Scratch example sample (featuring DJ Groovemeister Martin)
Although ‘DJ scratching’ is more than 35 years old, the genre has been afforded scant attention in academia. 
But now the first [?] doctoral thesis to provide a comprehensive analysis of ‘DJ scratching’ is available online.
“Scratching was first introduced in the middle of the seventies and has since then become the most recognizable musical feature of hip-hop alongside rapping. It is performed by a DJ, disk jockey, who uses one hand to change the playback speed on a turntable, and the other hand to turn on and off the sound on an audio mixer.”
– explains Kjetil Falkenberg Hansen, from the Speech, Music and Hearing department at the KTH School of Computer Science and Communication of Sveriges Största Tekniska Universitet in Stockholm, who has authored an in-depth study of scratching for his PhD thesis. The acoustics and performance of DJ scratching (an 86 page .pdf comprised of six individual papers) examines not only how scratching artists – also known as turntablists – accomplish their audio artworks, but also scientifically quantifies and analyses their output.
In an experimental study looking at the dynamics of typical scratching performances, test DJ scratchers used the now-classic “ahhh” sample from “ahhh, this stuff is reeeally fresh”, found on the B-side of the track ‘Change the beat’ by Fab 5 Freddie (1982). The DJs scratched their “ahhhs” on a specially constructed rotational measurement system attached to a vinyl record deck [see photo above]
“The acoustic analysis showed that the tones in this music bear little resemblance to the tones of other traditional instruments of Western music. They are very short, they have a particularly unstable pitch with big frequency glides, and sounds with a broad noise spectrum are preferred over periodic sounds. Such tones are not suitable for melodic music …”
Despite its predominantly unmelodious qualities, the studies determined that in the hands of an expert, scratching can nonetheless express anger, happiness, disgust, sadness, coolness, and even romance. Some have suggested, however, that scratching as a musical artform may have reached what could be called the ‘mature’ stage (as one DJ explains : “… there is only so much you can do with a turntable”.)
But according to the thesis, there is still plenty of room to (r)evolve :
“… the prospects of scratching are only limited by the availability of instruments, the DJ’s creativity, and the listener’s openness.”
And thus there are a wealth of further opportunities for future scratching researchers, or as Hansen puts it –
“Studies of DJ scratching have only just started.”