Autotuba measurements, plus shiva

[mwmkravchenko]

Hello John. I have posted with you and respect your opinion.

Hello RCW. I have read your posts and they have had a decent grounding in theory.

From what I have read there are valid points in both of the posts up untill you both get tired. Please! Your both intelligent men with lots to offer. Don't give in like this.

If you look at things from an objective point of view you come up with this.

1. A sealed enclosure when sized correctly has a better transient response to a signal input than does a bass reflex. It comes to a point of rest more quickly than does a reflex enclosure due to the air in the enclosure not wanting to stay compressed or rarefied for longer than it is forced to. Overall a theoretically better enclosure. They sound sharper and more lifelike if you have been exposed to the instruments you are listening to live and recorded.

2. A reflex enclosure has some things going for it.

For a given size the low end cutoff and the over all efficiency will be lower and higher respectivly. For many applications these are the only determining factors. Louder and lower.

For the part that is of most interest. Actually the part that offers both the most colouration and the most distorsion. Wether you try to think of it with formulas or just in a common sense manner there is a conflict in the output of the systems in a reflex enclosure.

In the region that the port and driver give up and take over there is a real and definite phase ( read time delay actually ) shift. The sound being reproduced actually goes from being produced from a piston firing at you ( the driver ) to a helmholtz resonator ( the port ) firing from inside an enclosure. As the two cross over from each other there is a slight null and then a shift in the time between the signal input and the sound output.

A helmholtz resonator is not magical. It is a derivative of the grand old pipe organ. A system well understood for centuries. It to must be driven. In this case by the driver. Even though the port is producing the bulk of the sound (in the region we are talking about) it still is in the region of 180 degrees out of phase ( same as listening to something face on and then turning your back to it and listening to it again). So where the two are working at producing sound output there are some conflicts! The two sources of sound cause each other to fight over a narrow bandwidth and there is a definite degradation of transient response due to the lag in time between the driving piston and the resonating port. ( there is more and I acknowledge that but for the puposes of this short explantion this is the crux of the matter )

I have built and listened to both and I still like horns better!

Mark

[Stocker]

Guys, have a smoke and relax! If each man is happy with his box, let's all be happy with each other!

Of course different boxes sound different. Of course the same box sounds different when the port is open or closed. Physics are physics. The only half-difference I want to make with anyones opinion is that bandpass boxes don't dominate, not because they can't sound good, but because they are difficult to design, and VERY easy to make sound bad.

[johninCR]

Ok I apologize to all.

Mark,

Your explanation of the difference resulting from the conflict in output was well stated and the wording for which I was searching. I hestitate to lay the bulk of the blame there, because my dipoles don't exhibit any SQ problem. With front and rear waves both in full play there is the combination of waves with varying phase relationships across the spectrum, which doesn't sound destructive.

It may be that the tonal characteristics of what comes out of the port vs cone radiation are different enough to have a destructive influence in the range where apples and oranges are combined. As an example I'll use my Decware designed Housewrecker which is an isobarik 6th order pandpass with a large sealed aircushion in the isobarik chamber. I compared it to the same drivers in sealed boxes and EQ'd in the same response for both. The housewrecker has a very different presentation in its operating range below 100hz. While I'm sure some will argue that all bass waves are created equal, I don't believe they sound the same with music just like 2 different midrange drivers don't sound the same even if they measure the same.

In the case of comparing my EBS box with the ports open and closed the sound difference is "loose and sloppy" as I said before, so I believe it's simply a matter of the cone movement stopping more quickly with the ports sealed. At least that's what it sounds like.

With higher tuned BR's like woofers the sonic difference is probably a combination of the factors you mentioned, but there is definitely a sonic difference and not just in the response curve.

[BassAwdyO]

I'm no expert.... But might there be some kind of "critically dampened" vented box with perhaps the tuning equal to the FRC of the sealed system of equivilant size which would make the port actually slow down the driver faster? Probably no such thing... but I would think in a rare case the port might actually help slow the driver faster, I may be dead wrong

It is mostly step response that we judge in the bass region as being "fast" or "slow" I believe... The're no such thing as transients down there for sure. Voice coil inductance instantly smoothes a transient into... a non transient

[BassAwdyO]

A helmholtz resonator is not magical. It is a derivative of the grand old pipe organ.

From my understanding a pipe organ is more/less a quarter wavelength pipe with whisle on it to excite the resonance... not a helmholtz resonator... I may be wrong here again

[rcw]

I think Stockers point about bandpass boxes is well made.
They are difficult to get right, and they got most of their bad reputation
In the days before computer simulation and the only things to go by were Earl Geddes "calculator bashing' models. People rushed to them because they were a cheap easy to build alternative to horn loaded subwoofers for sound reinforcement, and the results in most cases were woefull.
I do however emphasis my point that there is no intrinsically better way of making a subwoofer, double blind testing indicates that you can't hear the difference between various forms of loading when such things as room resonances and proximity to boundaries is taken into account, and as a scientist I have to trust the research even if it conflicts with prejudices that I might have.

[johninCR]

Originally posted by rcw
I think Stockers point about bandpass boxes is well made.
They are difficult to get right, and they got most of their bad reputation
In the days before computer simulation and the only things to go by were Earl Geddes "calculator bashing' models. People rushed to them because they were a cheap easy to build alternative to horn loaded subwoofers for sound reinforcement, and the results in most cases were woefull.
I do however emphasis my point that there is no intrinsically better way of making a subwoofer, double blind testing indicates that you can't hear the difference between various forms of loading when such things as room resonances and proximity to boundaries is taken into account, and as a scientist I have to trust the research even if it conflicts with prejudices that I might have.

I guess all amplifiers with .01% or less THD will sound the same too. Why did you even bother to make a quality sub when the general public can't tell the difference in blind testing? What differences the general public can or can't hear in blind testing is irrelevant because their ears are totally untrained in identifying the sonic differences involved. While of course the effects of the room are very important, they are not the only factor. As a scientist you should understand that, once trained, our ears and brain are far more sensitive in many respects than any manmade instruments. Believe what you want, it is of no consequence to me, however, you are doing yourself a disservice if you don't learn to trust your ears more.

[mwmkravchenko]

From my understanding a pipe organ is more/less a quarter wavelength pipe with whisle on it to excite the resonance... not a helmholtz resonator... I may be wrong here again

Tis not as simple as all that. If you whistle while you work than....

An open pipe or flue pipe sounds at a tone like this. Approximately an 8' pipe sounds at 64 hz a 16' sounds at 32 hz and a 32' sounds at 16hz. They are 1/2 length resonators. There are pipes that do have 1/4 wavelength resonators ( usually reed pippes. Think of a saxophone where there is a reed that vibrates into a column of air ) and there are pipes that are stopped ( the open end is plugged ) and sound an octave lower than their open counterparts ie a 16' stopped pipe will sound at 16hz. Then there are the overblown pipes that sound an octave or two higher depending on the amount of pressure that is being used to make them sqeak.

Crazy man Crazy OR sorry you asked ( in a evil mood

Hey check this out! http://www.sengpielaudio.com/calculator-wavelength.htm


Mark

[paulspencer]

I think we should all be a little less certain of the conclusions we are drawing here. A lot less certain and a bit more curious.

John you say your vented sub sounds less musical than the sealed version. Fair enough.

To give a definitive reason for this is not something I believe we can do just yet. There is a lot of theory to back up different points of view, but I don't believe sufficient to come up with definitve answers which all must accept without question. The reason I say this is I don't believe our understanding of what we can measure and how this translates into what we perceive has progressed far enough to be so sure.

example: RCW, you say that 3% THD is audible. However, I've seen some work by Geddes on distortion perception which indicates that THD measurements and our perception of distortion do not necessarily correlate. www.gedlee.com <<< downloadable sampes are available on their website to indicate this, including a brief discussion of a new way of measuring distortion which actually correlates to what we hear.

This is part of the reason why there appears to be disagreement between measurements and perception - we don't fully understand how the relationship between what we measure and hear.

Regarding sealed vs vented subs:

* vented subs have output in the range most critical for music accuracy coming from the driver itself, the vent contribution is not significant for most of the bass in music
* vented subs have much lower distortion where their entire bandwidth is being used, however the distortion is reduced down low where it is not as critical
* sealed subs have what we consider better transient response, however can we actually perceive this? At which point does this occur? Suppose you are looking at the transient response around 70Hz - will this actually be any different for vented vs sealed, or is the slightly superior (in paper) transient response of sealed boxes down at a point where our ears can't perceive the difference anyway?
* IB subs are considered by many to have the ideal transient response, however, is this the difference that is actually heard? Is it instead that what is heard is a degree of linearity not normally experienced due to the larger number of drivers? Or is it that there is actually a very audible and detrimental impact of a vent, as well as the distortion of a sealed box, but that both problems are eliminated with IB, also with an audible improvement in transient response?
* vented vs sealed subjective comparisons are rarely made in such a way as to coming even close to identifying a clear cause/effect relationship - use of fairly sophisticated eq and outdoors listening with blind AB testing would be a starting point if you really want to begin to argue about the impact of the vent - otherwise perception is coloured by the room, a different frequency response as well as one's expectations

As much as I like the way that a heated debate brings out at times a more in-depth discussion, is it possible for us to proceed without the need to prove which opinion is right? Is it possible that all (or at least most reasonably informed) opinions have a piece of the truth? Is it also possible that we could all learn more if we all put foward our views, subjective experiences as well as our understanding of theory on the table, not in opposition but in collaboration, that we could actually learn more? I think if we ever do get definitive answers, it will only be by learning to reconcile apparent contradictions that appeared to be fighting each other.

[rcw]

The thing that I am trying to point out Paul is that if you are going to design something you need a place to start, and a good place to start is a list of priorities.
For instance we have audio pundits banging on enlessly about the superior transient responce of sealed boxes, this is true but testing indicates that if it is below a particular threshold frequency you can't hear it. It is also true that all other things being equall a sealed box produces more distortion than a vented one, and testing indicates that you can hear it, even at the lowest frequencies.
So by touting the superior transient performance of the sealed box as being more desirable you are favouring an abstraction that you can't hear in favour of a real effect that you can, so much for living in the real world.
I could go on further but I wont, there is a point where robust discussion can degenerate into a slanging match, and this one if not quite reaching that level has become perilously close.