Using microphones successfully requires a thorough understanding of basic acoustics.

In this issue I will try to explain the principles behind reverberation and room acoustics. For a 'big' rock drum sound and for today's gated drum sounds, reverb is an essential factor. A good reason to ask ourselves just what reverb is all about.

Last time I suggested roughly how to mike up the drum kit: to go for a good acoustic sound in the first place - to experiment with microphone positions and let your ears and taste decide which is best, the same applying to the choice of microphones. Bear in mind the interactions between microphones (such as leakage, particularly with overheads) when building up the image of one single instrument.

Trends In Drum Recording

Recording the drums in an acoustically 'dead' (non-reverberating) booth, perhaps even completely separated from other instruments by a thick wall, gives absolute control over the kit's sound. There's no leakage so the dry sound can be deliberately 'enriched' with reverb etc. However, more and more musicians and engineers prefer a drum sound that contains natural reverb, that gives the feel of the recording room. There is a definite trend towards live end - dead end recording room design, and the use of highly reverberant rooms for the contemporary drum sound.

A relationship that is often completely misinterpreted is the one between direct sound and reflected sound (see Figure 1).

The inverse square law helps us to get a fair idea of what's happening: as you move away from a sound source (eg. speaker cabinet or instrument), each time you double the distance there will be a 6dB SPL loss of direct sound and the resulting sound perceived as about half as loud eg. 2" away - 80dB, 4" - 74dB, 8" - 68dB etc. This is obviously not the case inside a room (unless it has been excessively damped). An instrument can 'fill' a room with sound of about the same loudness throughout, simply because the sound will be bounced back off the walls. This is called reflected sound.

Perception Of Delayed Sound

One reason for misjudging the proportion of direct sound is a psycho-acoustical phenomenon. By first travelling to a surface and then to the ear, indirect sound has a longer distance to cover than direct sound. As sound takes time to travel (about 1 ms for 1ft), it will arrive at the ears with a certain delay. The ear not only uses the first arriving sound (the direct one) to locate the sound surface, but also perceives it up to 10 dB louder than delayed sound. This is called the Haas Effect and is used to set instruments or vocals in the background of the stereo image by delaying it.

These reflections of sound can be seen in Figure 2. If you generate a sound impulse in a room, a microphone or a listener will be confronted with a signal looking like Figure 3.

After the direct sound, distinct reflections (echoes) will arrive, becoming more and more dense, so that finally they form a continuously decaying diffuse soundfield which is called reverberation. This theory should help you to use your reverb unit to achieve a more realistic effect. By delaying the reverb by 30-90 ms, and maybe even adding one or two distinct echoes (delays) within this gap, you can simulate room acoustics better than by just adding reverb to a sound.

Room Acoustics

Part of the sound in a room will be reflected by the surfaces, the rest being absorbed. The degree of absorption varies considerably for different frequencies, depending on the material of the surface. Together with the room's resonant frequencies (most distinctive in the bass) this forms the room's 'frequency response'. The size of the room will become apparent acoustically through the amount and delay times of echoes and reverb.

Proper room acoustics contribute a lot to a great drum sound. For a live sound, place the drums on a hard, reflective floor. Carpet will produce a tighter sound. If you locate the kit in a corner, low frequencies will be boosted. Also, try varying the height of the floor tom(s), it will alter sound reflections from the floor.

When miking up your drums it's best to walk around in your recording room while the drummer plays and find a distant spot where the sound is fine. Place a microphone there and add it to the mix. If you have enough tracks, experiment with more mics.

Getting A Big Sound

Getting a 'big, live' drum sound is something often aimed at in recording rock music. Large sized drums seem to be popular with most drummers who get a big sound. For studio work, however, smaller tom-toms (10"-14" rack, 16" floor) will do the job as well, if tuned properly. Single-headed rack toms are more suited for live playing than for studio work, but they are out of fashion anyway.

Tune the kick drum as low as possible, just before it sounds flabby. Tune toms low as well. Try to dampen them as little as possible and go for a good acoustic sound. Locate the drums in an adequately live room with a hard, reflective floor.

Miking up the toms should be done from quite some distance, maybe 1ft away. For bass drum mic positioning it's best to experiment. Employing a room microphone supports the bass drum and often picks up the sound of the snares that tends to get lost, if the snare drum is miked too closely. A popular trick is to place other instruments' microphones a little further away and make deliberate use of the leakage from the drums to add a live feel to your mix.

Contemporary Sounds

Today's drum sound is basically achieved by applying gated reverb. Phil Collins, who pioneered this technique with Peter Gabriel, started out by recording the drums in a big, reverberant room and cutting off the reverb with a Kepex-gate. In the meantime, the technique has been widely diversified. The principle is portrayed in Figure 4.

First the drum sound is enhanced with reverb. This may be done by simply recording in a live room. More often, the dry signal is sent into a reverberant room via a monitor speaker and picked up with one or more microphones. Finally, the reverb is cut off, either by using the gate's key input or simply by the reverb level dropping below the gate threshold.

This processing may be applied both to individual drums (snare, toms) or to the whole kit. Advanced processing of a snare drum beat, including the use of a delay and an expander-gate, may look like Figure 5.

First the signal is delayed, then reverb is added via a reverberation chamber. The reverb's decay is compensated for by the increasing gain of the expander (set for long attack time) and as the reverb level drops below the expander's threshold, the signal is gated off ie. cut out completely.

If you use a reverb chamber, try different rooms, even the bathroom. For playing back the signal, use a good speaker cabinet (studio monitor, hi-fi cabinet) and check out different microphone positions. One example might be to have one mic in front of the speaker, and another one far away, pointing to and very close (less than 1") to a large reflecting surface to get the PZM effect. Use a microphone with a flat frequency response (probably a condenser) like the AKG CK1/C460. Adjust the playback level to the room size (the bigger, the louder) and your neighbour's sleeping hours!

Drum Machines

Drum machines, if played back in a room and recorded with a microphone, can be easily upgraded soundwise. If you process the components separately, run the bass drum through a speaker and mike it up close (less than 1 ft) with a bass drum microphone: AKG D12, E-V PL20. The snare might also be treated with gated reverb. Alternatively, a complete stereo mix of the drum machine may be played back into a medium dead room and miked up with 4 microphones: 2 in front of the speakers and 2 for ambience.

Lately drummers have started using drum machines on stage, triggering the digital bass and snare drum sounds with those from the acoustic kit, and mixing both. As machines sacrifice the normal dynamic range (usually only one dynamic step 'accent' is available), the digital sound adds impact and clarity to the live drums: a way of 'cleaning up' drum tracks in the future.