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The Compensating system

Why do we need it?


Dr. D. James Blaikley invented the compensating system in 1878. It was protected by copyright for a hundred years. Its a compliment of his work that as soon as the patent ran out, all the serious euphonium manufacturers copied his invention. The purpose of the system is to improve the intonation of the instrument. Before we can describe the compensating system we have to understand why we need it, and to understand why we need it we have to appreciate how instruments are made.

Intonation Problems
There are four regions where intonation manifests itself in instrument manufacture.
1. The inherent characteristics of the harmonic series, 6th harmonics will be sharp and 5th harmonics will be flat, we have little control over this.
2. Errors in the graduation of the bore. Not all instrument manufacturers graduate the bore throughout its length.
3. Discrepancy of tube length, due to the correction of the bell. ie. When does the tube cease being a tube and start becoming a bell or amplifier. `
4. Errors arising from the use of more than one valve in combination. The compensating system was developed to overcome the later.

How do manufacturers calculate the length of tube required to generate a specific note?
The formula for producing a length of tube that generates a certain pitch is: “The speed of sound in inches per second (13,440) over the vibrations required (in cycles per second) X 2” 13,440 2 x vibs. in cyc. per second. Pedal Bb vibrates at 58.27 cyc per sec. The Calculation : The length of tube required to generate a pedal Bb = 13440 /(58.27 x 2) = 115 .32 inches To assist our understanding of the compensating system and to simplify a complicated subject we will discuss the manufacture of a three valve compensating euphonium. How to make an instrument with three valves? 115.32 inches is the effective length of a three valve Bb euphonium played without depressing any valves. Provided the vibrations (in cycles per second) of a specific note is known, calculations can be made to find the required length of tube to generate that pitch. Bb vibrates at 55 cyc. per second and requires 115.32 inches of tube. A vibrates at 55 cyc. per second and requires 122.20 inches of tube. Ab vibrates at 51.91 cyc. per second and requires 129.45 inches of tube. G vibrates at 48.99 cyc. per second and requires 137.17 inches of tube. To calculate the individual valve slide lengths simply subtract the effective length of a Bb Euphonium (115.32 inches) from the calculated lengths of tube required to generate A, Ab and G. 2nd valve slide: 122.22 - 115.32 = 6.88 inches 1st valve slide: 129.45 - 115.32 = 14.13 inches 3rd valve slide: 137.17 - 115.32 = 21.85 inches When attempting to play a low F on a three valve euphonium we need to use two valves in combination. (the fingering for low F is 1st and 3rd). However, if we add the sum total of the tubes used when depressing the 1st and 3rd valves. What we have is: 115.32 inches (effective length of a Bb euphonium) + 14.13 inches (1st valve) + 21.85 (3rd valve) = 151.30 inches. What length of tube do we need to generate a low F? Low F vibrates at 43.65 cycles per second. 13400 43.65 x 2 = 153.95 inches It is evident by comparing, what tubing is available (151.30) and what tubing is required (153 .95) that there is a shortage of 2.5 inches of tubing when playing low F on a three valve (non compensating) euphonium. It is this shortness of tubing, when using valves in combination, especially in the lower register that the compensating system overcomes. How does the compensating system work? Via port holes in a designated “master valve” (the 3rd valve on a 3 valve instrument). Extra tubing is utilised when the “master valve” is used in combination with another valve. The master valve on a 4 valve Sovereign Euphonium is the 4th valve. Hence, the small knuckle tubing on the reverse of the valve block (the compensating tubes) are only activated when the 4th valve is used in combination with others. The advantages The advantages of this system over non compensating instruments are : The fingerings are the same in every octave. The performer can concentrate on the music, without the complication of having to move slides during performance. At most, the performer is only using 4 valves.

Conclusion
This is a difficult system to understand, but the benefits of using it are enormous. In short the only analogy I can think of which might simulate its use, is the difference between driving a manual gearbox car and that of driving an automatic.

autore

bob@bobchilds.co.uk

 

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