Al Limberg wrote on Mon, 05 November 2007 20:52 |
Hmmm, It has been nearly 5 years (hard to believe!) but I would be nearly willing to bet that the distortion measurements were made with a single LAB since we only had individual samples of the two Community cabs and pairs of the BassMaxx offerings. Perhaps John Halliburton or Paul Bell or David would be able to confirm or correct my supposition. I'll definitely call and harass Too Tall tomorrow.
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Might be interesting to find out. I just quoted what I saw on the Michigan Subwoofer website. Even if we are talking about only one box though, we can still see pretty clearly that the LABhorn has the same distortion at 25 watts that the 12Pi has at 1600 watts.
Al Limberg wrote on Mon, 05 November 2007 20:52 |
I guess maybe I'm still a little slow on understanding why we are making such a big deal out of distortion figures outside of the design parameter of the speakers in question.
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The reason is Tom jumped in on this thread (and many others) to say push-pull drive used in the 12Pi basshorn sub doesn't work. I don't see how he can look at the data and come to that conclusion. The evidence is overwhelming that it does.
Tom claims that the LABhorn measures lower in distortion. He later admitted that he had never measured one, so I don't know why he ever said anything in the first place. There was no hard evidence to make any assertion as forcefully as he has done. The only data I've seen of LABhorn distortion was made with only 25 watts input power. That's not pushing it very hard, really. We need to see what the LABhorn does at 100 watts, 200 watts 400 watts and 800 watts. These higher power levels are where the rubber hits the road.
Even with just the 25 watt chart, we can clearly see a difference in the LABhorn and the 12Pi basshorn subwoofer. Where the distortion rises below 35Hz in the LABhorn, it rises
much more slowly in the 12Pi. The same distortion you're getting from LABhorns at 25 watts, you don't see in the 12Pi until it reaches 1600 watts. That's pretty significant, don't you agree?
The push-pull arrangement is most effective at very low frequencies. It is designed to be of most benefit near cutoff, where the horn is starting to lose its ability to load the cone. It does that beautifully, as the data clearly shows.
After looking at this data, I can find absolutely no reason to make a basshorn with two woofers without configuring them in a push-pull arrangement. In any installation where more than one woofer is used, I would suggest that they be done in push-pull pairs.
Antone Atmarama Bajor wrote on Mon, 05 November 2007 22:25 |
And what harmonic is being tracked, I would think its probably 3rd Harmonic.
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At the Prosound Shooutout, the way we measured distortion was to track the fundamental and bandstop it, bandpassing the two octaves above it. That way we got the 2nd and 3rd harmonics. The window is rather wide though, so it can "see" a lot of noise too. That's why we call it a THD+N measurement, because I think most harmonics from loudspeakers are 2nd and 3rd and because noise is included in the data.
Antone Atmarama Bajor wrote on Mon, 05 November 2007 22:25 |
The Second Harmonic is quite simply the Octave, so it will always be Consonant, and less noticeable. 3rd on the other hand is more of a problem and can cause dissonance. Push pull does not help here. So the push pull may give superior results at reducing 2nd order HD but not Odd, I'm not sure if it does anything for the other Even Harmonics.
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The thing is, the second harmonic falls within the passband of the horn. Since horn output is strong to about 180Hz, second harmonics of fundamentals up to 90Hz are acted upon by it. Basically, the whole usable range of the sub is vulnerable to second harmonic distortion generated by the woofers. As much as possible, the drive units should be made free of second harmonics.
The third harmonic is higher in frequency, so the low pass function of the front chamber and horn folds serves to attenuate it. Since horn output starts to fall above 180Hz, third harmonics of fundamentals above 60Hz are attenuated by the low pass acoustic filters formed by the folds and front chamber.
To me, this makes a very good pairing of technologies. I think push-pull drive is ideally suited for basshorns. Some have said basshorns don't need it, or that it is better suited for direct radiators. I think push-pull drive is perfectly suited to folded basshorns. Push-pull works best at low frequencies, where summing is good and the cancellation of harmonics is best. Basshorns are acoustically small, so they start to lose their ability to load the woofer at the lowest frequencies. They can use the extra help down low. This makes the two technologies ideally suited for one another. Basshorns and push-pull drive work very well together.
I wouldn't do it any other way. Can't see why anyone else would either. Can anyone give me one good reason
not to use push-pull drive in a folded basshorn?