words and photos by: Telegraphy
One of the greatest inventions in modern times for the sound experimenter is the advent of the Piezoelectric Sensor or Piezo pick-up for short. These little disks of sonic discovery opened up a door to a whole new potential of recording sounds from solid and liquid objects. In the old'en days, if grandpa audio engineer wanted to record with in a violin or under and ocean, he was limited in the placement of and the types of microphones to be used. Non of them really came close to capturing the brilliance in sound as an internally instrument mounted microphone. Having a microphone directly placed on a sound surface will produce a more brighter sound, Piezo pick-ups are perfect for the job.
So what are these mysterious microphones and how do they work? In 1880 Pierre and Jacques Curie discovered that by applying mechanical stress on such materials as tourmaline, quartz, topaz, cane sugar and Rochelle salt, they were able to measure a surface charge or voltage on these elements. This is what has become known as the "Piezo Effect". Any crystalline element will in some magnitude, give off an electric charge when stressed. Weather it be physically stressing it, thermally (changing it's temperature) or electrically.
A Piece of quartz (through scientific experimentation) was found to have the most surface charge then any other element. It's sensitivity to mechanical, thermal and electric stresses made it an excellent candidate for electronically detecting pressure changes in the environment.
The first real scientific application of the Piezo Effect was made during world war I, when in 1917 P. Langevin and French co-workers devised an ultra-sonic under water submarine detector. A mosaic of thin quartz crystals was sandwiched between two metal plates. The worlds first "Pick-up" as it can be regarded, had a resonant frequency of 50Kc (that's 50 kHz for all of you kids out there).
Enough of my history lesson. As someone important so gracefully said, "The only thing we've learned from history is that we never learn from history". Lets investigate closer of how a Piezo Pick-up works. Your typical Piezo element uses a thin wafer of barium titanate and lead zirconate . Now, I'm not a chemist but when it comes to big long names like this, I go for the short and sweet definition - Piezoceramic. Ahh, that's better. A piezoceramic wafer is sandwiched between a brass base plate and a silver top electrode. The base plate is the mating surface in which the sound energy is passed from the source to the piezoceramic material. The top electrode's job is to electrically connect the piezoceramic to the + hook-up wire. The other - wire is connected to the brass base plate. Sound pressure hits and vibrates the brass base plate. This sound vibration is transferred to the piezoceramic wafer, where it mechanically stresses it and produces a surface charge proportional to the amplitude of sound pressure. Simply said, sound enters through the brass base plate as a mechanical force and ultimately exits out the silver electrode as electrical energy.
Now lets get physical! Investigating deeper into the mechanics of these playful Piezo Pick-ups, we find out that the sound energy impresses a longitudinal force upon the piezoceramic wafer. This means the whole disk bends laterally with the sound energy, squeezing and expanding the disk long it's axis. It's better understood view then described in words.
Size matters guys! Yes the size of your package makes a difference in performance. Because when it COMES to satisfying your --------EARS? Well what did you think I was talking about? Hee hee. Yes the size of the Piezo Pick-up has a lot to do of how well it responds to different frequencies. The Pick-up's basic constructed is similar to a capacitor. You have two metallic plates sandwiching a ceramic insulator, just like in a capacitor. If you took a capacitor and measured it's "resonate frequency" you will find out that they respond quite well to high frequencies. This is why most smaller Piezo Pick-up's sound tiny. Not because there cheap or manufacture poorly. They are just responding well to high frequencies. The stiffness of your package (Oh, here I go again). also determines it's natural frequency. As sound pressure pushes against the ceramic wafer, an opposing electrical and mechanical force from within counter acts the lower frequencies but higher ones resonate it with ease. The more the area, the greater the mechanical elasticity, the better it responds to low frequencies. This is a general rule of thumb to abide by when designing your own pick-up system. So to find a nice sounding Pick-up, try using the biggest one as possible. See, size does matter.
Resistance is futile
So, now that brings us to the last lesson of the inner workings of these perplexing Piezo Pick-ups. One of the most asked questions of the human race, besides what is the meaning of life, what are the impedance of these devices ? Eeehhhaaaa.......the truth is going to hurt most folks. The nominal impedance of a Piezo Pick-up is high......very high. We are talk'en up to one million Ohms of resistance. Impedance is the measurement of the electrical opposition to audio frequencies. The more opposition - The more impedance. So it is crucial for the output of a pick-up to be properly matched with a different impedance on the input of an amplifier. You want the electrical energy from the Piezo pick-up to flow evenly to the amplifier. The most common and easiest method to match the high impedance of the pick-up to a low impedance input of a amplifier, is to employ a "low pass filter" circuit in between the pick-up and the amplifier. Failure to match properly will result in poor sound quality that will sound tiny. This is one good reason to use a tube amplifier, not just because it sounds better but because of their characteristic high input impedance is perfect for amplifying Piezo pick-ups.
and now........A cheap pick-up line.
In my opinion, you should always use balanced cable to hook up Piezo pick-ups to any other piece of gear. Why? Balanced cable characteristically has lower capacitance per foot then unbalanced shielded coax. Piezo pick-ups are essentially a capacitor with a high resonant frequency. Adding coax with a high characteristic capacitance per foot will only increase our Piezo Pick-up's capacitance and therefor rise the resonant frequency (what we don't want to happen). Plus you gain the benefit of having a balanced system that isn't susceptible to ground loops, radio and electrical interference.
So what do you do with a pick-up ? Here are some suggestions of various uses for these profiting Piezo pick-up's:
1. As a guitar microphone. Experiment positioning them on the body of your acoustic. Closer to the bridge gets you more volume. Away from the bridge achieves a softer warmer sound.
2. The ultimate spy microphone. Jealous of your neighbors? Tape them on the window to hear their complaining about your dog or cat..........or child.
3: Cool body sounds are always a treat for an experimental sound. Stick one in your mouth and sing or burp but don't swallow. After a meal, tape it to your stomach to hear low frequency gulps and rumblings.
4: Can't go without whale sounds from the ocean for your ambient sound track? Well, if your like me and only have a small lake filled with carp, catfish and clams. Through one in with a small sinker attach to it. The water shouldn't harm it - that is if you are recording in the salt water of a ocean. Then I suggest getting some kind of a water proof container, attach the pick-up to the inside wall......the "inside wall" folks, not the outside. Gee's.
5: Attach them to a steal girder of a busy highway overpass and listen to creeks and bumps made by the bridge.
6: A steal radio tower will make whooshing noises on windy days.
7: Record your phone calls pleading to the IRS. Attach them to the receiving end of your phone. Yeah, it's the end closest to your ear.
8:Oh yeah. Use them for your next plate reverb project. http://ionosonderec.blogspot.com/2013/04/diy-plate-reverb.html