Given that a "formation musicale" can be translated as a "music group" then I am guessing that a "formation acoustique" could be translated as an "acoustic group" or "acoustic band".

This is pure guesswork but I hope it helps :)

In the ideal analog world, acoustic information is transported from the source to the listener using not- hing but air. Air is the reference carrier of acoustic in- formation, as it induces no inherent distortions, how- ever it does have two serious limitations. First, the


pretty basic information and pretty well understood. However even the best clocks will have long term frequency drift, short term phase noise and a certain amount of random noise. When these short term phase and random noise components are different between the A/D and the D/A, the result is, audible distortions in the repro-

listener of the acoustic formation must be at the same location as the source of the sound. Second, the listener must also be there at the same exact time.
http://www.opticaldisc-systems.com/sepOct2000/ARTICLES/DCA/M...

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Note added at 47 mins (2007-10-02 19:49:59 GMT)
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formation - 6 : an arrangement of a body or group of persons or things in some prescribed manner or for a particular purpose (Webster's)

IAV Engineering is an engineering and technology transfer company working in the acoustics and vibrations fields.
Acoustic Engineering
Acoustic Expertise
Acoustic Filtering
Acoustic Formation
http://iavengineering.com/keywords_e.html

Chladni's work was part of the inspiration for Jules Antoine Lissajous' 1857 creation of a harmonograph, a pen-based device for drawing oscillatory patterns on paper. The harmonograph would later inspire the oscilloscope, a basic instrument of scientific research. In 1891 Margaret Watts Hughes created an 'eidophone' that allows vocal sounds to cause **acoustic formation**, leading to the rendition of voice, chanting and other music.
http://www.newalexandria.org/thinkdirt/crescentbell/teamViz-...

Rosa creates music and music theatre in a small electro/**acoustic formation**. The ensemble is inspired by literature, techno, visual arts, traditional and experimental, classical and popular music.
http://www.rosaensemble.nl/

Whether as a stripped back 3 piece **acoustic formation**, or the fuller 7 piece band, these guys have been wooing audiences around Melbourne since 2006.
http://www.lemaroc.org/videos/author-tomtravelman.html

[acoustic arrangement may also work]
TYPHOON EASYLITE 80W SPEAKER
A no fuss speaker system, the Typhoon Easylite 80W speakers is a simple 2 piece **acoustic arrangement** for every usage. Easy to install and can be use on many devices as long it excepts a 3.5mm stereo
http://www.typhoonline.de/en/kat.php?k=13

“Rick Ball, who designed the architecture of the interior room, gave us the latitude to change ceiling design and side-wall acoustics to make the room more performance-driven,” explains Fullwood, the principal consultant with the firm and the lead audio systems and acoustic design consultant for the project. “From the other side, Danny [Slaughter] gave me a lot of latitude to change speaker layout. One of our philosophies at Design 2020 is to allow the speaker arrangement and the **acoustic arrangement** to go hand in hand.
http://www.midasconsoles.com/assets/pdfs/press-reviews/WESTO...

An angry "It's Alright" was followed by an **acoustic arrangement** of MTM that I hadn't heard but loved.


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Note added at 48 mins (2007-10-02 19:50:30 GMT)
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http://my.execpc.com/~billp61/102502r.html

sugg

Accoustic in this case could refer to accoustic instruments ie instruments with no amplification (eg guitar, violin etc not plugged in to an amplifier or sound system). The term 'accoustic band' for instance implies that the instruments will have no amplification. However these days many players of accoustic instruments choose to plug them in to a sound system. This means that the sound will be louder but the instruments will look as if they are accoustic.
Oh dear this sounds rather convolted so I hope you get my drift!
I have assumed that you are talking about some kind of musical group- after all those in the music business often put on spectacular shows
Hope this helps
PS my refs are a sound engineer/musician who has been in the business for forty years as a guitar & keyboard player!

another possibility

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Note added at 5 hrs (2007-10-03 00:19:25 GMT)
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I'm guessing about your client, but I think something like this might work:
"both onstage and as strolling musicians"
"amplified onstage and also as strolling musicians"


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Note added at 5 hrs (2007-10-03 00:24:08 GMT)
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If as I suppose, these are indeed sometimes strolling musicians, then "acoustic" is not really necessary. The surprise would be if they walked around with amplified instruments. But "strolling acoustic musicians" could also be used - I find it a little more awkward.