# Harmonic Series Interference

I have been analysing intervals to look at the frequency difference between harmonics.

Before my analysis I was under the impression that harmonics for 'consonant' sounding intervals contained more harmonics that lined up - as is the case with the octave.

I analysed the tritone and as expected many of the harmonics fall close to each other causing interference and 'dissonance'.

Now I am looking at the perfect fifth (C3-G3 in this case). I was expecting some harmonics to be shared between these notes. Instead it seems that some almost line up but to not exactly.

For example the third harmonic of C (392.438) and the second harmonic of G2 (391.995) - a difference of .44. It is my understanding that this would create interference between these partials every 2.27 seconds.

I have tried recording guitar and bass playing this interval if i isolate this area I think I can hear some interference however i can't tell.

Is this correct? Is this small amount of beating between these partials just adding to the character of this interval?

Thank you!

• Where did you get your frequencies from?
– user50691
Dec 8, 2020 at 18:01
• I am not familiar with this but it does not look like the harmonic sequence. Thing need to be judged in the correct context. Please see my answer for more.
– user50691
Dec 8, 2020 at 18:11

This is called "12-tone equal temperament", and it involves dividing the span from our fundamental note to its first harmonic – i.e. from a fundamental frequency to double that frequency – into 12 equal ratios. So from one note to the next we have a ratio x such that if we took 12 of them (that is, if we considered x to the power 12) we would get the ratio 2; so x is the 12th root of 2. G is the 7th note of the 12, so corresponds to multiplying the fundamental frequency by a ratio of (12-th root of 2)-to the power of 7. Since any n-th root which is not a whole number is irrational, no integer multiple of the fundamental frequency can equal an integer multiple of any of our other notes – and as you perhaps know, harmonics are given by integer multiples of base frequencies.

An alternative to equal temperament is what's known as Just Intonation; where we in fact define the note G to be the note given by the 3rd harmonic of C. This obviously has better properties for exactly the reason you have observed, however ultimately generates an unlimited number of distinct notes.

• Thank you for the answer. Perhaps my initial conception was assuming just intonation. So assuming equal temperament, my analysis of this interval is correct and due to the nature of this division of notes even more 'consonant' intervals will have some level of interference among parials? Thanks again. Dec 8, 2020 at 18:08
• @Yoppayoppa Well, the interference will have a period of ~2.26 seconds (I see you didn't use all the decimal places at your disposal, not that it really matters), as in, the continually changing relationship between these two tones will repeat itself every 2.26 seconds. This is not quite that same as saying interference will occur every 2.26 seconds – the interference is in the fact they are changing relative to each other. The closer they are in frequency, the longer the span of time over which this change repeats itself. Dec 8, 2020 at 19:36

In general your understanding is correct. The perfect 5th has quite a few of the harmonics of each note lining up and where they don't they are a 5th apart, which is perceived as a combination of two distinct notes. In contrast the whole tone has nothing lining up all, at least to the 8th harmonic, and there are several that are close.

There are a few concepts that are missing from your description. One is the critical band theory. If the frequency difference (or ratio) is large enough the combination is not muddy. This, by the way, also explains why the perception of dissonance is somewhat frequency dependent. That is to say a third or minor third will be judged dissonant in the bass register but not in soprano.

The next is that the alignment of harmonics is more exact in Just tuning, which is based on the harmonic sequence. Fretted instruments and the piano are closer to equal tempered tuning which does NOT follow the harmonic sequence. Hence the harmonics of intervals in this tuning will not be perfectly aligned. Many people with perfect pitch or very good relative pitch (after years of training) can hear the dissonance in 12TET intervals. I knew a man with perfect pitch who could not listen to equal tempered music due to the dissonance.

• Thank you for your response. I am looking at this in the context of Critical Bands actually and their impact on consonance/dissonance. Interesting to hear about the man you know. It seems like cultural factors play a big role in our perception of consonance/dissonance. Dec 8, 2020 at 18:32
• I would recommend reading a text like Physics and the Sound of Music by Rigden, or even On the Sensations of Tone by Helmholtz. Cultural forces may influence what we decide to accept as harmonious but the fact is our ear are tuned to respond to harmonics and create them when excited by a fundamental. This is probably an evolutionary feature common to all hominids. As a result, even pure tones excite harmonics in the ear and we cannot not hear them.
– user50691
Dec 8, 2020 at 18:46

This is a fascinating area and the answer you are looking for is subtle because it depends on the conjunction of the spectra of partials of the two notes being played at once.

The definitive text in this area is Sethares’ Tuning Timbre Spectrum Scale. He derives many interesting results from looking at the relationship between timbre (i.e. the harmonic series of a specific note played on a particular instrument) and the mucial scale being used, including non 12 tone tempered scales.

It’s a long book but quite accessible. In particular do check out the example recordings where, for example, he makes the octave sound dissonant, or the tritone consonant, by manipulating the timbre of the instrument being used.