Speakers

[Simon Hickie]

It's good to be back after a hiatus!

Martin J King has done a lot of work on open baffle systems. For the bass driver, it seems highish qts drivers work best. U frames push the qts higher, and Fs lower. H frames do the same but a bit more. Both U & H frame result in a loss of efficiency. Sensitivity guidance for the drivers a bass driver with 6-10 db more efficiency than a fullrange or mid/tweeter units. U frames ideally need to be rolled off electrically from 175hz, H frames from 150hz. Acoustically, those roll off points are higher due to both dipole roll off baffle and U/H frames boost in the upper bass/ lower mid region. Relying on driver acoustic fall-off alone is unlikely to work. This means one of three possible routes: second order passive (large value inductor & caps needed); analogue digital (lots of extra electrical gubbins to degrade SQ); implement a digital crossover with something like a miniDSP. I will be going with option 3, although I have the components for option 1.

For the full ranger, electrical high pass roll-off from around 500hz then should be sufficient. An Lpad may or may not be needed, depending on relative driver efficiencies.

The big 'must do' is to put at least a meter between the baffle and wall. Needless to say, this is very different from the semi-omni cube type approach. However, OBs are supposed not to excite room modes in the same way as box speakers, and those in adjacent rooms are far less likely to be disturbed by normal listening levels - in theory anyway.

Anyway, all of this stuff is a for dedicated post!

[Ali Tait]

Mucked about with OB’s for a good few years now, had good results in the past with a miniDSP 4x10hd.

[Noodledoodle]

Hello all, I tried to start a new thread but this one looks like it might fit! I have a pair of Goodmans magnum k, and a whole bunch of 70s mids and tweeters and have been doing fiddling. They now have new crossovers and a Seas midrange and a Philips tweeter. The thing is the bass is a little shy despite the 12" drivers. I spotted a man selling a pair of Goodmans Audiom 12" which have an identical cone and basket, but the magnet loolks to be double the size. Does anyone have any experience? I can find EVERYTHING about every other Audiom that existed, but not these. I THINK they are the high power versions but I'd love to hear if anyone has experience with these before making another mistake. Many thanks. Richard

(Ps, I tried to upload a picture but was told the file was too big which surprised me)

[slinger]

Hello Mr Doodle I don't know these speakers so can't help you much. So you have changed the mid and tweeter AND made new xovers which are electrically different to the original ?

You could make the xover frequency between the bass and mid drivers at the centre of the baffle step transition. You can work that out using the width of the front baffle and doing a simple calculation. That allows you to try a padding resistor on the mid and tweeter which will pull their level down so the bass stands out more.

Or you could just try positioning them right back to the wall or failing that in the corners.

More efficient bass drivers would also work but the xover still, needs to be at the centre of the baffle step transition and it would be a shame to ditch the originals.

Hope that helps you a bit, sorry if it doesn't.

[Noodledoodle]

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Hi karatestu,

It probably sounds more dramatic than it was - I bought the crossovers, tweeters and midrange units that used to sit inside a pair of B&O s45 (or something similar) so they are meant to be together. The bass driver is different from the original obviously but I figure it's ok for tinkering.

I have tried every geographical placement possible in my apartment and I have tried a bunch of good crossovers, and while the upper bass is full fairly full, the low bass is lacking. So when I noticed that the identical drivers are available with double the magnets, i'm like ' well surely that has to have some effect'.

I have attached pictures this time - though in rather an odd manner. but I think they are attached. So the first two pics are my drivers (one is a stock photo as they drivers are in the cabs). The second two are the Audioms - identical in every way apart from the obviously enormous magnet on the back.

You can see my point, but will they make much difference?

[karatestu]

I am not sure a bigger magnet will give you more bass. It is normally more sensitivity that a bigger magnet brings. I could be wrong. That would still, make the upper bass more prominent.

Are the enclosures completely air tight ? How much stuffing is in them ?

What is the crossover frequency between bass & mid ?

[Noodledoodle]

Yes I wondered myself whether a bigger magnet might have a different effect than what I had hoped, I'm intrigued to try it. The enclosures are very solid, probably 18mm ply and air tight. The factory put a piece of foam inside which fills the entire enclosure for stuffing. Not sure about the crossover frequency - i'll check.

[Noodledoodle]

I have some crossover information. I used a youtube rising frequency video to check where the frequencies crossed over using my ears. The bass crossed over to the mid between 1500 and 2000hz which seems high because the mid driver is rated from 500hz upwards. So I'd like to try lowering the crossover frequency as you suggested, but am not sure how. I'm just coming back to electronics after a break so I don't want to waste your time, but if you have a link I would be very grateful

[Noodledoodle]

Morning.

I don't know how effective that video you used is. How did you ascertain it was 1.5 kHz ? Is that where the volume started to drop or where it cut out ?

Really you need to get the xover out and take a pic. Also try and trace the circuit. I have no idea what you are dealing with here - first, second, third order low pass. There will be at least a series inductor.

If it's a first order low pass then you just change the value of the inductor. If it's a higher order then you're on your own I'm afraid as I don't mess with higher order filters. There are lots of things to consider. The mid's high pass would also have to be changed so the two sum in an audibly acceptable manner.

If you cross at the middle of the baffle step transition (-3dB point) then that will correspond to the width of the baffle. How wide is the baffle ? You will probably have to cross higher than that otherwise the mid will be in distortion because it is trying to produce too much volume below what it is good for ( 500 Hz according to what you say).

Saying all this if it is only the low bass which is lacking then maybe we are barking up the wrong tree. I'm no expert on these things and I would hate to send you on a wild goose chase.

[Geoff.R.G]

Rather than using videos and ears to determine the frequencies and levels may I suggest a couple of phone/tablet apps.?

First, f Generator which is a function generator and can produce audio signals up to 20KHz
Second, RTA Audio which is an audio spectrum analyser

I know this isn’t subjective but occasionally one needs objective measures and crossover design needs to be somewhat more accurate than the use of a video suggests.