By Alex R. Carruthers
Provided that the weather is not too uncomfortable for the piper, the best place for playing the Highland bagpipe is out of doors. It is unfortunate that most pipers have to content themselves with a room in their house for practice sessions. Indoors and out of doors are environments with very different acoustical effects which affect the sounds of the pipes and are worth a closer study by the piping enthusiast. This article investigates the outdoor scene and presents only a brief outline of how nature influences the transmission of the sound.
A previous article in the Piping Times discussed the change in sound level that a listener experiences at different distances from the bagpipe. While the theoretical calculation therein showed that the sounds from the bagpipe can travel extensively, it was only valid for an ideal acoustical environment out of doors. Such ideal environments are never really encountered in our part of the world except that freak atmospheric and unusual ground conditions can give the impression of an ideal situation and sound can travel over very large distances. In reality, there are a number of factors which prevent or assist the sound in reaching great distances and fig. 1, below, lists most of the common ones:
The following discussion highlights some of the acoustical ‘obstacles’ which affect the transmission of the sounds of the Highland bagpipe. It must be noted that the acoustical conditions surrounding the piper are more complex than suggested by the present work. Furthermore, many of them vary with time and space and can only be tied down in statistical terms and it is usual to quote an average for a given time of day, season or place.
The first effect that occurs when playing the bagpipe out of doors is that the sound waves or air vibrations are not confined and are free to spread out in all directions around the player. If the air and the ground were acoustically ideal, then the sound level would reduce in a predictable way as the distance from the piper to the listener increases. The sound pressure level would in fact fall off in such a way that every time the listener doubles his distance, the pressure reduces by a factor of two.
This is equivalent to a reduction of 6 decibels (6 dB) for each doubling of the distance. We will call this the 6 dB law (it is also known as the ‘inverse square law’). The 6 dB law is remarkable if it is noted that the fall in sound level from, say, the 4th to the 8th metre position from the piper, is exactly the same as the fall in sound level that occure between, say, the 256th and 512th metre distances.
The ground surface
The most common sound reducer is the ground itself, If a player is on a perfectly flat surface and the ground material is a perfect sound reflector then only the 6 dB law applies (on a calm day). An aeroplane runway is a close approximation to this kind of surface. Such a playing surface is far from common and more often the piping takes place on a surface which is a good absorber of sound. Grass, sand and soil are porous substances and when dry and loose, the airborne sound waves from the pipes (mainly from the chanter) are only partly reflected upwards. An appreciable proportion of the sound passes through the pores of the grass and soil and is changed from sound energy to a minute amount of heat energy.
The result of this loss of sound is that under real conditions the sound may fall off much faster than the prediction made by the 6 dB law for doubling of distance. The decrease may be as much as 12 dB instead of 6 dB and represents a reduction of the sound pressure by a factor of 4 instead of 2. The sound propagated by the piper thus diminishes rapidly as the listener moves away from the player and, in a short distance, the music is hidden by the background noise of people, nature, transport etc. As an example, if the distance for distinguishable pipe music, in a background noise of 40 dB above the hearing threshold, is about 512 metres for the 6 dB law then the equivalent distance for the 12 dB law is about 20 metres. This represents a significant reduction of sound due to the ground.
The amount of ground absorption also depends on which note is being played. The chanter sounds are more affected than those of the drones because the sounds of the chanter are of a higher frequency and also because the chanter is much closer to the ground.
A uniform 12 dB law is quite rare and other approximations are often made. For example, a useful estimate for the attenuation at far distances is 9 dB / 300m for the sounds of the chanter and 3 dB / 300m for the sounds of the drones. These are shown in fig. 2, above.
Fig. 3, below, is a hypothetical propagation path involving more than one type of ground surface. This shows that there is a striking difference between the 6 dB law prediction and a more accurate prediction based on the composite graph which takes into account the different ground surfaces. The net result is a reduction in the distance travelled by the sound.
If the piper is brave enough to venture into snow-covered ground, the reduction due to the snow depends on its consistency. Freshly fallen snow is a good absorber of the sound but, as it freezes and hardens, it becomes a better reflector and less of the sound is attenuated.
The transmission of sound over still water is near the ideal 6 dB law since water is a good reflector. Turbulant water increases both the ambient noise and also diffusely reflects the sound to impede its propagation over large distances.
Frequency selectivity effects
Because all notes of the chanter are equally likely to be heard when a tune is being played, it is usual to take note D (or E) as an average indication of the sound of the pipes. Most calculations and data refer to this note. However, acoustical effects are closely related to the actual note that is being sounded. For distant listeners, the high notes are less likely to reach them than the low notes. This is partly due to the high notes being less loud than the low notes initially, and partly because the ground, the air and obstacles inflict different effects on notes of different frequencies. The high notes are of a higher fundamental frequency than the low notes and attenuation effects are greater for higher frequencies. Also, all notes have a family of harmonic frequencies which accompany the fundamental. Some of these harmonics can be high enough in frequency to be affected more than the other members of the family and the result is a deterioration in the quality of the note heard. It follows that high notes suffer most in both attenuation and quality change.
Fig. 4, below, is an indication of how the sounds of different frequencies, in calm air and free from obstacles, are attenuated at far distances from the piper. It is mainly the ground and the air which causes the frequency selective effect in this case.
• First published in the August 1979 Piping Times.