• RobT

4. Lug Tuning and Clearing/Equalizing the Drumhead

Updated: Oct 14, 2020

Lug tuning is sometimes called 'equalizing the drumhead' or 'clearing the drumhead' and ensures that the tuning is even at every point around the perimeter of the drum. We often evaluate this at each of the lug positions around the drumhead, and make adjustments with the tension rods that connect the hoop and drumhead to the lugs and the drum shell. Accurate lug tuning is important because, when completed, it allows the drum to vibrate evenly and smoothly. An uneven (or 'uncleared' / 'unequalized') drumhead can cause unwanted frequencies to interact and cancel each other out, resulting in a pulsing, warbling or beating effect on the drum sound. If you can hear a tone that comes in and out, sounding like a 'wow-wow-wow' in the decay of the sound, then it is likely that the drumhead needs equalizing.

Before we start getting into the detail, let's have a listen to the difference in sound between drumheads that are tuned evenly and those which are tuned unevenly. The video here shows a few different tunings, all recorded from the edge at a lug position on a drum. In each case, the top (green waveform ) examples show a smooth clear decay to the sound. The uneven tunings at the bottom (red waveforms) have a definite modulation and acoustic 'beating', as the sound of different frequencies on the drumhead clash and interact.

We mentioned the differences between the fundamental (centre) frequency F0 and the first overtone F1 (excited at the edge of the drum) in the tutorial on drumhead vibration. The F1 overtone is the frequency to listen for when tuning at the lugs and clearing the drumhead. This is because, while the F0 frequency is the same regardless of where you hit the drum, it is actually possible for the F1 frequency to be different at different locations around the edge of the drum. As mentioned, if each point around the edge of the drum 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.

Unfortunately, because of the many different frequencies being excited on the drum at the same time, it can often be really hard to hear if different points are the same or not. It can even be hard to hear if one is higher or lower than another, because our ears get drawn to different frequencies and our brain finds it hard to interpret all the information. We call this the limits of ‘psychoacoustic perception’, but generally it means there are only so many frequencies our brain can hear accurately at one moment in time - so don’t feel inadequate if you find this aspect of drum tuning difficult!

The great thing about the iDrumTune app for tuning around the lugs at the edge of the drum, is that it’s programmed to ignore all of the other frequencies except the one that is most powerful, so it gives a reading and lets you know exactly which lugs are lower or higher than the first one that you hit. You can then make some fine adjustments and measure again. If you get each point to within 1 Hz then that is really pretty good, and not many humans can hear to an accuracy of 1 Hz anyway, apart from a few incredible individuals who have perfect pitch and can tune a piano with no assistance.

The video below explains the iDrumTune Lug Tuning feature, which shows how after a couple of attempts, it’s possible to get the frequency at each lug to within 1 Hz.

As shown with the Lug Tuning feature, if the deviation is much greater than 1 Hz higher or lower, then you can identify those lug positions and gradually even out the tension. This can be quite a challenge, as the drumhead is very sensitive to even small changes in tension. You will find that you only need to tension each lug very slightly up or down to even out the head, and it is very easy to go too far and make a position that was too loose suddenly too tight. Over time you will get a feel for how subtle you need to be to adjust the tuning accurately. If you get really stuck then we advise you to untighten all the lugs and then tighten them again with just your fingers (which gives a guaranteed even tension, albeit though slightly loose), before gradually increasing the tension with the tuning key and the Lug Tuning interface.

Something to be aware of, is that it is ESSENTIAL that you move the microphone to each lug point when taking readings around the edge of the drum. It’s a simple fact of physics, but if you leave the microphone in one place when you hit the drum at different locations around the drum, the results are likely to be very inaccurate. The reason is that the microphone acts as a very simple vibration sensor, and with lug tuning we are interested in how the vibration of the drumhead differs at different points on the drum - so if you leave the mic in one place, then the sound recorded will be either purely a reading of that fixed location (every time, regardless of where you hit the drum) or, at best, an averaged reading of all the vibration characteristics of the whole drumhead. It’s really important to get a very localised reading at each point for lug tuning, and that can only be done by moving the microphone to each lug as you take measurements - this is great, because it means we are taking accurate readings from much closer to the point of interest than our ears could ever get. It would be much easier to attach the phone to a single place on the drum and take lots of readings until you are happy, but, scientifically, that is not a valid approach and will lead you to thinking the drum is well tuned when actually it isn’t – you can only blame the laws of physics for that! (We'll mention this more in a future tutorial about verifying the accuracy of the iDrumTune Pro app).

Now, let’s just take a closer look at the issue we are correcting with lug tuning and clearing the drumhead. The F1 vibration mode is actually a kind of see-saw profile, where the edges go up and down, but the middle stays stationary (see the little animation here, to visualise).

Take a drum on a stand in front of you, and imagine the drum is tuned so that the frequency of the see-saw vibration is, say 150 Hz, which will be measured by iDrumTune at the very nearest and furthest lug points, i.e. at the due North and South points on the drumhead, if it were setup like a compass. Now, it’s totally feasible that the vibration in the direction perpendicular to this (i.e. from East to West) could be a completely different frequency, especially if those lugs are tighter or looser than the ones in the first direction – imagine it is a little tighter at say 155 Hz. In this condition, the drum attempts to vibrate at two very close but not exact frequencies, and in fact they start to cancel each other out and clash. As mentioned, the condition is called ‘beating’ in acoustics language, it basically means that the sound we hear beats up and down (or on and off), which ultimately causes a warbled, modulated sound. Because the two frequencies are very similar, they start off enhancing each other, but gradually as one vibrates faster than the other, they become more and more out of sync (or 'out of phase’ in acoustics language), to the point where they cancel each other out completely. Then they start to gradually come back into phase and the cycle repeats - this video below explains the concept of beating when two similar frequencies occur at the same time:

You can absolutely hear this sound, as we saw from the first video in this tutorial, in fact it is exactly the same effect as a tremolo guitar pedal or mixing plugin. If your drum has two or more clashing frequencies it can be very noticeable, and ruins the smooth decay of the drum when hit. It’s even possible to see these frequencies identified in iDrumTune, if you use the Spectrum Analyser mode, sometimes you will see two F1 peaks very close together, which is a sure sign that the drumhead needs clearing or equalising.

The image above shows the difference in frequency spectra between an evenly tuned drumhead (left) and one which is not evenly tuned (right). The well tuned drumhead on the left exhibits a strong F0 frequency at 102.0 Hz with a significant edge overtone at 170.5 Hz. In contrast, the drumhead on the right shows two very close frequency spikes in the F1 region (highlighted in blue), indicating that the drumhead is attempting to vibrate at close but different frequencies, and needs to be equalised with lug tuning. The evidence of two close spikes indicates there will be beating and modulation in the drumhead sound.

You can also clear the resonant drumhead with the same lug tuning approach, but just make sure you tap very softly, as to not excite too many of the batter head frequencies and confuse the reading (and your ears!). In fact, in the next few tutorials, we’ll be talking about the resonant drumhead in much more detail, so we can dispel some myths and frustrations related to the resonant head once and for all!

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.

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