Measuring Ported Speakers
Ported speakers are by their very nature more difficult to design properly, and even more important, can be extremely difficult to measure, as demonstrated by Don Keele in his 1973 paper to the AES entitled “Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement”. The engineer must measure the contributions of the port and woofer separately, and then correctly weigh their outputs, taking into consideration the ratio of their radiating areas and the overall system tuning. For example, if the diameter of the port is one-half the effective piston diameter of the woofer, the port’s output must be reduced by 6dB relative to the driver’s measurement before their outputs can be summed. (See figure 2 for an example of a well-designed early-‘70’s ported loudspeaker, the JBL L-100.)
Fig 2 JBL L-100 of 1972
This produces a complex composite curve of the system’s overall far field
low-frequency response. The system’s response must then be verified by
double-checking with other measurement techniques such as placing the
microphone in the near field equidistant between the port and the woofer. If
the port and woofer are on the same surface—which is often the case—it can be
impossible to reliably measure the separate woofer and port contributions
without severe crosstalk between the two. Poor sounding ported speakers are
often the result of something as infuriatingly simple as the engineer measuring
and reacting to the wrong data. Designing good speakers is not easy stuff, and
as much as most avid hobbyists would like to think they have it all figured
out, they don’t.
A parallel situation occurred recently when someone asked me why oval speakers sounded worse than round speakers. My reply was that oval speakers don’t have to be inferior per se to round drivers, they just end up being worse for reasons totally unrelated to their ultimate design potential. If my friend has heard bad-sounding ovals in the past, he's probably right, but not for the reasons he thinks. They're bad simply because they were poorly designed drivers. Maybe they were intended only as rear-deck original equipment speakers in a Chevy Caprice, where ultimate audio quality was not the primary goal. So the speaker didn't have an optimized cone profile to reduce destructive resonances. It’s surround didn't allow long, linear excursion, the voice coil couldn't handle much power so its distortion was high, etc.
This would certainly lead to a bad-sounding speaker. The fact that it was oval is coincidental. But since the oval shape fits in a lot of places where high-fidelity is not a priority (tablet computers, the thin side bezel of a 15” LCD TV, standard-issue factory car audio systems, etc), it's natural to begin to associate "oval" with "low-fidelity." (See figure 3, a cheap oval ‘all-purpose’ speaker.)
Fig 3 Cheap all-purpose oval speaker
This is specious logic. There is no automatic cause and effect. There is nothing that says the shape of a driver is the sole or even the primary determinant of sound quality. The original Infinity EMIT tweeter was a 3 x 5" rectangle (see figure 4), but it sounded great.
Fig 4 Infinity 3 x 5 EMIT Tweeter
The oval 9 x 13" KEF B139 “racetrack” bass driver of the 1970's was a great woofer (see figure 5).
Fig 5 KEF 9 x 13” B139 “Racetrack” woofer
Martin-Logan electrostatics are long and rectangular (see figure 6), but do they sound airy, natural, and transparent? They sure do, and they're not round.
Fig 6 Martin-Logan electrostatics
it rains three Tuesdays in a row, does Tuesday cause the rain? No, it's just a
coincidence. The weather conditions cause the rain, not the day of the week. If
you hear three bad-sounding speakers, is it because they're oval or rectangular
or round? No, it's because they're just bad speakers.
Similar reasoning can be applied to ported speakers. If you hear a series of
bad ported speakers, it’s natural to associate boomy, floppy bass with ported
design. But this isn’t necessarily the case. If the designer is good, if he
uses the proper measurement techniques, if the system is tuned correctly, if
the driver has the appropriate electrical and mechanical parameters, then a
superb system can be produced. That’s a lot of “IF’s”, and it’s much trickier
to do it well than with a sealed system. As I’ve said before, if you miss by a
little in designing a sealed system, you’re still ok. If you miss in a ported
system, welcome to Boomy, One-note City. My own feeling is that the lower 3dB
down point of a small ported system and its higher efficiency are thought by
many companies to be worthy advantages in today’s less critical, less
hobbyist-driven audio environment. When Mr. And Mrs. Gen X/Y’er wander into the
Big Box Mass Merchant and listen to speakers, chances are they’ll be impressed
with the one that has more “bass” and plays louder. Those are bad ported
speakers. The point is, they don’t HAVE to be bad.
Now I’m going to completely contradict myself and go against all this carefully laid-out reasoning I’ve just spewed forth. There was an intriguing article by the late Peter Mitchell in the December 1995 issue of Stereo Review on the subject of group delay in loudspeakers. Group delay is a measure of how sharply the phase of a signal changes with frequency. Mitchell was an acclaimed audio expert, reviewer, and commentator whose views and writings were very highly regarded. He was a founding member of the nationally-known Boston Audio Society. I remember he caused quite a stir at a Bose press conference several years ago with his pointed, relentless questioning and was summarily banned from all future Bose press events. His passing several years back was quite a loss for the audio enthusiast community.
Bass Reproduction & Group Delay
In this article, Mitchell puts forth the idea that there is a definite
correlation between the subjective quality of bass reproduction and superior
group delay performance. Speaker systems can be thought of as minimum-phase
filter devices, so the magnitude response (amplitude vs. frequency) of a
speaker will determine its phase, time (impulse) and group delay
characteristics. All things being equal, sealed speakers (2nd -order high pass
filters) have superior (lower) group delay characteristics than ported or
bandpass systems (4th - or 6th -order high pass filters). As Mitchell’s article
states, “Sealed-box…speaker systems consistently have the least group delay
(under 10 milliseconds), and they usually deliver the tautest bass transients,
the deepest-sounding bass tones, and the most clearly resolved bass textures.
Bass reflex and bandpass systems often exhibit substantial group delay [in
excess of 50 milliseconds]…and their sound tends to be thicker, fuller, and
This is very thought-provoking, and goes a long way to explaining why the “tightness and crispness” of the bass of an acknowledged “perfectly done” acoustic suspension system like the classic AR-3a/AR-LST (see figure 7 and 8), or AR-9 with dual 12-inch woofers (see figure 9) seems so much better than many other systems’.
Fig 7 AR-3a Fig 8 AR-LST
Note, however, that there are lots of factors that are not “equal.” The system designer has many choices. How he chooses to damp the system affects the magnitude response, and thus affects all the other factors, including group delay. An under-damped sealed system will exhibit poorer group delay characteristics than a properly-damped ported system.
There is of course the camp that believes that well-designed vented systems are superior. If well-executed they have many benefits, but a lot of the time they are not well-executed.
The “other side of the coin” continues: “The supposed benefits of lower group delay of closed boxes is rarely the reason why vented boxes sound different from closed boxes, since this relies on linear theory, where of course loudspeakers are rarely linear at low frequencies. It is the effects of non-linearities that mostly influence our impression of the bass from typical loudspeakers.
It could be argued that the benefits of extended bass of vented over closed is actually of more benefit, as, although the ultimate group delay may be higher, it is deferred to a lower frequency where it is of less audible importance.
Additionally, the earlier roll-off of a closed box changes the nature of the sound of the instruments, and this can definitely change ones impression of the sound of the instrument. By suppressing the strength of the fundamental compared to the harmonics (because of the earlier roll-off), the apparent timing of the instrument changes, with the harmonics "exploding" sooner than the fundamental. Therefore, the musical pace seems to quicken, and the transients become quicker. Some musicians know this instinctively, and a bass player might know to pluck the string earlier as he descends the register in order to maintain the timing of the music.”
The sealed box AR-3a and AR-LST have a Q of .707, which is optimally damped. The AR-9 has a Q of .5, bordering on critically damped. The choice of these Q values by the designer results in a very flat, non-peaked response down to the system’s –3dB point, which implies a very low group delay. The 9 actually begins to roll off a little before its 3dB down point, but it’s an excellent design choice, because the 9’s natural 3dB down point is so low anyway (an honest 28 Hz!) by virtue of its two 12-inch drivers’ 18Hz free-air resonance and the amount of bass energy the system produces with that big enclosure (a floor standing cabinet of over 4 cubic feet). By choosing a Q of .5 the designers have elected to intentionally “throw away” a little bass energy—which they can easily afford because the 9 has so much to begin with—in exchange for super tight, clean bass. A lower Q means greater damping, and lower group delay.
Fig 9 AR-9
This notion of low group delay is also presented by another well-respected source, Siegfried Linkwitz on his website linkwitzlabs.com. In a feature entitled “Frontiers,” in section F under the heading “Group delay and transient response,” he states: "…I am not certain what happens in the range below 100 Hz and I have strong suspicions that this is the region where delay distortion is audible. It is also the region where delay really accumulates via vented and bandpass woofers, and the great numbers of dc blocking capacitors in the signal chain from microphone to speaker terminal."
Conclusion: Sealed vs Ported Speakers, Which is Better?
So who knows? Maybe sealed is inherently better. Or maybe we’re just not measuring the right data. But I have heard terrific (and lousy!) systems of all types, so I'm willing to believe that it's more a matter of the designer's goals and their skill at execution that makes the difference, rather than any arbitrary design approach. What do you think? Tell us your opinion in the dedicated forum thread for this article below.
Unless you believe that the worlds finest, modern speaker companies are concerned primarily with speaker efficiency and the -3dB point, to the detriment of bass quality, the article does not really explain the reason that most of the worlds finest, modern speaker companies prefer ported designs.
It all comes down to one fact: cone excursion is vastly lower with ported designs than with sealed, acoustic suspension enclosures. This translates in a rather immediate way to reduced non-linear distortion in the deep bass. With the acoustic suspension approach, the strategy is to make the suspension very floppy, using the pressure differential across the cone to provide the force that opposes the motor force. The assumption is that the restoring force will be a linear function of cone displacement. But the obvious questions are with the linearity of the motor, and also the spider, which is part of the mechanical suspension. The voice coils are very long indeed, which leads to greater self-inductance, and also greater DC resistance and thus reduced sensitivity.
The alternative approach is to avoid the large cone excursion. This fundamental advantage gets lost in all the talk about -3B points and group delay. Group delay is accompanied by a time shift for output in the deep bass, relative to the rest of the spectrum, and becomes audible when, but only when, attenuation is severe. As long as the enclosure and port are tuned to adequately low frequency, such that you do not get the steep drop-off below the -3 dB point, it is not a problem at all. But there are other drawbacks. If the diameter of the port is not great enough, compression will occur, leading to reduced low-frequency output at higher volume levels. I.e., turn up the volume loud enough, and the bass disappears, and as it does so, the cone excursion increases. And this occurs regardless at frequencies below the -3 dB point, i.e., you still have high cone excursion and high distortion. There are other potential disadvantages that may or may not be significant, depending on who you ask: organ pipe effect and midrange leakage.
For medium-sized satellite speakers that rely on a subwoofer for deep bass, there is probably no significant difference between the two types, but it depends on the size of the satellite speakers, and also on whether equalization is used. With home theater speaker systems that use little tiny satellite speakers, the ported approach can be advantageous if used in conjunction with equalization, the reason being that you can use equalization to correct anomalies in the frequency response and extend the bass, while the port has the net, appreciable effect of reducing cone excursion, thereby reducing distortion. This same advantage, by the same reasoning, can apply to any speaker where porting is combined with equalization, whether the equalization is built in to the speaker as in the case of a powered monitor, or is accomplished by means of (ostensible) room-correction speaker setup in a modern home theater receiver.
Ported designs also make sense with very large speakers with very large woofers, since with these speakers, the benefit of reduced cone excursion still applies, as it invariably does. Most powered monitors are ported, because the driver itself does not displace sufficient volume to be capable of strong output in the deep bass. The required amount of excursion would be so great that high distortion would occur. With this type of speaker, you tune the enclosure and the port to a frequency well below the frequency that the alignment rules says that you should use, and then use equalization to correct the anomalies in the frequency response. This type of speaker is very smart from the additional perspective of the crossover, since by using separate amplifiers for each driver, the crossover is rendered immune from interaction with the complex impedance of the driver.
As a side story, in terms of sealed versus ported speakers, I first got into this hobby with acoustic suspension speakers. The first speakers I ever purchased were a pair of Cambridge Soundworks Model Six speakers. For those that are unaware, Edgar Villchur had a partner at AR when he released the AR-1, former student Henry Kloss. Kloss would go on to found KLH (he was the K), Advent, and Cambridge Soundworks. The CSW Model Six was/is (theoretically) loosely based on the old KLH Model Six. I couldn't qualify them as being great speakers by any stretch, but they were quite easy to listen to. On the other side of the coin though, I didn't find anything magical about acoustic suspension there either, and I've never caught myself pining for my old Model Sixes in spite of the fact that every speaker I own now is ported.
For any woofer optimal box sizes for a sealed and ported alignment are going to be very different. There are significant differences in driver selection and design for the two alignments.
I'll beg to differ on the 'lousy engineering' part - but you're certainly correct on the second half of your statement.
That is why we use a completely different variant of the Ultra 13.5" driver in our smaller SB13-Ultra subwoofer. That driver is optimized for operation in a sealed cabinet of that volume.
- We revised motor geometry (overhung) with a very high force factor (BL^2/Re). Extensive modeling and acoustic testing indicated that this is the preferred motor alignment for a sealed application in this size cabinet.
- We use a unique voice coil (bifilar wound, 3" diameter, aluminum, and with a longer winding height) optimized for a sealed application, resulting in higher power handling and excellent thermal management and heat dissipation.
- We also added a unique gap extension plate (nested in the top plate) for a more symmetrical force/displacement profile, resulting in lower distortion and increased linear stroke.
In the case of the PB13-Ultra, that variant of the 13.5" driver (which is completely different that the SB13-Ultra driver) is optimized for a large cabinet volume. As such, the performance remains very good, even in sealed mode. The PB13-Ultra driver is not terribly happy in a small sealed cabinet - but as you noted it performs very well in sealed mode in a large cabinet - and that's why we confidently offer this operating mode for the PB13-Ultra.
You'll note the very low Q knee and shallow roll-off profile. This minimizes phase change with respect to frequency, and the associated GD curve is very favorable. The PB13-Ultra sounds fantastic in sealed mode, and has plenty of output for mid-size rooms.
The sealed performance is respectable, but F3 is probably too high for HT enthusiasts who want to feel the explosions.
With respect to deep extension, the PB13U in sealed mode is only down about 7 dB @ 20 Hz, relative to the 40-80 Hz reference drive level. Due to the very shallow roll-off slope (something you won't often encounter in commercial sealed offerings), when used a mid-size enclosed room which exhibits a modicum of 'room gain' (i.e., the acoustic transfer function below the modal/pressure transition frequency of the listening space) the PB13U in sealed mode can/will exhibit remarkably deep in-room extension.
My personal reference HT room has rigidly constructed boundaries and is fairly tightly enclosed, and exhibits about 18 dB of room gain at 10-12 Hz. The PB13U in sealed mode in this particular room is flat to almost 10 Hz.
Further evidence of this can been seen in PBC's review of the SB13-Ultra (which has a very similar system Q and roll-off slope as the PB13U in sealed mode). Even in his mid-size and rather open/lossy listening space, the SB13-Ultra measured flat to 15 Hz in-room.
For any woofer optimal box sizes for a sealed and ported alignment are going to be very different. There are significant differences in driver selection and design for the two alignments.
Or a more balanced viewpoint is offering multiple tuning modes including the option to seal a ported design opens wider placement options and is in fact excellent engineering in doing so. The PB13-Ultra is this example one of engineered excellence as any owner or person that's actually heard one could vouch for.