BillFitzmaurice.info

Curious what a direct A/B comparison would reveal, so I've devised a plan to make a removable spacer ring and slotted baffle for one of my direct radiators (Berg EX112) and measure frequency response and power handling. I'm expecting to see a shift lower in frequency response, a possible small increase in mids, and a drop off starting at 800 Hz. due to the slot acting as an acoustic low pass filter. Slot loading is used extensively (along with horn loading) in Bill's plans, so I thought it would be interesting to isolate this one variable and measure it. Testing should be complete within a week or so. Any thoughts?

(oops, just noticed, I'll have to lop of the one corner to unblock the port )

Slot loading can be modeled in WinISD Alpha Pro as a 6th order bandpass.

Thanks Bill. I just googled WinISD Alpha Pro looking for something to download. It's all for Windows, is there anything for a Mac? I'm a bit of a putz with computers, which is why I kind of prefer the empirical approach.

Well, found out slot loading by itself reduces the low end by a small amount, adds a 4 to 5 dB narrow bump at 500 Hz and rolls off the response steeply at 800 Hz.

Since I was set up, I also decided to make the slot half size (2"x9") for contrast and possible confirmation of the previous test, which it pretty much confirmed.

SPL Slot Loading Test, Chart


A few more pictures.

I find your testing and building very interesting. It's an added bonus to this site, as far as I'm concerned, because I/we get to learn so much more about 'sound'.

Thanks for the kind words tammojsmit. Visiting this forum has been a big learning experience for me as well.

My only disappointment is I haven't been able to use this yet.

tammojsmit wrote:I find your testing and building very interesting. It's an added bonus to this site, as far as I'm concerned, because I/we get to learn so much more about 'sound'.

Yeah. What he said.

Is that what Trace Elliot did with this amp? (BLX-80)

WB wrote:Well, found out slot loading by itself reduces the low end by a small amount, adds a 4 to 5 dB narrow bump at 500 Hz and rolls off the response steeply at 800 Hz.

As with any alignment the result will vary with the size of the slot and the specs of the driver, so this one example is not going to be universally representative.

Your steep notches look like Helmholtz resonances inside the cavity.
What is surprising is the lack of change below 100 Hz.
My understanding of mass loading is the increased effective mass drops the Fs and extends the bass response.

bgavin wrote: My understanding of mass loading is the increased effective mass drops the Fs and extends the bass response.

It does if the rear chamber is large enough to take advantage of it. Seeing what the slot does via an SPL measurement is only half the story. When I conducted similar experiments a decade or so ago I concentrated on the effect on Fs.