Auditions are a part of musician's life so, as a violinist, expect to audition fairly regularly. Depending on the culture of a class or symphony, auditions are required if you desire to ascend in the fixed-chair line-up or if you're wanting to "challenge a chair" position. Then, there are the auditions held for ensemble groups, orchestras, bands, symphonies and - of course - music school auditions.
Any composition you learn and master is potential fodder for auditions. That being said, there are certainly special considerations when choosing a particular piece for your violin audition repertoire.
Many groups have already selected a range of violin compositions to be performed by all of the candidates. These may be used for pre-screening - often sent online via audio files - or for actual audition purposes.
Thus, it's imperative you review these pieces first. If no familiar or favorite compositions jump out at you, review the list with your violin teacher so s/he can help you find the piece that best demonstrates your ability and musical expression.
Failure to follow instructions means an automatic disqualification, and you certainly don't want to be thrown out, before you've even finished, for lack of reading the fine print.
It's easy to automatically want to play the "most challenging" piece (or the easiest one) on the list, in order to impress the judges by your technique and/or ability. However, musicality - your ability to truly embody and express the piece - is so critical when performing a mandatory audition piece.
Judges are going to listen to the same violin concerto or solo over and over again, which means (assuming you're technically adept) your ability to connect physically and emotionally with the piece will shine through rote note-by-note accuracy.
Don't stop with the title and composer. Sometimes, judges or audition members will throw in questions or queries you wouldn't have expected. The more you learn about the piece, it's history, its inspiration, etc., the better prepared you'll be. Even if you aren't asked a single thing, this knowledge will foster a deeper connection to the piece, which enhances your expression.
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The deeper your knowledge, the more you'll impress a judge or board member should s/he ask.
Fortunately, music schools are becoming increasingly liberal in their willingness to appreciate and support more modern or atypical violin pieces. This may give you more freedom of choice for your personal audition piece.
Of course, it's always best to learn a bit about the culture and/or preferences of the group your auditioning for to determine whether your first-choice piece is something they'll appreciate. If you're auditioning for a symphony that is known for performing pop pieces, for example, you'll have a greater range of freedom here than with one that is staunchly classical in its culture.
As a more intermediate violin student, it may be difficult to find a modern or non-standard classical piece that is acceptable for the audition and that demonstrates your proficiency. Again, your violin teacher will be your greatest ally in selecting a violin composition that reflects you and is worthy of the audition forum.
This piece will be the ideal merger of the musicality mentioned above, along with your own personal flavor and musical attributes.
Beginning to intermediate-level musicians often make the mistake of becoming so focused on their audition piece(s) they neglect the technical basics during rehearsals. Almost all auditions require violinists to demonstrate scale proficiency - ranging from "all the major scales," to a select, specific few, and may also include etudes or arpeggios.
Some auditions may require you to show and speak about bowing and/or related violin techniques, so including these aspects into audition practices will pay off when you're nervous.
Read every single line of the information provided about your audition so you're completely prepared and have practiced every item on their list. Nerves can do funny things to even the most talented musicians, so the more prepared you are for every potential scenario - the better.
Finally, never forget that, while the sounds and compositions that emanate from your violin are the top priority, appearance matters too. Dress and groom with respect for the auditioners to enhance the overall impression you make at any audition your musical career presents.