Dispersion?

[Phil Starr]

On 16/08/2021 at 20:53, agedhorse said:

Respectfully, to counter the common argument that the polar pattern (response versus frequency versus axial angle) of a speaker depends only on the diameter of the driver and the spacing to adjacent drivers, this is only partially true.

 

It is possible (and commonly done in the real world) to alter the polar patter of a driver at higher frequencies by changing the cone depth, shape and the size/material and location of the dust cap. All of these elements contribute to the non-theoretical aspects of a driver's radiation pattern, and is why 10 different 15" drivers will have 10 different polar patterns.

 

As an extreme example, the JBL D-130 utilizes a shallow, light weight curvilinear cone, and an aluminum dust cap that is directly bonded to the top edge of the bobbin. This dust cap acts to radiate the higher frequencies differently that the cone itself.

 

As another extreme example of what could also be called a secondary radiator, the "wizzer cone" concept is a dust cap with a radiation element that is bonded directly to the top of the bobbin and can extend both the frequency response and polar pattern by an octave or more.

 

Polar patterns are something that designers consider when developing transducers for practical applications.

@Pirellithecat if you think this is a thread derail let us know and we'll take this elsewhere. If you are interested I hope the nerdiness is interesting.

 

@agedhorse is right and it is something @stevie and I have talked about before. The 'ideal' speaker cone is a piston moving backwards and forwards without bending pushing the air exactly tracing the waveform of the current flowing in the voice coil. Of course in life nothing is ideal. Cones are heavy fixed on the outside to a springy suspension and are mainly made of papers so they are a bit bendy. As the frequency rises the coil will do it's best to keep up but the outside of the cone lags behind a bit eventually the cone can be seen to be flexing so that some parts are going forwards at the same time as other parts are going backwards. It's often said that the centre of the cone radiates all the treble and the whole cone the bass, but the truth is that it is messier than that. The bits travelling in different directions cancel each other and the bits going in the same direction reinforce each other and the frequency response suffers. Its a distortion of the original sound. I tried to find a laser interferometry picture of this on YT but all I found was a computer simulation. Loudspeaker cone breakup (in a typical loudspeaker) 3 - YouTube

 

 

At this point there are design problems/opportunities and both have been chased. One approach is to say 'I'll make a stiffer cone' it is distortion and a proper piston is what I want'. The problem then is beaming at the frequency where the wavelength corresponds to the diameter of the cone (ish, it doesn't all happen at once) Another problem is that stiffness usually raises the mass of the cone significantly and other things are lost. One significant thing is that when a stiff cone breaks up it usually sounds horrible. 

 

The other approach is to say OK I know this is distortion but I can live with a little and manage it. I'll try and control the flexing so that the middle of the cone does the treble  so I reduce beaming and get a flatter response up to higher frequencies. 'Heck, if I get it right I won't need a crossover'+. In a sense the original Barefaced Compact with the 15" driver took that approach, you can have a 15 with enough top end to sound good. Peavey had a Black Widow 15 with an aluminium dome which sounded good and in years long past I used PA speakers with a whizzer cone which I got away with.

 

There was endless experimentation with this in 70's hi-fi. At the time the goal was to make a 2-way speaker with an 8" bass unit and a 1" tweeter and with the technology of the time there was a gap between the point at which an 8 starts beaming and the point where you could reasonably ask a 1" dome to take over. That was a special problem because that point was right in the middle of our hearing range where even small distortions are noticeable. One solution of course is to have a 3way design with a mid driver but that creates further problems with the crossover. 

 

This is of very little help to someone choosing a bass cab however, cabs will differ in their radial response in very complex ways. there is plenty of physics to guide designers but no perfect solution and the real picture once you take a cab into real rooms with room reflections bouncing sound all over the place mean that a cab that works well in one room can be a nightmare in another space. Little of this information is available to anyone choosing a cab anyway, but I hope it is reassuring to anyone stuck in a venue struggling to get the on-stage sound right

[Dan Dare]

 

3 hours ago, Phil Starr said:

a cab that works well in one room can be a nightmare in another space.

 

Excatly. Room acoustics, reflections, resonant frequencies and cancellations and so on will all vary considerably depending on how many bodies are present to soak up the vibrations, atmospheric conditions, temperature, etc, etc. It's enough to make your head hurt, Brian.

[agedhorse]

Virtually everything in transducer and speaker cabinet design is all about balancing multiple tradeoffs simultaneously without leaving real world performance on the table.

[Phil Starr]

4 hours ago, agedhorse said:

Virtually everything in transducer and speaker cabinet design is all about balancing multiple tradeoffs simultaneously without leaving real world performance on the table.

Ain't that the truth

[nekomatic]

I do find myself thinking that the paper coned electromagnetic speaker in a wooden enclosure is the worst possible way of converting electricity into sound, apart from all the other ways that have been tried from time to time. 

[BassmanPaul]

On 16/08/2021 at 06:30, Pirellithecat said:

Thanks Downunderwonder. 

I'll re-look at Barefaced.   Probably need something like 2 of the 2x10 cabs.   And Old School to boot!!

If you go this route, stack them on their ends for a vertical 4x10. In my mind they are the best for dispersion around the room.

[Pirellithecat]

Thanks All, 

 

I will check out the Barefaced offerings.   Interestingly I had a rehearsal the other day - same room as where I first noticed the dispersion issue.  I need to check with other speakers, but I suspect it might actually be a problem with "interference" i.e. there seems to be Bass Cancellation going on as you move away from the speakers - sounds like there's been a reduction in output as one moves around the room.   I'll also change the layout of the room and see how it affects things.

 

 

[Bill Fitzmaurice]

That's normal, it happens in every room, and the smaller the room the greater the effect of boundary sourced cancellations. It's also the explanation why the bass can be louder in the back of a club than on stage. On stage the boundaries are close enough to the speaker and you for cancellations to occur, while even twenty feet out there are no cancellations. But this is something that happens in the bass and midbass, where there is no directional content. Dispersion only narrows in the mids and highs, typically above 300Hz.