From the point of view of the criminalist, auditory illusions are hardly
less significant than visual illusions, the more so, as incorrect
hearing is much more frequent than incorrect seeing. This is due to the
greater similarity of tones to each other, and this similarity is due to
the fact that sound has only one dimension, while vision involves not
only three but also color. Of course, between the booming of cannons and
the rustling of wings there are more differences than one, but the most
various phenomena of tones may be said to vary only in degree. For
purposes of comparison moreover, we can make use only of a class of
auditory images on the same plane, e.g., human voices, etc. Real
acoustic illusions are closely connected with auditory misapprehension
and a distinction between these two can not be rigorously drawn. A
misapprehension may, as a rule, be indicated by almost any external
condition, like the relations of pitch, echo, repetition, false
coincidence of waves of sound, etc. Under such circumstances there may
arise real illusions.
The study of auditory illusions is rendered especially difficult by the
rarity of their repetition, which makes it impossible reliably to
exclude accidents and mistakes in observation. Only two phenomena are
susceptible of accurate and sufficient study. For three summers a man
used to ride through the long street in which I live. The man used to
sell ice and would announce himself by crying out, “Frozen,” with the
accent on the Fro. This word was distinctly audible, but if the man came
to a definite place in the street, there were also audible the words
“Oh, my.” If he rode on further the expression became confused and
gradually turned into the correct, “Frozen.” I observed this daily, got
a number of others to do so, without telling them of the illusion, but
each heard the same thing in spite of the distinct difference between
“frozen,” and “oh, my.”
I made a similar observation at a bicycle school. As is known, beginners
are able frequently to ride by themselves but need help in mounting and
dismounting their machines. To do so they call a teacher by crying out:
“Herr Maier.” At a certain place this sound would seem distinctly to be
“mamma.” I was at first much surprised to hear people of advanced age
cry cheerfully, “mamma.” Later I discovered what the word really was and
acquaintances whose attention I called to the matter confirmed my
observation. Such things are not indifferent, they show that really very
different sounds may be mistaken for one another, that the test of
misunderstandings may often lead to false results, since only during the
test of an illusion are both auditor and speaker accurately in the same
position as before. Finally, these things show that the whole business
of correcting some false auditions is very difficult. Yet this work of
correction may be assumed to be much more easy with respect to hearing
than with respect to seeing. If, e.g., it is asserted that the revolver
has been seen somewhere, and if it has been known that the sight was
impossible, it becomes just as impossible, almost, to determine what the
object seen really was. In the rarest cases only will it be something
altogether similar, e.g., a pistol; most of the time it will be an
object which could not be inferred from no matter what combinations. In
hearing, on the contrary, if once it is determined that there has been a
false audition, the work of placing it, though difficult, need not be
unprofitable. This work is often compulsory upon the criminalist who
receives protocols which have not been read aloud, and in which mistakes
of hearing and dictation have been made. Such mistakes are considerably
disturbing, and if the case is important their source and status must be
inferred. This may almost always be done. Of course, strange, badly
heard proper names can not be corrected, but other things can.
As regards the general treatment of auditory illusions, it is necessary,
first of all, to consider their many and significant differences. In the
first place, there are the varieties of good hearing. That normal and
abnormal hearers vary in degree of power is well known. There are also
several special conditions, causing, e.g., the so called hyper-auditive
who hear more acutely than normal people. Of course, such assertions as
those which cite people who can hear the noise of sulphur rubbed on the
poles of quartz crystals and so on, are incorrect, but it is certain
that a little attention will reveal a surprising number of people whose
hearing is far acuter than that of normal individuals. Apart from
children, the class is made up of musicians, of young girls, and of very
nervous, excitable, and sickly persons. The musicians in fact have
become so because of their ears; the young girls hear well largely
because of their delicate organization and the very fine construction of
their ears; and the nervous people because of their sensibility to the
pain involved in loud noises. Many differences of perception among
witnesses are to be explained by differences of audition, and the
reality of apparent impossibilities in hearing must not be denied but
must be tested under proper conditions. One of these conditions is
location. The difference between hearing things in the noisy day and in
the quiet night, in the roar of the city, or in the quiet of the
mountains, is familiar. The influence of resonance and pitch, echo and
absorption of tones, i.e., the location of the sound, is of great
importance. Finally, it must not be forgotten that people’s ability to
hear varies with the weather. Colds reduce the power, and not a few
people are influenced by temperature, atmospheric pressure, etc. These
considerations show the degree in which auditory illusions can be of
importance even in tests of their nature and existence. They show above
all that the same object of comparison under the same circumstances must
be used in every test. Otherwise much confusion inevitably results.
The presence of auditory illusions in diseases, fever, hysteria,
nervousness, alcoholism and its associates, mental disturbances,
hyperæmia, diseases of the ear, etc., is well known, but concerns us
only as pointing to the necessity of calling in the physician
immediately. They have their definite characteristics and rarely leave
the layman in doubt of his duty in that direction. The great difficulty
comes in dealing with diseases or apparent diseases while it is still
impossible to know of their existence, or where the pain is of such
character that the layman does not know of its presence and thus has no
ground for consulting the doctor. For example, it is well known that a
large amount of ear wax in the aural passage may cause all sorts of
ringing and sighing in the ear, and may even produce real
hallucinations. Yet a person having an abnormal amount of ear wax may be
otherwise absolutely sound. How is the need of a physician to be guessed
in such a case? Again, the perforation of the drum, especially when it
follows a catarrh, may cause a definite auditory illusion with regard to
the sound of voices, or the illusion may be effected by the irritation
of the skin in the ear passage, or by anemia, or by a strong carotid
pulse and a distention of the bloodvessels, as happens in alcoholism.
Many people become abnormally sensitive to sound at the beginning of
fevers. Women at the time of their climacterium hear all kinds of
voices. Inasmuch as this soon stops, the abnormality and incorrectness
of their audition is hard to establish. Childbirth, too, makes a
difference. Old, otherwise conscientious midwives claim to have heard
unborn children breathe and cry.
Examples of this sort of thing are innumerable and they teach that
whenever any questionable assertion is made about a thing heard the
doctor must be called in to determine whether the witness heard it under
abnormal, though not diseased conditions. Again, merely accidental or
habitual general excitability tends to intensify all sounds, and whether
the witness under consideration was in such condition can be determined
only by the expert physician.
The illusions of hearing which completely normal people are subject to
are the most difficult of all. Their number and frequency is variously
estimated. The physician has nothing to do with them. The physicist, the
acoustician and physiologist do not care about the criminalist’s needs
in this matter, and we ourselves rarely have time and opportunity to
deal with it. As a result our information is very small, and no one can
say how much is still undiscovered. One of my friends has called my
attention to the fact that when the beats of the clock are counted
during sleepiness, one too many is regularly counted. I tested this
observation and my experience confirmed it. If, now, we consider how
frequently the determination of time makes the whole difference in a
criminal case and how easily it is possible to mistake a whole hour, we
can get some notion of the importance of this illusion. Its explanation
is difficult and it may be merely a single instance of a whole series of
unknown auditory illusions resting on the same basis. Another and
similar phenomenon is the “double beat of the hammer.” If you have an
assistant strike the table with a hammer while you hold both ears with
your fingers and then open them half a second or a second after the
blow, you hear the blow again. And if you open and shut your ears
quickly you can hear the blow several times. This is explained through
the fact that a number of reflections of the sound occur in the room,
and that these are perceivable only by the unfatigued ear. The
explanation is unsatisfactory because the experiment is sometimes
successful in the open. Taken in itself, this matter seems very
theoretical and without practical value. But this kind of action may
occur automatically. It is well known that swallowing closes the
Eustachian tubes for a moment, especially if done when lying down. Now,
if this occurs during a blow, a shot, etc., the sound must be heard
twice. Again, it may easily happen that because of the noise a man wakes
up half asleep and, frightened, swallows the collected saliva; then this
accident, which in itself seems unimportant, may lead to very
significant testimony. Such occurrences are not infrequent.
The intensity of a sound already heard may be of considerable influence.
Certain experimenters have indicated the remarkable character of
slightly intensive effects of sound. If you hold a watch so far from the
ear as to hear it clearly but weakly, the sound decreases until finally
it is not heard at all, and after awhile it is again heard, etc. This
may lead to hearing distinct sounds made up of many tones, and need not
evince any great illusion with regard to the ticking of a watch. But the
thing may occur also in connection with more powerful and more distant
sounds, e.g., the murmur of a brook, the rush of a train, the pounding
in a distant factory. Noises far removed are influenced by reflections
of sound, waves of air, etc., and it is possible that all kinds of
things may be heard in a completely monotonous noise. This can be easily
learned by listening to the soft murmur of a distant brook at night.
Given the disposition and supposing the existence of the brook unknown,
it is easy to hear in its monotonous murmur, human voices, sighs,
shrieks, etc.
Another remarkable observation shows that in the dark very distinct
things are heard during the playing of delicate instruments, such as
mouth-organs. The humming approaches and withdraws, then it comes on
various sides, and finally one has the feeling that the whole room is
full of humming and winging insects. And this may go on indefinitely.
There is a large collection of reasons for this reduplication of
monotonous sounds. Everybody knows the accord of the æolian harp which
consists of identical notes, and the melodies which seem to lie in the
pounding of the train on the rails. This can become especially clear
when one is half asleep. If ever thinking begins to be ousted by
slumber, the rhythmic pound begins to dominate consciousness. Then the
rhythm gets its appropriate melody which becomes progressively more
intense, and if one grows suddenly wide awake one wonders why the
clearly-heard music is missing. Similarly, it is often asserted that a
row of travelling wild swans make pleasant chords, although each swan is
able to utter only one cry. Difference in distance and alterations in
the air cause the chords.
The difficulties in distinguishing the intensity or weakness of a sound
are of importance. Fechner learned from the violinist Wasilewski that he
observed that a male choir of four hundred voices did not sound
essentially louder than one of two hundred. At the same time one clock
is not heard at a great distance, a hundred clocks are heard. One locust
can not be heard eating; when 1000 eat they are heard; hence each one
must make a definite noise.[320] Early authorities have already
indicated how difficult it is to distinguish the number of bells ringing
together. Even musicians will often take two or three to be five or six.
Certain dispositions make some difference in this respect. The operating
physician hears the low groaning of the patient after the operation
without having heard his loud cries during the operation. During the
operation the physician must not hear anything that is likely to disturb
his work, but the low groan has simply borne in upon him. The sleeping
mother often is deaf to considerable noise, but wakes up immediately
when her child draws a deeper breath than usual. Millers and factory
hands, travellers, etc., do not hear the pounding of their various
habitual environmental noises, but they perceive the slightest call, and
everybody observes the considerable murmur of the world, the sum of all
distant noises, only in the silence of the night that misses it.
Illusions of direction of sound are very common. It is said that even
animals are subject to them; and everybody knows how few human beings
can distinguish the source and direction of street music, a rolling
wagon, or a ringing bell. Even when long practice enables one to
determine direction with correctness, an accidental event, perhaps the
weather, especial sounds, a different grouping of individuals on the
street, may result in serious mistakes. I tried to learn to judge from
my office-desk whether the ring of the horse-car came from above or
below. I succeeded so well that I could not understand how it was
difficult not to learn the difference, and yet I failed many a time
altogether in judgment. The reason for it I do not know.
All these enumerated circumstances must show how very uncertain all
acoustic perceptions are, and how little they may be trusted if they are
not carefully tested under similar conditions, and if--what is most
important--they are not isolated. We are here led back to the old
principle that every observation is not proof but means of proof, and
that it may be trusted only when it is confirmed by many parallel
actions which are really consistent. That even after that mistakes are
possible, is true, but “after that” is when we have done all that lies
within human power.