Generic: How BIG for mid duty driver

[gedlee]

While I still cannot get my website open to updating, I can upload the pdf whitepaper.

Enjoy - the website will contain example files for Holm and for input to my analysis.

PS. Not completely edited yet - still several typos.

 

[Eldam]

Thanks Dr Gedlee for this "how to".... much appreciate.

 

[Overkill Audio]

Very Interesting white paper.

Thanks Earl,

Your white papers are always highly informative, I look forward to reading it detail next week.
I am packing for a weekend away, off to this Festival Number 6 - Festival Number 6
in Wales tomorrow ...Lots of good current good acts plus The Pet Shop boys are on tomorrow night...80's revival!

Also thanks for your comments on my comments.

Number 5 is an interesting option for bass and low midrange.
There is a fair bit of research around and most folks use it for subwoofers but it can be excellent from about 200Hz and below depending on the driver.
Bag End Subwoofers claim they have patented it but there is loads of prior art and Rod Elliot covers it in detail on his ESP website.

Its only for sealed boxes, no ports, ABR's or TL'...

Its main benefit is greatly reduced box size.
Second benefit is very even impedance.
Lastly it avoids the high driver excursions at resonance by only using driver below the Fb.

This entails careful driver selection as you need to use a lot of power and Eq to compensate for the loss of sensitivity, but the benefit is great low end from a small air volume, so you need big Sd and lots of Xmax... expensive drivers!

I'll dig out an example and post in a few minutes.
Cheers
Derek.

 

[Overkill Audio]

DBR (Drive Below Resonance)

Here is an example of the " DBR " (Drive Below Resonance)as I call it!

Sub-Woofer Controller
Has lots of construction details on this and also
Linkwitz Transform Subwoofer Equaliser Linkwitz has even more on his site.

Take a high quality Pro driver like the Precision Devices 15 inch PD 1550 (effective diameter = 337mm), normally would be used in very large ( 100 litres ish )ported enclosure....In such a box you would struggle to get it flat to much below 45Hz and face the mountainous impedance peak at Fb....

Use DBR to get fantastic smooth end even bass (no impedance peak!) in 23Litres sealed... With a lovely 0.707 Q!

Use a DSP crossover at or below 80Hz ( THX certification point) and use Eq and a powerful amp to apply the boost. Theory says 12dB per octave but with room gain its never anything like that ( See Rod's detailed example)
PD Driver TS:
VAS
Fs
Qts

Box Sims.
Qtc / F3 / Fb = 0.6 / 107Hz /88 Hz Bv = 33 Litres
Qtc / F3 / Fb = 0.65 / 105Hz /96Hz Bv = 28 Litres
Qtc / F3 / Fb = 0.707 / 104Hz /104 Hz Bv = 23 Litres
Its only for sealed boxes with expensive drivers as budget drivers go into VC melt down ( small sealed box over driven with Eq...) but it sounds fab and has huge WAF....Always a factor!

DBR has other applications to, but that's another story.
Hope this is of interest.
Cheers
Derek.

 

[gedlee]

I think that looking at bass response as a free field problem is just misleading. What you call DBR, I have been using since back in the 80's when we did that in cars. I still contend that it makes no sense to talk about what a LF source does outside of its room. In the room we just EQ it to be flat, whatever that requires. What we started with makes no difference at all as long as the source(s) can handle the EQ. My room goes down to 20 Hz, but the subs Fb is more like 50 Hz. Of what use is knowing the sub is 50 Hz, or 80 Hz or 30 Hz if it all comes out the same in the end.

We need to get away from talking about sources in the modal region as if their free field performance had anything to do with the problem.

 

[Overkill Audio]

Like I said...

I think that looking at bass response as a free field problem is just misleading.

" Theory says 12dB per octave but with room gain its never anything like that ( See Rod's detailed example) "

What you call DBR, I have been using since back in the 80's when we did that in cars.

" ...but there is loads of prior art and Rod Elliot covers it in detail on his ESP website."


I still contend that it makes no sense to talk about what a LF source does outside of its room. In the room we just EQ it to be flat, whatever that requires.
" Theory says 12dB per octave but with room gain its never anything like that ( See Rod's detailed example) ..
What we started with makes no difference at all as long as the source(s) can handle the EQ.
" if the driver can handle the power and displacement required from the low end boost with Eq "

My room goes down to 20 Hz, but the subs Fb is more like 50 Hz.
Mine to, but in a 25 litre box...!

Of what use is knowing the sub is 50 Hz, or 80 Hz or 30 Hz if it all comes out the same in the end.
It doesn't all come out the same in the end....DBR allows you to use small boxes!

Both distributed subs and bass systems attached to floor standing or stand mounts....Small boxes with smooth deep bass...
We need to get away from talking about sources in the modal region as if their free field performance had anything to do with the problem.

As I point out, twice, DBR is a real world in room solution....DBR is an very good, not perfect but very practical.

Cheers
Derek.

 

[gedlee]

Then I have no idea what you are talking about and I suspect that if I can't figure it out that no one else is getting it either.

 

[Overkill Audio]

Sorry Earl,

Maybe I am not explaining it very well...Please check out Rod Elliots ESP sound pages website I linked to...He does a far better job of explaining it.

Its not mega world audio news buts its useful...
Once I have all the BMR stuff sorted out ( getting you some good measurements is more important!) I will give specific examples that may be of use...No harm done either way.
I am signing off now, early start and live music ( some acoustic as well as PA's) weekend ahead...Yea!

All the best
Derek.

 

[gedlee]

Sorry Earl,

Maybe I am not explaining it very well...

All the best
Derek.

I think that we can agree on that.

 

[Pallas]

***
4) Agreed, I would never use a rubber surround driver

I'm curious, why?

And thanks for the measurement guide. I've been dithering on making my own measurement turntable for a while. Too long. Nice to see your design.

 

[RockLeeEV]

I'm curious, why?

I can't read his mind, but I definitely agree with his position too. Just a look at the frequency response and impedance sweep of a lot of these long throw surrounds reveals a significant low Q resonance right there in the midrange. People always talk about cone breakup, but I consider surround resonances to be just as significant in removing "resolution" and they're always IN the passband of the driver (aside from subwoofers).

 

[gedlee]

Rubber is to low damping. From what I have found only multiple role fabric surrounds with high damping impregnation will have a HF response that is smooth. This makes perfect sense because this type of surround will absorb outgoing waves in the cone body and not reflect them. A wave has to reflect to cause a cone resonance - no reflection, no resonance - no resonance, smooth response.

 

[Eldam]

I was believing coraguted suspension was a problem for sealed cabined only (read in the V. Dickason Book).

So a too high Qms in a pro driver for our mid range (below the 700 to 1000 hz discussed in the thread) is bad. At the same time nordic Europe driver if my understanding is correct use the opposite with their low efficienty drivers : the higher Qms the better for some SB Acoustic and others...

I'm going to look at the spec of the B&C !

TAD :

The suspension components in the TAD Reference One bass driver are a combination of optimum shapes and materials calculated by using the most advanced computer simulation techniques. In particular, the bass driver is equipped with high performance dual spiders that ensure consistent, stable operation with low distortion even during continuous high amplitude input. For the surround, which can also significantly impact sound quality, we chose the corrugated type as used in our professional TAD Drivers. Just as with the spider, this type of surround supports the cone during large excursions allowing accurate and stable motion.

I don't know what they call double spider ? My own two x 6.5" bass driver in my bandwidth cabinet are double inversed spiders (Lynnfield 400/500 L) ?

 

[gedlee]

My understanding of a "double spider" is two spiders glued together with a high damping glue. This would give a Constrained Layer Damping effect as well as a much more robust spider. B&C does this as well.

 

[Hifisound]

You do need to insure that the X-max is not exceeded in practice, but if this is ensured then yes, you are correct, HD is unimportant.

I pay very close attention in a woofer to how well it handles its breakup. This is critical to me, but it will be for anyone with a fairly high crossover points. I pay no attention at all to TS parameters, I just use closed boxes and multiple subs.

I am just not in the camp that drivers are all important - I used to be. It is the architecture and the crossover that makes all the difference, not the drivers. (OK now, bring it on because that position will aggravate a lot of people. Just be forewarned that I have a lot of data to support me.)

One basic question about the Xmax part above :
I guess this puts a minimum size limit for the driver if one wants to achieve reference level SPLs down till atleast 80Hz, right ?

 

[Hifisound]

I think I asked the obvious
But will be good to know what will be that minimum driver size for reference level SPL at 1m

 

[gedlee]

I am not sure that there is a pat answer to this. It depends on what the SPL level is, how the driver is made (Xmax, etc.) and will multiple subs be used. I tend to err on the size of larger - that's always safe.

 

[Hifisound]

Say THX levels (85db continuous, 105db peak) at 1m with subs (one or two) below 80Hz and sealed enclosure for the speaker.

If that can provide required xmax, will size be not imp as long as that driver has that xmax ?

 

[krabapple]

I do trust your measurements and agree completelly but i think that there are some things that aren't measurable yet... or they are measurable but they are neglected for some reason. We are witnesses that THD and IMD (as you said earlier in this thread) are not being considered as an important factor any more. If there is the case where something was considered very important and now is neglected i think that the other way arround is possible too. The problem of me not being an expert in acoustics is because i can't explain what it is i hear through measurements because i wouldn't know where to begin. If it was just me, i would rule it off as a brain manipulation as you stated earlier but number of people heard the difference in AB test. Actually all 4 of them.

My knowledge in acoustics is substantial for an average music lover (hifi enthusiast, audiophile) but does not reach that far into the exotic measurements that may reveal and explain what it is we all hear - if we rule out mass hypnosis

You are (training to be?) a biologist but you don't seem to accept why controls exist.

Would you run an experiment without them? If you did, would it ever get published? Would it matter if you claimed that you had 'trained yourself' to be 'objective'?

 

[gedlee]

Say THX levels (85db continuous, 105db peak) at 1m with subs (one or two) below 80Hz and sealed enclosure for the speaker.

If that can provide required xmax, will size be not imp as long as that driver has that xmax ?

If you are asking what is the minimum size that I would use in that case it would probably be 10", as long as it is a reputable design.