[JeffB]

I was looking at the following graph on the Single Driver Website.
It depicts the relationship between frequency and baffle width

http://melhuish.org/audio/baffle.html

I am curious if this holds for all drivers.
For example, I was looking at the Hawthorne Audio Duet
http://www.hawthorneaudio.com/speakers.htm
1 15" coaxial and 1 15" bass driver on a 24" baffle.
The website gives the impression of a speaker with excellent bass.

On the other hand the graph from the single driver website would indicate bass only down to 120Hz.

The two items are in apparent contradiction.

Can anybody provide some insight into driver size versus baffle size and the expected bass frequencies?

[OTL]

Here's a couple of good ideas...

http://members.myactv.net/~je2a3/open.htm

http://www.troelsgravesen.dk/briggs.zip

[Grumpy_Git]

Also, when talking about baffle width, what about the height? Shouldnt the surface area be considered? or maybe just the shortest dimension of width/height?

Nick.

[JoshK]

Jeff,

I think you are asking the right questions.  I am not sure I can explain it all well and succintly.  Do some digging on baffle step diffraction and try to understand what that is about and how it is compensated.  The laws of physics don't go out the window for niche single driver designs. 

Basically the size, in all dimensions, of the baffle determine the baffle step frequency and the shape and dimensions of the baffle also affect diffraction, a concentric circle being the most extreme diffraction at any particular frequency.

Troels's site give some nice empirical data to support intuition about how shapes and wings affect diffraction and frequency support. 

One thing to consider is that with lots of single driver open baffle designs, they make some compromises to adhere to their dogma that any passive crossover network (including baffle step compensation) is more dissastrous than the roll off due to baffle-step.   Some designs rely on room gain boost, which is good to take into consideration mind you, to improve bass response.

I think it is important to understand the facts and the trade-offs and them determine for yourself what your priorities are and what tradeoffs can you live with.  That is what speaker design, hi-fi and the entire hobby is about.  I just think (maybe this is my own dogma) that we should be honest with ourselves about what the tradeoffs and facts are. 

[hurdy_gurdyman]

The drivers Qts will have a big impact on what kind of bass you will get with a given baffle size. The chart on the Melhuish site is basically for a driver with a 0.7 Qts. Higher Q drivers will produce a bass rise, or bump, which can be put to use by having the baffle made smaller to flatten it out. This gives the same bass response as the 0.7 driver (of the same driver size) but does it on a smaller baffle. When considering all this, one must also keep in mind displacement. A larger driver can move more air than a smaller one, thus can produce more bass on the same baffle (not deeper, just more of it). Combined with a high Q, a large driver can, indeed, produce gobs of bass on a moddest sized baffle. The Silver Iris OB 15" Coaxial will produce a flat50 Hz on a 24 inch wide baffle, and the Augie will produce a flat 31 Hz on same baffle when used biamped (you need higher gain for the Augie than the coaxial).

Dave

[2litre]

New guy, first post, Hi,

Dave, don't wings and driver height off the floor have something to do with it too?
I mean they won't change the depth of the bass but they could affect how it is presented, right?

R/

Jim

[hurdy_gurdyman]

New guy, first post, Hi,

Dave, don't wings and driver height off the floor have something to do with it too?
I mean they won't change the depth of the bass but they could affect how it is presented, right?

R/

Jim

Jim,

Yes, baffle size affects where the bass cut-off frequency is. However, it will be different with drivers of different Q above 0.7.  After all, one box size doesn't act the same with different drivers in a sealed box design, either.

Wings are simply baffle extensions with a whole set of their own problems. They change the sound from pure dipole to a more omni sound in front. The rear wave is now different from the front wave. There is more pressure in the chamber behind the driver created by  the folded back wings. This can cause vibration in the wings (I use 3/4 inch thick MDF on my SI/Augies. The wings vibrate enough I sometimes think they'll take off and fly). All of this can be worked around, of course, but needs more thinking-through than a simple flat baffle. My next baffle will be flat.

Driver height also has an effect. The closer to the floor you get, the more help in the bass from the floor. However, imaging can suffer, so one has to experiment and see how things go in your own situation. A driver with weak bass can benefit a lot from being mounted close to the floor, while a driver with lots of bass to begin with is usually better up around ear height. My SI is a bit boomy when mounted close to the floor. If you mount a driver close to the floor, you can tilt the baffle back so the driver aims toward the listening position.

Open baffles, in general, have only started getting popular the last few years. Not a lot of serious research has been done by manufacturers yet. We are all still learning what does and doesn't work. Things that were considered correct a few years ago are being re-thought out. I look for more info, and more accurate info, to be coming out in the next couple of years as more manufacturers get on board and start doing some serious research.

Dave

[2litre]

Dave,

What you've mentioned I've experienced, but it's always good to ask for an informed opinion.

About a year ago I built a 99% dimensionally correct version of the NoBox (20" x 48" baffles with tapered wings 5" top to 13" at the base), but used old drivers pulled from consoles. On hand at the time were 2.5" cone tweeters, Russell wide band 8's and CTS 12's.

Speaking about bass only, I noticed a very big difference in presentation when moving up from a 12" as the augmenter to their current 15" augmenters. I'm sure both the 12" and the 15" are similar (both are made by CTS and pulled from Magnavox consoles) in that they're both high Qts drivers but, the 15" makes bass effortlessly where as the 12, in hindsight, was trying much harder.

I was wondering about height off the floor because I'm considering narrower baffles with shallower wings. I'm also looking at 2 x 12 instead of 1 x 15 for augie's for the next iteration of these. My thought is that the lower 12 will give the heavier tone because of the floor boundary and the one higher up will sound 'lighter' and will integrate better with the main driver. Sound right? Or at least plausible?

JohninCR has already advised me about bracing with the wings and I will address that.

As for Fs I think my intended 12's are very close to my 15's so I expect they will have virtually the same extension but will move more air. I'm currently using a 75w, -18db plate amp for the augie's (MCM Mission).

Currently they're extremely pleasing to listen to and fill a modest room like nobodies business. The notion of OB's don't produce bass without EQ isn't correct with these. Maybe the disclaimer for OB's should read "OB's don't produce sub-bass without EQ".

R/

Jim

[hurdy_gurdyman]

Jim,

Yea, bass below 30Hz is hard to do on OB.

I haven't tried two identical bass drivers stacked, so not sure just how it will sound. Sounds like you are going to have fun finding out, though. Good luck.

Dave

[opnly bafld]

Jim,
With your two 12" drivers, will you be able to control output/x-o point separately or will they be driven by the same amp/x-o?
From my experience what you want to do with the 2 drivers should work, but you might need to be able to adjust them independently to get the best sound.

Lin

[2litre]

Now there is something I hadn't thought of!!

Last weekend I had an amp for each 15, so they were playing in stereo but presenting 8 ohm loads to their respective amps. xo point at full, about 150Hz

This week I wired both 15's to only one amp. So now the amp sees a 4ohm load but sums to mono. I set the xo to the point where I can't discern what is being said by a male voice but I know one is present, approximately 100Hz or half way on the knob. Much more dynamic? I don't know, but wired in mono the bass appears more 'full'.

Now with (4) 12's I could actually wire them in dual mono, presenting 4 ohm loads to both amps and staggering the xo points on each L/R set of 12's.

Will be a neat test.
I hope I don't like it!
I really want to keep it as simple as 2 x 12, 4 ohms, xo set at around 100Hz, stereo.

But, at what point would mono signals become painfully obvious?
Some people say 120Hz and down, some say 100Hz, and some say even less.

R/

Jim