Aspects of speaker design

[andy19191]

8 years ago on the forum I tried to get a cross platform speaker design community project off the ground but without success. Here in the UK we have now entered what looks like a 6-8 week lockdown and I intend to start a somewhat different second project having hopefully learnt one or two lessons from my previous failure.

The project is intended to provide reliable information and guidance on some aspects of speaker design that are not currently widely well understood by those with a hobby interest. Passive crossover design for example is widely and well understood by those with a hobby interest and so lies outside the scope of the project. Sound radiation from speaker cabinets however is not well understood with many holding a range of doubtful and dubious notions that are not based on an understanding of the physical processes involved.

The project will start slowly, likely very slowly, as a personal one but if it gathers interest hopefully others may want to get involved. If nobody joins then the scope of the project will remain limited to areas in which I hold sufficient expertise and interest.

The main tool I will be using to quantify the relevant physical processes is 3d numerical simulation which has been a significant part of the day job for decades but rarely seems to be used by speaker hobbyists. In particular I hope to use the projects as examples to help develop software. Eating ones own dog food I believe it is called these days or even dogfooding.

Anyway I have spent the whole day finding ways to avoid getting on with it (including writing this post!) because I find writing computer code a chore these days. My initial list of aspects possibly worth covering are:

Cabinet vibration and sound radiation
- Physics
- Modes
- The model
- Example subwoofer
- Example 2 way (1" tweeter, 6.5" midwoofer)
- Example 3 way (1" tweeter, 5" midrange, 12" woofer)
- Conclusions/design approaches

Constrained layer damping
- The model
- Geometric parameters
- Damping material parameters
- Implementation in speakers
- Compare with extensional damping
- Compare with box in box
- Conclusions/design approaches

Room acoustics
- Modes and physics
- Summary of relevant sound perception
- Example of ideal room (studio)
- Example of room in home (my lounge)
- Monopole, dipole, cardioid,... mains in example rooms
- Distributed subs in example rooms

Porting
- Physics
- The model
- Effect of position in cabinet
- Slot, triangle, round, bottom,...
- Effect of flaring
- Effect of stuffing within cabinet
- Chuffing (more complex model -> separate study?)
- Conclusions/design approaches

Tweeter waveguide
- Throat effects
- Profiles
- Mouth effects
- Cabinet shape
- DXT vs smooth
- compare with existing measurements if geometry available

Transmission line
- Modelling sound absorption

Large chamfers/rounding study
- 3d effects not handled well by existing edge/baffle simulators

Have I missed something relevant under the headings above? What other areas might benefit from detailed 3d studies to extract quantitative details of the relevant physics? Is the project worthwhile?

[tvrgeek]

Most of these topics should not be any confusion to anyone who has either built a few, or bothered to do any research. Actually, you could easily spend two months compiling, testing and proving what we already know about ports and write a definitive white paper. What had not been often discussed it actual objecting of different distortion using a PR rather than a port. Two issues, one is any addition mechanical distortion, and the other is effects on transient response.

There have already been many papers comparing the same speaker system in cabinets of different construction.
There are entire forums on room acoustics, psychoacoustics, cabinet shape etc. Maybe a trip to the library to read the AES publications to see these have been addressed since 1957.

There IS very little on tweeter throat matching to wave guides. Dr. Geedes mentioned the importance, but gave no guidelines for a DIY builder. That alone is worth a couple of months.

Pick one line from one topic. You may find to do it completely a much larger task than you expect.

[gregrueff]

Is the project worthwhile?

Andy,

Your musings and investigations are valued.
That is more than I can say for other contributions on this forum.

Cheers!
Greg

[billfitzmaurice]

All of those topics have been extensively investigated and reported on, including the tweeter wave guide questions. They might not be familiar to the layman, but they are to the community of professional acoustical engineers and to serious hobbyists as well.

[andy19191]

Most of these topics should not be any confusion to anyone who has either built a few, or bothered to do any research.

Many DIY folk are not engineers and don't possess the relevant background knowledge to effectively perform research. If they look things up on wikipedia, forums and the web in general they will come across some good stuff but much, and in some area most, will be nonsense. How to know what is reliable and what is not?

Actually, you could easily spend two months compiling, testing and proving what we already know about ports and write a definitive white paper.

The intention was not to be definite but to present the physics in a reliable, understandable and quantitative manner. It is the understanding of what is going on and the relative importance when making trade-offs that is often poorly understood.

What had not been often discussed it actual objecting of different distortion using a PR rather than a port. Two issues, one is any addition mechanical distortion, and the other is effects on transient response.

Thanks for the suggestion. This is the type of things I was posting for. In this case I am not sure detailed 3d modelling has much to offer DIY folk? Speaker simulators using lumped models of drivers without the motor will provide the transfer function and hence the transient response. Distortion will be dominated by the nonlinearity of the suspension which is down to measurement and not something a DIY typically has control over.

There have already been many papers comparing the same speaker system in cabinets of different construction.

I am not aware of any? In addition, knowing how something measures is not the same as knowing how to design it which will require an understanding of what is going on and the relevant trade-offs and compromises.

There IS very little on tweeter throat matching to wave guides. Dr. Geedes mentioned the importance, but gave no guidelines for a DIY builder. That alone is worth a couple of months.

The issue is less that type of matching since many waveguides are not deep like Earl's or even use compression drivers. The objective is to provide some understanding of the physics involved so that, for example, folk machining a shallow waveguide into a baffle or 3d printing a waveguide have some guidance on what to look for in a tweeter and how to attach the two.

[andy19191]

Your musings and investigations are valued.

Thanks for the encouragement.

[andy19191]

All of those topics have been extensively investigated and reported on, including the tweeter wave guide questions. They might not be familiar to the layman, but they are to the community of professional acoustical engineers and to serious hobbyists as well.

The information is indeed reported on but rarely externally. A few years ago on another forum during a discussion an engineer from a speaker company said that when he returned to work he would post a relevant result from what he was currently doing. I PMed him to point out he was running a real risk of getting fired and taken to court if he did because the information was not his to give away. I didn't get a reply but he didn't post a result from his company's R&D.

Most speaker DIY folk are laymen when it comes to engineering and would likely benefit from reliable guidance. I was suggesting doing this on one or two topics in which I hold an understanding and if it gathers some interest hoping others might do likewise in other areas outside my area of competence. It may well come to little but since it is a hobby interest and I am hanging it off other activity it won't be a major disappointment if that is how it pans out.

[tvrgeek]

I suggested a reliable source for information: AES. (Audio Engineering Society) If you have a question about the viability of WEB information, ask a specific question.
My point is that you are not going to verify the entire subject of close to 100 years of world wide engineering efforts as a stay at home hobby. That is not to discourage taking on a specific topic to clarify or advance the DIY understanding.

Want to write some useful code?
We could use something like "Edge" except for multiple drivers and non-strait edges of variable radius.
We could use something as easy to use with real time graphical estimations like PSD-Lite, but allowing more complex filters. An added feature to graph excursion with the crossover would be slick. ( it is my belief, a great many designs over-tax the tweeter)
We could use the next generation to Win ISD with better guesses for BP-4 and BP-6 alignments. ( Possibly instead of a single estimation, a family of curves you could pick from)
We could use better modeling for D'Apolitto alignment.
We could use better room gain and mode analysis for rooms other than 4 square walls. Add in estimations of response from a selectable source. ( best place to place two subs etc.)
We could use a spectrum analyzer where we have a slider for frequency sweep to make finding room resonances easier.
We could use a simple stand alone tool for measuring AC. I still use my old Old Colony "pulser" and a scope.
We could use a model for adding mass to a driver. What does it do to efficiency vs Vb, vs Fs and what does that mean to Vb? Graphical results of course.

We do agree, there is very limited DIY useful information on matching a wave guide to a dome tweeter. The prospects of 3D printing open up many great possibilities not available before. Unfortunately, everything I have seen was based on blind guessing and posting pretty cad pictures over and over with no data or guidance on what factors are at play. I just bought a DTX tweeter to play with as if one believes SEAS hype, it has some advantages. Could this be applied to other tweeters? What would happen if one added this concept to a top end tweeter, like a Transducer Labs?


( If any of this is already out there if the freeware or in the very cheap domain, please point me to it )

[gregrueff]

Thanks for the encouragement.

To be clear, you and I have previously debated the topic of cabinet construction & cabinet radiation and finite element analysis techniques for analyzing them. You brought a lot of expertise and knowledge to the discussion that I really appreciated. Please don't feel like no one wants to discuss these things. More discussion on this is welcome, and although myself and others may debate you, please understand it is with respect and a desire to further understanding.

You may or may not be aware that "Augerpro"/Brandon recently did some experiments on cabinet construction techniques including CLD. They were illuminating and pertinent to this discussion, but certainly injecting the topic with some FEA modeling would be awesome.

Also, yes let's discuss passive radiators and bandpass systems! Can a BP-4 system have different HD as a result of the passive radiator? Its suspension nonlinearities are different from the primary driver's nonlinearities. Is a BP-4 system one way to use a cheap primary driver and reduce its HD through a PR?

Cheers!
Greg

[tvrgeek]

Greg,
I suspect there is not a lot of objective data on BP systems as it turns out they are a bit hard and most of the work was done for cars trying to get insane response from too small drivers. They seem to be mostly overlooked in the HI-FI market, though I believe JBL and Bose have marketed them. In modeling, it looks like BP-6 might be suitable for LFE but not for normal subs. If one had the space, a dedicated effects sub would take the excursion and power strain off the subs just as the sub does the mains. A BP-6 looks possible for one octave, but not two.

BP-4 has good possibility in masking HD for the first octave above LP. My recent (failed) build shows that. At two octaves, the artifacts just pass through the port.
My conclusion is that a BP-4 is viable IF you find a suitable driver (I have not) but there is no substitute for a better motor, stiffer cone, and less excursion. Excursion is something a BP-4 also does better than a ported or sealed* , so actually, yes a cheaper driver can be improved, but even better with a bit better driver. We are missing general guidelines for selecting a suitable driver. For second and third order, we have EBP or guides on Qts and Vas. BP-4 seems to want higher Fs, larger Vas but the high excursion usually found only in sub drivers designed for more "civil" box sizes. I may pursue this after I finish my new HT mains.

Actual data on a PR vs port as far as both generating it's own distortion due to suspension non-linearities and resonances as well as advantages in blocking the escape of upper harmonics would be an interesting path to follow. I have read a lot of "mine is great" but no objective data.

Another example is reversing one driver. It was all the fad for reducing even order harmonics, but an experienced member hear warned about an increase in odd. So I have it a try. Yup, even dropped dramatically, almost 10 dB across much of the band, with an increase of only 1 to 1.5 dB in odd. For my build, setting 80 dB as a reference making a tenuous assumption that higher dB woudl be masked so not quite as important, my build remained below -45 dB to the primary, so a viable tradeoff. Glad I tested rather than just took the word for it. The advice was correct, but not significant in this specific case. Again, a rough assumption, but I am for now considering any subwoofer artifact below -50 dB to be irrelevant. It may not be true as one approaches the LP crossover. What happens if you use two PRs, reversing one?
What happens if you built two subs, one with reversed driver and PR, one conventional, and they are separated in the room?

I also have not seen any discussion of transient response other than a comment that the flatter the baseband, the better the performance. It makes some sense that if the rear chamber is too small, it's Q will be higher and TR will suffer. But does it follow if low Q, as in a critical Q alignment, and you accept the peak before crossover, rather than at LF acoustic crossover, is the TR improved? Understanding this may guide if some solutions have realistic ports or not. Or, do you need to oversize both chambers giving up a lot of efficiency? Does moving a pass band peak to the high end really increase HD from the lowest pass band enough to matter?

There are entire forums dedicated to subs, so it is quite possible I am looking in the wrong place. All my questions may be well known in the right circle. So before I spend money on more drivers to play with, so I will do more research so I may stand on taller shoulders. In the mean time, I will just use my low Q, push-pull sealed box as it is about 4 times lower distortion than my old sub and 6 dB more efficient.

* In all cases, I am assuming at least a 4th order HP subsonic filter. I have come to the conclusion all drivers should have a HP filter.

[tvrgeek]

This would seem to be helpful, but their calculations do not match either my measured build or WinISD. More to be learned.
https://audiojudgement.com/4th-order-bandpass-design/

[tvrgeek]

Here is a Horn Res sim for a BP-4. Very close to what I measured. So a good tutor for me on using HR!. from https://www.diysubwoofers.org/bnd/
Though my port resonance problem was @ 200 Hz and totally messed up the response.


Have we got Andy interested in just one topic yet?

[billfitzmaurice]

This would seem to be helpful

It might have been 25 years ago, when we were still using slide rules and scientific calculators to design enclosures. Today it's just as irrelevant as those devices. For that matter it was when written.

[djg]

My high school had a Slide Rule Club for math nerds. I'm old.

[neildavis]

I still have my dad's 1945 K&E slide rule stashed in our attic. But the cool nerds used the circular slide rules.

[andy19191]

I
Want to write some useful code?

The software is being written for reasons other than a hobby interest in speaker design. The OP was about using it to hopefully illuminate some aspects of speaker design.

[andy19191]

You may or may not be aware that "Augerpro"/Brandon recently did some experiments on cabinet construction techniques including CLD. They were illuminating and pertinent to this discussion, but certainly injecting the topic with some FEA modeling would be awesome.

I made one or two posts early in the thread but they weren't wanted and so I stopped.

Also, yes let's discuss passive radiators and bandpass systems!

As mentioned earlier I don't think this fits particularly well as an example for the software. I also have no projects on the horizon using passive radiators or bandpass systems. But I do have a fun desktop transmission line project pencilled in for a midwoofer I bought for a dropped project. That should probably be added to the list particularly given some of the differences that seems to exist concerning some of the physics and modelling assumptions for transmission lines. I also forgot to include sound radiation from a subwoofer cabinet as an example which I largely completed a while ago albeit with open source software.