[JBspeakerman]

Hello all....

As usual with me... this will be long....

Have been reading with great interest Richard's recent post about potential OB amps and the topic of actual OB efficiency has been opened if you will.  Especially in the lower end of the bass spectrum.   So here is a observation or two .... kinda kicking this topic down the road on it's own.   

The basics on this are well understood and have been presented well by S.L., John K., Martin and others.   Basically ... the laws of physics are the laws of physics relative to F = and F -6 points and it gets down to the design compromises .... sacrafices one is willing to... or needs to make.   In what I consider a true OB (basically a baffle with out appreciable wings) it comes down to the length of the path between front and rear sides of the woofer cone.   As soon as you add wings and start morphing into a cardioid... things change.... there are front and rear cone differential loading issues, cavity issues, radiation pattern change and so on.   Each has it's advantages and costs.  Again well known territory.   

It seems that once you get a combo of OB baffle diameter, say 16" to 28" diameter and a woofer, or woofers  with a customary OB Q, say .5 to 1.0, getting below 40 HZ with any efficiency higher than 93-95 db is very difficult.  I am assuming a passive system here.  And most systems are usually below this by 10 to 12 db.  I have built a lot of OB prototypes.... most just experimental... fool around stuff... but no mater what mix of drivers or passive tricks I apply... this is where things end up.... that is Low 80's to low 90's... again assuming a flat response to around 40 Hz.  The higher Q drivers require less passive correction to get to 40 Hz... but the high Q drivers have less efficiency.  Another assumption here is we are talking about suitable woofers Fs, area and Q wise.  Something that can play loud enough at 40 Hz to provide a usable output. 

So... when I see claims of 98 and 100 db per watt with -3 db at 20 or even 30 Hz I am very skeptical.   And a quick check of S.L.'s OB woofer calculation work sheet reaffirms my skepticism.  The predictions I have found there very closely match what I see with the various designs I have built.

One last thing about speaker Q values... I have found that as predicted... that you can take low Q woofers (QTS = .2 - .4) and get excellent passive OB results.  You trade away a ton of efficiency but still have the control of a strong motor.  And on balance you end up with the same basic bass cut off given the same baffle set up.  The Low Q woofers are cheaper and easier but to my ear do not sound as good.  One man's opinion... but there it is.

I certainly have not covered all the potential issues here .... but it's a start.

Interested in hearing other thoughts on this.

Best...  John

[nullspace]

Hi John,

As you point out, it's all about tradeoffs. My personal decision was to aim for 98-100db efficiency & sans eq on the mains. This lead me to use high-Q, high-Fs drivers. Consequently, I have an f3 of about 100hz and use subs to fill in below... To each their own.

Regards,
John

[pedroskova]

I use moderately low qts (.375) woofers actively driven (4 per side), so I may not qualify (according to the title).  I guess I don't understand the reasoning behind passively EQ'ed low qts woofers.  It's inefficient nature causes a series of actions that throws away energy in the form of padding down the mid and hf drivers - something that goes against my bias toward high efficiency.  It's kind of like using a high gain preamp where the volume control never gets past eight o'clock.

[Magnetar]

The best way to handle this has been posted here a few times by me. Use lots of the right drivers. In my system I use two 21", six 15", and fourteen 10" mid QTS pro drivers and no eq, one set of 15 and the 21 per side wired in parallel in line arrays and the tens in their own line arrays (16 ohm each) wired in parallel. I use real power amps with big solid power supplies and low negative feedback. Done this way it's easy to get over 100 db with a watt 40 cycles up. Below that I use a horn. IOW if you want realistic bass don't use passive OB speakers.

[panomaniac]

"Best", alas, is not always practical.   Many of us don't have the room or the means to do what is best. So we compromise.
Smaller baffles, fewer drivers, etc.

What I've heard on the OB circuit is that once you get the tonal balance right, you've lost 8 to 12dB thru the midrange.  That means a lot more power to reach the same spl as your driver(s) would have in a box.  That's the law, the law of physics.  That rising midrange has to be dealt with somehow.  So you throw it away.

And you can not get around the issue with an active rig.  You may think that you have not traded away efficiency, but you have.  Whether you use active EQ or just run the bass amp hotter, it's all the same.  More voltage, thus more power, is needed in the bass.  If it were not less efficient you would not need the extra voltage.  Active or passive, it all comes out the same.

High Qts drivers (mostly high Qes) do tend to have a flatter response in the low end.  Generally they are somewhat flat out to Fs, then drop like a stone.  Makes EQ easier.  But you can get pretty much the same result by putting a big series resistor on a lower Qts driver.   But how is that going to sound?  Yes, like an underdamped driver....

So you can EQ a lower Q driver and end up with pretty much the same response and efficiency as a high Q driver.

The only way to get a really efficient OB bass rig is the way Magnatar mentions - lots of drivers.  If you can't do that for practical reasons, then you have to live with the low efficiency.   It's the law.

[pedroskova]

And you can not get around the issue with an active rig.  You may think that you have not traded away efficiency, but you have.  Whether you use active EQ or just run the bass amp hotter, it's all the same.  More voltage, thus more power, is needed in the bass.  If it were not less efficient you would not need the extra voltage.  Active or passive, it all comes out the same.

With active EQ of low qts drivers, yes, you are still throwing extra energy at the bass...difference being - you're not putting an additional source of resistance between your other amp (hopefully tubed) and your mid/tweets.

[Magnetar]

And you can not get around the issue with an active rig.  You may think that you have not traded away efficiency, but you have.  Whether you use active EQ or just run the bass amp hotter, it's all the same.  More voltage, thus more power, is needed in the bass.  If it were not less efficient you would not need the extra voltage.  Active or passive, it all comes out the same.

With active EQ of low qts drivers, yes, you are still throwing extra energy at the bass...difference being - you're not putting an additional source of resistance between your other amp (hopefully tubed) and your mid/tweets.

And when you EQ the bass you raise the distortion through greater excursion.

The bass driver(s) eqed or not should also be high passed electronically with a steep high pass to avoid distortion.

The reality of using enough drivers in a line high passed (and low passed or "bandpass") is obvious when you listen to a OB system that will move enough air at low distortion through brute force and surface area. It is very addictive and enjoyable, beyond word IMO, it has to be experienced...! The sound is free, as in the walls disappear and the bass sounds whole and organic real just like the real thing. I believe it's worth building a bass system like this if you have enough room. It's really not that expensive because the drivers need not be esoteric.

[pedroskova]

And you can not get around the issue with an active rig.  You may think that you have not traded away efficiency, but you have.  Whether you use active EQ or just run the bass amp hotter, it's all the same.  More voltage, thus more power, is needed in the bass.  If it were not less efficient you would not need the extra voltage.  Active or passive, it all comes out the same.

With active EQ of low qts drivers, yes, you are still throwing extra energy at the bass...difference being - you're not putting an additional source of resistance between your other amp (hopefully tubed) and your mid/tweets.

And when you EQ the bass you raise the distortion through greater excursion.

The bass driver(s) eqed or not should also be high passed electronically with a steep high pass to avoid distortion.

The reality of using enough drivers in a line high passed (and low passed or "bandpass") is obvious when you listen to a OB system that will move enough air at low distortion through brute force and surface area.

Yep, but there is no reason why one can't use multiple drivers and active EQ.  As for distortion, it's been my experience that with enough drivers and active EQ, distortion really doesn't come into play... until you get silly loud, and at that point, I'm not really thinking analytically....probably been drinkin', now that I think about it. 

[Magnetar]

And you can not get around the issue with an active rig.  You may think that you have not traded away efficiency, but you have.  Whether you use active EQ or just run the bass amp hotter, it's all the same.  More voltage, thus more power, is needed in the bass.  If it were not less efficient you would not need the extra voltage.  Active or passive, it all comes out the same.

With active EQ of low qts drivers, yes, you are still throwing extra energy at the bass...difference being - you're not putting an additional source of resistance between your other amp (hopefully tubed) and your mid/tweets.

And when you EQ the bass you raise the distortion through greater excursion.

The bass driver(s) eqed or not should also be high passed electronically with a steep high pass to avoid distortion.

The reality of using enough drivers in a line high passed (and low passed or "bandpass") is obvious when you listen to a OB system that will move enough air at low distortion through brute force and surface area.

Yep, but there is no reason why one can't use multiple drivers and active EQ.  As for distortion, it's been my experience that with enough drivers and active EQ, distortion really doesn't come into play... until you get silly loud, and at that point, I'm not really thinking analytically....probably been drinkin', now that I think about it. 

That's true however it just get's more complicated and expensive that way. You have to build the eq, you probably will spend more on the drivers, and it is true the added excursion of the drivers will raise the distortion in most case.

When you use enough bass drivers in a line array OB it sounds better at all levels, even a whisper.

[Bob in St. Louis]

Yep, but there is no reason why one can't use multiple drivers and active EQ.  As for distortion, it's been my experience that with enough drivers and active EQ, distortion really doesn't come into play... until you get silly loud

aa Amen Brother.  Rock on....

Bob

[fivestring]

For efficiency rating (2.83 V @ 1 m), for a certain driver effective surface area and baffle dimensions, measured at 40 hz (!), all drivers of equal SD will have the same efficiency (very low). They will differ greatly at higher frequencies, according to their motor strength, but that`s it. The only way to raise efficiency is and always will be, multiplying the number of woofers, period. I`m sick and tired of sensational dipole low bass sensitivity claims...
Panomaniac and Magnetar are right on track.

regards
miro

[Magnetar]

For efficiency rating (2.83 V @ 1 m), for a certain driver effective surface area and baffle dimensions, measured at 40 hz (!), all drivers of equal SD will have the same efficiency (very low). They will differ greatly at higher frequencies, according to their motor strength, but that`s it. The only way to raise efficiency is and always will be, multiplying the number of woofers, period. I`m sick and tired of sensational dipole low bass sensitivity claims...
Panomaniac and Magnetar are right on track.

regards
miro

I built my first open baffle in 1974 and physics haven't changed a bit. I will argue that there are sensitivity differences on an OB at 40 cycles between same SD drivers although none will be "high efficiency"

[panomaniac]

....physics haven't changed a bit.

Damn it!!   Don't you just hate that? 

...sensational dipole low bass sensitivity claims...

Well there must have been some reason to put drivers in a box back in the old days.  It wasn't just to make them pretty. =)   OB is great, as long as we understand the trade-offs, right?

[fivestring]

I built my first open baffle in 1974 and physics haven't changed a bit. I will argue that there are sensitivity differences on an OB at 40 cycles between same SD drivers although none will be "high efficiency"

Sensitivity (1W@1m) yes, but not efficiency (2.83V@1m) ...

[fivestring]

OB is great, as long as we understand the trade-offs, right?

If done right, OB bass is the most realistic and fatigue free of them all...