Given that this issue of MT is brass themed, and we have been looking in recent months at how to record different instruments, it seemed sensible to focus on capturing brass instruments in this issue's technology column.
Brass instruments can be captured in a number of different ways, but they all present the same basic problem. They can be loud. You'll note I said ‘can be’ rather than ‘are’, because obviously the way they are played will have a huge impact on the volume that comes out. However, because brass instruments have the potential to be loud, and a student recital or an A level recording task is likely to have a wide range of dynamics in it, we should look at the best way to approach brass with this in mind.
SPL
The fancy ‘music tech’ term for volume is sound pressure level, or SPL for short. This is the technical term that actually analyses the volume in decibels, between the lowest perceivable sound (a leaf falling) and the absolute loudest sound possible (probably Brian Blessed shouting). Sound is measured on a logarithmic scale, so it gets 10 times louder for every 10 decibel (dB) increase. So, 30 dB is actually 1,000 times louder than one dB. I hope that makes sense? Therefore, when I tell you that a cello averages around 85 dB and a trombone averages over 100 dB you can imagine the actual difference in volume. Plus – a trombone bell is right next the ear of the player, whereas a cello is some distance from the ear, and volume is affected by proximity of the listener to the sound source.
Proximity and axis
Sorry if the above got a little physics-y (actually I'm not sorry – I find this endlessly fascinating) but it's relevant to the actual techniques we need. As discussed above, volume is affected by the proximity of the sound source to the listener – or to the microphone.
There are two common types of microphones found in music departments: condenser and dynamic microphones (if you're lucky enough to have a ribbon mic, good for you!). Both of these microphones have their advantages and disadvantages. Condenser microphones are more sensitive and, therefore, can potentially get damaged by very loud sounds such as a brass instrument played fortissimo. Dynamic microphones are much less sensitive and can handle loud volumes easily, but they suffer more from something called the proximity effect. This changes the amount of bass captured depending how near or far the microphone is from the source.
When you position the microphone in front of the brass instrument, you can also change the angle that you position the microphone in relation to the instrument. When you face the microphone directly towards the sound source this is called ‘on axis’ and gets the loudest capture. It also tends to be a slightly brighter sound. However, when you move the microphone out of the direct line of fire at a 45-degree angle this is called ‘off axis’. This will reduce the impact of the volume from the sound source but will have a slightly darker sound than ‘on axis’. This is due to the way different frequencies travel, bend and reflect (I'll save this explanation for another time). So, both ‘on’ and ‘off axis’ microphones have their advantages and disadvantages – and both can be useful.
Using the knowledge
I've just covered quite a lot of maths and science in relation to putting a microphone in front of a brass instrument. But as you can see, choosing different angles and microphones will capture a slightly different sound, particularly when the source material is loud. If you have the time and the equipment, it is worth doing some sound checks before recording your brass players to find the best combination of microphone, angle and distance for your recording.
Obviously, this is quite a detailed set of combinations for basic recording, and you can of course record ambiently if you find this easier. And in fact, if you are working with a brass ensemble, this is probably the best thing to do anyway. For more on ambient recording, revisit the technology column in October 2019's issue.
