Anachronistic Audio Effect Unit: Spring Reverb

As part of my residency at the Pier 9 Workshop, I explored the analog roots of some audio effects that are very commonplace in the current digital music world. The goal was to re-create these effects as they would have been designed in the Florentine workshops in the 16th century; going a step beyond function, innovators like Galileo were exploring and demonstrating scientific phenomena as true works of art.

The first effect that I chose to tackle was reverb.

Reverb has been a standard in audio recording and live performance for decades, and is basically a short-tail multi-tap echo that is added to an audio signal. The effect can range anywhere from a nice lush, full sound added to vocals such as when you sing in your bathroom to a harsh, tinny, robot-like sound that sounds like you pushed a screaming Dalek into a canyon.

The earliest forms of reverb used in the recording studio were plate and spring reverbs. Basically these would work by mechanically transducing the audio signal into a metal spring or onto a large sheet of metal and then electrically picking it back up somewhere along the distance of the plate or spring. This would cause the metal to "reverb"-erate and the reflections of the audio within the material would cause the echoes. These early reverb units were usually mounted inside an isolated tank or box to both prolong the echo and keep out unwanted noises.

I needed to find a cheap way to couple the audio signal into a spring, so I hit up the electronics lab. Now actual production spring reverb units often a transducer that is kind of like a piston coupled to a spring that gets excited inside a magnetic field set up by the audio signal in a transformer. I wanted to go a little more low-tech than that with the components that we had in the shop.

I found a few small 8 ohm speakers and carefully removed the cones with an exacto knife, which leaves you with a speaker that can't make much noise. The inner area (called the spider) still moves with the magnetic fields set up by the voice coil though, so it's perfect to hot glue a little metal stud to the middle of the speaker and use this to turn the audio into linear motion.

For the pickup, I soldered the same little metal stud to the middle of a piezo element. This are effectively contact microphones that are super sensitive to vibration- the piezoelectric crystals produce electrical fluctuations when vibrated. By plugging this into a microphone or high-z input on a mixer, you can pick up the sound that is being transmitted through the spring.

Now one of the big problems with the prototype apparatus shown in the last step is that the tension on the spring is very important for sound quality. Too much tension pulls on the spider of the speaker meaning it starts with too much extrusion and the magnets can't pull it back. This results in poor bass and midrange performance.

If the spring is too loose, it vibrates loosely which makes huge thunderous bass crashes and sounds kind of like listening to music through a blown speaker at high volumes.

After laying out the wooden base in Fusion 360 and cutting it on the DMS 5-axis router, I addressed the tensioning problem by fabricating some brass turntables on the lathe. These would twist into tee-nuts in the base and allow the height to be moved up and down. Since the piezo attaches directly to these it would have to be able to be disconnected to allow it to spin, so I rigged up some banana plugs for this connection.

Here are the piezo elements attached to the brass tensioners. I used some fancy cotton braided cable for aesthetic reasons and soldered to the inner and outer ring of the piezos. This can be a little difficult as the piezo body requires quite a bit of heat to make a proper solder joint- it's really easy to cold-solder these if you aren't careful.

The brass pieces were turned on the lathe from a solid brass rod and then threaded by hand to fit into the nut-serts on the wooden base.

Unless you love 60Hz hum in your audio path, the outer ring of the piezo should be the ground connection going back into the mixer. Also, it's sometimes necessary to isolate the speaker from the piezo- luckily the metal stud that holds the spring has a nice layer of hot glue which provides this electrical isolation.

A quick job on the waterjet produced this holder for the speakers. I decided to use two with this reverb so I could have two different effects by using different sized/tensioned springs.

I drilled and tapped holes on three sides of each round speaker mount to hold the speaker in place. You technically want as little coupling as possible between the speakers and the holding bar to keep the two audio sources isolated.

Ok! Great! So we have springs tensioned between the speakers and the piezos, and all associated wiring running down into the base. Now how do we hook this thing up?

I decided that the Reverb unit should be self-contained like a guitar pedal, meaning a non-amplified input and output source. Ideally, you'll hook up the input signal whether it be a cellphone, guitar, or microphone and then hook the output up to two mic inputs on a mixer or to an amplifier.

This meant that I only needed a small internal amplifier to drive the input signal to the speakers. You can see the small PCB amplifier board in the second picture above. The volume is preset to a mid-range value leaving the option to control input volume externally; for example, I hooked the reverb up to the effects send of a mixer and used the level knobs for that send to control how much the speakers would vibrate.

To finish up, I bored holed for the connector jacks: A 1/4" TRS jack for the stereo input signal, a DC barrel jack for 9VDC input to the amplifier, and two 1/4" mono jacks for the separate outputs.

The effect that I ended up with was a tinny reverb with a fairly long tail, much like the effect you'd hear on a classic dub album by the likes of King Tubby or Mad Professor. It sounds great with simple beats, and gives your voice a nice robotic sound when you speak through it.

The first audio sample above is an example of a clean drum beat followed by playback through each of the two coils, and a version of the dry and wet sound mixed. You may notice the second coil lets much more treble through and is a less prominent effect due to the much smaller spring and less tension. The fatter spring from the hardware store (first wet sample) is much more tinny and robotic sounding with a longer tail.

The second audio sample is running a soundboard with a few common percussion sounds through the spring reverb: handclap, cowbell, claves.

Finally the third clip is the wet output of a few seconds of a fully produced track run through the reverb unit (Electronic Performers by Air).