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.

1 comment:

Anonymous said...


> Slapping a full-range on a wide
> baffle just doesn't work,
> unfortunately.

How true, HOWEVER, "slapping" the right fullrange driver onto the right wide baffle can work rather well.

One approach is the Linkwitz and co one you describe. Using drivers with a linear frequency response and wide dispersion in the pistonic range and all that, using active equalisers to fix the problems caused by the baffle and so on.

Another approach might be to select a driver (like a fullrange driver) that has reasonable frequency response depression in the range where the dipole peak of the baffle falls.

Also, while one might operate drivers strictly pistonic, this ends up leading towards 4, 5 and six way systems, which have their own interests (be they open baffles or large hornloaded sound reinforcement systems like the ones I have some experience with), but which have an implementation complexity not insignificant.

Instead one may use drivers that actually manage the cone breakup well in subjective terms (which BTW gets around the "pistonic range" or "beaming" to a good degree).

Then use the dipole to help to reduce the dramatic change in directivity index from around 0dB at 100Hz (4.8dB for dipole) to 10db.

So, the right full range driver in the right bafflke can work rather well, as can Altec VOTT, Tannoy GRF Autograph's, Urei's, Linkwitz/Beethoven designs or even my current favourite pair of Aluminum Enclosure 2-Way monitors (not MINI-Monitors one may add).

As always, it less what you do, but how you do it that matters.

Fiat L.V.X.