Re: [Phys-L] sound

[Leigh Palmer <palmer@sfu.ca>]

You have the mechanism correct; the player adjusts his lip tension to vary the airstream intensity at the desired frequency, but he is helped considerably by the impedance maxima of the instrument with resonances at frequencies you identify below. The player can feel the resonances and keep centered on correct pitch by "tuning" his lip tension for minimum airstream flow. If he attempts to keep his lips vibrating at any other frequency his efforts will be thwarted by the presence of a lower impedance at a nearby frequency. The bugle is a mechanical device that performs the same function now routinely heard from modern tin-eared musicians using the electronic device called "Auto-Tune".

I should note that The bugle is a fine example of a high-Q resonant system. Students have for many years been badly served by physics teachers who do not really understand resonance phenomena. These teachers seem to be eager to show the Tacoma Narrows Bridge collapse film as their example, leaving almost all the students thinking that the bridge exhibited a resonance. It didn't, of course; the oscillation of the bridge was in response to a steady wind, not an oscillating one. If one calls this oscillation a resonance, it must be one of very low Q. If one watches the film carefully it will be evident that the frequency varies with time. There is no "Auto-Tune" effect. A bowed violin string is a good example of a system with several high-Q resonances, but a modicum of skill is required to demonstrate the harmonics (octave, twelfth, etc.). The steady bowing of the string also seems to be analogous to a steady wind on the bridge, but of course it is not. The bow is actually dragging the string with a periodic series of pulses resulting from a "stick-slip" oscillation like the one that can be had when chalk squeaks on a blackboard*.

A simple analog of the bridge oscillation can be made by stretching a blade of grass between your thumbs and blowing hard past it**. The bridge collapse film is indeed impressive, but it is not a suitable example to use for an introduction to resonant phenomena. If it were simple, the problem would have been caught early, and the film would never have been made.

Leigh

*  Yep, I'm that old.

** Get someone to show you how to do this if you don't already know how.

On 2013-04-12, at 8:37 AM, Anthony Lapinski wrote:

> Does anyone know how a bugle produces different notes (as it has no keys,
> valves, slides, etc.)? I know the instrument when played is physically
> closed, but it behaves like an open pipe due to the bell's conical shape.
> So the harmonic series is fn = nv/2L The length of the instrument is
> constant, so does the player just blow/buzz his/her lips faster? or with a
> higher frequency? That changes the harmonics/frequencies, but does that
> change v in the equation? I thought v was the speed of sound in air (343
> m/s) for brass instruments. I'm a bit confused with some of the finer
> details of this "simple" instrument.