Comment on my Bass Bin

[coolhandjjl]

I have a trace and an RTA on a bass bin, I have this pair I may try to re-appropriate for a set of modified Cornscalas, I plan to bi amp and cross anywhere from 400~600 with Dave's Eliptrac 400 as my top. I'm still not sure when to do a trace, or do an RTA, just a beginner here.

[Chris A]

John,

It depends on what you are trying to achieve. PWK always talked about "dirty curves" with no smoothing at all: if you can post the raw curve with no smoothing, this will tell a great deal about what is happening in the bass bin and your room and your measurement techniques (microphone placement, etc.). The smoothing that your two examples show above is typically what I'd use for graphical EQ (if I used one) and crossover/balancing purposes.

IIRC, the Cornwall typically crosses at a higher frequency--about 800 Hz--than what you mention typically because of the matching of the polars of the bass bin to the polars of the midrange horn at crossover.

Chris

[coolhandjjl]

Trace with no smoothing

[Chris A]

Well, I'd say that you have about a 3500-4000 cubic feet room, one dimension of your room is approximately 24 feet, and perhaps another at 17 feet, - both dimensions are approximately +/- a couple of feet.

I really can't tell if you have the Cornscalas in the corners, but I'd guess not. Where did you take the measurement at with the microphone? Was is just in front of the speaker or at your listening position? Both speakers going at once, or one speaker only?

Chris

[coolhandjjl]

This is only one speaker, up on a 18" stand for measurement purposes. Not placed in listening position.

It's in the middle of my studio, a 10,000 cu ft, semi-partioned room into approx two equal halves. I didn't get even close to 100 dB, as it was getting loud, and I have no hearing protection. Mic was about 6' from bass bin.

They are a PA style bin with two Eminence 3012LF drivers in a semi horn loaded vented box of approx 10~11,000 sq in. I am trying to see if it has potential to be part of a hi-fi system should I add the Eliptrac for for the high frequency. If these bins are just too weird or unsuitable for critical listening, I'll just build a real Cornscala pair as the style D.

[Guest David H]

The 3012LF is no slouch of a driver, however the enclosure may be a limiting factor.

Here is a simulation on the 3012 pair in a ported 6 cf enclosure.

[Chris A]

This is only one speaker, up on a 18" stand for measurement purposes.

One or more of those dips at 170 to 300 Hz will probably be much less severe if you place the bin on the floor instead of a 1.5 foot stand.

It's in the middle of my studio, a 10,000 cu ft, semi-partitioned room into approx two equal halves...Mic was about 6' from bass bin.

Your lf performance will smooth out and deepen dramatically if you place the bin in a room corner (within 18 inches of both walls), and you will probably also see a lot less negative going spikes in response. That spike at ~47 Hz will also probably be less pronounced and at lower frequency in a room corner.

You could also test the speaker outside on the ground - pointed away from any building structures if you are trying to understand the bin-only response.

How is the room "semi-partitioned"? It looks to me that you've got two rooms that you're playing into, looking at those dips at 95 and 135 Hz--which look like two transitions into the sparse mode region.

It also looks to me that you could crossover up to about 800 Hz, but the width of the bass bin polars wrt your "Fastrac" horn is what you're trying match. But you will need to separately test the off-axis response of the bass and the mid horn/driver (i.e., 30, 45, 60 degrees off-axis) to understand the natural crossover frequency.

Chris

[coolhandjjl]

Dave and Chris-

Everything you state makes sense to me. Perhaps I am trying to fit a square peg into a round hole? The bin pictured has approx 8800 cubic inches of volume above the horn/port area (less the volume of the mid/hi you see behind the grill, which are not hooked up and will not be used.

Each bin has two 3012LF's in it, but those would have strong resale as they are in demand for folks building bass guitar cabs, PA's, etc.

Would the better way to go be to build a proven winner, a Cornscala design with Bob Crites dedicated 15" driver? His designs have both the LF and HF components in one box. If I build the Cornscala just as a bass bin with the HF outside the bin on top, would I still use Bob's dimensions for the box, or would I subtract the volume that the Cornscala HF components normally occupy?

The entire volume of my existing box is approx 11800 cubic inches, so I could re-purpose that cab it by removing the lower horn assembly, building a new motor board and making a larger grill.

[Chris A]

Would the better way to go be to build a proven winner, a Cornscala design with Bob Crites dedicated 15" driver?

Bob does good work, however, horn-loaded bass has much lower modulation distortion than direct radiator bass bins, including bass reflex boxes like the Cornwall box. It's just physics: horn-loaded drivers have to move much less than direct radiators, apples-to-apples. Low modulation distortion means that they will sound "real" at much louder SPL, something like 15-25 dB higher SPL for the same amount of modulation distortion.

I don't know the exact geometry of your bass bin box folding, but it looks to be horn-loaded in like fashion to the Khorn, but it also has one advantage over the Khorn bass bin: there's really only one mouth, so you don't get the diffraction problems that you get with two mouth bass bins at higher crossover frequencies. I'd keep the present bass bins and put them in the corners, if possible, then EQ them down to flat-to-slightly rising response in-room at your listening position).

If I build the Cornscala just as a bass bin with the HF outside the bin on top, would I still use Bob's dimensions for the box, or would I subtract the volume that the Cornscala HF components normally occupy?

I think that you could subtract the small amount of volume occupied by the midrange and tweeter, however, I'd not change the relative dimensions of the box, since it is designed to have good bass port performance.

Just my $0.02.

Chris

[coolhandjjl]

The drivers are facing down, mounted on a 1/2 in spacer ring directly behind those rectagular openings you see on the angled horn surface. The back edge of the angled horn panel does not meet up with the rear of the box, there is a opening of about 1-1/2" along the width of the box, that's the tuned port.

If I strip out the present mid and tweet, the interior volume goes from 8800 to approx 9100 sq in, wonder if the gain will be helpful, or insignificant?

I like my bins, they're cool looking, so I really should complete my project with the HF tops and start listening to music.