#1
Okay, I know this is extremely pedantic - but, I'm busy writing an ear-training program, to help students learn the different intervals by ear. The program will play a sound and they will have to choose the interval. Simple enough, right? Here is my reference for the intervals: http://en.wikipedia.org/wiki/Interval_%28music%29#Main_intervals I know the Augmented Fourth/Diminished Fifth are the same thing, and what I'll probably do is just give that option both names. What I want to know is whether or not I've missed something and there is some arcane reason to call the interval an aug4 as opposed to a dim5, or the other way round? (Rather safe than sorry :P)
#2
I'm pretty shitty at theory but I guess my reason is if it sounds like there should be a fourth there, call it an aug4 but if it sounds like there should be a fifth, call it a dim5?
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#3
Im also rubbish at theory, but isnt what you call it related to the note you play before it?
Like if the aug4/dim5 is higher tone than the one before it, its called a dim5?
Typing this, i realise i actually have no idea
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#4
The only reason it's called one or the other is based on the notes involved. For example, if you had C and F# you'd call it an augmented fourth, and you'd call C and Gb a diminished fifth. They'd sound exactly the same, so the only factor of influence here is the note names.
#9
Depends on context.

E.g. The C Lydian Mode

C D E F# G A B

The fifth is perfect [G], but the fourth is augmented [F#]

Here's the C Locrian Mode

C Db Eb F Gb Ab Bb

The fifth here is diminshed and is not an augmented fourth. This is because there already is a fourth, which is the F [perfect 4th].
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#11
I think DiminishedFifth should answer the OP... he'll probably be biased.
Last edited by mdc at Nov 26, 2011,
#14
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No reason whatsoever to bring modes into this.


I just needed an example.
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#16
Woffez actually had a good answer. It can depend on the context of the other intervals and scale degrees you're working with, doesn't even necessarily have to do with notation. If there already is a regular 4th (such as the case with locrian), it's a b5. If there isn't, or if there already is a regular 5th, it's a #4 (such as the case with lydian).

If we didn't refer to intervals this way, describing something like Lydian would be confusing or unecessarily repetitive: both a 5th and a b5? Nah, it's a #4 because of the intervalic context (there already is a 5th and it's the 4th scale degree), regardless of notation.
Last edited by Brainpolice2 at Nov 26, 2011,
#17
Alternatively: if it's a fourth it's a fourth and if it's a fifth it's a fifth. I.e., any G to D is always a fifth, so G#-D is a diminished fifth, however any A to D is always a fourth, so A#-D is an augmented fourth. There's or real need to apply this to scales at all, it's purely intervallic.
#18
Quote by jazz_rock_feel
Alternatively: if it's a fourth it's a fourth and if it's a fifth it's a fifth. I.e., any G to D is always a fifth, so G#-D is a diminished fifth, however any A to D is always a fourth, so A#-D is an augmented fourth. There's or real need to apply this to scales at all, it's purely intervallic.


A# - D is minor 3rd.

How often does one use A# instead of Bb anyway?
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#19
Quote by Woffelz
A# - D is minor 3rd.

How often does one use A# instead of Bb anyway?


A# - D is a diminished fourth

Bb - D is a major third

A#s are everywhere bro.
#20
Just call it a tritone. Semantics are a hot-button issue around here when it comes to enharmonic frequencies.

If that's not an option, just use both.
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Last edited by Hail at Nov 26, 2011,
#21
Quote by Hail
Just call it a tritone.


Sadly, even that doesn't necessarily encompass both intervals.
Last edited by Dodeka at Nov 26, 2011,
#22
Here's a simple rule of thumb:

Augmented 4ths expand
F, B -> E, C

Diminished 5ths contract
D#, A -> E, G


Also notice that the letter difference between F up to B is a 4th. The letter difference between a D up to A is a 5th. That doesn't do anything if you're doing ear training but it's a sure indication of one or the other when looking at the score.

...modes and scales are still useless.


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#25
Quote by Xiaoxi
Here's a simple rule of thumb:

Augmented 4ths expand
F, B -> E, C

Diminished 5ths contract
D#, A -> E, G


Also notice that the letter difference between F up to B is a 4th. The letter difference between a D up to A is a 5th. That doesn't do anything if you're doing ear training but it's a sure indication of one or the other when looking at the score.

Since no-one has acknowledged this post, I will instead.

The tritones that resolve are labelled enharmonically in perfect cadences in major and minor respectively.