Derek Walton

Open Baffle Speaker
pi one tower
Eminence Alpha 15A and Visaton B200

About open baffles.....

A couple of years back I was using open baffles and they produced the best sound that I had ever heard in my room. However they were BIG and eventually their physical presence over-powered me in my small room. But open baffles are to speakers, what single ended 300B is to amplifiers and is one of those things that once you use them it is hard, very hard to go back to anything else and although I have been very content with my current fullrange speaker set-up, I just could no longer resist once again the temptation of the open baffle.

Enter the Eminence Alpha 15A with its high Qts of 1.26 and that makes it a great candidate for this and together with the fullrange Visaton B200 has meant that I can use a smaller baffle size than I was using before. Their high efficiency and easy amplifier load makes them an ideal choice for my 300B S.E. amplifiers.

OK.. there is no need to go into a great deal about the specifics of this as it is all really self-evident and I guess that what you are mainly interested in are dimensions that I used. Other than that, suffice to say that the Visaton is run fullrange and the Alpha has a single series inductor to limit it to the lower end of the spectrum.

At a particular frequency that is related to the baffle size the sound radiation goes from a 2Pi hemispherical pattern( radiating sound forward) to a 4Pi spherical pattern ( radiating sound in all directions) . The result is an apparent decrease in bass output because the low frequency is reduced in the forward direction and goes in all directions. This is a loss of 6dB at lower frequencies.

A full range speaker radiates in to 2 Pi space at higher frequencies and into 4 Pi space at lower frequencies and there is as a result a loss of 6dB from the treble to the bass.

The Alpha is quoted as being 97dB, but these measurements are normally taken in 2Pi space and the 6dB baffle loss must therefore be factored in, giving us a nominal sensitivity of 91dB, which is in the ball park figure of the Visaton B200 sensitivity. We need to use a bass unit that has a greater sensitivity than the main full range driver by at least 6dB or more, because of this baffle loss.

F3=4560/Wb ( where Wb is baffle width in inches)

The f3 for my baffle is there fore 228Hz.




Ok, so this baffle step loss, means that the Visaton will start to loose 6db during the transition to the lower frequencies. We therefore need to fill in this loss with the Alpha bass unit. So what we want is for the bass unit to gradually drop off as the Visaton takes over in the midband. We therefore add an inductor to the bass driver that will allow it to operate without restriction up to a certain point, at which point we want it to start reducing its output or else it would be doubling up, or adding to the Visatons output.

Low Pass filters allow low frequencies to pass below a selected crossover frequency, filtering out all frequencies above it. In a first order (6dB per octave) filter, this consist of a coil in series with a loudspeaker. Just below the crossover frequency, the coil begins to add resistance to the circuit. At the crossover frequency enough resistance has been added to equal the resistance of the loudspeaker and reduce the power by 3 dB or 50 %. One octave above the crossover frequency, power has been reduced by 6 dB or 75%. Each octave higher reduces the power by an additional 6 dB. The size of the coil will be determined by the impedance of the loudspeaker(s) and the desired crossover point. The larger the size or amount of inductance (millihenries, mHy) is, not physical size, the lower the low pass frequency will be.

How these sound depends upon the room size and placement. I initially used them in my sitting room which is about 18' x 15' and there was not much bass at all....yes, with equalization ( or tone controls) I could dial in the nesessary bass, but I do not wnat to resort to this however. Next was to use them in the room that they were destined for and thats the room in the photos here.. that is a much smaller room and in this case I have perhaps too much bass! But that is easyily rectified by a change in the inductor to lower the frequency at which it starts to cut off at, as at present it is running a little too high in frequency and therefore gives the apparent impression of too much bass. It is not that there is too much... its just in the wrong place! Also, even at this stage I do feel that the Visaton does need a tweeter, that is for sure. I will add a Fostex FT17H for this coming in at about 10kHz. Also in hindsight, I should have placed the Alpha lower, much lower on the baffle and when I have finalised the set-up in a few weeks, I will remake the baffles again. Ah well, you dont expect to get it 100% right the first time, now do you.

The tilt backwards is approx 7 degrees and i now feel this is perhaps a little too much and that 5 degrees is better. Ideally, I will put an adjustable foot at the back to allow for further adjustments - just another something for you to think of and bear in mind.

Click for larger image

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front front front front
front front front open baffle dimensions
Visit the Audio Circles - Open Baffle Forum web site HERE


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