An Acoustics Question

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Aitch
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An Acoustics Question

Post by Aitch » Tue Jul 14, 2020 8:59 am

BBC Radio 3 are doing some concerts in Glasgow. They are just a couple of performers in an empty hall. I was under the impression that concert halls were designed so that the acoustics were partly dependent on the audience being present to absorb some of the sound and that live broadcasts/recordings used stereo pairs of mikes to get the best, most realistic sound.

This being so, how will they be dealing with an empty, probably very reflective hall?

For a rock band or whatever where everything is either closely miked or goes straight into the mixing desk I can see it would be no problem, but for basically acoustic instruments/voices? They could take the same approach and close mike the performers, add in reverb etc at the desk (possibly from ambient mikes), but that would give a different sound from the usual stereo pair.

Anyone work in the industry and have an opinion on this? How will they deal with it?

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Re: An Acoustics Question

Post by Martin Y » Tue Jul 14, 2020 9:25 am

Some concert halls have seating that's designed to be almost as sound absorbing as people sitting in them. Other venues seem to be considering loading the seats up with sacks of spuds or whatever.

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Re: An Acoustics Question

Post by Martin Y » Tue Jul 14, 2020 9:38 am

R3 restarted their lunchtime concerts a while ago using the Wigmore Hall just down the road from Broadcasting House. So far as I know they haven't loaded up the seating and it's a fairly bright sounding space, but the bits I've heard have sounded okay to me. So I guess it's do-able if you're careful with how you mic it up.

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Re: An Acoustics Question

Post by dyqik » Tue Jul 14, 2020 10:55 am

Martin Y wrote:
Tue Jul 14, 2020 9:25 am
Some concert halls have seating that's designed to be almost as sound absorbing as people sitting in them. Other venues seem to be considering loading the seats up with sacks of spuds or whatever.
Yeah, more modern halls will have done this, and maybe could have deployable stuff in the ceiling as well to adapt things.

Good mic setup and careful use of ambient mics etc. can transform the recorded sound, anyway. And then you can always fake additional ambience as required.

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Re: An Acoustics Question

Post by Aitch » Tue Jul 14, 2020 12:14 pm

Listening as I type. Sounds OK.
dyqik wrote:
Tue Jul 14, 2020 10:55 am
Yeah, more modern halls will have done this, and maybe could have deployable stuff in the ceiling as well to adapt things.

Good mic setup and careful use of ambient mics etc. can transform the recorded sound, anyway. And then you can always fake additional ambience as required.
I suppose it depends how modern the Glasgow hall is.

As for faking ambience, I suspect a decent sound engineer used to doing classical concerts might be a bit unhappy about having to do that. Then again, I don't know how much of that they do anyway.
Martin Y wrote:
Tue Jul 14, 2020 9:25 am
Some concert halls have seating that's designed to be almost as sound absorbing as people sitting in them. Other venues seem to be considering loading the seats up with sacks of spuds or whatever.
So long as they don't try the South Korean method. Might distract the performers... :o ;)
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Re: An Acoustics Question

Post by shpalman » Tue Jul 14, 2020 12:27 pm

What about house plants?
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Re: An Acoustics Question

Post by Martin_B » Tue Jul 14, 2020 12:43 pm

Sydney Opera House is supposed to be designed to have similar acoustics whether full or empty. Although I thought they did that through being clever with the seat design.
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Re: An Acoustics Question

Post by Bird on a Fire » Tue Jul 14, 2020 10:25 pm

dyqik wrote:
Tue Jul 14, 2020 10:55 am
Martin Y wrote:
Tue Jul 14, 2020 9:25 am
Some concert halls have seating that's designed to be almost as sound absorbing as people sitting in them. Other venues seem to be considering loading the seats up with sacks of spuds or whatever.
Yeah, more modern halls will have done this, and maybe could have deployable stuff in the ceiling as well to adapt things.

Good mic setup and careful use of ambient mics etc. can transform the recorded sound, anyway. And then you can always fake additional ambience as required.
This is probably a stupid question, as I know literally nothing about recording sound.

To what extent can the acoustics of a space be mitigated with tech (especially computery stuff - presumably there's a sound equivalent of photoshop where you can alter levels of different frequencies and stuff)? I'm assuming with echoes/reflections that's quite difficult, because they'll tend to be around the same frequencies as the original music?

But then again you can edit shadows out of photos on a mobile phone these days, and select different modes (including "concert hall") for playing music files, so presumably you could just try to record the instruments etc as drily as possible then add all the effects back in with an app?
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Re: An Acoustics Question

Post by Martin Y » Tue Jul 14, 2020 11:18 pm

Yes, it's a lot easier to add reverberation than try to remove it, so while the ideal would be to record the instruments with just enough natural reverb to give a good sense of the space, the practical method would be to mic the instruments as closely as you can to get a very dry recording and then add a proportion of natural reverb from other mics placed to pick that up, (or add reverb synthetically with whichever reverb processor you like best).

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Re: An Acoustics Question

Post by Bird on a Fire » Tue Jul 14, 2020 11:56 pm

Cool, thanks!
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Re: An Acoustics Question

Post by hakwright » Thu Jul 16, 2020 11:50 am

It's worth adding that, although using different microphone placement (closer mics) will help control an overly bright/reverberant sound for an orchestra playing in an empty concert hall (assuming that seat design or other configurable elements aren't compensating for the lack of sound-absorbing bodies) - the players themselves will have a very different musical experience. If there is much more reverb, then that could well affect their ability to judge timings or hear other players, and so their musical performance could well be affected (probably for the worse) - even if the quality of the broadcast sound can be controlled by the different mic placement.

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Re: An Acoustics Question

Post by Aitch » Thu Jul 16, 2020 11:59 am

hakwright wrote:
Thu Jul 16, 2020 11:50 am
It's worth adding that, although using different microphone placement (closer mics) will help control an overly bright/reverberant sound for an orchestra playing in an empty concert hall (assuming that seat design or other configurable elements aren't compensating for the lack of sound-absorbing bodies) - the players themselves will have a very different musical experience. If there is much more reverb, then that could well affect their ability to judge timings or hear other players, and so their musical performance could well be affected (probably for the worse) - even if the quality of the broadcast sound can be controlled by the different mic placement.
Which could explain why the four concerts just have a couple of performers. Or, if not, at least it will help.
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Re: An Acoustics Question

Post by Martin Y » Thu Jul 16, 2020 1:23 pm

I think the main reason for only having solo performances or duets is maintaining distancing for the performers, not just on stage but backstage too.

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Re: An Acoustics Question

Post by basementer » Thu Jul 16, 2020 4:15 pm

Martin Y wrote:
Tue Jul 14, 2020 11:18 pm
Yes, it's a lot easier to add reverberation than try to remove it
Do you know if the latter is regularly attempted? I remember reading an article about cepstrum analysis in the 1980s. One of the applications was separating out echoes in seismic data. Audio DSP has advanced a lot in the decades since then, and on the face of it, it seems a similar problem.
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Re: An Acoustics Question

Post by Martin Y » Thu Jul 16, 2020 5:11 pm

basementer wrote:
Thu Jul 16, 2020 4:15 pm
Martin Y wrote:
Tue Jul 14, 2020 11:18 pm
Yes, it's a lot easier to add reverberation than try to remove it
Do you know if the latter is regularly attempted? I remember reading an article about cepstrum analysis in the 1980s. One of the applications was separating out echoes in seismic data. Audio DSP has advanced a lot in the decades since then, and on the face of it, it seems a similar problem.
It was a hefty understatement - so far as I know nobody tries to remove reverberation in music recording. Goodness knows what capabilities exist in military stuff like sonar processing, but I've not come across any "peace dividend" crossover tech. Perhaps the current generation of smaller "shotgun" directional mics use something like that kind of processing to get their very narrow pattern from a physically short mic body, but I don't know how they work.

Audio DSP has definitely advanced hugely but that's mostly been in the areas of creating a convincing surround or binaural soundfield. That generally involves adding exactly the right reverberation and filtering to believably position a sound source, using not just the relative loudness in each ear (like traditional stereo balancing) but also adjusting time of arrival and adding appropriate echoes and filtering effect of virtual objects (mimicking both the room the recording is supposed to be in and even the shape of the hearer's ears).

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Re: An Acoustics Question

Post by basementer » Thu Jul 16, 2020 6:25 pm

You presumably know about the convolution reverb algorithms that appeared in the 90s, which take the impulse response of a room and apply it to an input signal using the mathematical technique known as convolution. Some, but not all, functions have an inverse under convolution. Cepstrum analysis is all about deconvolution.
I suspect that the fact that in audio, we deal with functions that are finite, continuous, with continuous derivatives, and bandwidth-limited, will mean they are quite tractable. If so it might be possible to record the impulse response of the room at each microphone and generate the inverse function of each one, but it would be accurate only for sounds coming from near to where the original impulse was generated. So maybe you could cancel the reverb if you had a soloist or small ensemble, but not a large group.
I speculate as a (former) mathematician with an interest in sound rather than as a professional sound engineer or designer, obvs.
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Re: An Acoustics Question

Post by shpalman » Thu Jul 16, 2020 6:29 pm

Deconvolution dereverberation
Many live venue sound enhancement devices work by deconvolving microphone signals from the room impulse response in real time, to make it possible for live sound engineers to “dry out” their live presentations on the fly.
So that's apparently a thing, as long as you have the impulse response of the room.
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Re: An Acoustics Question

Post by dyqik » Thu Jul 16, 2020 6:51 pm

shpalman wrote:
Thu Jul 16, 2020 6:29 pm
Deconvolution dereverberation
Many live venue sound enhancement devices work by deconvolving microphone signals from the room impulse response in real time, to make it possible for live sound engineers to “dry out” their live presentations on the fly.
So that's apparently a thing, as long as you have the impulse response of the room.
Which you just need a clapboard to record during set up.

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Re: An Acoustics Question

Post by Martin Y » Thu Jul 16, 2020 6:53 pm

I wasn't aware of its being used, but it makes a lot of sense that a venue which adds a bit of sound reinforcement would like to be able to "dry out" the sound from the stage before feeding it to their speakers.

It's a long time since my eyes glazed over while trying to read about convolution in DSP but I get the gist of that and deconvolution. I'm cautious about how well it can be made to work in practice when, even if you had a really accurate impulse response for the hall, recorded at exactly the point where the musician sits, the instrument isn't a perfect point source, and it moves, while the higher audio frequencies have wavelengths of only a few cm, which looks like a problem.

It's a bit like active noise cancelling headphones - they are really impressive these days, but I reckon they only need to be really impressive at removing low to mid frequency noise, since the physical mass of the headphones blocks out the high frequencies anyway. Trying to actively cancel the highest frequencies would likely be awfully tricky without precise control of how the transducers were spaced away from your ears, as the phase errors would tend to produce odd comb filtering effects.

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