Understanding directivity plots

The directivity plots shown in this blog will be unfamiliar to many, but they are not difficult to understand. They are essentially a frequency response plots  which measure from on axis right out to 90 degrees.

Here is one example:

The x axis shows frequency the same as a response plot, but the y axis shows the angle. Think of it this way. Imagine you want to depict a mountain range graphically. You could take a slice through and show a line at that point, but it would only show a small portion of the shape of the mountains. This is very much like a frequency response plot. They only tell part of the story in a very two dimensional way.

If you really wanted to show their shape in one illustration, you might use a contour map. It will have lines where the elevation at each point on the line is the same. It is as if horizontal slices were taken at different heights, and the line is the outline of each slice seen from above. A directivity plot is just like this, but we use SPL instead of elevation.

Interpreting the plot above

The plot above shows an open baffle speaker and I've chosen this one because it includes many of the features you will see with various designs, not normally all in one plot!

Region A - Here you see ideal constant directivity behaviour from 200 - 1k. Dispersion is around 100 degrees. This is very good performance from a small speaker. I don't know of any other type of speaker regardless of size that can control directivity in this way down to 200 Hz. Notice how all the lines are flat at all angles. This means the sound waves that are seen by the room are not altered in tone by the speaker.

Region B - Here the speaker begins to beam from 1 - 2k. This indicates a problem that needs some attention with this design. Ideally we'd like to see a gradual widening in this range, instead of becoming narrow then wider again.

Region C - Again it is constant with a wide dispersion of 150 degrees. Since this only occurs over a restricted range this is a problem. The room will see a little more energy here.

Region D - The most rapid narrowing occurs here then there is a narrowing of response. Above about 9k the tweeter is starting to beam. This is because this speaker used a 3" full range driver.

The most interesting part is the directivity control that comes from an open baffle arrangement.

What does the ideal directivity plot look like?

Something like this would be ideal. True constant directivity from about 200 Hz up to 20k. The ideal dispersion would be about 60 - 90 degrees where the response would be 6 db down. You can find that point by looking at the colour scale, 6db down equates to orange. On the vertical scale, look for 30 degrees off axis to find a total dispersion of 60 degrees.

See more dispersion plot examples on the waveguide shootout page >

Software used

Arta - download >

Arta was used for these plots. The demo version is free to use but saving files is disabled. You can export plots such as these on the demo version.

Looking inside a Faital pro compression driver

While working on an Econowave, I discovered the compression driver (Faital pro) had failed. Opening it up to investigate, I found the wire connecting the voice coil to the terminals had come loose. It was a simple quick repair job, but a little fiddly. In the process I had a look at the diaphragm and novel phase plug:

Here you can see the phase plug (orange). The centre hole does not let sound through. The radial slots around the outside allow the sound through.

Here you can see the diaphragm (lighter coloured part). The circumferential groove is the moving diaphragm, and you can see how the phase plug shown previously matches up. This unit was used in an Econowave.

Other compression drivers with a look inside:

BMS 4550

B&C DE250

Econowave deluxe - part 2

Both boxes are now complete enough for testing, with a port in the side.

Edges after round over and sanding.

Now they just need a little more filling and they will be ready for finishing.

Initial impressions

These have many of the qualities I like about my Miniwaves, however when it comes to achieving a certain effortless quality in the midrange, these go to another level. They could easily fill a large room. They can play very loud and clean.

Econowave deluxe

This is a deluxe Econowave that I've been involved in building. This one belongs to Roger, one of a handfull of DIY enthusiasts that have been getting together in Melbourne to test out some waveguide speakers and active crossovers. Compared to working in isolation, we get to experience more things without having to buy and it's a lot of fun. Above is the not quite finished first box. It needs some sanding, round-overs and finishing. 

Faital pro compression driver on an 18 sound waveguide. The compression driver has a novel phase plug and I believe the cost is similar to the B&C DE250 that I use. 

Drivers - Acoustic Elegance TD10M

This is one driver that I'm keen to hear. It does appear to be exceptional.

All the MDF panels CNC cut. This made it all very simple. 

We started by gluing the baffles. A larger cut means no routing necessary to flush mount the drivers. 

You can see the internal brace and the corner pieces that will help when the front edges are rounded over. Rather than use very large clamps, we opted to use screws to hold it together while the glue sets. The screws can be removed and filled later. 

The rear terminals will be flush mounted. 

Assembled first box.

Mostly complete first box. With the panels having been previously cut, all of the above took the better part of a day to complete. This speaker will be featured in the next waveguide shoot out.

Previous waveguide shoot out >

Continue to part 2 >

Where can I find reticulated foam in Australia?

You can source reticulated foam for waveguides and horns in Melbourne from Flex Foam in Box Hill North.

In contrast to other suppliers, they were a pleasure to deal with. One said that I would have to pay $300 - 400 to get a chunk of foam as they have to buy in bulk. Or use layered sheets, each 12mm thick, not a very attractive solution. Shown above is enough foam for 6 waveguides, ranging from 6" - 12" in size. The cost is quite reasonable.

Why foam?

Earl Geddes has demonstrated that horns and waveguides sound better when filled with foam. Not just any foam - reticulated foam is needed to have the right balance of absorption without too much attenuation. The foam is intended to absorb to a greater degree the reflections within the horn that cause sound quality problems. I have not yet tried it, but by all account it is a worthwhile improvement.

What to ask for 

The foam you want from Flex Foam is called "Mericell." It comes in different densities, you should ask for 30 ppi (pores per inch). It is also referred to as a filter foam, but it's technical generic name is reticulated foam.

Does it really work?

When I have had a chance to test it out properly, I will be reporting my results, both measured and subjective.  I have another shoot out planned where we will compare different waveguides and drivers. The challenge at this point is to find a good way to cut the foam.

Review: Linkwitz Orion open baffle speakers

For sale: The speaker reviewed here is currently up for sale on the StereoNet Classifieds >

In the world of speaker design, if you haven't heard of Linkwitz, you haven't heard of anyone.

Not too many designers can boast that they have a crossover named after them - arguably the most popular crossover is the Linkwitz-Riley 12 or 24 db/octave. After a long and distinguished career, Linkwitz has created a speaker which combines everything he has learnt. It is named the Orion.

Recently I had the chance to hear the Orion with all current upgrades included.

What is the Orion?

The Orion will be familiar to DIY enthusiasts. It is a 3 way dynamic driver open baffle speaker. It includes a rear firing tweeter so that dipole radiation is maintained full range.

Listening Impressions

Very refined, very high resolution, great clarity. The sound stage is very wide and deep and the sound is very immsersive and there is considerable envelopment. I can see how many have called this speaker their last. And one very strong point for those who will place them in a living room, they excel in a normal untreated room.

They come with a large multi channel power amp recommended by Linkwitz and an analogue active crossover, which is a very elaborate crossover, one of the most sophisticated you will ever see. The box is well built with solid hardwood and the system has been upgraded numerous times to keep pace with successively improvements discovered by Linkwitz.

How does it compare to other high end speakers?

There are two very distinct differences. The first is the sound stage, the second is the bass. Both are very different to conventional box speakers. The sound stage is larger than just about any speaker you are likely to hear. Even in a small room this can be achieved. The sweet spot is also larger. The bass seems to be less room sensitive and so high quality bass can be achieved in a normal room without treatment. One of the main advantages of a speaker such as this is that it can be placed in a room without any treatment.

SBIR - Speaker Boundary Interference Response

Speaker Boundary Interference Response (SBIR) is one aspect of speaker/room interaction that is often ignored in home audio. What is it? In a nutshell, it refers to one way in which the nice looking anechoic response of a speaker gets messed up in a real room.

Here is a simple illustration, showing the direct sound and a reflection off the front wall:

The reflected sound wave causes cancellation, resulting in a dip in the response where the difference is equal to a half wavelength. That dip can be calculated:

f = 172 / x where x is the distance (reflected - direct) in metres

For the example above with a 1m offset from the front wall:

Direct = 2.6m
Reflected = 4.6m
x = 2m

f = 172 / 2
f = 86 Hz

Hence with 1m offset from the wall, a dip will be seen at 86 Hz.

If you know the frequency

If you are looking at an in-room plot, and want to know if a particular dip is related to a boundary issue, use the formula to find the distance.

 Wall offset = f  / 86 (metres)


Avoid similar offsets

If the offset from each adjacent boundary is the same, the dip will be worse. Hence it's best to avoid having similar offsets. The worst case scenario is one in which the offset from the floor, front and side walls is the same.

Confusion

In looking at the frequency response, it's easy to misinterpret. Where you see a dip, is it caused by SBIR, room modes or phase shift between a sub and mains? The phase issue can be eliminated by measuring a woofer only so there is no crossover with a sub included. Then you can calculate the points where you should see SBIR dips. It is possible to combine a room mode peak with a boundary interference dip and get them to cancel, although often this won't be possible.

Dealing with SBIR

In recording studios where SBIR gets the most attention, speakers are often soffit mounted to avoid this issue. The entire speaker is flush mounted into an angled baffle. Woofers can also be placed near the floor to avoid floor bounce. Room modes should also be considered at the same time because the best position for SBIR considerations may not be better regarding modes.

Acoustic Treatment

One way to tame SBIR is with acoustic treatment, where the reflection is damped with some absorption that is adequate to work down to that frequency. Another strategy which might be useful in small rooms is to mount the speaker against the front wall and use some treatment to tame early reflections. It won't work well without treatment.

It's also important to balance imaging considerations. The challenge is that the best location for imaging might not be the best for bass. This is where 3 way systems using active woofers come into their own. The midrange driver can be placed for best imaging, while the woofer can be close to the floor. There is more flexibility to achieve the best of imaging and bass response.

Here a problem with many stand mounts and floorstanders becomes clear. When placed in a real room without paying attention to boundary issues, the lower midrange will often become recessed while room modes introduce peaks in the bass. This means the bass will boom, but the lower midrange will lack fullness. It's something that many audiophiles have adjusted to, but those who seek accuracy in their systems will often not know what they are missing.