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In the Forum: Audio Discussions
In the Thread: Constructing LF modules to the limits

Post Subject: Shift in directionPosted by haralanov on: 9/22/2011

Rakesh, during the last year, there were some very important moments in my own realizations about the methods of low frequency reproduction.
I totally abandoned my Scanspeak bass modules and now they are resting in my garage. I intentionally have my listening without the low bass channels, because I first have to make the rest of the audio-band to sound just the way I want it to sound. During the last couple of months practically all of my efforts are concentrated towards the construction of my wide-range 12” channel, because it is the core of my system.

Now let’s get back to the topic. There are some things that my 25W woofers cannot do, no matter how much of them one is going to use. This is also true for any 10” woofer existing on this planet. The restriction is entirely due to the cone size. The behavior of a 10” cone is very different in comparison to a much bigger, lets say 23” cone, because the bigger cone couples very effectively with the surrounding air (just like horns!) because of its large radiating area for a given voice coil. When you have larger area, you have better control over the cone movement, because now there is more “attached” air which causes this control. But the moving mass must be as little as possible! Make some very simple experiment: Take a piece of cardboard with a size of approximately 22”x22. It must weight around 80-100 grams. Now try to make some wind, while waving it back and forth. You have to note it is not so easy to move it, because you feel high air resistance against your attempts to move it easily. Now take your phone (its weight must be approximately the same) and perform exactly the same movements with in the air, while holding it in your hand. Is there any wind? No? That’s for a reason. Do you think it is harder to control your hand movement because there is no air resistance that damps the inertia of the phone in your hand? Congratualtions! Now you know how the voice coil feels while pushing a small cone :-)

This very effectively flattens the impedance peak at Fs, and one has much more accurate tone at the range where the Fs of the system occur. It considerably lowers (nearly removes) the monochrome bass taste. The improperly reproduced range at and around Fs kills the entire sound of that channel. What happens at Fs? Usually the cone continues to radiate AFTER the signal passes. Everybody knows what it means. Imagine you are a box trainer (he is in the role of a voice coil) and you tell your boxer (he is the cone) you want him to demonstrate how fast he is able to kick. But imagine that boxer is lazy just in that moment, so he says to himself: "Why should I hurry, I’m so lazy – I will not kick fast and while I’m kicking, I’m gonna eat some sandwitches”. That’s what one gets from small coned woofers. They are not well controlled by the driving force, coming from the voice coil and they smear the accuracy of the bass notes. Using 4, 6, 10 or 20 ten inch woofers per side, does not improve the cone behavior of any of them – it only allows higher sensitivity and better dynamic range due to the raised total radiating area. But the whole trick is to keep the control at woofer’s Fs without using hard suspensions and too much electrical damping (because it makes the bass notes to sound dry and atonal). As a second benefit of all this, the big woofer will be very phase linear in the audible range of the bass, because there are no impedance anomalies which shift the phase of the signal (if the Fs of the woofer is out of the audible range, so it must be 16-17-18Hz). So now you have proper harmonic structure across the bass range and the bass channels are very easily integrated with the upper bass channels. But one have to high-pass his upper bass channel at least one octave higher than the Fs of that channel to keep it phase linear too. If one avoids phase anomalies, he will be rewarded with very realistic sounding system.

My bass channels will use 2x23” underhung woofers per side. The scale and authority of the big woofers (if they are properly made!) is absolutely unmatched by a stack of several smaller ones due to the above mentioned reasons. Take a look at the cones:

The second reason why I switch to considerably bigger woofer units is because they have much better tone (if properly made!). It is the same as reproducing a midrange with an optimized for midrange reproduction driver, instead of array of 10 tweeters. Well, using 10-20 tweeters to reproduce midrange will give you some midrange sound, but that sound is very far behind the sound of the dedicated midrange driver in context of tonal department.

There is another reason why I switch to bigger woofers, but this reason is indirect, because I can get the same effect by using a lot of 10 inchers. This is because sometimes I really love to listen at ultra loud levels, and I don’t want to have compression, distortion and dynamic restrictions in the low bass range when listening at 110-115dB. There will be around 2500 (there is no mistake in the number) liters of air behind EACH of the 4 woofers. May be in future I will add another pair of 23 inchers per side in order to have better dynamic reserve when listening at 120dB levels.
And not at the last place, I kicked the 25W woofers out of the system, because they tend to have just a slight taste of very soft rubber in the bass tone. This taste is very delicate, because the soft rubber is not directly in your mouth, but you feel the rubber as some kind of after taste after eating rubber in the previous day (just like eating garlic) but you can still feel it in your mouth/ear...

Best regards,
Petar Haralanov

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