The Thayer Axial-Flow Valve is a replacement for the traditional rotary valve found on trombone

Trombone

The trombone is a musical instrument in the brass family. Like all brass instruments, sound is produced when the player’s vibrating lips cause the air column inside the instrument to vibrate...

s with F attachments. Invented by Orla Ed Thayer (with assistance from Zig Kanstul) in 1976, it was the biggest advance in the design of the trombone since the rotary valve

Rotary valve

A rotary valve is a type of valve in which the rotation of a passage or passages in a transverse plug regulates the flow of liquid or gas through the attached pipes. The common stopcock is the simplest form of rotary valve...

 was added (creating the F attachment) in the mid-19th century. The valve was the subject of a protracted legal battle between Orla Ed Thayer and James Nydigger, an early business partner. A U.S. patent (No. 4,469,002) was filed in May 1982 and issued in September 1984, and is now expired.
Orla Ed Thayer died on June 3, 2009, at the age of 89.

The Problem

When engaged, a rotary valve deflects the air flow through the instrument at least four times, and at least twice at rather sharp angles, leading to a stuffy response and audible tone colour

Timbre

In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the...

 changes, compared to the "open" instrument.

These drawbacks are especially severe with the trombone, because the instrument's characteristic timbre is a direct result of its construction, which is mostly completely straight: Apart from any possible valve tubing, there are only two bends, and the bore is almost completely cylindrical.

In contrast, the softer sound of "horn-like" brass instruments, so readily distinguishable from that of the trombone, is at least partially a result of their more or less conical bore and their intricate construction with many bows and bends.

The change in timbre and response was made even more severe by the traditional wrap of the valve tubing in narrow curves between bell and gooseneck that was the norm until the 1980s.

The Solution

The Thayer Valve uses a conical plug, allowing a deflection of the airflow by only 25 degrees or less. This also lets a smoother wrap of the F attachment's tubing (open wrap) become far more efficient. As a result, the response and tone colour are nearly unchanged from those of the open instrument, even for twin-valve bass trombones using the in-line configuration. Some trombonists even describe instruments whose response they experienced to be "more free-blowing" with the valves engaged.

The Criticism

Some trombonists, as well as servicing craftspeople, claim the Thayer valve has higher cleaning and maintenance requirements, sometimes to the degree of impaired reliability. Others attribute these problems not to the valve's construction but to a lack of expertise on the part of the critics.

Some trombonists, especially bass players, claim that more resistance enhances the response of low notes on the valve side (below E♭). This group finds playing more modern designs to be more of a strain and also sometimes criticizes their tone, which they find "less warm".

("Open wrap" designs are sometimes criticized because the valve tubing is more exposed and thus less well protected when compared to traditional designs. This, however, is not a complaint specific to the Thayer valve as it may also be employed with other valve types.)

The Aftermath

The Thayer Valve kicked off an "arms race

Arms race

The term arms race, in its original usage, describes a competition between two or more parties for the best armed forces. Each party competes to produce larger numbers of weapons, greater armies, or superior military technology in a technological escalation...

" among trombone designers and manufacturers that continues today. Newer valves (Hagmann

Hagmann valve

The Hagmann Valve is a rotary valve for trombone that was developed by René Hagmann in the early 1990s. His intention was to address some of the problems with the popular Thayer Valve, in particular the maintenance issues caused by its relatively complex design....

, Laetzsch-FullFlow, Greenhoe, the Kanstul "Controlled Resistance" valve, Balanced Valve System, Millervalve, among others) have become the de-facto standard

De facto standard

A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces...

, at least in the upper price range and among symphonic trombonists where consistent timbre is a highly-prized attribute.