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RE: Why do Bass Amps us Four 10's and Tweeter?




Hi Claude
I have to say your the only person that ever asked me that question and i did not expect to have the answer derail a bass amp project a couple years ago.
After playing through monitors and no bass amp, i decided i needed a bass amp to hear more details.

I made a small portable cabinet, a pair of 10 inch woofers and an 8 inch mid driver aimed at my head, closer to my ears..
This was small compared to the olden days and nice to play through, had a very satisfying low end heavy sound.
So the crossover to the mid was the next step and here was the answer to your question.

When you measure a home speaker at the couch and have a woofer that is up off the floor, you usually see a deep notch in the upper bass range, a floor bounce causes this by producing a delayed second arrival that arrives out of phase so often around 150Hz or so is that notch.

To get the loudspeakers actual output / response in a room, one can put the mic on the floor boundary and not see the room notch which is the darker CURVE in the response.
The lighter curve is what you get at the same distance from the speaker BUT now at head height.

Now, each woofer's output arrives more separately in time and each woofers reflection arrives at it's own time and the lighter curve is the sum of both direct and reflected sound and the result of the destructive interference reduces the level.

This result of the constructive and destructive addition that results when one signal arrives (delayed) at several different times aka comb filtering..

Now if one wanted flat spectrum response where your head is, what you need is a lot more mid and hf that offsets the cancellation loss the separate arrivals cause.
In other words where your head is, only the lowest notes are produced as a single radiation in time and above that, the destructive interference rolls off the actual level where you are.
The bigger the speaker stack is, the more pronounced this effect is, it's the same thing that gives a line source a reduced fall off with distance, as you move farther way from it on axis, there is less and less destructive interference and the closer it becomes to having a single radiation in time.

My conclusion was something close to the floor avoids interference and directivity is still your friend haha.
Hope that helps.
Tom


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