies, where battles are always loud. But when it comes to killing an unsuspecting or unarmed opponent, it can be done silently.

Answer 2

I would say vice versa: the more skillful a wizard is, the quieter are his spells. Hence, the same spell can sound different from different caster and probably if it's bangs and loud noises - the caster is not a mighty one.

We often see that when students just learn spells or do something wrong it's accompanied by loud sounds:

...Ron plunged his hand into his robes, pulled out his wand, yelling "You'll pay for that one, Malfoy!" and pointed it furiously under Flint's arm at Malfoy's face. A loud bang echoed around the stadium and a jet of green light shot out of the wrong end of Ron's wand, hitting him in the stomach...

And when you look at fights of the most powerful wizards (like Dumbledore vs Voldemort or Snape vs McGonagall) - their spells are mighty, but silent.

Another good example is Apparation - looks like the more skillful a wizard is, the more silent is his Apparation. Don't want to still other's good research, so here is the link to an answer with lots of quotes.

Answer 3

It depends

I don't think this was addressed in the books, but there are good arguments for both yes and no.

If the sound is a side effect of the magical energy, then it is likely that more magical energy might produce a louder sound, or at least that a louder sound is caused by more magical energy (notice that these are two similar but different relations, and are not equivalent).

But there may also be a different effect that the loudness of the sound is not just related to the magical energy the caster puts into the spell, but also by the caster's skill. A skilled caster may be able to cast more efficiently, converting more magical energy into the spell and waste less on undesired side effects like sound.

I also don't think that the compared loudness of two different spells indicates a difference in the magical energy involved. You mention the lightning spell, and lightning is always associated with thunder. So it is reasonable that a lightning spell also creates a sound similar to thunder, because it heats the air the same way lightning does.

On the other hand, for other spells there seems to be just the sound of something disturbing the air as it travels to the target.

There was a flash of blinding green light and a rushing sound, as though a vast, invisible something was soaring through the air - instantaneously the spider rolled over onto its back, unmarked, but unmistakably dead. (GoF)

There is no mention of the sound being loud, we just know that it was noticeable in the presumed silence of the class room, having watched the other curses and knowing which curse was to follow.

But on the other hand, the curse is described as just a flash of light, followed by something invisible soaring through the air. As there are many passages in the books where people sidestep curses aimed at them, they must be able to see the curse coming, so it can't be invisible. Maybe just nobody noticed the green light traveling because the distance was so small that the time in travel was too short, with Moody/Crouch standing at the desk and the spider on the desk. So it is possible that this description of the spell is not entirely reliable. It may be formally written as the narrator talking, but it may describe the perception of the students and not that of an objective observer.