I love the look of slots, but some of those are too small.
I hear some people complain about audio quality in arcade cabinets... This is why. Those slots are clean and nice, but have a detrimental effect on the sound. Blocks the front radiating sound from exiting properly. This will change frequency response and the speakers ability to accurately reproduce sounds. It may not matter for bleeps and bloops, but can assure you, it can sound better. There is actually a portion or audio community who uses slot loading to their advantage, but they do so intentionally with an ability to get certain results. Bigger slots are better, and you want to restrict as little as possible... So go with thick slots with more than 50% sound travel allowed. The more open the better, but figure most will have to balance aesthetics with these principles. I've never tested this in an arcade cabinet, but have in HiFi, and was always something that stood at back of my mind regarding cab builds...
... and I like slots. Not hating. Just throwing out ideas.
When you say 50%, do you mean that for your speaker size you effectively want to make sure that at least 50% of the speaker is showing. So either with wider slots or more of them
Speakers are black magic. They can "just work" or you can get them to sound good.
I like that you chose to do audio the hard way, and not just slap a PC speaker in there... not that there is anything wrong with that.
Ideally, we don't want to block any sound. If we block it, we stop sound from traveling unimpeded. Blocking the front sound wave of a speaker can be good or bad. The speaker is a piston making pressure, when you have a small opening, the pressure builds up and affects the speed of the air that leaves the opening. From an audiophile standpoint, this is bad if we are working with speakers that already have an inherently good sound. One time we use this is when we want to shape the sound of the speaker, as we do with bass woofers in open baffles... It's a big read, but if you google "Nelson Pass Slot Loaded Woofer" you will find a white paper on his research. He's the man when it comes to audio. Basically bass speakers in a box is what you're all used to. Bass outside a box is tricky, sounds different, and requires different approaches.
Now the purpose of slot loading is to get better bass. What makes Mr. Pass' example work, is that he is trying to boost low frequency. While this sounds good to us, we have to remember that most of us use a coaxial speaker (high and low together). So blocking the speaker 50% may help us get a bit better bass (minimal when talking about 3-4" speakers) but at the price of blocking out the high frequency of the tweeter. In an arcade cabinet, this seems counterintuitive. That said, anyone adding a mild subwoofer (won't be hammering 20z) to their cabinet could use this principle to their advantage - not needing to make a box inside another box. But that is for another day.
The reason car speakers work so well in arcade cabs is that they are 1) cheap, 2) small, 3) sensitive (4ohm vs 8 ohm, another whole lesson) and 4) they are found in doors, dashes and rear decks - big leaky enclosures. You will notice that some car speakers have a high QTS spec under the Thiele-Small parameters. What makes a high QTS speaker good is that they have a somewhat exaggerated bass response, which means that when put in a big leaky box (which hurts bass), the speaker will still have good bass response. So it's not to say that 50% of your opening is wrong. If thats what looks good and you're happy with it, then no problem. Depending on your speaker, it may sound better, it may not.
Your ears will have to be the judge, there is not right or wrong way to do it... just a compromise is usually required. Nintendo Cabs have them sexy slotted speaker grilles that I love to look at, but the audio guy in me is whispering in my ear "that better be bass only"
If you want a cab that will have great sound, it's not necessarily rocket science, but it's not just a matter of throwing a speaker in a hole. Caution : It has occurred to me in writing this that some games may have been designed or modified to accommodate a speaker mounted in such a way. Could explain why some games are so high pitched - they expected those sounds to be attenuated by speaker grill. Or I am just overanalyzing it... totally possible, but felt the need to caution you
I am working on an audio tutorial for arcade cabinets that I hope can help folks in picking audio parts etc for their cabs. I cannot say when it will be finished, but hope its something I can contribute to the community.
Feel free to PM with more questions/clarifications, as it's possible I just made this worse