midrange response/baffle question

lowmass

Member
2016-01-26 12:46 am
Sooo I have this pr of old midrange cones, not sure what the mfg is. There response is confusing me. I mounted on a large approx 2 ft by 3 ft 3/4 in mdf board to take some measurments using the Omnimic. Going down in freq. the response is reasionably level then shelfs down approx 5 db from 2khz to 1khz then is level again untill about 500 hz then falls off . HOWEVER when I mount one on the cabinet ( 9 inch wide x 33 tall) I get a flat response down to about 700 hz. What happened to the shelf down at about 1.5k?? It seems to me if it was going to show a shelf down it would be on a small baffle not the large one ?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
If you are taking a single measurement then you are likely seeing effects from the baffle. The driver on the larger baffle is around a wavelength from the edge at 1kHz. The response will be lumpy especially around the transition.

A better way to make the transition from a flat baffle to open radiation at the lower frequencies is to curve the baffle. At any rate a roundover can ease the response.
 

lowmass

Member
2016-01-26 12:46 am
Thaks Allen, yes it was a single measure on the big baffle. I then threw the cone on some old cabinets to see what would be. The plan was to x over at around 500 so im fine with the response I was getting both on and off axis on the 9 inch wide cabinets. Was just confused by the shelf down on the large baffle and thought maybe I was missing somthing important.
So I hear ya but still not sure I understand. I get that we are about 1 wave L away from edge BUT what phenom exactly is at work here? I mean I would have expected a flat response down to like 150 hz or so if driver capable anyway. What am I missing, what is this "effect" ?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
If you measure at other locations you might find that energy that appears to be missing in your first measurement isn't missing there. This unevenness isn't optimum but isn't necessarily devastating either but steps to alleviate the problem are recommended.

The problem is that the baffle is trying to hold all the energy in half space but fails to do so to some degree, especially at lower frequencies. You can try to ease the transition by making it smooth. The reason this helps is because pressure that moves suddenly around a sharp edge represents such a disturbance (over a short time and distance) that energy 'dislodged' from the wavefront at those points, which will act like another speaker all around the edge, can interfere with the direct sound in this way.

There are two basic things to aim for. One is to smooth the response, and the other is to have the next driver down in frequency radiating into a similar portion of space.