Buzzer between free-throws

Question

In the NBA, I regularly notice a short buzzer sound played between the first and the second free-throw. What's the reason for this?

Answer 1

Buzzer is to note that subs are coming in.

Buzzer can be used for any substitution. I can not find any referee notes for the NBA regarding in-between free throws (probably because they would be in the notes for person running clock).

Anecdotally I refereed since I was 11 years old. The scorer's table will hit the buzzer to alert the referee OR players for any substitution. Meaning if I call a foul and at the scorer's table, I might motion the substitution in before I get set and before the rotating referee in-bounds the ball.

The scorer's table can hit the buzzer there. They usually don't (pee-wee through small time college).

The most common reason the scorer's table will hit buzzer for subs are:

1. Referee missed seeing the subs and about to start play. For sure the number one reason outside the NBA. There are some gyms where kids are on a knee in front of their bench (not scorer's table) so it is hard to infer they are coming in. Been in so many gyms (even small colleges) where I was constantly asking players if they were subbing - "no talking to coach" so you tend to stop looking and wait to hear. The scorer's table if working properly will buzz the referee so they stop and motion subs in.
2. Players not paying attention. Referee motions for subs and players just sit there. Little buzz gets them going.
3. And in some gyms they do the automatic 2nd free throw buzz. Maybe 1 out of 5 gyms in my area. I think this is just to get things moving faster. Ref and player hears the buzz, player runs in, other player runs off. With no buzz then ref holds up hand walks in the vicinity of subs and motions them in, given they are paying attention. It is faster for sure. But we are talking about maybe shaving off 3-4 seconds if that. I can see the NBA doing it because that is for sure the 2nd worst part of watching an NBA game is the 60 seconds for 2 free throws (the worst is the 10 timeouts in the last two minutes).
4. Some people running the clock just love to hit the buzzer. I mean seriously I will work a game where I hear the buzzer at the end of each quarter and that's it. The next game the guy hits it 40 times.
5. This goes along with number 4 and not applicable to the NBA. Some smaller gyms have bad buzzers. Some are excruciatingly loud and some sound low or weird. So the scorer's table might refrain from using them unless they have to. Those gyms you will get the "hey ref" instead of the buzzer. Note that the buzzer is much more efficient as some gyms I might not hear a "hey ref" even if yelled from 20 feet (ref's are also conditioned to not listen to any phrase from the stands with the word "ref").
6. For specifically the NBA I think the buzzer serves two purposes - make things quicker and alert fans.

Also in case you have another follow-up. In a two free throw situation you only can sub on second free throw, three free throw only on third, and one free throw or a 1 and 1 you can sub before the first. This is solely so there is only one set of subs per stop in play. If you are a new referee and let in subs on the first free throw - it is very natural to do it when you are giving scorer's table the number of the player committing the foul and subs are 3 feet from you - and then the coach decides to put in another sub on the second free throw... you will get majorly dinged in any critique if someone is watching your game.