Questions: Perfect Intervals by Ear

Question 1

A student hears two notes played simultaneously and describes the sound as 'hollow, open, and stable — like a power chord.' The interval is most likely:

• A. A major third (4 semitones)
• B. A minor third (3 semitones)
• C. A tritone (6 semitones)
• D. A perfect fifth (7 semitones)

Answer: (D)

The perfect fifth's defining perceptual quality is exactly this: hollow, open, and acoustically stable, due to its simple 3:2 frequency ratio. Major thirds sound warm and bright; minor thirds sound darker and more intimate — neither is 'hollow.' The tritone (augmented fourth/diminished fifth) is maximally dissonant — the opposite of open and stable. Rock power chords are literally root + perfect fifth, chosen specifically for this hollow, powerful quality.

Question 2

Why are parallel perfect fifths forbidden in traditional counterpoint?

• A. Perfect fifths are dissonant intervals that create too much tension between voices
• B. Moving two voices in parallel by perfect fifths makes them sound acoustically fused, destroying voice independence
• C. The perfect fifth is too widely spaced for voices to sing simultaneously in parallel
• D. The fifth is an unstable interval that requires resolution and cannot be repeated

Answer: (B)

The perfect fifth is highly consonant — its 3:2 frequency ratio causes the two pitches to blend strongly. When two voices both move by a perfect fifth in parallel, this blending effect compounds, making the voices sound like a single merged unit rather than independent lines. Traditional counterpoint prizes voice independence above all else, so parallel fifths undermine the entire polyphonic texture. Ironically, the same acoustic property that makes isolated fifths so powerful makes parallel fifths structurally destructive.

Question 3

Perfect intervals sound 'hollow' or 'open' because they lack the third that creates major or minor color.

• T. True
• F. False

Answer: True

Major and minor intervals derive their characteristic brightness or darkness from the presence of thirds or their related intervals. Perfect intervals — unison, fourth, fifth, octave — contain no third and thus have no major or minor coloring. This absence is what produces the distinctive hollow quality: the interval is stable and consonant, but empty of the warmth (major) or darkness (minor) that comes with a third. The perceptual openness maps directly onto the simplicity of the frequency ratios involved.

Question 4

The perfect fourth and perfect fifth are acoustically indistinguishable by ear because they are interval inversions of each other.

• T. True
• F. False

Answer: False

Although the perfect fourth and fifth are interval inversions (each inverts to the other), they are distinctly recognizable by ear. The fifth (3:2 ratio) sounds more stable, settled, and complete — it 'locks in' acoustically. The fourth (4:3 ratio) sounds slightly more tense and expectant, as if leaning toward resolution, particularly when heard above the bass voice. Reliable discrimination between them is a fundamental goal of interval ear training. Their shared hollow quality does not make them identical — it is the difference in stability and directional pull that distinguishes them.

Question 5

Why do perfect intervals have a 'hollow' or 'open' quality, and how does this connect to the physics of sound?

Model answer: Perfect intervals have simple integer frequency ratios: the octave is 2:1, the fifth is 3:2, and the fourth is 4:3. The simpler the ratio, the fewer beating overtones occur between the two pitches, producing strong consonance with minimal tension. This strong blending without the complexity of thirds yields the hollow quality — plenty of resonance but no warm major or dark minor color, because no third is present in the interval.

This connection between physics and perception is why ear training is not merely memorization: understanding that simpler ratios = more open and stable helps explain why octaves are the most 'pure,' fifths are powerful, fourths are slightly more complex, and thirds and sixths introduce color. The physical model predicts the perceptual hierarchy of interval consonance.