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Ask Rudy

June 28, 2000

Q: Do I need a shotgun mic to record sound effects?

Dear Ray,
The Audio-Technica AT822, which has unbalanced 1/4" outputs, and its balanced XLR-equipped twin, the AT825, are good choices for "point and shoot" sound gathering. They're also a good value at $399 retail. The somewhat oversized head of these condenser mics houses a pair of cardioid mic capsules angled about 90 degrees apart. These two capsules create a left-right stereo image from a single mic. This type of stereo mic arrangement is often called "crossed pair," or "X-Y" stereo micing.

Click here to listen to some Chicago street ambiance I recorded using an Audio-Technica AT825 stereo microphone.

For those with deeper pockets, I suggest a Shure VP88 ($1,194). I use this model for much of my field recording work. It's a "mid-side" stereo mic, which I find very natural sounding. An M/S mic uses a pair of capsules. The first, (usually a cardioid) faces straight ahead, to capture a relatively directional "mid" image of the sound. The second capsule is a figure-8 pattern that faces side to side, to capture the relatively spacious "side" image. (A figure-8, or "bi-polar" pattern is equivalent to two cardioid mics facing in opposite directions.) Through an electronic circuit called an "M/S" decoder (built into the VP88, but also available as an external processor or software plug-in), these two signals are combined into a normal stereo image that has excellent mono compatibility. The M/S decoding process also gives you control over the relative "width" of the stereo image, another advantage of this type of stereo mic technique.

Neither of the stereo mics I've mentioned are as directional as a shotgun mic -- typically a long, narrow microphone designed to isolate a sound from the world around it, such as the chirp of a lone bird, with pin-point precision. The high directivity of such mics is useful, but if you could only pick one good mic for recording sound effects, a shotgun isn't your best choice; furthermore, they're mono, which means for stereo recording -- a must for most sound effects gathering -- you'd need two of them. Granted, they do have the ability to reject unwanted sounds, but you can often get suitable results by simply moving a slightly less directional mic closer to the desired sound source.

Shotgun and other highly directional mics are especially useful when you have no choice but to work some distance from your subject. For instance, if you were recording actors' dialogue during a video or film production, you'd need to keep the microphone out of the camera's field of view. In this case, a shotgun mounted at the end of a boom-pole would let you get sonic close-ups from a distance. Cheers!

-- Rudy

Q: Can I run more speakers from my power amp?

Hey Bobby,
Good question, since adding more speakers will make your power amplifier work harder. By calculating the overall impedance load on your amp, you can figure out whether you're asking your amp to work too hard -- which might send your amp on an one-way express trip to Meltdown City.

To make this calculation, you must first know the impedance value of each loudspeaker cabinet connected to the amp. Most cabinets will have this value (in ohms) stamped on their rear panel. The impedance of your JBL MR925 cabinets, 8 ohms, is the most common value. The lower the "ohmage," or impedance load, the greater the power drawn from the amplifier.

It would be nice if you could calculate your system's total impedance load by simply adding up the individual ohmage of each cabinet -- but in fact, when multiple cabinets are wired together in parallel (as is the case here), the total ohmage goes down, which means your power amplifier has to work harder! Fortunately, as long as each speaker enclosure you're connecting has the same ohmage, the formula used to determine their combined impedance is easy: It is equal to the impedance of an individual cabinet divided by the total number of connected cabinets.

OK, so let's hook up some more speakers. With another 8 ohm cabinet added to each channel, the amp "sees" a load of 4 ohms per side, as shown below. Your amp is rated for 4- and even 2-ohm operation, but it will run warmer with this increased load, so make sure it has adequate ventilation during the show. Other cool tips? Leave an open rack space above a warm amp and/or add a cooling fan to your rack. And if your amp has a built-in cooling fan, remember to clean out its air filter!

Now, a word of caution. Not all amps are able to drive 4-, let alone 2-ohm loads, so it's a good idea to do these simple calculations when designing your sound system -- and then check to make sure your amp is rated for the load you're asking it to drive.

A final note: as mentioned above, this formula applies for speaker cabinets wired in parallel. Serial and series/parallel arrangements -- which are sometimes used when you're hooking up raw loudspeaker drivers within a single cabinet -- use different formulas to calculate impedance. But when you connect multiple speaker cabinets to the same amplifier, regardless of how you run the cables, the result is a parallel connection and the math described above applies.

Hope you've enjoyed our homage to ohmage, and thanks for writing!

-- Rudy

Q: Can I recondition the heads on my tape deck rather than replace them?

Dear Alex,
If wind and rain can wear the nose off the Sphinx, I suppose we shouldn't be surprised when flimsy magnetic tape shaves away at our metal tape-heads. Fortunately, there's an alternative to head replacement: re-lapping. At roughly 10% to 20% of the cost of new heads, your old analog heads can be lapped and polished back to new (and often better-than-new) performance.

But when is it time to re-lap? "There are two indicators to look for when evaluating head wear," says Tony Pretto of International Electro-Magnetics. "Examine the wear pattern on the tape head. When it begins to flare into a 'hourglass' pattern, you're a candidate for a re-lap."

The second indicator is more apparent on wider multitrack heads. As wear progresses, the top and bottom edges of the tape stop making solid contact with the heads. You can hear this as high frequency tones that seem to flutter or momentarily drop out. It can also affect the reliability of time code placed on these "edge" tracks.

Most re-lappers will have you remove the head assembly and ship it to them. Prices per head range from $35 for a cassette deck, to $45 for a 1/4" 8-track, to around $100 for 2" 24-track. Most inexpensive recorders have combination record/play heads, while most "pro" machines have separate record and play heads; erase heads usually last twice as long as the record and play heads and likely won't need re-lapping as quickly. Understandably, you'll pay more if you need to send the entire recorder rather than just the heads.

After the process is complete, you'll need to have your machine reassembled, its heads realigned, and its electronics recalibrated (for bias, record levels, etc.).

Depending on your chops, these steps could require a visit to a local repair person. But once that's done, you'll be back in business, on a budget.

-- Rudy