How expensive are busses?

How computationally expensive are audio busses? I know, the general answer is “it depends on the plugins,” but what I’m really asking is how much computational overhead there is for a (relatively) empty audio bus.

As mentioned in another post on this forum, I’m building an ardour project template for virtual orchestral scoring. I currently have the following, with an elaborate routing scheme (feel free to substitute “insane” for “elaborate”), generally routed from top to bottom :

  • Dozens of midi tracks, one for each orchestral instrument/articulation
  • 5 “sampler” busses, each having a linuxsampler plugin instance with up to 16 midi input channels and up to 32 audio output channels
  • Dozens of audio busses representing orchestral sections (to control panning, stereo width, and gain for each section)
  • A handful of audio busses, each having an early reflection reverb plugin instance (to give forward stage presence to woodwind and string sections)
  • A single bus with a concert hall reverb plugin

I keep wondering if all of these busses are going to result in way too much load on the CPU. So far I haven’t seen any evidence of that happening; testing on a rather commodity machine with a quad-core i5 processor, my system monitor reports very low usage of each core. I haven’t filled all of my midi tracks with content, so it might just be that there isn’t much work happening yet. I still have a number of things that I would like to do with this template, but before I put more time into it, I want to make sure that it isn’t going to crash and burn.

I can’t speak to he deeper technical reasons, but I’ve certainly never noticed an increase in DSP load after adding an empty bus (although obviously it uses CPU once I add a reverb).

Thanks adotm, that matches my experience. I should clarify that I’m really only using a small number of plugins, even though I have a proliferation of busses. My project currently has six instances of Invada Early Reflection Reverb, which fortunately seems to use very little processing power, and one instance of TAP reverb set to the “Medium Hall” preset, which might take a little more CPU than Invada, but still isn’t too bad since there’s only one instance. Other than that, it’s just the five instances of Linuxsampler, which I also hasn’t given me any trouble.

All of my other busses are just for panning, stereo width, gain, along with sends and routing to give each section its own stage presence. All of which feel like they shouldn’t be too expensive because they’re built in to Ardour, but I wanted to check and see if this kind of usage is really as computationally inexpensive as I think it is.