Alpair 7.3 Bookshelf Bass Reflex?

[Bigun]

Dave is the guru on this, but I'll take my shot at what he's saying.

The first derivative is a measure of the slope of the curve, of the FR curve. If the slope is up then the first derivative is larger than zero, if the slope is down the first derivative is negative, if there is no slope and its dead flat then the first derivative is zero.

You also get a flat spot on the FR curve at the top of a bump or at the bottom of a hollow. And either side of the top of a bump or bottom of a hollow, the slope of the FR curve changes from going up (or down) to going down (or up). So over the course of a bump or hollow in the curve the first derivative will change sign (from + to -, or from - to +) and in doing so it 'crosses zero'.

So no first derivative rule means no bumps or hollows in the FR plot. The 'wrong' choice of box size and/or port dimensions will product bumps/hollws in the FR plot.

To play with this I use this excellent BR calculator which shows the FR plot: Loudspeaker enclosure calculating with Thiele Small parameter

I used Dave's approach to tuning up my Martello enclosure and to my ears it sounds good.

[planet10]

Originally Posted by StevenZ

The chart was done on the work computer. I'll see if I can blow it up tomorrow and change the color to something easier to identify.

I replicated it in my modeler. The read arrows point to where the 1st derivative crosses zero (it also does up above 200 Hz, but that one doesn't really concern us). You will also see how the blue line (group delay) zooms off the top of the chart. In the 2nd chart is the 14/50 again, Jeff's 7L/55Hz, and what i'd do 9L/60 Hz) as well as 7 & 14 L sealed.

When looking at bass extention pay attention to F10, Toole determined that F3 doesn't tell you much (which was pretty much understood already).

You can't tune the 7L much higher with a round vent as it will become underdamped. My tuning assumes a high-R vent which knocks the bump down (increased damping -- the highR vent also makes for higher tolerance to dynamic T/S changes as they move along their curves).

dave

[StevenZ]

Okay. I think I'm following you Bigun. Where I was getting lost is that at no point after 100hz does the FR go above 0+. Though you see two bumps they're below zero as stated in a previous post. I'll see if I can smooth out the response to have zero bumps and a gradual roll off. So what is the general consensus here? Are we aiming for something about 8liter tuned to 55hz? Someone shoot me a target enclosure size and tuning and I can play around from there. I'd like to start slingin' some saw dust by Friday.

Thanks,
Steven



Edit: Posted at the same time as Dave's post which answers the questions I asked in this post.

[StevenZ]

Awesome, thanks Dave! So by high-R vent, you mean using a high resistive vent, something like the 1" you advised of in a previous post, correct? Looks like I'll be knocking down some cabinets pretty soon.

-Steven

[planet10]

To have a high R vent it needs to have a high aspect ratio and as long as you can get away with.

dave

[StevenZ]

So, here we are today. I'll go with your recommendation of a 9L box tuned to 60hz with a 1" port. When I plug all this into my box calculator it tells me that the port should be 1" long. Could you confirm this?

Thanks,
Steven

[planet10]

Originally Posted by StevenZ

So, here we are today. I'll go with your recommendation of a 9L box tuned to 60hz with a 1" port. When I plug all this into my box calculator it tells me that the port should be 1" long. Could you confirm this?

I get 29mm. Note that i'd use this tuning oonly with a high ratio slot vent (in which case you can use the CGR as a reference. With a cylindrical port i would tune a bit lower -- say 40mm x 1" vent.

dave

[vinylkid58]

FWIW, here's a screen shot of a 9 litre box with 1.5" (38mm) long vent.

heff

[Studio Au]

My cabinets based on the reflex port design listed on Mark Audio's website.

So nearly finished...

[pesli58]

Is the br-cabinet for the A 7.3 also useful for the A 7.2? If not: does anyone know a custom bookshelf br-design for this speaker?
Thanks,
Peter.

Originally Posted by Studio Au

My cabinets based on the reflex port design listed on Mark Audio's website.

So nearly finished...