Editorials

Speaker Isolation, The Tweak That Actually Works

by Chris Eberle

Close up of speaker feet at bottom of speaker

n HiFi, many tweaks claim to improve sound quality, but the isolation of speakers and their component parts is one that truly makes an audible difference.

Audiophiles know from their research that there are countless devices, components, bits, and bobs (some, rather creatively marketed) that claim to improve the sound quality of our systems. From cable elevators to little metal bowls, enthusiasts have sought to find ways to make music reproduction more realistic. We can pour money into electronics and advanced speaker designs but sometimes, seemingly minor details can make a huge difference.

I am not typically one to promote the use of special objects in the listening room, but isolation is a concept that has real science and logic behind it. And it’s something I have recent experience with. Almost by accident, or perhaps by fortuitous timing, I have found a simple and inexpensive way to improve bass balance and control in my speakers. I’ll talk about my application shortly but first, some background and context.

What Is Isolation?

When I use the term isolation, I am referring to the literal usage. Many speaker designs are based around the isolation of their component parts. The drivers are isolated from the enclosures. The front baffles are isolated from the rest of the enclosure. The enclosures are isolated from the floor. Why is this good? If we remember that sound is no more than the movement of air from a source to our ears, then it follows that when vibration is precisely controlled, the sound will be better.

When a driver cone or ribbon is excited by electricity, it vibrates. The moving cone or ribbon pushes air, hopefully in the direction of our ears, and we hear the result as sound. Obviously, controlling that vibration is the key to more realistic sound reproduction. We can have super precise drivers and a crossover built from premium components but that’s not enough. The driver itself must be physically restrained, otherwise more than just the cone or ribbon will vibrate. Remember that Newton guy? He said, “Every action has an equal and opposite reaction.” If you don’t lock that cone down, the cage, motor, magnet, and everything else that makes it up will vibrate too. Driver cages aren’t engineered to reproduce sound. So, you don’t want them spoiling the party.

Most speaker designs bolt the driver firmly to the front baffle. But this introduces another variable. Now, the baffle will want to vibrate. The energy from the driver’s cone must go somewhere. Good speakers combat this with a thick baffle, usually made from MDF or some other heavy inert material. But what if the driver had some way of dissipating that energy before it reached the baffle? Hmm, I think we’re on to something.

But we’re not done yet. The enclosure is a major source of unwanted vibration. The best way to deal with this is mass and structure. Good speakers have enclosures with internal bracing made from heavy material. The enclosure should be resonant inside but not to the point where it vibrates on the outside. Sound from the back of the driver cone will resonate inside the enclosure so the best scenario is one where the enclosure itself cannot vibrate.

The last part we need to control is where the enclosure meets the floor, or the stand if it’s a bookshelf model. Feet are an important and often overlooked aspect in speaker design. But audiophiles already have lots of options there. You can get better rubber or silicon feet. There are two-piece designs like the ones from IsoAcoustics that prevent any remaining enclosure vibrations from reaching the floor. Some people like spikes for carpeted floors. The effectiveness of spikes is a hotly debated topic and I won’t take up that argument here. I’ve used them in the past but right now, I like big soft feet and I’ll tell you why shortly.

paradigm founder 120h floor standing speaker

Examples of Isolation in Modern Speaker Designs

A few of my recent experiences are what led me to write this editorial. First was Paradigm’s introduction of the new Founder Series of speakers. These are ground-up designs, not evolutions of existing models. And they feature isolation as a primary design focus. To start, their drivers are not directly bolted to the baffles. Rather, they are isolated from the material by elastomer mounts. That means they are only in contact with the baffle via soft, vibration-absorbing parts. This prevents any energy generated by the driver cones from vibrating anything but the air around them. No energy is transferred to the baffle.

Paradigm then isolates the baffle from the enclosure in the same way, with elastomer mounting. Any sound resonating inside the enclosure is controlled by extensive bracing which keeps the enclosure from moving any air of its own. Then the enclosure is isolated from the floor by two-piece feet. The end result is that sound is produced only by the movement of the driver cone and nothing else.

Dissected view of PSB Synchrony T600 floor standing speaker

PSB takes the same approach with its new Synchrony Series. The drivers are decoupled from the baffle; which is decoupled from the enclosure; which is extensively braced. And the feet are two-piece components from IsoAcoustics.

Not only did I learn about isolation from researching these products, but I also experienced two examples from PSB, the T600 towers and B600 bookshelves. Both impressed me with their incredibly precise bass control and overall tonal balance. The T600s were especially nice with their full range capability, and I was able to enjoy bass without the bloat. Despite not using room correction, they did not excite the modes I usually hear in my primary listening space. This tells me that the modes were being triggered by uncontrolled vibration, not just a particular frequency.

How I Addressed the Issue

Now we arrive at the point of my discourse. What can we do if we don’t want to buy a new pair of Paradigm Founder 120Hs or PSB T600s? Believe me, after hearing the T600s, I wanted to buy them. But perhaps I could try and emulate one aspect of their design, the feet. PSB uses IsoAcoustics Gaia II feet that retail for $300 a set. I could buy these for my Axiom towers but that would mean spending $1,200 for my speakers in two listening rooms and another $600 for my subs. Instead, I picked up SoundPath feet from SVS that run $50 a set. Now, they are not as fancy or mechanically complex as the Gaias two-piece design. The SoundPath feet are large elastomer donuts with a metal flange and large machine screws that bolt onto anything with a threaded fitting in it. They are marketed as subwoofer isolators, but they’ll work with any large speaker.

I bolted them to a pair of Axiom LFR1100s and a pair of Axiom M80s along with my EP800 and EP350 subwoofers. I also found silicon half-spheres in different sizes on Amazon and stuck them under my center channel speakers, an Axiom VP180 and a VP150. I was immediately struck by what I didn’t hear, namely, the room modes. I have found that I prefer to avoid room correction, no matter how good the technology because it takes away some of the liveliness from the sound I experience in my room. Even though I experienced some bass bloat, I enjoyed the midrange more and was willing to live with the occasional boominess.

Adding the feet was almost like dialing in a response filter. The bass modes weren’t completely erased but they were so much quieter that I quickly stopped noticing them. If you want to try this for yourself, cue up the nightclub scene in the first Matrix film. It has a pounding bass and drum track that will expose poorly controlled bass no matter what your room or system is. Bolting on the feet made this scene far more watchable and I could now hear Neo and Trinity’s conversation without straining.

The Logic Behind It

I’m always partial to logic and science in everything I do with audio and home theater. If something sounds better to me, I want to know why. My research and experience with speaker isolation was no different. To find an analogy, I turned to my past automotive endeavors. About 20 years ago, I had a Mazda Miata that I took to track day events around the Northeast. I made many modifications to this car but the one that applies to this editorial is the decision I made regarding engine mounts. A full-on race car has almost no rubber bushings in its chassis and the engine is bolted directly to the frame. By contrast, a street-going vehicle has soft material between the engine and frame to prevent vibration from reaching the cabin. Trust me, if your car had solid engine mounts, it would literally vibrate itself to bits in short order. But there is a performance advantage there. When you rev an engine up and down, it shifts from side to side. This movement affects balance and handling, especially during cornering. It’s simple logic – the engine weighs a lot, and a lump of cast iron or aluminum will shift the entire car as it moves, regardless of the driver’s inputs. This is a bad thing.

To affect compromise, I installed aftermarket engine mounts. They are stiffer than the stock rubber pieces but still soft enough to keep vibration at a reasonable level. Yes, I felt some buzzing in the cabin. But the car was much better behaved in corners where I had to get on or off the throttle.

How does this translate? It’s simple. When a driver cone moves air, it produces sound. It’s precisely controlled by its motor and magnet. If that driver also vibrates the cabinet, it dissipates some energy that could be used to, you guessed it, produce sound. If less energy is wasted on extraneous vibration, the sound is better controlled and balanced.

One of my pairs of speakers, the Axiom M80s, is up on a wooden bench rather than on the floor. This piece of furniture will vibrate. Clearly, that removes some energy from the speaker’s ability to produce sound. By isolating the M80s from the bench, I was able to turn the volume down by 2dB to achieve the same perceived loudness. My other speakers and subs? They are on the floor, carpet over concrete. While concrete might seem to be an inert material, it is not. It will vibrate at high volumes enough that you can feel it through your bare feet. Adding the SVS SoundPath isolators allowed me to stand right next to my EP800 sub as it thumped away while I felt nothing from the floor. Again, I had to adjust the speaker and subwoofer levels to maintain the correct balance.

Conclusions

If any of this makes sense, I urge you to try a set of vibration-isolating feet with your favorite speakers. Smaller models can benefit from the half spheres I mentioned which are very inexpensive and can be stuck onto the speaker cabinet or the plinth of your stand. For large towers, I recommend the SVS SoundPath isolators or one of the two-piece designs from IsoAcoustics. The benefits of these items are real enough that PSB includes them with all its new Synchrony models. And Paradigm has put something similar on its Founder Series speakers. If you currently have spikes or speaker feet that are either hard rubber or very small, a more substantial solution may very well deliver an audible improvement. The best part is it doesn’t cost a lot to try, and you won’t have to make any permanent modifications to your speakers. The science behind this tweak is hard to ignore. Physics that creates better bass, what’s not to like?

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