Recording Brass & Reeds |

Brass and reed instruments present some challenging problems to the recording engineer, whether played by soloists or sections. Hugh Robjohns offers some hints and tips. Recording any musical instrument is a challenge if you want to capture a faithful rendition of the performance without distorting the tonal quality of the instrument or veiling it with undesirable mechanical noises. The full family of brass instruments as we know them today, including the cornet, trumpet, trombone, tuba, and euphonium, date from about 1850, by which time their designs had been optimised. The last major evolution was the invention of the valve as a means of introducing or bypassing sections of tubing and thereby allowing the performer to alter the tuning of an instrument whilst playing. Previously the only means available was a slide (like the modern slide trombone), and although short‑slide trumpets were popular in the 19th century, the valve offered a more practical solution.

The basic principle of the trumpet (and its cousins) is that blowing into the tube through taut lips sets up an audibly resonant vibrating column of air. Figure 1. Trumpet.A characteristic of brass instruments which everyone knows is that they are all very loud! However, their loudness varies with pitch, because it requires much more energy to force the tube to resonate at the higher harmonics. For example, measured at four metres, a trumpet can produce well in excess of 96dB SPL, and easily over 130dB within 0.5m of the bell! In terms of the frequency spectrum, brass instruments all tend to be rich in harmonics. For example, although the fundamental of a trumpet is centred around 1200Hz, it generates strong harmonic components beyond 8kHz for the upper notes, with the implication that the harmonic series continues well into the ultrasonic region. The harmonic balance also varies enormously with volume, the fundamental and first overtone remaining relatively constant but the upper partials exhibiting a much greater dynamic range.

Thus the trumpet (see Figure 1) can sound quite mellow when played softly, but brilliant when blown hard. Figure 2. Trombone.The trombone, on the other hand, has its fundamental centred around 550Hz (see Figure 2), but can produce frequency components above 10kHz when played hard. When played more softly, however, the balance of harmonics doesn't change quite as much compared to the trumpet, so it retains much of its rich and bright quality even during quieter passages. The lowest brass instruments are the bass and contrabass tubas (and their relatives) which are capable of generating notes as low as 29 Hz (a low Bb), although the first overtones are generally much stronger than the fundamentals. Reed instruments include the clarinet, saxophone, oboe and bassoon amongst others, but I will concentrate mainly on the clarinet and sax as these are the most common in popular music. However, the principles discussed here apply broadly to the whole family.

Blowing across the reed forces it to vibrate, and thus stimulate a column of air within the tube of the instrument to vibrate in sympathy. The effective length of the tube is determined by the keys which reveal holes setting the acoustic length of the resonant pipe. The key mechanism is merely a practical method of allowing human fingers to cover and control the numerous widely spaced holes. The clarinet produces a sound in which the odd‑numbered harmonics tend to predominate over the even‑numbered ones, although in the very highest registers the even harmonics become stronger. The prevalence of odd harmonics creates the characteristic 'hollow' sound quality, and these extend well beyond 12kHz for the higher notes. Saxophone.The saxophone is, by any standards, an odd instrument combining a conical brass tube vaguely similar to that of a trumpet, with a reed mouthpiece like a clarinet's! Capable of a very wide dynamic range and possessing a rich combination of harmonics which extend well up to the 12kHz region and higher, the saxophone is, to all intents and purposes, a metal‑bodied clarinet.

However, the increased size of the pipe bore makes it capable of greater volume, and the bell means that it radiates sound differently. Figures 1 and 2 (above) show how different bands of frequencies radiate from trumpets and trombones. The high frequency harmonics tend to beam directly to the front of the bell, with the lower frequencies being radiated over a wider arc. Although it might seem the most obvious place to position a microphone, the most common mistake is to position microphones directly on‑axis and far too close to the bell. In a live PA situation, this may be the only possible solution to achieving enough isolation, but it certainly isn't the best technique for recording! I would recommend starting with a mic placed at about two metres from the bell and about 30 degrees off‑axis. With trumpets it is usually easier to mic from above because most players angle the instrument downwards, but with trombones I would generally come in from below.