Ribbon Tension—Too Loose? Too Tight? Just Right?

Over the years, I’ve often received one question from DIYers and major microphone manufacturers alike: “how do you tune your ribbons?” While some of our methods remain trade secrets, I think it would be both interesting and important for ribbon mic lovers to understand the value of proper ribbon tensioning.

Before we get to tuning, let’s briefly go over the basic physics of a ribbon microphone. Thanks to the great Michael Faraday, we know that if you move a conductor (like a strip of aluminum foil) while it’s placed in a magnetic field (like magnets on either side), you will generate a small electrical current across the conductor. This phenomenon is known as electromagnetic induction, or Faraday’s Law. Now, if your foil is thin enough, even sound will be able to move it. And if it is moved by sound, then that motion will be translated into an audio signal across the ribbon. Voila! You have a device that converts sound into an electrical signal—a “transducer”, and more specifically in this case, a “ribbon motor” (Fig. 1). Of course, this signal is not yet ready for your console, interface, DAW, or reel-to-reel and will need an appropriate transformer and preamp...but those are topics for another time

Fig. 1 — Diagram of Ribbon Motor

Now if you were to use a plain old strip of household foil, or even a gum stick wrapper (yes, you wouldn’t believe how many times I’ve seen this), you would get very low output and a thin, honky tone from your ribbon mic. This is for a number of reasons, but the biggest one is that the ribbon is not corrugated. Applying lateral ridges known as corrugations along the full length of the ribbon adds several benefits to the microphone’s performance, including higher output due to increased ribbon travel, and broader, flatter frequency response due to damped resonances. Appropriate corrugations also reduce shear and rotational movement, maximizing axial directivity. Of course, there is a science behind these corrugations as well...but again this is a topic for another time.

But let’s take a step back to discuss “resonances”. Resonance is a property of objects or spaces that intensify and prolong sound at a specific frequency, and often multiples of that frequency (harmonic overtones). This property can be highly desirable in guitar strings, pianos, and violin bodies for example. But, in the case of transducers like speakers and microphones aimed for high-fidelity sound reproduction, resonance is not so desirable. It colors and limits the frequency response of your microphone. Luckily, the corrugated ribbon in a ribbon microphone is naturally effective at damping (mitigating) any resonances...but only when tuned properly!

Bonus Science

I can’t help but go deeper into some physics here, so consider these next three paragraphs optional. The most effective analogy for illustrating resonance is the simple mass-spring system (Fig. 2). Put an object of mass m on a spring with stiffness k (fixed at one end), then displace and release the mass. What happens? In an ideal world, free of frictional losses, the mass will oscillate (or bob up and down) forever. This is a condition of pure resonance, and the frequency f of the resonance is determined by the value of m and k in the following equation:

Fig. 2 — Simple Mass-Spring System and Frequency Equation

Thus, your resonant frequency f will increase with stiffer springs and/or smaller masses, and decrease with more flexible springs and/or larger masses. Ideally, a microphone’s primary resonance should be as low as possible to avoid peaks in its working frequency range. This is where ribbon mics excel beyond dynamic or condenser microphones. Because the ribbon is corrugated and only bound at two points, it maintains a very low mechanical stiffness, and thus an incredibly low resonant frequency. Dynamic and condenser microphones on the other hand are bound along the entire perimeter of their circular membranes/diaphragms, resulting in higher mechanical stiffness and noticeable mid/high frequency resonance. This is not to say that ribbon microphones do not experience any resonances throughout their frequency range, but that they are generally softer in a way that is superior to other microphone technologies.

Fig. 3 — Mass-Spring System with Damper

If you take that same mass-spring system and add a damper with damping constant c, the mass would still be able to move just as freely but without any resonance (Fig 3). This is the condition of “critical” damping—the perfect balance between resonance and damping—which closely approximates the behavior of a correctly tensioned ribbon. Other microphone elements with supplemental damping and resonators also sacrifice output level and leave resonances free to color the microphone’s natural tone.

Tuning Methods — Experience vs. Science

So how are ribbon mics tuned? First a target tension is determined. In the case of vintage RCA microphones, this tension was specified by the legendary Harry Olson and his team of engineers after exhaustive research and testing. With this tension in mind, RCA engineers then trained technicians—like the great Clarence Kane—in ribbon installation procedures and techniques.  Because of this combination of calculation and tradition, ribbon microphone tensioning remains both a science and an art that can take years to master.

Traditional ribbon tensioning was done primarily by experience, originating with RCA’s original procedures. My mentor and RCA original, Clarence Kane, taught me how to properly tune a ribbon based on look, movement, and tone alone. With many decades of experience, he was able to re-ribbon microphones with unbelievable consistency. On the rare occasion that he  didn’t get one quite right, he would simply start over until it was. Thanks to him, I was able to learn and quantify these original techniques. Though I’m always working to get my “do-over” rate down to his numbers, I’m proud to say that Pitman honors this high standard for traditional re-ribboning.

Fig. 4 — Tensioning a Ribbon

So how are ribbons tuned without such training? Is there another way? Yes, in fact, most modern ribbon microphone manufacturers use an approach known as “impedance resonance testing”. This approach involves connecting a function generator and oscilloscope to the ribbon motor and sweeping through frequencies to find the impedance rise (via a Lissajous curve). The frequency at which this rise occurs is the resonant frequency of the ribbon. As stated before, this frequency is very low—commonly near or below humans’ lowest audible frequency. This still leaves much to be desired in terms of ensuring proper ribbon travel and maintaining appropriate corrugation shape, but this method provides an excellent way to check your work with precision. At Pitman, this method is used to ensure the consistency of our traditional methods so as to truly offer the best of both worlds!

Fig. 5 — A Ribbon Connected for Impedance Resonance Testing

Fig. 6 — Oscilloscope During Impedance Resonance Testing

Some will argue that experience is less precise, but that is not necessarily the case. A master of this craft can tune a ribbon within a few Hertz of a target frequency without checking an oscilloscope, all while balancing the shape and spacing of the ribbon. It’s also no secret that this method has gotten the result preferred by musicians for decades. Still, impedance resonance testing offers a clear advantage for fool-proof tuning, something especially important for modern large-scale production.

So what happens when the ribbon is not tensioned properly? Is it really that important? If you want to maximize frequency response, output, and minimize low frequency anomalies, the answer is yes! Here’s what happens in each case...

Too loose — You’ll get a nice broad frequency range, but lower output and susceptibility to ribbon sag and magnet contact. This means that your frequency response may change unexpectedly when tilted forward or back, and that the ribbon will begin to distort at a lower SPL threshold.

Too tight — You’ll get very high output, but the high end will be dulled. You’ll also get boomier low mids with pronounced low frequency resonance anomalies.

Just right — The perfect balance! You’ll get high output and a broad frequency response including a more detailed top end. The ribbon will retain its shape and tone in any position, and the low end will be smooth—no boomy low mids or low frequency resonance anomalies.

Fig. 7 — Frequency Response of a Microphone at Various Tensions

Fig. 8 — Variations in Frequency Response from Normalized Baseline

Hopefully you can see and appreciate the difference a well-tensioned ribbon can make. In many cases, ribbon tuning can change a microphone’s tonal character by more than 3 dB. Such a difference is not insignificant to the discerning ear, and is certainly problematic in “matched” pairs of ribbon mics. Ribbon tension is also not permanent, as high SPL sources and long-term use will eventually stretch the ribbon and lower its tension below what’s acceptable. The good news is that ribbon microphones are a truly durable good, something you don’t see much in our modern throwaway consumer culture. With proper use and care, they need only be re-ribboned every handful of years for multiple lifetimes of use. This is probably my favorite aspect of ribbon microphones, their definition and their underlying philosophy: a functional assembly of entirely naturally-occurring and fully-recyclable conductive and magnetic elements that will last forever with relatively inexpensive repairs done by the hands of a real humans. How many products can make that claim?

Until next time,

—Luke

Have a ribbon mic that doesn't sound quite right? Consider a reribbon for optimized performance! Visit pitmanmic.com or contact us for more information.