Bass reflex vs. passive radiator bass response?

[codhead]

[DrWho]

The efficieny of a port is the same as that of a passive radiator. If anything, the passive radiator could be considered slightly less efficient because it drops at 18 dB/octave versus the port's 12 dB/octave below the tuning frequency.

Because they both use the same woofer, it's my guess that they tuned the chorus II slightly lower. I noticed that the mid crossover frequency is at 600Hz versus 650Hz. This probably allows for a slightly lower tuning while maintaining mid-range clarity. That 1dB difference below 40Hz is basically inaudible...it just happens that the +-3dB window falls in those specific ranges. You could even argue that both speakers have useable resonse down to 30Hz, but you won't see that in the specs.

[WMcD]

I was reading Professor Leach's book. He's out of Georgia Tech.

He shows that the mathmatical analysis of a passive is different than a vented. I believe this arises from the fact that the passive radiator does have a spring action in its surround, where the mass of air in the vent does not.

Also, the passive has more radiating area than a vent.

I pass this along because we had a long discussion going about vented boxes and pretty much equated the vent with a passive. Same overall idea but not identical, per the good professor.

But I agree with your point. When specs gets married to the plus or minus (particularly the minus) 3 dB window, we are not getting all the data. Some speakers do go lower and as you point out, may continue to perform just under the F3 cutoff for stating bandwidth.

This sort of thing is particularly true in the sealed versus ported devices. Generally, the ported ones go a bit lower and then roll off very quickly. But the sealed don't roll off as fast and there may be "more" output as you go farther down below the F3 cut off.

We have occasional inquiries to the forum where someone is worried about just a few more Hz of bass in the stated bandwidth. Almost as if one speaker is a silk purse and the other a sow's ear . . . or that some aspect of Dark Side of the Moon is going to be missing.

Obviously that is not the case. Thanks for pointing out your experience.

Best,

Gil

[formica]

I'm not sure how you measured the "Chorus I" but that can effect your results more than the 1db difference you are finding.

If I'm reading correctly, you currently own only the "Chorus I" and are comparing your in room measurements of it versus the stated anechoic measurements of the "Chorus II"? I'm just asking as room peaks and valleys are in the ±10db range, and it's not uncommon (in most standard sized rooms) to have a resonance around 40Hz. Without a RTA with adjustable gate times... it's really difficult to even get a ballpark figure of frequency response.

OTOH... you are correct that the published frequency response curves we see are usually derived by the marketing dept, and aren't always representative of the speaker's capability. I think most people (present company not included) would be astonished by the variances in the engineering dept FR curves.

Rob

[John Warren]

What bothers me about both pasive radiatiors AND vented ports is that they are resonant systems, designed to reproduce "one" frequency, Fb. What bothers me, in paticular about ports, is that the rqmnts for laminar flow in the port are difficult to achieve in practice making distortion inevitable.

[codhead]

[formica]

----------------

On 3/28/2005 10:15:52 PM codhead wrote:

The 4dB down is just a hypothetical scenario, as I was trying to venture a guess as to why this speaker's specifications would not be rated for frequencies which it is clearly capable of reproducing.

----------------

In which case your experiment did show you that there is alot info which the marketing specs don't indicate. BTW, I'm sure this applies to both the Chorus "I" and "II".

Another casual eye opening test you can try is to simply play some fixed frequency (single tone) bass test tones, and slowly walk around the room

----------------

On 3/28/2005 6:30:00 AM John Warren wrote:

What bothers me, in paticular about ports, is that the rqmnts for laminar flow in the port are difficult to achieve in practice making distortion inevitable.

----------------

Have you gotten your sub installed, tuned and tested? ... I'll post back in your original thread to avoid side tracking this one...

Rob