This is an explanation for our potential clients why they need more than one conference simultaneous interpreter per language for their conference.
Simultaneous interpretation is correctly considered the most strenuous linguistic exercise. The ability to listen to the original text and render it in another language does not come naturally: it is a skill that needs to be learned and constantly honed.
Our ability for intense concentration is limited even if you have proper skills and training.
In professional simultaneous interpretation conference interpreters work in pairs and alternate every 30 minutes. It is a standard in our profession and there are natural reasons for it.
It has been found empirically (since the inception of simultaneous interpretation in the 1920s) that after about 30 minutes your brain begins to play an interesting trick: you continue to interpret and you may even be thinking that you are doing all right.
Imagine quite justified frustration of the client who will be getting this “product!”
Another role that the off duty interpreter plays is helping the active interpreter. Looking up terminology, jotting down digits and numbers, passing notes from audio technicians, monitoring sound quality, opening a needed document on a correct page are just a few of the tasks. Interpreters work in tandem, and it is our professional obligation to support each other as needed.
During very difficult texts (the presenter has neck breaking speed of delivery, accent, hardcore terminology is used) interpreters may even choose to alternate every 15 minutes.
At high level international conferences 3 interpreters per booth may be engaged.
Caveat emptor: beware of any individual interpreters or companies that will agree to offer you one simultaneous interpreter for an entire day or even for a few hours. The results may be abysmal. Tested on humans!