Is there any decibel level to set instruments to in doing a small venue for about 100 people? Our bass and guitars are through the amps. The singers, keyboard and acoustic guitar through a PS system. No monitors.
The appropriate volume levels will often vary according to the venue and the type environment and the type music and the audience. Also there are many things to take into consideration to provide optimal sound distribution to the majority of the audience.
For example, if you are playing to a 100 person venue and there is a dance floor and the 100 people are there to dance, and you are playing dance music, then you would want the overall volume relatively loud.
On the other hand if you were playing at a 100 seat restaurant, and you were playing jazz instrumentals as background music, you want to keep volume levels low so as not to drown out conversation.
The acoustical environment can vary greatly and affect the perceived volume. An acoustically live venue with tile floors and a metal ceiling often necessitate the lowest volume you can get and still keep your instruments and vocals balanced in the mix. A carpeted venue with and acoustical tile ceiling will call for a higher volume setting on your PA and amps. Outdoor venues require a completely different approach as none of the sound is reflected.
One thing to keep in mind is that you are trying to achieve the appropriate volume level for the venue and the setting while at the same time, making adjustments so that electric guitars, bass, drums (if any), acoustic guitars, keys and vocals are appropriately balanced in the mix from where the audience will be sitting.
Remember that low frequency sound such as that coming from the Bass Cabinet will be multi-directional and will be heard throughout the room regardless of where the speaker is pointed. Higher frequencies are more directional and it's important to angle the speakers so the sound will reach the ears of the audience. That means if you are on a high stage and the audience is seated, you might want to tilt the mains on the PA down slightly. Conversely if you are playing in a venue with stadium seating where the audience is looking down at the stage, you may need to angle the mains up.
The height of the main PA speakers can be adjusted if placed on speaker stands and you want to consider how high they should be for the situation. If there will be folks dancing close to the speakers and other folks listening in the back of the room, you will want to raise the speakers slightly above the heads of the dancers so their bodies don't absorb all the sound and prevent it from reaching the audience in the back and also you don't blow out their ear drums.
If your audience is seated, you might try keeping the speakers slightly lower.
TTW was wise to suggest having a person check the levels from around the room. I try to get set up enough ahead of time to do a sound check with one of the other band members who is more experienced than the average non-musician, stand at different parts of the venue in places we expect the audience to be and we bring up the volume of each instrument one at a time and then pull down the volume of any that are too loud in the overall mix when multiple instruments are playing.
When I play with my band - the first song is announced as a "sound check" song and we recruit a few folks we know to listen from various parts of the venue and give us feedback on the overall volume as well as the mix. They might say things such as "not enough acoustic guitar, vocals a tad muddy, lead guitar not cutting through, Bass too loud, more cowbell".
During breaks I always visit with members of the audience and ask them how the mix and volume sound. If I get a consensus that it's too loud, or not loud enough, the appropriate adjustments will be made prior to the next set.
I personally prefer to use stage monitors if the room is not too acoustically live. I can't afford an in ear monitoring system which might be more versatile. By using monitors on stage for the musicians, I can place them in such a way so that all the band members can hear what they need to hear and I can place the PA speakers for optimal sound distribution to the audience. If the musicians and vocalists have to hear the mix through the PA mains, you might have to position them so that the musicians can hear the music they are playing with or singing to (or hear themselves) all while avoiding feedback. Placing the PA mains so they double as monitors without feedback from the mics will often preclude positioning them for optimal sound distribution to the audience. But in some venues, it is best not to use monitors and you don't need them.
Ultimately, your objective is to deliver an enjoyable experience for the audience. In order to do that, you must be sure that as much of the audience as possible hears as much of the music as is appropriate for the situation - and that the mix is balanced so they hear the music the way the band intended to convey it. But it's also important that the band deliver the best performance they are capable of and the performance will suffer if the musicians can't hear what they need to hear at the volume they need to hear it at.
To summarize - each situation may call for different overall volume levels and the acoustic properties and characteristics of the venue itself will dictate what methods must be employed to achieve the appropriate sound levels for the particular audience and situation.
I've played from very large to very small venues. The only way I could set volumes reliably (for solo or ensembles) was to have someone walk around and tell us how things sound. Of course, as the venue fills with people, lots of sound energy is absorbed by clothing (and hair and ear drums ...) so some adjustments may be called for. Groupies and band spouses can be useful.
When setting up, the old stereo rule of thumb is useful. A speaker (or instrument or singer) suspended in air generates a level of loudness X; the same speaker on a floor or the middle of a wall (or ceiling) generates approximately 2X loudness; in a dihedral (wall to floor or two walls meeting) one gets 4X and in a corner about 8X. If you are playing from a corner, the music will be much louder than first expected. Klipsh used to use this in high end speakers (maybe they still do.)
There are some omitted important factors that go into answering this question for yourself:
You want the level to reflect what the audience expects. Teenagers dancing will expect and tolerate louder music than retirees eating brunch.
You want to respect the venue. Bar owners and managers will get annoyed if their bartenders can't hear drink orders over the sound of your band. Venues are also responsible for complying with noise ordinances.
Different genres create different audience and venue expectations. You will probably annoy both of the above categories if you try to play folk-rock at a heavy metal volume or vice-versa.
What the music is being played for also interacts with the above factors. If you're playing at a bar, the ultimate intention for your music is that it will attract more customers to buy more beer. You want it to be loud enough to excite the drinkers and make them dance and get thirsty, but not so loud that bartenders can't work. If you're a worship band, then making sure the people in the back can hear and sing along with the music without deafening the devoted in the front is your balancing act.
So we can't answer your question but it's an important question for you to answer. A dB meter is one of my important live sound tools. You can get one for not too much and use it to calibrate your ears and expectations. In general, you don't want any part of the room to be louder than 120 dB since that can cause pain and rapid permanent hearing loss. I like to get my hard rock bar mixes to be between 100 and 110 dB at mix position and under 100 at the bar, if possible (usually bars are far from the stage in the better laid out bars).