A sound system consists of a microphone for input, the amplifier circuitry, any volume and tone controls, and a loudspeaker for output.
Every component of the sound system is equally important, therefore engineers have coined the term “High Fidelity”, or HiFi, which refers to the ability of a sound reproduction or recording system to accurately reproduce original sound in a wide range of frequencies.
This article is an overview of the design and operation of microphone and loudspeaker acoustics and is intended to highlight the essential workings in terms of sound physics.
Microphones are transducers that convert sound energy into electrical energy. A vast variety of microphones have been developed for specific purposes. They range substantially in quality:
From the very simple carbon microphone that needs to respond to a limited frequency range of the human voice
To the moving coil or ribbon microphone that provides the broad frequency response required for recording music
Note that the moving coil microphone has a diaphragm that causes a coil to move between the poles of a magnet, creating an alternating induced voltage. The ribbon microphone has a metallic ribbon that moves in response to a pressure fluctuation, and in the same manner, an electrical current is produced.
Loudspeakers are also transducers but they work in the opposite sense to microphones. That is, they convert electrical energy into sound energy. Many specialist speakers have been developed applicable to particular frequency ranges.
The loudspeaker cone system shown is a dynamic loudspeaker that has a cone made to vibrate by the movements of a coil, which undergoes an alternating current while sitting in a strong magnetic field.
Limiting Sound Distortion
Speaker enclosures or baffles limit the diffraction effects that can occur for low frequencies, where sound waves coming around the back of the speaker can cancel out waves from the front. A baffle limits these resonance effects inside the speaker but its use is not always practicable, whereas an enclosure is often more appropriate. With a better-quality loudspeaker, the sound pressure level becomes more uniform in the room.
A sound system is made up of many parts, each needing to be of high quality. Microphones are transducers that convert mechanical energy (pressure waves) into electrical energy. Loudspeakers work in reverse, converting the electrical signal back into mechanical pressure waves, which our ears can decode. Baffles and enclosures around speakers improve the speaker quality, making the sound more uniform and enjoyable in the room.
How to Record a Drum Kit
The piece of string method is also known as the “Recorderman” technique. An informative video demonstration can be found on Colin Fairbairn’s website. In the interest of keeping things simple, this article deals with one microphone at a time.
Microphone 1 – Kick Drum
Common budget choices for the job include an AKG D112 or Shure Beta 52A, but any microphone with a good low-frequency response that can withstand high sound pressure will work. Put a close mike down in front of the kick drum. This can be anywhere from 12 inches outside the shell to right inside the drum almost touching the beater.
Microphone 2 – Snare
Probably the most widely used microphone for this job is the Shure SM57. It’s an affordable mike and solid enough to take a beating from the odd wayward stick. Adjust the microphone about an inch from the top rim, pointing towards the center of the drum where the stick hits it.
Microphone 3 – Overhead 1
The choices here are almost limitless, but for the sake of argument, a good recommendation is a cardioid pattern condenser microphone, like a Rode NT-2. For those who are feeling upmarket maybe even a Neumann U87 can be used. Place the mike just in front of the kit and above the cymbals, angled down so the center of the mike is pointing at the center of the snare drum.
Microphone 4 – Overhead 2
This can be exactly the same type of mike as number three, but it might be worth experimenting here. Sometimes a ribbon microphone with its natural figure-8 pattern can bring in a little more of the “room” sound as well as soften harsh-sounding cymbals if necessary.
This microphone can be the most difficult to position. Depending on the position of microphone 3 it can tend to work best quite close to the drummer’s right ear. Point the middle of the mike towards the snare drum, ensuring it can also capture the cymbals and toms. It should be roughly the same distance from the snare as mike 3.
Recording with String
Now, here comes that piece of string.
The assistant might want to do this before the band arrives because it can tend to make the head engineer look a little uptight, or at worst, like a complete amateur.
One end of the string should be tied to the beater of the kick drum so it stays in one place when the drummer’s foot is pressed down on the pedal. The string is used to measure the distance up to microphone 3, then over the finger and down to the center of the snare drum.
Keeping the finger on the same part of the string so the distance measured doesn’t change, move it towards the microphone 4. The idea is that both microphones should be equidistant from the center of the snare drum and kick drum.
Spending some quality time moving the overhead mikes around until this is the case is vital to achieving a well-balanced sound.