Can a notch filter improve phase lag? [message #19898] |
Wed, 31 March 2004 12:26 ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
akhilesh
Messages: 1275 Registered: May 2009
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Illuminati (3rd Degree) |
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HI Everyone, Here is a speculative argument as to why a notch filter may improve phase shift. in a BR box, the bass frequencies will come from the vent, and there is a phase lag. A notch filter will add afurhter phase lag. THe difference in ear-driver and ear-vent distances will also come into play. If the former is more, then it will lead to phase lead for the bass. In some cases, the combination of the factors that cause phase lag (box & notch filter) and phase lead (distance difference) can mayeb cancel each other out. In other cases, if the ear-driver distance is smaller enough, it may lead to a 360 degree phase lag, whioch may lead to a better integration of the music. What do you all think? -akhilesh
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Re: Can a notch filter improve phase lag? [message #19900 is a reply to message #19898] |
Wed, 31 March 2004 18:24 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
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Wayne Parham
Messages: 18758 Registered: January 2001
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Illuminati (33rd Degree) |
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There are a lot of things that act as filters. The box, the mechanical movement of the driver, the electronics. So you're on the right track here. But the fact remains that whatever filters are used to modify response have the same result, no matter if they are mechanical, electrical or acoustic. Basically, whatever you do to modify the amplitude response also modifies the phase response. So if you do something that equalizes the characteristic response curve, you have also modified its phase response too. It really doesn't matter if the equalizer is an electrical, mechanical or acoustic filter, the laws of physics apply to them all the same. If you change the amplitude response by any means, then you must have also changed the phase response.
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Re: Can a notch filter improve phase lag? [message #19906 is a reply to message #19901] |
Thu, 01 April 2004 05:17 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Martin
Messages: 220 Registered: May 2009
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Master |
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The phase shifts from the BR enclosure will be extreme near the tuning frequency, least say 50 Hz. By the time you get to the baffle step center frequency, probably near 400 Hz or higher, the box is no longer an issue. In the frequency range where the baffle step occurs there is a smaller phase shift due to the summation of the "sources" along the edge of the box combining with the driver's reponse. For my Lowther DX3 ML TL enclosure this amounted to about 30 degrees maximum. If you apply a Zobel across the driver, the combination behaves as a pure resistor. Then placing a BSC circuit in series the baffle step is corrected including the phase. The BSC circuit has the opposite phase shift whne compared to the baffle step response. Above the baffle step region the BSC circuit acts as a pure resistor so you have a simple voltage division with the driver/Zobel to pad down the SPL magnitude. There is no impact on the phase. This is how the circuit worked for my Lowther DX3 ML TL design. I measured the response and plotted the SPL and phase (after subtracting out the time of flight phase shift). The phase response was improved with the BSC circuit in place! I verified this with some calculations in MathCad. This is just one data point but I think it is accurate for most situations, I am sure somebody could dream up a situation where this was not the case. They probably reside over at AA. The purists at AA have completely closed minds and very little technical understanding beyond the rhetoric. Our frined TC is a prime example. Hope that helps, Martin
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Re: Can a notch filter improve phase lag? [message #19907 is a reply to message #19906] |
Thu, 01 April 2004 06:23 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
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Wayne Parham
Messages: 18758 Registered: January 2001
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Illuminati (33rd Degree) |
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I second Martin's analysis. Sometimes - often times - a system like this is made more pure with a couple of components. The Zobel is a good example. A speaker driver is a reactive device, so compensation is sometimes in order. I understand the idea of minimalist solutions and low parts count. But remembering that the speaker motor/diaphragm/cabinet is a system that has many filters, sometimes having a part of two in the circuit counteracts reactances and makes the system as a whole more pure. I really like keeping it simple, and sometimes that's best. Phil's little speakers sound great just bolting the speaker in the box. But there are other systems that are definitely improved with passive compensation. And it's not necessarily a matter of quality, or of artificially "fixing" a driver's response. It's just the way linear systems act.
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