# Uniform Directivity - How important is it?



## Wayne Parham (May 5, 2013)

I must say that I'm in the middle on this subject of directivity and how much it matters. Back when directivity wasn't even considered by most audiophiles, I made more a point of discussing its importance. But now that constant directivity has become more popular in hifi circles, I tend to find myself reminding the new converts not to throw the baby out with the bathwater.

That's why I often refer to speakers with uniform directivity rather than constant directivity. It is more important to me that the power response be uniform than it is for directivity to be constant. They are similar, but not exactly the same.

To illustrate what I mean, consider two loudspeakers, one designed for uniform directivity and another designed solely for flat on-axis response.

The "traditional" loudspeaker designed for flat on-axis response has a woofer that is omnidirectional at LF and crossed-over where convenient. Not that it is a design criteria, but at this point, woofer beamwidth has narrowed, probably in the vicinity of 90°. The tweeter is omnidirectional at crossover and its beamwidth doesn't narrow to 90° until the top octave. Measured response is flat on-axis.

The speaker designed for uniform directivity is crossed-order where woofer and tweeter directivity match. So instead of beamwidth narrowing through the woofer band, then widening as the tweeter is crossed-in, the tweeter directivity matches the directivity of the woofer and remains relatively constant at that point. Alternatively, some designs have the tweeter beamwidth continue to narrow.

_Provided there is no abrupt beamwidth jump, I would consider both approaches to be "uniform directivity" designs._

Why does this matter?

Because people listening at at off-axis angles hear smooth response from the speaker that provides uniform directivity, but they don't from the other design. Also, the reverberent field is more natural when the uniform directivity speaker is playing, and that's important because that's the sound that surrounds you. It's as simple as that.

My experience with constant directivity designs started by making Klipschorn-style cornerhorns with constant directivity horns. I learned early on that they did something special where imaging was concerned. The walls confined the midrange and midbass down to the Schroeder frequency, so the sound was uniform throughout the room. Directivity is constant through the entire audio band in a design like this.

There is no way to be outside the pattern in a design like this. It is unique in that regard, and has always been my favorite design approach. But rooms with the right layout to support constant directivity cornerhorns are rare.

Another option presented itself, which is the matched-directivity approach. Physically, it is the same thing as the large Altecs and the JBL 4430. Those were my inspiration for this second design approach. They don't provide constant directivity, because the radiation pattern is omnidirectonal at low frequencies and beamwidth narrows as frequency goes up. But the directivity is at least uniform, and that means off-axis response is still flat. It may have a downward slope, but it is relatively smooth.

Speakers with uniform directivity sound more natural because the reverberent field has spectral balance.

One problem presents itself, and that is horns that provide constant directivity sometimes don't sound all that good. The best example is early CD horns with sharp edges inside. The discontinuities created by the sharp edges cause internal reflections and response anomalies. So I switched back to radial horns early on. These provide nearly constant horizontal beamwidth and gently collapsing vertical beamwidth. That is a very good characteristic for home hifi, in my opinion.

Prosound horns have a different set of priorities where constant directivity if concerned. Prosound horns not only need high-efficiency and constant beamwidth, but they must also be arrayable. This means they have to concern themselves with things like astigmatism, pattern flip and waistbanding. As an example of why those things are important, consider an installation where horns are splayed. In this arrangement, the primary lobe of one horn gets interference from the secondary lobe of the adjacent horn. So in this case, the characteristics of the sound radiated outside the pattern is as important as the sound radiated within the pattern.

A prosound horn design will allow some response ripple and other anomalous behavior within the pattern for "good behavior" outside the pattern. This makes sense, because horns in a prosound environment will be arrayed, so it doesn't make sense to optimize the horn for single use.

But there are a different set of priorities for a horn/waveguide designed for studio monitors or for home hifi use.
For home hifi, we want the response in the pattern to be as smooth and clean as possible, and ideally we want the beamwidth to be as constant as possible too. But where a trade-off must be struck, it doesn't make sense to optimize the response at the extreme edge of the pattern or outside, like we might choose to do in a prosound horn. The home hifi horn will not be arrayed. So the best approach is to make the radiation pattern uniform and to optimize response within the pattern.

This brings me back to what I said in the first paragraph about some people throwing the baby out with the bathwater. I'm all for uniform directivity, and found myself regularly arguing its virtues over the years. But it seems like lately, I see guys posting polars and sonograms, looking for a holy grail in its virtues. Some have gone way too far with that, in my opinion, and are using prosound techniques to build home hifi speakers. Their polars look wonderful, but their response curves and distortion performance is only so-so.

Compare the two curves on the chart below and you'll see what I mean. These are two waveguides that are approximately the same size, designed to be used over the same passband. One is optimized to provide smooth response, the other to provide constant beamwidth. The one with smoother response has about 2dB ripple, but directivity narrows slightly below 2kHz, before pattern control is completely lost around 1kHz. It is also 3dB louder with the same input signal, so drive voltage requirements are reduced, lowering distortion. The other waveguide has about 5dB ripple, but is able to maintain beamwidth down to its lower cutoff around 1kHz.








I've measured several different horns over the years, from radials to tractrix, and a lot of them are captured in the link below. Look through the list and you can see measurements of various types of horns and waveguides:


Measured Datasets
Remember the examples above, the traditional loudspeaker with the 90°-to-omni shift compared with the speaker having uniform directivity? In this case, when we compare response at 45° off axis, we see the traditional speaker has a 6dB dip at the crossover point, because the 90° beamwidth is defined by its -6dB point. The speaker with uniform directivity has no dip, its off-axis curve is a straight diagonal line.

But what about a speaker with a smidge of waistbanding? This is something to avoid in the prosound world, because a horn creates a secondary lobe in the waistbanding region, and this secondary lobe interferes with the primary lobe of another horn in an array. Not so in a home hifi setup, because there is no other other horn to interfere with. So what other consequence do we see from wasitbanding?

Waistbanding is a "pinch" of the pattern at low frequencies. An example is a horn/waveguide that provides constant 90° beamwidth from say 2kHz up, but that narrows to 70° below that, before ultimately opening wide up approaching omnidirectional radiation at low frequencies below 1kHz where pattern control is completely lost. A sonogram will show this "pinch" in the 1kHz to 2kHz region.

What does it mean in a home hifi environment? Practically nothing, it's inaudible. What is really happening is the sound at 45° is reduced _slightly_ between 1kHz and 2kHz, and by slightly, I mean about 2dB. It is minor, nothing like the 6dB dip off-axis of a traditional loudspeaker designed solely for on-axis response.

Does that mean waistbanding is completely insignificant? Of course not. Would we want to design to reduce it? Certainly, provided there are no other trade-offs. But there _are_ trade-offs, there are _always_ trade-offs. The most common mechanism to counter waistbanding is a secondary flare, and this increases horn size, which increasese center-to-center distance, which in turn brings the vertical nulls closer, limiting the size of the forward lobe. Or worse, if the mouth size must remain constant, then adding asecondary flare requires shortening the main body of the horn, modifying the flare profile and possibly truncating it. This introduces extra ripple. That may be worthwhile in a prosound horn, but it probably isn't in a waveguide designed to be used in a studio monitor or home hifi speaker.

Which leads me back to the radial horns. I personally would rather have a radial horn that provided constant directivity in the horizontal, gently collapsing directivity in the vertical and smooth respponse in the pattern than I would a so-called "waveguide" that had peaky response. I've seen some out there that have 5dB ripple, and that's about twice what I would be willing to live with.

A good hifi horn offers response flat within a 2dB window, and a good studio monitor waveguide is able to do this too. The waveguide usually cannot be used to as low frequency as a similarly sized (exponential) radial horn, because the waveguide's acoustic loading isn't as good at low frequencies. It has this in commmon with a tractrix horn, which also loads relatively poorly down low. But it need not increase response ripple like prosound horns do, otherwise it has thrown the baby out with the bathwater.


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## Wayne Parham (May 5, 2013)

To offer a little more information, consider the following sonograms:

First, the ideal directivity pattern would be constant through the entire audio band. A constant directivity cornerhorn comes close to this, because the walls confine the beam down low, and the midhorn and tweeter waveguides set the pattern up high. So basically, the whole room is the "sweet spot."








Of course, that's an idealized sonogram, and in the real world, room modes will break up that nice pretty picture and make pockets of hot and dead spots throughout the room below the Schroeder frequency, around 200Hz or so. That's what flanking subs and distributed multisubs seek to mitigate.

But that's a whole different subject. For now, back to tweeter waveguides.

Now the opposite end of the scale, a sonogram that is audibly deficient. This shows approximately 90° beamwidth up to 1.6kHz, then widening to over 120° to 6kHz, then narrowing to 40° above that.








The problem is the response off-axis. A listener that's sitting on-axis gets a different sonic presentation than a person sitting at 30° off-axis, and another person at 45° gets another completely different presentation. There is no spectral balance for listeners off-axis more than about 20°.

Look at the legend to the right. See the colors, and how they relate to SPL. At 45°, the sound is -6dB compared to the on-axis level below 1.6kHz. It rises nearly to the on-axis SPL, maybe -2dB in the 1.6kHz to 6kHz region, and then falls rapidly above that, being approximately -12dB at 12kHz.

Said another way, imagine the response curve for a listener sitting 40° off-axis. For that person, the response blooms about 4dB from 1.6kHz to 6kHz, and then it falls rapidly down to about -6dB by 12kHz.

This is non-uniform directivity, and it is something I would personally not want.

But now let's look at some popular horn/waveguides that provide good directivity.

This is a horn/waveguide that provide near-ideal beamwidth. As you can see, the sonogram is reasonably flat. It stays locked-on at 45° beamwidth across the band.








But what if there are design features that cause this horn to have 5dB ripple on-axis, as well as off-axis? In a way, it doesn't matter that the polars are nice and pretty, because there is a lot of ripple no matter where you sit.

Here are some sonograms of horn/waveguides that show some waistbanding, but that are still very good.


























In each case, you will notice the beamwidth is 45° from midband up. But down low, there is a little bit of a pinch in the beamwidth.

Examine each chart closely, and look at the legend. Remember that the beamwidth is defined as the angle where response is -6dB down from the on-axis level. So since each of these waveguides is a 90° device, and since each shows some waistbanding down low, look and see what the SPL is in the "pinched" region. What you will notice is that instead of being -6dB, it is a little more, like -8dB from the on-axis level. What you are actually seeing in these charts are devices that have about 2dB less output at 45° off-axis in the waistbanding region.

In some cases, this may be in the crossover overlap region, where the woofer and tweeter directivities blend. If so, the waistbanding may be masked. But even if it isn't, or if it is only partially blended, we're still talking about a relatively minor anomaly. It isn't as though waistbanding is entirely trivial, but it is more important in prosound applications, where arrayability is important. For high-fidelity monitoring applications, I would prefer the waveguide have smooth response through the pattern as its primary design goal.

In the end, the matched-directivity design will look something like this, with woofer and tweeter blended:


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## AudiocRaver (Jun 6, 2012)

Great posts, thanks for the detailed information and the sonograms. Gives a speaker designer a lot to think about.

I have had little exposure to horns in a serious listening environment, only 1 set of mid-level Klipsch bookshelf speakers, which sounded pretty good with a bit of tweaking.

My prime goal in speaker setup is always imaging first, all other considerations follow, since frequency response can generally be improved a bit with EQ, but imaging very little. To accomplish tight, localized imaging, the LP always ends up off-axis (still trying to figure out why), which leads me to prefer working with "uniform directivity" speakers, to use your well-described term. You mentioned imaging at one point in your writeup. In your experience, is sharp imagine more easily attained with well-designed horns? Or is directivity the main consideration? Have you experienced laser-sharp imaging with on-axis horns before? Appreciate your feedback.


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## Wayne Parham (May 5, 2013)

The best imaging I've ever heard came from constant directivity cornerhorns. Those are a special breed, all on their own. But of course, you have to have a room with suitable corners or that option is out. The next best imaging is probably from electrostatic panels, but it is delicate and finicky, and definitely room/placement sensitive. I think the best general-purpose setup is matched-directivity two-way speakers with flanking subs, which would be third on my list but gives much more placement options than the other two. It has much greater dynamic range than the electrostats too.


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## Danny Richie (Jul 12, 2009)

Just to build on what Wayne is sharing a little...

I too look very closely at horizontal and vertical off axis responses. This is going to have a lot to do with whether or not the total power response or in room response is smooth or not. Sure the room acoustics play a major role in the in room response. But that is not within the control of the loudspeaker designer. As the designer I have to make sure that the speaker itself maintains a smooth and even response in all directions.

Too often I see speakers designed based on the on axis frequency response alone. Or I might see the designer posting horizontal off axis responses from time to time, but the vertical off axis is almost completely overlooked. The vertical off axis is by far more important of the two. Side walls are rarely bare walls. Treatment of various kinds or room furnishings help minimize side wall reflections. So even if there is a couple of db of variance in the off axis horizontally, the room response will further break it up, make it slightly worse, or average out small peaks and dips. Ideally you don't want a broad range peak or dip in the side wall reflections. If this were the case then the slightly delayed time arrival with it's peak or dipped area will not match the on axis response and skew the sound stage and imaging ques. A hot spot from a peaked area can also cause listening fatigue as it will appear bright.

Back to the vertical off axis importance. The vertical off axis response is going to be what is reflected from the floor and ceiling. While floors are typically treated with carpeting, the ceilings are most often not treated at all. There are no furnishing on the ceiling. And rarely are diffusers or absorbers mounted on the ceiling. They should be in a serious listening room, but in a typical home environment they are not. 

The main problem seen with vertical off axis responses are time alignment issues with the drivers. Since one is stacked over the other in a vertical arrangement there is a potential for cancellation patterns that form from time delays. This means that as the point of reference (measuring position) moves up or down, then one driver becomes further away in time to the point of reference than the other. Then as the delay in time (or phase rotation) is delayed enough compared to the other then comb filtering effects cause a cancellation.

So if the vertical off axis is smooth then so is typically the reflection from the floor and ceiling. There will be some reflections (typically from the floor) in the mid to lower range that can be canceled to some degree by the time delayed floor reflection verses the on axis response. The distance from the floor of low frequency drivers and floor treatment will cause this to vary. And speakers are always much closer to the floor than the ceiling. So this can be an issue.

That being said, I always look closely at the vertical off axis response especially moving up above the listening height. This is what will be reflected by the ceiling. A smooth response in both the vertical and horizontal off axis will give the customer the best chance of obtaining an even in room response.

Have a look at the measured responses of these two pages of measurements: http://www.stereomojo.com/Small Spe...omojoSmallSpeakerShootout2007Measurements.htm These are all mini-monitors. Have a good look at the vertical off axis of these speakers and think about what the ceiling reflection will sound like. Most of them have a cancellation in the vertical off axis. So the ceiling reflection will often have a hole in the response.

Some people will say that they only listen on axis, and they try to disregard the hole in the response that they hear when they stand up. Folks, you don't just hear the on axis response. As Wayne as correctly pointed out above. We hear not only the on axis but the off axis response as well. And not just when you move off axis, but also when you are seating in the sweet spot.

There a re a few things to keep in mind that will help minimize vertical off axis issues. You want to pay close attention to two things. You want to look closely at the driver spacing (center to center) and the wavelength at the crossover point.

Looking at a wavelength chart like the one seen here: http://www.soundoctor.com/freq.htm will help you determine what the wavelength is for a given frequency. Now, you will want to keep the center to center spacing of the drivers to a distance that is less than the wavelength of the crossover point. This is a good rule of thumb. Closer in this case is better.

So for instance. Let's say that your crossover point is 3kHz. That wavelength is about 4.5". So you will want the center to center spacing of your drivers to be within 4.5" or less. And if the crossover point is at 2kHz then the wavelength is about 6.5". So you will want the center to center spacing of the drivers to be 6.5" or less.

Keeping the phase rotation at the crossover point minimized over a wide vertical range means an even vertical off axis response and a smooth (or potentially smooth) in room response.

So I typically shoot for low crossover points where wavelengths are longer, and close driver spacing. This is why we often mount drivers in such a way that the frames even overlap on top of one another in an effort to keep a close center to center spacing.

And it is easy to see how co-axial designs can have advantages as the center to center spacing of drivers do not change in the off axis.

Make sense?


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## Wayne Parham (May 5, 2013)

I definitely agree with you, Danny. Vertical beamwidth is very important, because off-axis response in the vertical plane contributes to the spectral balance of the reverberent field, just like horizontals do. In fact, in some ways, verticals may be more important than horizontals since the vertical boundaries are almost always the ones closest together. Rooms are almost always wider and deeper than they are tall.

I think the main reason we focus on horizontals is listeners move along the horizontal plane, but stay pretty much at the same height in the vertical plane. Listeners stand up and sit down, but that's just a couple feet of difference. Movement along the horizontal spans much greater distances than that.

Still, I'm in complete agreement about the importance of uniform radiation at vertical off-axis angles.

I also agree with you about the importance of minimizing ceiling slap, by minimizing vertical beamwidth. I've looked at this for a long time, and in my opinion, an asymmetrical coverage pattern makes the most sense for home hifi. A device that creates an asymmetrical radiation pattern does a better job of being "fit" to the room, being wider than tall.


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## Danny Richie (Jul 12, 2009)

> verticals may be more important than horizontals since the vertical boundaries are almost always the ones closest together. Rooms are almost always wider and deeper than they are tall.


Exactly!

However, I think the importance must be on an even and balanced vertical off axis response rather than limited.

Limiting the radiation pattern of a tweeter only softens the top octave and some of the second octave down. The line source designs that I have designed do this considerably well. Because of the length of the line (being so long) upper range information is limited to the length of the line. Slightly above and below the line and the top end clearly drops out. As frequency decreases though the pattern widens back up (vertically) and starts to become more omni directional.

But there is really a very limited amount of music information left to the top octave. Even the overtones are well down in output. Basically what is left is air and spacial ques that contribute to imaging and sound stage information. So typically there is not a lot of ceiling reflections in those areas because there is little output to begin with, and even a small dome tweeter will have limited off axis response in the top octave. We see that in a typical 40 degrees off axis measurement.

If anything imaging can be improved by getting back some of that upper range signal that was lost by limited dispersion patterns. This is why adding an up firing or rear firing tweeter often helps in sound stage layering and spacial ques. The same goes for open baffle designs that allow the tweeter to play equally to the rear as well as to the front. It helps create an effect of an instrument being in the room with you rather than playing at you from a box.

The real brightness that can occur from a ceiling reflection is down in the 2kHz to 3kHz range or area. Check out the ranges on this chart:










So even a driver with limited vertical dispersion can still have a considerable ceiling or floor reflection as its vertical off axis can still be quite strong in the 2 to 3kHz range. The real key is will the ceiling reflection be a smooth and balanced response that accurately reflects the on axis behavior contributing to a balanced sound. Or with the ceiling reflection have a peak or dipped area with missing information? If there is a large dipped area (common) in that reflection then it will cause an uneven room balance. This is why driver alignment or phase related issues are so important.


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## Wayne Parham (May 5, 2013)

I think we're pretty much in agreement here. I've always kind of been a stickler about verticals as well as the horizontals:


Matching directivity in the vertical and the horizontal planes
You know, I'm a point source waveguide guy but I really like some of the arrays I've heard, and was mighty impressed with the 1970s Infinity models with arrays of EMIT tweeters. That's a great configuration, in my opinion - midwoofer/ribbon arrays.

Raven Audio is running a speaker of that configuration at audio shows these days - Is that one of your designs? It's nice.

One thing I always noticed about arrays is the floor bounce notch is mitigated (as is the ceiling notch). The vertical boundaries pretty much just mirror the line.

In fact, it was this understanding that prompted me to start using what I call a "flanking sub" approach. It is like a minimal truncated "array" that smoothes the lower midrange notches that would otherwise occur from self-interference from reflections from nearest boundaries.


Room modes, multisubs and flanking subs
Helper Woofer Location


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## Danny Richie (Jul 12, 2009)

> Raven Audio is running a speaker of that configuration at audio shows these days - Is that one of your designs? It's nice.


Yep, that was one of mine. I still offer that one as a kit. 



> One thing I always noticed about arrays is the floor bounce notch is mitigated (as is the ceiling notch). The vertical boundaries pretty much just mirror the line.


Yep, this is true. 



> In fact, it was this understanding that prompted me to start using what I call a "flanking sub" approach. It is like a minimal truncated "array" that smoothes the lower midrange notches that would otherwise occur from self-interference from reflections from nearest boundaries.


That's true. On a typical stand mounted speaker one can calculate the delayed reflection from the floor verses listening distance and speaker height to get an out of phase cancellation. It is usually 5 to 6 feet of delay so it typically falls in the 180 to 200Hz range. But the side wall or ceiling reflections are easily below 100Hz and the sub can be used to fill those in nicely. 

If you really want to take it to another level add another pair of subs in the rear of the room and run them out of phase from the subs in the front of the room. This can effectively do away with most of the low frequency room loading effects and result in deep bass with no room boom trailing it.


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## Wayne Parham (May 5, 2013)

I do generally recommend distributed multisubs in addition the the flanking subs. And while I call them flanking subs, they are really run fairly high for "subs" so they can provide smoothing all the way up into the lower midrange. I usually rolloff the flanking subs around 150Hz but some run as high as 250Hz, depending on proximity to the mains and the type of helper woofer used.

The whole reason I started doing them was I wanted to smooth the higher end of the modal range - between 100Hz and 200Hz - where distributed multisubs couldn't reach. Distributed subs would be localizable if run that high but helper woofers or "flanking subs" can be crossed higher because they are in near proximity to the mains. They are sort of like a 2.5-way system, with the helper woofer detached so it can be placed a couple feet away in all three planes.


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## Danny Richie (Jul 12, 2009)

Too bad you didn't make it over to our room at LSAF. It sounds like you would have really liked the speakers were were showing. You have been practically describing them. 

The Serenity Acoustics Super-7's incorporate a lot of what you are talking about. The upper section featured all planar magnetic drivers in an open baffle crossing at 200Hz to a pair of 12" servo controlled woofers also in an open baffle. 

http://www.serenityacoustics.com/products/thesuper7.html

The 12" woofers have no floor delayed cancellation as they go from the floor to the upper drivers. And the control system built into the servo amps allow adjustments that will compensate for room related effects. They also do not load the room the way a sealed box sub would. Being an open baffle design they cancel out at 90 degrees off axis. Typically only the very lowest octave loads the room a little. So we place a pair of sealed box 12" servo controlled woofers in the back corners of the room covering from 25Hz and down. We run these out of phase from the mains speakers. This effectively nullifies the low bass gains that tend to gather in the corners of the room and balance out the room load even further. 

Here is the pair that went to the show burning in, in my listening room. 










Back to the original subject of uniformed directivity. The Serenity Acoustics Super-7's also incorporate a shallow wave guide on the tweeter to increase some lower frequency range output of the tweeter, minimize surface reflections from the baffle, and maintain a uniform off axis response pattern. 

Here are a few pics of the upper section. 



















Back side: 



















These pics are without the grills of coarse. This pair had a split grill










And the No Rez damping material had not been installed yet into the lower section. 










So horizontal and vertical off axis responses are very uniform. 

And Wayne, I guess I will have to get you to try some of our servo controlled woofers. Once you go servo control you never go back. Did I mention they also play flat to 20Hz? And that is in a small sealed box or open baffle.


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## ALMFamily (Oct 19, 2011)

Danny Richie said:


> Yep, that was one of mine. I still offer that one as a kit.


Is that the CLS-9?



Danny Richie said:


> Too bad you didn't make it over to our room at LSAF. It sounds like you would have really liked the speakers were were showing. You have been practically describing them.
> 
> The Serenity Acoustics Super-7's incorporate a lot of what you are talking about. The upper section featured all planar magnetic drivers in an open baffle crossing at 200Hz to a pair of 12" servo controlled woofers also in an open baffle.
> 
> ...


I was very impressed with the Serenitys - great all-around speaker.

And, thanks guys for having this discussion here. Some of it has been way over my head, but what I was able to understand so far has been very informative.


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## Wayne Parham (May 5, 2013)

Looks like a great speaker, Danny.

My all time favorite configuration is the constant directivity cornerhorn. The concept is generally the same as the Klipschorn, or at least it borrows from that. But there are a few differences, the biggest one being that constant directivity waveguides are used to maintain constant directivity from the Schroeder frequency upwards. The bass bin uses the expansion form the apex as its waveguide, the midhorn is placed within 1/4λ so the corner acts as an extension and the tweeter is a waveguide with matching beamwidth. It's my favorite configuration.


Speaker placement and wavefront launch 
As for subwoofers, I have always loved hornsubs but they're just too large for home systems, especially when doing a multisub configuration to smooth room modes. So for use in home, I generally suggest direct radiating subs. But my "all out" subwoofer is a basshorn with push-pull drive and a patented cooling system.


12Pi basshorn subwoofer
When you mentioned servo subs, do you mean something with a feedback loop? I ask because I know of at least one manufacturer that called their commutated motors "servos", but they aren't, they're an open loop system. Not that I think closed loops are necessarily required, provided the system is used within its linear range. But I do understand the attraction to closed loop systems - They're needed when the load is excessive or unusual. Is that what you're talking about?


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## Danny Richie (Jul 12, 2009)

> Is that the CLS-9?


It was just an LS-9 kit that was built out to a finished pair by a highly skilled builder up in Canada. 



> And, thanks guys for having this discussion here. Some of it has been way over my head, but what I was able to understand so far has been very informative.


Thanks for that. I hesitate some to explain such things that have lead me to where I am. Eventually it also leads to current products that I offer, and I don't want this to sound like I am trying to sell something. 



> My all time favorite configuration is the constant directivity cornerhorn. The concept is generally the same as the Klipschorn, or at least it borrows from that. But there are a few differences, the biggest one being that constant directivity waveguides are used to maintain constant directivity from the Schroeder frequency upwards. The bass bin uses the expansion form the apex as its waveguide, the midhorn is placed within 1/4λ so the corner acts as an extension and the tweeter is a waveguide with matching beamwidth. It's my favorite configuration.


I still keep an open mind about such designs. I know there are those that really like them and I try to get a grasp for what it is that they like about a particular design. 

Thus far though I have not heard a corner loaded design that didn't sound like it was blaring at me from a corner. Everything typically sounds like it is playing from a box with a megaphone around it, and is very localized. And there was no imaging, image depth, or sound stage layering. Maybe I have just not heard a good one. 



> As for subwoofers, I have always loved hornsubs but they're just too large for home systems, especially when doing a multisub configuration to smooth room modes. So for use in home, I generally suggest direct radiating subs. But my "all out" subwoofer is a basshorn with push-pull drive and a patented cooling system.


My experience with most horn loaded type subs is the huge amount of panel resonances and coloration that they add to the music. I am used to those things not being present and so when I hear them they stand out like a headlight in the dark. I tend to want a brace about ever three inches and use thick walls and No Rez damping material to kill box resonance issues. 



> When you mentioned servo subs, do you mean something with a feedback loop? I ask because I know of at least one manufacturer that called their commutated motors "servos", but they aren't, they're an open loop system. Not that I think closed loops are necessarily required, provided the system is used within its linear range. But I do understand the attraction to closed loop systems - They're needed when the load is excessive or unusual. Is that what you're talking about?


The servo control system is a patented technology from Rythmik Audio. GR Research is the only company that I know of licensed to use it. And we use it with our own drivers. 

Check it out: http://www.rythmikaudio.com/technology.html

Basically it does a couple of things very uniquely. It constantly compares the input signal with the cone movement. In doing this it maintains a linear response by adding gain as needed. Then when the input signal stops it electrically slams on the brakes to the driver and allows it to return to rest 7 to 10 times faster than an uncontrolled woofer. So it allows one to easily hear detail and resolution levels that go well beyond an average sub. And the control system allows for adjustable extension filters with the lowest allowing a flat response to 20Hz and a -3db in the teens. And the sealed box version does this in just 1.5 cubic feet sealed. It also allows for variable damping setting (variable Q) and that can also have an effect on sound stage layering (oddly enough). It also has a rumble filter and a one band EQ. The one band EQ has adjustable frequency and bandwidth of that filter. So one can attenuate a wide band or narrow band peak. Or one can fill in a wide band or narrow band dip. It has a variable phase control too. So matching it to your mains speakers to a near perfect phase relationship is just a matter of turning a knob. 

Here is a pic of the controls on the amp.










The servo system is so sensitive that while the system is on and playing you can press on the cone and it will sense the pressure that you put on it. It knows that the cone movement you made doesn't match the input signal and it kicks it back at you to return it back to center. It does this very quickly and can scare you if you are not ready for it.


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## Danny Richie (Jul 12, 2009)

While we are on the sub topic. Here are some build pics of a 1.5 cubic foot sealed box for a single SW-12-04 Direct Servo sub. 










This is the inner box. It is built from 3/8" thick MDF on four sides. 










The braces are 2.875" apart and are made from 3/4" MDF. The woofer mounts in the front (as shown) and the plate amp will mount on the panel on the rear. 










Next some 3/4" wide and 3/8" thick MDF strips are cut and laid in line with the braces below. 










the outer box is then built around it leaving a 3/8" thick space between the walls. 










The outer panels are all 3/4" MDF. 










Additional 3/4" thick pieces cover the front (where the woofer mounts) and the back panel where the amp will mount. This makes the front and rear panels 1.5" thick. 










Then comes the fun part, pouring in the sand. You have to shake it, rock it, and beat the sides with a rubber mallet insured that the sand was forced in as tight as possible and as much as it could hold. 










Then the last panel is glued on sealing it up. 










This makes a rock solid box. The more solid the box the less it will color the music with panel resonances. This is what separates an average sounding sub from a great sounding sub. 

Next is to line the box with some No Rez and then that will just about do it. 

So all you DIY guys out there can apply this to any sub design and get excellent results.


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## Danny Richie (Jul 12, 2009)

I was just looking at the 12Pi basshorn subwoofer. Man, I bet that thing will play loud. It reminds me a little bit of the Bassmaxx woofers. I heard this one ion a home one time: http://www.bassmaxx.com/products/subs/motiv-horns/z-5d/ it would play really loud too, but it was very one note heavy and did not play real low. 

Our servo subs simply target a different application than those SPL monsters.


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## Wayne Parham (May 5, 2013)

David Lee, the proprietor of BassMaxx, came out with me to do the Prosound Shootout, three years in a row. We measured various subwoofers there:


Prosound Shootout
I think the servo concept used for speakers is interesting. Maybe you should create a thread on that topic and show the response/distortion differences between a linear motor with a servo feedback loop compared to the same linear motor without servo feedback.

What about Hsu and Velodyne subs? Do those use a closed loop servo mechanism too?

As for constant directivity cornerhorns, as I said, I personally think nothing else sounds as good. I do like some other configurations but none as much as that one.

But you're right, some horns have way too much panel resonance. The original Klipschorn is a perfect case - It runs the bass bin all the way to 400Hz, which means vocals and fundamentals of almost every instrument have to go through a labyrinthene passageway. They sound throaty and "woody" to me, as a result. Not only that, but the path length difference between the bass bin and the midhorn is several feet. So they have serious problems. But they were designed in the 1940s, after all.

A good constant directivity cornerhorn is nothing like that. It's basically just a large format waveguide, large enough to control the pattern down to the Schroeder frequency, around 200Hz or so. It uses the room to its advantage, making what would be the biggest problem for other loudspeakers become its greatest strength. The only real downside is there are so few rooms that are laid out to support this configuration. But where it is properly realized, it is magic like nothing else I've heard.


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## Danny Richie (Jul 12, 2009)

> I think the servo concept used for speakers is interesting. Maybe you should create a thread on that topic and show the response/distortion differences between a linear motor with a servo feedback loop compared to the same linear motor without servo feedback.


Here is a review of one of our woofers in one of the Rythmik Audio boxes: http://www.hometheatershack.com/for...h-f12g-direct-servo-subwoofer.html#post605320

Distortion levels especially in really low ranges are almost impossible to measure with accuracy for good comparisons outside of a large anechoic chamber. The real difference would be in measuring stored energy, But again doing so at a really low frequency range is not easy. 

It is very easy to hear the differences though. 



> What about Hsu and Velodyne subs? Do those use a closed loop servo mechanism too?


Older servo technology using accelerometers are not really comparable. 



> A good constant directivity cornerhorn is nothing like that. It's basically just a large format waveguide, large enough to control the pattern down to the Schroeder frequency, around 200Hz or so. It uses the room to its advantage, making what would be the biggest problem for other loudspeakers become its greatest strength. The only real downside is there are so few rooms that are laid out to support this configuration. But where it is properly realized, it is magic like nothing else I've heard.


I'd like to hear that.


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## gottavtr (Oct 24, 2011)

I love the sand idea. I was going to do that until my plans changed to an attic mounted sub. The idea is to add mass and dampen the energy transferred to the outside panel correct? Would it not be better to isolate the inner and outer box more? Instead of strips you could space them just in the corners. Am I incorrect here?
Zach


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## AudiocRaver (Jun 6, 2012)

gottavtr said:


> I love the sand idea. I was going to do that until my plans changed to an attic mounted sub. The idea is to add mass and dampen the energy transferred to the outside panel correct? Would it not be better to isolate the inner and outer box more? Instead of strips you could space them just in the corners. Am I incorrect here?
> Zach


Here is how I THINK it works, correct me if I missed something...

I think of the purpose of the sand as being more about helping make the walls of the box as sturdy and unshakable, or non-resonant, as possible so no acoustical energy is absorbed in the act of shaking a box wall at a resonant frequency. A resonant panel would change box characteristics at the resonant frequency, affecting frequency response. No resonant panels - flatter frequency response.

Rockport Technologies make their full-range speakers double-walled and fill the space between with epoxy resin for super density. Sand sounds cheaper, cleaner, simpler. I suppose brick and mortar would work, too, but I can see drawbacks.


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## Wayne Parham (May 5, 2013)

Based on my observations over the years, I have made three basic assumptions that largely drive my design choices:

My first working assumption is that speakers with constant beamwidth always sound better than speakers that don't, _provided everything else is equal._

A second assumption is that speakers with non-constant beamwidth but uniform-directivity - those having just gradual change - sound better than speakers with directivity that shifts radically somewhere in the passband. This is especially true if the directivity shift happens in the peak of the Fletcher-Munson curve where we are most sensitive.

When I say "directivity shift", I'm not talking about 20%, by the way. I'm talking about at least 50%. Beamwidth that stays constant within 20% is perfectly fine, certainly, at least for home theater or hifi. It's the shift from 90° to 180° in less than an octave that you'll hear, not the little bulge of 10° or 20° in a pattern that averages 80° or 90°. A 20% shift amounts to about 2dB at the very edge of the pattern, which is completely inaudible. But a 90° to 180° shift is _6dB_ at the edge of the pattern, which is most definitely audible. A shift like that screws up the spectral balance in the reverberent field.

So speakers with collapsing directivity (like DI-matched two-ways) can sound very nice provided the directivity change is smooth and gradual. The DI-matched two-way approach is a worthwhile compromise where constant directivity is impossible or impractical.

My third assumption has to do with the "provided everything else is equal" part. This assumption is where sound radiators are concerned, those with truer (flatter) amplitude response sound better than those with peaks and dips. This is true not only of direct radiators but also of horns.

Waveguides offer the promise of smoother response than constant directivity horns, but at the expense of slightly less pattern control. For example, without a diffraction slot in the throat, they cannot maintain beamwidth in the top-octave, narrowing instead to the compression driver exit angle. They sometimes waistband a little at the bottom end of their range too, depending on the shape of the flare nearest the mouth. But in general, a waveguide provides constant directivity and _also_ provides much smoother response than a constant directivity horn.

Waveguides are designed to provide smooth wavefront propogation. The wave, where it contacts the waveguide, is always perpendicular to the surface of the flare. This provides a nice, clean spherical section as the wavefront exits the mouth. It makes them act something like tractrix or LeCleach flares, but with nearly constant directivity. But different shapes and flare profiles offer different optimizations, and correspondingly different performance metrics. Some geometries provide smoother response than others.

An argument can be made that as long as response ripple is constant across all axes, then it can be equalized flat. The idea is that if directivity is constant, then the power response is the same shape as the on-axis response, so equalization in one plane is appropriate to all planes. I think there is merit in that argument, but I do not agree that just because a horn is equalized flat, it will sound as good.

There is a big difference between equalizing for mass-rolloff and using a series of tank circuits to tame response ripple. The conjugate filter for mass-rolloff is a simple single-pole high-pass, and is not a resonant condition. That is quite different than the conditions that cause ripple, and I have not found any cases where the underlying mechanisms that create this ripple come without additional penalty. Sound quality suffers.

You can always take a constant directivity horn and EQ out the ripple. Take the JBL 2370 and 2380 horns, for example, which exhibit 5dB peaks in the passband. They can be equalized flat, but even so, those kinds of horns still sound harsh. And we have seen that some waveguides also generate a peaky response chart, such as the SEOS family of devices we compared with earlier. The SEOS device produces some ripple, a result of its geometry. Here's some discussion about it, where Bill Waslo claims it to be audible, but suggests a way to correct for it in the crossover, using multiple notch filters:



Bill Waslo on Sat said:


> It's those Inductor-Capacitor-Resistor (LCR) strings that go across the CD driver. Can't just take them out, other stuff would need to be adjusted to compensate or it would sound awful. I did run some designs without those LCR, but really think they should be left in. One of the bumps they deal with is at 2kHz, which is a terrible frequency to have a bump at (near where ears are most sensitive). Why go to all the trouble of waveguides and CDs and then cheap out on a few components?



Bill Waslo comments on the audibility of the ripple inherent in the SEOS device
Consider that 5dB represents a 3x increase in power. Equalization requires a significant power shift - To remove a 5dB peak means the power is cut 3x at the peak, which also means that it must be raised in comparison by 3x everywhere else. This also means excursion is increased and everything else that goes with it. That is not the only issue, in fact, it may not even be the most significant issue. But whatever it is, there can be little doubt that a constant directivity horn is nowhere near as smooth sounding as a properly designed waveguide.

I have said many times before, I even prefer a good radial horn to many constant directivity horns, purely because of their sound quality. I can remember so many discussions over the years with tractrix horn guys, many that use a simple first-order capacitor and nothing else. They trade everything to get smooth response - out goes directivity, power response, excursion at the low end, etc. And when I say "out goes directivity" I don't just mean the horizontals, but even more so the verticals, because with a single cap, the forward lobe becomes a paper-thin strata. But still, they love the pure sound they get in that one pinpoint spot.

What I like about a good waveguide is we can achieve this kind of smoothness, and still provide nearly constant directivity. It really is a design approach that has one foot in the constant directivity world and the other in the audiophile response purity world. Of course, there is a continuum of optimizations one can choose, spanning between those two worlds. The waveguide can be more constant directivity or more smooth, or somewhere halfway in between.

Which brings me back to the first observation/assumption, that speakers with constant beamwidth always sound better than speakers that don't, _provided everything else is equal._ Or more precisely to the qualification, the "provided everything else is equal" part. It's why I said earlier that a good waveguide should not "throw the baby out with the bathwater." We do not want to use a waveguide that is excessively peaky in a high-fidelity loudspeaker. I'd say a worthwhile criteria is no more than 3dB variance in an octave, i.e. +/-1.5dB. Above that, and the response ripple becomes audible. Why settle for audible ripple when you don't have to?


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