You would want it loud enough for the back seat people to be "entertained" but not over powering for those up front.
Initial research with pink noise through the non-sync seems to be around 70 db, maybe even a bit lower.
Thanks
Paul
With "normal conversation" being about 60 decibels, you seem to be in the ballpark.
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
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As with the movie sound, non-sync music needs to be played louder for a full house than for a weekday matinee show.

)
Just my .02 cents worth.
The following may be of interest to anyone using Dolby CP-45s (or any other similar Dolby processor)...
I set the Non-Sync levels in my CP-45's as though I am setting Dolby tone. Here's what I do:
In your music distribution system...
1. Set the input levels for Left and Right. The input levels in my Smart DA-226 are about 3/4 of the maximum.
2. Set the Left and Right output levels for each cinema. Mine are about 1/2 way. They are all set to about the same level for uniformity.
The following should be done with ALL of your cinema processors at their usual fader settings (i.e. "7" or whatever volume setting you've settled on for film playback).
In the first cinema processor...
1. Playback a sample selection and set a preliminary comfortable listening level using the Non-Sync input level pots inside the processor...You will have to set this by ear; Non-Sync volume is really a judgement call.
2. Play back a 1kHz tone on a CD and look at the level LEDs on the Cat. 222 card.
3. Balance them so that Left and Right signals are at equal loudness. Double-check this by ensuring that the LED for the centre channel is brightest on the front panel. (When the Left and Right non-sync signals are balanced, the centre channel LED should be the only one lit.)
4. Since you've been tweaking your input levels, you will have to play your sample music track again and re-verify the playback level in the theatre.
5. If necessary, repeat steps 2 thru 4 until you are satisfied with the result in the theatre.
6. Play the 1kHz tone again and note the LED pattern on the Cat. 222 card. For example, my particular tone should playback with only the lowest red lights illuminated (Left and Right).
For the remaining auditoria:
1. Leave the 1kHz tone playing (step 7, above).
2. Move on to each processor and set the Left and Right levels so that all processors are uniform (copy step 2 & 3, above).
3. Play the sample music selection again and verify a comfortable listening level in each auditorium.
If your cinemas are roughly the same size, then this should give you pretty much the same result in each room.
If there are much larger or much smaller cinemas then you may want to do this by establishing an SPL with a pink noise recording, but I think that's going overboard for Non-Sync music...then again, some people think my suggestion is overkill already.
Oh...I don't use the "matrix" setting. If you set the jumper to the "normal" position then your Left + Left Surround loudspeakers are used for the Left signal and Right + Right Surrounds are used for the Right Signal. It just sounds better to my ear.
This may seem like a lot to go through but, really, you only have to do it once. Right?
~Manny (The Champion of Value.)
Josh
It becomes a delicate balancing act of not making it "too loud" for the patrons, while also making it "loud enough" that the sponsors don't complain.
Even though it is true that I was not at the movies to hear the music program, I have been to theatres where I thought the Non-Sync was "too quiet." They were playing a song that I liked and I couldn't really hear it. And, besides, anything would have been better than hearing the conversation coming from two rows behind me.
Personally, I think it's important to know - once and for all - what your Non-Sync system is putting out there. The exact manner in which one decides to go about doing it is another matter.
Thanks. As always, you confirm my suspicions.
Paul.
Depending on the recording level of the Pink Noise, that CD may or may not be very helpful.
To your ear (or VU Meter) is the noise about the same as a typical CD you'd be playing?
"People don't come to theatres, stores etc. to be entertained by the walk-in music."
I agree----but tell those companys that specialize in charging fees for any kind of background music. It will fall on deaf ears. One of those companies flipped us crap once because we were playing cassette tapes for walk-in music in one of our theatres. They were babbling about royalty fees or some happy HS like that. They wanted us to play their garbage and charge us money for it.
So, we bought a radio instead, and tied that in for a non-sync source. That made them equally mad. The radio station didn't care - in fact, they liked it! It was the station I do engineering work for..
They have already flooded the Restaurant Industry with their hogwash.
A friend of mine who owns a restaurant told me that one of these companies threatened a lawsuit on him for playing cassette tapes in his establishment, even though they were his own personal tapes he purchased from a music store.
Anyway, with the exception of one theatre that uses one of the background music companys, we use audio screen ads with DVD and LED projectors, and we get paid to do it. That sure beats paying a fee to a company that makes a walk-in music CD with some RAP music selections.... 

With TRN, they also said to play it at "foreground" level. We usually just left well enough alone with faders and such and everything was fine. I always found this line in the memo funny: "Under no circumstances is your TRN CD to be played at background levels!" (or it will explode?
)
We would also occasionally play non-sync themed to whichever movie we were doing a big promotion for. Usually we would ask the studio if we could play the film's soundtrack CD (if available) and they would be more than happy to send us a whole box of them to use as giveaways or whatever.
IMHO, non-sync should be all instrumental. 
=TMP=
When I went in there one night to run movies, I heard that crap and astonishingly asked myself, "What the F***???" It is light rap without the obsenities, but it is still RAP music.
I would sooner listen to someone farting in the bathtub than listen to that stuff. I think it would be more entertaining.
True in most places -- not ours. We program a 20-25 min Preshow Concert, mostly movie scores and short classical pieces, much of it requested by our patrons (some even bring in their CDs). The program is an integral part of the whole show. It ends precisely as the WB cartoon logo hits the opening curtains w/warmer lights dimming. People come specifically to hear our preshow music concerts.
We run the CD L & R output through a Sansui Studio VarioMatrix Type II Decoder (used years ago in recording studios for Quadraphonic playback). It gives us Lf, Rf, Lr, Rr output (and I derive a L+R sum for the center channel to eliminate the center hole). Most music sources, even if they weren't mixed for quad, have plenty of ambient surround information so that the whole theatre is enveloped in the music. Our patrons love it. And they can still fart away and roll their bottles to the front (thanks for that imagery, Bill)
As for the legal BS, yes, ASCAP and BMI want their pound of flesh. Going with prepackaged services USUALLY relieves the theatre from any licensing burden, but not always. Instead of hassling with them -- they have pitbull lawyers who, if they decide to make an issue of it, will sink their teeth into you and won't let go until you are up to your eyeballs in legal fees and harassment with a capital H. We decided to just give in and pay the damn yearly blanket license fees. That allows us to play just about any music we want (between the two licensing orgs, they represent 90% of all recorded music out there) without even thinking about any royalty issues. The yearly fee isn't too bad, it is based on the number of seats in the theatre. But beware, these scum sucking parasites like to get paid two and three times for the same exhibition. We found out that they were going to many of the artists we book for our live shows, demanding that THEY pay a fee to perform any song covered by their rosters. They even tried to get a fee from the musicians playing for the same artist (musicians local flipped them the bird on that one). So they were getting paid a licensing fee from the artist as well as from us for the playing the very same song. I suppose we could fight it, but the Center decided it would cost us more than just eating the fee, besides, it was not my call. I think if I owned the theatre, I would fight it just on principal.
As for level....we set the non-sync the same as for the film -- full, robust, rattle your fillings and massage your prostate -- tactile sound level; after all what kind of hype would it be if on the lobby card you proclaim that the audience will hear a Preshow Stereo Concert from Digital CD in 6 Channel Total Surround MegaSound, and then play it as background musak? 