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MaVo 24th February 2009 07:45 PM

Horn mouth bubble size

If you take a in room measurement of a closed box woofer and place the mic not further than ~20cm away from them, the room modes have nearly no influence on the measurement. If the mic is placed further away, the room modes influence on the measurement grows rapidly. Too bad, that it is really impractical to sit so close to the woofers.
Thomas Danley recently introduced his SPUD sub, which seems to be designed around this concept. He mentioned, that the listener should be placed really close to the horn, in the "mouth bubble". This term describes that the horn is virtually longer than its physical size, shaped like a bubble around the mouth. The effect i observed seems to be the same or a similar thing, just with a closed box.

Here is a link to a post where Tom mentiones the effects of mouth bubbles: (cant find the SPUD related post)

Is the room mode negating effect, considering horns, larger in size than the ~20cm of my closed box woofers? Which factors influence the size?

MaVo 25th February 2009 11:59 AM

I did some rough experiments with the orientation of 4 subs. Placed in a 1*4 standing pile and mic placed at the height of the upper rim and slowly dragging it away from the boxes, room effects seem to get influence alot faster (meaning, you have to place the mic very close to see none), than in a 2*2 pile with the mic placed in the centerline. This seems to indicate the presence of some kind of "bubble" around the woofers.

Of course, i could easily be totally wrong here, but it seem to be the size of the membrane and maybe the closeness of the adjacent planes, that influence the size and shape of the "nearfield" zone. This way, horns would indeed have a benefit, since their mouth area acts as the membrane, giving a bigger nearfield area.

I think this could be an alternative to mutliple sub placements for single or maybe two close listener setups, since the nearfield response of the woofers is alot smoother than every multiple sub configuration i tried, in my room. Yet, one would need quite the big horn mouth to make it work well.

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