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 Post subject: OB Technical question
PostPosted: January 1st, 2014, 3:22 pm 
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Joined: October 21st, 2013, 6:53 pm
Posts: 270
Hey guys,

I have a good question about OB that might get Roscoe and Tom talking.

Suppose I have a driver in an OB, and it has a steep rolloff below 100 cycles.

My amplifier however is providing signal info down to 20-30 cycles just suppose.

Is the actual cone of my driver responding to those signals way below the 100 cycle rolloff, but just not translating it into energy that projects into the room?

Or, as a result of the OB arrangement, is the cone not even responding to those lower signals?

I have not devised a way of figuring out the answer to this. Does anyone have an answer for me?

Chris


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PostPosted: January 1st, 2014, 8:04 pm 
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Joined: March 5th, 2013, 9:35 am
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Location: Highland, MD
A completely inexperienced response:

Place a microphone on the front side of the baffle, and one on the back side, both very close to the driver and see if each microphone registers a response? Hopefully they will be far enough from the edge and close enough to the driver to not suffer from cancellation.

And you can try to measure the voltage response of your amplifier to verify that it is producing a signal below 100 Hz.

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PostPosted: January 1st, 2014, 8:50 pm 
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Joined: February 28th, 2013, 3:31 pm
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The following discussion assumes the theoritical behavior of a dynamic driver and does not take into account the many anomolies that can affect a particular driver. If the driver is above resonance the excursion will increase by a factor of 4 for every octave decrease in frequency with the same voltage drive you apply. Once you get below resonance, the excursion will not increase as the frequency goes down. That is because above the resonant frequency, the movement of the cone is mass loaded which varies with frequency and below resonance the movement is suspension loaded which does not vary with frequency.

With an infinite baffle, the acoustical output for frequencies above resonance for the same drive level will theoretically be the same. That is because as the frequency drops for example by one octave the cone movement will be 4 times of that of the lower frequency but the impedance of the air will drop by a factor of four at that frequency. The less efficient coupling of the cone to air exactly offsets the increase in movement. When you drop below below resonance, the impedance of the air will continue to drop but the excursion will not increase as the suspension, not the mass dominates at that point and it is not frequency sensitive.

The problem with an open baffle as compared to an infinite baffle is that the rear pressure wave can travel around the edge of the baffle and cancel the pressure wave coming off the front. The point where that happens depends on the size of the baffle in relation to the wavelength being reproduced. If the frequency is high enough the pressure wave can move out from the driver before the cancelling rear wave can arrive. As the frequency goes down, because of the longer times involved, the rear wave begins to reach the front wave as it is still building and cancellation results.

If the driver and baffle are fully suspended in air away from all surfaces, the rolloff will be more severe. If the driver and baffle are mounted against one or more surfaces, those surfaces will reduce the amount of rear energy reaching the front and cancelling. In any case, whatever the percent of cancellation that occurs, those cancelling areas will continue to cancel regardless of how hard you drive the woofer.

If the dipole is mounted in free space, the cancellation will result in a 12 dB/octave drop in output. If it is mounted on a floor you might have an 8 dB/octave drop. For the sake of discussion, if you want to compensate for a 12 dB drop you would have to increase the cone excursion by a factor of 4. Add to that the 4 times excursion you would need in a non-dipole situation to maintain the same pressure and you can imagine how quickly you will run out of excursion. Above resonance you would need 4 times the power to get that displacement to overcome the mass loading. Below resonance with the 12 dB/octave dipole drop and the additional 12 dB/octave below resonance drop off for the same drive power, you would now have to overcome a 24 dB/octave drop in response. In reality that is a very difficult situation both in terms of linear cone excursion and power available.

In my opinion, dipole bass is very effective at frequencies above around 90 Hz. Below that the situation gets very difficult. I think it is better to use multiple sealed box subs to minimize standing waves up to around 90 Hz and then transfer to dipoles. Roscoe and I have been talking about a way that we can demonstrate this at a meeting and will keep you informed. Like anything else, you have to find out what works best for your desires given the acoustics of your listening space.

Tom


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PostPosted: January 2nd, 2014, 4:56 pm 
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Joined: December 14th, 2013, 2:19 pm
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I think the short answer you are looking for is, YES, the woofer cone is moving, but you don't hear any bass. The front and back waves cancel, as tomp explained.

You can make life easier on your amplifier and get more usable power out of your amplifier by installing a high pass filter in line, before the amplifier, to filter out those low frequencies that your OB speakers cannot reproduce, i. e., below 100Hz. That is a huge power savings and will allow more upper frequency sound to be cleanly reproduced. Another benefit of such filtering is reduction of the intermodulation distortion created as the woofer cone pumps, trying to create those low frequencies while simultaneously producing those midrange sounds you CAN hear.

Stuart


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PostPosted: January 3rd, 2014, 2:32 pm 
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Joined: March 12th, 2013, 12:45 pm
Posts: 39
Location: Annapolis, MD
This article may interest you >> Dipole Bass <<

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