Acoustic recording techniques

Where should I put the microphones when recording my acoustic guitar?' ‘What's the best way to record a sax?’ ‘Can you recommend the best mics to use for recording piano?’ These are typical of the queries which pop up in online sound-recording forums. Fellow contributors often provide excellent advice, but what seldom appears in their response is the question: ‘What sort of music are you recording?’ This is important. As listeners, our expectation of what an instrument should sound like on a recording varies depending on the musical genre, and that sound is a consequence of recording technique.

To be fair to the online correspondents, the reason the question of genre doesn't arise is the assumption that everyone in the group is talking about pop or rock music, which they usually are. In school, however, you'll face the challenge of helping students record a wider range of styles, be it for HE or national youth ensemble auditions, or for digital music exam submissions. Thankfully, there's not a radically different way of doing things for each style of music, but rather two distinct approaches. They are commonly referred to by recording engineers as ‘pop’ and ‘classical’ techniques, as they are clearly recognisable in the commercial recording of those two common genres.

Pop versus classical techniques

Interestingly, the recording equipment used by engineers recording classical and pop music – microphones, mixing desk, recording device, and loudspeakers – are often identical. The key difference which separates one style of recording from the other is microphone technique, summed up in one word – distance. In pop recording the mics are close, sometimes no more than a few centimetres from the sound source. For classical, that distance increases significantly, with mics placed anything from a metre to several metres away.

There are a few other differences. Classical music is recorded in large spaces with notable acoustics: concert halls, large studios, churches and cathedrals. Those venues have reverberation times of 1.5 to a few seconds long, and these acoustics are an important part of the sound. In contrast, pop music is usually recorded in smaller, acoustically-dry studios with reverb times of less than one second.

One further contrast in approach is the number of microphones needed. It's perfectly possible to make a broadcast-quality, stereo recording of a symphony orchestra of 50 players with two high-quality microphones. However, it would be almost impossible to produce the authentic sound of a rock band of just four players with the same setup.

Orchestral recording

In practice, few commercial recordings of classical symphony orchestras are made with two microphones. However, although the engineer might rig 20 or more mics, by far the greatest part of the sound the listener hears will be coming from just four or five main mics placed along the front of the orchestra at a height of about three metres. All of the other microphones, known as ‘spot mics’, will be dotted around within the orchestra, closer to individual instruments or sections, and blended into the main sound to focus quiet or distant instruments, enhance solos or add detail.

Pop band recording

In pop/rock band recording, there's much more creative intervention in the sound by the engineer. Processing such as gating, compression, EQ, reverb and echo are added to individual instruments at the time of recording and later, during the mixing. Placing a separate mic close to each instrument and recording in a non-reverberant space creates the close dynamic sound which the audience expects. It also enables the recording of instruments cleanly onto separate tracks and thus the choice of which sounds are processed, without affecting others in the mix.

Why genre matters

It's worth pausing to consider why these two approaches to microphone technique exist; why is it that we have an expectation of difference in recorded sound between the two genres?

The answer, in large part, is to be found in performance practice at live concerts. Classical music is performed in acoustically rich halls with no amplification. Audience members can be tens of metres away from the performer. They hear the natural sound of instruments in the hall – a combination of the direct sound from the ensemble, and reverberant sound reflected from the walls, floor and ceiling. In contrast, at pop gigs the common use of drum-kit demands that other instruments and voices in the band are amplified, simply in order to be heard properly. The only way to achieve that successfully (if you're to avoid the dreaded ear-piercing feedback), is to mic each instrument and singer closely, with directional microphones. Most of the sound the live audience hears will be from loudspeakers, projecting the very direct sound of those microphones. Of course, in practice, even the drums have microphones, and then everything's cranked up to satisfy the genre's insatiable appetite for the very loud!

How it's done

Let's take a closer look at the techniques which reflect those live performance traditions in action, using solo piano recordings as an example: the same grand piano, the same studio, two very different set-ups for microphones.

The idiomatic pop/rock/jazz piano sound is achieved by positioning the microphones inside the piano, with the lid open full or on half-stick. The microphones sit roughly 20–40cm above the strings, one covering the treble end, the other taking care of the bass. The microphone directivity pattern is usually ‘cardioid’, meaning it picks up from only the front.

Not surprisingly, this set-up produces the aural effect you would get if sitting right next to the instrument: a close, acoustically dry sound, no matter if the instrument is in a small studio or concert hall, such is the dominance of the direct sound of the instrument.

For the classical sound the microphones will be anything from two to four metres away. They will often have a more open directivity pattern such as ‘figure of eight’, which picks up in front and behind, or ‘omnidirectional’, which, as the name suggests, picks up all around. The combination of the distant placement and fewer directional microphones produces a result that captures the classical concert-hall performance sound.

Try it out

Realising that recording facilities and expertise will vary across schools, it's worth saying that even with a small portable stereo recorder with built-in microphones, it's perfectly possible to adopt aspects of these classical and pop recording techniques. Here are some guidelines for trying them out with a piano recording.

For all recordings, try to work in a space or at a time where you won't be impacted too much by external noise. Use the best instrument available; hopefully, the tuning will be in decent shape. Have your portable recorder on a sturdy stand. If the recorder is the type with small external microphones, ensure that one is pointing left and the other right, with a right angle between the two.

If recording classical music (in its broadest sense), try the following:

• record in the school hall or a large classroom, for more appropriate acoustics
• place the portable recorder on a stand at about head height, or a little higher if you can
• place the stand about two or three metres in front of the piano
• have the mics point towards the strings and more towards the hammers than the tail

If recording jazz, pop, rock and so on, try the following:

• use the best piano available, but avoid ‘churchy’ or large acoustics
• place the portable recorder on a stand at about chest height
• place the stand right in front of the piano
• have the mics point towards the strings and more towards the hammers than the tail

If you have time, experiment with the mic stand at different distances from the instrument and with pieces of different genres so that you can hear the range of effects possible. You could also try with other instruments, ensembles and singers. For example, if recording an unaccompanied vocalist, you might have the mics about 30–50cm in front of them for a musical theatre piece, but achieve a more appropriate sound for an operatic aria a metre or so away.

For schools with modern recording studio facilities, it's possible that it's still better to record classical repertoire in the school hall for the reasons discussed above, and to use the studio for other styles more suited to a close mic'ing.

Adopting elements of these genre-related recording techniques into student recordings should result in a more idiomatic and authentic sound.

Online audio examples

• Jazz piano sound, ‘pop’ microphone technique: tinyurl.com/2a4dayzm
• Classical piano sound, ‘classical’ microphone technique: tinyurl.com/py9dfa5e