Originally Posted by buckchester
Dr. Toole, I found this post very informative. However, I don't think I understand everything. Would you mind elaborating on a few of these points you made?
You mentioned that the difference in amplitude response between sealed and ported designs is detectable. Could you elaborate what this means? I don't think I understand.
You also mentioned the the lower roll off rate of sealed designs is an advantage. Can you explain why?
If a ported sub is tuned below 20hz (the threshold of audibility) would the lower roll off rate of a sealed design still be an advantage?
There are very likely other internet sites and forums that focus on this topic. There are certainly large sections of books that do - but not mine
. If you look at the comparative frequency responses of the designs, and the variations possible by playing with the driver and box parameters it is easy to see potentially audible differences. However, as I have said before, room modes swamp a lot of this if they are not "handled". Chapter 8.
Reflex designs have a range of performance options with respect to resonance and rolloff frequency, and Q (damping). The attenuation in rolloff is 12 dB/oct. You suggest that so long as the rolloff happens below 20 Hz all should be well. But 20 Hz is not the limit of hearing, it is around the lowest frequency we hear as being tonal, having pitch. Below that bass exists as a pressure, whole body, experience - a compression wave in an explosion for example. If it is there you know it, and it is nice. Movie bass in cinemas is almost always "booms", loud to be sure, but in my experience often with pitch. Through a system that extends persuasively to and below 20 Hz explosions and gunshots are a different more credible experience - no pitch. This can be achieved with reflex systems tuned very low, or in small closed box systems with large amps. In multi-sub installations that offer efficiency gains those are logical choices. But, it is a choice.
My early research showed a correlation between low-frequency cutoff and sound quality ratings. There was no trend based on - 3dB frequencies, a little trend based on -5 dB , and a persuasive trend based on the - 10 dB level - obviously adjacent boundary room gain was involved. See Figure 5.3 in my book. Olive's subsequent work showed that bass extension and smoothness accounted for about 30% of the factor weighting in overall sound quality judgments - See Section 5.7 in my book. Sealed boxes that roll off at - 6 dB/oct. have an advantage, especially if size is a consideration.