|Have any of you been paying attention to the current trends in larger multi-woofer speakers that cost multiple thousands of dollars? So that many of you can follow along, i'll use the Legacy Focus 20/20's at $6K, the Piega C8's at $15K and the Aerial 20T's at $23K as points of reference. All of these have been reviewed in Stereophile over the last few months. If you're not familiar with these, all of them are vertical dynamic designs using multiple woofers in vented cabinets. |
If you look at the response of of these speakers, they all have very pronounced bass peaks with elevated low frequency plateau's taking place. Of these three, the Legacy's are by far the worst of the bunch. Not only do they diverge from neutrality the most ( +7 dB peak @ 100 Hz ), their elevated bass output or "low frequency plateau" levels out at 40 Hz and at 400 Hz. That is over 3+ octaves of "extra" output that wasn't on the recording. Above 400 Hz, the output levels off with very noticeable rippling slightly above that point in the midrange and multiple large peaks with a dip up in the treble response. Below 40 Hz, the output drops like a rock. The reason that the plateau levels out at 40 Hz is because of the associated sharp roll-off associated with vents below their point of resonance.
To sum things up, this speaker, which Paul Bolin raved about in Stereophile, is anything but "smooth" or "linear" in reproduction. As can be seen in the graphs, there is a very definite "boom & sizzle" type of response taking place here. As a side note, i found that the Legacy Signature III's showed a similar large bass peak centered at appr 100 - 110 Hz, so this would seem to be a consistent design attribute / "house sound" / "family voice" to Legacy speakers.
Moving onto the Piega's, their overall response looks to be measurably smoother than the Legacy's from the midrange on up. As far as bass goes, the Piega's peak occurs at an amplitude of +5 dB's and is centered at appr 85 Hz. Their "bass plateau" is quite wide, actually just as wide as that of the Legacy. Both show the same appr "elevated output" aka "bloat" from about 40 Hz to 400 Hz. Much like the Legacy's, the Piega shows the typical sharp roll-off below 40 Hz due to the output of the vent being out of phase with that of the undamped woofer. Even though both speakers show very similar plateau's and a similar F3 ( -3 dB point ), the Legacy's bass plateau has both a higher peak and a higher average.
Moving up to the $23K price range, we've got the Aerial 20T's. Similar to the Piega's, the Aerial's are reasonably smooth in response from the mids on up with a few low amplitude peaks and dips. Side by side comparisons though, it would appear that the Piega's are a little "flatter".
When it comes to low frequency performance, the Aerial's produced a +5 db peak centered at appr 60 Hz. Of the three speakers mentioned here, the amplitude of the peak is the same of the Piega's ( +5 dB's ), which is much lower ( 66% reduction ) than that of the +7 dB peak of the Legacy's. Even with this 66% reduction of the peak amplitude at resonance compared to the Legacy's, we are still talking about a divurgence of +5 dB's here!!!
As far as the "bass plateau" goes with the 20T's, this speaker is much more linear than either of the above. While the Aerial's also level out at appr 40 Hz and drop like a rock below that point, the upper end of the bass region is MUCH smoother. Whereas the others were contributing added output up to appr 400 Hz, the Aerial's are leveling out at appr 120 Hz or so. In effect, the Aerial's appear to offer the most controlled bass with the least amount of bass colouration. Then again, they are by far the most expensive also.
As far as low frequency extension is concerned, the Aerial's resonance peak is centered the lowest of the three i.e. 60 Hz for the Aerial's vs 85 Hz for the Piega's and 100 Hz for the Legacy. Even though the Aerial's have a resonance that is 25 Hz below that of the Piega's and 40% lower in frequency than the Legacy's, all of their -3 dB points are within a very few Hz of each other. While the graph's aren't completely legible, it appears that the F3 ( -3 dB point ) for all of these speakers are right about 34 - 38 Hz or so. How do such different designs achieve similar F3's? It has to do with the tuning of the vents and the amplitude of the peaks at resonance.
By creating a huge peak at resonance, it takes longer for the amplitude of the signal to fall off. As such, the Legacy's much larger peak at resonance allows it to achieve appr the same F3 on paper that the other designs worked harder to achieve. As such, were the Legacy's designed this way because they like the sound of massive bloat? Were they designed this way so that they could claim a lower F3? Could it be a combo of the two? We'll probably never know.
What does all of this add up to? Judged in comparison to each other and strictly talking about bass linearity, the Aerial looks the best on paper by far. Why just on paper? Because we have to factor in the added gain associated with in-room response. Our ears hear the entire presentation i.e. the speaker and how the speaker loads up / pressurizes & excites the room. As such, what looks the best on paper may not be what you like the most in your room. If you're room is properly set-up, the results on paper and the results in the room should pretty well jive. That is, at least as far as frequency response & linearity go. There are a LOT of other factors going on here though, not to mention personal preference.
What happens if the room isn't properly set up? Compared to anechoic responses, all speakers will have greater output / added extension when placed in an average listening room. While specific speaker placement comes into play in terms of the extension and amount of boost, most rooms will produce maximum ouput somewhere in the 50 - 80 Hz range. Obviously, this varies with the size and shape of the room.
The net effect is that these speakers are going to produce even MORE bass than what they already show in these graphs. Not only are we picking up low frequency output from what is called "room gain" ( "cabin gain" in a vehicle ) by pressurizing the room, we are also going to be exciting the resonances of the room too. All of this adds up to GOBS more "apparent bass". Add in the fact that this bass lacks speed and control* and you've got "bloated, ill-defined thump" running rampant.
Other than that, one has to wonder just how extended the bass response of these designs would be if they didn't have such HUGE peaks? After all, the higher the peak at resonance, the lower the -3 dB point of the speaker appears to be. Do we have to add "bloat" to get extension? How do you get around all of this and still keep good sound? That's easy but it is a completely different subject : )
What i want to know is, what do you folks think about this type of performance at these price levels? Is there anything that we can learn from this? Do we see a specific trend taking place here and in other parts of the audio market? Inquiring minds want to know : ) Sean
* vented designs all suffer from a lack of transient response, increased ringing, over-shoot and phase problems. In this respect, a well designed port is typically "more linear" than a passive radiator.