Looking to improve polar and power response

bcodemz

Member
2014-02-21 1:19 am
I've been thinking about improving the polar response, power response, and directivity for my next speaker. Here is what I'm thinking of, can anyone please comment if this works, and if what I'm saying is correct?

So I'm thinking of using a Scanspeak D3004/6020-10 deep chamber tweeter and a Scanspeak 10F/8424G as the midrange. I'm thinking of using a 1700Hz LR4 crossover between the two for the following reasons:

1) The midrange's off axis response starts dropping at above 2000Hz at 60 degrees, by crossing at 1700Hz, the midrange is well below the beaming point which should hopefully result in a very flat power response all the way up to around 5000Hz.

2) The 1/4 wavelength at the crossover frequency is just slightly smaller than the centre to centre spacing between the tweeter and the midrange, which should minimize both vertical and horizontal lobing.

Question: I'm not quite sure about this part. I'm not sure if it is the 1/4 wavelength or the wavelength of the sound wave that I need to watch out for lobing.

Next, I'd like to get better off axis performance on the high end of treble. Would a waveguide do what I want?

It seems Pellegrene Acoustics is no longer in business, so I'm not sure how I can do a waveguide. But what I'm thinking is to get a small waveguide that'll give me controlled directivity above 5000Hz.

Any comments would be greatly appreciated.
 

eriksquires

Member
2013-05-10 4:11 pm
Um, ok, but you are giving up a lot of smoothness in your FR. Also, WHY do you want to achieve such wide dispersion? I mean, are you sure it will be good for you? Wider = more reflections. Make yourself happy though. :)

Audax and Peerless make some really smooth 5 1/4" mid-woofers that are pretty flat and extended. They would let you use the smaller D3004 which in my opinion, is also damn easy to integrate. :) Just some thoughts. Scanspeak also makes a number of 5"-ish woofers with smoother responses than the 10F/8424G you areconsidering, but are much pricier.

Best,

Erik
 

TMM

Member
2007-09-01 8:37 am
Australia
A waveguide narrows the beam width of a tweeter - it makes a tweeter droop even harder as you move off axis. I'm not sure if you consider this 'better' or 'worse' ?

There are two reasons people use waveguides on tweeters; the first is that it boosts the low end (1-5kHz) of the tweeter so the tweeter doesn't have to work as hard to deliver the same SPL in that region compared to without the waveguide. After you shape the response of the waveguided tweeter back to flat the non-linear (harmonic, intermodulation) distortion goes way down.

The second reason is that waveguides (can) give controlled directivity - ideally as you move off axis the response will droop down but in a smooth and controlled way (as opposed to the nasty comb filtering of a larger cone speaker). As a general rule of thumb if you use a 6" waveguide you get directivity that is comparable to a 6" woofer, therefore matching the beam width / power response between the woofer and tweeter. Basically you avoid having a lumpy power response due to the woofer starting to beam and then power response spiking back up at high frequencies due to a non-waveguide tweeter which has a much wider beam width. With a 4" cone driver crossed at 1.7kHz the beaming is avoided so you don't need a waveguide to match directivity because both drivers are operating as point sources where they cross.

I don't think it will be worth using a waveguide since you plan to cross low to avoid the beaming/directivity problems of the mid and the tweeter already has very low distortion.
 
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To expand in a few words what TMM said:

A waveguide doesn't widen the high frequency dispersion, it narrows the lower frequency dispersion of the tweeter to match the narrower dispersion the tweeter does inherently at its high frequency limit. And to match the dispersion of the midrange (at the mid's upper frequency end).

The way to widen a tweeter is to (1) bounce it off something like the B&O lense or (2) squeeze it through a diffraction slot (which is generally acknowledged these days to sound terrible).
 

bcodemz

Member
2014-02-21 1:19 am
What I was thinking is that above 5KHz or so, the tweeter is going to become more and more directional. So above that frequency, I'm OK with only say a 90 degree coverage.

I know a waveguide will deliver a flatter power response because the off axis response is smoother. But one thing I'm not quite clear on is whether if the waveguide widens the dispersion at the very top end, say 10KHz and above. Say if I have a 90 degree coverage waveguide, then it'll narrow the dispersion the lower range of the tweeter to 90 degrees, but I'm not sure if it helps the top end, where the tweeter is starting to beam.

What about the earlier parts on crossover and center to center spacing. Is what I said correct?
 
A few baffle simulations with variations in area, shape, driver location, and edge diffraction effects will provide insight into TM directivity, and required crossover equalization. At 1700Hz, the 10F/8424G should be close to 180 degree = baffle-shape polar response, and a direct-match the bare D3004/6020 better than within a waveguide. The baffle shape will set the polar response until the 1" dome starts to physically beam(narrow).

-Minimum area for a lower frequency 2PI --> 4PI transistion (reduce direct and increase room)
-Tweeter offset on baffle to spread diffraction effects over wider freq range.
-Truncated pyramid with edge control can improve tweeter clarity (like Avalon speakers)
-Sphere with time aligned tweeter minimizes diffration (like B&W 801)


A tweeter waveguide usually best services the lower crossover frequency that a 5"-7" midbass would require.
 

bcodemz

Member
2014-02-21 1:19 am
How does a waveguide achieve much more constant directivity without increasing the dispersion of the tweeter up high? I understand it narrows the dispersion of the tweeter down low, but a typical tweeter has a drooping off axis response as the frequencies go up, and will not going to have more than 90 degree of dispersion say about 5-6KHz. However, looking at the polar responses of many waveguided speakers shows fairly constant directivity to about 10KHz. I don't see how a waveguide is not helping with dispersion at high frequencies.

Here's the polar of a DIYSG Fusion 15, it still has 60 degrees of directivity at 20KHz. There is no way a direct radiating tweeter could have 60 degrees of dispersion at 20KHz.

Fusion-15%2Bv2%2BPolar.png
 
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The more I started to look at directivity control for multi-way loudspeakers in a box using a dome tweeter and cone midrange/bass unit the less excited I became about building them. In the end I have turned to open baffle type speakers, particularly using the midrange and tweeter "nude", that is without any baffle around them. This can provide smooth, frequency-independent directivity, however, it creates some new challenges - the frequency response drops at 6dB/oct starting at a frequency that is related to the driver diameter. This might be at 1-2kHz for a 6" driver for example. You need to do some extensive equalization, however, with DSP that's just a matter of adding the necessary corrections. Also using more bands (e.g. a 4-way speaker) can help ameliorate the need for extensive frequency response corrections.

So, if you (bcodemz) are looking to try a design that has smooth directivity you might look into the nude-OB type loudspeaker.

If you really want to stay with a more conventional route, look into tweeters that include small waveguides. For example the DXT tweeter by Seas (27TBCD/GB-DXT). There is a new cheap waveguide tweeter by Dayton audio that I am checking out as well:
http://www.parts-express.com/dayton...odymium-tweeter-with-waveguide-4-ohm--275-051
It's currently on sale for a whopping $16. Not for crossing low, however.

You could go the horn route. DIYsoundgroup offers some smaller SEOS horns, including one that is meant to be used with certain dome tweeters to make their response more CD-like (SEOS-8 for dome tweeters). See:
http://www.diysoundgroup.com/plastic-seos-12.html
Paired with the RS-28 tweeter this can be crossed over down to 1.5kHz no problem.
 
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diyaudnut

Member
2015-02-24 5:41 am
The more I started to look at directivity control for multi-way loudspeakers in a box using a dome tweeter and cone midrange/bass unit the less excited I became about building them. In the end I have turned to open baffle type speakers, particularly using the midrange and tweeter "nude", that is without any baffle around them. This can provide smooth, frequency-independent directivity, however, it creates some new challenges - the frequency response drops at 6dB/oct starting at a frequency that is related to the driver diameter. This might be at 1-2kHz for a 6" driver for example. You need to do some extensive equalization, however, with DSP that's just a matter of adding the necessary corrections. Also using more bands (e.g. a 4-way speaker) can help ameliorate the need for extensive frequency response corrections.

So, if you (bcodemz) are looking to try a design that has smooth directivity you might look into the nude-OB type loudspeaker.

If you really want to stay with a more conventional route, look into tweeters that include small waveguides. For example the DXT tweeter by Seas (27TBCD/GB-DXT). There is a new cheap waveguide tweeter by Dayton audio that I am checking out as well:
Dayton Audio ND25FW-4 1" Soft Dome Neodymium Tweeter with Waveguide 4 Ohm
It's currently on sale for a whopping $16. Not for crossing low, however.

You could go the horn route. DIYsoundgroup offers some smaller SEOS horns, including one that is meant to be used with certain dome tweeters to make their response more CD-like (SEOS-8 for dome tweeters). See:
The SEOS? Project DIY Sound Group
Paired with the RS-28 tweeter this can be crossed over down to 1.5kHz no problem.

Useful pointers Charlie. After a visit to Mr. Linkwitz's home to hear the Lxmini and Lx521, i am unable to listen to my box speakers (4way with Seas Excel drivers, DSP controlled, hypex ncore amps).

the low price Lxmini simply beats an over engineered box speaker with high quality parts and meticulously measured and tweaked.
 

bcodemz

Member
2014-02-21 1:19 am
Useful pointers Charlie. After a visit to Mr. Linkwitz's home to hear the Lxmini and Lx521, i am unable to listen to my box speakers (4way with Seas Excel drivers, DSP controlled, hypex ncore amps).

the low price Lxmini simply beats an over engineered box speaker with high quality parts and meticulously measured and tweaked.

What do you think is the reason for this? Upon first glance, it is difficult to believe the LXmini can sound that much better than your very high end 4 way build without even using a tweeter.
 

diyaudnut

Member
2015-02-24 5:41 am
What do you think is the reason for this? Upon first glance, it is difficult to believe the LXmini can sound that much better than your very high end 4 way build without even using a tweeter.

I am no expert on this. But have attained a fair understanding.
I am unable to obtain a satisfactory result with the 4-way, even though they seem to measure fairly well on-axis, off-axis, low uniform distortion etc etc.
Continuing to investigate with help from the forum, but a clear issue is still evading me. But something is definitely off, and even a "BOX" speaker with such high quality parts needs to sound better.

the 4ways do sound spectacular as is, but its in comparison with the best that i have heard (lxmini, lx521) that they fall short.

I think one of my issues is a fairly low sensitivity 5" (seas W15CY001) metal driver used as midrange in such a large speaker. The other issue is definitely too reflective a room.
The speakers sound dull and recessed not as dynamic and smooth as the lxmini.

I think LXmini has a few things going well for it, based on my own listening impression and from discussion with few competent folks on this forum

1. LR2 at 700KHz puts both drivers (very closely spaced) in a very comfortable region. This yields very smooth power response.
2. The directive full range actually reduces room issues. It lacks the HF extension of a dome tweeter, but trust me it hardly sounds like its missing anything in the HF.
3. Dipole pattern of the full range again controls off-axis energy.
 
How does a waveguide achieve much more constant directivity without increasing the dispersion of the tweeter up high? I understand it narrows the dispersion of the tweeter down low, but a typical tweeter has a drooping off axis response as the frequencies go up, and will not going to have more than 90 degree of dispersion say about 5-6KHz. However, looking at the polar responses of many waveguided speakers shows fairly constant directivity to about 10KHz. I don't see how a waveguide is not helping with dispersion at high frequencies.

Here's the polar of a DIYSG Fusion 15, it still has 60 degrees of directivity at 20KHz. There is no way a direct radiating tweeter could have 60 degrees of dispersion at 20KHz.
The Fusion 15 does not have a waveguided dome tweeter, it uses a horn loaded compression driver. Although they both use a type of horn they are very different beasts.

For studying the effects of horn/waveguide loading a normal dome tweeter see here:
Zaph|Audio

Note how with the 6.5" waveguide the power response of the top octave is largely unchanged. With the 8" and 10" waveguides, power response is improved in the top octave however it's more that the on-axis response has been attenuated rather than the off-axis response being increased.
 
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AllenB

Moderator
Paid Member
2008-10-18 11:31 am
Hi,

there are three unique solutions for flattening power response and at the same time making beamwidth more narrow than Pi (half-sphere):
* Horns (waveguides are short and wide horns). Disadvantage of horns is size and building effort.
* Keele's Constant Beamwidth Transducers with convex baffle: Disadvantage is, one needs many tiny fullrange drivers.
* Classic Constant Beamwidth Transducers with strait baffle and first-order filters: Alleviated disadvantages of Keele's.

Best for HiFi (high but not best quality reproduction) is the last solution, possibly with some flavouring from the second one. One important design mantra (as with every rule, there are exceptions):
* Do not leave much space between adjacent diaphragms (rules out most dome tweeters)
 

Face

Member
2007-05-23 8:38 pm
I haven't met anyone who's heard #2 and enjoyed them.

As for horns, I don't see how building them is difficult as there are many excellent off the shelf waveguides and horns available to us. If horn loaded mids or bass are desired, then size becomes an issue very quickly. :)
 
I actually thought Keele's CBT sounded very good. Too much floor space for me to use, though. An advantage of waveguide speakers is they can be designed to live up against a wall, though if the waveguide has to control down below the room's modal frequencies they can tend to stick out pretty far from said wall!

One way a horn can widen the dispersion of a tweeter up high is by diffraction. Let it zing off of irregularites down by where the driver attaches and HF sound will go off in all forward directions. Diaphragm breakup is another way a small dome can radiate wide -- not much radiates like a plane wave near the highest octave...

BTW, that polar plot is clearly "normalized" (unless the designer did a monumental job of designing a crossover for dead flat). Pretty sure that it isn't going to be that hot up near 17kHz.

But, really, I wouldn't worry about the angle coverage of 17kHz too much. Unless you're so young that your dad is reading this to you, you probably won't be hearing it very much (yeah, you might hear it when you drive with a tone generator, but at how many dB down??)
 
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jojip has like LXmini and is not happy the his boxed 4-way which obviously dos not have waveguide for tweeter.

I think that the "natural and balanced" sound of good dipoles comes very much from the backside radiation at high frequencies and appropriate/balanced 3D-full sphere radiation.

A speaker with smooth frontside radiation (achieved with beam control with waveguides/horns) will never sound the same in a room! They lack backside sound radiation. This difference can be seen in room decay measurements with eg. REW. Dipole-guru John Kreskowsky is happy with backside radiation up to 6kHz, perhaps because higher frequencies will largely be absorbed in wallls and furniture before reaching the ears or the measurement microphone!