[Magnetar]

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So the operating range of any bass dipole is restricted to the dipol-loss-range below the lowest dipole peak. JohnK explains this in some of his Tech Studies: http://www.musicanddesign.com/tech.html

If you build a 21" wide H frame with a depth of 21" (front to back), the first dipole peak will be at ~320 Hz. This peak will in no way be "peaky" - just the upper end of the 6 dB/oct dipole roll off turning down to the first dipole null. This H frame will give you ~6 dB more efficiency than a 21" wide OB. Crossing over to your line array at 80-100 Hz you will be well away from that peak too, even with a low order x-over.

That´s what I would recommend for your 'bass' dipole array.

Thanks for the link - I need to read this --------- 6 db more efficient is almost unbelievable. You have built and measured this comparision?

I read what I thought was related and found the author found a damped U frame was just as efficient as a longer length (36"!) H frame but I saw no comparison to a flat board.

[Graham Maynard]

Is the *efficiency* (per Watt) increased by 6dB,

or is it the improved impedance match which leads to increased radiation, such that increased transduction requires increased drive, maybe 6dB more for the cubing dimensions suggested ?

(This is likely to vary through the resonance though, and that too will be modified compared to plain OB !)

Cheers ........ Graham.

[Rudolf]

Thanks for the link - I need to read this --------- 6 db more efficient is almost unbelievable. You have built and measured this comparision?

I read what I thought was related and found the author found a damped U frame was just as efficient as a longer length (36"!) H frame but I saw no comparison to a flat board.

For what I measured look for the first diagram in my comparison: http://www.audiocircle.com/index.php?topic=49940.msg448752#msg448752

I took MJKs worksheet to compare a H frame of 16" total depth for the Eminence Alpha 15" with a OB of same front area. That resulted in +5 dB from 20-35 Hz and +3 dB up to 150 Hz.

[Rudolf]

Is the *efficiency* (per Watt) increased by 6dB, or is it the improved impedance match ...

I can´t tell. It was with the volume knob remaining in the same position. I don´t have impedance diagrams for those measurements, so I don´t know if there was a difference.

[Magnetar]

Thanks for the link - I need to read this --------- 6 db more efficient is almost unbelievable. You have built and measured this comparision?

I read what I thought was related and found the author found a damped U frame was just as efficient as a longer length (36"!) H frame but I saw no comparison to a flat board.

For what I measured look for the first diagram in my comparison: http://www.audiocircle.com/index.php?topic=49940.msg448752#msg448752

I took MJKs worksheet to compare a H frame of 16" total depth for the Eminence Alpha 15" with a OB of same front area. That resulted in +5 dB from 20-35 Hz and +3 dB up to 150 Hz.

I think what's going on here are two things, the H frame is loaded / coupling with the floor better and the H frame is loading up the beet freq's in pipe resonance.

[Rudolf]

I think what's going on here are two things, the H frame is loaded / coupling with the floor better and the H frame is loading up the beet freq's in pipe resonance.

We certainly can´t separate the effects from one another completely, so by and large you are right. But there is more to it. This H frame with a 8" long pipe has its resonance at ~280 Hz, which corresponds to a virtual pipe length of 12".  This in turn is the effective dipole separation. Practically the H frame is donating a physical length that is optically not visible.

Since you don´t exploit the H frame up to it´s resonance, that loading up is not part of the game. AFAIK the extra bass is generated by mass loading the driver with the lumped mass in the H frame.

[Magnetar]

I think what's going on here are two things, the H frame is loaded / coupling with the floor better and the H frame is loading up the beet freq's in pipe resonance.

We certainly can´t separate the effects from one another completely, so by and large you are right. But there is more to it. This H frame with a 8" long pipe has its resonance at ~280 Hz, which corresponds to a virtual pipe length of 12".  This in turn is the effective dipole separation. Practically the H frame is donating a physical length that is optically not visible.

Since you don´t exploit the H frame up to it´s resonance, that loading up is not part of the game. AFAIK the extra bass is generated by mass loading the driver with the lumped mass in the H frame.

I'm guessing that a Karlson Coupler chamber on both the front and back would be even better if you are using a single driver. It will spread out the resonance plus load the driver to the air better plus will help in cancellation because the wing depth or 'h frame' woud be extended. I built one and it worked well except I like using more than one driver for the bass.

[johnk...]

Just though I jump in for a moment re a couple of things. If you make a careful comparison between and H frame and a flat baffle yuo will find that there are basically two differences. One is the added mass effect ont he H frame which lowers Fs, raises Qts, and also lowers effency. These effects are more significant for high effency drivers with low moving mass. Drivers with high moving mass, like the Peerless XLS woofer, show less of this effect. The second is the resonance peak of the H frame which is absent from the flat baffle. Also, the frequency of the resonance peak is a function of the length of the H frame and the cross sectional area.  However, this does not alter the effective length of the H frame with regard to front to back separation or delay. For an H frame (or U frame for that matter) the delay associated with front to back separation is purely the path length difference. Coupling to the floor is not going to be significanly different for either.

[Magnetar]

Just though I jump in for a moment re a couple of things. If you make a careful comparison between and H frame and a flat baffle yuo will find that there are basically two differences. One is the added mass effect ont he H frame which lowers Fs, raises Qts, and also lowers effency. These effects are more significant for high effency drivers with low moving mass. Drivers with high moving mass, like the Peerless XLS woofer, show less of this effect. The second is the resonance peak of the H frame which is absent from the flat baffle. Also, the frequency of the resonance peak is a function of the length of the H frame and the cross sectional area.  However, this does not alter the effective length of the H frame with regard to front to back separation or delay. For an H frame (or U frame for that matter) the delay associated with front to back separation is purely the path length difference. Coupling to the floor is not going to be significanly different for either.

So you are saying the H frame is less efficient.

The slanted Karlsn slts (front and back) I mentioned above actually raises the efficiency in the midbass. If I was gunf to build a single driver B midbass system I think it's the way to go. It should be usable to a higher freq too due to the driver looking at the expo slot instead of a straight pipe

[Rudolf]

Also, the frequency of the resonance peak is a function of the length of the H frame and the cross sectional area.  However, this does not alter the effective length of the H frame with regard to front to back separation or delay. For an H frame (or U frame for that matter) the delay associated with front to back separation is purely the path length difference.

John,
I appreciate your clarification and extension. But I am not convinced yet WRT the effective separation length. If you are right, why don´t I measure any dipole peak or at least some plateau above (in frequency) the resonant peak? Do you know of any measurement or geometry configuration where one could separate the resonance peak of a H frame from its dipole peak?   

Will the path length be a "sharp" distance? Or could it be "smeared" between the length of the H frame and double the lambda/4 resonance wave length? I´m not sure whether the delay of an impulse response for instance would tell the true story?

[Graham Maynard]

Hi Rudolph,

Just a quick comment.

If you had measured voice coil current with your fixed voltage input, it would likely have increased when the mass loading frame was added.

Indeed a meter in series with the driver would be another way of checking for the increased LF SPL due to addition of a frame. 

It might even be possible to further increase electrical drive and SPL (fractionally) around 25Hz by inserting a 1000 to 4700uF capacitor in series with the framed driver, and adjusting the value whilst monitoring for peak current consumption.

Cheers ........ Graham.

[johnk...]

John,
I appreciate your clarification and extension. But I am not convinced yet WRT the effective separation length. If you are right, why don´t I measure any dipole peak or at least some plateau above (in frequency) the resonant peak? Do you know of any measurement or geometry configuration where one could separate the resonance peak of a H frame from its dipole peak?   

Will the path length be a "sharp" distance? Or could it be "smeared" between the length of the H frame and double the lambda/4 resonance wave length? I´m not sure whether the delay of an impulse response for instance would tell the true story?

First let me clarify the efficiency thing. Of course the efficiency of a dipole or H frame system is frequency dependent due to the baffle or H frame transfer function. What I was getting at is that if you consider the infinite baffle efficiency of the driver and then consider the efficiency of the same driver, again in an infinite baffle but with added mass equivalent to the mass loading of the H frame you would observe a lower efficiency. When the driver is in the H frame this lower efficiency would likely be observable at lower frequency. However, as the frequency rises towards the dipole peak the efficiency of the H frame can actually increase to levels above that of a flat baffle due to the "1/4 wave" resonance. All this will depend on the specifics of the baffle/H frame construction as well as the driver and driver directionality.

Now, with regard to the effective path length, the 1/4 wave frequency of an 8" duct of very, very small cross sectional area would be 423 Hz which would also be the frequency of the dipole peak. However, when the duct has a finite area the duct resonance will be much lower. This is the  first resonance peak, however and above that frequency there are a series additional resonances which lead to valleys and peaks in the response. If you observe the first peak at 280 Hz, then my rough calculations indicate that there would be a valley in the response at somewhere around 500 Hz. You can see this in Martin's results by looking at either the acoustic impedance for the front, or by looking at the SPL response for the front. Then the next thing to observe is the effect of directionality. Again, look at the difference between the front and rear SPL response as computed from Martin's codes. In his default case the rear response as see from the "measurement point" drops off significantly above 300 Hz. So when the front and rear responses are summed with the appropriate delay the absence of the dipole peak, which would be around 425 Hz, is well above the point where there is a loss of symmetry between the front and rear responses with the result that the theoretical dipole peak is not observed. All this has less to do with the effective source separation than it does with the resonant structure of the H frame and lack of symmetry between front and rear responses.