Soundlab (Chinese) 204mm (8") drivers in open baffle array

[timmillea]

I am returning to the world of DIY speakers after almost 30 years away! Not much seems to have changed - closed box, ported box, crossovers, etc. - but one new development is open baffle designs which simply are not mentioned in my DIY speakers books from that time. Something else has changed - cheap drivers have got even cheaper!

While I have managed with commercial small tuned boxes + subwoofer for many years, my interest this time is to provide music at a Summer garden party a long way from any electric supply. I looked into super-efficient design amp & speakers to run off batteries (& perhaps solar power) and it appears that the Chinese Tripath amps selling on eBay for around £15 are just the ticket, getting almost 90% efficiency into 8 ohms (nearer 80% into 4 ohms). Of course, these 'class D' amps did not exist either in my youth. I settled on a Lepai Ta2020-based unit because 1) it has (bypass-able) bass & treble controls which may be of benefit 2) the 2011 revised version has addressed much of the criticism of the previous model and 3) everyone says they sound great.

I chose an open baffle design also because everyone raves about the openness, transparency and that 'in the room with you' realism, because I have not yet heard any and because they are very easy to make. The figure of 8 polar pattern also appealed for their directivity in an outside scenario. I settled on a line array because they break the inverse square law, i.e. they exhibit cylindrical rather than spherical directivity so the SPL drops off in linearly with distance rather than the square of the distance - handy when people may be scattered over the large area of a garden rather than sitting in the ideal listening position.

For drivers, I found these Chinese units listed on many UK shopping sites. I have trawled and trawled and could not find any reviews or tests of them or designs using them. They are listed as either Electrovision or Soundlab 204mm (8 inch) full-range speakers and seemed like good candidates. Here is their full spec.

Colour: Black
Diameter: 205mm
Dimensions: 86 x 205 x 205 mm
Frequency Response: 40Hz-18KHz
Impedance: 8Ω
Magnet Weight: 0.34kg
Peak Music Power: 20W
Power RMS: 15
Resonant Frequency: 40Hz
Sensitivity: 96dB
Weight: 0.997 kg
(from SoundLab 204mm (8") Round Speaker 15W ONLY 6.36)

Now 15W RMS may not seem that much but just look at that 96dB sensitivity. I bought ten units for £60.76 (under $10 each) with free delivery from DFB Sound & Light Warehouse. My project is to put four units, configured two in series x two parallel to maintain the ideal 8 ohm load for the Tripath amp in a vertical line array. This should yield a whopping 102dB SPL @ 1W, 1m! This means I can keep the output of the amp well down to avoid distortion, have loads of headroom and run the system all day off 10 AA rechargeable batteries. The line array also helps to mitigate manufacturing tolerances by averaging out the different peaks and troughs between the drivers.

Of course the great weakness of open baffle designs is the weak bass. An open baffle has to be at least the size of the half a wavelength of the lowest frequency, e.g. a minimum of 3.5m at 50Hz, or 7m at 25 Hz, to benefit from the 'boundary effect' 3dB lift. I looked into H-frames and U-tubes but in the end developed a simple design to use the boundary effect in reverse, i.e. to take it away from all but the lowest frequencies and thereby provide additional low bass support.

Above around 1KHz, corresponding to the half-wavelength equal to the width of the drivers, the dispersion should be highly directional and benefit from the 3dB boundary effect lift. This is fine as high frequencies lose their power faster over distance than low frequencies. By putting the drivers on the smallest possible baffle, i.e. the same width as the drivers, we deny the boundary effect until the difference in direct and reflected sound (from the ground) becomes small relative to the wavelength. The difference is maximal right next to speaker and lessens with distance. With a bit of simple trigonometry and a spreadsheet, the distance from the ground to the centre of the speakers should be the order of 1-2 m for a listening distance between 2m and 8m to reinforce only the bottom end of the response range. The final height will be determined by experimentation as I cannot be confident that these drivers will go smoothly down to the claimed 40Hz.

I sourced some reclaimed scaffolding timber which is 9" wide and 35mm thick for the baffles. Cleaned, stained and waxed it looks stunning. The drivers are due tomorrow, the amp is on its way from China and the 10 AA battery box arrived today. It is two weeks to the party - wish me luck!

Any thoughts? Have I missed anything?

Tim.

[Jim Griffin]

Originally Posted by timmillea

Any thoughts? Have I missed anything?

Eight inch diameter drivers--even full range ones--will have very limited high frequency capability because of comb line effects. It is associated with center to center spacing. Please read my white paper for insight to line array design. I also wonder if you really will have enough bass without a large baffle.

ArraySpeakers Forum - New Home for 'Near Field Line Array White Paper' - Jim Griffin, April 19, 2008 at 00:47:55

http://www.greatplainsaudiofest.com/...ineArray01.jpg

[timmillea]

Many thanks to Jim for the reply and link to your paper. There is plenty in there for me to chew over and I am grateful that you have included the maths and graphs too.

Every loud speaker design is of course a compromise. My favourite sounding speakers have always been wide-range driver boxes (usually + sub). I don't know why that is, perhaps something to do with my brain, but all the speakers I have liked the most, upon investigation, have turned out to have wide-range drivers. I know there is a 'full-range' community but also plenty of reasoning why a single driver can never adequately reproduce the whole 10 octaves of the hi-fi spectrum. I think my brain must not like the effect of crossovers.

So I accept that the higher frequencies will be hurt by comb-filtering and not just the higher ones. However, to me, this is a price worth paying for the lack of crossovers and the dispersal patterns for outdoors where SPL could be lost very quickly. I think my brain is better able to cope with comb-filtering, something that we constantly rely upon to locate us in a physical space, than changing phase shifts which I think are the aural equivalent of sea sickness :-)

[sreten]

Originally Posted by timmillea

Any thoughts? Have I missed anything?

Tim.

Hi,

An error people make is open baffle is simply a roll-off in the bass, it isn't,
and OB bass outdoors will be utterly and totally pathetic. Its hard enough
outdoors as it is, and OB cannot use rear walls or corners for affect.

Outdoor design is far different to indoor design, OB's a complete no-no.

The reason for this is its baffle loss, not roll-off, roll-off changes
the excursion of the driver, loss does not, but effectively makes
the drivers size, or excursion limits far smaller with the more loss.

9.5" wide baffles ?????!!!!!! don't what planets theory your coming
from but not a chance in hell even domestically for decent bass.
You won't even get upper bass + lower mids, nevermind bass.

There is very, very big hole in your understanding of the situation.
A very poor point to be planning on building the thing from.

rgds, sreten.

You need full TS specs to know what is going on, Fs = 40Hz,
sensitivity = 96dB is all I can find. Given that and some
intuition put the line array in a box vertically offset
from the centre line, around 50 - 75 L internal.
Add vent tuning 40 - 45 Hz.

[timmillea]

Originally Posted by sreten

Hi,

9.5" wide baffles ?????!!!!!! don't what planets theory your coming
from but not a chance in hell even domestically for decent bass.
You won't even get upper bass + lower mids, nevermind bass.

There is very, very big hole in your understanding of the situation.
A very poor point to be planning on building the thing from.

rgds, sreten.

I didn't mean to offend anyone with my speaker design. I am just a re-enthused speaker builder from 30 years ago.

As I tried to explain, my plan is not to support the midrange or bass until the last couple of octaves. Hence zero baffle and use of the ground reflection only to boost the bottom. This means withdrawing the drivers to a height above the ground until the boundary effect only supports the lows, rather than supporting the mid range and high bass as in conventional open baffle designs. I am giving up 3dB in SPL from 1Khz downwards to around 50Hz precisely so that the bottom end gains 3dB. Everything is relative. Perceived bass response can be achieved by taking away something above.

O.K., this is now the 'zero baffle ground design' experiment. I am a little taken aback by the replies so far. I was only trying to share my ideas in the hope of refining them for mutual benefit. There is more opinion than i bargained for. As a result of the feedback, I have ordered parts for a measurement mic and will now be doing tests which I will share here.

Tim.

[glt]

Originally Posted by timmillea

I didn't mean to offend anyone with my speaker design. I am just a re-enthused speaker builder from 30 years ago.

... I am a little taken aback by the replies so far. I was only trying to share my ideas in the hope of refining them for mutual benefit. There is more opinion than i bargained for. As a result of the feedback, I have ordered parts for a measurement mic and will now be doing tests which I will share here.

Tim.

I believe there is large silent majority excited about your project... Keep going...

[gvimhoof]

Hello Tim,

Don't let the tone of the replies negate the information in them, or your desire to achieve a positive outcome. Since you stated that you did do DIY in the past, I would assume you've dealt with the following scenario: I put all this work into this and I didn't get the results I wanted. The "enthusiasm" that you received so far looks (to me) like fellow DIY'ers trying to steer you away from that disappointment.

As someone who dived into DIY speakers with a FAST OB design, I still smile every time I listen to a concert recording and think about what I put together and the results it has given me. I love the open sound.

I have been a sound engineer for numerous outdoor events, but I've never used OB for sound reenforcement in any type of venue. I encourage you to proceed with your measuring and sharing your results.

Good luck and remember to have fun!

later,
G

[timmillea]

Many thanks for the support of gv & gvimhoof! I am so grateful and privileged just to be able to take part in such a discussion.

My Lepai amp arrived today. I have married it, with snips and a soldering iron, to the 10 x AA battery box and they look very happy together. I also commissioned the baffle maker. Here is the design. My very rough sketch is not to scale!

The drivers should arrive Tuesday.

Bestist,

Tim.

[woodturner-fran]

Very interested in this project because I actually have a pair of these drivers - mine have a chrome coloured whizzer. Always thought it would be worth playing with them in an open baffle...

[chris661]

Hi, with 4 of these drivers, you'll probably have sufficient cone area to reproduce some reasonable bass.

If you find them to be lacking, a small (4ohm or less - you'll need to experiment) series resistor can, by altering the T/S parameters, give more bass on open baffles.

Be prepared to experiment with room placements and things like that.

Best of luck,
Chris