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Identifying Legitimately High Fidelity Loudspeakers: The Tweeter Resonance Frequency

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One of the ways in which the designer may choose to create a smoother system response is by lowering the resonant frequency (Fs) of the tweeter.  Since we all loathe to add mass to a tweeter for fear of killing the top end, the best way to achieve this lowering is by using a softer edge compliance.  However, even with the edge very soft, the air behind the diaphragm will exert a stiffening force on the tweeter (just like the sealed box does for the woofer).  For this reason, some designers will put a hole in the pole-piece, and enclose the back of the tweeter with a plastic housing to allow for a much lower resonance. 

The holes in the tweeter’s neodymium coin magnet/back plate have to be reasonably large in order to engage the air behind it to reduce the stiffening effect of that air.  Using a thicker neodymium magnet or doubling up on a standard-thickness magnet will compensate nicely for the loss of magnetic strength that results from the holes.  Also, making that rear chamber out of, say drawn/ribbed aluminum instead of plastic will make the entire rear chamber’s surface area function like a heatsink and add very significantly to the tweeter’s power-handling ability. There is no easy, inexpensive way to do a good low-resonance frequency tweeter. You need a larger, more compliant surround for the longer excursions needed at 1500Hz compared to 3000Hz, and you need to have very robust power-handling/heat dissipation, since a tweeter tasked with reproducing full-strength signals at 1500Hz receives a far higher voltage drive through the crossover than a tweeter that only operates from 2500 or 3000Hz on up.  It costs real money to make such a tweeter design which is why you typically only find them in the very highest caliber products.  That said, all low Fs tweeters aren’t created equally.  In speaking with a prominent loudspeaker designer, he was pretty vocal about his views that most low Fs tweeters he has measured didn’t have sufficient power handling, or motor or suspension linearity to handle the excursions needed to play below 1kHz.  Though he admitted his testing was limited to metal dome designs and most designers would typically not cross a tweeter over that low to begin with.

Most designers agree crossing the tweeter over 1 octave above its Fs helps produce the smoothest response.

While making the edge of the tweeter softer will often create issues of frequency response at very high frequencies, lessening the back chamber air pressure carries very little cost save for that of weight, and some magnetism loss in the circuit. (Caused by the loss of material in the pole-piece.)  What you will find when taking apart tweeters made old school, is that the designer is placing damping material behind the diaphragm, very much like the treatment of the inside of a woofer box.  Reflections from the metal, are very close to the dome, so with a wavelength short enough, a potential problem can be avoided by use of felt or foam.

At many of the of the speaker industry’s major companies, the prevailing design approach has been that it’s best to use a driver at least an octave above its resonant frequency. This tends to be the frequency range where its response is the smoothest and its distortion will be its lowest. The head of design at one company went so far as to mandate that tweeters were “always down 18dB at resonance.” In other words, when you ran the voltage curves of the crossover, the High-Pass section curve would have its curve be 18dB below “0dB” at the tweeter’s resonant frequency. Granted, this made for some pretty high tweeter crossover points and almost totally prevented the use of first-order HP sections, but as that company’s Design Head often boasted, “We hardly ever replace a burned-out tweeter under warranty!” 

That could certainly be seen as an unrealistically conservative approach that actually had some negative performance implications for two-way speakers in terms of dispersion and midrange beaminess, but many respected designers say that 12-15dB down at resonance works really well with very little real-world thermal danger to the tweeter (in higher-end systems that will be used with good-quality amplification).

Scan Speak 9500          Dayton Tweeter

 

Scan Speak 1” Silk Dome Tweeter with Rear Chamber (left pic);  Dayton Audio DC25-T-8 Tweeter (right pic)

 

Tweeter

Power Handling

Fs

Price

Scan Speak 9500

150 watts

500Hz

$150

Dayton Audio DC25-T-8

50 watts

1659 Hz

$17

The above example comparing the Scan Speak and Dayton Audio tweeters is to illustrate the benefits a rear chamber have on both lowering resonance frequency and increasing power handling.  This is NOT a knock on the Dayton Audio titanium tweeter as its design goals are for a more budget minded system.  It does however show you that the type of design executed in the Scan Speak example, typically doesn’t come cheap.  You can’t put a pair of $150 tweeters in a bookshelf speaker retailing for $400 but you can and should expect a pair of $3k bookshelf speakers to utilize a tweeter of this caliber.

Stweet_shallowome manufacturers have great success implementing a shallow horn on the tweeter instead of a conventional flat faceplate.  The advantage in doing so extends the low frequency output of the driver to better match the directivity of the midrange.  It also means the tweeter can be rolled off earlier, thus increasing power handling.  However, there are no free lunches and the designer needs to be mindful that doing so does NOT lower the resonance frequency of the tweeter because the horn typically unloads below the resonant frequency. Thus, care must be taken NOT to lower the crossover point within the region of the drivers Fs.  Also another potential drawback of employing a shallow horn is it can reduce the high frequency dispersion because the tweeter is now "sunken" into a cave of sorts, so the ultra high frequencies get blocked off-axis.  This is why the best designs have a much shallower cavity than the example shown here.

Bottom Line on Tweeter Resonance Frequency:
The tweeter resonance frequency is a critical metric in determining where to set a loudspeaker’s crossover frequency.  Budget minded designs tend to utilize tweeters with a higher Fs and thus have to rely more on the woofer to handle the critical upper midrange frequencies.  Cost no object systems typically use more elaborately designed tweeters to lower Fs, increase power handling and open up more design flexibility options to improve the transition between the woofer and tweeter for a more seamless blend.

Conclusion

There are always trade-offs in loudspeaker design approaches in all but the very most elaborate and typically most costly designs.

Cast basket drivers enjoy a multitude of advantages over drivers that employ stamped-steel baskets. As we’ve shown, there are advantages in rigidity, precise alignment of the driver’s moving parts, resistance to shipping damage, and even improved aesthetics, for what that’s worth.  The biggest disadvantage is the initial tooling expense, which makes it difficult for smaller speaker companies to absorb. But with the large number of independent driver suppliers who offer cast-basket drivers in their lineup, there is really no reason why any speaker company with high-end aspirations can’t use cast basket drivers. 

There are many benefits to venting woofers such as reducing mechanical noises and distortion by reducing the air pressure build up at the dustcap and arguably increasing power handling by better managing heat. Venting is essential for bass drivers and beneficial for midrange drivers as well.  The downside to a vented pole piece is you have to effectively add more magnet power for the loss caused by the drilled out hole in the backplate.  This of course adds cost.  Alternatives to rear venting in woofers are punch holes in the former/cone, phase plug physically connected to the pole piece and coupling an aluminum former to an aluminum cone.  Affixing an aluminum voice coil to the diaphragm can act as an effective heatsink but it doesn’t address the potential of mechanical noise build up due to too much air turbulence under the dustcap.  Employing a phase plug helps to reduce cone mass and on-axis beaming and acts as a heatsink more than it does a vent. However, as a result of the lost cone area, these drivers tend to be at a disadvantage for producing ultra low bass frequencies, hence why they are typically more popular to use as midrange or midbass drivers. 

Horn loading a tweeter does NOT lower Fs, because the horn typically unloads below its cutoff frequency.

The benefits of exotic diaphragm materials are a slightly less clear-cut situation. There are numerous materials that can deliver excellent performance, and the selection is based more on the specific system goal at hand, rather than an overriding “this is always better” scenario.  How the designer deals with the actual damping of the driver to result in a more gentle break up mode (preferably above the crossover frequency) is truly a paramount metric in determining its sound quality. 

Lowering the resonance frequency of a tweeter by venting it and coupling it to a rear chamber has many benefits, such as wider midrange dispersion, improved high frequency power handling and smoothing out system frequency response.  The penalty of such designs is added cost which cannot be justified in budget speaker systems.

Getting a peek at the guts inside a speaker system you are considering purchasing can tell you a lot about the budget allocated towards the drivers in the design.  If the manufacturer doesn’t supply such images, it doesn’t hurt to ask.  If not, request a review from us of the speaker system you are interested in as we always dissect the products we review.  Manufacturers that truly stand behind the quality of their products love our review approach.  Those that don’t, well, they typically don’t submit review samples.  Better parts truly can yield better performance in the hands of a competent designer which are more common these days with the advent of inexpensive measurement equipment and knowledge of the basics in loudspeaker mechanics 101.

Acknowledgements 

I would like to personally thank the following people for their contributions and/or peer review of this article, all of whom are true experts in their respective fields. Their contributions enabled us to make the most comprehensive and accurate article possible on the very complex topic of loudspeakers cabinets dealt with herein.

  • Paul Apollonio, CEO of Procondev, Inc
  • Philip Bamberg of Bamberg Audio
  • Steve Feinstein, Audio Industry Consultant
  • Tim Galdwin, Loudspeaker Driver Engineer for Warkwyn Associates Inc
  • Ed Mullen, Directory of Technology & Customer Relations of SV Sound
  • Shane Rich, Technical Director of RBH Sound
  • Mark Sanfilipo, Audioholics.com Resident Speaker Expert and Writer
  • Dr. Floyd Toole, PHD & Chief Science Officer of Harman

 

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Recent Forum Posts:

phaser78 posts on August 12, 2021 13:05
Your article is great…after years of searching for hi-end sound on a budget i think i've learned that price isnt always a guarantee of quality construction! Every time i bring someone into my living room to play vinyl they always remark about how great my system sounds and they oooh and awww over the marantz (or the other good gear: rotel, technics, tascam) but nobody gives the Realistic “Optimus 400” speakers any of the credit…in my opinion, the Optimus 400's are MUCH better speakers than anyone gave them credit for…i refoamed mine when i got them and i was flabbergasted to see what the build quality is like: internal bracing, lots of damping material, separate midrange enclosure, cast aluminum baskets, super heavy magnet structure and that amazing planar tweeter…and they are gorgeous with real walnut veneer! i think this was a magical speaker that was manufactured at the right place/time with good materials and just never got the credit because it was Radio Shack competing with all the big US names at the time but these speakers run circles around any “rack system” speakers of that period from Pioneer, Kenwood etc..they easily compare with AR, Polk, Advent of the same era but audiophiles wouldnt be caught dead shopping in a Radio Shack! 49713
Ponzio posts on July 29, 2020 08:29
Regarding resonance issues with AMT tweeters, do you believe the RAAL tweeters fall into that category Gene?
ajemeditor posts on July 28, 2020 23:41
Gene- what about cast polymer baskets ? Presumably they will have the least resonance, and yet with mOderN chemistries, be as or more stronger than aluminum cast ? I think boers and Wilkins highlights polymer as a feature of many of their drivers (also who makes their own drivers other than Focal?)
gene posts on January 03, 2016 15:09
Check out our recently added YouTube video discussion on Loudspeaker Drivers with Shane Rich, Technical Director of RBH Sound.

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Mark of Cenla posts on September 22, 2015 15:44
Thanks for the informative article! Peace and goodwill.
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