Of Tubes, Slides, and Valves--How Brass Instruments Work
Basic Brass Acoustics
Lip-reed instruments share in common the following components: an airflow interruptive system (the player’s tensed lips, which provide resistance to the airstream the player produces with his or her diaphragm and results in periodic bursts of pressure in the form of a ‘buzz’); and an articulated air column, either open at both ends or only at one end, that broadens over its length. The lip-reed interruptive system is situated either at the narrow end of a tube open at both ends (see End-Blown Lip-Reed Aerophone below), or over an opening on the side of an instrument’s body that is located near its narrow, closed end (see Side-Blown Lip-Reed Aerophone below). A cup-shaped mouthpiece can be built or inserted into the narrow end of a tubular air column (see the two End-Blown Lip-Reed with Mouthpiece images below), in which case the player’s lips come into contact with it rather than the actual end of the air column.
Sounding a lip-reed instrument involves the activation of its articulated air column into an audible standing wave form, an orderly pattern of pressure waves that produces not only a fundamental frequency or partial, but also secondary partials the frequencies of which are simple multiples of the fundamental frequency. A clear, pitched sound can be produced in an air column of a given length not only at its fundamental frequency, but also at the frequencies of its partials (see ‘fundamental partial’, ‘harmonic partial’, and ‘harmonic series’ in Definitions). Which frequency, fundamental or partial, sounds will depend on the intensity of the airflow and the rate of the ‘buzz’ being produced by the player with her/his lips. Therefore, a basic challenge of playing lip-reed instruments that must be mastered by a player is to accurately select the naturally available partials of an air column.
Instruments with air columns closed at one end and that are therefore side-blown (e.g., aben, turu, mbiu, hwamanda) are generally used to produce a single, fundamental pitch but not any of the pitches at their harmonic partials. Lip-reed instruments with tubular air columns and that are end-blown (e.g., bugle, Renaissance trumpet, Baroque horn), however, are almost always used to produce multiple frequencies--the pitches of their harmonic series. Interestingly, in general, the fundamental frequency is not used on these instruments because it is difficult to get it to ‘speak’; instead, it is the frequencies of the harmonic partials immediately above it that have proven to be most musically useful. In the Western world, instruments such as these--end-blown tubes of a given length--are labeled ‘natural’, perhaps in reference to their constraint of sounding only the pitches of the natural harmonic series.
Focusing for the moment on Western end-blown lip-reed instruments, a lot of science has gone into designing their air column shape, or bore profile, over the past few centuries. This is because this facet of an instrument’s design has been found to contribute significantly to its sound quality and to the ease of playing it with accurate and dependable intonation. As mentioned earlier, an instrument’s bore increases in its diameter over the length of the tubing, starting out narrow at the lip-reed/blowing end and terminating with a much wider bore at its distal end, which is often referred to as its bell. Decidedly imprecise terminology is used to describe bore shape; the three most commonly used terms are cylindrical, conical, and flaring. In reality, all bore shapes are a mixture of all three of these forms--they all start out cylindrical, eventually become conical, and terminate with a pronounced flare. What proportion of the tube’s total length is cylindrical, conical, and flared, how gentle or severe is the widening of its conical portion, and how subtle or dramatic is its final flare are features of bore design that are often simplified down to two general bore profiles that can be understood only in relation to one another--cylindrical bore and conical bore instruments. The cylindrical bore instruments have a proportionally longer initial cylindrical section than do conical bore ones (see Cylindrical Bore Instrument below), conical bore instruments have proportionally longer conical sections with a greater overall increase in diameter than do their cylindrical counterparts (see Conical Bore Instrument below); flaring is often, but not always, more dramatic on cylindrical bore instruments. When an instrument’s design incorporates slide and valve mechanisms, these are located in the initial cylindrical section.
Chromatic Brass Instrument Design--Mechanisms for Instantaneously Adding Length to the Basic Tube Length of an Instrument
Two basic mechanisms to quickly add tube length to an instrument’s fundamental length (and as a consequence change its pitch) while playing have been incorporated in Western brass instrument design--the telescoping slide and the valve. The former mechanism is historically older, dating back to Renaissance Europe, and is still in use today on the trombone. The latter, dating back only to the first quarter of the 19th century, has two common types--the rotary valve and the piston valve--versions of both of which are still in widespread use today.
The modern tenor trombone (without any rotary attachments) and its predecessor the sackbut come equipped with a slide mechanism that allows its performer to change the length of the instrument’s initial cylindrical tubing section either in an instant or gradually. The slide section consists of (see Parts of a Trombone image) two long parallel cylindrical tubes made of nickel silver connected near their top ends with a cross-stay (the bottom unit in the image); the final four inches or so at their bottom ends are slightly thickened to form sleeves or stockings. The outer slide (the middle unit in the image) is also made from two cylindrical tubes (of brass), these connected at their bottom end with a U-bend tube and near their top end with a cross-stay; the internal diameter of the outer slide tubes is slightly greater than the external diameter of the inner slide sleeves, providing a close fit that is lubricated to minimize friction. The ends of the inner slide tubes are inserted into the openings at the ends of the outer slide tubes to complete the slide mechanism, which in turn is inserted into the narrow end of the bell section (the top unit in the image).
While holding the cross-stay of the outer slide, the performer can lengthen the tube by pushing the slide out from its set shortest length (called ‘first position’). If moved away from its shortest position by two inches, four inches are added to the instrument’s total length because the slide tubing doubles back on itself. A trombonist must learn just how far to extend the slide in order to produce a total of six further fundamentals along the length of the slide, each successive position producing a frequency a minor second (m2) lower than the previous one. These positions are labeled ‘second position’ through ‘seventh position’, and above each position’s fundamental frequency its harmonic partials can be sounded. Therefore there are basically seven ‘natural’ brass instruments, each with a unique length, rolled into this one instrument. Performers must learn how to coordinate the positioning of the slide and the ‘settings’ of the airflow interruptive system (their rate of exhalation and the tension of their lips) in order to successfully sound a desired note. For many notes within the range of the trombone there will be two or more slide positions in which a desired note may be sounded, the choice of which position is used being left to the player. (See below the Position, Partial, and Pitch chart.)
Valves are mechanical devices built into brass instruments that allow a player while performing to add and remove fixed lengths of tubing, singly or in various combinations, to the basic length of an instrument. Valved brass instruments have at their core a grouping of three closely-positioned valves through which the instrument’s air column passes. The length of the air column is at its minimum when the valves are in their normal position (not depressed). Only when one or more valve is activated (by depressing its button) is extra tube length added to the instrument’s air column. One of the valves (the middle one, called the ‘second valve’) will add enough tubing to lower the fundamental pitch of the instrument a minor second (m2), another (the one closest to the performer, the ‘first valve’) a major second (M2), and the third (furthest from the player, or ‘third valve’) a minor third (m3), the tubing of which is as long as the sum of the other two valves’ lengths combined (valves 1+2) (see Relative Lengths image below). Various combinations of these three lengths will lower the fundament a major third (M3, valves 2+3), a perfect fourth (P4, valves 1+3), and a tritone (A4/d5, valves 1+2+3). There is a direct correlation between the slide positions discussed above for the trombone and the additional tube lengths brought into play by valves; therefore a brass instrument with valves, like the trombone with its telescoping slide, is several ‘natural’ instruments in one with it being possible at each tube length/fundamental option to sound the notes of the harmonic series above its fundamental (see below the Position, Partial, and Pitch chart).
Two basic valve designs have been developed since the 1810s when the first patent for such a mechanism was registered. Piston valves were developed first and the rotary valve was introduced only in the mid-1830s. Both mechanisms involve a smooth-moving airtight cylinder that fits inside a stationary casing--for piston valves, the internal cylinder slides up and down in its casing; for rotary valves the internal cylinder rotates or cranks in its casing around a vertical axis. Casings for both types of valves have a number of holes puncturing their sidewalls to which tubing is attached. These holes must line up exactly with the openings at the termini of channels in the internal cylinder both when the valve is in normal and activated positions. For piston valves in particular, it is a bit of a three dimensional puzzle to get all these channels, holes and tubing to align, but when they do it allows for instantaneous rerouting of the instrument’s air column to produce the correct tube length necessary to sound a particular pitch.
The piston of a piston valve is made from thin sheet metal rolled into a cylindrical tube and coated with a low friction metal (see Valve Piston below). It will have perforations in its side and tubes connecting pairs of these perforations across the interior of the piston will be welded into place. A shaft protrudes from the piston’s top and extends, after passing through a hole in the threaded cap screwed to the top of the casing, an inch or so before terminating in a flat button. The piston is shorter than its cylindrical casing and a spring is located either above or below it to keep it at the top of the casing. The piston moves downward when its button is depressed, but as soon as that pressure is released the piston springs back to the top of its casing. When at the top of its casing, the airstream passes directly through the piston; when depressed the airstream is redirected through the supplemental tubing attached to the valve casing before exiting the valve. Two piston valve designs are found on instruments in the collection and will be discussed in the following two paragraphs. The first design is primarily of historical interest, the second is the standard design used on all piston valve instruments today.
Stölzel piston valve: Two of the three valves (the first and third) on the mid-19th century cornet à pistons (see Cornet image below) in the collection are Stölzel valves, named after the individual, Heinrich Stölzel, credited with their invention in around 1815. The Stölzel valve design is immediately identifiable by where the main tubing is connected to the valve--at the bottom of a valve’s casing. The bottom section of the piston is hollow and just below where the air passage is blocked there are two perforations in its wall across from one another; one of these perforations is blocked by the valve casing when the piston is in its normal position, the other is blocked when the piston is in its depressed position. Two more punctures are made in the upper section of the piston that are connected by a single tube channel running through its interior; this channel is in play only when the valve piston is depressed. See the Grove Music Online diagram of the Stölzel valve design for an illustration of how this type of piston valve operates. The second/middle valve on the collection’s cornet à pistons is of a different design, called a Périnet valve, which would eventually win out over all other piston valve designs to become a fixture in the world of modern brass instruments.
Périnet piston valve: Introduced in 1839 by Parisian instrument maker Etienne Périnet, a new valve design with tubing configurations that improved on those of earlier designs was introduced. Périnet’s design eliminating right angle bends in the air column and kept a more consistent and smooth bore diameter throughout the entire valve section of the instrument. The path of the basic air column enters and exits the sides of each valve casing (see 2nd valve in the Cornet à Pistons image below) and passes through them in a relatively straight line with all valves in normal position. Each valve piston has two perforations connected by an internal tube channel near its bottom end that is in play as part of the basic length of the instrument’s air column. Above this pair of perforations are four more that come into play when the valve piston is activated (depressed)--these function to bring into play the extra length of tubing attached to the valve casing wall. See the Grove Music Online diagram of the modern valve design for an illustration of how this type of piston valve operates.
Brass instrument makers started designing valves with rotating cylinders rather than piston action cylinders in the 1820s. The rotary valve design that is still with us today was introduced in 1835 by the Viennese maker Josef Reidl--he called his valve the ‘Rad-Maschine’. A rotary valve might be thought of as a wheel with an axle enclosed in an airtight metal casing. One end of the axle or spindle rests in a depression in a screw-on cap that closes the bottom of the casing. The other end of the axle passes through a central hole in the top of the casing where it becomes part of a mechanism that rotates the cylinder a quarter revolution when its finger-plate is depressed by the performer and springs back to its original position when the finger-plate is released (see Rotary Valve Cylinder images below). The casing sidewall has four equidistantly-placed round perforations in it each of which has attached tubing; the tubing leading into and exiting two of these holes is part of the basic length of the instrument’s air column; the other two are part of the supplemental tubing that is brought into action when the valve is activated (see the Grove Music Online diagram of the rotary valve design for an illustration). Two curved channels are machine cut into the sidewalls of the internal cylinder. The openings at the end of one of the channels align with the tubing for the instrument’s basic air column when the valve is not engaged, and the openings of the other channel are aligned with the inactive supplementary tubing. When activated, the rotor turns 90 degrees so that the outflow hole of the normal position is now the inflow hole and the channel directs the air into the supplemental tubing. The other channel is now aligned so that one hole serves as the outlet for the supplemental tubing and the other connects back up with the main tubing of the instrument. As with piston valves, the three main rotary valves add different lengths of supplemental tubing. On the double horn (with F and B-flat sides), the rotor is twice as deep (see Double Horn Rotary Valve Casings image below) because it serves two sets of supplemental tubing, one for the F side of the horn, the other for the B-flat side (see Double Horn Supplemental Tubing and Double Horn Supplemental Tubing Sleeves images below). A fourth rotary valve with a different tubing and channel design and operated with the thumb is used to direct the basic air column to one or the other side of the horn and to the appropriate tier of rotary valve channels and their supplementary tubing.
Position, Partial, and Pitch
In summary, lip-reed instrument players must select at any given moment a length of tubing (with the aid of a slide or valves) and a harmonic partial (by controlling airstream pressure and the buzz of their lips) in order to produce a desired pitch. In the chart that follows the resultant pitches from many (but not all) length-and-partial combinations that a brass performer of a B-flat instrument must internalize are laid out. Accuracy of pitch selection and intonation is attained only after years of practice during which a performer develops the nuanced ‘feel’ of interaction with her/his instrument and its acoustical behavior that is necessary to bring that otherwise inert metal tubing to life.
|Slide position||Valves depressed||Harmonic partials|
Pitches resulting from interacting variables of tube length (as determined by slide position or valve combination) and harmonic partial selection for the tenor trombone, euphonium, baritone horn, and the B-flat side of the double horn. [By raising all the octave indicators up one digit, this chart also covers the B-flat trumpet, cornet, and flugelhorn, and its first line only the B-flat bugle.]
Baines, Anthony. 1976. Brass Instruments: Their History and Development. New York: Charles Scribner’s Sons.
Bate, Philip, and Edward H. Tarr. “Valve (i),” Grove Music Online, accessed October 5, 2015: https://doi.org/10.1093/gmo/9781561592630.article.28961
Campbell, Murray, Clive Greated, and Arnold Meyers. 2004. Musical Instruments: History, Technology, and Performance of Instruments of Western Music. Oxford: Oxford University Press.
(by Roger Vetter)