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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Why are ported designs ubiquitous? posted by Peter on December 22, 2009 at 18:19:36:
Peter,
This is from Roy Johnson, the designer for Green Mountain Audio speakers. Allow me to share a perspective of many years' experience in retail and then in loudspeaker manufacturing.
During the 1960s, most woofers still required large boxes, ported or 'infinite baffle', even though Acoustic Research (AR) had already shown what was possible for a small sealed box. As the decade progressed, we saw KLH, KEF, Leak, and finally Advent validate Villchur's work (of AR) on sealed boxes (I am sure to be leaving out some).
At the end of the `60s, the impact of stereo on the (USA) market encouraged firms to make better use of the now-available ported-box and sealed-box formulas, in order to produce more 'bookshelf'-size models (which were less expensive, since they were smaller). JBL chose to produce their ported 8" 2-ways and then their ported L100 model, to satisfy their goals of sensitivity and max SPL. For firms entering this expanding USA marketplace, that generally meant they designed around sealed boxes, for their smaller size and audibly better bass-tightness-- models from Infinity, Celestion, Mitsubishi (Diatone), Yamaha, Dahlquist, and Epicure (and Bozak?), followed later by ADS, Allison, and Genesis, to name many of the largest-selling manufacturers.
During the early `70s, we also had the influence of Bose's 901 with its active EQ, the Servo-Statik's bass-module design from Infinity, and JBL's 212 3-piece system. By the mid `70s, passive radiator design was also understood and a few companies went in that direction, at least with some of their models.
However, during both the 1960s and `70s, I witnessed that a speaker's bass tightness affected someone's purchase decision much less than did the speaker's overall tone balance and its clarity, as those were where the large differences lay. Imaging was something yet to be sought out (other than by those already exposed to Quads or other electrostats).
In the late `70s, imaging became more important thanks to the efforts of Harry Pearson and J. Gordon Holt. To that end, we started to see slimmer cabinets, taller too, which then made speaker stands unnecessary (not even used until the mid to late`70s)
By the end of the 1980's, home theater became possible, and its growing market influence caused many different speaker-solutions to be offered. At first, the goal of a manufacturer was to produce very-small boxes, which had to be ported to produce sufficient bass (regardless of its quality). Later, tall towers with multiple smaller woofers (usually ported also) were offered as the logical step-up from small boxes, at least to the majority of audio customers. Here I am writing only of what most of the USA public could readily hear on the market, as regular stereo-stores were quickly being replaced by mega-stores, where brand, price, and 'new technology' counted for as much or more than 'the sound' itself.
When Dolby 5.1 finally took hold in the '90s, we saw the introduction of subwoofers-- even less reason to buy 'big' speakers, again, as far as folks who spend well-under $1000/pair for any one pair of speakers-- the sales which drive the largest companies such as Infinity, B&W, and Paradigm.
Today, the concept behind using multiple small woofers falls away with the more expensive audiophile speakers, and we see most designs rely on only one woofer in the cabinet. The reason to go back to one woofer in the pursuit of higher fidelity is rather simple, as Presto has pointed out below: One large woofer can be made to have a frequency response that goes lower, naturally.
This is because one can only make any woofer's suspension so soft before it falls apart, and therefore a small-diameter woofer does not have a heavy-enough cone to produce a naturally-low self-resonance (unless you want to throw away a lot of sensitivity by using a really heavy cone/voice-coil assembly on that small woofer, which is what Phil Jones does).
And that one larger woofer may still be ported, because that does increase the speaker's overall sensitivity-- not because of that port, but because the port keeps a woofer from stroking, and this means a woofer's voice coil does not have to be wound as long. A short voice coil weighs a lot less, reducing a woofer's moving mass by a significant amount, lifting the woofer's output by several dB.
More sensitivity is now important to two types of designs-- ones that use the 'high-tech'-appearing higher-order crossover circuits and those that use heavy plastic cones. High-order crossovers, and frankly, ANY passive circuit crossing over below 200Hz, eat up a lot of the woofer's signal, which reduces sensitivity (and damping factor, which is why many require an amp with a big power supply).
The idea of a ported design perhaps being more 'open-sounding' in the middle ranges, as Duke suggests, is I think related to his woofers also handling the voice range, where any reduction of stroke on bass notes is a good thing. Just speculating-- Hi Duke!
Anyway, after designing many sealed and ported designs for several decades now, I can say that sealed always sound better, especially if the crossover circuit is not getting in the way, if the boxes for both are constructed equally well, and if the woofers themselves have the right kind of suspension. Remember, we are asking a sealed woofer to stroke farther and for that, 'the right kind of suspension' is one that allows clean movement on very small signals and on the very large ones. The latter is now easy to achieve, while the former is offered by only a handful of woofer-driver manufacturers, and expensive, such as those found in Scanspeak's line.
So why do we not currently offer a sealed design? Because in today's 'medium-price' high-end market, size still matters, and no one is purchasing a physically-wide speaker, especially in Europe. So my current models are ported with the lowest useful Q. The sonic penalty is now small, compared to what I hear from the best sealed-box design: The port makes the lowest bass a little 'sloppier'-sounding, for any type of bass impulses that happen NEAR the port's tuning frequency. So best to tune the port as low as possible, to get it out of the way of as much of music's tones as possible.
The overhang is not so much a matter of a longer 'group delay' in the bass of a ported speaker (which is a complete misnomer for what is happening with the port's output), as it is the Q of the port's tuning-- which describes how much overhang will be produced by the port after each note that occurs near its tuning frequency.
While that overhang can be made small, it is still audible, but far more audible is what happens to tones near the port's tuning frequency: By definition, ANY resonance always drags nearby tones up or down to its frequency AFTER the input tone (stimulus) has stopped. So, pitch definition can suffer (the dreaded one-note bass being the most flagrant example) along with the pace and rhythm of the performance. Again, a reason to keep the port's tuning as low as possible.
But a port tuned too low makes a little woofer stroke quite far on its way down to that port frequency, and also its output will dip before the port kicks in (because you are listening ten feet way). And that makes speaker-placement more critical, to fill in that middle-bass tone-range with suitable reflections off the wall behind.
Multiple small-woofers 'fix' that issue and provide mucho impact (from their greater 'radiation resistance'- a good thing), but at the expense of low-level signal resolution (caused by each woofer moving less which exacerbates the natural 'stiction' of their suspensions, whihc prevents their cones from moving properly on small signals).
Multiple woofers also create unusual standing wave patterns in a room (from their radiation patterns being different at each frequency). They also change in tone balance relative to the midrange as you move away from them (because their radiation patterns at each frequency do not match that of a single mid).
So, its a big complicated mess, and for any speaker design to be small in footprint, we are not likely to see a resurgence of sealed woofer boxes. I hope I have not created more questions, but let me know if I can explain anything more clearly.
For the record, I do agree in large part with several folks' experiences and comments below, including those of Presto, Duke, Morricab, caspian, Audioquest, hahax, and John Marks. Thank you for your contributions to this thread. Perhaps the one AA thread that has not deginerated to name-calling!
Best regards,
Roy Johnson
Designer
Green Mountain Audio
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Follow Ups
- Evolution- sometimes 'sideways'... - RoyJ 17:30:22 01/14/10 (0)
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