[gitarretyp]

I'm currently using IB15s in an OB and they produce no untoward self-noise under high excursion conditions. Also, I agree fully with the Johns regarding excursion requirements for OB woofers both mathematically and experimentally.

[David Weil]

what about mechanical noise? I have dayton HF subs and they have quite a bit of noise when cone starts to visibly move.

Very improbable in OB. Quite probable however in vented systems. There you can get noise from the air moving in and out of e. g. the BR tube, especially, if the tube is very narrow, angled or if the edges are not rounded off.

[AK]

David, it's easy to check. just play loud pink noise with woofer(free air), low passed at low frequency ~100hz and down. and you will hear noises that come from its suspension, unless of course you have very very good woofer

or even better, try single 20 hz tone, you'll hear suspension noise only.

[Magnetar]

David, it's easy to check. just play loud pink noise with woofer(free air), low passed at low frequency ~100hz and down. and you will hear noises that come from its suspension, unless of course you have very very good woofer

or even better, try single 20 hz tone, you'll hear suspension noise only.

In my experience, open baffle bass drivers need high passed third order or higher above FS. If you don't you'll really be compromising the sound.

[gitarretyp]

I also recommend testing for mechanical noise with test tones, but i usually use tones below 20Hz so the high volume levels aren't as bothersome.

In my experience, open baffle bass drivers need high passed third order or higher above FS. If you don't you'll really be compromising the sound.

The IB15s work quite well with a second order high pass at FS (16Hz). What would compromise the sound besides excursion on the low end?

[Magnetar]

I also recommend testing for mechanical noise with test tones, but i usually use tones below 20Hz so the high volume levels aren't as bothersome.

In my experience, open baffle bass drivers need high passed third order or higher above FS. If you don't you'll really be compromising the sound.

The IB15s work quite well with a second order high pass at FS (16Hz). What would compromise the sound besides excursion on the low end?

I suppose a 2nd order would work with the right woofer. Two things - with dynamic drivers the more movement or excursion the higher the distortion - it muddles up the upper bass/midrange. 2) trying to produce the bottom octave in an average size room with an open baffle is almost impossible (pressure build up, dipole cancellation, low efficiency, ect...) and is better left to a subwoofer that has some type of loading. A bass horn works good for me - low excursion, high efficiency, low distortion- open baffle bass/midbass can be so clean I feel it's best to use it where it's practical and possible with low compromise. This normally means an octave (maybe a little less) above FS and up rather than forcing it to do something it really isn't best at

[David Weil]

David, it's easy to check. just play loud pink noise with woofer(free air), low passed at low frequency ~100hz and down. and you will hear noises that come from its suspension, unless of course you have very very good woofer

or even better, try single 20 hz tone, you'll hear suspension noise only.

Or you might be hearing
a) a 20 Hz tone sounding just like that.
b) noises that are due to driving the woofer over its mechanical limits. 20 Hz at listening level on OB is asking for quite some excursion. It takes 70 dB to even cross the hearing threshold at 20 Hz, and 95..100 dB to perceive something like conversational noise level of 50..60 dB(A). Even with very big drivers that is hard to achieve. A 12" woofer probably needs something like 60 mm of excursion for that. Even two 15"-ers will still need something above 20 mm, make that 10 mm for two speakers with 2 x 15" each. A little less, if room and positioning help them.

Maybe it's not your woofers being bad, but you being bad to them. 

[Graham Maynard]

Hi AK,

I so agree.  The larger drivers shift more air *quietly* and generally with much less harmonic distortion.

Running an unmounted driver to X.max at 20Hz quickly shows whether it distorts (which includes mechanical noise) or not.
If you hear more than you feel it is often generating parasitic harmonics.

Cheers ......... Graham.

PS  Didn't notice some of above before posting.

[John_E_Janowitz]

There are several things to consider with mechanical noise.  The most common is noise from the air moving in and out of the pole vent.  How much air moves in and out is determined by the diameter of the VC.  The coil former is the cylinder and the pole essentially becomes a piston pumping air in and out through the pole vent with the motion of the driver.  The larger the diameter of the coil, the more air is pumped in and out of the pole vent.  The IB15's and OB15's use a small 2" diameter coil.  So with a small coil and the pole vent adequately sized and flared properly at both ends, you can play a sine wave, filtered pink noise, etc, free air at full excursion with almost no audible sound.

Two things - with dynamic drivers the more movement or excursion the higher the distortion - it muddles up the upper bass/midrange

This is where the Lambda motor excels compared to any driver available.  The coil acts as an inductor and the pole is the core.  In a typical driver the core varies over excursion from pure iron core on the inward stroke to nearly air core on the outward stroke.  This greatly changes the inductance and upper frequency response of the driver on the inward and outward stoke.  It leads to high distortion and the upper frequency response is modulated with low frequency excursion.  A second effect is that the flux field is also modulated by the eddy currents created as the VC moves through the gap.  This is another great issue in regards to distortion.

The full copper sleeve on the pole of the Lambda drivers, and the IB15's does two things.  First, it greatly lowers inductance and makes the coil operate nearly as an air core throughout it's entire travel.  The core is now the same at all excursions and the inward and outward stroke are the same.  Secondly it shorts the eddy currents and eliminates the flux field modulation.  The Dipole15's go a step farther with an underhung coil for nearly perfectly linear BL throughout the rated travel.  They also don't have a pole vent so there is no vent noise to worry about.

John

[John_E_Janowitz]

I suppose a 2nd order would work with the right woofer.

The main thing is having a driver with high enough Q to limit it's own excursion.  The higher the Q, the more it's excursion will be limited below Fs and the lower order filters you can get away with.

trying to produce the bottom octave in an average size room with an open baffle is almost impossible (pressure build up, dipole cancellation, low efficiency, ect...) and is better left to a subwoofer that has some type of loading. A bass horn works good for me - low excursion, high efficiency, low distortion- open baffle bass/midbass can be so clean I feel it's best to use it where it's practical and possible with low compromise. This normally means an octave (maybe a little less) above FS and up rather than forcing it to do something it really isn't best at

I definitely agree with this.  Infinite baffle subwoofers really give you everything an open baffle will, but without the dipole rolloff.  There is still no coloration from the enclosure and no reflected sound from the rear wave.  Again, how low you can go is determined by the design of the driver.  The IB15's were intended originally for infinite baffle use their name states.  In an IB you get an F3 of 16Hz, which is the Fs of the driver as well.  As you get down to this point though you get some VLF gain that starts to bring up the response.  We've had many people with 4-8 IB15's that have nearly flat response to 10hz.  You want to have a high Q driver both to make efficient use of Xmax down to Fs and to limit cone excursion below there.

John

[jkelly]

Martin,

Just wanted to give you an update on my project.  My friend John finished
the cabinets for me.  I used a single baffle board for both drivers.  We put
groves in the H and bottom boards for the baffle to slide in. 

I have placed the drivers in for a test drive this weekend before I apply the
final finish.

News at 11:00!

Jeff

[jkelly]

OK this is only after a few hours but I have to say the bass from the single 18" Goldwoods
is amazing.  This is compared to running 2 Eminence Alpha 15A's per side in another system.
(total of 4).  I would recommend these in H frames as subs.  This is the bass I always wanted
but never got, until now.

The system is sounding a little warm so I hope things will open up after break-in.

I like the 20 - 100 LR12 and LR24 352 - 20k crossover settings for now.

I would like to thank Rudolf for sending these files for the DCX2496.

Edit: actually I like the 20 - 100 LR12 and LR12 250 - 20k better now!

[MJK]

The bass from the 18" Goldwood woofer is really amazing. I think that the Alpha 15A's and one of the other Goldwood 15" woofers would also work really well in a slightly smaller H frame if size is an issue.

The Jordans will open up with more time. They produce a very laid back sound, as apposed to a forward sounding driver like my Lowthers, and do all of the different frequencies but not with a lot of flash and sparkle. I have played with a few adjustments in my set-up to re balance the sound. I have been adjusting the amount of boost to the Goldwood down between 0.5 and 1 dB to bring the midrange up a little and I have also added a dB or two of EQ at the very top end to try and bring out some more sparkle.

I am also going to start swapping out full range drivers to try other combinations. First up is the Fostex F120A which I received this week and then the Fostex FE-126E which I have had for a while but never got around to using in a speaker design. Doing some comparisons should be an interesting exercise. When I decide which full range driver I like best I am going to add a passive crossover to the mix.

[jkelly]

Martin,

Please keep us posted on your experiences with the other drivers.
I would probably want a little more sizzle as well - but first I want
to break the drivers in - and enjoy this strong OB system.

Thanks again and it really is a nice design.

Jeff

[mcgsxr]

I have lusted after 18's for bass for some time, glad to see some folks playing around with the drivers I centered on years ago, but never bought.

What are the H baffle specs - depth is?

[ecir38]

See the link at the bottom

http://www.quarter-wave.com/Project08/Project08.html

MJK, I think your SPL thiele/small measurements on the Alpha and Goldwood are off in the pdf above.

BR

[MJK]

MJK, I think your SPL thiele/small measurements on the Alpha and Goldwood are off in the pdf above.

Not sure which measurements you are referring to, can you tell me what you think is off?

[MJK]

What are the H baffle specs - depth is?

The external dimension of the H frame are 21 inches wide by 21 inches tall by 16 inches deep.

[ecir38]

Not sure which measurements you are referring to, can you tell me what you think is off?

In your pdf here page 3
http://www.quarter-wave.com/Project08/Jordan.pdf


I thought the spl db/W/m for the alpha 15 was 97 and think the goldwood is 94.2

BR

[MJK]

Where are you getting those values?