By "ear" we mean our total hearing and interpretive system, an intricate and ingenious complete system of perception and interpretation of what is heard: ears, nerves, and mind.
The ears themselves are sensitive over a very wide range of intensities, but have their maximum sensitivity and selectivity at low volume levels. Setting the sound volume level just high enough to be clearly readable, both protects the hearing and improves performance. The ear responds to what it hears first.
The ear is sensitive to pitch. Few people can accurately remember pitch ("absolute pitch"), but most have no trouble detecting changes and differences in pitch. Not many seem actually to be "tone deaf." The usual pitch range used for CW is between 500 and 1000 Hz. Some find the best pitch for copying in interference is about 500 Hz.
Those with serious hearing losses -- who cannot hear certain pitches, or who cannot distinguish code signals in the usual pitch range because their ears "ring" where the spaces should be -- may find a lower pitch (e.g. 300 - 400 Hz) helps. Sometimes using a buzzer tone, or adding white noise to the tone may enable them to hear properly. (Note: Avoid the use of an actual buzzer in teaching as it has a delay in starting to sound. This distorts the timing.)
The usual narrow bandwidth of tone for CW is uncomfortable to some people and may become monotonous, uncomfortable or unpleasant. The narrower the pitch range the more frequent the complaint. They find a more complex tonal pattern far less tiring and even pleasant. However, when interference is present more complex tones become a hindrance.
In the perception of rhythm the human ear will adapt itself within rather wide limits in the actual duration of sounds. Our judgment of the duration of a brief sound is poor, perhaps because of a persistence of sound (like persistence of vision), yet we can judge the relative length of brief silent intervals rather well. (This is probably why the telegraph sounder has worked so well for receiving American Morse, where rhythm patterns are complex.) Thus, "If we take care for the spaces, the 'marks' will take care of themselves." Some students may have difficulty distinguishing dits from dahs. (The normal ratio is 1:3.) For them it may help to overemphasize the length of the dahs at first by lengthening them from 3 units to 4. (It is interesting that in American Morse the dahs tended to become shorter than three units, to contrast with the longer dahs of L and zero. Again, it may be the nature of the sounder that led to this.)
There are good reasons for believing that we must distinguish between conscious perception of duration and what the brain actually is capable of perceiving at subconscious levels. Support for this belief comes from the experiences of those operators who can receive code signals accurately at speeds which far exceed the point where dits and dahs all sound alike. See Chapter 10.
The ear is remarkable in being able to make sense out of some pretty badly mangled code, such as is often heard on the air. It is a forgiving organ: by mental adjustment one can quickly learn to recognize and read quite poorly timed code-- code whose glaring defects would stand out prominently if traced out on paper. Within fairly wide limits the actual duration of the sound in a rhythmic pattern may vary and still be recognized. However, the spacings within and between characters and words is a highly significant factor.
Some distortions of proportion are less unintelligible than others. Better discrimination exists when the dits are too fast as compared to the dahs than when the dits and dahs begin to approach the same length (easily confused). The ear can often read this kind of stuff when "machinery" fails.
The normal ear can learn to separate between signals nearly, but not quite, identical in pitch. For many people the ear-brain filter can focus on a bandwidth as narrow as 50 - 100 Hz. If one can focus on a 50 Hz. bandwidth with a receiver having a 3 Khz. noise bandwidth, a CW signal nearly 18 dB below the noise level can be heard. However, a bandwidth of about 500 Hz., rather than a very narrow one, makes the mechanics of tuning easier and gives freer rein to the ear-brain filter.
It is usually only when the going gets quite rough that we need an extremely narrow receiving filter -- and then there is the risk of losing the signal entirely if anything shifts just a little. It has been said that "The amateur ear, trained to dig out signals buried six layers deep in murderous QRM is the most prized ear in intercept work in all the world."
Headphones effectively double the power of received signals compared to a loudspeaker. The muffs on the phones keep out extraneous noises and keep the weak sound energy in. The signal-to-noise ratio can be increased by reversing the phasing of the phones: that is, the noise at one phone is 180 degrees out of phase with the other and the brain tends to cancel the noise. Noise type ear plugs can also help with phones and/or filters to reduce spurious noises.
The Art and Skill of Radio-Telegraphy
©William G. Pierpont N0HFF