Horn length-Mouth size

correct me if im wrong but as far as i am aware the cut off frequency of a horn is a quarter of a wavelength of the lowest frequency that the horn can play.

A good rule of thumb for mouth size is, assuming a round horn, the circumference needs to equal the wavelength of the lowest frequency, if used in full space. The area can be halved for half space use, halved again for quarter space and so on.

If the mouth is too small there will be a reduction in level at the lower end, and peaks and dips above cut off.

For a more detailed explaination look at Keele's paper

If you are using an exponential horn you will also have a flare rate, or flare frequency (called F12 in hornresp) which is linked to the ratio of throat to mouth. This also determines how low a horn will play.

There's a lot more to it than that, look at some of the horn design links on the main speakerplans site.

Kieran

cheers for the link. have actually been flicking through some of his AES papers today.

thanks again doober, some cracking links there

So then what effect does wadding / foam on walls have on horn behaviour? I assume it probably has no useful effect as its never used...

There's a formula which calculates the cutoff frequency of a exp horn. It uses

1) horn length
2) mouth area
3) throat area
4) horn exp coeff (eg 1 = hyp exp)

I cant remember it offhand. but basically if you make the mouth/ throat ratio smaller you get a lower cutoff for a given horn length at the expense of a more peaky response and less gain. simple.