Cloning a $3200 Speaker for $400

[Patrick Bateman]

I am very happy with my home speakers, but I've been working for the past few years to get some of that magic in my car. I've tried everything from tractrix horns to Unity waveguides, and everything in between. Recently I stumbled across a design which looks promising, so thought I'd document how to clone it.

The speaker is a Beolab3. It uses an unusual enclosure shape to reduce diffraction, and an acoustic lens and waveguide to shape the polar response of the tweeter.

[Patrick Bateman]

To give you an idea of how I stumbled upon this solution, here's a pic of my home speakers. The waveguide works down to 900hz, but the enclosure is about three feet tall! Way too big for a car, clearly.

For comparison's sake, here's the Beolab 3. (It's the smaller one.) While the two speakers look dramatically different, they both share some things in common:

• Both are two ways
• Each uses a waveguide on the tweeter
• Both enclosures used curved surfaces to reduce diffraction
• Both designers focus on the power response over the frequency response

In my opinion, these are all excellent goals for a design.

Here's a quote from an article about the company:

"There was a lot of talk about the importance of the power response (the power response of a loudspeaker signifies the sound pressure averaged over all directions of radiation rather than just one - basically the combination of the on and off axis response in 3 dimensions ) over the frequency response. They believe that having a smooth power response is more important than a smooth frequency response for a speaker."

In a nutshell, I'm hoping that the Beolab 3 might give me a fraction of my home speaker's performance, but in a much MUCH smaller footprint.

Note that cost is no object here; I'm simply trying to reduce the size to a point where it will fit in my car.

[doug20]

Pretty home speakers!

Cool project also. Thread linked

[Patrick Bateman]

The "secret sauce" in the B&O speaker is clearly the "Acoustic Lens Technology" (ALT) licensed from Sausalito Audio Works (SAW.)

After studying their patents, I have a fairly good grasp of how it works. But first, some background.

A conventional piston begins to "beam" at a frequency equivalent to it's diameter. For instance, we'd expect a 1" dome to "beam" above 13500hz. (speed of sound/diameter) In the graph above, we see that the "rule of thumb" is true. The dome measures flat on axis, but the response falls quickly off axis.

The reason why this is A Very Bad Thing is that a great deal of the sound that we hear is reflected, and the rolled off response off-axis contributes to the unnatural "hi-fi" sound endemic to conventional loudspeakers.

Waveguides have been discussed in-depth on this forum, and they're an excellent solution. In the graphs above, we can see that the on and the off-axis response of a waveguide is consistent. This helps to create a realistic soundstage, and reduce listening fatigue. Unfortunately, waveguides are BIG, and there's no getting around that.

Or is there?

From what I read in the patents, I believe the SAW acoustic lens is small because it works over a narrow bandwidth.

The measurements above show the polar response of the ALT device. The top pic is the response at 20khz, and the 2nd is at 10khz. You can see that the response off-axis is stable out to 90 degrees.

On the other hand, the waveguide in my home speaker words down to 900hz. 900hz is 15" long, and that's why the waveguide is that big. From what I'm seeing in the SAW patents, their lens isn't working at all frequencies. It's basically tuned to work at frequencies where the piston is beaming.

For instance, the 1" Scan Speak tweeter is beaming from 14000 and up; therefore you only need a lens that works in that narrow range, just a fraction of an octave. In the rest of it's range, it's basicaly omnipolar, and no lens is needed.

That's why their lens/waveguide is so small.

[edjosh23]

looks cramped in there

[chris661]

It's a curiosity they get any bass at all from in there - enclosure volume is the minimal it could be.

Interesting work on the waveguide though.

Chris

[edjosh23]

Volume is small, but that is a passive radiator (on the desk).

Josh

[freddi]

those cavities remind me of the Karlson coupler but with driver firing all the way up (and sitting ~1/2 way in) plus a parabolic aperture - a clone should be lots of fun

[norman bates]

Patrick, how is the xt 1464 horn ? what compression driver do you have with it ?

I have a 1.4" ev dh7 on an ev hr90 horn (crossed at 1khz) and was thinking about trying that horn. Using 24db LR, I think 1khz crossover point should work (-6db at crossover point). Looking at the di for that xt horn, its freq response should drop off below 1500hz due to the height of the 12" mouth (but shown is 2khz knee). My hr90 has a 13.7" high mouth, it loses directivity below 1khz, thats where I cross it.

I've switched to the klipsch rb75 with 2khz crossover 90 x 60 degrees.

norman

[John Sheerin]

The reason the waveguide is small is because it is not trying to constrain the radiation to a small angle like the waveguide on Earl's speakers. From what I can see it's a reflector that reflects high frequency sound omni-directionally but only over a given vertical angle (at least at high frequencies). The lows are obviously already omni, so it's just the highs that need to be reflected.