Duke: Traditional vent systems typically tune the vent at the same appr frequency that the woofer resonates at. This produces two smaller "humps" rather than one larger hump. Bandwidth is increased and overall bass sensitivity also climbs over a wider region.
By doing so, you've now got two resonant points, which are really nothing more than undamped oscillation from the driver and the vent. These are two points of oscillation that the amplifier can't load into or control ( due to impedance peaks ), producing twice the potential for music to excite them with the resultant increase in "slop" i.e. undamped / uncontrolled output. Couple this with the elevated output due to the increased sensitivity in this region and the "room gain" that naturally takes place and you can see why we've ended up where we are today with MOST vented designs.
The other alternative to tuning a vent is to let the woofer resonate where it wants to in that cabinet and then tune the vent for whatever frequency that you want to try and extend output to. While this does increase extension somewhat and produce a shallower roll-off ( with resultant improvements in transient response ), the lowest region isn't as efficient as the upper region. This "can" produce a more natural presentation IF properly implimented, BUT, you've still got the associated phase shifts, lack of damping below the vent tuning frequency and reduced power transfer / lack of control associated with higher impedance peaks that vents typically demonstrate. In plain English, we've still got plenty of side-effects / problems to deal with using such an approach, even though they aren't as prominent as the more conventional designs.
As far as various alignments offering a different set of trade-offs, I know this and you know this, but in order to educate and share with those less familiar with the subject, you have to start with a base-line that they may be familiar with. As you mentioned, this type of tuning is not commonly used. That is why i didn't base my descriptions / argument on this type of vented design i.e. it's not widely used. Having said that, there is one very well known ( but NOT widely respected by audiophiles ) manufacturer that advocates such designs. That manufacturer is Cerwin-Vega.
As a side note, Stewart Hegeman used a very unique approach when designing vented speakers that some might consider to be similar to the above. I can see how it could have quite a bit of merit if properly applied. There is only one "manufacturer" that i know of using such an approach and that is Don Morrison Audio. While i've never heard these speakers, they do seem to try and tackle quite a few important aspects of speaker design. Don has been working with this design over time and seems to be on the right track from a technical perspective. If interested, Don also makes a small two piece preamp that some rave about. I am NOT "endorsing" or recommending either of these products although i will say that Don seems to be more than technically competent. Having said that, his preamp should go lower in frequency response though : )
Other than that, i know that Richard Shahinian of Shahinian Acoustics has acknowledged that Stewart Hegeman was a primary influence on his designs. Given that Hegeman was also one of the first that i know of to use metal cones ( not just for tweeters ) in his speaker designs, i would have to say that his thinking was ahead of many others. The first speaker that i know that used actual "metal" in it ( but not the whole cone / piston area ) was the Ohm A. While this was a revolutionary design in itself, i could be wrong here and there might have been others using "metal" in their drivers prior to the first Walsh based design. Sean
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By doing so, you've now got two resonant points, which are really nothing more than undamped oscillation from the driver and the vent. These are two points of oscillation that the amplifier can't load into or control ( due to impedance peaks ), producing twice the potential for music to excite them with the resultant increase in "slop" i.e. undamped / uncontrolled output. Couple this with the elevated output due to the increased sensitivity in this region and the "room gain" that naturally takes place and you can see why we've ended up where we are today with MOST vented designs.
The other alternative to tuning a vent is to let the woofer resonate where it wants to in that cabinet and then tune the vent for whatever frequency that you want to try and extend output to. While this does increase extension somewhat and produce a shallower roll-off ( with resultant improvements in transient response ), the lowest region isn't as efficient as the upper region. This "can" produce a more natural presentation IF properly implimented, BUT, you've still got the associated phase shifts, lack of damping below the vent tuning frequency and reduced power transfer / lack of control associated with higher impedance peaks that vents typically demonstrate. In plain English, we've still got plenty of side-effects / problems to deal with using such an approach, even though they aren't as prominent as the more conventional designs.
As far as various alignments offering a different set of trade-offs, I know this and you know this, but in order to educate and share with those less familiar with the subject, you have to start with a base-line that they may be familiar with. As you mentioned, this type of tuning is not commonly used. That is why i didn't base my descriptions / argument on this type of vented design i.e. it's not widely used. Having said that, there is one very well known ( but NOT widely respected by audiophiles ) manufacturer that advocates such designs. That manufacturer is Cerwin-Vega.
As a side note, Stewart Hegeman used a very unique approach when designing vented speakers that some might consider to be similar to the above. I can see how it could have quite a bit of merit if properly applied. There is only one "manufacturer" that i know of using such an approach and that is Don Morrison Audio. While i've never heard these speakers, they do seem to try and tackle quite a few important aspects of speaker design. Don has been working with this design over time and seems to be on the right track from a technical perspective. If interested, Don also makes a small two piece preamp that some rave about. I am NOT "endorsing" or recommending either of these products although i will say that Don seems to be more than technically competent. Having said that, his preamp should go lower in frequency response though : )
Other than that, i know that Richard Shahinian of Shahinian Acoustics has acknowledged that Stewart Hegeman was a primary influence on his designs. Given that Hegeman was also one of the first that i know of to use metal cones ( not just for tweeters ) in his speaker designs, i would have to say that his thinking was ahead of many others. The first speaker that i know that used actual "metal" in it ( but not the whole cone / piston area ) was the Ohm A. While this was a revolutionary design in itself, i could be wrong here and there might have been others using "metal" in their drivers prior to the first Walsh based design. Sean
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