Some help with lobing

[Trdat]

I think I understand the concept but can't figure out the solution.

"Lobing occurs with any two drivers producing the same frequency range in parallel (identical signal) where the distance between the drivers is greater than 1/4 wavelength of any frequency they are reproducing."

So if I am going to crossover around 3000hz or therabouts for a typical tweeter to midrange crossover than what would be the minimum centre to centre distance between them? I ask cause I want to put a waveguide on my tweeter and it seems there will a minimum centre to centre distance of about 13cm which is longer than what it could be between it without the waveguide.

I tried calculating it myself but the 1/4 wavelenght on a wavelenght caclulator is giving me 25000 metres so I must be doing something wrong.

Then there is the distance to the listening position but again lets say around 2 to 3 metres.

 

[kimmosto]

what would be the minimum centre to centre distance between them?

Minimum c-c is 1.0 x wave length and maximum about 1.4 x wave length at XO frequency assuming that design is conventional uni-directional box (not open baffle) with phase matching (acoustical 4th order) slopes. Good and quite flexible initial/design value for c-c is 1.2 x wave length at XO, giving smooth combination of power and early reflections i.e. balanced sound without significant power dip at XO due to bump in DI and dip in vertical early reflections. In other words, this concept aims lobe nulls to directions which are the least significant for power response and vertical early reflections - and listener sitting in sweet spot of course.

 

[Trdat]

Minimum c-c is 1.0 x wave length and maximum about 1.4 x wave length at XO frequency assuming that design is conventional uni-directional box (not open baffle) with phase matching (acoustical 4th order) slopes. Good and quite flexible initial/design value for c-c is 1.2 x wave length at XO, giving smooth combination of power and early reflections i.e. balanced sound without significant power dip at XO due to bump in DI and dip in vertical early reflections. In other words, this concept aims lobe nulls to directions which are the least significant for power response and vertical early reflections - and listener sitting in sweet spot of course.

So basically I multiply the wavelenght by 1.2 and it gives me what exactly? The distance between the c-t-c distance? And also how do I find out the wavelenght at that crossover frequency?

The wavenght calculator gave me 100 metres for 3000hz have missed something simple?

But overall thanks for explanation I will use the 1.2 as the bases of working it out.

 

[hex168]

Are you using an electromagnetic radiation wavelength calculator?

Sound travels 343 meters per second, so:
(343 meters/second) * (1/3000 seconds) = 0.114 meters (about 4.5 inches)

 

[kimmosto]

With sound speed = 344 m/s and XO frequency = 3 kHz, c-c = 1.2 * 344000 / 3000 = 138 mm.

 

[Adam_M]

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!

 

[Trdat]

Greatly appreciated. I had scoured the forums reluctant in starting a new thread but the above has made it so simple. Ill stick to those rules...

 

[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.

I will take that in consideration, I probably need to do some further reading in understanding what the difference is between choosing 1 wavelenght opposed to say 1.4. In saying that I am limited to the size of the waveguide which is aluminium and I can't route it out and with the 4 inch woofer the closest I can get is 140mm and any further is already pushing it to 1.4 which according to Kimmosto is the maximum.

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.

Myy scanspeak 12W/8524G00 starts beaming around 3000 ish so I personally wouldnt go higher than that.

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.

There is an Aussie guy who has performed tests on the tweeter in question ill check out the distortion graphs and see how low I can push it. In any case, I will be using DSP to tie it all in so I can play around with it. I just got to get the lobing right as DSP doesn't mitigate that issue at all.

 

[kimmosto]

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.

 

[puppet]

Dispersion matching and complimentary phase characteristics are more important to a coherent overall response .. I think.

 

[Trdat]

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.

So if I am pushing towards the 1.4 wavelenght is there anything I should be worrying about?

I am using DSP to tie it in which gives me the opportunity to use a lower crossover point. Unfortunately I am limited by the waveguide size, but I am lucky that the 4 inch doesn't beam till about 3000 ish. Closest I can get is the 140mm c-t-c distance.

The speaker is a normal 3 way that is layed on the side with the tweeter above the midrange and I will be using it as a centre speaker. It is basically the same as my L/R but I am adding a waveguide to the design on the tweeter.

 

[Trdat]

Dispersion matching and complimentary phase characteristics are more important to a coherent overall response .. I think.

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.

 

[kimmosto]

c-t-c distance.

What is c-t-c? c-c in my messages stands for distance between center points of tweeter and mid(woofer). You can have two mid-woofers i.e. MTM but c-c is still from tweeter to mid (not from mid to mid).

 

[kimmosto]

coherent overall response

What do you mean by "overall response"?

 

[Trdat]

What is c-t-c? c-c in my messages stands for distance between center points of tweeter and mid(woofer). You can have two mid-woofers i.e. MTM but c-c is still from tweeter to mid (not from mid to mid).

I saw c-t-c somewhere but yeh same as c-c centre to centre distance.

 

[kipman725]

If your using DSP you are not so restricted as you can apply delay to compensate for distances between acoustic centers and have acess to very steep slopes. With DSP you would generaly want the drivers as close as possible together. With the delay you make the drivers sum perfectly as the desired listening point and by using steep crossover slopes you reduce the range of freqeuncies for which there are directivity anomalies.

 

[Trdat]

If your using DSP you are not so restricted as you can apply delay to compensate for distances between acoustic centers and have acess to very steep slopes. With DSP you would generaly want the drivers as close as possible together. With the delay you make the drivers sum perfectly as the desired listening point and by using steep crossover slopes you reduce the range of freqeuncies for which there are directivity anomalies.

I just want to make sure there is nothing I missed that DSP doesn't mitigate but in general I'm confident I have covered everything. It was just the lobing as c-c would be larger with the waveguide. DSP does magic so lets see if I can get it right this time around as well.

 

[Trdat]

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.

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

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...

 

[kipman725]

I just want to make sure there is nothing I missed that DSP doesn't mitigate but in general I'm confident I have covered everything. It was just the lobing as c-c would be larger with the waveguide. DSP does magic so lets see if I can get it right this time around as well.

It can't make the response good everywhere if the acoustic sources are more than 1/3rd wavelength apart as even if you delay appropriately such that on axis the vector sum of the two sources results in positive supposition there are points off axis where there is destructive supposition. 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.

 

[kimmosto]

DSP does not change much. 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). Also possibilities to cross-smooth directivity differences between M and T is reduced...totally lost with steeper slopes so designer should verify that directivities of M and T are compatible. Once again: lobing is less problematic than possible power & early reflection issues due to bad c-c if listening elevation is quite constant.