Two LSPCad questions

[morbo]

I had a couple of questions re modelling my first design in LSPCad:

1. LSPCad/JustMLS both seem to give me only relative SPL values (ie -5, -10, -15) rather than the absolute values I see some others getting (80, 85, 90 etc). Any idea why this would be? I am using 2 channel measurement mode, calibrated soundcard, and a decent microphone. How can I get an actual sensitivity/efficiency for my modelled speaker and an absolute SPL curve? Sorry if my terminiology is incorrect and/or confusing here, not sure how to describe it correctly.

2. I am purposely crossing over at the spot the baffle step starts to roll off the bass. My idea was to use an LPad on the tweeter or in some other way adjust the relative levels of the 2 in order to have a crude sort of adjustable baffle step compensation. My question is, how would I model putting this Lpad on the tweeter in LspCAD? I am currently modelling the system as a 'simple 2 way', do I use the 'attentuator' field in the tweeter XO?

Any help would be much appreciated!

[Paul H]

Jakub,

LspCAD will only give relative SPL, not calibrated actual SPL. You can manually shift the graphs so that they are close to your real levels, and/or do somewhat of a calibration by checking sound levels with a sound meter and shifting the graph to adjust.

Baffle step compensation and tweeter level adjustment are different concepts. I'm not sure why you're setting the crossover where the bass rolls off. Perhaps you could explain that further.

Assuming you need to adjust tweeter level downwards, insert resistors in the tweeter crossover circuit, after the other components, closer to the driver. Place one resistor in series, and a second resistor in parallel (after the first resistor, from the end of the first resistor to ground). I have no idea what specific values you'll end up needing (it's very specific to your drivers), but start with say 2 ohms in series and 8 ohms in parallel and work from there.

Paul

[morbo]

Paul - actually I am not using a proper tweeter, I am using a Tangband 871 as a mid-tweeter along with a Dayton RS 225 for the bass. The concept was to build a very inexpensive, shielded, full range speaker for home theater use in small rooms without the need for a sub.

I simmed the RS225 in-baffle using EDGE, and the rolloff started at ~ 600hz, so I thought that if XO at 600hz as well (which models quite nicely in terms of power handling on the TB 871), I can just adjust the relative levels of the drivers to get the desired BSC lift in the low frequencies. I know it will not be perfect, but it doesnt need to be - the speakers will eventually be used flush on either side of a projection TV, and when I model that in EDGE, it doesnt seem to need much BSC at all. The adjustment is to allow for taste (I'm building 2 pairs of these for 2 different rooms/users), and to allow for freestanding (ie not flush with a 50" TV) operation if necessary. I hope that clarifies a bit, if not let me know and I'll rephrase it.