Slinky Spring Reverb

There are a number of different ways to send vibrations travelling down a spring, but most spring reverbs use torsional (rotational) motion, which is less sensitive to interference from external vibrations. Here is an experiment that The Peasant did with a Slinky Jr (R) and some old speaker drivers which delivers loooonng delay low frequency DIY reverb for almost no cost.

The reverb assembly consists of a 1 5/8 inch diameter Slinky Jr spring suspended on a four foot long frame, mechanically coupled to speaker drivers as transducers, and installed in a foam lined packing case.

The spring is mounted with tiny ball bearings to allow for free rotational movement. A four inch full range speaker drives one end of the spring while smaller speakers pick up the delayed vibrations at two other points further along the spring. A couple of pieces of foam rubber are inserted inside the spring to control a low rumbling bass resonance in the slinky.

The drive speaker has a round plate glued to the cone with silicone rubber, which is then coupled to a rocker arm with an "L" bracket and small bolts. All of these parts are made from plastic to prevent magnetic interference with the drive speaker. The other end of the rocker arm is clamped to the slinky near the end bearings. These bearings (BTW, most old VCR transport mechanisms have a few of these bearings buried inside) hold the rocker arm center of rotation at the center of the slinky's diameter, for most efficient transfer of drive speaker linear vibration to slinky torsional motion.

Further down the slinky are the pickup speakers and more support bearings.

The pickup speakers are smaller 1 1/2 inch full range drivers that have had their cones removed with a knife to control microphonics. They are mechanically coupled to the spring through short wooden rods (cut from small cleaning swabs/Q-tips) glued with silicone rubber to the cap of the speaker voice coil. The other ends of the wooden rods are split down the center for about 1/2 inch with a sharp knife and then clamped on either side of the flat side of the slinky spring with a small tie-wrap. This clamp as well as other attachment points on the assembly were coated with clear nail polish to control unwanted vibration.

The most basic wiring setup can be done using almost any type, quality, and combination of common audio components. A low wattage power amplifier is all that's required to drive the spring, and a high-pass filter similar to a speaker crossover (C1) helps to prevent an overload of bass energy. A 4.7 uF capacitor is a good starting point for experimentation here.

The pickup speakers drive a regular phono preamplifier quite well, as there is sufficient gain, although the RIAA equalisation does cause a bit of loss in the higher frequencies. The high frequency response does roll off quite quickly, but the bass response can be quite scary at times, as well as the many other "interesting" sounds that can be created by overdriving, adding external vibrations, etc. And the delays are nice and long compared to conventional spring reverbs, the sounds are somewhat like the resonance of a very large indoor arena.

The next challenge is how to make the mechanism smaller, and more robust. After experimenting with various possible mechanisms, small dc permanent magnet motors proved to be superior to all other tested transducer types. One advantage they have is they are a rotary device to begin with, so they are easy to use as a rotary transducer.

However, in order for one to use these as transducers, they require modifications. To do this, the electrical brushes must be removed and bypassed with a direct flexible wire link to the armature. This can be a delicate and challenging task. To see what is involved, check out Building a DC Motor Rotational Transducer but be careful, it's not for the faint of heart!

Here are some pictures of these type of transducers and their applications.

The electrical application of these transducers is basically the same as for the previous speaker drivers. They sound similar as well, and function well in driving the spring. As pick-ups they are sufficient, but their low impedance presents gain problems. Future research may find a better solution here.

To see a complete reverb effects unit built with rotary transducers by The Peasant check out
The Thermiomniverb

To see further work with slinky reverbs by Grant Richter and The Peasant click here.

To see a really huge slinky sound generator, check out Robin Whittle's Sliiiiiiiiiiinky