by David Gauger II

"Shofarsogood"

(See the previous article for microphone selection, placement and geometries when recording impulse responses.)

Now that your mics are set up and the lines are run, there are 3 main methods to generate an impulse that will garner the response we're looking for. The oldest is a simple gunshot. There are several problems with this method. You need to carry what looks like a firearm (not always a great idea) which has gun powder residue all over it (a problem if you fly a commercial flight!) and is comparetively expensive. And on top of it all, the impulse it makes (the gunshot) is not always consistant and might not contain every frequency at equal energy which is what we're after. In addition, not every gunshot has the same exact intensity. On the plus side, it's loud (we need lots of volume in order to get a decent signal to noise ratio), it's small, and the resulting digital recording IS the IR: no post processing necessary other than to edit the beginning and end of the IR.

Another method is called the swept sine. The basic idea is to take the frequency spectrum and spread it out over time. The result is a sine wave that starts at 20 Hz and sweeps up at a predetermined rate to 20 Khz. On the plus side: if you've got a good playback system, this method yields extremely accurate spectacular sounding IR's. The downsides: much more equipment to set up (amps, monitors, CD player, etc.) and you must have accurate monitors capable of filling whatever room you are impulsing with a lot of sound. The pros use omni-directional speakers to send out energy in all directions. Also, after you're done recording, the sweep tones must be run through a piece of software called a deconvolver. Audioease Altiverb has one, as does Voxengo and a few others. The output of the deconvolver then often needs to be edited (heads and tails) also.

The third method, MLS sequence, uses a specific sequence of pulses that are then run through a deconvolution-like process to recover the IR. This method is least used by amateurs due the the software needed to utilize it.

First try: starter pistol. They’re not readily available these days. Besides, showing up at some church and shooting what looks like a real gun (even if it’s a starter pistol) with all the noise and smell of spent gunpowder is enough to make people really nervous these days. They also cost about $100, at least as far some cursory web research went.

Bottom-feeder solution: a powder actuated power nail device. The Remington company makes one for about $30 and it’s designed to drive nails into concrete. Basically, you put a .22 blank in the top, a special case-hardened nail in the bottom, then tap the top of the device with a standard hammer. The charge blows (making the big bang that we’re looking for) driving the nail into concrete.

The .22 blanks come in strengths numbering from 1 to 4. Use the #4 charge which is the strongest. Strength number 3's didn’t sound very good to my ear. The problem with #4 charges is that they really drive the nail hard. One solution is to fire the nails into a double thickness (about 7 or 8 inches thick) of a 2 foot long piece of “Glue-Lam” board. All the laminations and glue holding them all together slows the nail down better than just a 6 by 6 board.

Problems notwithstanding, we now have a legal, unobtrusive device that makes a lot of noise similar to a starter pistol. The tool, .22 blanks, and nails can all by purchased at your local ACE or Sears hardware stores. Here is the device: http://www.cornerhardware.com/productdisplay.pl?item_id=215436&AID=10283904&PID=1255551

One issue here is the tone quality of the resulting “gunshot”. The shots are fairly consistent although there are some that could be considered “duds” or at least they lack power and sound different than normal. Do at least 2 “gunshots” at each impulse location to account for these variances. In the editing stage, choose the gunshot that sounds most like all the others in the set in terms of tone and volume.

As for the frequency content of the sound, they’re a little bass shy and perhaps a tad mid-range heavy but still very useable. Perhaps an RIAA-like emphasis EQ curve could be designed and used on each impulse in the editing stage. The frequency content mimics the fact that many engineers roll off the bottom end of the feed to the reverb anyway to avoid bass buildup.

With all the talk about Gigapulse's ability to add reverb from different originating locations, you can blanket the stage doing a nailer “gunshot” every couple of feet or so. The idea is that if you want to have a sound that is panned hard left, you can use an IR where the gunshot was recorded from a hard left position as well, and you'll get the hard left early reflections and coloration. That is the way it occurs in real life, as we all know.

Use whatever you have, as long as it's digital because the signal to noise ratio is not favorable to analog equipment noisiness at the end of the reverb tail. You can even use a computer if you have enough mic pres and can quiet the fan noise somehow. The Fostex VF16 16 track digital recorder works well for a number of reasons: