Citation
Meijerman, L. (2006, February 15). Inter- and intra-individual variation in earprints. Barge's Anthropologica, Leiden. Retrieved from https://hdl.handle.net/1887/4292
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1.1 General introduction and goal of research
The value of the external ear – also called auricle or pinna – as a means of identification
was recognized centuries ago. It has been studied and described as part of procedures to
establish the identity of criminals and victims of crimes and accidents (Bertillon, 1885;
Hoogstrate et al., 2001; Hunger and Hammer, 1987; Iannarelli, 1964; Swift and Rutty, 2003),
or to prove paternity (Schade, 1954; Tillner, 1963). Not only the auricle itself showed
potential for establishing the identity of criminals, but also its prints (Dubois, 1988; Hammer,
1986; Hirschi, 1970a,b; Kennerley, 1998a,b; Van der Lugt, 2001, Pasescu and Tanislav,
1997). W hen perpetrators of crimes listen at, for instance, a door or window before breaking
and entering, oils and waxes on their ears leave prints that can be made visible using
techniques similar to those used when lifting fingerprints. These prints appear characteristic
for the ears that made them.
The majority of scenes of crime where earprints are discovered involve burglaries. M ore
rarely, cases concern sexual offences or murder cases (Van der Lugt, 1997; Kennerley,
1998a). A historical case in the Netherlands is described by Dubois (1988). He reveals how
earprints became part of the evidence used to convict the suspect of an armed robbery.
According to a survey carried out by the Rotterdam Police, in 15 % of domestic burglaries,
forensic crime scene investigations showed the presence of latent earprints (Cor van der Lugt,
personal communication). The percentage differs greatly per region, as it is not common
policy to actively search for earprints. In absence of a national database, the chance of an
earprint leading to a potential suspect, or the prospect of linking different cases of burglaries
to one burglar, depends greatly on the effort – and memory – of the investigating policemen.
Among the goals of the ‘FearID’ research project are the development of a system for
the recovery of seemingly similar earprints from a database, and the standardisation and
harmonisation of procedures to detect, recover, lift and record earprints. Presumably, this will
improve the practical implementation of earprints in forensic research. The preparation of a
Apart from practical implementation matters remains the issue of uniqueness. We cannot prove that no two ears are alike. It is a working assumption, comparable to the assumption that no two people have a similar pattern of ridges and furrows on their fingers. But presumed uniqueness of ears does not guarantee uniqueness of prints, as many ears are only partly imprinted due to differences in elevation of the various parts. So the question that needs to be addressed is whether there is enough variation in earprints to be able to distinguish between prints of different ears. An attendant complication is that not even two prints of the same ear are exactly alike. Differences in the way prints are left, or the material they are left on, may cause variation in prints by a single ear. The feasibility of earprint individualization, therefore, depends not only upon the amount of variation in prints of different ears (inter-individual), but also upon that which occurs in prints of a single ear (intra-individual).
To justify the claim that we can match an earprint uniquely to an ear, we must establish that the print resembles other prints from the same ear more than it resembles prints from another ear. We may attempt to do so by analysing multiple prints from a large sample of ears, comparing inter-individual variation with intra-individual variation over a suitable set of measurable features. An experimental feature set is suitable only if the inter-individual variation is significantly greater than the intra-individual variation. For such a comparison, we may use, for instance, cluster analysis or formal concept analysis (Dr. M. Ingleby, University of Huddersfield, UK, personal communication). The outcome of this analysis will be probabilistic. This means that we may estimate the probability of encountering seemingly indistinguishable prints from different ears.
relatively silent? Does applied force, and therefore the appearance of an earprint that was left on a crime-scene, vary depending on whether there was noise coming from inside the house or not?
Other factors to be considered for potential sources of intra-individual variation are, for instance, the duration of the listening time and the amount of secretions that are present on the ear. Are earprints affected by the time of day a person has listened or by outside temperatures? Furthermore, to what extent does the material the print is left on influence its appearance? And to what extent do the materials used to lift and secure it? Lastly, how stable are the features of the actual live ear? The imprint of a newly acquired piercing or the outline of an additional crease in the skin in front of the ear may be recognized as possible sources of intra-individual variation in earprints. In addition, dimensions of the imprinted features may change because the auricle itself increases in size throughout adult life. If the matching system to be designed would base its classification at least partly on these dimensions, it would help to know just how much the auricle may grow within a certain period of time, and moreover, if the various parts of the auricle expand equally. Is it mainly the earlobe that stretches? Or does the cartilaginous part of the auricle expand to a similar extent?
Such basic questions must be addressed before embarking upon a study of earprints aimed at determining the extent of inter- and intra-individual variation. The answers will allow us to make informed decisions regarding the circumstances under which earprints should be collected. A study of earprint variation that includes a number of prints from each of many ears – collected under varying conditions known to influence their appearance – will eventually enable us to determine the boundaries to ‘natural’ variation in prints by a single ear. It will allow the extraction of features that are sufficiently stable for a classification system that is capable of distinguishing between inter- and intra-individual variability. Finally, it will permit an informed estimation of the probability that two prints originate from the same ear.
1.2 Morphology of the external ear (Excerpt from Meijerman et al., 2002)
larger end directed upward. Its lateral surface is irregularly concave, directed slightly forward, and presents many ridges and grooves that serve to pick up sounds. It is made up of elastic cartilage, connective tissue and skin, with muscular attachments in the back. The skin overlying the lateral surface adheres firmly to the perichondrium, which carries the blood supply to the cartilage. The medial surface however is attached more loosely. The auricularis posterior muscle is surrounded by fibroareolar tissue, which adds to the flexibility of the ear.
An overview of the morphology of the external ear is provided in Fig. 1.1.
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triangular fossa anterior crus of anthelix crus of helix anterior notch anterior knob (of tragus) tragus intertragic notch
helix
auricular tubercle superior crus of anthelix scaphoid fossa
cymba conchae anthelix (stem) cavum conchae
posterior auricular furrow antitragus
lobule
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Fig. 1.1 Morphology of the external ear.
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(1954) called this supernumerary ridge crus cymbae, after Quelprud (1934) (Fig. 1.2a). Schade further noted that incidentally a similar ridge can be found in the lower part of the concha. He called this ridge crus cavi.
Posterior and superior to the concha is a prominent ridge: the anthelix (erroneously also
called antihelix in literature). It is divided superiorly into two crura: crus superior (upper crus)
and crus anterior (also called crus inferior or lower crus). The two crura define a shallow
depression: the triangular fossa (fossa triangularis, also called fossa digitalis). At times, a third
crus of the anthelix is present, running from the crus superior to the supero-posterior part of the helix. This crus is called crus posterior, or also crus tertium (Fig. 1.2b). The depression between the anthelix and the helix is the scaphoid fossa, also called scapha or fossa navicularis.
Anterior to the entrance to the external acoustic meatus lies a prominent cartilaginous process, the tragus. At times, the tragus has a forked appearance. The superior tubercle of a forked tragus (or tuberculum supratragicum) also appears in literature as the anterior knob. Opposite the tragus, and separated from it by the intertragic notch (incisura intertragica), is
another prominent tubercle: the antitragus. The antitragus is an extension of the anthelix, and
is separated from the latter by a depression called the posterior auricular furrow (sulcus auriculae posterior), also known as the oblique furrow (sulcus obliquus).
Note. Appendix I contains a list of definitions for the gross anatomical features of the external ear.
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a b
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Fig. 1.2 External ear showing crus cymbae (a) and crus posterior anthelicis (b).
1.3 Variability of the external ear (Excerpt from Meijerman et al., 2002)
A great amount of variation is found in the morphology of all anatomical structures, and ears are no exception. Variation in the anatomical structures of the ear, their position, elevation or indent, will affect the print left by a given ear. It is, however, by no means necessary to try to ‘recreate’ in our minds a 3-dimensional structure from the 2-dimensional print in order to be able to individualize a print, as we may compare a print with other prints and not with the ear itself. Also, variation in ear morphology does not automatically lead to a similar amount of variation in earprints. It is nonetheless very informative to know the extent of variation in ear morphology, as it will enable us to interpret the features of a print, and facilitate the recognition of variation in prints made by the same ear from those made by different ears. In addition, knowledge on the frequency at which different ‘printable’ morphological structures occur may allow a prediction of the significance of an imprinted feature.
Several authors have described and/or classified the variation in morphology of the
external ear (e.g., Hunger and Leopold, 1978; Imhofer, 1906; Van der Lugt, 2001; Oepen, 1970, 1976; Olivier, 1969; Schade, 1954; Södermann and O’Connell, 1962; Tillner, 1963). In the following text, different structures and classifications of their forms are treated. When available, the frequency at which the various forms are reported to occur is provided. Peculiarities and anomalies are also discussed.
General form of the external ear
The overall shape of the auricle is determined by the contour of the helix and of the lobule. A common way of classifying the shape of the auricle is by defining it as either ‘oval’,
‘round’, ‘rectangular’ and ‘triangular'1.
1
The oval shape appears to be most common. Van der Lugt (2001) found 68.7% of examined (male) ears to be oval, and Iannarelli (1989) 65% (both sexes). For triangular ears a frequency of 21.9 % was observed by Van der Lugt, and 30% by Iannarelli. For rectangular ears this was respectively 9.1% and 3%, and for round it was 0.3% and 2%. When taking the shape of the earlobe into account, the ear may be further classified as ‘kidney-shaped’ (oval with an unattached earlobe) or ‘shaped as half of a heart’ (oval with attached earlobe). Hunger and Hammer (1987) classified the shape of the ear as ‘elliptical’ (32.2%), ‘oval’ (59.1%), ‘quadrangular’ (3.2%) or ‘triangular’ (5.5%) (no definitions provided). Hildén (1929) compared human ears to those of other primates, suggesting that in 0.96% of cases humans may exhibit the ‘macaque form’ (pointed supero-posteriorly; Fig. 1.3a) of the ear and in 1.98% of cases the ‘cercopithecus form’ (pointed superiorly; Fig. 1.3b).
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a b
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Fig. 1.3 Uncommon shapes of the external ear: the macaque form (a) and the cercopithecus form (b).
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Length and width of the auricle
the posterior part of the helix, on a line perpendicular to the ear base.
Sexual dimorphism is evident in measurements of length and width of the auricle. Among males and females belonging to the same age group, males exhibit higher values for both auricle length and width (Ito et al., 2001; Meijerman et al., 2004b). Measurements of auricle length and width have also been used by various authors to explore differences between populations. The work of Vorobev has been cited in Gerasimov (1940) and Van der Lugt (2001). Vorobev studied ear length differences and suggested that different populations display different phenotypes, hence different averages for ear dimensions. Similarly, Farkas (1994) measured length and width of ears of various populations in a series of studies aimed at recognizing populational differences necessary for facial reconstruction in plastic surgery. The studies conducted by Farkas included Afro Americans, Chinese and North American Caucasians. It was observed that ears of different ethnic populations varied in overall proportions.
Physiognomic Ear Index
In addition to simply measuring ear length and width, these can be put in relation to one another as to give what is called the Physiognomic Ear Index (EI), which provides an immediate reference of the proportions of the ear in a single value. It is calculated by the following formula: EI = (Width * 100) / Length. The index provides – to some extent – information about the shape of the ear, but not about its contours. Hrdlicka (1920, cited in Farkas and Munro, 1987) emphasized the greater importance of indices over absolute measurements when comparing various groups of people or populations. The ear index has therefore been used to classify the overall shape of the ear into narrow (EI<60), medium (60<EI<65) and wide (EI>65) (Facchini, 1988). According to Topinard (1885), the EI is relatively low among Caucasians as compared with African, Polynesian and Micronesian populations. This indicates that Caucasian ears are comparatively long and narrow.
Helix
one or both ears in 12.6% of the males and 18% of the females. The degree of rolling (or inward folding) of the helix varies greatly as well, and may be classified as either strong, normal, weak or not rolled at all. Depending upon how the rim is rolled or folded, the lateral surface of the helix will be flat or convex or even concave.
Helix width may also vary greatly, both among various different ears and within the helix of a single ear. As differences between the various regions of the helix are great, the helix is best divided into sections when describing its variation. Commonly the helix is subdivided into a superior and an inferior part (also called cauda helicis). This division is useful for comparing the local degree of inward rolling of the helix, as the lower end of the cartilage in the helix is partially separated from the main body of cartilage (Fig. 1.4), and the degree of inward rolling of the helix in these sections is often quite different. Unfolded parts are characteristic: Oepen (1970) found the superior part to be unrolled in one or both ears of 2% of the examined males and in 0.2% of the females. The inferior part was unrolled in one or both ears of 6% of the males and 17.8% of the females. Oepen further noted the occurrence of a notch in the outer margin of the helix between the superior and inferior parts. It was observed in one or both ears of 11.8% of the males and 7.6% of the females.
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Fig. 1.4 Cartilago auricularis and part of os temporale.
1. Os temporale (temporal bone); 2. cartilaginous part of the auricle; 3. cauda helices.
A special characteristic of the helix is the presence or absence of an auricular tubercle, also known as Darwinian tubercle or Darwinian nodule. This tubercle is positioned postero-superiorly, and may be very conspicuous and characteristic. It may be pointed or more rounded, and it can be positioned on the inner edge, outer edge, or both, or even on the lateral surface (Fig. 1.5). Keith (1901) illustrated the diversity of forms of this tubercle by stating that “between every type distinguished there occurs a chain of intermediate forms”. Hildén (1929) found that the tubercle occurs more frequently on the right side than on the left side, and also more frequently in males than in females. More rarely, two or even more tubercles are present. Oepen (1970) provided numbers for the occurrence of various forms of the auricular tubercle. A very pronounced auricular tubercle occurred one-sided in only 0.2% of the examined males. It was not observed in any of the females. A presence of two smaller auricular tubercles occurred in one or both ears of 5.6% of the males and 4.8% of the females. The one-sided occurrence of three auricular tubercles was noted in 0.2% of the males and 0.4% of the females.
Tubercles are also frequently found on the superior part of the helix. Oepen (1970) observed them on one or both ears of 9.4% of the males and 4.4% of the females. They may be very characteristic. Finally, Geyer (1928) and Jürgens (1961) described a ribbon-like, flat helix: the helix taeniata.
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Fig. 1.5 Various forms of the auricular tubercle.
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Anthelix
elevated. The superior section is usually divided into two crura: crus superior and crus anterior (also called crus inferior). The superior and anterior crura of the anthelix define the triangular fossa. This area varies in size depending on the curve and width of the two crura. The curvature of each of the sections of the anthelix varies greatly, as does the elevation of the various sections. Oepen (1970) found the superior crus to be flat (one- or both-sided) in 7.8% of the males and 4% of the females.
At various places, knobs may occur, particularly in the inferior section, the stem of the anthelix. Oepen (1970) observed a knob at the anthelix stem in one or both ears of 19.2% of the males and 11.2% of the females. A knob at the anterior crus was observed in 0.2% of the females and none of the males. A knob at the superior crus occurred in 1% of the males and none of the females. A knob at the junction into crura was found in 0.8% of the males and 0.2% of the females.
The presence of a third crus, running from the crus superior toward the supero-posterior part of the helix (see Fig. 1.2b), may further be characteristic. Oepen found this additional crus, known as crus posterior, in one or both ears of 2% of the males and 0.6 % of the females. Imhofer (1906) found it in 1% of the studied ears of both sexes, and Tillner (1963) in 1% (males) and 1.4% (females) of the ears.
Scaphoid fossa
The anthelix and helix define the scaphoid fossa, also known as scapha. It can be wide or narrow, and may taper toward the earlobe, or broaden. It can be short or long. Oepen (1970) observed that when the scaphoid fossa broadens toward the earlobe, or when it has an angular (ventral) apex, the posterior auricular fossa is often deep. She found the scaphoid fossa to broaden in 35.4 % of the males and 37.4% of the females. A scaphoid fossa with an angulated apex occurred in 18.2 % of the males and 13.6% of the females.
Crus helicis, crus cymbae and crus cavi
crus of the helix toward the anterior crus of the anthelix. This ridge, the crus cymbae, is often found to be only one-sided. Quelprud found this crus in 5 to 6% of the people, Tillner in 1.03%, while Loeffler (1940) did not encounter it at all. Cor van der Lugt, during his extensive study of ears and earprints of the Dutch population, did not encounter it either (personal communication). Schade (1954) quoted Loeffler (1940) in saying that the occurrence of this feature varies greatly between different geographic regions. He furthermore introduced the crus cavi, a ridge that also runs from the crus of helix, but through the lower part of the concha. Schade did not provide information regarding its frequency.
Tragus, antitragus and intertragic notch
Oepen (1976) provided the following classification for the different forms: simple, spherical, flat or with a second protuberance. The latter form, the ‘forked tragus’, was observed by Imhofer (1906) in approximately 21% of 500 studied ears. The second (superior) protuberance is called anterior knob. The tragus may be arched outward, or more inward, and it can be smaller or larger than, or of equal size to, the antitragus. Excepting the forked type, the antitragus may be similarly characterized. It can further be categorized as either horizontal or oblique.
The tragus and antitragus define the intertragic notch. This notch can be wide or narrow, and is classified by Schade (1954) as either V-shaped, U-shaped, horseshoe-shaped, angulated or arched. The edges may be flat or swollen, and may give rise to a lobular furrow. A furrow originating at the intertragic notch was noted in 26.6% of the studied males and in 21.4% of the females by Oepen (1970).
Anterior notch
The tragus (with or without the anterior knob) and the crus of the helix define the anterior notch, which may be more or less pronounced.
Concha
or cymba smaller than cavum.
Posterior auricular furrow
This is a depression between the antitragus and the anthelix, which may be absent or present, deep or shallow, and oblique to various degrees. Oepen (1970) found a very deep posterior auricular furrow in 22.8% of the males examined and in 11.4% of the females.
Lobule
Much variation is found in the length and shape of the lobule (earlobe), as well as the extent to which it is attached to the head (free, totally attached, or anything in between). Van der Lugt (2001) recognized 4 shapes of the lobule: round, triangular, square and lobed. Hunger and Leopold (1978) and Hunger and Hammer (1987) refer to this division similarly as arched, triangular, square and tongue-shaped. Schade (1954) recognized six forms, dividing the triangular lobule into the following: ‘attached, rounded’, ‘attached, straight’ and ‘attached, tapering to the cheek’ (See Fig. 1.6).
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a d
b e
c f
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Fig. 1.6 Various forms of the lobule: unattached, tongue-shaped (a), unattached, arched (b), attached, square (c), attached, triangular, rounded (d); attached, triangular, straight (e); attached, triangular, tapering to the cheek (f).
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of the earlobe as well, and found the ‘round' earlobe (similar to ‘unattached, arched’ (b) in Fig. 1.6) to occur in 63.6% of 1000 studied (male) ears, the ‘triangular’ earlobe (including type d, e and f in Fig. 1.6) in 22.6%, the ‘lobed’ earlobe (similar to ‘unattached, tongue- shaped’ (a) in Fig. 1.6) in 9%, and finally the ‘square’ earlobe (resembling ‘c’ in Fig. 1.6, but unattached) in 4.8%. Hunger and Hammer (1987) recorded percentages of 38.8 (arched), 31.8 (tongue-shaped), 15.3 (triangular) and 14.1 (square).
An unusual form, the lobe inclus was depicted by Jürgens (1961). In this form, the auricular attachment (or ear base) curves more or less abruptly to the front. According to Jürgens it is found particularly in persons of multiethnic descent.
Peculiarities, minutiae and anomalies
Furrows may be very characteristic. They are often present in the lobe, tragus and the pre-auricular area. Moles, freckles, as well as an irregular skin structure may also be distinctive. The occurrence of pre-auricular appendages and (pre-) auricular sinuses, also called auricular pits or fistulae auris congenitae, may be very characteristic as well. Appendages and sinuses are skin tags and depressions or tubes respectively. (Pre-) auricular sinuses may indicate abnormal embryological development of the auricular hillocks, whereas pre-auricular appendages may be due to accessory hillocks (Sadler, 2003). Appendages may usually be found anterior to the tragus, while sinuses more often occur on, or close to, the anterior margin of the ascending limb of the helix.
mentioned the occurrence of congenital cleft earlobes and earlobes with holes, resembling pierced earlobes. Both phenomena are rare.
1.4 Recognizing the representations of gross anatomical features in an earprint
An earprint is a two-dimensional reproduction of the parts of the auricle that touch a surface, like the print of a stamp. Unlike in a regular stamp, the elevation and the flexibility of the various structures of the auricle vary. Due to this fact, the imprint of some morphological structures will appear in the overall earprint, but some may not, or only partly. This all depends on the position and elevation of each morphological structure in relation to the position and elevation of the other structures. Imprinted features in prints of different ears may therefore not only differ in shape and size, but also in relative intensity, or they may be absent from a print. Absence of the imprint of (part of) a gross anatomical feature from a print may therefore also be informative regarding the origin of the print. Fig. 1.7 shows three examples of left-earprints, in which imprints of the various gross anatomical features are indicated. Depressed areas of the ear show as voids in the print. In the first and third prints, features of the external ear are almost completely imprinted. In the second print they have a very patchy appearance.
Anatomical features most frequently found in a print are the helix, anthelix, tragus and antitragus. If visible, the imprint of the helix will be the most peripheral part of the earprint. It may be continuous or interrupted. The imprint of the anthelix usually runs parallel to the imprint of the helix. At times, the imprints of the helix and anthelix are connected in the posterior section of the print. The imprint of the anthelix may have one or more branches extending from its upper area. It is frequently continuous with the imprint of the antitragus and varies greatly in shape. Extensions from the upper area of the imprint of the anthelix are called anthelix branches. Not all branches of the anthelix are necessarily imprints of actual crura of the anthelix. An inferior branch (extending downwards) may for instance result from the inner margin of the anthelix-stem, or from a knob at the junction of the superior and anterior crura. For convenience we do not distinguish between branches on the basis of their origin. Branches may extend anteriorly, superiorly, posteriorly or inferiorly.
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Fig.1.7 Examples of left-earprints in which imprints of the various anatomical features are indicated.
1. helix; 2. crus of helix; 3. anthelix; 4. anterior branch of anthelix; 5. superior branch of anthelix; 6. inferior branch of anthelix; 7. tragus; 8. antitragus; 9. (outline of) intertragic notch; 10. lobe; 11. apex of schaphoid fossa; 12. auricular tubercle.
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auricular area. The projection produces a curve that may range from pointed to shallow. It may also occur as a separate patch. The imprint of the antitragus is situated inferior to the imprints of anthelix and tragus. It may appear as a separate patch, but it may also be continuous with the imprints of tragus, lobe and/or antitragus. The notch linking the tragus and antitragus imprints the outline of the intertragic notch. Its shape may vary. If the tragus and antitragus imprints are not connected, traces of this notch may still be visible.
der Lugt (2001) a print is depicted in which the entire crus of helix is imprinted. This print was made while using excessive force. A complete imprint of the crus of helix as such is thought to occur unlikely in a functional print, i.e., a print resulting from the act of listening.
The entire outline of the concha is usually not visible in a print, and the same can be said for the outline of the scapha. At times, however, the outline of the – often characteristically shaped – inferior apex of the scapha is clearly imprinted. Finally, a crus cymbae, which is already a rare feature in the actual auricle, is even less likely to be found in a print because of its low elevation. Part of the pre-auricular region is, however, often represented in a print, and may provide valuable information.
Note: When describing and comparing the various components of an earprint in which the imprints of two or more of the gross anatomical features are connected, it may be useful to have an agreement on where to put the boundaries of each of the features involved. Appendix III provides directions for the transitions between the gross features zones in earprints.
1.5 Materials and methods
A list of the images used to illustrate this thesis is provided as appendix IV. Acknowledgments of sources are provided with this list.
Details on materials and methods used during the various studies are outlined in the appropriate chapters of this thesis. In this section, some general remarks are made on the manner of collecting the earprints that were used for the various studies.
All prints but those of the identical twins (chapter 9) and those used when exploring the effect of the amount of oily substances present on the ear (chapter 7) were collected using
a preliminary version of the FearID listening box. This apparatus had a weight scale2
incorporated in one of its surfaces. The surface of the weight scale served as the listening surface, enabling the measurement of (perpendicular) force applied by the ear during listening. Loudspeakers were placed on the inside of sound box, making it possible to supply
2
sound during a listening effort. After the sound box had been adjusted for the appropriate height, the subject was asked to approach the sound box and to listen for sound.
Latent prints were enhanced with ‘Special Silver’ fingerprint powder3 – referred to as
aluminium powder in the remainder of this text – using a brush made of squirrel hair. Prints were secured using black gelatine lifters. These lifters comprised of three separate layers: a removable transparent plastic cover over a layer of low tack gelatine with a backing of white rubberised linen. Firstly, the transparent top layer was removed, after which the gelatine layer was applied onto the powdered earprint. The lifter – now having the fingerprint powder adhered to it – was removed from the listening surface and the transparent top layer was placed back on the gel to protect the print. Prints were scanned at 600 dots per inch, using a Hewlett Packard ScanJet 7400c.
When collecting the earprints used for the study of variation in earprints of identical twins (chapter 9), and those used when exploring the effect of the amount of oily substances present on the ear (chapter 7), an acetate sheet was attached to a hard (metal) vertical surface. This acetate sheet served as listening surface. After subjects were asked to listen, the acetate sheet was dusted, and prints were secured in the same manner as described above.
Prints that were used for the studies mentioned in chapters 2, 3 and 8 were collected by Ruud van Basten (Police Academy of the Netherlands). Cor van der Lugt (Police Academy of the Netherlands) collected the prints used for the studies described in chapters 7 and 9. For the study described in chapter 6, prints were collected by Cor van der Lugt and Ruud van Basten (Police Academy of the Netherlands), Marta Giacon and Francesca De Conti (University of Padova, Italy), and Zale Johnson (Centrex National Training Centre, UK).
1.6 Chapter outline
Chapter 2 contains the results of a literature study into the classification and individualization of earprints. Results from this literature study were combined with results from a preliminary study of earprints and suggestions for future research.
In chapters 3 to 7, various (potential) causes of intra-individual variation are addressed. Chapter 3 contains a study into the effect of the level and frequency of the target sound and
3
the level of ambient noise on the force that is applied by the ear to the surface when listening. In chapter 4, the effect of presence or absence of a target sound on the force applied by the ear is explored. Chapter 5 contains a study aimed at determining the rate of growth of the external ear during adult life in order to evaluate the extent to which the anatomical features may shift position in earprints with passing time. In chapter 6, it is investigated whether the duration of listening affects the size and intensity of the imprinted surface. Chapter 7 contains the results of a study to test whether the amount of oily substances that is present on the ear affects the size and intensity of the imprinted surface.
Chapter aims to provide insight into the level of detail to which prints are compared. We show for five individuals (examples of) the variation that may occur between different prints of the same ear.
In chapter 9, realistic intra-individual variation is compared with a very small degree of inter-individual variation, i.e., that in earprints of identical (monozygotic) twins. A possible method for the classification of earprints is suggested. This method may be applied to design an automatic matching system, and to calculate the match likelihood for any two prints.
Chapter 10 discusses the use of earprints in forensic investigations. This chapter provides an overview of literature on the practical use of earprints as well as a summary of the results from previous chapters. It contains the concluding remarks.
