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Old 2nd February 2004, 06:59 AM   #1
brianon is offline brianon  United States
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Default Open Baffles :- )

A message I posted about open baffles on joelist... thought it might be relivant here,

Hi...

Folks, please remember that most modern drivers are not designed to be used
in an open-baffle configuration. The cones will over-modulate, which causes
distortion.

Anyone know of any companies that specifically design drivers for
open-baffle?

I have direct experience in this. I bought an NHT 1259 bass driver. Found
that the designer had specs for using the driver with a port. The designer
specifically stated that the driver generally could not be used in anything
but a closed box. BUT, he stated that it could be used in a ported
enclosure but the results could not be guaranteed.

So I made a ported enclosure and tested the configuration. The driver was
rated at 16 hz -3db 1 watt 1 meter in the ported config compared to 25
hz -3db 1 watt 1 meter, both measured directly in front of the driver (
on-axis).

When I turned the amp up beyond 1 watt, the driver cone fluttered like a
goose-wing in the ported config. You could actually see it occur below 20
hz!

The lesson is, drivers today are engineered to be in enclosures, not
baffles. The lack of back pressure will distort the output, not matter what
the subjective listening experience is. Open Baffles are extremely hard on
today's speakers. Of course the subjective is what is important but the
wear and tear of forcing a mechanical object to do things it was not
designed to do is not good engineering or design... and the driver is
distorting no matter if you notice it or not while listening.

The better route is to work within the laws of physics. Or buy drivers that
are designed to be used on an open baffle. I'm writing this so people do
not spend vast amounts of money to buy 12 8" bass drivers per channel to
defeat the laws of physics and concurrently tear-up the drivers by
letting their cones flop back and forth in a sloppy way.

A soundfield full of holes caused by destructive interference is not the
direction I would move toward, but, that's just me. Open Baffles cause
that, don't believe me, refer to any intro book on physics. If you are
looking for a pure sound, a realistic sound... open baffles are a dead-end
pathway.

The funny thing is, the listening zone that is full of holes caused by
destructive interference is a two-edge sword. There will be no bass
response unless dramatic efforts are used and the bass sound will not be
better because spreading more bass over the holes will naturally cause weak
bass at the empty spots. AND, as frequencies become higher, the
interference patterns become closer together. Find that sweet-spot and dont
move your head even a tenth of an inch or one ear will dip into a zone of
destructive interference.

This is the reason why loudspeaker are in boxes


Those that rave about open baffles probably are unconsciously biased due to
their efforts building the baffles and the cost of purchasing massive
numbers of expensive drivers--- they unconsciously NEED to believe that
their investment sounds good, physics be damned.


I guess if one wants to listen to a solo flute instead of regular music, an
open baffle COULD be ok. If you can ignore all the destructive interference
and also ignore all the damage occuring to the loudspeaker cones.

I totally agree with the concern that people have about box coloration. The
answer is not in eliminating the box, because physics demands a box to
seperate the out of phase soundwaves broadcast from the rear side of the
loudspeaker cones at equal sound levels to the front side of the cones.

Box coloration is a real effect in a major way. Soundwaves bouncing inside
the enclosure causes a reflective effect on the loudspeaker cone a few
millaseconds after the initial sound is broadcast, which muddy's the sound
output. Also, soundwave diffraction occurs whenever a soundwave passes a
sharp corner-- even a corner at the edge of an open baffle.

The soution was first detailed in 1938 by Olsen. Merely curve the
loudspeaker enclosure. Additionally, curving the enclosure adds stiffness
to the material used in the loudspeaker enclosure. This is actually a good
thing because it raises the resonant frequency of the enclosure, requiring
more dampening material to be applied. Result-- more headroom for dampening
mistakes. But the key benefit is reducing external diffraction effects.
Open Baffle does not do this because the baffle has a sharp edge.
Curving the inside of an enclosure also reduces the amount of bounceback
hitting the back side of the loudspeaker driver because the soundwaves are
aimed inward where they harmlessly dissolve into heat... two pressure points
exist, along the inside walls of the enclosure and at the center of the
enclosure. These effects are all well-documented.

However, building curved enclosures is really difficult. Not as easy as
cutting various sized holes in a baffle. But as more than one poster on
this thread has pointed out, true sound quality has not been the alpha and
omega when building loudspeaker enclosures with commerical loudspeaker
companies. If one really wants sound quality from their speakers that
matches the work they put into their amps, their musical decoders, etc.
they sould go the proper mile and investigate the "final mile" at the
enclosure end. Why take so much care on the back-end and then pump the
carefully attuned signal into a mushmaker?

Bri

PS: Don't try the baffle. It will hurt or destroy your drivers if you
listen to anything more than classical flute. It will cost you a fortune to
buy enough bass drivers to defeat the laws of physics and your soundstage
will still be full of interference-caused holes. Baffles have razor sharp
edges that cause external diffraction into the listening soundstage. There
are far better yet seldom-used alternatives.
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Old 2nd February 2004, 11:32 AM   #2
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oh well in that case I will be first to say damn physics. I enjoy my OBs with a massive numbers of expensive drivers that is if a JBL2225H and a fostex FE206E per side can qualify as such.
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Old 2nd February 2004, 11:47 AM   #3
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Quote:
Folks, please remember that most modern drivers are not designed to be used in an open-baffle configuration
Hi,

Your post has some interesting thoughts but you seem to build your whole thesis on this one sentence...

My question is... do they really?

How do you know? Apart from the one driver you tested that is..

I am planning a MCM 55-1870 open baffle line array. Uses little 5inch drivers...can't imagine them flapping about..

I imagine your driver did cause it was a subwoofer....type of driver

Cheers,
Bas
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Old 2nd February 2004, 02:57 PM   #4
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Default Re: Open Baffles :- )

Konnichiwa,

Quote:
Originally posted by brianon
A message I posted about open baffles on joelist... thought it might be relivant here,
Well, then, a message I posted on the Joelist, I think it is relevant here....

> Folks, please remember that most modern drivers are not
> designed to be used in an open-baffle configuration.

Sure, so don't use them. Normally drivers are aimed at a specific application. For example, many car audio drivers are designed for "open air" use, also most pro-audio drivers will happily operate without rear load, especially under domestic conditions. Of course, those ****-poor $ 2 each in quanteties of 1,000 excuses for "High Fidelity" drivers commonly used for so-called High Fidelity Speakers and often also sold expensively to DIY'ers cannot operate well in open baffles, but let's face it, theyr don't do well in boxes either, so why bother using these jokes?

> Anyone know of any companies that specifically design
> drivers for open-baffle?

Many companies make drivers aimed at operation without acoustic rear load. They usually are not specifically aimed at Audiophool HiFi though, so one muct look in the Car Audio and Pro Audio ranges of such manufacturers.

> I have direct experience in this. I bought an NHT 1259
> bass driver.

Now here is an example how NOT to do it.

The 1259 is a specially desigend custom driver, intended for a specific Speaker and not a general use one. The result is that it is highly optimised and in effect cannot be operated sucessfully under different conditions. That is a result not of being a modern driver (the parameters of the 1259 at it's time where VERY unusual for a commercial driver) but of being a driver highly optimised and customised for a specific application while manufacturerd in large quanteties cheaply for OEM use (where the application is a strictly limitet one).

> The lesson is, drivers today are engineered to be in enclosures,
> not baffles.

You make mental leaps that are surprising.

Because of your experience with ONE SPECIFIC driver and thereby one that was not designed as generic driver for a wide range of applications as most of the "sold to the general public" drivers are you conclude that your experience applies generically to "All modern drivers". Superb.

> The lack of back pressure will distort the output,

With a specific driver OEM developed to offer for a very specific application an optimised set of parameters, disbaring effectively any other use.

> Open Baffles are extremely hard on today's speakers.

You make again a general statement. The Drivers on my open baffles are modern and work fine, without flopping in the air and so on. Open baffles are not hard on them. So clearly, open baffles are "extremely hard" on SOME drivers (I could also think of certain old ones), but in general many suitable drivers exist.

> The better route is to work within the laws of physics.

Which is exactly what Open Baffle speakers do and do quite well if I may add.

> Or buy drivers that are designed to be used on an open baffle.

Of which there are plenty (either designed or suitable), if you know where to look.

> I'm writing this so people do not spend vast amounts of
> money to buy 12 8" bass drivers per channel to defeat the
> laws of physics and concurrently tear-up the drivers by
> letting their cones flop back and forth in a sloppy way.

I am sure your intentions are pure consumer protection, but even fairly generic 8" drivers when used 12pcs aside will work fine. Now if you had advocated to avoid using a single, unsuited expenbsive woofer driver (NHT 1259 for example), that would make sense.

> A soundfield full of holes caused by destructive interference
> is not the direction I would move toward,

Nor would I, hence the use of dipole radiators over the modal range of the room (which excites room modes minimally and which offers one of the best directivity of any simple structure at low frequencies, with a directivity pattern moving towards a cardiod/hypercardiod one above around 500Hz - 1000Hz as observable with Cone Drivers on open baffles. In fact, using open baffle cone drivers MINIMISES the destructive interference of soundwaves observable with common socalled "High Fidelity" Speakers.

> Open Baffles cause that, don't believe me, refer to any intro
> book on physics.

I find that statement hard to accept, simply based on basic acoustics.

> If you are looking for a pure sound, a realistic sound...
> open baffles are a dead-end pathway.

That seems a conclusion based not on actual empirical evidence and proper acoustic research. It seems more a prejudice than a conclusion based on actualities. Did you ever get bitten by an open baffle as child?

> The funny thing is, the listening zone that is full of holes caused
> by destructive interference is a two-edge sword.

And one that is minor and small compared to that of the intererence caused by sound "bouncing" off room boundaries and room modes.

> There will be no bass response unless dramatic efforts are
> used

Why? Did you ever build a dipole?

> and the bass sound will not be better because spreading
> more bass over the holes will naturally cause weak bass
> at the empty spots.

Actually, based on experience the "bass holes" and even more the "Bass peaks" caused by room are much more trublesome. Extensive research (not by me - but which included a number of AES Papers and other publications) showed a much reduced excitation of room modes, both in terms of constructive and destructive interference at low frequencies.

> AND, as frequencies become higher, the interference patterns
> become closer together. Find that sweet-spot and dont move
> your head even a tenth of an inch or one ear will dip into a
> zone of destructive interference.

By the time you get to frequencies where a few inch make sucha difference the radiation pattern of a cone driver on a baffle is no longer a figure 8, but a hypercardiod, which means there is very little desructive interference. Again, basic acoustics.

> This is the reason why loudspeaker are in boxes

Are you ABSOLUTELY CERTAIN? The reason speakers are in boxes have much more to do with the fashion for very small speakers with reasonable low frequency extension than any physical or acoustical considerations.

> Those that rave about open baffles probably are unconsciously
> biased due to their efforts building the baffles and the cost of
> purchasing massive numbers of expensive drivers

Funny. My baffles where both the easiest and cheapest speakers I ever build, plus in many ways the best sounding.

> --- they unconsciously NEED to believe that their investment
> sounds good, physics be damned.

I find that mostly true about people who build boxes. They must sound good, physics be damned.

> I guess if one wants to listen to a solo flute instead of regular
> music, an open baffle COULD be ok.

Well, my open baffles handle home cinema just fine, pretty big explosions etc. Rock, Rap and the like too, at volume levels definitly neighbour unfreindly. Solo Flute is fine too, of course.

> If you can ignore all the destructive interference

Which actually does not happen in reality.

> and also ignore all the damage occuring to the loudspeaker
> cones.

Which actually does not happen in reality.

> I totally agree with the concern that people have about box
> coloration. The answer is not in eliminating the box, because
> physics demands a box to seperate the out of phase
> soundwaves broadcast from the rear side of the loudspeaker
> cones at equal sound levels to the front side of the cones.

Hmmm. A suitable size baffle does this just as well and provides a big advantage, namely the effect that the net pressure change in the room is zero.

> The soution was first detailed in 1938 by Olsen. Merely curve
> the loudspeaker enclosure.

This solves very few of the main issues. Diffraction for example is excessively overrated, a competently designed driver offer reasonably controlled directivity once the baffle depth becomes acoustically large (compared to the wavelength) and edge diffraction becomes an issue. And no other problems are really solved.

> Additionally, curving the enclosure adds stiffness to the
> material used in the loudspeaker enclosure.

Which would not be needed if one was not suceptible to the delusion that the rear wave must be "imprisioned" at low frequencies (at higher frequencies it's not an issue anyway).

> But the key benefit is reducing external diffraction effects.

Which are not an issue with competently designed transducers.

> Open Baffle does not do this because the baffle has a sharp
> edge.

Yet with a competently designed transducer there is no problem to start with.

> Curving the inside of an enclosure also reduces the amount of
> bounceback hitting the back side of the loudspeaker driver

To a modest degree...

> However, building curved enclosures is really difficult.

Is it? A simple moulding suffices and is quite easily done, actually.

> Why take so much care on the back-end and then pump the
> carefully attuned signal into a mushmaker?

Like the kind of speaker you get if you place a modern "HiFi" driver into a curved enclosure, with very high levels of compression and distortion?

(msg cut - too long)
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Old 2nd February 2004, 02:59 PM   #5
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Default Re: Re: Open Baffles :- )

Konnichiwa,

Quote:
Originally posted by Kuei Yang Wang
Well, then, a message I posted on the Joelist, I think it is relevant here....
continues.....

> PS: Don't try the baffle. It will hurt or destroy your drivers
> if you listen to anything more than classical flute.

That is chemically pure male bovine excerement. Full stop.

> It will cost you a fortune to buy enough bass drivers to
> defeat the laws of physics

That is chemically pure male bovine excerement too. Full stop.

> and your soundstage will still be full of interference-caused
> holes.

More chemically pure male bovine excerement.

> Baffles have razor sharp edges

Oh goodies, I hope my folks at home don't cut themselves on them.

> that cause external diffraction into the listening soundstage.

How so, if the transducer does not radiate any sound towards them at frequencies where such interference can happen?

> There are far better yet seldom-used alternatives.

Which would be?

Would you mind to suggest a type of speaker "enclosure" that matches or improves on the behaviour of the open baffle in terms of directivity at low frequencies, middle and high frequencies, an enclosure that shows the same low degree (or a lower degree) of room mode interaction and an enclosure that illustrable causes the same low degree or less distortion of low frequency transients than an open baffle? I am all ears.

Sayonara
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Old 2nd February 2004, 03:27 PM   #6
Nuuk is offline Nuuk  United Kingdom
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I would second eveything that KYW says if I understood half of it

Seriously though..

Quote:
Those that rave about open baffles probably are unconsciously biased due to their efforts building the baffles and the cost of purchasing massive numbers of expensive drivers--- they unconsciously NEED to believe that their investment sounds good, physics be damned.
My own open baffles cost me 25UKP to build and the drivers another 35UKP so I don't think that they sound extremely good because of their cost. And they are by far the easiest speakers that I have built so it's not the effort that's 'fooling' me.

The bottom line is that every loudspeaker design has some compromise, and yes, if you use drivers not suited to the job, that's a recipe for disaster but that doesn't just apply to open baffles!

PS I can't hear my hi-fi when I'm unconscious so I doubt that I am "unconsciously biased"
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Old 2nd February 2004, 04:22 PM   #7
kan3 is offline kan3  United States
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dayton ib drivers are suppose to be good for the money
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Old 2nd February 2004, 08:16 PM   #8
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Default quick question

I am wondering if these would work well, they have a qts of .78. would they work better in a Sonotube type TL, or just a plain ol 2 cubic ft sealed box. (btw i have 16 of them in order to make a left and right line array)
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Old 2nd February 2004, 08:17 PM   #9
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sorry i forgot the link http://www.partsexpress.com/pe/pshow...number=299-721
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Old 2nd February 2004, 08:20 PM   #10
Sjef is offline Sjef  Netherlands
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Brianon, have you actually ever listened to a good designed open baffle system ?

I have heard pretty much all of your comments on them a hundred times before but always from people who have only tried a random speaker unit on a random sized baffle. If you have tried that and you didn't like it I think you are more than perfectly right with your statements. THAT DOES NOT WORK. That's the same as putting a random driver in a shoebox and tell everyone that closed enclosures don't sound good, is it ? Don't take it as an offence but you clearly should study a bit more open baffle systems and listen for once to a good designed one before judging them. (there are a lot of bad designs out there, but that also counts for reflex, closed or whatever designs)

>>>So I made a ported enclosure and tested the configuration. The driver was
rated at 16 hz -3db 1 watt 1 meter in the ported config compared to 25
hz -3db 1 watt 1 meter, both measured directly in front of the driver (on-axis).

That's not the same load as an open baffle load. With a reflex enclosure you are creating a resonator wich can be a very difficult load for some drivers especially when driven with any material below the tuning frequency.

>>>The lack of back pressure will distort the output, not matter what
the subjective listening experience is

For a lot of modern midranges the present of back pressure causes more time delayed cone break-up, curving the enclosure helps a little but not much, air pressure doesn't behave in the same way as light as many people think. The curves only help at frequency's where the driver beams like a laser beam. It's got totally no effect on low frequency's.


>>>The better route is to work within the laws of physics. Or buy drivers that
are designed to be used on an open baffle. I'm writing this so people do
not spend vast amounts of money to buy 12 8" bass drivers per channel to
defeat the laws of physics and concurrently tear-up the drivers by
letting their cones flop back and forth in a sloppy way.


I totally agree on that one, not every driver is suitable for open baffle bass. Twelve 8" drivers wich where designed for reflex enclosures will not work, this way is a complete misunderstanding of the way open baffle systems work.

>>>The funny thing is, the listening zone that is full of holes caused by
destructive interference is a two-edge sword. There will be no bass
response unless dramatic efforts are used and the bass sound will not be
better because spreading more bass over the holes will naturally cause weak
bass at the empty spots. AND, as frequencies become higher, the
interference patterns become closer together. Find that sweet-spot and dont
move your head even a tenth of an inch or one ear will dip into a zone of
destructive interference.


Again, a complete misunderstanding of how an open baffle actually works. The size and shape of the baffle determines the off-axis response, wich, if good designed will actually be better than that of a closed box.

As for open baffle bass, yes it does have directivity, but that's also one of the main advantages of the system. I have compared my woofers in a reflex enclosure and in a electronically compensated open baffle system and did a lot of measurements on both of them.

First of all, the open baffle had a resonance of 31 Hz, the reflex enclosure, while being five times the size only went down to 45 Hz. Maximum output before distortion became over 10% at 30Hz !!!!!!!! for the reflex enclosure was 124dB and for the baffle 116dB. There is no way I'm gonna reach these levels while listening to them. (I use PRO Audio drivers)

Measuring frequncy response at 4 meters distance showed something very very interesting. The reflex enclosure showed peaks and dips of +/- 9dB, the baffle of +/- 4dB. That's food for thought isn't it ??

>>>Those that rave about open baffles probably are unconsciously biased due to
their efforts building the baffles and the cost of purchasing massive
numbers of expensive drivers--- they unconsciously NEED to believe that
their investment sounds good, physics be damned.

Bullocks, my last speakers where about as twice as expensive as my current open baffle ones. And again, you don't need massive numbers of expensive drivers, that's not the way to do it as mentioned before, you clearly don't have a clue about designing a good sounding open baffle system.


At last, I don't want to tell that open baffle are always the better way to go than any other type of loudspeakers. Every, and by that I mean EVERY loudspeaker principle is a combination of compromises. There are many way's that lead us to Rome. It's just a matter of choosing the right compromises. For me, in my room untill now the open baffle principle has the most advantages, maybe I would have come to other conclusions in a bigger and acoustically more optimized living room.

Most people forget that the speaker and the room is actually one system. One of the most important things in speaker design for me is room integration, that's highly overlooked by most designers.
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