There has been loads of progress on Betty this week, probably most obvious (as the organ builders are still working on balancing the sound, etc.) has bee the return of the decorative zinc pipes on Betty’s South side, facing the chancel – but they probably warrant a post in themselves – to follow.
One of the less visible, and hopefully un-noticeable things that’s been done is checking the sound from the pipes going in and, where, necessary making adjustments. The swell box has changed size and shape significantly. On the whole it is much larger, but where it was a triangular cross section previously, with the longest pipes in the middle, it is now a rectangle, with the largest pipes on the outside and footboards in the middle for maintenance and tuning. This put the longest pipes at the outside. It also meant that the maximum height of the box had to be reduced to fit under the sloped roof (which it does within 30mm!) The longest of the swell pipes therefore needed to be mitred.
Mitring is the process of bending the pipe, not done simply by passing it round a corner, but by taking two 45º cuts and welding the pipe back together, making a 90º bend. By mitring with the two 45º cuts the bend is gradual so the length contained in the bend and the continued section after it are part of the overall effective length of the pipe – a sudden bend would do something far more complex and undesirable to the sound. This is similar to the use of winglets on an aircraft wing’s (but not the same as a wing-tip fence which serves a
different purpose). By bending the tip of the wing up the effective length and therefore effective lift of the wing is increased by the length of the winglet – there are some additional benefits that can be achieved, but increased lift in the same wing span is particularly desirable.
One of the mitred pipes sitting in the corner of the swell box sounded perfect outside the box, but the impact of having it in the corner and mitred created a sound not unlike something that you get from doppler effect, where a sound is moving towards or away from you and so sounds like it is undulating. Pipes need space around them at the top for the air to flow freely and the sound to be stable, but the result of having the pipe in the corner was causing an instability in the sound.
In aerodynamics there is a similar effect. The ground acts like a mirror when a plane is close to it, thus a plane gets additional lift (and reduced drag) when taking off or landing (this is known as ground effect). Those running wind tunnel tests have to take into account these reflections of the test specimens in the walls of the wind tunnel (thus defining a maximum size of model for any particular rig) and some engineers have sought to make the most of ground effect by designing planes that fly very low over the sea (so low they are actually categorised as boats).
The team from Nicholson spent lots of time ensuring their was sufficient flow, that the mitre was pointing in the correct direction and that the entry of air to the pipe from the sound board was the ideal shape and size to eliminate the undulation. Alternatives might have been to use a stopped pipe, thus doubling the effective length as the sound wave goes there and back, this reduces the flow of air so would require the rank to be adjusted to get the balanced sound.