While going through some questions given in my book I came across the following:
Why is the note produced by an open organ pipe sweeter than that produced by a closed organ pipe?
The answer given was:
"The note produced by an open organ pipe consists of both odd and even harmonics but the note produced by a closed organ pipe consists of only odd harmonics. Due to the presence of larger number of overtones or harmonics, the note produced by an open organ pipe is is sweeter."
I disagree with the answer. If we assume two organ pipes of the same length $\ l$ the first one open and the second one closed, the harmonics of both the organ pipes are:
Open: $\frac{v}{2l}$, $\frac{2v}{2l}$,$\frac{3v}{2l}$, etc., with a difference of $\frac{v}{2l}$ between successive harmonics
Closed: $\frac{v}{4l}$, $\frac{3v}{4l}$,$\frac{5v}{4l}$, etc., with a difference of $\frac{v}{2l}$ between successive harmonics.
In both cases the harmonics form an arithmetic progression with the same common difference. Just the first term is different. Then how can we say that the open organ pipe would have larger number of overtones? According to this logic the very statement that the note produced by an open organ pipe is sweeter then that produced by a closed organ pipe seems false. Then, why does my book claim that quality of sound from an open organ pipe is sweeter than that from a closed organ pipe?
Best Answer
If you start with the same fundamental frequency for both open and closed, say $f_0$, you'll notice that for the open pipe, the harmonics are: $$f_0, 2f_0, 3f_0, ...$$ While for the closed, $$f_0, 3f_0, 5f_0, ...$$ And you'll notice that the closed pipe has greater spacing of harmonics compared to the open pipe.
That is for a given single note. You have to lower the note of the closed pipe by $\frac{1}{2}f_0$ (1 octave) to produce the same spacing of harmonics as the open pipe.