Some help with lobing

[kimmosto]

I think I missing the point of the last sentence in which you say 1/4 wavelenght at XO is just a utopia?

1/4 wave length at 3000 Hz would be c-c = 28.7 mm which is utopia with conventional M and T.

My design will have it at around 1.4 times the wavelenght would that cause any problems that DSP cannot mitigate? You said max 1.4 so I am guessign it should be okay...

1.4 x wave length is okay. DSP has nothing t do with this.

 

[headshake]

VituixCAD thread on diyaudio contains three examples with real life data and simplified theoretical study about c-c = 1.2 x wave length concept. It's actually quite common in practice. Traditionally XO frequencies 2.5-5 kHz were common, and sound of those speakers was typically smoother and more tolerable than (modern) low XO point. So I'm not trying to invent anything new or provoke. Just giving an answer why some sound features were better in the past; no blood from ears while listening 80s' Gary Moore or Iron Maiden.
Common (modern) opinion/statement is that c-c should be as short as possible. With "normal luck" it hits c-c = 1/2 wave length at XO which causes the worst possible power dip and balance break with conventional unidirectional box speaker. Also risk of power bump above XO point increases with conventional tweeters without wave guide. c-c = 1/4 wave length at XO is just an utopia - worthless to mention for other than XO between mid and woofer, or woofer and small full-range as a tweeter.

Is a 1.2xWL @ XO ideal for nearfield too? What if you are sitting closer to your speaker than the wall, floor, and ceiling? I am wondering if there is any benefit to going lower than 1.2xWL @ the XO.

Thanks for posting and for creating VirtuixCAD.

 

[kimmosto]

^Near field has different requirements because power response has not much effect and early reflections should be avoided. So very short c-c or coaxial would be better.

 

[puppet]

What do you mean by "overall response"?

I meant that the loudspeakers drivers response having a seamless quality to their summed output. Sounding more like one source (drivers combined response) than an apparent (glaring) transition between sources.

 

[puppet]

Amatuer hour but with a 3 way the dispersion will more evenly matched right? And phase will be mitigated by the DSP. Of course we aim to get the best before using DSP.

Matched dispersion has to do with the drivers directivity at the individual xo points. A 3-way doesn't automatically achieve that. As for phase ... a DSP isn't a magic bullet. XO points along with EQ have to be considered as they impact phase.

 

[kimmosto]

This is why in most speakers the drivers are arrayed verticaly; so the veritcal polars are messed up and not the horizontal. The ear is much less descriminating in the vertical plane and usualy a smaller solid angle needs to be covered. The only speakers I know of that can have good horizontal and vertical polars are multiple entry horns as other types of speaker have too large driver spacings.

Good points.
Reverb and reflections are also very random; mixture of multiple signals with multiple delays and multiple spectrums due to possible diffuse & damping surfaces so other than individual 1st order reflections from straight walls, floor and ceiling do not care much about non-systematic errors such as lobing. Also early reflections total is combination of reflections from all (typically six) surfaces. Effect of vertical lobing alone could be significant if lobes are towards 1st order reflection points and directivity in vertical plane is not much greater than directivity index. Otherwise vertical lobes are there, but quite harmless assuming constant listening elevation.
So the tools to avoid problems due to vertical lobing are:
a) Design huge vertical directivity, or at least vertical DI >> horizontal DI assuming that walls are not much more damped than floor and ceiling.
b) Aim vertical lobes with c-c and XO to somewhere else than 1st order reflection areas on the floor and ceiling, and optimize spectrum of directivity index (DI) and early reflections directivity index (ERDI). This item suggests c-c ~1.2 x wave length at XO. MTM could be a bit asymmetric e.g. c-cMT = 1.15 x WL@XO and c-cTM = 1.25 x WL@XO.
c) Use coaxial driver for MT.

P.S. Strong directivity cannot override bad room acoustics so the smartest move is to improve room acoustics (shorten early reflection decay [EDT]) first instead of increasing directivity. Directivity and concentric technologies are not problem free either so increasing directivity or concentricity could damage some sound features or force to choose a lower quality driver or DSP with numerous digital biquads or FIR to play with.
Synergy/unity horn is also directivity technology so it's not directly comparable with lobing control by c-c and XO and small coaxial drivers.
I prefer separated radiators i.e. more willing to compromise remaining lobing than more constant and reasonable overall directivity and sound quality of the drivers and less acoustical intermodulation.

 

[Trdat]

Steeper slopes make lobing problems narrower, but power & early reflection dips at XO will be as deep as with shallower slopes (exception is FIR brick-wall).

So when you say steeper slopes compared to shallower, you mean the steeper is a quicker roll off at XO right?

A power dip can be mitigated with DSP but can you tell me more about reflection dips or link me to a read, it would be great to have a quick read to keep following this great post. (I jsut saw your latest post which has info on this, I am reading it I think it covers a lot)

Also possibilities to cross-smooth directivity differences between M and T is reduced...

Just to confirm when you say is reduced meaning it "is not improved" and is this with shallower or steeper slopes?

Once again: lobing is less problematic than possible power & early reflection issues due to bad c-c if listening elevation is quite constant.

Yes, but with a decent c-c such as 1.4 times wavelenght would early reflection issues be okay? Just trying to understand if max 1.4 would mitigate the secondalry problems you mention that are worse than lobing itself.

 

[Trdat]

1/4 wave length at 3000 Hz would be c-c = 28.7 mm which is utopia with conventional M and T.

So just confirm when you say Utopia meaning that conventional M and T barely reach 28.7mm and getting c-c distance of 28.7 is good for conventional M and T?

 

[kimmosto]

No

 

[kimmosto]

1/4 wave length of 3000 Hz is 28.7 mm. Diameter of mid driver and tweeter should be less than 28.7 mm to get c-c <= 28.7 mm. So that's not an option in real life without some sort of cup (micro-synergy) or pipe radiators. Typical 5.25" mid and 1" dome with 104 mm flange can be installed c-c >= 130 mm. 130 mm supports XO frequencies from 2646...3705 Hz by kimmosto's c-c law.

 

[Trdat]

Matched dispersion has to do with the drivers directivity at the individual xo points. A 3-way doesn't automatically achieve that. As for phase ... a DSP isn't a magic bullet. XO points along with EQ have to be considered as they impact phase.

Yes, but it is easier to match dispersion with a 3 way but I understand the point your getting at.

Phase is one of my least researched topics in audio no particular reason but I have some reading to do.

 

[Trdat]

I've always tried to keep it to less than 1 wavelength CTC at the XO frequency and it's worked out quite well. More often than not, I end up cutting into the tweeter flange so I can further minimize the distance - I typically target just under 1 wavelength.

In your example, I calculate the wavelength to be about 2,650Hz at that distance, so I'd probably start in the 2.5k range (which is more or less as high as I'd go with a 6.5"-7" driver - maybe even a touch higher than I'd like depending on the specific driver in question), assuming the woofer won't be beaming at that frequency - check the spec sheet to see where off axis starts to break down.

I generally push the XO point as low as I can before the tweeter distortion starts to be an issue, while using as small of a tweeter as is feasible for the performance I'm looking for.

...With that said, Kimmo has probably forgotten more than I've ever known about speaker design and the math to it, so don't overlook his suggestions in favor of mine. Get some parts and give it a try!

So my tweeter can easily play down to 2600hz which allows me to choose options of a lower XO below the 3000 with the the caclulated c-c. Appreciate the calculation above helps me see what the ideal XO point will be.

 

[Trdat]

1/4 wave length of 3000 Hz is 28.7 mm. Diameter of mid driver and tweeter should be less than 28.7 mm to get c-c <= 28.7 mm. So that's not an option in real life without some sort of cup (micro-synergy) or pipe radiators. Typical 5.25" mid and 1" dome with 104 mm flange can be installed c-c >= 130 mm. 130 mm supports XO frequencies from 2646...3705 Hz by kimmosto's c-c law.

Great. Now, I get it! Appreciate it!

 

[BenB]

Instead of worrying about what's radiated into the room, it makes more sense to me to focus on what is perceived in the listening position. The direct path, and early specular reflections arrive at the listening position with the most energy. It's important for these to have balanced spectra. Reverberant energy from diffuse reflections and multiple bounces arrives with less energy, and our hearing actually expects these sounds to arrive decorrelated from the original source; comb filtering in this reverberant soundfield is not an issue, and narrowband dips from crossover cancellation will be completely benign, or very close to it.
In typical rooms, the floor reflection is the first and loudest reflection (except perhaps at frequencies absorbed by the carpet), and typically comes from energy radiated about 30 degrees below the horizontal. The ceiling reflection often arrives next, with energy radiated at 45 degrees elevation. Assuming these reflections aren't mitigated with directivity or acoustic treatments, the balance of these reflections should take precedent over the balance of the late arriving, diffuse/reverberant energy.

 

[headshake]

Newb here,

(driver + baffle image) I was curious about a seas dxt + sub 2khz XO + the largest PE box when it comes to Kimmo's suggestion for spacing. He mentioned a higher XO for convenience but I wondered what a lower XO would look like when it followed the 1.2X the WL of the XO. If the PE Box is the boundary, I can't fit anything larger than a 5" and still keep the XO sub 2khz.

(image of my lobes) The lobing for the dxt + 5" is on the right. My nearfield XO that is 0.5WL of the XO is the top left image. The bottom left is what happens if I added 140mm of space between my tweeter and mid.

I think it is neat to see the lobing Kimmo mentioned.