TH: Two Drivers vs One, Power, Xmax, and Compression?

[ghettosynth]

Hi All,

In a tapped horn xmax peaks above fb and in almost every variation of small tapped horns driven by 10s or 12s that I've simmed, the sim seems to show that I run out of xmax long before I run out of power handling capability.

So, would anyone actually drive a real Danley TH-Mini with 1200 watts? Depending on where I try to put fb, xmax seems to run out in most clones between 200 and 450 watts (using the B&C 12PS100). What I'm asking is how can I relate the sim to real world with respect to maximum displacement?

I realize that power compression can become an issue also so perhaps this isn't all that bad? I just hear that people drive a pair of TH-Minis with 2500 watts and everything I've simmed looks like it would blow the cones out of the box at that power level. I'd like to get a handle on some rules of thumb here.

Ok, so, in searching for the optimum power handling/spl/size etc. configuration I'm trying to understand how xmax and multiple drivers relate. Is it as simple as xmax is halved per driver with two drivers? Or, is there a more complicated relationship? When I sim two drivers in hornresp, (using the menu option), is it giving me the xmax per driver? It seems to be somewhat inconsistent. I've noticed sometimes when copying an existing sim that it will retain some driver parameters.

I've also noticed that, holding other drivers constant, increasing surface area decreases xmax. Granted, you can't do this so trivially, that is, any change in surface area of a real driver will change other parameters as well, but ok, larger drivers produce more bass for less xmax.

Last bit before I tie this all together, I've noticed that really shrinking S2 can often smooth out the peaks in sims but this increases the SD/S2 ratio. I've been able to get some nice looking curves with high compression ratios, say 4:1. From everything that I've read, however, that only small drivers and very stiff cones can handle ratios this high. I've also noticed that I often get better looking sims with S1 larger than S2, is this related to the "cone correction" that I've been reading about in other threads? What are reasonable compression ratios, especially if one is looking at "cheaper" drivers?

Alright, so the real question is, for small PA tapped horns, it seems that multiple smaller drivers might produce better results? Granted, as you go smaller, power handling goes down and multiple drivers probably drive the weight up more than a single larger driver. But, modulo extremes, individual weight is less important than physical size. If one is trying to control one dimension of the cabinet so as to make the cabinets a one man lift, what's the optimum? One 12, two 10s, three 8s, four 6s?

I welcome all discussion, but I'd really like to hear Patrick and Art weigh in on this, if you guys have the time/interest.

tnx,
gs

 

[m R g S r]

Are you saying 1200 watts with the proper high pass? That makes a big difference. And the driver in the TH mini has a very strong motor with a 4" voice coil.
Danley may even have the 12ps100 modified to his specification.

I'd also rather have 2500 watts available with the amp limited -3db as opposed to having only 1000rms available and running the amp into heavy clipping.

And yes Xmax is listed as 8mm, but xmech is much higher. (21mm winding length on the coil). So one could technically drive it past Xmax without an issue (other than increased distortion)

 

[ghettosynth]

Are you saying 1200 watts with the proper high pass? That makes a big difference. And the driver in the TH mini has a very strong motor with a 4" voice coil.

Yes, of course, I'm assuming that the high pass is set properly and I'm only looking at xmax above the excursion minimum.

Danley may even have the 12ps100 modified to his specification.

Sure, but if that's the case, I can't really factor that into my consideration.

I'd also rather have 2500 watts available with the amp limited -3db as opposed to having only 1000rms available and running the amp into heavy clipping.

Sure, certainly.

And yes Xmax is listed as 8mm, but xmech is much higher. (21mm winding length on the coil). So one could technically drive it past Xmax without an issue (other than increased distortion)

I see, so the question then becomes, how far past xmax is reasonable/tolerable?

 

[ghettosynth]

So, from here: http://www.diyaudio.com/forums/newreply.php?do=newreply&noquote=1&p=3402003

If I compute Vd = Sd * xmax, I get the "bass-ability" more-or-less of a driver. If I now compute Vd/cost, I get some sort of cross driver "bass-ability" per dollar unit comparison. Granted, there's variability and I get that this is just an approximation, but, it seems to help think about driver tradeoffs, especially when considering similar quality drivers.

So, 15PS100 = (855*8)/223.8 = 30.56
and 12PS100 = (531*8)/202.58 = 20.97

Which suggests that, for whatever tradeoffs I endure for going to the 15 over a 12, I get about 45% more "bass-ability" for my dollar.

Or, considering inexpensive Dayton 10s as compared to the B&C 12.

(346.5*5)/43.80 = 39.7

Meaning that, assuming that they're suitable, the cost per "bass-ability" unit of the Dayton is about half that of the B&C, of course, these are not comparable quality.

2*Dayton 10 will produce 3465 Vd as compared to 4248 of the 12PS100, or, about 80% of the Bass for 43% of the cost. Again, assuming that the reduction in quality of the driver, over whatever parameters, is acceptable.

Now, my modeling efforts suggest that the Dayton 10 isn't well suited to a TH. I can get a decent looking response, if I'm willing to really constrict S2.

Patrick talks a lot about the new Alpines, but, I haven't had good luck modeling them in a TH. Moreover, when I do, I find that the inefficiency shows. That is, I don't run out of xmax, but I do need to feed them a lot of power in order to get decent SPL.

Then again, I'm new at this, maybe I'm just not doing it right.

best,
gs

 

[sreten]

Hi,

Generally drivers have a thermal limit. Generally they excursion limit
in the bass and thermal limit somewhat further up. Generally only
low efficiency dedicated subwoofers approach their their thermal
limits handling low bass, often small subs with built in bass EQ.

For a sub you can ignore the silly power ratings of (car) drivers
and use an amplifier related to the drivers excursion limits and
efficiency, maximum SPL should be matched to the mains.

There is no point having SPL you will never use.

There is no real difference between two drivers and
one that is ~ 1.4 times larger (twice the area).

rgds, sreten.

 

[epa]

as far as i can tel by reading about it and experimenting,the excursion of a th is far less then predicted.
hr shows the excursion if you aply sine waves,but music is never only sine waves.
take for instance my latest bild ,the double 15"(testing/measuring wil be done next weekend)
acording to the sim it runs out of x-max @ 100 volts,however whe had a qsc 3k6 and a powersoft digimod 3000 amp module on it al the way up to clip level,and the excursion was around or just a litle over x-max.(not near the predicted 16mm)
this was with various types of music.

otoh you have to setup you rms limiter carefully to not fry your voice coil.
i think thermal is the limit,when the hpf is setup correct..

 

[Kindhornman]

ghettosynth,
I'll give you a few ideas that work in the real world with bass horns, not necessarily related to the tapped horns. Surface area of the cone is your friend. it never hurts to use a larger diaphragm but there is always a tradeoff. As the cone size is increased the the upper frequency response will generally roll off at a lower frequency. Two device such as two 15" drivers would be basically equivalent to one 18" speaker in output by the surface area equation but typically they will both have the identical motor in pro-audio speakers, just a larger frame and cone area and the two 15" speakers can therefore handle more power as they can dissipate twice the heat. Look at some JBL or other pro-audio speaker companies and you will see this is fairly common that they use the same motor. I build double 18" bass horns and there is an advantage over using two 15" devices in a few regards. Typically since you are trying to make maximum bass output the upper frequency cutoff is not the issue, you will use other cabinets above the bass horns that easily can cover the lower upper frequency limitations of the larger drivers. When starting with the larger cone area the start point with an equivalent mouth size will mean a shorter horn with the same flare rate and this can be used to reduce horn size, or length between the two designs. I usually use a 25% loading of the cone area and this does increase efficiency in the low frequency output but comes at a cost of a narrower band width, that is the real result that you will see in a frequency response, it always works that way, the bandwidth narrows and is not as flat in the pass-band but is easily corrected with a small amount of eq. People have different ways to create this loading and that has as definite effect on the upper pass-band. I use a molded section for the throat section that matches the cone area to the flare without any sharp edges or slot loading of the cone. What you will find is that many people use a very large rear chamber and this is not the best way to do things as this will allow excessive excursion and with a slot loading and if there is not a thick spacer ring the cone will often slam into the back of the horn. I could go on but you can start to get the idea of a real horn vs an idealized simulation. A smaller rear chamber will extend the lower frequency cutoff below the typical 1/4 wave calculation by up to 1.2 times the normal cutoff. This takes an extremely small rear chamber to achieve this result.

Steven

 

[weltersys]

I see, so the question then becomes, how far past xmax is reasonable/tolerable?

That depends on the driver.
Lightweight cones sound like crap driven to Xmax, heavy cones can be driven past Xmax and not sound too bad.
Above Xmax, the magnet has less "push", and most suspensions far more resistance, so additional power does not increase excursion in a linear fashion.
It takes 6 dB more power to double excursion up to Xmax, above Xmax even more, so doubling power may have little effect (other than increasing distortion, which sounds way louder) on excursion past Xmax.

Two to one is probably as high a compression ratio advisable for a high excursion (greater than 8mm) lightweight cone driven to Xmax in a TH.

As far as all the questions in your OP, most come down to displacement, whether you use multiple small drivers, or a single large driver to add up to the displacement needed for the SPL you desire.

Obviously, small drivers can use smaller enclosures, but long path length is needed for a low FB, so more wood (and weight) per enclosed volume is used in multiple small enclosures.

Art

 

[Djim]

Hi Art,

Adding more drivers will also increase the total moving mass of such system. In other words, the moving mass of a "heavy weight" cone can be compensated by multiple drivers of a lower moving mass if all other important parameters are more or less equal.

Hi Gettosynth,

In general you could say it is a matter of motor control over the movement of the cone. Drivers with a low Qes have more control over their excursion. Usually, this is where car and 'weak' drivers differ from Pro PA drivers.

The amount off usable movement beyond Xmax is matter of driver design/balance and how the figure of Xmax is calculated. Since there are different methods for calculating Xmax, comparing Xmax (or determining maximum usable excursion) between brands can become tricky.