2. Drumhead Vibration and the Science of Sound!
Updated: Oct 14, 2020
Drums generate sound through vibration, but what actually is vibration, how does that relate to the sound we hear when a drum is hit, and what does that have to do with drum tuning?
Let’s start at the beginning… vibration is quantified by the number of times something moves backwards and forwards in a single second, i.e. its frequency. The measurement unit for vibration frequency is known as Hertz or Hz. So, if a drum is tuned to 100 Hz, its drumhead will vibrate up and down 100 times in a single second - this is obviously too fast for us to see, but the vibration happens with a very small movement. So, when it is hit, the drumhead vibrates, and that vibration causes a very small change in pressure of the air molecules around it. The pressure disturbances also vibrate and transfer through the air to our ears or a microphone, so we can hear and record the information from a drum, or any sound source.
We call analysis of sound vibration ‘acoustics’, which essentially refers to ‘the physics of sound’ and is really useful for any musician or studio engineer to have a basic grasp of. Here's a cool little video that explains the very basic principles of acoustics, sound and vibration in 25 seconds!..
In the world around us, large heavy objects tend to vibrate more slowly than small lighter objects, so it's no surprise that a large diameter drum gives us a low frequency sound, and a small diameter drum generally has a higher frequency. This applies equally to guitar strings, where the thinner the string the higher the frequency of vibration. In musical terms, we refer to the strongest vibration frequency as the 'pitch' of the note being played. The tension of a string or drumhead also affects frequency. The tighter the drumhead, the higher the frequency, as applies also to guitar strings! So, by tuning the tension rods positioned at lugs around a drum, we can increase and decrease the frequency to achieve our preferred sound or pitch.
In fact, there’s one key acoustics theory that is absolutely fundamental to drum tuning. Understanding this simple theory will immediately improve your knowledge and ability with drum tuning, and as a result, you will learn the skill of drum tuning faster, with more knowledge of the instrument and you’ll find it much easier to get the sound you want from your drum kit. You’ll be able to tune more precisely, more consistently, and if you wish, to musical notes and intervals on the drum kit.
The theory applies to all musical instruments, but is particularly relevant for drums and drum tuning. It's quite simple, basically the drumhead vibrates at many different frequencies all at the same time. So when you hit the drumhead, many different frequencies are heard as the drumhead attempts to vibrate in different ways and shapes, we call these frequencies and vibration shapes 'vibration modes', and depending on where you hit the drum, some modes/frequencies resonate stronger and louder than others.
Two of these frequencies are the most important for drum tuning:
The fundamental frequency, which we call 'F0' is excited most when the drum is hit at the middle and sounds like a BOOM
The first overtone frequency of the drum, which we call 'F1' is excited most when the drum is hit at the edge and sounds like a PING
Here’s a short video by iDrumTune inventor Professor Rob Toulson to show you how to identify the two main vibration frequencies on a drum and to explain how these are used in drum tuning:
So, it's really easy to hear the difference between these frequencies, just by hitting between the middle and the edge repeatedly, you'll hear the BOOM at the centre and the PING at the edge. This applies to all musical instruments - if you take a guitar and strum it in the middle of the strings, the sound is warm and full, then if you strum closer to the bridge, you'll hear more overtones and the sound becomes quite thin with more treble. It’s exactly the same concept as hitting the drum in the middle or the edge.
We use the F0 fundamental (centre frequency) to set the overall pitch of the drum. This can be high for jazz type drumming or low for rock style drumming, and anywhere in between too. Every drum has a range between the drumhead being too slack or too tight, so there are lots of frequencies and tunings to experiment with, based on the type of music you are playing and your personal style too. The benefit of using iDrumTune here is to take a reading of the pitch and identify what frequency you prefer for each of your drums. Then you can make sure your drum is always tuned to this frequency every time you play, or if you play in different bands you might use different tunings for different music genres, so the app helps you quickly get the sound you need at any moment in time. It's also useful if changing drumheads, or to make a record of the tunings used in a recording session.
The F1 overtone frequency, which is most prominent when hitting the drum at the edge, is important to ensure that the drum is tuned evenly and vibrates with a clean and consistent profile. If one point has a slightly different frequency, this causes a vibration clash on the drumhead which makes the sound of the drum warble or modulate. In acoustics we call this condition ‘beating’ which occurs when two or more close but not exact frequencies occur at the same time. Beating in the drumhead means that the sound is not smooth, and you don’t get the pure tone of your drums when they are hit.
In acoustics and music production, we often use what’s called a ‘spectrum analyzer’ to see a graph of all the different frequencies that are heard in a sound signal. iDrumTune also has a spectrum analyzer feature built in, so you can see what vibration frequencies are being emitted by the drum, and you can then be more certain which is the F0 fundamental and which is F1 overtone. For example, if we hit a drum in the center, we expect F0 to be excited most, which is seen as a big spike on the spectrum analyzer at that frequency (image A). If we hit the drum at the edge, F1 is excited most and so we see the spike in a different place at a higher frequency (image B). What’s really cool is that if we hit the drum somewhere between the centre and edge, we excite both at the same time (image C) – our ears might not be able to pick out or distinguish between both frequencies, but the spectrum analyzer is more than capable of showing us what is happening in terms of drumhead vibration. It’s no surprise to see there are a few other much smaller spikes too, representing the other vibration modes of the drumhead that are less powerful. Generally for tuning, we can ignore these other modes, as the F0 and F1 frequencies tell us everything we need to know in order to tune a drum so that it sounds amazing!
It's good idea to go now to your drum kit and pay a little focused attention to each drum. First hit the drum in the centre and then hit the same drum at the edge. Listen to the difference in the sound and get used to hearing those two different frequencies in the drum sound. Now hit the drum repeatedly while moving gradually from the centre to the edge, notice how the sound changes and how somewhere inbetween you can hear both frequencies at the same time. Now move around the kit and try this on all your drums - after this you'll have just become a bit more knowledgeable and more intimately connected to your drum kit, which can only be a good thing!
In the next lesson, we’ll look in more detail at the F0 fundamental frequency and discuss how it relates to tuning different drums, before looking at the F1 frequency and some concepts relating to the resonant drumhead in later lessons too. But for now, you have just picked up some great knowledge that makes you a little more at one with your drums - understanding how they ‘work’ and what scientific theories relate to the sound, and ultimately the creativity, of the instrument!
Happy drumming, happy tuning!
By Professor Rob Toulson - Professor of Musical Acoustics and Inventor of iDrumTune Pro.
iDrumTune Pro is available in the Apple and Google Play App Stores for iOS and Android.