Posted by Paul R. Ogushwitz on January 10, 1999 at 11:01:25:
In Reply to: Re: Recording Bell Size posted by Jay Bertolet on January 09, 1999 at 10:05:21:
Doug Elliot has put the matter nicely and correctly in two postings above. There are two things to add.
(1) The ideal "impedance matching" of lips to atmosphere is only achieved by a horn of infinite length. Practically speaking, however, such a horn would also have infinite weight. The bell diameters of recording basses (and large sousaphones) are, therefore, a compromise between ideal impedance matching and weight limitation.
(2) The larger the aperture of the horn (i.e., larger bell diameter), the less pronounced the "edge effects". Edge effects are physical variations in the sound field in the immediate vicinity of the bell. Edge effects are not terribly important when the tubist is more than a few wavelengths from the listener. But when the listener is close to the instrument, edge effects are measureable. In the early days of recording, the tuba was at most one wavelength (approx. 19 feet) from the recording horn. So, the larger horn helped to reduce distortions in the bass voice in that environment.
We don't do recordings that way any more, and haven't for quite some time, so who should care about edge effects? Anyone who makes studio recordings with nearby microphones, for one; the tubist him/herself, for another. What we as players hear from our own horns is not the same as the audience hears, because of edge effects (and because of other acoustical effects, like spreading and absorption).