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Gervais Currie: Sun 31st Aug. 2014

People entered the classroom, and while waiting, spontaneously picked up the objects we’ve laid on the table, and started playing them. Suddenly, each corner of the London Hackspace Classroom was filled with the echoing sound waves of mbira, bowls, and the percussive rhythms of the tables and metal bits. It was truly surreal, and rather entertaining to witness.

Unpredictability has always been a thrilling aspect of our meetups, as the tone of the meeting widely varies between one to another. In this one, we’ve have somehow managed to create an ideal environment for experimentation and true self-expression.


This meetup, we had the pleasure of meeting Gervais Currie; an instrument repair expert at Portobello Music. Gervais has been repairing amplifiers, fixing instruments and installing pickups for over 20 years [ link ].

To summarise, here are some tips and the most discussed points of the evening:

  • When designing a pickup system for a stringed and bridged instrument, use two Piezo discs instead of one. One is placed under the bridge and another is placed over the bridge. The two Piezo Discs must be wired in the same polarity (to avoid phase cancellation). This would provide an extra 6dB of gain, and among other benefits this can be used as a ‘differential’ setup as described here [ Wikipedia ]
  • The popular Panasonic WM-61A capsules have been very well known all over the internet, and you don’t have to dig deep to find out why. These beautiful little omnidirectional electret condensers have brilliant flat frequency response [ as claimed in the datasheet ], cheap as chips [ eBay ], and their tiny size factor means you can make them fit virtually anywhere.


The main idea is to use one of these capsules to mount directly onto the top plate of an instrument, in this case Gervais’s Banjo, and here this tiny creature really comes alive.

These capsules obviously have to be phantom powered or plugin powered ( Here is an example of such setup on Wildlife Sound ) – if you don’t have a mixer with phantom power. Although among us, we have yet to test these out.

Note, most handheld recorders provide Plugin Power.

  • The Art of Electronics is highly recommended [ Buy ]
  • Class D Amplifiers crept up in our discussions, and the reason is because they don’t require much power, and are very musical sounding. Therefore they would make for a great portable general purpose amplifier for use on the go.If interested in reading, here are Class D Amplifiers explained in Detail [ Link ]
  • Vaguely touched on what’s called ‘Acoustic Transformers’ which deals almost exclusively with matching ‘Acoustical Impedance’ rather than Electronic Impedance. This can take the form of for example, a horn [ Horn Loudspeakers and Acoustic Impedance ].If interested, have a read this Audio Express Horn Theory Paper from our resources folder [ link ].
  • Gervais recommended to trace Harvey Gerst’s tips around the internet for thorough understanding of the technology and history of microphones. [ his profile ] Harvey Gerst has 50+ years of experience as a gold record songwriter, studio musician, recording engineer, producer, musical instrument designer, and manufacturer.Make sure to browse the stickies by Harvey Gerst on the Home Recording Forum [ link here ].


These photographs mark the end of this megapost. Warm thanks to everyone who attended, and our special guest for making this meetup truly fun memorable. This one has definitely been the most ‘musical’ meetup we’ve had!

We’re encouraging people to participate in our friendly discussions on our own forums [ link ]. Hop over there, and you can click join to enter the Acoustic Hacking circle, and you’d also receive latest updates of our discussions. We’re also constantly adding resources such as papers and links to the Goodies Folder, so have a browse if you fancy doing some general research or even solve some mathematic equations [ link ].

Join us next time on Sunday 28th September 2014 7 pm at the London Hackspace. Agenda will be uploaded soon! See you.

Any queries, please email acoustichacking@gmail.com

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Tony Hardie-Bick: Sun 25th May 2014


Last meeting on Sun 25th marks one of our most successful meetings yet. It was overwhelming to witness such great attendance, with such a variety of people from different practices, ranging from visual artists and musicians to engineers and product designers. Below is an excerpt of the discussions that took place.

piezo impedance

Piezos are an essential tool to pick up surface vibrations, and here at Acoustic Hacking, we love them for their abundant availability, simplicity and versatility in a wide array of applications. As you can see, this simple circuit [drawn by Tony, a guest practitioner], may be needed to remove the ‘tinny’ sound, an issue commonly associated with piezo discs. The piezo-electric material has very high impedance, and this circuit is designed to match the impedance to mic level input on your mixer or audio interface.

Another tip is to use a shielded cable, to further prevent hum or electromagnetic interference being picked up along the cable.


Other discussions included how to design a ball for the blind. Some example of such a device/ball are photographed above, sitting on the table. The idea is to design a highly resonant structure embedded at the core of the foam ball. This structure can either be activated by movement of the ball, or possibly the vortexes of air can be harvested through the crevices of the ball to vibrate this ‘strucutre’; much like a whistle — example of whistle footballs commercially available.


Hacked Watkins Copicat Tape Echo by Tony Hardie-Bick

Quoting Tony from an email:

The machine is a Watkins Copicat Tape Echo, made in the 1970s and 80s, there were a number of slightly different designs, some with valves, some with variable tape speed, some using op-amps etc. My modification was to put the four playback heads at non-equal time differences along the tape, and to put two of them on a separate channel, to get an uneven stereo echo, which builds to create complex distorting stereo textures.

The thing that’s perhaps interesting, electro-acoustically, is the piezo mic on the tape arm, which picks up the sound of wheel-scratching and actual physical tape hiss, which can then be mixed in with the main signal, or just use on its own so the thing becomes an instrument in its own right.

The piezo idea was prompted by a performance in Vienna in 2011 by Martin Blazicek and Andras Blazsek. The performance can be viewed here.

Also I wanted to share this link as part of the discussion. This is Michael Vorveld, from Berlin, who I also saw in Vienna. He uses light bulbs in series with bimetallic strips to generate performances like this.

I think the sound also comes from piezos, picking up the vibrations from the bimetallic strips as they switch on and off.

The concept of using bimetallic strips is very interesting and unexplored, possibility to look into this in future workshops!


Again, huge thanks for the people who attended and contributed to the workshop this time. Please stay tuned here for an updated agenda for next meeting, and possibly another guest practitioner. The meetings are held last Sunday of the month, so ext meeting is going to be on:

June 29th 2014 at 7pm

It is a possibility workshops might be arranged in between those meetings, those will involve more making! this will also depend on people’s desires.

Any enquiries, please email acoustichacking@gmail.com