The different projects


I have lagged behind in updating the blogg
This is how the finiched box looks like there is a cutout to clear the magnet in the internal baffle. Note that there is no damping material att all added to the cabinet. The cabinet is not flush with the floor but stand on 19mm high feet.

Turned the right way with the driver facing the wall or corner it looks like this
And now for some measurements. I started with a Philips 9710 fullrange driver. It has a 10 dB lift above 1-2 kHz on axis and sound really bright and piercing in that position.
Black is with driver facing forward measured 1 m away at driver hight. Some of that lift can be seen around 1 kHz but the angling upwards tames it (Black trace) facing the wall (red trace) is more even if I would apply third octave smoothing.

In the corner boost below 200 Hz. Both positions give good output to about 40 Hz.

I fully expected a terrible boxy sound from that flimsy cabinet with no damping material. To my surprise it sounded really good. Sure the lower half of the front vibrates a lot, as can be felt by hand, when playing loud. The rest of the cabinet vibrates less.

If I look on the time domain, with the driver facing forward, with two sets of near field measurement of frequency of amplitude and time domian respons it look like this:  The 9710 has a double cone and seem to be sensiteve to the precise location of the microphone, that is the reason for very different frequency response in the top end.
 This is the pipe output from empty pipe and from having the first section of the pipe filled with rolls of BAF fibers
Near Field response of the Philips 9710 in empty pipe

This is two ways of looking at time domain response the lower one is decay in milliseconds the upper one is the same data set but decay is expressed in cycles instead of ms. This slant the graph to the high range as there are more cycles per millisecond the higher the frequency. It is not a pretty sight but not so bad either. The slow decay is quite evenly spread.

Next I added 3 rolls of BAF (Biltema) of 82 grams each. Two rolls were put into the closed end on each side of the center brace. The third roll was cut up to fit covering the the back and side walls behind the driver ( when driver is facing forward)
At this slightly different measurment position I got a peak in the 8 kHz range compared to previous measurement.

Time domain wise it is huge improvement. This rather crude sets of measurements suggests why it can so good even without damping material with good supression of the third harmonic of the pipe and decent higher harmonics. Adding bulk damping up to the driver improves the time domain response a lot but was a let down with regard to the harmonics. The optimal placement of damping material should be at pressure minima/velocity maxima of the pipes fundamentals.

I will try to get some decent images to post the drawings of the orginal article by Ralph West in HFN &RR about the Decca Corner Horn.

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