Open Baffle Speaker With Active Bass

[SteveTheShadow]

Hi everyone.
I thought I'd post a bit of DIY on the forum. The design pictured below is an open baffle speaker I have been developing over the past couple of months. One reason I have for posting this is the fact that, in this month's HFW, Noel keywood, in answer to a reader's letter, and with some justification, talks about the fact that OB speakers are not for those with small rooms. I won't go into why here because Noel explains it in his answer, in the letters page.



The design I have done is a bit of a cheat really as it is a hybrid design using active bass reinforcement, from a subwoofer bolted into the base. However what this means is that the delightful, fast open qualities of dipole midrange and treble can be enjoyed in a small room without the subsequent bass issues that a full dipole radiator would enforce. The laws of physics state that the speaker still needs to be relatively large to accommodate the big twelve inch lower midrange/upper bass driver, in this case a Goodmans Axiom 401, but not so large as to be overwhelming in a small space.

Why use such a big driver when there is a sub to handle the bass? The answer is that the sub needs to cross over at a relatively low frequency, certainly well below 200Hz, otherwise its monopole radiation pattern will interfere with the dipole baffle above and cause a discontinuity in the bass register, which will be audible at the listening seat. Plus the fact that a driver mounted on an open baffle needs to be at least 6dB more efficient than the upper mid/tweeter, otherwise searing treble and an overall tinny sound will result unless the tweeter is padded down. The large driver satisfies these requirements much easier than a small unit.

Personally I like to keep crossovers as simple as possible but this in turn means that you need very high quality drivers, matched for efficiency and radiation patterns otherwise you are wasting your time.

The monopole bass end also means that with a bit of tweaking for bass level, the speaker itself can be placed closer to the wall than would otherwise be the case for a full dipole radiator, again handy in a small space. My speakers are placed roughly half a metre from the rear wall and sound great in that position.



Here is a view of the rear of the baffle.
Looking carefully, one might see the slot at the base of the side wing through which the subwoofer vents to the sides.
There is one on the opposite wing too.

The towel is a temporary arrangement until I get some filling to put down the sides of the sub. This stops the sound, coming from the OB, above from resonating in the gap with the subwoofer output, until I eventually fit a board to isolate the sub chamber from the baffle.

The phase is set so that the sub radiates in-phase with the baffle's rear radiation. No direct front radiation of the out-of-phase bass from the sub is allowed. However, slots are cut into bases of the baffle wings to allow the bass to escape to the sides and of course the sub is free to radiate rearwards.




And here they are seen in-situ from, the wider angle, with my PX25 SE amp driving them. I like the look of them and so does the missus.

As to sound quality, they suit me down to the ground, with a beautiful, clear, open mid range that is fast and detailed without descending into shoutiness. the Fostex FE108EZ and the Goodmans Axiom 401 are a marriage made in heaven and together, produce a sound that is powerful and organic.
The presentation might be a little too "neutral" for rock music but given my diet of jazz, soul, blues, folk and classical, they suit me perfectly and are completely unfatiguing to listen to. The sub simply fills in the bottom octave from around 70Hz downwards and as a result, its radiation pattern causes no weird effects when combined with the dipole baffle.

I'm very pleased with them. If one is prepared to cast aside any prejudice one might have against subwoofers then they can be very successfully employed to give listeners the benefits of open baffle sound within a much smaller space than would be normal for such a speaker.

Steve

[SPS]

Hi everyone.
, in answer to a reader's letter, and with some justification, talks about the fact that OB speakers are not for those with small rooms. I won't go into why here because Noel explains it in his answer, in the letters page.



As to sound quality, they suit me down to the ground, with a beautiful, clear, open mid range that is fast and detailed without descending into shoutiness. the Fostex FE108EZ and the Goodmans Axiom 401 are a marriage made in heaven and together, produce a sound that is powerful and organic.
The presentation might be a little too "neutral" for rock music but given my diet of jazz, soul, blues, folk and classical, they suit me perfectly and are completely unfatiguing to listen to. The sub simply fills in the bottom octave from around 70Hz downwards and as a result, its radiation pattern causes no weird effects when combined with the dipole baffle.

steve these look great , and your room is not exactly big?

its funny what you say about being too neutral for rock,

I bough led zep 2 yesterday and it sounds very well with my baffles
and 3 watts /channel ..105db's my room was rocking..
but it was not the sound i expected.. i was used to hearing it with a level of distortion in the bass.. thickening it up... and without all the peaks in the music being heard..
but i was more than happy.. as i feel my system plays it fairly accurately

can't wait to hear them next week...

steve

[SteveTheShadow]

steve these look great , and your room is not exactly big?

its funny what you say about being too neutral for rock,

I bough led zep 2 yesterday and it sounds very well with my baffles
and 3 watts /channel ..105db's my room was rocking..
but it was not the sound i expected.. i was used to hearing it with a level of distortion in the bass.. thickening it up... and without all the peaks in the music being heard..
but i was more than happy.. as i feel my system plays it fairly accurately

can't wait to hear them next week...

steve

Hi Steve and thanks for the positive comments. Given your comments about trying Led Zep with your baffles I thought I might as well bite the bullet and try a bit of rock with them.

Using Thin Lizzy's "The Boys Are Back in Town" from their "Jailbreak" album The result was very impressive. Everything was separated out and as you say the bass was well portrayed without any thickening up, plus the track really moved along at a brisk pace. Much better than I remember it sounding on other systems I've had.

So it looks like my baffles are performing in a similar manner to yours.

That's what I like about baffles...you seem to be able to play anything you want on them.

Steve

[John]

Lovelly looking speakers Steve and I bet they sound great I heard the Bastanis Atlas and was blown away by them I wish had the room but alas only 4m by 3m
They really could play rock very well and did everything else as well

[SteveTheShadow]

Hi John,

They really work well.
I guess the hardest part about building them was the crossover.
Trying to marry together two ostensibly full-range drivers with rising responses meant I had to throw the crossover rule book out the window, by putting a big hole in the mid-range, so that the rising responses would flatten, come together properly and fill that hole back in again.

The results however have been well worth the effort.

Steve

[SPS]

Lovelly looking speakers Steve and I bet they sound great I heard the Bastanis Atlas and was blown away by them I wish had the room but alas only 4m by 3m
They really could play rock very well and did everything else as well

I would think steves listening room is about 4x4m.. so would not be put off too much, the real issue for many is can you accept the size of the speakers in the room,
open baffles can be very good ..i have not heard the bastinis but i would guess they would have a similar sound to steve's and my own..

steve

[aquapiranha]

Lovely! I especially like the Fostex drivers.....just kidding.. I have done a very modest amount of diy audio myself but I have never tried an OB - perhaps it is time I gave it a go? I do like the look of the Hemp acoustics drivers, but they woulod be best in a cab..then there is the Siver Iris...now I have started myself off again!

Like I said, they look brill and I bet they sound it too!

steve.

[SteveTheShadow]

Hi Guys,

To give an idea of the size..they are 41 inches high by 16 inches wide by 13 inches deep, at the bases, where the wings are at their widest. The measurement from the back of the Fostex driver to the rear wall is around 2ft, which means that the subs at the base are positioned about a foot from the rear wall.

In a 4m x 4m space, the baffles become a feature of the room... no getting away from it, so they have to be designed to look good, otherwise the better half will not accept them (and you can't blame her either)

Steve has bigger baffles in his room with 2 x 12 inch Goodmans drivers below a Lowther EX4 full ranger. I think his are about 2ft 6in across the front and about 4 feet high with foot deep side wings. Although he has a bigger room than I do, his baffles still become a feature of his room, simply because of their increased size compared to mine. But he has also designed his to look good to minimise complaints from the distaff side.

That I think is the secret of getting open baffles into a room that is shared with the family, make them look good, complement the fixtures and fittings, put a few pot plants around to soften the impact and half the battle is won. The sound takes care of the rest, because even the most cloth-eared person can tell that, in the right circumstances, open baffles will blow away most box speakers.

Steve

[Marco]

The sound takes care of the rest, because even the most cloth-eared person can tell that, in the right circumstances, open baffles will blow away most box speakers.

Hi Steve,

Nice speakers, btw!

Much as I like good open baffles (and there's plenty to admire in their presentation) the ones I've heard so far (at Owston, etc) lack the sort of scale and visceral bass I get with my Spendor SP100s (with 12" bass units) when playing music at the sort of silly loud levels I do when friends pop round. It's that gut-wrenching, feel it in your chest, 'physical' quality to the bass that I miss with the open baffles I've heard so far, and I want that, big time!

I find almost every other aspect about the presentation of open baffles better than large well-sorted box speakers (and otherwise), apart from the above. I also find much the same thing with good push-pull amps compared to SETs, but that's for another discussion.

Like always though, I'm always open-minded about these things and if or when I hear a pair of open baffles which do the 'visceral' thing in the bass that I like I'll openly acknowledge it!

If you're bringing your speakers to Owston this weekend then I'll bring my p/p 30W Class A Copper amp and some suitable music to see whether your babies do it for me in the bass and scale department

Marco.

[SteveTheShadow]

Hi Steve,

If you're bringing your speakers to Owston this weekend then I'll bring my p/p 30W Class A Copper amp and some suitable music to see whether your babies do it for me in the bass and scale department

Marco.

You know of course they won't

Notice I said most box speakers. There's no way a 12 inch driver on an open baffle will hit the chest like a 12 inch driver in a box will. Laws of physics innit? I'd probably need a pair of 15s to get anywhere near.

You're welcome to have a go with your amp but we need to remember that these things are efficient, so the power handling is not that of a modern driver. The Goodmans Axiom 401 will only take 15W before it blows up and I love these drivers too much to risk them in any way. Also they are not mine (not yet anyway )

The Fostex FE108EZ driver is nominally an 8W max power speaker with a 0.28mm xMax and again I'm not prepared to risk destroying them in a power game.

I'm not trying to be awkward or anything Marco, but you can see the problem.

Steve