Differences Between Enclosure Types?

Veleric

Member
2016-02-26 6:26 pm
This is my first post, so I apologize for being so naive.
That said, I'm thinking about building a pair of FR speakers. A few weeks ago I was at my brother's place and heard his Fostex BK 12m Folded Horn set and I thought they sounded and looked great, plus I was really intrigued by the folded horn concept.
So I decided I'd like to build some myself, but maybe take a crack at designing my own rather than simply buying a kit. But the more I read the more confused I get.
At first I was just thinking about a backloaded folded horn. Then I discovered front loaded horns too, and oh there's transmission line too. And TQWT too. And BIB. Others?
Are there some generalizations that can be made about the differences (pros and cons) between the types?
I know the backloaded horn is very efficient and hence especially good for low power tube amps. Are some of these other types more or less efficient? What other differences are characteristic of the various types?
I'm kind of leaning towards the horns, but mainly because I like the look (curvy, sometimes). Any particular cons of front loaded horns?
Thanks
Eric
 
At first I was just thinking about a backloaded folded horn. Then I discovered front loaded horns too, and oh there's transmission line too. And TQWT too. And BIB.

Any horn is typically only good for 3, maybe 4 octaves. This makes a front loaded FR somewhat problematic*. Horns, TLs, TQWT (Voigts), and BIBs are all quarter-wave enclosures with many of the same pluses & minuses.

* (the marrige of a BLH, front horn, and direct radiation at the highest frequencies can work well, but are a bitch to design)

Are there some generalizations that can be made about the differences (pros and cons) between the types?

As a generalization the box only affects the lowest frequencies. The following is simplistic.

Sealed: most drivers are not optimum for sealed boxes so they often need bass support. They roll off slower and as such tend to have fewer LF artifacts. Very tolerant of the detail of implementation
Reflex: uses the output of the back of the speaker to extend the flat response lower, but roll-off rate is 2x that of sealed bringing more artifacts
QW lines: Use of a ¼ wave resonance to extend the bass. The output is delayed by n+½ periods (n=0,1,2 …) which drives some people nuts. One also has to deal with the higher orders of the ¼ wave resonance which are unwanted as they cause response ripple (just look at a BIB sim)
Open Baffle: if you want bass it needs a helper woofer. Very simple. The output from the back of the driver brings its own issues. Liked by many because people like the lack of boxiness (not always the case, and my take that is from people not ever having heard well built boxes -- partly driven by the penchant to use MDF, a material not well suited to speaker boxes)
Aperiodic: Can be looked at as either a heavily damped reflex or a leaky sealed box. Often the easiest way to achieve is a heavily damped TL (there is a school that believes a TL has to be such a box, taking the title of Bailey's article as the whole and not paying any attention to the details of the article)

And then there are hybrids, boxes that fit in between these categories.

As always the end result comes down to execution.

I know the backloaded horn is very efficient

Not true. The BLH only operates over the lowest frequencies and does not affect the efficiency of the enclosure. It does affect the gain in the bass, and this can be used to compensate for baffle-step so vrs a box that does need baffle step compensation, one does not need to depress the output at mid & higher frequencies.

The Frugel-Horn Mk3 is another similar size small BLH from the same designer, and is better & much more versatile (ie works with more drivers), so it is possible to take things well beyond your brother's BK12.

dave
 
Horns, TLs, TQWT (Voigts), and BIBs are all quarter-wave enclosures........

Open pipes [such as a vent] and tapered pipes [TQWT, horns] are 1/2 WL resonators: Resonances of open air columns

Typical TQWTs, horns are so acoustically short though that only a small portion of its HF BW is usually horn loaded, so at a glance seem to be 1/4 WL resonators.

GM
 

Veleric

Member
2016-02-26 6:26 pm
Dave,
Thanks for your reply. However, I'm even more confused now but that's okay.

So now I understand that all the "types" I mentioned are all types or quarter wave enclosures (or half wave, per GM).

But beyond that, I'm confused. Let's start with these two things:

First, you wrote that it's "Not True" that a back loaded horn is efficient. But I feel like I've read about the great efficiency of horns so many times in so many places that it's taken for granted. For example:

Oswalds Mill Audio - About / Why Horns | OMA

Regular loudspeakers, such as a cone in a box, or even worse, electrostatic or planar speakers, are incredibly inefficient. A typical speaker (85dB/1w/[email protected]) is .1% efficient! That means that for 1000 watts input, you get exactly 1 acoustic watt output. An acoustic watt is actually a lot of sound, but compare a horn loaded loudspeaker, which can easily be 60% efficient (108dB/1w/[email protected]) or 600 times more sound from the same input as the regular box speaker. Imagine we have two loudspeakers, a conventional one (the 85dB one) and a typical high efficiency horn speaker (105dB/1w/1m @8ohms). To reach a realistic sound level of 96dB at one meter, the conventional speaker needs 50 watts of power. The horn speaker needs less than half a watt.


I feel like I'm missing something simple, what is it?

Second, since my brothers speakers are back loaded horns, I started my research by reading about horns. And articles like this one:

https://www.grc.com/acoustics/an-introduction-to-horn-theory.pdf

When you do this, you can take radiation
from the driver in the form of plane
waves and transform the high pressure,
low velocity vibrations at the throat into
low pressure, high velocity vibrations that
can efficiently be radiated into the air.
Depending on how the tube flares, it is
possible to present a load to the driver that
is constant over a large frequency range.


gave me the impression that a flared tube was an ideal shape, and that the trick was really in the details of the flare (conical vs. exp vs. tractrix, etc).

But then I start seeing designs that flare but then have a small opening, and others that don't flare at all, and even others (I think) where the flare is reversed. Is the flare important or not?

Thanks,

Eric
 
I feel like I'm missing something simple, what is it?

Is the flare important or not?

Except for reactance annulled [RA] BLHs that are only useful over a narrow range with no useful mids, HF output from the driver [in short, an open back compression FLH], they are only as efficient as required to tonally match up to the driver, i.e. normally used to add acoustical baffle step compensation [BSC], so if corner loaded for instance, the horn must be less efficient to keep it from overpowering the driver's front radiation, ergo you want an efficient BLH, then start with an efficient driver.

A compression [FL or RA BL] horn OTOH can be up to 50% efficient with the other 50% being 'boiled off' as heat.

Both a horn's flare and frequency factor is essential to a successful horn alignment, with 'successful' meaning it meets the needs of the intended app; making horn design the most flexible of speaker alignments, ergo a lot harder to get right without needing a lot of 'after the fact' tweaking same as like designing a F1 racer's complex, fully adjustable suspension system and getting it anywhere near 'spot on' before powerful computing, measurement systems became available and even then it's taken decades of accumulating on track racing data to take full advantage of it.

Horn design isn't that complex of course, but still was no trivial pursuit until fairly recently with the advent of powerful computer programs to do all the 'heavy lifting' math required, though even now the best designs still need some tweaking in room due to being unable to input enough of its acoustics [yet] and of course hand's on experience helps as a computer is only as 'smart' in choosing the most correct driver specs, horn alignment as the designer.

WRT BLHs, MJK's write-up is the best I've read for the novice, so a 'must read' for budding horn designers, ditto Don Keele's for reference: AES Papers -- Official website of D.B.Keele

If you can 'wrap your mind around' the compression horn design theory the pioneers of audio created though, the late Prof. Leach's design routine can be used to design any type of horn alignment [including tapped] worth building: http://users.ece.gatech.edu/mleach/papers/HornPaper/HornPaper.pdf

There's a minor followup to this, but couldn't find the link and currently don't have access to my tech 'library', but hopefully someone will post it.

GM
 
sreten,
Thanks for your reply, but what exactly are you saying is strangely unpopular?
The MJK article? Or Backloaded horns in general? or both?
Eric

Hi,

The MJK article has received a very lukewarm reception, rather than acclaim.
In calling a spade a spade, the fans of BLH's seem not like his implications.
Possibly because as he alludes, most BLH's (/FLH's) are somewhat flawed.
The messenger has been shot to a degree.

Personally I like the design and believe he is due more credit than given.

TBH for a 4" FR its the only compact BLH design I'd seriously consider.

rgds, sreten.

The fly in the ointment seems to be his use of the cavity absorber.
I think its clever, others seem to think it destroys design purity.
 
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Hi,

Allthough the FF125WK full range driver was used the
FE-126E, FE-126En, FF125K, FX120, and F120A are
all quoted as possible. Me, I'm no FR expert, at all.

I'd presume any right sized FR with Fostex like
specifications / parameters would be suitable.

rgds, sreten.
 
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Veleric

Member
2016-02-26 6:26 pm
Except for reactance annulled [RA] BLHs that are only useful over a narrow range with no useful mids, HF output from the driver [in short, an open back compression FLH], they are only as efficient as required to tonally match up to the driver, i.e. normally used to add acoustical baffle step compensation [BSC], so if corner loaded for instance, the horn must be less efficient to keep it from overpowering the driver's front radiation, ergo you want an efficient BLH, then start with an efficient driver.

I'm sure this is right, but I have to admit I have virtually no idea what this means. My fault, not yours. Gotta keep reading. Thanks for the links!
 

Veleric

Member
2016-02-26 6:26 pm
Just so you guys know where I'm coming from, let me share this:

What I'd like to do is build some "crossover-less" speakers using full range drivers and a well designed enclosure. If they turn out better than my bro's BK12m kit that's all the better!

I've never built speakers before (obviously) or studied speaker design, so this is all new to me. But I'm an engineer (materials, not EE or acoustic) and I do have some woodworking experience (I build my own bikes with Baltic Birch).

Obviously, I'd like them to sound good (duh) but also I hope to make something that looks good too and maybe even be somewhat original. Because of that, I was kind of drawn to the horns because, it seems to me anyway, that the curves of a horn offer a better opportunity to be creative in terms of aesthetic design.

But what I'm starting to understand (I think anyway) is that a horn that isn't huge is actually really a quarter (or half) wave TL, regardless of it's flare (or lack thereof). So maybe the horn thing would really be more for appearance than performance? Is that right?

Thanks,

Eric