The Science of the Recoil Stabilizer

Sound travels through air and other mediums in the form of waves. These waves compress at various frequencies called the wavelength. Longer wavelengths or low frequencies generally have more energy and tend to travel further while shorter wavelengths or high frequencies travel in a more direct line.

Longer wavelengths require more energy to generate and once in motion, are very difficult to contain. With sufficient energy, long wavelengths will cause walls and floors to vibrate. Thunder shaking your house is a very good example of the tremendous energy that low frequencies can contain.

Simply put, bass travels through just about anything while high frequencies are fairly simple to control. So what has all of this got to do with near field monitors or reference loudspeakers? Speakers produce bass frequencies that will transfer vibrations through the speaker stand to the floor, the walls and what ever else is physically connected to the speaker. This is called acoustic coupling.

When sound vibrations couple with the speaker stands or the shelf, they resonate and produce unwanted sound coloration. To sum up, coupling causes a frequency bump at the resonant frequency which in turn changes the speaker's performance. What happens is that the resonance will either be in phase or be out of phase with the loudspeaker which will invariably cause a frequency shift commonly known as comb-filtering.

Engineers will attempt various solutions such as placing foam pads or other insulating materials under their near field reference monitors as a means to eliminate the acoustic resonance that occurs when the speaker and the shelf are coupled. Good news: this does in fact effectively decouple the two devices. Bad news: it introduces a new problem that is best described as recoil.

Recoil – The New Problem

Think of a cannon firing. The power charge explodes propelling the projectile forward. As the projectile pushes forward the cannon recoils backwards to absorb the energy. This is exactly what is happening when you place your speakers on a soft spongy material: during a transient such as a sharp kick drum thump, the forward momentum of the speaker is dulled by the foam as the soft malleable substance allows the speaker to tilt backwards and recoil. This 'recoil' may be desirable when firing a cannon, but with a loudspeaker, the movement blurs the initial transient at the start of the wave, and the result is predictable: you lose clarity and detail. So instead of the bass resonating into the shelf causing low frequency disruption, it is now being lost via a thermodynamic transfer as the loudspeaker wiggles around on the foam. Essentially, you fix one problem while creating another.

Solving the problem entails decoupling the loudspeaker from the stand while providing a stable platform so that the loudspeaker fires accurately - as it was designed to do. You invested in high performance monitors… it only makes sense that you hear what paid for!

Recoils Stabilizers To The Rescue!

The Recoil Stabilizer begins with a high density isolation pad. Made from open cell acoustic foam, the isolation pad is sufficiently dense to hold the loudspeaker level while being soft and thick enough to provide the required isolation from the shelf or stand. We employ dark grey foam that is embedded with carbon to lessen the effects of oxidization and UV rays as a means to lengthen the foam’s life span.

Stabilizing the loudspeaker is achieved by introducing a heavy mass on top of the isolation pad. By significantly increasing the mass of the base platform, the loudspeaker's forward momentum is no longer sufficient to move the platform backwards. It is stabilized and the recoil is significantly reduced. If you think about it, this is exactly how a gyroscope works. Gyros are used in airplanes, ships and torpedoes to stabilize the navigational system so that the device stays on course. By rotating a heavy mass very quickly (like the earth around itself - around the sun) the centrifugal force retains the gyro's position relative to all axes. Since we need only control the effect from the speaker excursion on a single axis (front to back) we do not need a centrifugal force to steady the device. We can achieve stability with a simple, heavy steel platform that acts as the stabilizing mass.

The heavy platform is in turn, sandwiched between the isolation foam and a no-slip neoprene pad. The pad works double duty by providing the speaker with a comfortable place on which to sit while also working in sympathy with the bottom urethane foam and mass of the speaker to dampen the steel plate. To further reduce self-resonance, a curved front panel aids in the process by varying the impedance thus eliminating internal standing waves. This also adds more mass to the Recoil Stabilizer without increasing the size of the footprint.

The Benefits of proper Isolation

When the loudspeaker manufacturer designed your near field monitors, they painstakingly ensured that the speaker enclosure was both rigid and heavy so that the energy being produced is maximized. They likely tested the loudspeaker on a large heavy slab of granite to ensure the base was rigid so that the resonance from the loudspeaker could not transmit into the granite block.

And what do you do? You take your brand new near field monitors out of the box and put them on a stand, shelf, meter bridge, foam pad or bass trap. What happens? Simple: the loudspeaker was designed (tonally balanced) to sound right when placed on a solid, stable platform. By introducing variables that inevitably will resonate, not only have you gone away from the original sound that was intended, but you have introduced a tonal change in the form of resonance and phase. Let’s consider some of the more popular mounting options:

  • Shelf or Desk
    Bang your fist on a desk and it will make a sound. The loudest is likely the fundamental resonant frequency of the desk top. When your loudspeaker is sitting directly on the desk or directly coupled, as soon as you play musical passage that contains the same resonant frequency as your desk, the desk will vibrate in perfect sympathy. And depending on the phase relationship, the frequency will either get louder if in phase or be attenuated if out of phase. A shelf is no different: as these are usually rectangular, there will be two fundamental resonant frequencies (and harmonics!) relative to the length and width of the shelf. As soon as your speaker causes the shelf to vibrate, energy transfers into the shelf.

  • Speaker Stand
    The most common refrain we hear regarding speaker stands is: “My speaker stands are heavy and therefore I do not have a problem with resonance.” Truth is, unless you have huge—wide—stable speaker stands that are filled with sand you will likely have a problem. In fact most speaker stands are made from tubes of steel. Tubes ring just like a tubular bell. The other problem is ‘sway'. If you can push on your monitor and the speaker sways, you will experience a kinetic energy shift as the forward momentum of the loudspeaker causes the stand to sway backwards.

  • Meter Bridge
    Most mixing consoles are equipped with one of two types of meter bridges: steel or wood. If your meter bridge is made of wood, then it will resonate just like a shelf. If it is made of steel, then bang your fist on it and it too will resonate. Same problem—only at some other frequency. Engineer/Producer Ed Cherney (Rolling Stones, Eric Clapton, Bonnie Raitt) describes the effect: “I used to rely on 'feeling the fundamental' of the kick and bass coupling with the mixing console. This necessitated cranking the volume to get that happening. With Recoils, the speakers are now decoupled from the console and I am really enjoying the balances, especially the low end which holds up at lower volumes as well as when played loud.”

  • Foam Pad
    We are just as guilty as Auralex. We sell the IsoPad which is comparable to their MoPad. Both do good by eliminating resonance, yet both are harmful in that they dissipate the energy that should be produced by the monitor as it wobbles and wiggles around like a shock absorber on a car. Energy is lost. What do you do? You compensate by maybe adding bass. But what bass frequency are you adding? Who knows… you have introduced a variable that was not designed into your monitors at the lab.

By minimizing ‘variables', the Recoil Stabilizer introduces a ‘constant' that will ultimately ensure that your recordings are more consistent and will translate better to other systems. This is all the more important as today, more and more engineers and producers are ‘mobile' and find themselves working in multiple studios. This in fact is what truly makes the Recoil Stabilizers so cool. And is why they have been adopted by the ‘who's who' of the recording industry.

Primacoustic is a division of Radial Engineering Ltd. Copyright 2014. All rights reserved.