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In the Forum: Horn-Loaded Speakers
In the Thread: Another time aligned 5-way horn project

Post Subject: Pretend you are a diaphragm.Posted by Romy the Cat on: 10/22/2015

Pretend a driver diaphragm, any driver. From front diaphragm "sees" the horn throat and the mass of air in the horn that diaphragm needs to "push". This creates let call front spring. If a driver has a front chamber than let discard it for now. What a driver diaphragm "see" on the back side? A feedback from back plate that most of compression driver have sealed and the back chamber is "tuned" to the some kind of frequency. The driver has own stiffness of suspension and diaphragm that are made to work in specific frequency range with Fs right under the bottom of that range. This creates let call back spring. That all is typical.

Now we attached to a driver two horns. One let say 500Hz short horn and another 30hz long and deep horn. In case of 500Hz horn the mass of air in there is negligible and the front spring force to the driver is irrelevant. Then we load 30hz horn and now the front spring force is very substantial and it impact an movement of a diaphragm in case the diaphragm move forward. What however happens with back spring? We can't not keep the size of back chamber identical as in two our cases the diaphragm will be experience a different balance of back and front forces. For sure we would like that at any given moment the balance of forward resistance and back resistance to the diaphragm would be identical. Now a few point. We never be having a balance at "any given moment" and it is reasonable to pick one single balance point (similar to anti-skating). it is not a surprise that we pick a point of max excursion - the resonance frequency of the driver. So, the objective is that at the time the driver moves at max distance front it experience the same front spring force as it would be from back spring force in case the driver moves to it max excursion back. In this case the driver suspension work naturally in push/pull mode and do not oppressed by anything. A front moving of diaphragm pull it back with the same force as the back moving at the same distance. Another point to mention that in real world there is no movement of diaphragms of cause per se. In a compression driver we do not deal with microscopic cone movement and we rather talk about rife or pressure on back and front instead of the actual velocity. Still, it does not change anything and conceptually it work the same.

Now, pretend that we have a 100Hz horn that is driven by a driver with 20Hz resonance. The suspension is too soft and as the cone moves forward the throat reactance (or air in horn) pushes it back. However, as the cone moves forward moves back there is no opposing force to the cone. that is not good. To fix the problem we need to shrink the size of a back chamber to the level when moving a diaphragm front a default position back would created the same force (compression in back chamber) as if the diaphragm loaded to trout resistance world be moving forward. You might think about back chamber as if it is air suspended sealed box. As the diaphragm move forward and fights the front resistance (suspension + air mass in horn) the back chamber creates a contra force (suspension + negative pressure of back chamber). As the diaphragm hit the own resonance then the back chamber contra-pressure damps that resonance.

Well, in reality it would never happened as resonance of driver way too low? However a horn loaded driver has a few resonances: driver resonance, throat reactance, back chamber reactance. They all very well measurable with impedance jumping. How, how about if we bring all those resonances at one single point and make it exactly at a very lowest frequency we want the horn reproduce? AT MF it would not be important but with bass horns a few extra dBs of EQ at the lower knee it is very desirable. Now we are killing (damping) the throat reactance and make the system to resonate at the lower frequency that our given horn can still pass. So we do not waste any diaphragm movement and implement a completely symmetrical (back and front) load to the diaphragm.

With all theory the main proof is pudding and you need to make your horn your driver and begin to shrink the back chamber, measuring the impedance and listening the sound. The very last 1-3Hz when the Fs will be approaching the horn rate you will hear a very dramatic change in bass structure. It is not absolutely "better" but it is different and until you try and understand it is hard to explain. It requires some thinking about meaning of bass, more cultural thinking about Sound then thinking about the sounds but it is a whole another subject.

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