1.1 Vibrato and Tremolo

We use the term "vibrato" to denominate a periodic undulation of a tone (here a singing tone) in pitch, volume, and timbre. For a better differentiation, we distinguish between a pitch vibrato, a volume vibrato, and a timbre vibrato respectively, although the last one will not be discussed further. The shape of the vibrato resembles a sine, by which it can and will be described mathematically. There are at least two important parameters shaping the vibrato, which are its rate (or speed, which is usually between five and nine hertz, meaning between five and nine vibrations per second) and its range (which will be given in pitch cent or sound pressure level, dBSPL, respectively). In the following, we refer to the term "range" to describe the distance between the upper and lower reversal point of the vibration, whereas the "amplitude" describes the deflection from the zero or fixed point in each direction. Therefore, it is the same if we say that a pitch vibrato has a range of two semitones, or an amplitude of plus minus one semitone. To avoid confusion, we will henceforth use the term "range."

Pitch and volume changes of vibrato do not behave strictly proportionally to one another. This means that two vibratos can have the same pitch range but very different volume ranges. Thus, commonly used adjectives to describe the vibrato, such as "great" or "small," are problematic, for it remains unclear whether these adjectives relate to the rate or the range, or if the adjective refers to both dimensions, volume and pitch.

The term "tremolo" is used as a historical term only, and we assume that both vibrato and tremolo were used to describe the same phenomenon around 1900. One might object that a tremolo is a volume vibrato without a pitch vibrato, because this is precisely what today's effect units for tremolo do. There are also certain early singing techniques that substantiate this viewpoint; the trillo for instance, which is a rapid sequence of notes of – or a trill on – the same pitch, is particularly important in so called historically informed performances. It was described by Caccini in the 17th century and has been mentioned by Hugo Goldschmidt (1890, 87f). Therefore, it was known at the time in question, but as we will explain later, most sources of the late 19th and early 20th century do in fact describe what we call vibrato, when they use the term "tremolo." Like many others, Edison only used "tremolo." In the following, we use the term tremolo if the source we refer to used it, otherwise we use the term "vibrato."

1.2 Edison and the Implications of His Record Policy

Edison did not only invent the phonograph; his record company was also one of the most important providers of records until its decline in the late 1920s, and its insolvency in 1929. Edison's records are still valued as exemplary documents of early 20th century singing, including artists such as Frieda Hempel, Leo Slezak, Lucrezia Bori, and Jacques Urlus. For Edison, a recording of high quality was the most important criterion (see Martensen, 2019). He explicitly made that clear in a letter to Thomas Graf, head of his recording studio in Berlin, in which he wrote:

I propose to depend upon the quality of the records and not on the reputation of the singers. There are, of course, many people who will buy a distorted, ill-recorded and scratchy record if the singer has a great reputation, but there are infinitely more who will buy for the beauty of the record, with fine voices, well instrumented and no scratch. (Harvith & Edwards Harvith, 1987)

This attitude towards singers and their reputation was quite unique. To ensure that his staff knew what he wanted, he wrote a general policy in 1912, in which he demanded:

All that we desire is that the voice shall be as perfect as possible, free of conspicuous tremolo, clear and without ragged sustained notes, free of subsidiary and false waves on these notes. (Edison, 1912)

It is unclear whether the adjective "conspicuous" was meant as an attribute or a modifier. It could be read as "free of tremolo, which is always conspicuous" or as "free of those particular tremolos that are conspicuous"; as we will demonstrate further, the former interpretation is more likely.

Edison's policy is only one instance showing that his selection of artists was based on his idea of a perfect voice, and he ensured personally that his criteria were met. In doing so, he pressed for technical improvements and evaluated singers and their voices. Thus, the voices we hear on old Edison records are not a cross section of the artists of that time, nor do they provide a representative sample of the singing technique of the early 20th century. This makes the use of Edison records problematic, if we use them as historical sources and take them as exemplary instances of a cultural practice. What we are listening to is a distorted image of the past. As this paper will show, that distortion refers to the fact that the artists selected became representatives for their generation, as well as for the sound of the voices that we hear in those recordings.

Edison inspected each sound recording made in his studios over the years, listening to them and writing memoranda for the evaluation of his singers. All artists who wanted to record in Edison's studios had to undergo voice trials. Only those artists who had passed the trials successfully could make recordings for the market. Edison frequently demanded that singers or the accompaniment were replaced, and he even dreamt of freeing himself from the laborious selection of singers, when he wrote that "[w]ith a regular corps of singers we will be enabled to rehearse, change the voices and style of accompaniments and make several duplicates of the tune and thus adopt the most perfect one" (Edison, 1912). Edison and his staff went even further and made analyses of records from competitors, such as Victor Records, Columbia Records, and Fonotipia. If Edison cylinders or discs shall be used as historical sources, it is crucial to know more about Edison's concept of vocal quality. For him, one of the most important aspects of voice quality was the tremolo, the meaning of which will be central to this paper.

1.3 Outline

In the following section, we look at different sources from around 1900 to figure out what was meant by "tremolo," and especially how Edison used that term. The investigation of writings alone, however, cannot clarify what Edison meant, but it will raise questions which serve as the basis for our main approach. The main aim of this paper is to combine archival work and performance analyses to investigate both Edison's comments, and the recordings Edison referred to. In Section 3, we will describe the archival material we gathered, and how we edited the comments in Edison's voice trials for further use. With an analysis of the pitch vibrato described in Section 4, we will take a first step towards a better understanding of the relationship between the two vibrato parameters, rate and range, and Edison's positive or negative reactions he recorded in his voice trials. Given that the audio files we used were of different origin, we distinguish two corpora of the voice trials, namely Records Trials and Probe Trials, which are later merged into one larger corpus called Test Trials. The preliminary analysis of the pitch vibrato allows for constructing hypotheses regarding the volume vibrato, which we analyze in Section 5. Finally, the main results are summarized and discussed in Section 6.

2.1 Edison's Perspective

As Ann-Christine Mecke (2016, p. 672) argues, the nomenclature for "vibrato" or "tremolo" was not standardized until around 1900. And even after the turn of the century, there is hardly any agreement amongst the sources dealing with the vibrato. Therefore, it is not surprising that Edison could not rely on a consistent definition, as in his time there was little information on the vibrato.

For Edison, the tremolo was one of the most important features of the singing voice, as it was one of the singers' most serious flaws. This reading is based on statements in several sources, including his personal pocket notebooks. For instance, in one of these notebooks, Edison referred to the Grove's "A Dictionary of Music and Musicians" (probably the first or the second edition and presently known as "The New Grove Dictionary of Music and Musicians"):

Walter Miller[:] Groves Dictionary of Music[,] page 327 article Cavalieri middle of column on right says[:] "Certain vibratory production of the voice, a fault common among bad singers and known as Tremolo, shake." (Edison, 1910–1912)

Here, Edison obviously addressed this entry to his staff member Walter H. Miller, head of sound recording in the Music Room in New Jersey. One possible explanation would be that he wanted to help define the term tremolo. It is also possible, however, that he wanted to provide some arguments supporting his personal view on the matter. The latter interpretation rests upon the observation that Edison altered certain aspects of the original article by omitting information. Originally, the article deals with Emilio de' Cavalieri (1550–1602), and the author of the article asks, whether Cavalieri's "trillo" must be regarded as a "shake" or as a "vibratory production of the voice":

It may be questioned, however, whether the last-mentioned [trillo] was the true "shake"; that is to say, a rapid oscillation between two tones or semitones; or whether it was only a certain vibratory production of the voice, probably considered an elegance in early times, but now more fitly as a fault common among bad singers, and known as the "tremolo." (Grove, 1879, 1900, both p. 327)

Here, Edison did not refer to the difference between "tremolo" and "shake." He simply treats both equally as "a fault common among bad singers." Further remarks suggest that Edison thought of the tremolo as a defect:

The tremolo is the greatest defect of the human voice and is never put in by the singer …Singers are unaware that they have a tremolo …As a rule if the strength of the voice is called 100% the defects outside of the tremolo seldom exceed 10%. Hence if singer is in a very large Auditorium the defects in many cases are not heard if the Orchestra is large and the hum of the audience Considerable [sic]. (Harvith & Edwards Harvith, 1987)

In an interview, Edison tried to explain the tremolo as a faulty alignment of the vocal cords, which underlines his view on the tremolo as a defect rather than a means of expression:

[B]oth cords must be in the same tension. If one cord is a little looser, it will have fewer vibrations than the other. This conflicting of the vibrations of two different tones produced at the same time is what causes the tremolo with its "out of tune" effect. (Anonymous, 1916b)

In another interview, he differentiated between a "trill" and a "tremolo," again suggesting that the tremolo is a fault in comparison to the trill.

A singer's trill is quite a different thing from a tremolo. A trill can be and is controlled by the brain, but a tremolo is not within brain control and, so far, there has been no means found for correcting it. Most singers cannot sustain a note, without breaking it up into a series of chatterings, or tremolos. The number of waves varies from two per second to as high as twelve. When at the latter rate, the chatter can just be heard and is not particularly objectionable. When at a lower rate, it is very objectionable. (Anonymous, 1916a)

The range of the vibrato rate between two and twelve hertz, to which Edison refers, is very large. His remark that a slower tremolo is worse than a faster tremolo is, however, noteworthy, and it is a rare case in which he makes a more differentiated statement about the tremolo and the perception of its quality.

The "tremolo" is an object of examination in most of his voice trial memoranda. Too strong a tremolo seems to be the main reason for Edison to expel a singer from further trials, and from making a master disc for the market. A few examples may provide an insight into Edison's brief but clear diction:

MISS FANNY ANITIRA MEZZO-SOPRANO. Scala. This lady has a very pure scale – Has some tremolo but not very great – […] but her voice is pure and if she didn't have the tremolo she would be a GOOD SINGER. (Edison, 1912–1917)

FRANCO TURNENELLO. TENOR. […] – A fine sharp extra set of vibrations all through his voice. His tremolo is too much for us. […]. (Edison, 1912–1917)

PHILLIPS, JOSEPH A. […] You can use this man. […] His tremolo is so rapid that it will not be objectionable under many conditions. […]. (Edison, 1912–1914b)

FERN, HARRY […] This man we can use. He has no tremolo at all. […]. (Edison, 1912–1914b)

88335 VICTOR [RECORDS. ENRICO] CARUSO is getting a bad tremolo. Record is horribly distorted. This record is a parody on any voice. Tune N[ot]. G[ood]. Victor loud record N[ot]. G[ood]. (Edison, 1912–1914b)

Murphy, Lambert This voice is fine – but spoiled by his tremolo. Voice is so rare you had better get him to come in and ask him if he cannot improve his tremolo. If he can we can use him extensively. If he cannot we don't want him. His voice is exceedingly sweet and rich. (Edison, 1912–1914b)

Adolph Fowler […] Tune N[ot].G[ood]. Voice awful sharp, perhaps it's the recorder. Seems to have no tremolo, might make a trial record of good tune having sustained notes. (Edison, 1912–1914b)

The last example shows that Edison was aware of the possibility that the recording procedure, particularly the recording devices, might influence the sound and timbre of the voice. He also assumed that the recording apparatus could exaggerate or even cause audible vibrations. In a letter to his staff member Thomas Graf, Edison wrote that the competing record company Victor Records:

make[s] another mistake and think[s] that because a singer is good on the stage he will be good on the phonograph. This is not the case, because the phonograph exaggerates the tremolo which is present in nearly every voice. (Harvith & Edwards Harvith, 1987)

There are nevertheless a few examples indicating that Edison accepted the necessity of a vibrato for artistic reasons. In an interview, he said: "Such a use of the tremolo [for dramatic purposes] should be tolerated. It is like a trill. It intensifies. It is the constant use in lyric passages where a beautiful even tone is desired that I deplore" (Anonymous, 1916b).

In conclusion, we can say that Edison's comments raise more questions as opposed to clarifying what the tremolo might have meant to him. Although the tremolo served as an important criterion for the assessment of singers, his remarks lack stringency, or are contradictory. Obviously, for Edison the tremolo was a fault, and it was mainly related to the singer's voice and singing style. However, he showed a certain degree of awareness that the recording process (including the recording equipment) might also play a role. This is further substantiated by his insistence on listening to the voice trial recordings and not to the singers on stage instead. Nevertheless, he remains somewhat unclear about his ideas on how these two factors, voice and recording process, might interact. From the way he judges his singers, it seems reasonable to believe that he attributed his perception of tremolo mainly to the singer's voice, despite his awareness that the recording process distorts the voice.

For a better understanding of the tremolo as used by Edison, it is important to understand what was known about the tremolo and the vibrato in his time in general, and to assess what could have been known at that time. Therefore, other contemporaneous sources might help, or at least narrow down the scope of possible meanings.

2.2 Treatises on Singing

The literature reviewed here reflects the positions on tremolo as they circulated around and before 1900. Certainly, these views on vibrato and tremolo originated in the 19th century. We will therefore reflect on some 19th-century literature but refrain from older sources, because we are interested in the view on the matter, at the time in question. Edison's writing suggests an affirmation of more physiologically based schools, which are most prominently represented by Manuel García. The intense use of German authors may seem unbalanced, but these works were highly influential, and many were translated and published in the U.S. immediately.

As mentioned above, vibrato and tremolo were not defined consistently before 1900, and even in the years after 1900 there is hardly any clear explanation for either term. In his analysis of treatises on singing written by 100 authors, Brent Monahan (1978, p. 222) investigated the use of the terms vibrato and tremolo. The meanings of both differed, and often no distinction was made at all. Tremolo was frequently described as an exaggerated vibrato; the latter showing barely noticeable pitch oscillations but loudness oscillations, the former showing apparent pitch undulations. Monahan (1978, 96f) also refers to the Harvard Dictionary of Music of that time, in which the frequency of the vibrato is specified at eight hertz. The physiologist Henry H. Curtis wrote in his "Voice building and tone placing, showing a new method of relieving injured vocal cords by tone exercises" (1914), that the vibrato is produced involuntarily through the process of breathing, whereas the tremolo results from a weakness of the muscles related to the vocal cords (Monahan, 1978, p. 97). John Andrew van Broekhaven's "Some unfamiliar facts concerning the functions of the larynx in singing" (1908) and George Dodds' "Practical hints for singers" (1927) are directly opposed to one another: Van Broekhaven (1908, p. 45) distinguishes between an avoidable tremolo, which results from a weakness, and an artistic vibrato, which is the result of voice control. Dodds (1927), on the other hand, describes both in an opposite fashion (Monahan, 1978, p. 97). However, von Broekhaven's view seems to be in accord with most other sources, according to which the tremolo is the worse vibrato.

In treatises on singing, statements about the use of vibrato are rare; if there are any comments, they are mostly about avoiding it. For instance, the soprano singer and singing teacher Lilli Lehmann (1902a) thought of the vibrato as a dangerous symptom and first step towards the worse tremolo. In her view, both were the result of a faulty vocal technique:

Big voices […] are easily disposed to suffer from the tremolo, because the outflow of the breath against the vocal chords occurs too immediately. (Lehmann, 1902a, p. 170)

Even the vibrato, to which full voices are prone, should be nipped in the bud, for gradually the tremolo, and later even worse, is developed from it. (Lehmann, 1902a, p. 174)

Vibrato is the first stage, tremolo the second; a third and last, and much more hopeless, shows itself in flat singing on the upper middle tones of the register. (Lehmann, 1902a, p. 174)

If the first and second stages of tremolo are difficult to remedy […], the repair of the damage is nothing less than a fight, in which only an unspeakable patience can win the victory. (Lehmann, 1902a, p. 175)

But then, she opens the following section "The Cure" with the sentences:

There are no magic cures for the singer. Only slowly, vibration upon vibration, can the true pitch be won back. (Lehmann, 1902a, p. 175)

When listening to Lehmann's recordings, one can easily find that she made use of the vibrato and did not modulate it by chance; Figure 1 shows the pitch and dBSPL of two segments of her rendition of the aria "Casta Diva."

This example demonstrates that she indeed used the vibrato as a means of expression, despite her condemnation of it in her writings. It is unlikely, however, that Lehmann omitted dealing with the positive side of the vibrato deliberately. Probably she was not aware of her own vibrato's intensity, or she – as others, too – took the vibration we call vibrato for granted, and mainly spoke against an intense tremolo (whatever this would mean). A hint might be found in her passage about regaining the voice when damaged. Although the quotation above is taken from the official translation of Lehmann's book, some subtle nuances were lost in translation. The original German version of the last sentence is:

Wunderkuren giebt es nicht für den Sänger. Ganz langsam nur kann Schwebung für Schwebung wieder gewonnen werden. (Lehmann, 1902b, p. 26)

Usually, the German term "Schwebung" is not used for the vibrato but for the beat that occurs by the superimposition of two very close pitches. Even though the phrase "vibration upon vibration" is somewhat unclear, for it can mean the vibration of the vocal folds, the term "Schwebung" clearly indicates something, the impression of which we would relate to the vibrato. Unfortunately, neither Lehmann nor other writers used the term "Schwebung" to designate a vibrato; however, the phrase indicates that Lehmann might have had an idea of a "good vibrato," which she could not explicate.

Other authors were equally clear as Lehmann in their dismissal of the vibrato. Manuel García (1894) wrote that the tremolo is "[t]he trembling of the voice. This intolerable fault ruins every style of singing," and referring to the slur, he added that "[n]ext to the tremolo, it is the most universal and most distressing fault.[…]" (García, 1894, p. 18). August Iffert (1906) wrote that "Das Tremolieren ist ein schlimmes Anzeichen davon, daß das Organ übermäßig angestrengt wird; […]" (Iffert, 1906, p. 73, translates into: "The trembling is a severe sign of the fact that the organ is exerted excessively"). Oscar Guttmann (1861) wrote:

Bevor die Muskeln der Stimmwerkzeuge die gehörige Kraft erreicht haben, wird die Stimme immer mehr oder weniger nach oben oder unten abweichen (detonieren), sowie tremolieren (zittern) (Guttmann, 1861, 28f)

Before the muscles of the vocal organs have attained the necessary strength, the voice will always more or less vary from the correct pitch, as well as tremble (Guttmann, 1884, p. 77)

One rare exception is Ferdinand Sieber (1862), who speaks positively about the vibrato. In his view,

[…] every radical note should be accompanied with a swelling of the tone […] The Italians call this swelling and sinking, this ebb and flow, the vibration of the voice, and they never find a song beautiful in which this is wanting. (Sieber, 1872, 146f)

Not only did he differentiate between tremolo and vibrato, he also used the vibrato positively as a necessary ingredient of the voice and as a means of expression:

Es herrscht in Betreff der Ausdrücke für die verschiedenen Gesangsthätigkeiten noch immer eine große Begriffsverwirrung. So lesen wir täglich, es sei zu rügen, daß der oder jener Sänger mit seiner Stimme vibrire u. dergl. m., während mit dem getadelten Fehler das Tremoliren gemeint ist. Die Vibration der Stimme ist ihr innerster Lebenspuls, ihre Triebfeder, ihr Alles - ohne sie giebt es nur Monotonie, Töne, wie wir sie aus den Orgelpfeifen gewinnen. Wird aber aus dem vibrando ein hörbares Beben und Erzittern des Tones, ein tremolando, gemacht, so verfällt der Sänger in einen (als Gewohnheit) unerträglichen Gesangsfehler, der nur in den seltenen Fällen eine Berechtigung erlangen kann, wo das Tremolieren eben als Ausdrucksmittel für die höchste Erregung gebraucht werden soll. (Sieber, 1862, p. 130)

It causes great confusion in regard to the expression among singers of different degrees of ability.

We read daily that it is reprehensible in this or that singers to indulge in this vibration, while it is really the tremolando which is blamed. The vibration of the voice is its inmost life-throb, its pulse, its spring; without it there is only monotony, simple notes, as we obtain them from the pipes of an organ.

But if the vibration is changed into a tremolando, the singer falls into an intolerable fault, which is warranted only in very rare cases, when it serves as a means to express the very highest degree of excitement. (Sieber, 1872, p. 147)

The interesting point here is that this passage is found in the section on dynamics. This leads to the suggestion that the vibrato was perceived as a periodic change of volume rather than of pitch.

2.3 The Vibrato in Music Psychology

It was not until the rise of empirical musicology, particularly the intense vibrato studies of Carl E. Seashore (1936) and his colleagues (Easley, 1932; Hattwick, 1932; Hollinshead, 1932; Lindner, 1932; Reger, 1932), that tremolo and vibrato became more precisely defined. In their empirical investigations, they used a "strobophotograph camera and a vacuum tube voltmeter, with which it was possible to gain reliable data about the pitch and amplitude of a sound" (Tiffin 1932, as quoted in Hähnel 2013, p. 11). Simply put, Seashore and his colleagues defined the vibrato as a sine-like fluctuation of pitch, sound pressure and timbre quality, the last of which they called "sonance". For their investigations, they used what they called a vibrato apparatus, which allowed for independent control of the two parameters of loudness and pitch variance.

Most importantly, the experiments of Milton Metfessel (1926) showed that listeners were unaware of the vibrato parameters. When not told what they were listening to, they rather reported a certain timbre. When told, they could not say whether pitch or loudness was manipulated. Since Metfessel (1926) wanted the term "timbre" to be reserved for the description of the multiple frequencies and their proportions within a static sound, he tried to introduce the term "sonance", which should describe the dynamic changes of the parameters constituting the vibrato, including rapid changes in fundamental frequency, amplitude and timbre. Unfortunately, the term "sonance" was never established, so today we are still facing similar terminological problems. When listeners were told that they were hearing pitch vibrations and that they should estimate the range between the upper and lower boundaries of these vibrations, they underestimated the pitch range of the synthetic vibratos heard. They perceived about half the range, which corresponds to the deviation from the mean (i. e. the amplitude) and not the overall range; the same was true for loudness. Moreover, the effect of the underestimation was more pronounced when the pitch or volume change became larger, when the vibrato rate became faster, and when the tone had been richer (Seashore, 1932).

2.4 Towards a Better Understanding of Edison's Tremolo

It seems likely that around 1910 – and therefore for Edison – there was no way to understand the characteristics of the vibrato. Thus, there was no solid base for a differentiation between vibrato and tremolo, at least not until the late 1920s, when Seashore and his colleagues started publishing their findings. As stated earlier, Edison's impression of the "tremolo" served nevertheless as a selection criterion for singers. In the following sections, we will therefore investigate the vibrato in recordings Edison mentioned in his voice trials. We assume that if we find some systematic characteristics of certain vibrato parameters, which can be related to Edison's comments, we will be able to better understand Edison's perception.

4.1 Method of Pitch Vibrato Analysis

For analyzing the vibrato, we used a two-step approach, which consisted of (1) the extraction of a pitch curve related to the singer's voice, and (2) the automatic detection of sine-like undulations, which were interpreted as vibratos. Pitch curves were extracted using the Sonic Visualiser and the Smoothed Pitch Track algorithm of the 'pYin' plugin (Mauch & Dixon, 2014; Mauch et al., 2015), which estimates the fundamental frequency of the signal as an indication for pitch. Afterwards, the pitch curves were manually cleaned of any trills and areas unrelated to the vocal melody, because in these areas the data could be mistaken for a vibrato in the following automatic procedure (e.g., if a trill is played on a flute and recognized by the pitch extraction algorithm). The detection of vibratos within the pitch curve was done in a script written in Gnu Octave. 1 After converting the hertz values into pitch cent, the script narrows down the areas of possible vibratos by measuring the distance between succeeding local maxima. Since vibratos are assumed to have a rate between 4 and 10 Hz, which corresponds to wavelengths between 0.25 and 0.1 seconds, the script selects maxima that show these distances to their next maximum as well. Since the pitch curve is in fact a series of discrete points, the true maximum is likely to be positioned between the measured maximum point and one of its neighbors; therefore, the maximum and its two neighboring points are used to construct a quadratic function, the maximum of which is taken as the position of the corrected maximum. The distance between the corrected maxima is then used to calculate the frequency of a sine, which functions as the ideal vibrato, to which the pitch curve will be compared later. The comparison takes place within a short time window the size of 2.5 vibrato cycles, and the range of the pitch curve within this time frame is used as the range of the sine. Before the correlation of both curves, they are convolved with a Hanning window, which fades out the borders so that the resemblance of both curves is more important near the center of the time frame, which is the local maximum. The resemblance of the pitch curve and the sine wave is measured with the coefficient of determination D, expressed in percent, where D = 100 · r² and r is the Pearson correlation coefficient. To count as a real vibrato, the D must be at least 90 percent; when this criterion is matched, frequency, amplitude, D, and time position of the vibrato are all printed into a file. All pitch curves were analyzed in one step in a batch process, which provided a summary showing several location parameters and the total vibratos found in each recording.

4.2 Exploration of the Pitch Vibratos of the Records Trials Corpus

4.3 Pitch Vibrato Range in the Probe Trials

4.4 Merging Records and Probe Trials into a Combined Test Trials Sample

After merging the corpora of the Records and Probe Trials, we had a better-balanced number of samples in the henceforth called Test Trials corpus. The mean values for range and rate in the Test Trials are shown in Table 3, but differences between the groups in the Test Trials corpus could not be tested. This would have been a methodological flaw, since our hypothesis for range differences was partially based on the exploration of the Records Trials corpus. Since the Records Trials are part of the merged corpus, logic forbids that a hypothesis is tested on the same material used for its construction (Section 4.2). The merged corpus is thus only used for the following analysis of the volume vibrato.

5.1 Preliminary Considerations

Measuring volume of vibrato is virtually impossible for different reasons. First and foremost, the voice is mostly accompanied by instruments, which have a volume of their own and are intermixed (particularly in mono records). Secondly, in the recording process, the level of the singing voice is not transformed evenly, which means that the sound pressure level (dBSPL) produced in the playback of the record does not reflect the real volume that had been produced in front of the recording horn. The volume we hear depends on the frequency which is put in, or in other words, how a tone is sung. The pitch of the voice may be of lesser importance than its timbre and the vowels it produces, because a tone has many overtones, which are all influenced by the transformation, but only one fundamental frequency. The third major problem is the noise of the recording, which always accompanies the singer. What we can measure is the dBSPL the record provides. Since the singer's voice is usually louder than the accompaniment, it is possible to detect changes in dBSPL that correspond to a vibrato. These level undulations may nevertheless be more important for the impression of the recorded voice than the actual volume vibrato; for instance, one can imagine that an intense volume vibrato might be perceived to be less prominent when it is embedded in a loud orchestral accompaniment, so that the volume produced by the singer does not stick out of the general level of the recording. A similar voice might be perceived as having a strong vibrato, when the accompaniment is only soft or even absent, because the general level of the recording is highly influenced by the singer's voice alone.

5.2 Measuring dBSPL Ranges and 2f-Vibratos

In contrast to the pitch vibrato, which could be analyzed automatically, the loudness level of the vibrato was analyzed manually to a greater extent, because it requires a listener to decide if the loudness level corresponds to a vibrato, or to something else. This personal estimation was based on a combination of close listening and observing loudness and pitch curves. We had our assistant use the Sonic Visualiser's spectrogram view, the pitch curve and the RMS curve, which was created by the RMS Amplitude Plugin (Version 4) of the libXtract Plugin by Chris Cannam and Jamie Bullock. The assistant marked each local maximum of the RMS curve if it corresponded clearly to a loudness-vibrato of the RMS amplitude curve.

5.3 1f-Vibrato Analysis

For clarity, we use the term 1f-vibrato for the normal dBSPL vibrato and 2f-vibrato for the special case, in which the dBSPL vibrato has a frequency that is two times the frequency of the pitch vibrato. The following subsections deal only with the 1f-vibrato.

5.4 2f-vibratos

Up until now, we have analyzed the differences in dBSPL ranges as an indication for volume vibrato. Now we also have a look at the phenomenon of the 2f-vibrato. Before the analysis, we will elaborate the Artefact Theory further, to which the 2f-vibrato seems to be related.

5.5 Phase-Shift-Exploration

The analysis of the phase shift between the pitch vibrato and the volume vibrato (1f-vibrato) was exploratory in nature, since we had no basis on which we could create a hypothesis. Figure 7 shows the distribution for all median phases from the recordings of the Test Trial corpus. Except for the "Non-Bad" Tremolo group, in which the median phase shift was 0.525 π, the median was always 0.5 π. The results are in accordance with our subjective impression. However, the median hides the fact that the phase is not always 0.5 and, for now, we do not have a plausible theory explaining why the phase differs in so many cases. Small deviations from the median phase shift may be explained by the accuracy of the measurement or the alignment procedure, but not to that extent, because there are substantial amounts of recordings with a median phase shift between 0.25 π and 0.75 π. If we suppose that the phase shift is determined biophysically by the production of the vibrato in the vocal tract or other parts of the body, then deviations from this 'natural' phase shift may be the result of additional resonances caused by the recording and reproduction chain, or other influences. If produced artificially, we could further assume that such phase shift deviations might sound implausible or strange. However, since there is no difference in median phase shift between the groups, this assumption is not supported by the data. We present these observations regardless, as they might encourage further research on this matter.

Table. 7. Results of the analysis of 2f- and 1f-vibratos. Contrary to the hypothesis, the number of 1f-vibratos was even less, but not significantly less in the "Bad" (B.) groups, than in the "Non-Bad" (N.-B.) groups. The number of 2f-vibratos is significantly increased in the B. groups. The ratio of 2f-vibratos/1f-vibratos is significantly increased in B. groups, indicating that the proportion of 2f-vibratos influenced Edison's dislike of the tremolo. Together the results support a hypothesis which is based on the Artefact Theory, according to which the volume changes that are caused by the recording and playback devices are probably more responsible for the "Bad" comments than the vibrato as it was sung.

	Tremolo				Overall
	B.	N.-B.	W	p	B.	G.	W	p
1f vibratos (amount)	57	66	677	.735	59	65	516.5	.795
2f-vibratos (amount)	65	33	1018	.002	58	28	840.5	.002
2f-vibratos / 1f-vibratos	1.09	0.57	988.5	.005	0.83	0.29	813.0	.004