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The perfect fourth is the last of the perfect intervals, after the unison, octave and perfect fifth. Of these, it is the only one that does not fundamentally appear in the harmonic series. The unison is the fundamental frequency. The octave is the first harmonic, and you can play it at the twelfth fret. The second harmonic is a fifth above that octave, and you can play that at the harmonic on the seventh fret. The third harmonic is again an octave, two octaves above the fundamental, and you can play that one on the harmonic at the fifth fret.
The perfect fourth however, does not appear in this series. Rather, we get the perfect fourth as the distance between the fifth and the octave. The perfect fourth completes the octave. Early musician were in a quandary about what to do with this interval. Many people treated the fourth and the fifth both as the middle consonances, and allowed the use of both of them. Some people, however, considered the perfect fourth to actually be dissonant. The advocates for consonance won out. After all if you have people sing a root, the fifth, and the octave, the fourth is going to appear, so it was going to be hard to extinguish it from music altogether, however unnatural it might be on its own.
Deep Purple must be very thankful for that development. The opening of Smoke on the Water simplifies the oh so complicated power chord to a lick that is nothing but parallel perfect fourths.
In just intonation, a perfect fourth has a frequency ratio of 4:3. It is 5 frets on the fretboard. It is also the interval between the E/A, A/D, D/G, and B/E strings on a guitar in standard tuning. On the guitar, which is in equal temperament, the perfect fourth is tuned 2 cents sharp of what it would be in just intonation.
The pentatonic scale has four perfect fourths in it. Take the A minor pentatonic scale. The notes in it are A C D E G and A. A/D D/G E/A and C/G are all perfect fourths. Thus, you will hear the fourth all the time in blues solos, especially when a player is trying to break out of straight scale wise patterns.
The perfect fourth is also used in place of the third in sus4 chords. Take a Dsus4 chord. This is formed by playing a Dchord, while covering the first string, third fret with your pinky. So the G rings instead of the F#. The reason this is called a suspended chord is because in the key of D, the G chord would be the dominant. So your typical cadence would go from the G chord to the D chord. But with the suspension, the G from the previous chord is left hanging, or suspended, over the D chord. Thus the full resolution of the cadence doesn’t occur until this fourth slides back into a third.
People trying to break out of classical harmony and its dependence on the triad have turned to the fourth as a basis for building chords, which leads to a much more open sound. Here’s a Debussy study in fourths:
In Jazz, voicing in fourths became very hip with guys like Bill Evans, and even more so, McCoy Tyner. Here he is using voicings that emphasize fourths all over.
Melodically, Amazing Grace and Love Me Tender begin with an ascending perfect fourth. I’ve Been Working on the Railroad starts with a descending perfect fourth.
Finally, the parallel perfect fourth has also been used to invoke/parody oriental music, maybe most famously in the opening riff, by Nile Rodgers, in David Bowie’s version of Iggy Pop’s China Girl:
It’s Smoke on the Water again, but done in an entirely way, in a different context. It’s amazing how music can take what is basically the same thing, and make it sound so completely different.
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What about the 5 ? I understood that both the 4 and 5 are perfect? I’m just asking,,I’m not sure but , I watched this https://youtu.be/nmtYaj59slU
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There are different characters of intervals, which have to do with how consonant they are. The simpler the frequency ratio between two notes, the more consonant it is.
There are four perfect intervals: unison, octave, fifth and fourth. They have frequency ratios of 1:1, 2:1, 3:2, and 4:3 respectively. Those are the intervals that people tend to find the most universally pleasing.
After this, there are the major and minor intervals. These are the 3rds, 6ths, 2nds, and 7ths. The major third has a frequency ratio of 5:4. The minor third is 6:5. See how this is going. Maj 6 is 5:3, Min 6 is 8:5.
After this there are augmented and diminished intervals. Only one appears in the major scale: the tritone. It was also called the devil’s interval. It was considered too dissonant to use in any sacred music. (It also is the fundamental basis for the blues.) Compared the the simple ratios above, the tritone has a ratio of 45:32. Not simple at all, and it is the most dissonant single interval in diatonic music.
It will also be the subject of my next topic, because its really cool and important.
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