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I know about $1$st, $2$nd or other overtones in the formula of frequency in a one-sided open system (specifically in closed organ pipes) that is

$$ f = \frac{\left( 2 n + 1 \right) v}{4 L} \tag{1} \label{1}$$

where $f$ is the frequency of the wave, $v$ is the speed of sound and $L$ is the length of the organ pipe. Most importantly, $\left( 2 n + 1 \right)$ represents the $(2 n + 1)$th harmonic and $n$ represents the $n$th overtone.

My first question is: What is an overtone in a real physical sense?

Secondly, putting $n = 0$ in \eqref{1}, does that mean that it is the $0$th overtone (or is it the same as commonly fundamental harmonic in a closed pipe)? Or is it called something else? Or does the term ‘$0$th overtone' even exist in one-sided closed systems?

enter image description here

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    $\begingroup$ Oh thanks for telling ,I have changed it now! $\endgroup$
    – phymestri
    Jan 14 at 20:33

4 Answers 4

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Question 1: What is an overtone?

Initially let’s clarify the term. The nomenclature overtone is used primarily in the “world” of musical acoustics. This is because there is some cross-disciplinarity with music and musicians are more familiar with tones (which is informally related to the fundamental frequency - tonality) and harmonics.

The harmonics or overtones (one and the same) are the natural frequencies supported by a system, an open-closed tube in our case. The equation you provide is valid for an ideal lossless system, which is usually used as a first approximation to systems in musical (and not only) acoustics.

For practical systems with losses, the pressure amplitude will be maximised when the system is excited at these frequencies. Please note that the system can be excited in all frequencies. The excitation frequency depends solely on the external force that acts as the “cause of excitation”. However, at the natural frequencies, the system will exhibit resonant behaviour, and the pressure amplitude will be maximised. This least to the characteristic peaks in the spectrum of tonal instruments, such as that shown in the figure below (spectrum of a piano, taken from: http://www.met-lab.org/magnetic-resonator-piano/).

Piano spectrum

Question 2: Is the $0 \textrm{th}$ overtone the fundamental?

Based on the “definition” of the overtone provided above, the $0 \textrm{th}$ overtone does not make much sense, but it does coincide with the fundamental frequency. Essentially, if two people agree to call the fundamental the $0 \textrm{th}$ overtone then it would make perfect sense (to them).

Now, the term $0 \textrm{th}$ overtone is “valid” in the sense that in the equation you provide $n$ can take any non-negative integer value. Thus, $n = 0$ is a perfectly valid value, which leads to the term $0 \textrm{th}$ overtone. But, the same equation could be written like

$$ f = \frac{m v}{4 L}, \quad \quad m = 1, 3, 5, 7, \ldots \tag{1} $$

What has changed here is the fact that there are only odd-valued harmonics/overtones incorporated in the possible values of $m$. The reason I used $m$ is to make clear that the allowed values are not the same as in your equation. The results you get though, are identical to the equation you provide. However, the notion of the $0 \textrm{th}$ overtone does not come naturally.

It may have been a long intro but the point I want to make is that the numbering of overtones is not always so clear and quite often depends on the context. For example, many times, people who do not deal with musical acoustics use equation \eqref{1} and denote the harmonics as $1^{\textrm{st}}$, $3^{\textrm{rd}}$, $5^{\textrm{th}}$ and so on, instead of $1^{\textrm{st}}$, $2^{\textrm{nd}}$, $3^{\textrm{rd}}$, etc.

Remark

Please note that what is shown in the figure you provide are not the modes of an open-closed (or one-sided open as you call it) tube or similarly a fixed-free string. The amplitude of the depicted quantity cannot be the same at the two boundaries for such a system as the boundary conditions are different. The pressure amplitude for a closed-open/fixed-free one-dimensional system is shown below (taken from: https://phys.libretexts.org/Bookshelves/Waves_and_Acoustics/Book%3A_Sound_-An_Interactive_eBook(Forinash_and_Christian)/11%3A_Tubes/11.01%3A_Standing_Waves_in_a_Tube/11.1.01%3A_Tube_Resonance).

Closed-open tube pressure

As you can see, at the closed end (right-hand side), the pressure is maximum as the movement/displacement of air “particles” is restrained at the boundary. This causes particles to concentrate at the boundary effectively resulting in a pressure maximum. On the open end (left-hand side) where the boundary is an open/free boundary the air “particles” are free to move and so the pressure there is at its minimum.

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  • $\begingroup$ Firstly, if I say that the notes of a piano (C C# D D# E F F# G G# A A# B ) are basically the resonating frequencies , will I be correct? $\endgroup$
    – phymestri
    Jan 14 at 20:47
  • $\begingroup$ Second what I understood is that "overtone" is a relative term and that it is based on mutual understanding. $\endgroup$
    – phymestri
    Jan 14 at 20:48
  • $\begingroup$ Lastly, What I observed is that you have mentioned that overtones and harmonics are one and the same. But I had read from many sources that there is a slight difference between the numeric values of them. What I mean is that if it is the first harmonic( in open-closed system) it will be fundamental frequency or what I said as '0th overtone", if it is the third harmonic, it must be first overtone and so on. Similarly, in open-open organ pipe, if it is the second harmonic, it must be first overtone... Although in physical sense, both harmonic and overtone are same. $\endgroup$
    – phymestri
    Jan 14 at 20:55
  • $\begingroup$ Well, the pitch of the notes of a piano (and any real tonal instrument really) correspond (to a high degree but they are not identical) to the fundamental frequency of the vibrating system. In order to make a system vibrate you have to excite it, leading to resonant behaviour (in the steady-state of vibration, after the initial transients “die away”). Thus, in a sense, yes the notes you mention are some of the resonating frequencies; those with the lowest supported frequency. $\endgroup$
    – ZaellixA
    Jan 14 at 23:30
  • $\begingroup$ Regarding overtones, I would suggest to think of them as the harmonics of a fundamental frequency. Although this is not always exactly correct. For example, when the harmonics of a guitar or piano string are used to tune it the spectrum contains higher harmonics too, but their amplitude is quite low so they can be “neglected”. $\endgroup$
    – ZaellixA
    Jan 14 at 23:32
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An overtone usually occurs when an antinode is situated at the open end of an organ pipe. Physically this represents a sound which has a larger amplitude (or loudness) which can clearly be distinguished as well. When the frequency is $\frac{v}{4 L}$ (as you mentioned when asking the $n = 0$ case), it is indeed called the fundamental harmonic of the closed organ pipe. Usually, the term $0$th overtone isn't used but it essentially means when the frequency is $\frac{v}{4 L}$.

Also, the derivation of the frequency of an overtone considers the position of antinodes (places with maximum amplitude). This is because Wavelength/$2$ = Distance between two antinodes (this can be located using the pipe's dimensions). Using this and Velocity = Frequency $*$ Wavelength, we can say Frequency = Velocity/(4 L) This can be extended further for the following overtones

This image shows where exactly we find the antinodes

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  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Jan 13 at 19:10
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The $2^{nd}$ overtone has two waves. That means the pressure has $2$ peaks. The $1^{st}$ overtone has $1$ wave with $1$ peak. The $0^{th}$ has $0$.

That means the pressure is constant. That is, it represents the static air pressure in the room when there is no sound.

This is $14.7$ lb/sq inch.

Usually people don't mention it, because because it doesn't change all that much, and knowing the value it isn't important for sound.

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It is all to do with labels.

A pipe has a number of resonant frequencies.
These are called harmonics and are labeled $1$ the lowest frequency, 2 for the next highest resonant frequency above harmonic $1$, $3$ for the next highest resonant frequency above harmonic $2$, etc.
Another way of labeling the frequencies is to call the lowest resonant frequency the fundamental and the next highest resonant frequency above the fundamental is called the first overtone, the next highest resonant frequency above the first overtone is called the second overtone, etc.
Thus you could call, but nobody does, the fundamental frequency the zeroth overtone.

The nice integer progression of the resonant frequencies is not always followed with perhaps the best example being the drum for which the progression is: first mode/harmonic (fundamental) $ = f_1$, second mode/harmonic (first overtone) $= 1.59\,f_1$, third mode/harmonic (second overtone) $= 2.14\,f_1$, etc.

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  • $\begingroup$ ok so it means that 0th overtone is technically/theoretically correct but does not make any physical or real sense, am I right? $\endgroup$
    – phymestri
    Jan 14 at 20:57
  • $\begingroup$ No it is the zeroth overtone above the fundamental, ie it is the fundamental, and hence it is nonsense label. $\endgroup$
    – Farcher
    Jan 14 at 23:05

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