What is better? Big diameter or long throw?

[wilsonj]

Hi,

I am looking at building a subwoofer, but am a little unsure of a few things. Hopefully someone here can help.

If I had 2 subs to choose from. One being larger in diameter, but smaller xmax, both with the same piston volume. Which would be ther better choice? why? All other things considered equal.

I am after low bass more so than brute SPL. I am considering using the Adire Tempest in the 214L vented enclosure. But from the few models I have messed with on LspCAD I can get smaller larger throw drivers to extend lower. Is it purely SPL where the larger driver will prevail?

Last question is about frequency response. Any tips to help avoid boominess at low freqs. I read that you should avoid a curve that is flat all the way to 20hz. Due to room gain.

Any input would be great.

Thanks

Regards
Jamie Wilson

[VEC7OR]

Its not the matter of big dia. or long exc. but the volume of air your driver can push around.
If you want something 20-30 hz at 0db your "generally" have to go with bigger driver(bigger Vas - enclosure will be bigger, thus it'll be easier to tune it lower, futhermore smaller eclosures tend to "lag"(group delay drastically increases as frequency decreases) if tuned low)

[SY]

VEC7OR's comments about air volume are right on the mark.

Other thoughts:

Consider how high you want to take the sub. Higher lowpass frequencies may call for smaller diameter drivers.

Long throws are nice, but nonlinearities are often a problem. Large dameters are nice, but beware cone breakup if you try extending the frequency range upward.

Multiple smaller diameter drivers can often be a good compromise.

Room gain is real and often (usually) unpredictable. EQ is not to be feared- it can be your best friend.

[AudioFreak]

A few points worth mentioning:

long throw....
higher distortion due to nonlinear excursion

large diameter....
cone breakup and generally less pistonic nature of the radiating area.

[wilsonj]

Thanks guys for your insight.

To clarify a couple things.

1. Looking for a good response from 80-20 anything below that is a bonus.

2. Need a box no bigger than 200L

3. Will be using HS500 amp. 700wrms into 4ohm.

What are nonlinearities? What do they SOUND like??

OK, and group delays are bad why?

Boy what a learning curve!

Any particular drivers I should consider given my requirement?

Thanks again.

[SimontY]

Hi Wilson,

I have a Tempest in a 250l vented box, it sounds very good, but a box this big you will need to brace so dam well for it to stay even nearly still.

I have a 150w/4ohm plate amp and it goes very loud. The HS500 is probably a little bit overkill, but certainly a fine choice, I bet it will sound great with all that headroom, and probably damping factor - to keep things tight.

One point though, where do your speakers roll off at, at -3db? My kit speakers supposedly do so at around 45-50hz, and 50hz is my minumum cut-off for the sub. This creates slightly too much overlap and makes things just a tad thick in the region. It is worth considering your x-over requirements, before buying an expensive plate amp.

I think group delay is the phase difference/time lag between parts of the audio band reproduced. Presumably the lower in frequency the problem area is the better - ie. one reason a nice low tuning frequency is better than a higher one (where ears are more sensitive). Tuning with a port can cause that excessively detached bloated bass around the tuning frequency and I think group delay has something to do with it.

One more point - will two Shivas (ie. 12" version of Tempest) not be far better than one Tempest? Use them in a push-push arrangement, so firing apart from each other. Iirc, 2 Shivas only need roughly the same space as 1 Tempest, and I bet they sound tighter, because they will have more magnetic force per cone mass. Furthermore, operating in a push-push stylee will cancel some vibrations - useful in a huuge box like we have.


-Simon

[EC8010]

Group delay is the time by which frequencies within a group are delayed. If you had a group delay of 1s for all audio frequencies, it wouldn't matter (except for when cueing the stylus). There's only a problem when you have non-constant group delay - some frequencies are delayed by more than others. Filters are not only characterised by their amplitude vs frequency response, but also by their delay vs frequency response. The low-pass filter needed for a sub-woofer always implies substantial delay, and unless you also delay the audio to the main loudspeakers, the sub-woofer will always lag behind. Additionally, the LF alignment of the sub-woofer and its enclosure forms a high-pass filter and as as the volume of the box decreases relative to VAS, Q rises and delay between different frequencies increases. There's nothing like a big box.

A larger diameter driver can move the same volume of air as a small diameter driver, but with less excursion. Less required excursion means the coil doesn't need to be so overhung, and that increases efficiency. A larger cone will break up at a lower frequency because break-up occurs when the circumference of the cone is equal to one wavelength. The velocity of sound varies with cone material, but break-up frequency is broadly inversely proportional to cone diameter. You have to try quite hard to get a cone to break up at <200Hz.

[amo]

Originally posted by EC8010


A larger diameter driver can move the same volume of air as a small diameter driver, but with less excursion. Less required excursion means the coil doesn't need to be so overhung, and that increases efficiency. A larger cone will break up at a lower frequency because break-up occurs when the circumference of the cone is equal to one wavelength. The velocity of sound varies with cone material, but break-up frequency is broadly inversely proportional to cone diameter. You have to try quite hard to get a cone to break up at <200Hz.

Great Info!!! What does this say about the TAD 12 inch midrange? Is is sensible to use such a large driver to go so high, and use a shallow xo slope on top of that?

[owdi]

If box size isn't an issue, I would pick the larger diameter speaker. As has already been mentioned, the larger driver will probably be more linear. What hasn't been mentioned is that the larger driver will also be more efficient than the smaller counterpart. You will be able to use a smaller amp to push it to xmax. These are however generalizations. There always seem to be exceptions...

What are nonlinearities? What do they SOUND like??

This is my uneducated understandig on nonlinearities and compression, so everyone feel free to correct me.

If you play a test tone at 1 watt, your speaker will play that tone at some volume, say 80db. Double the power to 2 watts, and the speaker should play at 83db. Double again to 4 watts, and it will play at 86db.

Now imagine you are playing the tone at 200 watts and getting 100db. If you double the power to 400 watts, the speaker should play at 103db. But because you are nearing the capabilities of the driver, it may play the tone at 102.8db. At higher power levels 'compression' becomes a increasingly important factor. Doubling in power does not result in 3db gain. The way you will hear this is by noticing a reduction in the dynamics of the music.

Hope that helped...

Dan

[Richard C]

I vote for diameter every time plus very big magnet assembly. If you look at driver linearity it is only reasonably so over an excursion of a few milimeters. Undersized subwoofers are not nice and should only be used if there is no other option.

Remember cone area increases by r squared so even relatively small increases in cone diameter result in substantial increases in area.