S7 Open Baffle Loudspeakers

The S7:
- Fully dipole radiation from 35Hz - 20kHz
- Vifa P13WH midrange
- Eminence Alpha 15"
- Fully active 4th-order Linkwitz-Riley crossovers M-T at 1.5kHz
- 2nd order LR W-M at 300Hz
- Time aligned using phase correction
- Tri-amped using 3 pairs of LM3875 Gainclones

Follow the design process here.

Practical OB design practices

What has been learnt. Slapping a full-range driver on a wide baffle just doesn't work, unfortunately.

  1. Drivers

    Operate all the drivers at their pistonic properties as much as possible. This is roughly below their beaming frequency which is 1/2 wavelength of the effective cone diameter (0.5 * 345 / diameter) where 345 is the speed of sound in m/sec.

  2. Baffle size

    - All dipoles will exhibit a dipole peak. The 1st peak can be tamed using notch filter, but the next peak-and dips are not. The idea is to use the response before and just above the dipole peak. The consequence of this is usually the need of tweeter which can be crossed low, at about 1.4kHz.

    - The baffle size should be less than 2.2x effective midrange cone diameter. This way the dipole peak will be pushed higher in the frequency response, ready to be crossed with a tweeter. Using very wide baffle will push down the peak (more bass) but also put the peak-and-dip in tweeter crossover region.

  3. Dipole peak equalization

    - Get 1m gated impulse measurement. Then select using 10ms time window. Notice the 1st peak. (Edit: I have since trusted outdoor, ungated measurement more. Raise the speakers to at least 2m from the ground and measure from 1.5m).

    - Obtain Fo, Q, and depth of the notch filter. This can be really easy using JohnK's spreadsheet , but not entirely required.

    - Build the active notch filter. The components must be as close as possible, and better yet simulate them using Spice.

  4. Time alignment

    - Measure on-axis the woofer and tweeter, both at exact same distance from baffle surface e.g. 30cm. Measure using impulse response.

    - Select same time start and time window, e.g. 5.5ms with window of 6ms. Compare the phase of the woofer and tweeter, obtain the degrees difference. Then calculate:

    e.g. xo freq = 1500, difference = 43.6 degrees. Then:
    For LR4 the difference should be 0
    Phase = 43.6/360 = 0.1211
    time = phase/xo = 0.1211/1500 = 0.0807ms
    equal distance = speed of sound * time = 345 * 0.0807 = 27.855mm

    then create the circuit based on equal distance.

Open Baffle (OB) Loudspeaker Articles

Collection of well-written articles and investigations about Open Baffle Loudspeaker design:

Advanced Articles:
  1. Measured Monopole and Dipole room response
    By Elias:
    I did some measurements with monopole and dipole bass in a room.
    Particularly I try to pay attention to temporal behaviour since as I believe steady state measurements in this case are pretty useless when considering human perception. Also I try to match the measurement excitation signal to represent the final situation, that is music.
    Music is no impulse nor steady state sinusoid, thus I'm using short tone bursts with shaped envelope. It will give quite good visuality to what is happening in the room due the reflections.

  2. Monopole vs Dipole bass
    Elias gives emphasis on temporal fidelity of the reproduced signal. Real music signal contains time domain energy variations that must be reproduced accourately before high fidelity is achieved. This is also true for bass.

Past Projects

Various speakers I built in the past which I remembered to take pictures of, not in any particular order.

Why build so many speakers? Because only transducers and speaker topologies makes acoustical difference to a hifi sound. Not CD players, not DAC, not Amplifiers, and most definitely NOT cables!

S11 Bookshelf OB speakers.

S9 with H-Frame woofers

Two 10", Vifa P13, and tweeters in dipole configuration
Sufficient output for a desktop OB!
Sept. 2008

Same thing as above before painting

Building Pluto Clones.
May 2008

Trapezoid 3-way OB speakers.

One 10" woofer was not sufficient in terms of bass output

How would dipole-monopole hybrid sound like? In short...not good.
May 2008

Experiment with U-frame dipole woofer.
Good output but not so good sound, most likely due to distance to the wall.
April 2008

What the H-frame setup look from the front.

Trying out H-Frame baffle.

Also not too good due to distance to rear wall avoid dipole radiation pattern.
April 2008

Small dipole desktop speakers. Passive XO. There's no bass output.
March 2008

The small desktop dipole from the front.

The woofers are taken from $12 dicksmith bookshelves.
They eventually used in the Pluto clones.

My first amplifier build for almost 20 years of HiFi hibernation.
Oct. 2007

My first ever Active Crossover.
Dec. 2007

"Number One". First dipole speakers!
It was finished in Christmas eve of 2007.

Very simple crossover.

Number Zero. The first time I ever heard of dipole speaker was this one, and not even a pair. I knew right away that this is 'the way'.
Dec. 2007

The drivers were from kerbside rubbish and the baffle was from unused kitchen cabinet.

Eminence alpha 15" + $4 Jaycar midrange.
Nov. 2008

Investigation on influence of driver offsite (time alignment).
The experiment was failure as the resonances kept from actually making good judgement.
Oct. 2008

Eminence Alpha 15 + Jaycar mid + TangBand 3" as tweeters.

Eminence Alpha 15 + P13WH + Tangband 3-way

The "S9". Most refined OB I built.
Aug. 2009

The "S8". Bass output was great but the 30cm baffle is too wide acoustically for 5" P13WH
March 2009

Another pic of S8. It requires 10 separate amps, but I ended up using 6.

The "S7", the right progress for smooth polar response and revisited as "S9"
Jan. 2009

P13WH mounted on very narrow baffle.

S7 XO work in progress!

Eminence 15" measured to investigate if it's worth to clone the Physics CS2.
The answer was NO.
It was a very hot summer night.
Jan 2009

Experiment in horn-loading tweeters.

Eminence Alpha 15" + Horn loaded tweeters.

10" woofer + Tangband fullrange 3".

These were actually promising for future revisit to "Desktop OB"
Nov. 2008

Simple xo


I don't know why I have so much shots taken.

Investigation of omnipolar tweeter + Dipole mid and low.

Testing in the living room.
I went back between Omni and Dipole numerous times. Always ended up having the Dipoles as perferred speakers.

15" Eminence Alpha + P13WH Midrange + Tangband 3".
Big enough for a desktop system ;) ?

Ridiculous Audiophile Products

  1. The "Belt Pen"
    review: http://www.positive-feedback.com/Issue1/beltpen.htm

    As I said, I cannot discuss the scientific theory behind the pen, but here is a quote from the literature provided with it: "The P.W.B. ‘X’ Coordinate Pen has been induced with complex messages and when a human writes a message with the pen, the conditions on the object change in a similar way to the events in the ‘double slit’ experiment, in which the essential ingredient is the pure energy form of the photons and electrons which readily interact with the human observer."