The biggest thing to understand about an Orchestra is that they operate "at a loss".

To have a successful orchestra, you need to be able to have donors, ticket sales, and subscriptions.

Donors make up the most of the monetary substance. Subscriptions come in second, and ticket sales dead last.

On the other end (money out), you need to pay staff, musicians, and such for time to practice, perform, and hone their skills. Being a professional orchestra level musician is a life style more then anything else.

So now you have a large money in, large money out "business" that makes no money itself. Most orchestras struggle just to "survive", but even "thriving", can mean just breaking even.

So, the question becomes, what do you need to be able to play the most varied selection of music? Would you donate $10,000 to an orchestra that couldn't play "Beethoven's 5"? How about one that couldn't play Opening theme from Star-Wars? What about something like Vivaldi's Four Seasons?

Ok so now you have a pretty set list of players you need to pay. You have your strings, your percussion, your brass, and you wood-winds.

Now, be need a Trumpet player. Guess what there's a lot of those so we can afford 4 trumpets at around $70k each. The better ones more, the less skilled less, but averaging around $70k.

We should have some french horns. Hmm there staring around $100k on the low end. Arrg. Ok let's get 1.

Now lets get an oboe player. Lots of those again, They start around $62k. Lets get 4.

You know what. I want to play a piece from Zelda, Link's awakening. I need an ocarina player. Hmm, well theres only 3 of those in the whole world, and the only one that wants a full time gig wants $1,200k a year. Hmmm, let's re-write the ocarina part for clarinet. WOOT $36k a year. Still only need one, but wow was that better then $1,200k.

And so on.

Most pieces call for a similar set of instruments. Pieces that call for "special" instruments can be arranged to not need them. Thus I don't have to pay for a full year just to play 1 piece.

If this year's schedule calls for 30 different pieces, and they all have the same instrumentation, that's great. If 29 pieces have the same instrumentation, and 1 piece needs something special, then maybe I should rethink that one piece.

In the real world, most pieces have mostly the same instrumentation. It's generally close enough to pay for the 4th trumpet to just play the 3rd part with the 3rd instrument on the 4 pieces that only have 3 parts. But an odd/new instrument that's only around for one piece? What are they supposed to do the rest of the time?

This is even more true if the piece the new instrument plays is part of a concert that is only given once or twice a year.

Including this player in the orchestra becomes a waste of money. Better to just contract them out for that one performance. However doing so could be very expensive.

So while an orchestra may not actively avoid having new instruments. It also doesn't make a lot of sense to have a player on the books that isn't going to perform in many pieces. Better to have them as a "guest" then a "member".

That said, most orchestras are more then willing to have "new" instruments that are replacements for old ones. But an electric violin, doesn't sound the same as an "acoustic" one. Sound is everything in music, so....

Would you want to listen to a MIDI keyboard over a Grand Piano?

No, totally new instruments simply wouldn't get enough playtime to be worth the cost, while new version of old instruments are more accepted, they also don't sound the same, and that's important. Combined, it means that orchestra companies tend to favor the same (rigid) set of instruments.

coteyr has already accounted for why it's hard to add permanent new instruments to an orchestra in general. Let me give a slightly different perspective on short-term additions to the orchestra:

As opposed to the United States, orchestra financing is very different in Europe. Once again, orchestras operate "at a loss", but instead of being financed through donations, they are usually financed through taxes. This implies that they just like their American counterparts have to fill their concerts as it's hard to argue why to keep an orchestra if nobody is listening, but they often have a bit more room to wiggle. For instance, there is often some money to hire additional musicians just for an evening even if they are not soloists.

This way, you can actually include new instruments in an orchestra and it is routinely being done for contemporary music: I've personally seen wide arrays of modern instruments and objects used as instruments (such as a saw), most of them percussion instruments (not the saw), added to an orchestra for particular presentations. It's possible and it is being done.

Why are they not added to the permanent staff? Well, coteyr already argued that it's too expensive. I just want to add here some statistics: It only makes sense to add a permanent member to your staff if that person will play a considerable amount of your performances. Let's put the bar really low and say we want to have more than 10% of the performances.

Now, let's have a look at what orchestras play: There are extensive statistics out there: https://bachtrack.com/classical-music-statistics-2015 If you have a look at the infographics, you'll find out that nowhere does the amount of contemporary music played in the 2015th season reach 10%. More than 2/3 of all pieces played by an average orchestra in the Western world are more than one hundred years old. The amount of 20th century music seems large, but remember that it includes people like Sibelius, Strauß and Shostakovich. In other words: The pieces played by your average orchestra are dominated by Beethoven, Mozart and Bach.

Since the current orchestra already contains all instruments to play pieces from 100 years ago, this means that your new addition to the orchestra will never play even 10% of the time even if all contemporary music used it.

Maybe, in a hundred years time, orchestras will play enough pieces that need some new instrument such as a saxophone, but it will take quite long. Even then, there will only ever be slight changes to the orchestra, which will continue to be dominated by strings:

The reason is that at least in Central Europe, compositions are usually payed for by orchestras or orechstra-affiliated institutions. And even if they aren't you need to have an orchestra to play the piece. As I said, it's often possible to add some new instruments to the orchestra, but when you have to pay upwards of five professional musicians for the evening in addition to the orchestra, it gets expensive. This is why - at least in Central Europe - most new compositions either work with (parts of) a classical orchestra and few additions or compose for very small ensembles. Clearly, this limits the possibilities to change the composition of the orchestra.

That doesn't means that other type of music - even for larger ensembles - doesn't exist and that there aren't orchestras to play them. There is for instance a German ensemble that gives performances with instruments invented by Harry Partch only. Those smaller ensembles are often much more likely to experiment with different instruments, so you should listen to them if you like experiments in this direction.

An "orchestra" tends to consist of permanent players covering a broad enough repertoire to keep enough of audience and sponsors interested in keeping the orchestra running. As a result, the vast majority of the played pieces has to involve most of the players. In contrast to smaller ensembles, many pieces are played in the available readily orchestrated version without significant adaption.

As a result, you need core competency in standard instruments for every player, with "stranger" instruments being additional elements of certain players' skill sets.

There are certain half-standard instruments like saxophones and saxhorns and euphoniums and the ophicleides primarily known as instruments used in Berlioz' Symphonie Fantastique these days.

There are other instruments like the natural corno da caccia which have mostly single-piece use these days (like in the "Quoniam tu solus sanctus" bass aria of the Bach B minor mass) even though their repertoire was far wider at their own time.

Other instruments like the cornett or zink (instruments of the brass instrument class but built from hard wood and keratine) are only played by specialist ensembles even though their sound quality is a lot more pliable and pleasant than that of the modern brass instruments having replaced it.

But in a large orchestra, it is important to cover a lot of ground in sound qualities, and in particular cover the spectrum the composers anticipated.

So as a result, the "money skills" of orchestra players tend to constitute a mainline orchestra, with other instruments being left to hired soloists or played as secondary instruments. However, since those secondary instruments are also secondary in hiring decisions, they more often than not are only supported by single players or pairs rather than whole sections.

Ennio Morricone was the first thing that sprang to my mind. He included many new instruments to his orchestral compositions, such as the whistle, harmonica, electric guitar, and so on. Maybe the movie industry is the way to look for these orchestra settings with new interesting instruments. They probably also have the budgets.

The make-up of the orchestra is dictated by the pieces they want to play and therefore by the composer.

If a composer wants to highlight a brilliant new keyboard instrument that allows you to play in any key without retuning he may be moved to write "The Well Tempered Clavier". It all starts with the composer. The composer writes the music that needs a particular set of instruments and the composer will know that he/she has access to strings, woodwind, brass & percussion so can include them without issue. Anything more exotic needs to be considered carefully.

So, the drivers here are:

1. An unknown Composer wanting to maximise the chance of their music getting played by not making it unduly expensive/complicated for an orchestra to do so.

2. The orchestra wanting to play music that people want to pay to hear. There is leeway in a program to sandwich something unknown with a couple of popular pieces but if the new piece needs a lot of unusual instruments as well, that's two strikes and it's less likely to happen.

In order for new instruments to be added permanently to an orchestra, a big enough composer needs to start writing wildly popular music that demands this extra new instrument. So we have two issues here - there aren't any composers working today popular enough to achieve this sort of critical mass. No Mozarts, No Beethovens, No Berlioz'. Also, what credible new instruments could be added into an orchestra that have been invented in the last hundred years and will still be around in a hundred? Zappa (probably the nearest an artist working in the classical field got to "critical mass" since the death of Stravinsky) wrote for the Synclavier 25 years ago - they're already in the "Vintage synth" category and getting hard to find and keep maintained.

I think for a while, saxophone which is a woodwind instrument has been under consideration to be added to the orchestra, normally brass was being used but a few here and there have incorporated sad. You know orchestra music is rigid. It has fixed rules. Then I also watch a school orchestra include two rock guitar soloist in their performance of " carol of the bells "

Euh, il y a beaucoup de questions hautement votées ici qui ne répondent vraiment pas aux questions du PO qui était

Y a-t-il eu de telles inclusions d'instruments modernes ces derniers temps ou est-ce que l'ensemble n'a pas changé depuis longtemps déjà l'heure?

La réponse est oui. L'orchestre standard est fondamentalement défini, et ce sont de bonnes raisons pour lesquelles les orchestres tardent à étendre leurs instruments, mais il y a au moins un instrument moderne qui l'a fait. Je vous présente le Steelpan.

Le steelpan avait essentiellement son style et sa construction établis au milieu du 20e siècle. Dans mon pays, il y a des orchestres qui ne jouent rien d'autre, employant au moins un millier de personnes en saison et des centaines à plein temps. Il y a des gens qui étudient les casseroles en acier à plein temps dans les universités locales et régionales. Voir UWI Panoridim Steel Orchestra. Des gens du monde entier viennent aussi à Trinité-et-Tobago pour jouer chaque année dans Panorama.

Je ne trouve pas d'enregistrements de musique classique sur youtube mais j'ai personnellement assisté à plusieurs spectacles à orientation classique. Voici un gars qui joue du Bach.

Le steelpan et d'autres instruments ne font peut-être pas partie de l'orchestre standard, mais pour ne rien dire d'autre que les trucs européens sont joués dans des orchestres simplement ce n'est pas vrai. Je chercherai des exemples d'orchestres américains et autres utilisant un plateau en acier dans le cadre de leurs percussions. Encore une fois pas standard mais pas complètement absent non plus.

The most recent addition in the non-percussion area looks like the Saxophone, invented in 1840, but not introduced into orchestral music until the 1930s by the likes of Ravel, Prokofiev, Rachmaninoff and others.

In the percussion section, the Drum Kit was not introduced into the orchestral section until about the same time, although its history goes back into the 19th century. The Queensland Symphony Orchestra, for one, regularly uses a Drum Kit -- and a bass guitar sits near it when used!

And the Marimba, although an old instrument in Africa and South America, did not have a chromatic form suitable for orchestral use until the turn of the 20th century.

You could look at it this way.

If you run a full-time orchestra then what you do is perform concerts. Most orchestras will perform a wide range of repertoire and the majority of that will require just the standard range of instruments within the four groups.

For any other "unusual" requirements you will hire professionals on a one-off basis because for the majority of what you play they are not needed. So when you need a zither or an Ondes Martenot you hire one in just for that performance. Until a time comes when there is so much repertoire calling for these instruments that you need one at almost every concert they will remain on the sidelines I think.

Les orchestres peuvent varier et varieront leur instrumentation selon les besoins pour jouer la musique qu'ils choisissent de jouer ou sont invités par leur direction à jouer. Si vous voulez un exemple relativement courant, vous pouvez régulièrement voir des groupes de pop et de rock embaucher des orchestres bien connus pour des concerts d'occasions spéciales. Pour un exemple moins trivial, voir les symphonies pour orchestre et combo jazz de Wynton Marsalis.

La raison pour laquelle l'instrumentation de l'orchestre symphonique reste si standardisée est simplement que les orchestres sont en mesure de vendre des billets et de satisfaire leurs donateurs plus efficacement en collant au répertoire orchestral standard qui n'a pas changé sensiblement au cours des cent dernières années environ, et ce répertoire repose, pour la plupart, sur l'instrumentation standard.