Acoustic Q/A , all the responses you need

A compression driver is a plane wave generator, a regular tweeter is not, so the solution cannot be the same and the usage is not the same too.

We can constrain the radiation of a compression driver and choose a radiation angle, but with a regular tweeter it’s more difficult to do it without create a lot of problems.

It’s why we talk about wave-guide for tweeter and not horn, with a tweeter we just “guide” at 110° horizontally the tweeter.

But with the use of modern phase plug we can reach 100° without problems:

It’s why we dedicated tweeters for close distance or monitoring usage and compression driver for medium/long distance usages, as see in the guide page.

At mid-distance, so with a 90° coverage, horn with compression driver will be more adapted.

As we see upper, the throat size will dictate the top end control of the horn, so the CD behavior. larger the throat size is, lower the top end control we will have, but there is some tricks, like on our bi-radial horn.

1" compression driver: They are relatively cheap, they cannot go very down, the best of them have a limit of 950/1000hz (BMS and 18Sound), the throat is tiny so the radiation control is high in frequency.

2" compression driver:

It’s an historic size, the control in high frequency is limited, today we continue to see some of these throat size for very specific usage : Have a high SPL, low end device (for replace a tinier compression + medium couple) without taking account of top end control.

These needs find an use case Line Array, so for a very far distance, it’s typically the usage of the Celestion Axi2050, it’s have been create for this very specific purpose.

The usage of 2" is not recommended for our usage.

1.5" or 1.4" compression driver:

It’s an ideal, they goes down and the throat size is not too big. In fact some 2" inch driver are just 1.5" driver with build in adapter, like the JBL 2450 or TAD TD4002. For the perspective of the horn it’s way more efficient to use an 1.5" version of a driver rather than the 2".

1.4" drivers have a 3" diaphragm, so the break-up is upper. 1.5" drivers have a 4" so better max SPL capacities and lower THD at the bottom range.

On our 1.4/1.5 Bi-radial horn we use a smooth pinch for push the control like it’s an 1", this way we have all the advantages.

A compression driver has a exit angle, sometime we talk about rapid flare vs slow flare it’s the exit angle of the device in degrees, we talk in semi-angle, for example an TD 4002/4003 have a semi-angle of 4/4.5°, so the “full” coverage is 8/9°.

More the flare is important less the horn is deep because you we have to smoothly bring the flare to reach the coverage angle of the horn, semi-angle 45° for example, so coverage 90°. As more and more compression drivers are designed for Line Array the flare with an important semi-angle, it’s rapid flare, that is not an advantage for our usage.

In an OS horn where we can choose the throat angle of the horn, it’s not a problem, but for HCD and bi-radial we prefer a slow flare, aka an angle as close as possible to 0°, the JBL 2450SL is 0°, it’s a no-flare device, it’s why I advice it for 1.5" usage generally, it’s also find-able used at an acceptable price.

Note: about diaphragm materials, Truextends Be is embossed Be, not vaporized Be like TAD does (more expensive), so even if it’s Be it’s far that TAD does.

It’s hard to set precisely a new diaphragm in a compression driver, even if modern driver facilitated it. Diaphragm characteristic is taken into account in the full device, sometimes changing by another doesn’t bring a lot of improvement (and sometimes it’s worse), so be careful with this.

The goal of a horn or wave-guide is to control the sound radiation of a device (tweeter or compression driver).

When a directivity is not constant, the device loose more and more coverage when the frequency goes up, the polar aka the energy in dB relative to the angle of radiation show us the problem :

On this polar map, that represent how the sound energy radiate, we can see it become tightened after 5Khz. The listening experience will be a speaker that sound “tiny” because there is a radiation incoherence according to frequency.

The ideal response is :

It’s completely straight and the orange color (-6db) define the coverage angle in degrees, of course a speaker cannot be straight on all this bandwidth :

• On bottom frequency it will be the end of control of the horn directly dictated by horn width.
• On top frequency it will be stopped according to the throat size (2"/1.5"/1"), tinier the throat, upper the directivity control will be push.

So a good horn will look like this :

The game is to push the CD behavior upper we can and lower we can thanks to the allowed width and throat size. We will have to reduce the midrange narrowing and the midrange beaming as good as we can, see the “What is mid-range beaming or narrowing?” Q/A.

The listening experience will be a lot better in constant directivity, in a room around half of the sound you listen is a reflection from wall, so even if you listen alone in front of the speaker, a constant directivity horn will change everything and will bring a more natural and fidelity listening experience.

One things that it’s important to not forget, we always choose a frequency cross-over between two devices where the radiation pattern is similar between these two elements, see “What frequency I should use for my crossover?” Q/A.

Here we can see a not fully optimised end ogf profile resulting a narrowed mid-range in 1khz region :

It’s an JBL M2, even if the profile is optimized, it doesn’t have a proper complete return on the baffle sides, generating this problem:
A mid-range narrowing, even id its relatively acceptable here, and mid-range beaming as some accident we can see in mid-range region.

That will imbalance the listening experience.

How to deal with it ?

Integration of the horn is as important as the Horn himself, the baffle is in fact a flat 180° horn. So in free air we have to continue the profil smoothly for avoid mouth diffraction and baffle diffraction.

Theses diffraction will cause accidents and a large mid-range narrowing on the global energy radiation behavior that will seriously impact the listening.

Whatever the horn, wageguide, integrated on baffle or not, the contour must be fluid even on reverse side and in all directions.

Here is a very good elliptic wave-guide with a slightly rounded traditional baffle where mid-range beaming and mid-range narrowing are still visible :

Then the same wave guide (but 0.5" tinier) integrated with a fluid profile :

It’s why a fluid profile everywhere matter : A round or elliptic horn into a square baffle even with a round-over will brings accidents and problems.

For solve this we pass from round to rectangular smoothly with the garanti to keep the good radiation behavior of the horn, with this way the horn and the baffle become the same element.

Historically bi-radial horns with fins wasn’t very good on this point mainly by the absence of rounded curve and the fins conception, today the new technology and FEA simulation allow us to completely solve theses previous down sides.

Here we will talk about an even more complex behavior on horn, inside a coverage angle, for example 90°, how the energy is distributed ? More precisely, the energy is constant in frequency but how fast this energy goes constantly down inside the 90° ?

Well not in a uniform way even in a Constant Directivity horn, when we move off axis the energy lost it’s not linear, it’s a little bit exponential.

Let’s analyse an OS 1" horn, every colored line is the response each 15° relative to the 0° in black :

We can see that the 15° is almost on the 0° but not the 30°.

A horn with fins like our Next Gen bi-radial horn doesn’t have this behavior, on all this kind of horn the 30° is almost on the 15° and the 0° and the position of the 45° is where the 30° is on the OS horn, it changer everything.

In one word the energy is way more concentrated on +/-30° when on a horn without fins it’s +/-15°.

The two horn are CD horn but one is more efficient in term of constant SPL, the listing experience is a step up with these kind of horn, and very well adapted for mid-distance listening in room.

The distance between each acoustic center crossed together should be, inferior or not to far from 66% of the frequency wavelength.

Then it’s very important that the coverage must match at frequency :

In 2 ways it’s not a good idea with two woofer as the two acoustic centers will be further so we will create lobs, see here the difference of vertical off axis response with one woofer then with two woofer in two ways :

On axis no change, but vertically off axis, there is cancellation, so you will have the listening sensation that the horn and the woofers play separately at close or medium distance listening position.

A MTM layout will bring a similar problem and will need to cut lower for avoid it.

Putting two woofers horizontally will also brings the same problem but worse, an horizontal accident.

The solution if we absolutely want bottom vertical or horizontal dual woofer placement is the 2.5 way :
With the first bottom woofer crossed with the horn and the other one crossed lower when we loose the +6db that give us the baffle (a baffle is a kind of 180° horn), on axis it will be :

Context: with a subwoofer, that is a very good idea, the interest of dual woofer or MTM is very limited, we use it if there is a SPL max limitation that have his origin in the use of a single woofer.

Conclusion

The regular 2 way + subwoofer is a good solution for close/mid-distance, like a combination of the bi-radial with a 15" vented.

3ways + sub will be useful more for long range with high SPL usage. But this approach bi-radial + 12" waveguided sealed + 18" vented works in medium range listening distance :

For MTM we will avoid 2 way and use 2.5ways filtering, with a look’s like the JBL K2 6500 :

But in 2.5way filtering the JBL K2 9500 is a 2 way speaker.

Like these ones, IPN style:

The Hypex Fusion Amp line-up, state of the art Class D + DSP :

Active crossover with DSP is advised for horn speaker, they need EQ on axis. It would be very hard to do a crossover for a high efficiency horn speaker.

Yes! And even in Hi-Fi, if well integrated and in phase positioned in the room it will bring more bass and less IMD.

Just be careful of the temptation to put it in a corner: It will be more bass but by exciting room mods so these more bass will be flowing (the “vrooom”, not a “dry” sound).

Prefer a bigger subwoofer on the line of the speakers and to not excite too much the room mods.

The acoustic center of the horn must be at the same height of your ears at your listening position (generally sit-down).

So generally the acoustic center of the horn must be close to 94 or 96cm height from the ground.