TEMPO

Tempo is one of the most aurally salient parameters of popular music. London (2004) amalgamates recent research on tempo perception and confirms humanity's ability to perceive a pulse from between roughly 30 and 240 BPM, preference of pulses between 86 and 120 BPM, and a maximal salience for tempi around 100 BPM. In order to be consistent and consider a salient tempo indicator across all songs, tempo was measured by choosing a pulse which always bore the same relation to the underlying audible drum beat. This was done by measuring tempo with respect to the alternating kick and snare of the basic backbeat pattern. For instance, Figure 2 details the drum part to "Nuthin' but a G Thang", showing how the tempo was determined. Tempo was measured using an online tempo-tap application, which records tempo by continuously averaging all previous taps.18

TEXTURE / ORCHESTRATION

Instrumental texture was analyzed with respect to functional layers, a concept outlined by Moore (2012, pp. 20–21). He advocates for the analysis of song texture with respect to the functions of each textural layer, rather than the instruments which comprise each layer. Moore's layers are four in number: explicit beat, functional bass, melodic, and harmonic filler. In hip-hop music, these four layers are normally present in some combination, but where the melodic layer in other popular genres might be occupied by a singing voice, in hip-hop this does not always occur. Therefore, the melodic layer was defined purely by melodic instruments, reserving the analysis of vocals for a separate parameter category. A fifth textural layer was added to account for any sound effects that do not fit into Moore's four categories. Thus, for each song, the transcription data was parsed into these five layers and represented the data as shown in Figure 3. Here we see how the aurally-identified instruments of "Nuthin' But a G Thang" occupy the five textural layers. Texture was analyzed only in the verses of each song so it could be compared consistently across the corpus. Song texture varies greatly in hip-hop music, especially in songs from the Golden Age. For instance, "The Choice is Yours" (Black Sheep, 1991) has a comparatively minimal texture, as seen in Figure 4 (Audio Example 2). The verses of this song only utilize three textural layers: explicit beat, functional bass, and sound effect, forming a thinner, sparser texture.

FORM

The main goals for formal analysis concerned content (what types of formal sections were present in a song) and balance (what proportions existed between these sections). Hip-hop music tends to exhibit cyclical, repetitive song forms. The alternating sections common to rock and pop are also present in much hip-hop, but with certain notable differences. Unlike rock or pop, in hip-hop, singing occurs almost exclusively in the hook-type sections interposed between rapped verses: only three songs in the corpus had substantial amounts of singing in their verses. Moreover, hip-hop music does not heavily rely on textural or harmonic changes to mark the different sections of its songs: only half the songs (48%) in the corpus exhibited noticeable textural changes between sections, and only 27% exhibited harmonic changes. Thus, lyrical content alone often suffices for determining where sectional boundaries occur, and what types of sections those boundaries separate. Verses are fairly easy to identify in hip-hop, but the hook sections that interpose them are less consistent in content. Take, for example, the differences between a refrain and chorus. For rock music, Stephenson (2002, p. 135) defines a refrain as consisting of "one or two textual lines that recur periodically" and a chorus as "a musical section that recurs numerous times with a fixed set of several lines". De Clercq (2012, p. 57) suggests that a refrain does not form a standalone section, rather it "exist[s] within a section". Two dicta can be extracted from these points: a refrain is sub-sectional and much less lyrically substantial than a chorus. With these dicta in mind, we arrive at the following definitions of hip-hop hooks (illustrated in Figure 5 and Audio Examples 3, 4, and 5):

• Refrain: a brief, recurring lyrical entity that typically appears at the end of a verse, and/or is interspersed throughout the period before the next verse begins.
• Extended Refrain: largely similar to the refrain but with more substantial lyrics, which are still formally connected to the verse they follow.
• Chorus: the most substantially lyrical and autonomous hook type, characterized either by a significant amount of lyrics disconnected from each verse, or by sung vocals. Singing often suggests a chorus.

Yet, categorizing formal sections in hip-hop music might itself be viewed as problematic, in that contrasting sections characteristic of many other popular genres are not prerequisites for a hip-hop song: In an interview with producer Mr. Supreme, Schloss (2004, p. 154) notes that "there are no rules in hip-hop…you don't have to have a chorus, you don't have to have a bridge…". Furthermore, no consensus on sectional nomenclature exists in hip-hop discourse: for hook sections, Ohriner (2013) uses "hook", while Adams (2008) uses "chorus". Despite the lack of consensus in published scholarship, the definitions presented here are robust enough for the goal of capturing aurally salient formal parameters. The formal transcription of "Nuthin' but a G Thang" in Table 2 can be converted into quantifiable or binary data: measures are used to calculate the length of all sections, the above definitions are used to define the type of hook section, and the presence (or absence) of intros, outros, and fadeouts is noted.19 Intros and outros were defined as the sonic content on each track that preceded the entry of or followed the cessation of the beat supporting the song.20

PITCH COLLECTION

The small body of literature studying hip-hop's musicality focuses primarily on texture, rhythm, and timbre, and rarely on pitch collection. Considering how much sampling occurs in hip-hop and the tonal ramifications of this practice, the tonality of hip-hop beats are a rich source for inquiry into the characteristics of this music's overall sound. The following data was gleaned for "Nuthin' But a G Thang", as shown in Table 3: tonic, pitch collection, and presence, length, and composition of harmonic progression. 25% of the songs in the corpus exhibited no discernible pitch collection, and 7% exhibited no perceivable tonic. Songs such as "Bonita Applebum" (A Tribe Called Quest, 1990) challenged the pitch collection scheme in that they were clearly based on a chromatic pitch collection (as shown in Figure 6 and Audio Example 6), but were not objectively classifiable in an established mode. They were thus classified simply as "chromatic". Even though labelling every non-modal song as "chromatic" ignores intricate aspects of these songs, separating songs with chromatic or diatonic pitch collections reflects a salient aspect of their sound.

LYRICS AND VOCALS

The lyrical content and sonic aspects of vocal delivery are prominent elements of hip-hop music; songs are mixed so vocals are easily heard over the beat. As an MC's contribution to a song consists of both lyrics and flow (the rhythmic and articulative delivery of these lyrics), the analysis considers these aspects separately. Adam Krims (2000, pp. 46–92) has written on the relation between lyric subjects and hip-hop genres of "party rap", "mack rap", "jazz/bohemian", and "reality rap". These genres are associated with specific lyrical topics: party rap tends toward lighter topics dealing with celebration, pleasure, humor, romance, and sex, and often laced with self-deprecating irony. Mack rap topics focus on success, braggadocio, money, and sexual prowess. Jazz/bohemian topics are much more serious, self-conscious, yet tend to assume a more positive tone. Reality rap includes gangsta rap, chronicling the harrowing aspects of street life, gangs, and drug use. Extrapolating from Krims' subgenres, 12 distinct topics were enumerated with which the lyrics of each song were coded.21 These topics comprise partying, sex, romance, humor, braggadocio, dissing, the generation gap, stories or messages, reflections on social issues and street life, reminiscence, autobiographical storytelling, and eclectic topics (lyrics which didn't fit into any of the other 11 topic categories).

Sonic aspects of an MC's vocal delivery, such as timbre, articulation, tessitura, and rhythm, are all highly developed facets of flow, and can be used to measure the creativity or virtuosity of an MC.22 Flow supports the conveyance of meaning in lyrics: numerous studies investigating flow and its semantic and/or embodied potential abound, including Woods (2010), Adams (2009), Ohriner (2013), and Rollefson (2015). Despite this interest in flow and its timbral, rhythmic, articulative, and pitch components, objectivity remains elusive. Heidemann (2016) says of vocal timbre: "[it] is a facet of musical experience that cannot be denied any easier than it can be explained" [1.2]. While respecting the subjectivity of salient parameters of vocal delivery, the analysis here was made as broadly applicable as possible by only making very simple observations. For vocal timbre, smoothness or roughness of an MC's vocal delivery was evaluated. Roughness denotes a "gravelly" or guttural vocal quality, such as that typically heard in performances by 2Pac. The analysis of tessitura consisted of judging where in an MC's voice (how high, i.e. in what vocal register) he or she most commonly vocalized. 2Pac again exemplifies this well: in the song "Dear Mama" (1995, Audio Example 7), he adopts a low vocal register, while in "California Love" (1996, Audio Example 8) he raps high in his voice. Vocal articulation was measured according to the general level of enunciation or suppression of consonants, again approximated by ear. For example, Cypress Hill's B-Real raps with articulate, sharply enunciated consonants (Audio Example 9). Vocal rhythm was also generalized into two permeable categories: speech-like or metric, or a mix in between.23 As Adams (2008) points out, the lyrics of rap songs are often composed and recorded after the instrumental track, or beat, and MCs perform across a wide rhythmic spectrum from total synchronicity with the beat to an almost free-flowing poetic style that has very little rhythmic alignment with the beat. By reviewing an excerpt (Audio Example 10) from "Nuthin' but a G Thang", we hear Snoop Dogg rapping in a smooth timbre, in the middle of his vocal range, enunciating his consonants, and adopting a rhythmic style that mixes metric and speech-like rhythms.

Other vocal and lyrical aspects were easily converted to binary data, such as whether singing or profanity were present in a song.24 While this methodology for analyzing lyrics and vocals is undoubtedly subjective, prior scholarship has demonstrated that these musical aspects are resistant to objectivity: timbre and articulation are difficult to systematize, rhythm—with all of its microtemporal deviations—is labor-intensive to analyze, and the analysis of lyrics opens a Pandora's box of interpretive possibilities. That said, gathering a generalized representation of vocals and lyrics across a small but broad set of parameters allows for a general formulation of vocal and lyrical profiles for each song.

COMPONENT ANALYSIS, STEREO IMAGING, AND PRODUCTION EFFECTS

The first and most important part of production parameter analysis was to identify every distinct audible element (henceforth called instrument) over the course of each song.25 Distinctions were made between instruments that were sampled and those that were recorded directly. If a sample featured multiple timbres (e.g. a brass section), it was considered one instrument. Conversely, if a drum loop was comprised of individual samples (e.g. unique samples for kick drum, snare, and hi-hat), these were considered individual instruments. Direct-recorded instruments were also treated individually. Some songs had very limited lists of instruments—as low as three—and others reached well beyond ten (still very few by today's production standards). For organizational purposes, instruments for each track were named in a consistent manner; 121 unique instrument names were used. For example, the seven instruments identified in "Straight Outta Compton" (N.W.A., 1988) are shown in Table 4, and consist of sampled drum loop, hi-hat, guitar, orchestra hit, sampled saxophone, vocals, and turntables.

In Audio Example 11, we can hear the instruments listed in Table 4. The drums come from a single sample (limited in frequency bandwidth, mono point-source within the stereo image, and custom equalization has been used to add low end to the kick). A hi-hat has been added to this drumbeat, likely from a pre-recorded loop: it does not form part of the drum sample (full/unlimited frequency bandwidth and panned to a slightly different position in the stereo image than the drums). The guitar appears to be directly recorded: its timbre sounds clean and no background noise matches its stereo image. The orchestra hit comes from the classic sample heard on many electronic keyboards. Due to its unnatural timbre, the saxophone was determined to be a sample. Finally, vocals are present throughout (without any backing vocals) and turntables (scratching) make consistent appearances.

Each instrument was qualified with a stereo image position (panning position) and a list of any production effects used on it. To determine the panning position of each instrument, a mono source of static noise was generated in Pro Tools and panned around the stereo image until it was perceived to line up with the instrument in question. Numbers represent this panning position: -100 left, 0 center, +100 right. In "Straight Outta Compton" (Table 4), the vocals appear in the center of the stereo image, so they receive a 0. For stereo sounds which occupy a spread of space along the stereo image two numbers are recorded: the left extremity and the right extremity. For example, the orchestra hit has been panned in stereo full left and right, which gets a value pairing of -100 and 100. Only certain instruments seem to be treated with audible production effects. 16 unique production effects were identified across the corpus. For example, in "Straight Outta Compton" the hi-hat has been treated with a short reverb and the vocals with a long reverb.

VOCAL EFFECTS

Additional attention was given to special production effects used on vocals, since they are typically such an aurally salient part of the production. The presence or absence of backing vocals and vocal doubling was documented. If either of these production techniques was used, the number of vocal layers was approximated. The distinction between vocal doubling and backing vocals was made using the following principles: a vocal double must be done by the same voice as the "lead vocal". The vocal double must also be at the same pitch level as the lead vocal. All harmonies and other voices were considered backing vocals.

TRACK LOUDNESS, COMPRESSION, AND PHASE COHERENCE

In addition to instrument-specific analysis, production parameters pertaining to the composite track mix were analyzed. These parameters included dynamic range, master bus compression, and overall stereo image width (or phase coherence). Dynamic range was measured in loudness units measured to full scale, or LUFS, which are calculated in negative numbers with zero being the highest possible. Compression was measured with the DRC meter from PitchTech (2016) and reflects the overall loudness range between the weakest and strongest audio signals in the track.26 Phase coherence values reflect an overall measurement of how wide or narrow the stereo presentation of the program is (-1 extremely wide, 1 extremely narrow). Across these production effects, critical listening and audio engineer analysis tools allowed for the development of a quantifiable and comparable set of data with which to complement the musical parameter data.

TEMPO

Considering its mean value per year, song tempo trends downward over time. The highest mean tempo occurs in 1986 (112.71 bpm), while the lowest occurs in 1995 (87 bpm). Figure 7 shows the distribution of tempi in a scatter plot: each dot represents a song classified by its year and tempo. Though there are notable outliers in both high and low tempi, the majority of songs cluster around the line of best fit, which illustrates the downward trend. Condit-Schultz (2016) found a similar trend in a corpus that covered a much broader timespan. Hip-hop's crossover with rock, exemplified on albums such as License to Ill (Beastie Boys, 1986) and Raising Hell (Run DMC, 1986), might account for some of the faster tempi in the early years of the corpus, as do some of the songs that Krims (2000) might classify as "party rap", such as those by MC Hammer, Sir Mix-a-lot, and Heavy D. and the Boyz. Subsequently, the rise of G-funk-influenced West-coast artists (particularly Dr. Dre, Snoop Dogg, and 2Pac) may speak to the slower tempi of the early nineties.

HOOK TYPE

Though nearly all hip-hop songs contain verses (where rapping typically occurs), a wide variety of section types—or none at all—can be interposed between these verses.27 The content of these interposing sections in each song was evaluated and assigned one of four labels: chorus, refrain, extended refrain, or none. The percentage of songs in each year receiving these labels is illustrated in Figure 8. The best-fit curves in this graph appear to neatly partition the time period in two: from 1986 to 1991, each section type encountered brief moments of dominance in the corpus (except "neither chorus nor refrain", which declines slowly throughout the whole time period). From 1991 onwards, the chorus rises to unchallenged prominence as the preferred hook type. In fact, in 1995 and 1996, all songs in the corpus had choruses. How can this trend towards total chorus prevalence be explained? One hypothesis suggests that when hip-hop gained more visibility and a broader audience base, it became subject to the same sort of commercial requirements as other mass-produced popular music genres: it is no accident that top-selling hits contain catchy hooks. Indeed, Krims (2000, pp. 85–86) notes that prior to the arrival of G-funk and "don rap" (which he describes as being heard on Notorious B.I.G.'s releases), R&B-style sung choruses were rare.

SAMPLE USAGE

In 1991, Grand Upright, Ltd. v. Warner Brothers, Inc. named Biz Markie and his label Cold Chillin' as having unlawfully used a sample from Gilbert O'Sullivan's "Alone Again (Naturally)" in Markie's song "Alone Again". Markie et al. lost the case, and a new precedent was set whereby hip-hop artists and their labels were much more cautious in their use of sampled material. Demers (2006, p. 97) outlines the ramifications for this and subsequent sampling lawsuits: top-level artists who could afford to pay the clearance costs still used samples, and indie artists, who were essentially below the radar of the big industry, continued to use samples relatively undetected. Mid-level hip-hop artists resorted to shorter samples (which were more difficult to identify), real instruments, and samples with lower clearance costs. Figure 9 shows this trend by plotting sample usage over time, again with dots representing individual songs by year. Note that this graph only reflects the portion of the corpus where samples were audibly present—48 songs—and that these samples were tallied by ear. This method of sample taxonomy was chosen because it reflects how the samples would have been identified by legal parties pursuing compensation for unfair use; the less audible the samples were to them, the less they are to the average listener. By the mid-nineties, the use of real instruments was notable on releases by Dr. Dre, Snoop Dogg, Notorious B.I.G., and others.

SINGING AND PROFANITY

The presence of singing in hip-hop consistently increases over the 11-year period considered in this study: only two of the earliest 30 songs contained singing, while 11 of the latest 20 songs did (as detailed in Figure 10a). This trend can be explained by the fact that singing almost always occurs during the chorus, thus a rise in chorus prevalence suggests a concomitant rise in singing. In many cases—"Nuthin' but a G Thang" (Dr. Dre ft. Snoop Dogg, 1992), "Tennessee" (Arrested Development, 1992), and "Tha Crossroads" (Bone Thugs N Harmony, 1995) being notable exceptions—guest artists do the singing. (Contemporary hip-hop has seen an increase in MCs doing their own singing; Drake and Kendrick Lamar are notable examples of this.) Figure 10b shows the use of profanity over time in the corpus, which exhibits a notable upward trend from 1990, tapering off around 1993. During this time, the West-coast dominance of hip-hop led by Dr. Dre and Snoop Dogg was beginning to occur. These rappers were steeped in the L.A.-bred gangsta rap tradition, pioneered by acts such as N.W.A. (and its members' solo releases), Ice-T, and Too $hort. Besttickets.com contributor Andrew Powell-Morse (2014) has undertaken a large-scale survey of profanity in hip-hop since 1985, and his findings suggest that before the 1994 East-coast releases Illmatic (Nas) and Ready to Die (Notorious B.I.G.), profanity in rap was almost wholly a West-coast phenomenon.28

LOUDNESS AND COMPRESSION

The term "loudness war" refers to a historic trend in popular music production where songs were produced with ever-increasing overall volume. Producers and psychoacousticians agree that if two songs are essentially equal in every way except for loudness, the louder song will sound better to the listener.29 Music producers were (and still are) "warring" to be louder than one another. Recording mediums have a limited amount of headroom, which the audio signal cannot exceed, so producers would apply additional dynamic range reduction so the entire signal could be made louder. This trend can be identified in the earliest years of the corpus with an increase in both overall loudness and dynamic range reduction (compression) shown in Figures 11a and 11b.

PITCH COLLECTION

Pitch collection in hip-hop music resists simple analysis: in a beat, samples may not be exactly in tune with one another, or might be shifted from their original pitch. Determining the home note or key of a hip-hop track may not always be possible, but determining its pitch collection (the underlying scale or mode) can be a more productive pursuit. The proportion of songs with no discernible pitch collection differs drastically between the earlier and later halves of the corpus: of the 46 songs released before 1991, 17 (37%) had no discernible pitch collection, compared to just 5 (9%) of the 54 songs released in 1991 or later. This trend may also be linked to the decline in sample use: as producers experimented more with real instruments, beats were more often composed ex nihilo, or were interpolations of existing compositions, re-recorded in place of being sampled.30 However, there are still outlying examples that buck this trend: "Check the Rhime" and "Award Tour" (A Tribe Called Quest, 1991 & 1993) have aurally salient pitch collections in sample-heavy environments.

HARMONIC PROGRESSION

54 songs in the corpus exhibited a salient harmonic progression in their verses. Similar to the trends observed for pitch collection, the use of harmonic progressions increases over time. In addition to this trend, songs with four or more chords in their harmonic progressions outnumbered songs with two chords, but only after 1991. Notable are the years 1995 and 1996, in which 10 of the 11 songs exhibit harmonic progressions with four or more chords. Some of these progressions go as far as displaying directional tonality or parsimonious voice leading, as shown in Figures 16a-c and Audio Examples 12, 13, and 14. "Big Poppa" (Notorious B.I.G., 1995) uses harmonic devices such as applied chords and half-diminished ii chords, momentarily obfuscating the tonic by shifting between F major and A minor. "Gangsta's Paradise" (Coolio, 1995) follows a common minor mode progression: VI - iv - V - i. Finally, "I Wish" (Skee-Lo, 1995) chains together four major 7^th chords, with parsimonious voice leading via multiple common tones between each chord. These are just a few examples of how hip-hop harmony began to get increasingly sophisticated toward the end of the Golden Age.

SUMMARY

Trends of change paint a vivid picture of how hip-hop music and production entered and exited its Golden Age. During this time, hip-hop became louder, more compressed, more chorus-friendly, slower, less reliant on crude sampling techniques, featured more singing and profanity, and became more tonally grounded with intricate harmonic progressions. These observations confirm those made by Condit-Schultz, Powell-Morse, and Demers for hip-hop music over a longer time period, suggesting that the trends of change observed in the Golden Age continued well onward into subsequent eras of hip-hop's history. Trends of change show how the songs in the corpus relate to general sonic parameters, demonstrating ways in which this repertoire evolves and develops through those parameters. The Golden Age sound can thus be defined, in part, by its ancestry and legacy.

TEMPO

Given London's (2004) findings, which suggest that listeners prefer tempi between 86 and 120 bpm, we might expect the tempi in this corpus to fall mainly between those two numbers as demonstrated in Figure 13. 86% of the songs have tempi between London's boundary values, and 95% have tempi between 80 and 130 bpm. Furthermore, just as London reports a maximal pulse salience of roughly 100 bpm, the mean tempo for the corpus is 100.42 bpm. These correlations suggest that a listener's ability to maintain an embodied response to hip-hop music, be it through dance, toe tapping, or mindful entrainment, is related to the music's critical or commercial success.

TEMPO V. LYRIC TOPIC

Correlating tempo with lyrical topics reveals some subtle trends of prevalence. While many groups of songs classified by lyrical topic had mean tempi close to the overall mean, several groups stood out. Songs with lyrical topics such as reminiscence or moral-based storytelling trended toward slower tempi, while songs with topics like partying and humor trended toward higher tempi. These correlations may not seem surprising: up-tempo songs are more suitable for dancing, and thus party-themed lyrics might seem apropos. We recall that Krims's (2000, p. 56) "party rap" genre uses faster beats with the goal of "moving the crowd". The somber emotional songs about reminiscence would suggest that slower tempi are appropriate. Furthermore, if moral-based storytelling aims to impart a clear, unambiguous message, less complicated and slower rapping atop a slower beat might strengthen the clarity and potency of this message.

PITCH COLLECTION

We have already observed the trend of change toward greater clarity of key in the later years of the corpus. Taking stock of the 78 songs that displayed salient pitch collection reveals the data found in Table 5. Before dissecting this data, it should be noted that certain inferences had to be made with some songs. In some cases, an incomplete number of pitches or harmonies were noted and thus a scale or mode was assigned without 100% certainty. In these and all other situations, labels were decided upon by ear: if a specific pitch collection was perceived, even if it was not completely represented in the transcription, this was deemed a strong enough indicator. Often the missing pitch was scale degree 6, thus making differentiation between Dorian and Aeolian mode difficult.31 Table 5 partitions the pitch collection schemes at larger levels, which yields a more interesting observation: of the 77 songs that were classified as representing a mode or diatonic scale, roughly two-thirds (51 of 77) were in minor modes (those with a lowered scale degree 3): Dorian, Aeolian, Phrygian, or harmonic minor. 42 of these 51 were Dorian or Aeolian.

VOCAL PROFILE

By investigating parameters of vocal delivery—timbre, tessitura, articulation, and rhythm—we can observe which types of these parameters occur more often (e.g. do MCs more frequently vocalize high or low in their voice), and whether specific combinations of these parameters occur more commonly than others (e.g. does rapping high in the voice typically exhibit a rougher timbre). The abbreviations shown in Figure 14 were used when analyzing vocal delivery parameters. Table 6 lists the most common "vocal profiles"; that is, specific combinations of the four parameters. For instance, the most popular vocal profile includes a gravelly timbre, high tessitura, dull consonant articulation, and rigid vocal rhythm, or in short form, GHDR. To get a sense of what this sounds like, Audio Example 15 contains an excerpt of "Hey Ladies" (Beastie Boys, 1989). Overall, certain aspects of vocal delivery are dominant: gravelly vocal timbre, high and medium tessitura, and rigid vocal rhythm.32 Woods' (2010) observations on connecting projections of masculinity to vocal timbre support this paper's findings: many of hip-hop's lyrical topics are connected with masculine imagery (e.g. braggadocio, financial success, the tough nature of surviving street life, and sexual prowess), and the use of a masculine-sounding, rough timbre might enhance the delivery of these topics.

FORM

Several formal characteristics were widely consistent across the corpus. First, of the 69 songs that contained hook sections of consistent length (i.e. each hook section in a song is the same metrical length), 52 (75%) contained eight-measure hooks, 9 (13%) contained four-measure hooks, and the remaining 8 songs had hooks of 6, 9, 10, 12, and 14 measures. The mean value for the number of distinct verses in each song from the corpus was 3.11. 77% of the songs had either three or four verses, while 92% of the songs had two, three, or four verses. As many of the songs in the corpus received significant radio play and were commercial hits, formal consistencies such as eight-measure choruses and three or four verses per song are perhaps not surprising.

SUMMARY

To summarize, the trends of prevalence reveal several tendencies across the corpus. Lighter lyrical subject matter seems to correlate with faster tempi, and the converse is also true. Tempi trend toward a mean of 100 bpm, with a median of 99 bpm. Vocal delivery trends towards a prevalence of features such as gravelly timbre, higher-tessitura vocals, and metrically rigid vocal rhythm. Form and tonality trend towards three or four verses per song, usually in minor modes. Similar to trends of change, trends of prevalence reference general sonic parameters. However, instead of relating the Golden Age to its ancestry and legacy, trends of prevalence represent the era's defining characteristics.

METHODOLOGY

Measuring similarity between songs in the corpus involves taking any song pair, comparing the musical and production parameter data, and determining, on a numerical scale, how closely similar the two songs are. To do this, we need first to establish whether each analyzed parameter should contribute equally to the similarity measurement. For instance, do two songs with the same tempo sound more similar than two songs with the same chorus type? Parameters that are more aurally salient were given more weight in the similarity computations. Salient parameters included tempo, hook type, lyric subject, instrument list, and vocal production, while parameters such as stereo image width and verse length were considered less salient. With a relative weighting scheme in place, similarity tests could be run on the 4950 unique pairings of the 100 songs, thereby generating a similarity rating for each pair.

To accomplish this task, the Daisy application from the cluster package in the programming language R was used (R Core Team, 2015; Maechler et al., 2015). This application facilitates the combination of different types of data (including numerical and categorical types) into a single computation of similarity rating. Daisy also permits the use of a custom weighting scheme. All 4950 pairwise similarity ratings were combined into a single dissimilarity matrix, which was then used to develop a divisive hierarchical cluster, shown as a dendrogram in Figure 15. A divisive hierarchical cluster graph creates smaller clusters of entries in stages from a parent group of entries. In this case, Figure 15 shows how the computer program determined a similarity-based categorization scheme at a level that created eight groups, as denoted by color. Pairs of songs that lie very close to the left-hand edge of the dendrogram are statistically the most similar. For example, such ultra-similar pairings can be seen between "Hey Ladies" and "Shake Your Rump" (both Beastie Boys, 1989), and between "Shook Ones Part II" (Mobb Deep) and "Flava in Your Ear" (Craig Mack et al.).

Daisy was run again with the 100 songs, this time using genre data collected from www.acclaimedmusic.net (Franzen, 2016). (This website classifies each song and album according to a genre label system, the origin of which is unclear.) The same hierarchical clustering algorithm was used to form four song groups based on genre similarity, as shown in Table 7. In general, these groups resemble the four genre categories laid out by Krims: group 1 appears to be a mix of Krims' reality rap and jazz/bohemian genres; group 2 resembles his party rap genre; group 3 consists almost exclusively of gangsta rap and G-funk (aspects of Krims' mack and reality genres); while group 4 seems like a 'leftover' group, bearing no consistency with any Krimsian genre. The goal of running this categorization process on both the analyzed data and the Acclaimed Music genre data was to see if the results produced similar groups.

CAN TRENDS OF SIMILARITY BE HEARD?

In general, the eight groups produced by the analyzed data (Figure 15) and the four categories produced by Acclaimed Music's data (Table 7) were extremely uncoordinated. Figure 16 shows a dendrogram for the four Acclaimed Music groups, using the color of each song title to indicate the eight analyzed data groups. (If the groups were similar, the colors would be clustered together.) This discordance suggests either that the weighting scheme used in this study was erroneous for determining similarity, or that sonic parameters were not use to produce the genre labels found at Acclaimed Music. If the latter holds true, we are presented with a conundrum. If Acclaimed Music categorizes its music by genre, then it would be reasonable to believe that artists or albums in the same genre might sound somewhat similar. Why, then, are the grouping strategies so dissimilar? To be sure, we do not know how Acclaimed Music defines its genre labels, but with such stark differences between the two groupings, the ones generated by this study should be re-evaluated to determine if trends of similarity can, in fact, be "heard".

Consider the yellow-colored group of nine songs from Figure 15. These include "How Ya Like Me Now" (Kool Moe Dee, 1987), "I Know You Got Soul" (Eric B. and Rakim, 1987), "Follow the Leader" (Eric B. and Rakim, 1988), "Mama Said Knock You Out" (L.L. Cool J, 1991), "Hey Ladies" and "Shake Your Rump" (Beastie Boys, 1989), "They Want EFX" (Das EFX, 1992), "Vapors" (Biz Markie, 1988), and "Wild Thing" (Tone Loc, 1989). Audio Example 16 chains together short clips of each of these songs. With perhaps the exception of "Wild Thing", each song in this group shares aurally salient similarities with another: six have very similar tempi, eight have eight-measure choruses, each has a vocal profile almost exactly the same as one another song in the group, four are in the Dorian mode, six have refrains, all have relatively low levels of dynamic compression, and they all mainly comprise complex combinations of sampled and machine-produced drum sounds. These features are not unique to the yellow group, but their specific orientation in these songs caused Daisy to group them as maximally similar. Audio Example 17 assembles the black-colored songs from Figure 15. Again, we can hear convincing similarities between these songs, which were all released in 1995 and 1996. (This is particularly notable since the similarity ratings generated in this study did not consider release year.)

However, even in the small groups presented here, the elements of similarity between songs are not wholly consistent. If the groups were made larger, the similarities connecting songs would become even more diverse. Trends of similarity do not necessarily identify stylistic norms across a whole repertoire. (Of course, the fact that all songs in this corpus have vocals and beats is itself a trend of similarity, but a rather useless one to consider here.) While a song in a repertoire may not sound like all other songs in that repertoire, it will sound similar to one or several other songs. Thus, trends of similarity are an important component to the definition of a repertoire's sound because they represent the actual individual songs which comprise that sound.