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Posted by on Nov 10, 2011 | 5 comments

HISS and a ROAR Contact Mic, Tim Prebble Explores the Lack of Acoustic

CONTACT MIC is a new library of sound design source material recorded by Tim Prebble and released by HISSandaROAR, including 1,556 sounds recorded at 192kHz/24-Bit using contact microphones.

Recording with contact microphones is fascinating territory to explore due to the unpredictable results; it encourages experimentation! Inspired by the incredible work of Alan Splet and Anne Krober on Dune (as described in this article) I bought a custom built Trance Audio Inducer contact mic and preamp back in 2001, and have been experimenting with it ever since. A couple of years later I added a pair of Barcus Berry Planar Wave contact mics and while these mics featured in Tortured Piano this library is something else again: it is literally the result of hundreds of hours of experimenting.

One aspect of contact mic recording that is fascinating for sound design is the lack of acoustic – there is no reverb due to the sounds being recording via direct vibration, not through the air. This feature alone makes for very malleable sounds, excellent as components and layers in complex, composite sounds or for when you need an abstract organic sound but where the actual source must remain concealed…

The library is available at Hiss and a Roar for $99. Also check this article at themusicofsound, where Tim talks about the release.

Now here’s our Q&A with Tim, who shares some details about the recording process, props used and the inspiration behind the library.

What inspired the library?

Back in the 2001 a friend gave me an old copy of American Cinematographer magazine from 1984 which had an interview with Alan Splet & Anne Krober about their work on the film Dune. One section described their use of the FRAP contact mic and it totally set off my imagination! [article link]

What model contact mics did you use?

After reading that article I went hunting for a FRAP only to discover they were no longer available. I pursued some DIY approaches but the results just were not that useable – it was obvious the frequency response was not full range. But then I found the Trance Audio Inducer, which came with a matched preamp. This was a revelation to me, especially with regards to the tonality of the sounds and the low frequency response. A year or so later I bought another contact mic for my double bass, but it didnt have the same sensitivity so I went back to Trance Audio to buy a second Inducer only to discover that model was no longer available either! Next I tried the Barcus Berry Planar Wave contact mic, which also came with its own matched preamp and I ended up buying two of them as they produced similar great results to the Trance Audio. The library is all new recordings, but its based on my experiences recording with these for the last ten years.

Are there any special techniques or advice for using contact mics that you can share?

Well, the first & most important aspect is the matched preamp. Like most sound editors I own many different recorders & preamps but it seemed the only contact mics I got great results from were the Trance Audio and the Barcus Berry when used with their own preamps. I learned this through experimenting but only recently discovered why that is. I’m sure the reasoning behind is very basic for anyone who has studied electronics but this article spells it out…

“”The problem with piezo guitar pickups and contact mics is that they are not well matched to typical audio inputs. By their nature they can generate a lot of signal, but they cannot drive a 50 kilohm typical line input. The pickup needs to work into a much higher impedance, typically 1 megohm or so.

The reason why these devices often sound tinny is because the piezo sensor presents its signal through a series capacitance which is small, typically 15nF or less. When wired to a normal 50 kilohm line input this forms a 200Hz high-pass filter, which eliminates the bass.

If wired to a consumer plug-in-power microphone input of about 7 kilohms impedance, the result is a 1kHz high-pass filter. Hence the reputation for poor bass performance….”

So impedance matching is the key and explains why many DIY contact mics sound thin – people buy a cheap piezo element & plug it into a line or mic input. They get a signal & it is useable, but you don’t have to be a rocket scientist to appreciate what a 200Hz High Pass Filter does to the sound.

What about props? Contact mics were traditionally created for musical instruments, what else do they respond well to?

Psychologically using a contact mic makes you look at the world in a different way. We are all so used to hearing and interpreting the world through air molecules and acoustic spaces. And an important part of being a sound recordist is being able to hopefully predict how sounds will react in a space. But using a contact mic makes you put your assumptions aside and experiment because the results can be counter-intuitive. Sometimes I’ve found a prop and thought: “THIS will sound AMAZING with a contact mic” and then hooked it up & been thoroughly underwhelmed. What seems resonant & complex through the air may well have a singular pitch & be far less interesting with a contact mic. But its when the reverse is true that things get exciting.

I’m sure I’ll work out the pattern  at some stage, but in many ways the mystery is actually a pleasant side effect. Not knowing the outcome means when you do stumble across a beautifully resonant body, time slows down & you become completely absorbed into physically manipulating the sounds. I totally agree with Dave Farmer about not always wearing headphone when recording, with the one exception being when using contact mics. The exact placement can make a huge difference in the tonality of sounds, especially on hollow resonant bodies. Depending on what the object is made from there is also often a huge range in dynamics, which means setting levels can be tricky and requires constant monitoring. But it also fascinates me that some of the sounds recorded with a contact mic that you perceive as being big, loud, aggressive sounds were actually created through very subtle movements. The big blue water bottle in the video is a good example: hitting it hard with a mallet produced a harsh sound, but hitting it very gently with a soft bass drum mallet produced a sound that would excite your subwoofer!

With such diverse sounds, how did you go about categorising them?

About half way through recording the library I came to realise that a good way to think of the sounds is ‘resonant body’ versus ‘activator’. Over the years I’ve slowly built up a collection of props that respond well to contact mic recording, many of which feature in the new library. But I’ve also slowly collected up a suitcase full of activators, so the files are named based on this approach. But I also spent time making sure every file in the library has a photo of the prop included so you get an actual image of how the sound was created when browsing in SoundMiner.

What are some of your favourite sounds form the library?

My favourites tend to be the unexpected new sounds I discovered while recording. One good example is the rubber recording. I had never thought of sticking contact mics to a balloon but I did and spent half an hour getting fairly familiar sounds from them and I didn’t include those sounds in the library. But it got me thinking, what if I used a much bigger balloon and only partially filled it with air, so it is a large resonant body but with very low air pressure? I gave it a try & discovered some really expressive sounds which I can only describe as sounding like a whale with indigestion! The low frequencies are quite amazing, because the pitch variation was totally controllable and they sound unreal at half speed!

Like most sound effects editors I have a lot of metal recordings in my library but during these sessions I managed to record some metal sounds that I have simply never heard before. Due to how I was applying pressure the two metal elements produced almost rhythmic stutter effect!

Another funny sound I stumbled across reflects an aspect of why contact mic recordings can be so useable. Because the sounds are recorded through direct connection with the vibrating material, there is no room acoustic or reverberation. This means the sounds can be used as layers or components in composite sounds without the listener being able to identify the source, but it also has a less obvious benefit for recording. One example was when I was recording different scrapes & movements across a metal bowl. I was performing them with a knife and a drill grinder bit, but I was interested in more complex sounds & remembered I had my battery drill at the studio. Normally if I recorded anything that involved the drill, it would have the sound of the drill motor all over it. But if the contact mic is not attached to the drill it doesn’t hear it at all! So I started putting different activators in the drill & trying them…

Whats next for HISSandaROAR?

Having spent so much time in the studio lately I am craving some exterior recording, and as summer is just starting in New Zealand I am going to record a library of beach ambiences before the cicadas start & make life difficult. I’m also working on a library of metal spring recordings and am pursuing another creature vocal library. Onwards!

5 Comments

  1. thanks Brad!

  2. Good advice about the preamps. I use a piezo film pickup and Fishman preamp from windworld.com, run through a DI and another mic pre, and the results are very nice.

  3. Hello! Please note that we are again custom making the mono Inducer System that Tim mentions, and we can also do a custom modification of our stereo Amulet System to make it more useful for SFX work; just email us for info.

  4. Really enjoyed this article, Tim!  I love your balloon recording and description of how you came about it. That’s a surface I hadn’t thought of trying (in the bazillion years I’ve played around  with contacts)  So agree with you about giving up expectations and going for discovery, and well… everything else you said here.

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