Tbf, they don’t have to fill the full stadium. Although the stadium could have an overall capacity of 60,000+, for Birmingham homes games they could easily limit tickets to 40,000.
In the states, Atlanta makes changes to the Mercedes Stadium when the MLS club are at home. As per the video below, you can see they cover the tiers and it still looks good. Birmingham could easily do the same, but with a bit of forward planning, they could still keep a very good atmosphere
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The atmosphere at big open bowls is bad enough but imagine closing an entire top tier.
Having been to some super stadiums in the USA, they have dreadful acoustics for a football chant. A bit like Wembley.