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Old 11th April 2009, 07:34 AM   #5191
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Hi John,

That's a nice set of amplitude responses you present there.

What is most often NOT said however is -

that these are the *steady sine* responses, and

that these do NOT become established or applicable until AFTER the drive waveform has become a steady sine at the frequencies measured;

ie. after the LS/cab/baf energy exchanges have become fully stabilised in Time.

However, Music is not steady sines, and it has its basis in Time.

During the first 90 degrees of waveform, say around 150Hz, all of the exampled loudspeakers will be transducing relatively linearly, though the boxed driver will be driving against additional resistance as it compresses enclosure air.

After 90 degrees (say 3mS) the cone motion of the boxed driver becomes considerably modified over a range of frequencies centred upon system resonance, this by energy exchanges between the driver itself and the enclosed air as cabinet returned energy either destructively decreases or resonantly increases the cone's amplitude response.
This is why we *hear* the rear pressure reflections upon cone transducers when they are 'boxed' and reproducing Music (as compared to test sines).

Your responses show that even the U-baffle sides will become audible circa 300Hz wrt the flat baffle via ongoing Music reproduction.

Music Waveform Amplitudes become modified in Music Time by energy returned from within the box or frame; this assymetrically and independently modifies the reproduced amplitude response of the driver cone in Box or Frame Time !!!

If there are any cabinet/frame induced pressure reflections impinging upon the rear of any driver cone, then every time there is LF content (especially percussive drum and guitar) the linearity of amplitude transduction and harmonic relationships of all reproduction becomes inconstant in Time.

With your boxed example this is first noticeable at 60Hz which makes for 16mS (related to Q) of typical waveform amplitude distortion in Music Time; while for the exampled U-frame similar audibility would likely appear at 240Hz / 4mS.

Such time constants have considerable bearing upon what we hear. They modify the amplitude response of ALL frequencies reproduced by the same driver with respect to the waveforms reproduced by any other drivers !!!!!

For this reason the only LF drivers I have are large OB, because I can no longer stand the sound of those entirely *avoidable* internal box pressure 'reactions' which so distort music reproduction.

Quality is what really counts; not quantity .

Cheers ........ Graham.
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Old 11th April 2009, 08:13 AM   #5192
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Teh



...
Furthermore, Lynn, himself, has comments on the absolute precision and stunning dynamics of the Summas. That's quite a commentary. I've also read reviews comparing the Summas to other OB speakers and the reviewers actually commented that the Summas were, not just better, but, FAR superior.

...
I would be interested in reading the reviews that you mention. Are there any links to those reviews? Up to now, I have not been able to find any reviews that specifically addresses music source + other speakers compared + specifically addresing music passages where people can listen for on their own systems.
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Old 11th April 2009, 09:41 AM   #5193
Rudolf is offline Rudolf  Germany
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Quote:
Originally posted by Graham Maynard
Hi John,

That's a nice set of amplitude responses you present there.

What is most often NOT said however is -

that these are the *steady sine* responses, and

that these do NOT become established or applicable until AFTER the drive waveform has become a steady sine at the frequencies measured;

Cheers ........ Graham.
Graham, don΄t you read?
Those measurements are done with IMP (which stands for Impulse), and very purposefully they avoid any '*steady sine* response' you are talking about. So your conclusions simply are not valid - at least not in this case.
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Old 11th April 2009, 10:18 AM   #5194
Telstar is offline Telstar  Italy
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Quote:
Originally posted by Graham Maynard

For this reason the only LF drivers I have are large OB, because I can no longer stand the sound of those entirely *avoidable* internal box pressure 'reactions' which so distort music reproduction.

Quality is what really counts; not quantity .

Cheers ........ Graham.
I wholeheartedly agree with Graham here.
Even if the premises are wrong, the results are like described. Boxed LF sounds horrible.
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Old 11th April 2009, 10:45 AM   #5195
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Quote:
Originally posted by MBK

John K to offer a comment on your graphs: the different FR bumps for box and u-frame vs. truly open baffle can be explained with not too much trouble by resonance of the box (the air that is) and the cavity resonance of the u frame. This does not have to mean that the cone itself is "leaking" rear radiation. Not to claim it doesn't exist but if such leakage truly was a major problem then compression drivers would have to be deemed completely useless due to their rear chamber no?

The near field response curves I show are directly indicative of the cone motion. That the sealed box and U-frame results show clearly the effect of the enclosure resonances is an indication of the "acoustic" transparency of the cone (or maybe it should be called translucent). Lynn said cones a transparent. Earl said they are not. I said it not so black and white. Also as I said a while back, when the term transparent is used it does not mean the box/u-frame resonances passes through or leak through the cone. It means the rear resonances which impinge upon the back side of a cone transfer some of their energy to the cone to excite it to vibrate in a different manner than it would in the absence of the resonance. This difference in vibration is then reflected in the front side SPL response. To what extent this happens depends on many factors including the difference between the acoustic impedance of the cone and and air, the thickness of the cone, the internal damping of the cone... to name a few. Your comment about compression drivers applies equally to any driver with a closed chamber, like a dome mid, or tweeter. In all cases the rear chamber must be correctly designed, and possibly damped, so as to minimize or eliminate the effect of chamber resonances on the response in the desired passband.
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Old 11th April 2009, 11:15 AM   #5196
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Graham,

You always seem to take a simple process and convolute it into something incomprehensible. What you are looking at in my plots is the representation of the system impulse response in the frequency domain via FFt. The addition of the resonances does not change the linearity of the system. Given the impulse response I measured, it could be convolved with any input signal to get the output. It has noting to do with SS sine waves, except that in the input was a SS sine wave the output would be reproduced with amplitude as shown in th FR plots.

Any system which does not have a perfect impulse necessarily add some decay tail to the input. Therefore, if you like, any system with a non perfect impulse adds time smear. This goes back to the discussion on stored energy and linear distortion.

I would get in to low frequency sources and what the box does. It's been don't to death in these discussion groups and isn't relevant to midrange reproduction.
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Old 11th April 2009, 11:51 AM   #5197
mige0 is offline mige0  Austria
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Regarding the discussion about cone transparency two things got constantly mixed that should more clearly be distinguished from each other IMO.

1.) first part that is related upon mirroring the back wave
2.) the other part is about the different acoustic resistance a driver has to work against in OB, back open or closed box or whatever


Ad 1.) I have shown in

http://members.aon.at/kinotechnik/di...ucing_BDMD.htm

that this actually shows the "real" transparency of diaphragms (if you refuse to accept Lynns great arguments about building cardboard speakers and monitoring rooms with paper walls)

For this part we have to be aware of the nature of the mirror actually in duty – if you have not only one mirror in the back but rather many of them - the effect is different (smeared it is) as outlined with several simus – also giving different impression of box coloration with different shapes of boxes
(Horn honk due to reflection at points of strong impedance change actually is of the same nature - only difference - its a reflection from the front not from the back)

The main thing to keep in mind with this part also is that the distortion occurs after a certain time – related to the time of flight diaphragm > mirror > diaphragm.

Meaning there actually *is* a time window where there is *no* distortion through that effect


Ad 2)
This is a completely different part as the driver "sees" the acoustic impedance form the very beginning of cone movement.
Here there is *no* early time window of cone movement that is *not* affected by the shape of your enclosure.
Regarding this effect the cone movement is throughout determined by the shape of the box (in overlay with its own parameters).

For this effect we also could say the cone is transparent, but more precisely we possibly should say its coupled – what in the end turns out to be roughly the same from a perception point of view.

Michael
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Old 11th April 2009, 12:17 PM   #5198
MBK is offline MBK  Singapore
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Quote:
Originally posted by john k...


Also as I said a while back, when the term transparent is used it does not mean the box/u-frame resonances passes through or leak through the cone. It means the rear resonances which impinge upon the back side of a cone transfer some of their energy to the cone to excite it to vibrate in a different manner than it would in the absence of the resonance.
This is what I meant exactly in my comment above (also corresponds to Michael's "ad 2)"). But this also means that the graphs do not show anything related to a "transparent" or translucent cone in this situation. It just means the cone as an electrodynamic spring mass system is influenced in its own resonant behavior by the ordinary and usual resonances / impedances it encounters. Additional bracing or adding tons of lead to the box won't change any of that because it's the air volume really that does it - that's the fundamental spring. And, if the design of the box / u-frame etc takes proper account of the resonance it must not necessarily be a problem.

The discussion started along the lines of "disorderly box modes may leak through the cone". I just wanted to point out that John K's graphs showed something else, it showed the response of the driver's resonant system to coupling with the box or u frame resonant system. I think we are finally in violent agreement here. And, I don't even like boxes - I am not using any (OK OK I do: my tweeter has a rear chamber).
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Old 11th April 2009, 12:43 PM   #5199
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Quote:
Originally posted by MBK


But this also means that the graphs do not show anything related to a "transparent" or translucent cone in this situation. It just means the cone as an electrodynamic spring mass system is influenced in its own resonant behavior by the ordinary and usual resonances / impedances it encounters.
Yes, exaclty, this is what is meant by "transparent". That is, the effects of the box resoances alter the mechanical motion of the cone such that these resonances, or there effect on the cone, are then re-radiated from the front side. Perhaps it is the term transparent that is the source of confusion which is why I tried to explain (9define) it a while back.

The terms transparent and opaque are just lables being applied to indicate that either the cone reacts mechanically to the internal resonances, thus their effect impacts the radiated sound, or the cone does not.
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Old 11th April 2009, 02:00 PM   #5200
mige0 is offline mige0  Austria
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If we
- are accepting my theoretical work, simus and measurements about BDMD "back diaphragm mirror distortion" to be valid
- and we also accept to distinguish between the two effects (delayed and non-delayed overlay of cabinet behaviour) described in my last posting

- then we possibly also can state some conclusions of interest that apply to the discussion at hand, regarding impacts of "box cabinet resonance" (more so about mechanisms involved than its sonic significance though).

If we take a standard closed box, all walls can be considered to be mirrors for the back wave. If the walls vibrate
– one effect is that there is sound radiation towards the surrounding - ie. directly into the listening room.
– the second effect is that the mirror is vibrating – hence modulating the reflection of the back wave and also re-radiating sound towards the driver

For that part of box cabinet vibration created by the reaction forces of cone acceleration (rather than by pressurising the cabinet) - that ideally could be avoided by massive construction or free suspension of the driver - we also can state that there is *additional* energy radiated from the diaphragm.

The part of "from box wall generated sound" that's transitioning through the cone my easily be perceived as distortion of the driver – what isn't correct in a more precise understanding of second order effects.



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