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In Reply to: RE: Why are ported designs ubiquitous? posted by peter on December 22, 2009 at 18:19:36
That sounds silly but that is the reason.
Back in the day there were lots of acoustic suspension designs with the all-too-common 12" woof, 5" mid and tweeter. The cabinet width for a 12" (14-16") resulted in the design easily achieving the requisite box volume for a sealed box design, which is higher than that required for ported.
Along came "tower" designs which were thinner. Dual 8" woofs and even 7's were used. Then 6-1/2" and even five inchers. 10 or 12" drivers were side firing or found in a separate sub or plain non-existent.
So here is my theory: First, as you get into smaller and smaller drivers you get less and less favorable sealed-box bass performance. It's a lot easier to built a sealed-box 12" than smaller drivers. I think a big factor is the basic Fs of the driver. It's relatively easy to get a 12" driver to have an Fs of 20-25Hz. Try that with a 6-1/2" driver. Or a 5" driver. This is why, IMHO, the port became a necessity - to get some semblance of low bass out of smaller drivers in the popular thinner cabinets. This is also why the use of side-firing woofers became popularized, and even moreso the use of a sub.
Myself, I would rather have a sealed box design that rolls off at a predictable sealed-box rolloff, augment that with an active filter, and use a single ported sub.
To me, having a ported sub and a ported main speaker is to have far more complexity in the low end than is desireable. Too much phase distortion and other issues. Not even subs need to be ported anymore as well. One could have an all sealed box system, get wickedly low bass, and have optimum pulse response and no port-induced noise or phase distortion.
I think ported designs are over-rated these days. Especially now that we have subwoofer drivers that offer serious performance in a sealed box.
Sealed box subs sound more musical to me anyways. The imrpoved transient response is quite audible IMO.
Just my $.02
Cheers,
Presto
Follow Ups:
"Sealed box subs sound more musical to me anyways. The improved transient response is quite audible IMO."
I read a paper in the Journal of the Audio Engineering Society that investigated the audibility of group delay (transient distortion) on the order of what we would find in a decent vented box. Group delay was found to be barely audible on test tones, but not audible on music program material. This implies that a subjective preference for sealed boxes is probably not caused by their superior bass transient response.
In a home listening environment, the room itself is a huge factor in the perceived bass. Because of boundary reinforcement, rooms tend to boost the low frequencies. The deeper we go in the bass region, the more room boundaries will be within 1/4 wavelength of the speaker and the more bass reinforcement we get. A term that some writers have used for this is "room gain", and I've seen charts created by two different authors which suggest that typical room gain is +3 dB per octave below 100 Hz or so.
A vented box tends to be "flat" anechoic down to a much lower frequency than a sealed box, while the latter tends to have a more gentle rolloff characteristic. Factoring in room gain, a vented box that is "flat" to the 25 Hz will be +6 dB at 25 Hz in-room, which will sound bloated and slow. On the other hand a sealed box that's gently rolling off to -9 dB anechoic at 25 Hz will be -3 dB at that frequency in-room, and will sound quick and tight. In other words, the net result of vented box + room gain is likely to result in excess bass energy which is perceived as poor transient response, while the net result of sealed box + room gain is not.
I'll admit that the above is a rather extreme example, but the principle it illustrates is valid. And relative to group delay, differences in the in-room frequency response are vastly more audible.
So I don't dispute your observation, Presto, but the actual cause may be something else.
Ironically, achieving an un-equalized response that is the approximate inverse of typical room gain is easier with a vented box than with a sealed box. I've been doing it for years, and I'm sure others did so long before me.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
What about energy storage? Most vented boxes have a Q of 0.9-1.1 (gives a fat underdamped bass) whereas the optimal sealed box design has a total Q of 0.6-0.707. The higher the Q the more energy storage and the longer settling time a system has.
I used to have the Infinity IRS Beta, which had separate bass cabinets with 4 12 inch woofers per side in a sealed box. These were servo controlled by the use of an accelerometer on the voice coil of one woofer per box. The signal was sent to the "brain" that compared the motion with the signal and applied a corrective signal to make the bass match the signal. Now, one of the cool features was a Q control that allowed you to adjust Q from about 1.2 down to 0.5 or so. The difference in the bass quality was shocking (now remember this is in a sealed box so it is purely electronic control) and went from loose/bloated at the highest Q setting to techno beat tight at the lowest Q. Not surprisingly, the optimal Q came around 0.7 and this gave a tight yet well rounded bass. Optimal sealed boxes always shoot for this goal. I know of very few vented systems that come close to a Q of 0.7. I don't know this for a fact but by listening I would guess that the Wilson Grand SLAMM MkI had a Q closer to 0.7 as it is quite tight and fast sounding for a vented system.
Notes sound slow when energy is built up and sustained for (relatively) long periods. Bells have very high Q values and as you know they store and reradiate energy for quite some seconds and in a narrow bandwidth. Vented systems with a high Q tend to sound "one notey" for the same reason.
Even though, as the other knowledgeable poster pointed out to me, the port means more variables. (Albeit a variable that can be reasonably approximated by computation.)
A vented design offers more degrees of freedom so in that sense it may be easier to meet my performance goals with a vented box, but at the same time the variety of choices to be made in optimization multiplies along with the opportunites for screwing up.
With a vented box there are more things I have to watch out for: Port airspeed; overexcursion from out-of-band signals, port tube resonances; finding room for the port (sometimes not as easy as it sounds); greater sensitivity to variations in driver parameters within a normal production run; less leeway with where damping material can go inside the box; greater sensitivity to minor air leaks in the enclosure and/or drivers; taking into account where the ends of the port will be relative to drivers, cabinet walls and braces, and room boundaries.
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
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