Usher S-520 Review Addendum: Woofer Bottoming Out Issue

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.