[JohninCR]

Since we seem to be getting some great technical guys dropping by who really know the physics, can someone please explain in layman's terms the difference between velocity and compression waves and how it relates to OBs.

[Rudolf]

JohninCR,

we better stick to some appropriate wordings here:
Regarding sound there is nothing like a velocity wave.  Sound is always about pressure changes. Our ears can´t hear anything else.

I believe what you want to talk about is the difference between a so-called pressure transducer (a monopole) and a velocity transducer (a dipole). But this unfortunate difference in naming stems from an incomplete look at the way how drivers apply pressure changes to the air IMHO.

When the cone of a "boxed" driver moves outward, it sends a compression impulse into the room. In the next moment, when the cone moves inward, it sends a rarefaction impulse into the room. And so on... This has lead to the immediate (but incorrect) impression,that a monopole is somehow working like an air-pump, pressurizing (and de-pressurizing) the room.

When a "dipoled" driver moves outward (whatever direction that may be) it sends a compression impulse to one side of the room and a rarefaction impulse to the other. In the next moment, when the cone moves inward, it again sends both impulses into the room but with 180° turned direction. And so on...  This has lead to the (incorrect too) impression, that a dipole is somehow working like a fan, "blowing" air from one side of the room to the other - and vice versa.

All room-related differences between monopoles and dipoles (and cardioids in between) can be explained with this difference in radiation. But there is absolutely no difference in how the cone transmits its kinetic energy to the air.

So you may need to redefine your question.

[johnk...]

Yep, a wave is a wave is a wave. The propagation of a wave through any elastic medium, air in our case, travels at the wave speed, of the speed of sound. Wave transmit information about disturbances. The strength of an acoustic wave is generally measured by the magnitude of the pressure variation it creates. This is a quantity called the condensation, S = const x (P-Po)= const x p, where Po is the normal atmospheric pressure and P is the disturbed pressure, and p = (P=Po). How the air responds to the propagation of a wave results in the movement of the air at velocities, u,  related to the spatial variation in P. S, p, and u are all governed by the same equation, the wave equation. Additionally, u can be related to a velocity potential and the velocity potential is also governed by the wave equation. If you solve for the velocity potential around a single or multiple sources the fluid velocity is given by the gradient of the the velocity potential and S (or p) is given by  the time rate of change of the velocity potential. Thus you can see that any wave generates both a pressure and a velocity filed.

In acoustics books what you generally see are equation for the pressure as a function of frequency and distance from the source. Similar equations are usually presented for the fluid velocity. However, in acoustics typically only the velocity in the radially outward direction from the source is presented. For example you might see the expression for the pressure or velocity far from a dipole written as U or P = (const/r) x cos(theta) x exp(j(wT-kr)) where r is the radius from the source w is the frequency, k is the wave number (2PI/wave length) and theta is the off axis angle. BUT this is only the radially outward component of velocity which, obviously goes to zero at theta = 90 degrees. For a simple point source this makes sense since the pressure wave travel out in a spherical form and there is only a radial component of velocity . But with a dipole, the velocity in not just radially directed. in the plane of the dipole the potential pattern looks like the dotted lines in this figure. The velocity follows the lines with the arrows. notice that the velocity is always perpendicular to the potential lines. As you can see, at 90 degrees the velocity is front to back and there is no outward component. Far from the dipole the potential lines will form a figure 8.
Not exactly laymans terms, but I hope it helps.

[JLM]

Maybe you're referring to compression drivers vs. dynamic drivers?

IMO compression = bad because of the various distortions inherent to compression.

[AJinFLA]

Maybe he's talking about this type of compression/velocity wave?

http://www.youtube.com/watch?v=7Wb-so9yuUQ

All kidding aside, I believe JohnCR has come up with both a pressure source (RLH) and a velocity source (OB) with his OB/RLH http://www.audiocircle.com/index.php?topic=34399.msg313229#msg313229 , similar to a cardioid?


cheers,

AJ

[JohninCR]

Thanks Rudolph and John,

Is it only dispersion and room interaction that explains the following?
My desk is my usual listening position and my desk vibrates much more
playing a set of monopole speakers at moderate volumes than playing
my dipoles with deeper bass extension at much louder volumes. 

Also, even with deep powerful bass extension my dipoles seem to have
less physical impact than monopoles.  I can feel[/b] the bass
more with boxes.  Is that because of the lack of room gain, which Dan
Wiggins measured actually starts at well above 100hz, or is it because
with the dipole the rear wave arrives so closely behind the front wave
that the impact feels different on our body, or is it my imagination, or
a combination of these factors?

[JohninCR]

AJ,
It doesn't exhibit much cardiod behavior, probably because the U-baffle on
top is unstuffed since I didn't want the low pass filter of damping.  At a minimum
what seems to happen with the alignment is that the Helmholtz style slot extracts
a portion of the LF pressure from the rear wave and sends it on a much longer
path (ie about a 2m delay).  I won't know if any acoustic transformation occurs
in the expanding pathway until I, learn to do measurements and do them, and
figure out how to interpret those measurements. 

[Rudolf]

My desk is my usual listening position and my desk vibrates much more
playing a set of monopole speakers at moderate volumes than playing
my dipoles with deeper bass extension at much louder volumes.

A desk will typically vibrate at (some) resonant frequencies. We need to specifically look at those frequencies when discussing an excitation by the speakers. What makes your desk rattle does not need to do the same to your ear. So your subjective perception of "loud" could be quite different from what the desk "hears".
But different dispersion and room interaction could explain why the monopole speaker excites those resonant frequencies better than the dipole.

Also, even with deep powerful bass extension my dipoles seem to have
less physical impact than monopoles. I can feel the bass
more with boxes. Is that because of the lack of room gain, which Dan
Wiggins measured actually starts at well above 100hz, or is it because
with the dipole the rear wave arrives so closely behind the front wave
that the impact feels different on our body, or is it my imagination, or
a combination of these factors?

Leaving real shockwaves (explosions, sonic boom etc.) aside the physical "impact" of bass on the human body (stomach and chest area) is a resonance thing too, mainly in the (kickbass) region 80-120 Hz afaik. You may look with a sweep generator, if and how your different speakers excite different room or cabinet resonances in that region. I wouldn´t be surprised if the dipoles do it to a lesser extend than the monopoles.

Generally it is always risky to compare one physical phenomenon (what your ears hear) with another one (what your body feels) and jump to conclusions that seem obvious but may not be valid because your measuring instruments are too different.

[JohninCR]

Rudolph,

Thanks, if both you and JohnK really don't notice a difference in physical impact with OB bass, then I must
be mistaken in my perception, because you guys have a lot more experience.  It seemed kind of obvious to
me, so stimulated resonance and/or SL's 4.8db dipole bass advantage or something similar must be causing
my misperception.  It is going to nag at me until I can figure a way to prove it to myself.

[JohninCR]

Update.  The nagging wasn't subsiding, so I did what I usually do when in doubt....go to Linkwitz since the answers are usually there.  http://linkwitzlab.com/rooms.htm about a third of the way down the page, the 1 2 3 items in the directivity topic.

"1 - An open baffle, dipole speaker has a figure-of-eight radiation pattern and therefore excites fewer room modes.
2 - Its total radiated power is 4.8 dB less than that of a monopole for the same on-axis SPL. Thus the strength of the excited modes is less.
3 - A 4.8 dB difference in SPL at low frequencies is quite significant, due to the bunching of the equal loudness contours at low frequencies, and corresponds to a 10 dB difference in loudness at 1 kHz.
Thus, bass reproduced by a dipole would be less masked by the room, since a dipole excites fewer modes, and to a lesser degree, and since the perceived difference between direct sound and room contribution is magnified by a psychoacoustic effect...."

It seems pretty obvious to me that a significant difference in actual radiated power for the same SPL plus some psychoacoustic effects due to directionality results in feeling less physical impact from OB bass partly because there is less actual power plus directionality makes it sound louder at the same SPL.

BTW guys, thank you for straightening me out about there being no such thing as a velocity wave.  Why are dipoles often referred to as velocity sources and monopoles as pressure sources?  Are those incorrect terms too, or are they correct (however misleading) because a dipole introduces no net pressure into a room, but a monopole does at a point in time?  There's so much mumbo jumbo out there regarding OB's, and I don't want to be part of the problem.

[Rudolf]

Rudolph,
Thanks, if both you and JohnK really don't notice a difference in physical impact with OB bass, ...

Sorry John,
I certainly did not want to give THAT impression. And certainly I would not want to be contradictory to SL in such an important matter 

Of course I hear and feel a difference. To me dipole bass seems to be more accurate/detailed and less punchy. My personal view is that this difference is due to less resonances excited in the room.
I would question room gain as a main source because I feel room pressurization below the lowest room mode as kind of nauseating, working in the head but not in the body.

Regarding velocity and pressure source: I believe JohnK has shown that it is like the difference between both sides of one medal - different views of the same object. So there is nothing really wrong with velocity source and pressure source. But I have been in too many discussions where people believed in different physical working principles for both sources. That´s why I might have been a bit cautious when you brought up those definitions.

[johnk...]

Rudolph,

Thanks, if both you and JohnK really don't notice a difference in physical impact with OB bass, then I must
be mistaken in my perception, because you guys have a lot more experience.  It seemed kind of obvious to
me, so stimulated resonance and/or SL's 4.8db dipole bass advantage or something similar must be causing
my misperception.  It is going to nag at me until I can figure a way to prove it to myself.

Hey no body is saying the dipoles done sound different. But there are differences of opinion as to why. With dipoles fewer room modes might be excited assuming you are in a sealed, recangular room. It's a little antidotal to generalize about room modes. Another perspective is that while dipoles may excite fewer room modes, they are also more sensitive to speaker and listener placement, as reported by Backman in the JAES. But for my money the real issue is room pressurization. Dipoles can not pressurize a room. Monopoles can. Monopoles will typically start to sound boomy when the cut off frequency of the woofer approaches or is below the room fundamental. This results in a bass boost that you don't get with a dipole.

What sound better also depends a lot on the music and venue. Listen to an orchestral piece recorded in a large hall and dipole bass might sound more natural. But listen to a kick drum or plucked stand up bass  recorded in a small club and that dipole woofer won't hit you like the real thing. That is one thing people often over look. Room modes don't just occur in the listening environment. They occur in the recording venue as well. only the recording engineer know how the recording is really  supposed to sound.

[AJinFLA]

Speaking of dipoles/monopoles, what ever happened to the CRAW? I can't find on your products page 
Any chance you rear horn load the NaO's cardioid section to improve output the way JohninCR has with his unique OB/RLH design?

cheers,

AJ

[JohninCR]

Thanks guys, the world is round again.  It always bothers me when I don't agree with the guys who know the physics.  Just to reconfirm, there's no such thing as a velocity wave as a sound wave, correct? 

The reason I brought up the physical impact of the sound from dipole, wasn't about whether they sound different.  I consider that a given and even think I understand most of the reasons.  I was asking wrt physical impact.  For some people feeling the bass is a high priority and a common term they use is "slam".  I typically caution them that OB may leave them unsatisfied because they have less physical impact and you don't "feel" the bass, even at high volumes.  eg  I would hesitate to recommend dipoles to a dance club, even though proper placement could take advantage of the nulls to keep the prime output on the dance floor leaving other areas much more quiet for conversation.  I was under the mistaken impression that it was a difference in the sound waves (the whole velocity vs pressure wave thing that I've read about elsewhere and never quite grasped).  Am I still flawed in my understanding, or on the right track?  I try to boil this stuff down to a simple form that anyone can understand, because I believe that most of us are intimidated and get lost in a lot of SL and JohnK's technical explanations.

Rudolph,
Also, note that I'm assuming flat FR.  I realize that peaks in tuning can increase the slam feeling too.  A good example is the Decware Housewrecker subwoofer.  It's a 6th order bandpass with a double peak tuning inherent in the design, and the upper peak in tuning around 80hz makes it a great party cab with plenty of "slam".  It not only sounds different, but it feels different too, but that's outside of the topic of my questions.

[johnk...]

Speaking of dipoles/monopoles, what ever happened to the CRAW? I can't find on your products page 
Any chance you rear horn load the NaO's cardioid section to improve output the way JohninCR has with his unique OB/RLH design?

cheers,

AJ

I discontinued the CRAW. The recommended plate  amps have been discontinued and I wasn't in the mode to rework another amp. It was designed as a research tool and served its purpose. Plus, it was expensive for what it did.

No, I won't be changing the NaO for the foreseeable future. It is a stable product gaining an audience little by little.

[AJinFLA]

Shame. One would think there would have been a market given the number of Maggie or other dipole panel speakers out there with restricted bass dynamics.
Did you ever perform subjective evaluations of a CRAW in dipole mode vs a single XXLS in the same depth H at modest output levels?
I understand that this is not an exact comparison, but I'm curious as to how much of the subjective impression being formed at (subwoofer frequencies) relates to radiation pattern vs a driver actually being enclosed.

cheers,

AJ

[johnk...]

I never compared the CRAW in dipole mode to an H. Though I will say that in dipole mode I did find, both subjectively and objectively (measurements) that in dipole mode it was the most sensitive to placemant. Measurements comfimed that the dipole seemed to excite fewer room modes. The problem was how those modes were excited. I did compare the U-frame to cardioid. The results were substantially the same. I would have no reason to expect otherwise for a ture dipole and H. Here is one comparison between monopole(=yellow), dipole (=red) and cardioid (=white).



You can draw your onw conclusions.

[AJinFLA]

With the closed box CRAW dipole in mode, I would expect the resonance peak to occur at higher frequency than the H, affecting the filter, etc., hence my not exact comment. But it should be close, at moderate levels, where aerodynamic noise, etc. are minimal. It would have been interesting to compare them subjectively, although the placement issue/room effect would have made that next to impossible. Perhaps outdoors?
I may have to investigate this further myself.

cheers,

AJ

[johnk...]

Oh, I see what you were referring to. You meant the dipole peak. It occurs at the same frequency in both cases. In the dual driver case there is only a dipole peak. For the CRAW that would be at about 300 Hz. For the H frame configuration with same front to back length there is a dipole peak at the same frequency but there is the possibility of the peak being augmented by the 1/4 wave resonance of the H, which is also at the same frequency. The peak is at WL = 2D and the 1/4 wave resonance is at WL = 4D/2 = 2D, WL = wave length.

The effect of the resonance (H) is typically to make the roll off locally a little shallower than it is with the dual driver dipole. For instance reducing the slope from 4 th order to 3 1/2 order. It is easily compensated for with a notch filter is necessary. Above the peak you generally start to see steeper roll off due to the effects of VC inductance.

I haven't heard any significant subjective differences between the CRAW and a similar H used with the same crossover filters.

[tubamark]

John,

So would you say - just from a sonic standpoint - that the differences between a pure symmetrical double-driver dipole and (same driver, but single in) an H-frame are negligible, assuming crossover well below the peak -- Efficiency, response, polar, etc about the same?

I am constructing a symmetrical double driver unit, simply because I have 4 woofers I like on hand, and because it is theoretically pure - no rear cone coloration & no open pipe resonance . . . But if the same drivers could be used for a double H or W without sacrificing sound quality . . .

I'm very open to the fact that not everything measurable is sonically significant.  99% performance at half price sounds good to me.

There has been little posted on this forum about symmetrical doubles. I assume efficiency & driver cost is what keeps H's and U's in the forefront.  Driver cost aside, what has been your experience with the symmetrical double versus H and even U formats?
(others chime in too, if actual experience)

Maybe I should have posted this as a new thread . . .

Thanks,
Tubamark