Why you should enlarge the cabinet hole of your pinball machine.

Copyright (c) 2006 by TheKorn
Last Updated: January 15, 2006




So it seems to come up about once a month where someone on RGP is doing a speaker modification to their pinball machine (that's good!) and is trying to decide whether or not he should take the plunge and enlarge his cabinet hole.

Invariably, some dumb yahoo who doesn't know sh*t from shinola comes along and proclaims that he is both the second coming of Christ, and oh by the way he also has the answer to your question. (I find it comical that the answer is, more often than not, entirely wrong!)

It's time to end the argument, debate, or whatever you want to call it. And we're going to do it with science, not some hearsay that some jackass heard second hand from his mother's hairdresser's cousin's... Well, you get the idea. :)



TEST SUBJECT:

Williams Rollergames, (c) 1990
System 11C
Roughly 7-10w of available power


Rollergames was my second pinball machine, and the first where I tried experimenting to see if I could get the machine to put out better sound. Like many people out there (and since I was "new" to this whole pinball "thing"), I was reticent to do anything to the machine that would radically alter it.

Up to that point, to my knowledge nobody had put any energy into "juicing up" speakers on pinball machines. This was before that guy started selling speaker and subwoofer kits. So as far as I knew, I was in brand new uncharted territory. And since I wasn't sure at that point if a pinball machine would be worthy of good equipment, I decided to go the "two buck chuck" route of improving the sound. (Remember, this is way back around 1997 or 1998!)

So I started rummaging around in my junk pile of stuff I had pulled out of the boomin' crappy Jeep. I put in a two-way crossover inside the game for the panel speakers (so that the mid frequencies went to the woofer on the left, and the high frequencies to the whizzer on the right), and a subwoofer with choke coil in the cabinet. At that point, I did not cut the cabinet. I simply placed the new speaker over the cutout for the old one, and moved the mount points for the subwoofer.


Subwoofer and choke installed. Mmmmm, the sound was so Realistic!


Subwoofer removed to show original hole still there.



So, the subwoofer and crossovers went in. And they helped. A *lot*. And I was happy! For under $30 all told, they made a marvelous change to the sound. Definitely the "two buck chuck" of audio solutions, but simply put I wasn't in the mood for fine wine!

And the years went by. I did other audio projects such as the Booming Black Knight 2000, the Next Generation Ster Trek, and the Crazy Centaur. (Check RGP if you want to read about all those!)

But I always had this nag in the back of my mind. I knew that I had broken several rules of speaker building in Rollergames by putting that woofer over a too-small hole. (There are several other rules of speaker building that were violated, but I couldn't do anything about those. :) )

So, one day I set out to rectify the situation and revisit my love, Rollergames. But I also wanted to settle, once and for all the debate about whether one "should" cut the cabinet hole larger or not. And to do that, I was going to need science.



TEST METHODOLOGY
The Control

Since I had always wanted one, this was a good excuse to buy a new toy. I bought a digital SPL meter so that I could obtain hard data. Nothing too elaborate, I paid somewhere between $50 and $100. A decent meter, but more importantly, I could mount it to my camera tripod!


Meter mounted to tripod


Here are the steps taken for each test:

As a control, I measured the room's ambient noise. As anyone who has been to my house before can attest, my house is built like a bomb shelter. So it was with little surprise that across five runs, the ambient noise in the room (with all games off) was measured at 25.9 dBA. (no AC, no heat, holding breath, 10 second sample time... :) )

Then I ran the tests on Rollergames, with a stock cabinet hole to get a 'baseline'. (i.e. cut for the 6" woofer that normally comes with the game.) The result was that the cabinet speaker was putting out 76.1 dBA.



The Modification

Now for the fun part! Grab some carboard, make a template, cut it out...


Template made...


Hold it up, trace it out...


All done! Now we can let that Realistic fury fly!

Keep in mind, I didn't touch the volume control between the first set of tests and this set. In other words, the excat same amount of power was being put through the exact same woofer. The only thing changed was the size of the hole!

The result? The cabinet woofer was now producing 82.6 dBA of sound. A net gain of 6.5 dB!

For every 3 dB of measured power, twice the accoustical energy is being output by the driver. So, in short, four times the accoustical power is being output by the woofer! And that's without using any more power than it was before. Quite literally, I was getting four times the power "for free". (This does not mean it was twice as loud... more later.)



BEYOND THE CONTROL!

As I was preparing to dismantle the experiment, for giggles I turned up the volume. Much to my absolute astonishment, the clipping point had changed! Up to this point, I had made the blatent assumption that it was the amplifier that was running out of steam and causing the clipping that I had heard while setting the original volume level. Turns out, I couldn't have been more wrong.

It turns out that the amplifier was not clipping, the speaker was distorting. In other words, the undersized hole was restricting the airflow enough in front of the woofer that it was causing the sound to distort. Needless to say, I knew that I had to take a new set of measurements, to see where the "clipping point" now was!

So I readjusted the volume so that it was as loud as possible with clipping (from any source) inaudible. ( <3% THD) Again I ran five tests and averaged the results: 87.5 dBA!!! Remember, this is without adding any amplifiers, using the same speaker!

Let's sum up the results here... Remember these results are all using the same speaker with the built-in amplifier, the only change was the size of the speaker hole, and in the last test, the amount of power fed to the woofer without clipping:


MesurementA-weighted Maximum
Room ambient
25.9
Original cabinet hole
76.1
Enlarged hole
(same power)
82.6
Enlarged hole
(new max power)
87.5


Summary

I would think that, by now, the results should be obvious. Should you cut the cabinet? BY ALL MEANS YES!

Remember when I said that 6 dB, despite putting out four times the power would not sound twice as loud? It's generally accepted that an increase of 10 dB sounds twice as loud. Obviously, an increase of 11.4 would sound slightly louder than that.



So I think the results speak for themselves. Do yourself a favor and cut that cabinet!!