Previews routinely describe an intimidating atmosphere without saying where it comes from. A large part of it is set weeks earlier by ticket allocation rules that have nothing to do with the fixture.
Visiting support is capped by rule
Competition regulations typically entitle the away club to a minimum share of capacity, and the host club is generally not obliged to provide more than that.
The allocated section is placed where segregation is manageable, usually a corner or an upper tier, rather than where the acoustics are best.
A traveling support of a few thousand in a large stadium can still be clearly audible, because it is concentrated in one block rather than spread through the crowd.
European ties follow a different standard
Continental competitions set their own away allocation requirements, which are often larger than the domestic norm for the same stadium.
That is why a European night can sound different from a league fixture at the same venue, even with an identical total attendance.
Clubs occasionally reduce allocations further for security reasons, and those decisions are made by authorities in the host city rather than by either club.
Home atmosphere depends on layout, not just numbers
Stadiums with steep single-tier stands behind a goal concentrate the loudest supporters together, which produces sustained noise rather than intermittent reaction.
Bowls with a running track or wide corporate tiers separate the crowd from the field and disperse the sound, which is why some large stadiums feel quiet.
Safe standing sections, where permitted, keep supporters in the same place throughout and materially change how a stand sounds during a match.
Ticketing policy changes who attends
Clubs that allocate heavily to season ticket holders get a consistent crowd, while clubs selling more single tickets get a more varied and generally quieter one.
High-demand fixtures against major opponents attract occasional attendees, which is why a stadium can be fuller and less noisy for a big match than for a routine one.
Kickoff time matters as well, since a late midweek start reduces attendance from supporters traveling any distance on a work night.
What a preview should actually say
Claiming a hostile atmosphere is only meaningful with the specifics: the size of the away allocation, the stand it sits in and whether the home support is concentrated.
Those details are knowable in advance, and they support the claim in a way that repeating a stadium's reputation does not.
Reputations also lag reality by years, since a stand can be rebuilt, a supporter group can relocate and a ticketing policy can change without the descriptions used in previews being updated at all.