Effect of bore diameter on sound

[NickN]

A question for the technical experts, but what is the theoretical effect or difference in the sound caused by the differing diameter of bore / throat between the K bell (on the left) and P bell (on the right)?

Curious to know what is or was the reasoning behind such a big difference. Cost?
Also, does the resulting difference in diaphragm diameter account for the buzzier sound of a P horn?

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[Mafarnz]

The smaller the bore diameter, the higher pitched the note. The larger the bore diameter, the lower the pitch. Notice that the P bells are much longer than the K bells.

The reason for the P horn squeak or buzz is the fact there is no metal to metal contact inside the P horns. The rubber nozzle allows for a much lower starting pressure, and the horn starts to sound even as pressure is building inside the airline and manifold. With other horns the pressure is much more even throughout the whole system (air lines, manifold, ect) when it starts to sound.

[NickN]

Thanks for the comprehensive answer, Maddie.

I suppose the legendary 'big bore' 5 bell was confusing me as presumably that still played the same pitched note after it became small bore, and I have to assume that would be down to bore depth being altered, or throat length?

And the rubber nozzle explanation for the P horns, yes, that does make absolute sense.

[Mafarnz]

I suppose the legendary 'big bore' 5 bell was confusing me as presumably that still played the same pitched note after it became small bore, and I have to assume that would be down to bore depth being altered, or throat length?

The big bore K #5 bells are a tad bit shorter. The newer small bore 5 bells do play more or less the same note.

[NickN]

Ah, that's how they did it. Good to know, I have a lot to learn.

[Flat Train]

Great photo of comparing the p and k bells.

[cold-steel-iron]

Here is a side by side comparison of a small bore vs a big bore K#5 bell

[b2major9th]

The big bore and small bore 5 bells are nominally the same length. The reason they blow the same note is that the area inside the nozzle is bored deeper on the small-bore bell to lower the pitch. If it were not bored, it would sound something like 660 Hz. That little difference in depth lowers the pitch a significant amount, 38 Hz

The manufacturer did this to save money by making use of the "African" 5H bell for both applications, thus eliminating a separate mold for the American 5 bell, and a separate inventory.

Mike

[Mafarnz]

Thanks for the correction Mike.