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

If 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 ‘slower’.”

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.