Usher S-520 Review Addendum: Woofer Bottoming Out Issue
Usher S-520
We recently reviewed the Usher S-520 bookshelf speaker system and found them to be an uncommonly good value because of their fidelity, build quality, and aesthetics. If fact, they fared quite well in their price class in our most recent Bookshelf Speaker Face Off.
Just as we were gearing up to ship these speakers back to the manufacturer, Musikmatters – US Distributor of Usher Audio products, requested that we retest these speakers after replacing the woofers since we reported the woofers were susceptible to bottoming out during large bass transients in our face off review.
The Claim
According to Musikmatters - the coil that surrounds the phase plug can warp from excessive heat or power. Once this occurs the woofer sounds as if it’s bottoming out. What's happening is the woofer cone is not tracking properly and catches on the phase plug. The space between the plug and the coil is at a very tight tolerance.
They assured us that replacing the woofers would completely eliminate the problem - especially since they had two reported cases of such occurrences that were resolved in a similar fashion.
Our expressed opinion was that this was nothing more than a case of a small woofer attempting to play down too low at higher SPL levels and that the simple solution was to apply bass management. We wanted to give them the benefit of the doubt, however, and acquiesced to their insistence that we had defective woofers and that this problem would disappear if they were replaced.
I decided someone that passionate about their products deserves the benefit of the doubt so I had them ship me new woofers to retest for this bottoming out issue.
Set-Up
Using two bass heavy tracks, one from Fourplay titled “The Chant” and the other from Donald Fagen Morph the Cat CD “Security Joan”, I proceeded to conduct my listening and measurement tests on the original stock woofers to use as a comparison for the replacement woofers that were sent to me.
I placed the speakers on 30” stands about 5ft from my backwall. The tone controls on my receiver (Denon AVR-5805) were set to “defeat”. I then placed the mic from my Sencore SP395 Audio Analyzer 2 meters away from one speaker on axis at its acoustical center. I took two types of measurements; Avg SPL and Peak SPL, using C-weighting. I increased the volume level until audible bottoming out was clearly heard.
Editorial Note on the Sound of a Bottoming Woofer
The sound of a woofer bottoming can range from a snapping or popping sound, to a gross distortion and/or breakup of sound as the voice coil of the woofer attempts to exert beyond the magnetic gap. Prolonged driver excursions can ultimately stress the driver causing permanent damage and even burning out the voice coil if it cannot dissipate the heat. This is called blowing the speaker out. If you hear a woofer bottoming out during a listening session, it is strongly advised to turn the volume down, or bass manage the speakers to minimize its occurrence during loud listening sessions.
The Results
On the Fourplay track “The Chant”, I was able to bottom the system woofers out with an average drive level in the upper 80dB range two meters away once the peak meter hit around 95dB during transients of the kick drum. There was a clear sign of distress prior to the popping sound I heard. These little 5-1/4” woofers were being pushed too hard for their own good. I noted the master volume level on my AVR-5805 receiver so that I could set it to the exact same position with the new drivers installed.
On Donald Fagen's Morph the Cat track #6 “Security Joan”, I experienced similar issues at around the same SPL levels as I did with the Fourplay track. Once bass peaks hit in the mid to high 90dB range, again measured 2 meters away, the woofers bottomed out. This time the bottoming sounded very distorted, almost like someone was intermittingly connecting and disconnecting the wire behind the speaker. Again I marked the master volume level as a reference to compare once I had the new drivers installed.
I established the minimum volume levels on two separate sources to cause the Ushers to produce audible distress from the woofers. It was time to see if the new woofers offered an improvement to this condition.
I got my hex screwdriver and began pulling the old
woofers out. I was surprised to see no
rubber gasket between the rim of the woofer basket and the cabinet. In their defense, I have taken apart a few
speakers in this price class all of which didn’t feature a woofer gasket. Although I don’t consider it a show stopper
by any means, it is a great and relatively cheap preventative measure of further
reducing driver resonance caused by vibrations of the frame against the
cabinet.
I got the new woofers installed in a jiffy and I liked how Usher utilized different sized mating connectors on the speaker wires to ensure the assembler makes no mistake on driver polarity. We’ve gotten speakers in for review from other companies that had drivers wired out of phase so it’s nice to see a manufacturer take an extra step to minimize this occurrence.
I set up the speakers once again on my stands and began the listening tests. As I turned up the volume to the reference levels I established before for both sources, I achieved identical results. The woofers were bottoming at the same drive levels and to the same extent. Thus, the woofer swap didn’t make a difference. They were still bottoming out with bass heavy program material when being run full range and moderately loud listening levels.
Conclusion
Simply put, the woofer bottoming issue has nothing to do with improperly tracking woofer cones but more to do with a limitation of the woofer and the application it is being used for. Phase plug drivers certainly have their advantages (reduced cone mass for higher frequency extension, reduction of on axis beaming), but they do pay one penalty, driver sensitivity which is largely due to leakage caused by the air gap between the cone and phase plug. Combine a lower driver sensitivity, a small 5-1/4” woofer, with system tuning a speaker in the low 50Hz range such as that of the S-520’s and you’ve got a bookshelf speaker with a nice warm bass sound character that simply cannot play at loud SPL’s in reasonably large rooms without bottoming out. By weight of comparison, I am currently reviewing a similarly priced and sized speaker system from another company whose 5-1/4” woofer was not nearly as susceptible to bottoming out under the same or even higher drive conditions. Both speaker systems measured the same sensitivity at one meter, but the later speaker was tuned at 60Hz and didn’t utilize a phase plug driver.
When an engineer designs a bookshelf speaker he/she must decide if they wish to sacrifice loudness for bass extension. My personal opinion here is to design a bookshelf speaker that won’t bottom out as easily, allowing it to be played at louder SPL levels, but ultimately producing less low end bass extension. The user can than bass manage the speakers and mate them with a good subwoofer or two to get the full range sound without stressing the system's woofers, thus preserving dynamic range and vocal clarity.
I can certainly see the appeal for making a small speaker produce full range sound. By itself in a small room at moderate listening levels, most listeners will usually prefer a speaker system with lower bass extension (all other things being equal). This is a design trade off that the buyer must be aware of when choosing a speaker to meet their specific needs.
It’s important to note that our opinion of the Usher S-520’s is unchanged from our two very positive reviews. We still feel these are some of the finest sounding and looking speakers in their price class provided that they are used within their design limits specified in our reports.
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This implies that if the speaker with its bass extension is similarly combined with a subwoofer, that full range sound without stressing the woofers cannot similarly be achieved, and that dynamic range and vocal clarity cannot similarly be achieved. This does not make a whit of sense to me.
It seems to me that with a small speaker of this size and its itty-bitty woofer, that it is manifest that there will be consequences if it extends down below a couple hundred Hz. My intuitive expectation is that the woofer will either bottom out or else exhibit very high levels of harmonic distortion starting at SPL much lower than the level at which the woofer bottoms out. The question becomes whether you design the speaker so that its natural response curve rolls off before it gets into deep bass, vs. design it so that it will extend into deep bass at low levels but exhibit high levels of distortion when played at moderate levels, or design it so that it will extend into deep bass at low levels and assume that owners that want to turn it up to eleven are familiar with the operation of the bass control on the amplifier or the receiver. Evidently, Gene has issues with tone controls, and feels that rather than make it necessary for him to turn down the bass when he wants to crank it up to eleven, the manufacturer of the speaker should design it so that the response curve rolls before it gets into deep bass, even when played at low and moderate SPL.
Basically, it was turned up way high, and instead of exhibiting a steady increase in harmonic distortion, it continued to behave in a linear manner up until it bottomed out, and this is as bad thing?? Perhaps the reason why it is looked at this way, by the person having this complaint, is partly that they don't actually bother to take measurements of distortion and study how those measurements interact with other factors.
Many speakers do not bottom out no matter how high they are played, but you have to ask why this is so. If the force applied to the voice coil increases linearly, should not the displacement of the coil increase linearly as well? If it does not, then doesn't this imply non-linear distortion? In most speakers, one or both of two things will happen to prevent the coil from reaching the hard limit of its travel. In the ideal, the surround and the spider behave as ideal springs (and dampers), which means that there is a linear relationship between displacement and force. In reality, as they are stretched to their limits, the ratio of force to displacement increases, i.e., doubling the force no longer doubles the displacement. Additionally, and contrary to what I often read, overhung voice coils do not ever actually overhang the gap, because of all the stray flux. This is especially true when the magnet assembly is joined to a basket made of stamped ferromagnetic steel. The strength of the magnetic field experienced by the voice coil diminishes as it moves away from the midpoint of the gap, and as it does, the ratio of force to displacement changes. The electromagnetic force weakens as the coil displacement increases, the effect of which is equivalent to the opposing force (due to the surround and spider) increasing in a non-linear manner as the displacement increases.
These non-linear effects conspire. The net effect is that instead of the driver hitting a hard bottom, it hits a soft bottom, and it starts to do this long before the hard bottom would otherwise be felt. In practice, it is these distortion-inducing effects, and not the intentional application of high-pass filtering, that prevents most speakers from bottoming out. The reason they do not bottom out, in other words, is because they are designed in a way such that the distortion increases gradually from a moderate SPL and increases more rapidly than the SPL. It is analogous to "soft-clipping" circuits, which some people like. You can design an amplifier so that it will exhibit "soft clipping", but the price paid is invariably that the level of non-linear distortion will increase dramatically and before the hard clipping level would otherwise be reached. It is a dumb idea, and a better idea is to put idiot lights on the front of the amp so that audiophiles who actually are deaf because they always play stuff too loud can tell when the amp is clipping. By analogy, it makes sense to think of the woofer bottoming as an idiot light of a similar sort. The alternative would be to alter the driver so that it is inherently inferior and so that instead of bottoming out, it exhibits distortion that a good many audiophiles are evidently incapable of hearing. A loudspeaker review that does not at least attempt to measure flatness of frequency response and non-linear distortion is about as useful as human excrement, and the reason, for the lack of meaningful insight into the reported behavior of this woofer, is due in no small measure to the fact that no meaningful testing of any sort was done with this speaker. All they actually did was open it up and take some pictures, and offer some purely subjective and inherently worthless opinions on the quality of its sound. Well, given that they are selling these speakers and that they represent themselves as providing a greater service (which they do), I suppose that's the least they could do for us.
Matt34;594571
The audioholics store has them for $400 with free shipping.
Nice! I totally missed that...now where can I put another pair of speakers...
Matt34;594571
The audioholics store has them for $400 with free shipping.
Hey Matt, did you listen to them personally?
droht;594566
I think the Ushers are $479 now, but still may be the best bang for buck speaker out there. They come up on the used market fairly often for <$325. Hard to beat that.
The audioholics store has them for $400 with free shipping.
