The acoustical environment of the highland bagpipe out of doors, part2

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In part 1 of this article, it was seen that, to the listener of the Highland bagpipe, there are a few visible and invisible obstacles in the way of its sound. The piper standing on the ground is capable of sending the pipe music over great distances if the ground material will allow it to do so. The higher notes of the chanter, apart from being quieter than the low notes, are of a higher frequency. This makes them intrinsically less able to travel as far as the low notes.

The discussion continues to expand upon a few more obstacles and effects which influence the Highland bagpipe out of doors.

By Alex. R. Carruthers.

Physical obstructions
All propagation paths involve physical obstacles in the point to point path of the sound. Bushes and trees absorb some sound. The more dense the foliage or the more trees, the greater is the loss of sound. Walls and fences which are taller than the listener can decrease the sound level by as much as 20 dB. The higher the wall the greater the decrease in the sound level. These and other physical obstructions such as buildings and hills also inhibit most sounds from reaching the other side. The degree of attenuation depends on the initial strength of the sound and its frequency. Behind obstructions there is a sound ‘shadow’ region as illustrated in fig. 5, below. The extent of the shadow region depends on the size of the obstacle and the frequency structure of the impinging notes. The shadow is ‘darker’ and ‘longer’ for high frequency notes and large obstacles. The notes with frequencies below about 500 Hertz (500 Hz), such as the fundamentals for the drones, G, A and B, can bend easily, for example, around walls. The higher notes, especially G’ and A’ are very unlikely to be heard behind obstructions. A change in the quality of the notes is also probable since the sound heard behind an obstruction may be depleted of one or more of the harmonics which give the note its timbre.

People are also good absorbers of sound. The human body can absorb 10 dB or more. If someone stands between the ear of the listener and the piper then this factor comes into play (especially for the sounds of the chanter).

The atmosphere
The atmosphere or, more specifically, the air is the transmission medium that carries the sound from its source (the piper) to the listener. Without air there would be no sound. Also if the air is disturbed in any way and caused to move then the sound being carried by it will also be disturbed and moved in the direction of the air movement. The most important atmospheric conditions most likely to affect the sounds of the pipes in this way are wind and temperature differences.

If the air is foggy, rainy or there is hail or snow, then the amount of sound lost in transmission through it is small. Most attenuation effects during this kind of weather can generally be attributed to something else like the wind or temperature. The effect of humidity (water vapour) of the air is also small.

Strong wind and temperature variations will cause bending of the sound because they move the air. In doing so they may limit or extend the distance travelled by the sound. A local temperature change in the path of the sound can directly influence it to change its direction. In the daytime, the sun’s energy strikes the ground and heats it. The temperature reached by the ground depends on its composition. For example, on very hot days a tarmacadam road can reach 60 Celcius (i.e. 60˚C) and a grassy surface can reach 44 Celcius. The hot ground heats the air layers near to it and causes the air to rise. The hotter the ground the faster the air rises. Thus the horizontal sound bends upwards away from the ground surface and the warmer the ground the greater the bending. An opposite effect occurs at night-time when the sound bends towards the ground. The bending of sound downwards makes it possible for sound to bypass obstacles and so reach places quite far from the piper. The sound of the pipes at night-time is thus often better than in the daytime and can be heard over very long distances; aided by the reduced noise level at night-time. It is obvious that the heating effect discussed above will depend on weather conditions, and it is much less when the sky is overcast with clouds. It is also dependent on the time of year.

Winds are the result of pressure differences in the atmosphere and can bend sound waves by displacing the air. The type of wind and its strength depends on the time of day and the local geographical terrain. Fig. 6, below, shows the bending effect on the sounds from the pipes. The sound will bend downwards if in the direction of the wind and upwards if against the wind. Sounds can therefore travel farther when they are carried by the wind.

The atmospheric pressure has no effect on sound, and pressure changes do not affect the sound from the pipes.

Directive aspects
An important point about musical instruments is that they do not propagate their sound equally in all directions but tend to concentrate sound in some directions more than others. Because the Highland bagpipe is normally played in the standing position the player’s body acts as a sound absorber and a sound obstruction. This results in the sound from the chanter being louder in front of the piper (95 dB at 1 metre) than at the rear (83 dB at 1 metre). The drones, on the other hand, have about the same level of sound all round the piper because they are above the body and have a low frequency structure which casts a very small sound shadow. These are interesting observations that are best discussed at another time.

Background noise
The environment around the piper and listener is a sea of extraneous noise, some of which is natural and some of which is man-made. In either case the noise acts as a mask and hides the true bagpipe sound so that after a Short distance from the player, the noise becomes comparable in level to the bagpipe sound. Beyond this point the noise predominates and the music becomes inaudible and confused.

Conclusions
Although the sound level of the Highland bagpipe is remarkably high and has the potential for sending its music over enormous distances, there are many natural barriers ever present to hinder the propagation of its sound.

This article has tried to outline a few of these barriers and how they affect the Highland bagpipe. It has to be stressed that every barrier and every obstacle presents an unsteady, imprecise amount of influence on the transmission of sound. Some effects vary from point to point and from instant to instant. Along the point-to-point path between the piper and listener the ground surface changes and the background noise and meteorological conditions vary daily or hourly. It is because of this changing environment that the sounds of the Highland bagpipe out of doors are never absolutely predictable as far as their propagation ability is concerned.

Although there are no fixed rules for calculating the exact acoustical properties of the path between piper and listener, the reader can make a first approximation by assuming a 6 dB law to apply. More accurate prediction is more difficult except in a few simple situations and over very short distances.


REFERENCES:

(1) Piping Times. Vol. 29, No. 12. Sept. 1977.

(2) Journal of Acoustical Society of America. Vol. 25, No. 3. May, 1953, p405.

(3) Acustica, Vol. 38, 1977, p201.

* Read Part 1