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Room Acoustics - The Problem

Room Acoustics - Research Room Acoustics - Solutions Room Acoustics - How it Worked Room Acoustics - Evaluation Calculation Table for Different Absorption Coefficients/Sabines
Instrument Acoustics- The Problem Instrument Acoustics -  Research Instrument Acoustics - Solutions Instrument Acoustics - Practical Designs Instrument Acoustics - How it Worked Instrument Acoustics - Evaluation

Building an Instrument (Part 2)


As well as online research we undertook some experiments. We investigated the effects of the valves on a trumpet and how the valves extended the run of tubing through which the air passed. We examined a violin and an electric guitar. We performed measurements on a glockenspiel to identify the lengths of the bars and the distance of the fastenings from the ends of the bars, which should correspond to the nodes. We also noted that a number of different depth and width indentations had been drilled into the underside of the keys. These had been used to fine-tune the exact pitches.

We also used investigated some other percussion instruments but while these produced different pitches they were not tuned pitches. We also blew across tubes with open and sealed ends. Sealing the ends resulted in the pitch dropping by an octave. This is because we doubled the node length and the wavelength. In an open-ended tube there are two nodes and the wavelength is half the tube length, but once the end is sealed and the air cannot escape the air vibrates differently with only one node and a wavelength which extends over the full length of the tube.

I identified a number of different methods of controlling the pitch of materials used. In blown instruments the length and width of the tube were the primary factors. The length could be blown by covering or uncovering holes, as in a flute or clarinet, or by adding additional lengths of tube using valves as in a trumpet or by using s telescopic tube as in a trombone.

In some brass instruments the pitch can also be controlled by the embouchure of the players lips.

In stringed instruments the length and thickness of the strings is again important but here we add the additional factor of how tightly the string is stretched between its two ends. The string length can again be varied, by the player pressing the string against a fingerboard of some kind at the required point.

In instruments designed to be hit the length, width and depth of the object being hit are the determining factors. Drilling small parts out as in the glockenspiel described above can do fine-tuning. The pitch cannot generally be varied. Instead a tuned series of hittable objects would be available.

One exception is the pedal timpani, which is a drum where the tightness of the drum skin is controlled by a pedal. The tighter the skin is pulled across the drum the higher the pitch. Interesting effects can be gained by changing the pitch during a note.

Establishing and creating the precise lengths of tubes, bars, etc. would be very tricky as the precise relationships needed to create the exact pitches in a given scale would need to be reproduced very exactly.

Also establishing the point at which to fasten any vibrating objects to their fastenings would need to be very precise in order to not reduce the capacity for the object to vibrate.

In order to do this the fastenings would need to be placed at the node points of the wavelengths, i.e. the point at which the waveform crosses the zero axis and therefore the point at which little vibration is required. This is roughly one quarter of the way from the end of the vibrating object and attaching it here will enable the centre and ends of the object to vibrate around the fastenings.

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