Bass Djembe

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Re: Bass Djembe

Postby e2c » Thu Mar 17, 2011 8:17 pm

A little bit of shea butter goes a long way... it's mainly about the playing time required to get the head broken in. (Same with newly-skinned duns.)

You're aiming for a different sound than you get with "solo" djembes headed with cow - a full, rich, mellow one, as with dununs.

(I guess it's kind of obvious that I favor the low end sound... :))
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Re: Bass Djembe

Postby michi » Thu Mar 17, 2011 11:49 pm

Rhythm House Drums wrote:There are lots of variables to control.. however, it's not a matter of opinion here... it's the facts. In audio engineering think of a subwoofer, a smaller port means lower frequency.

Yes. The resonant frequency of a Helmholtz resonator is given by the following formula:

Resonant-Frequency.jpg
Frequency of a Helmholtz resonator
Resonant-Frequency.jpg (22.08 KiB) Viewed 1008 times

V₀ is the volume of the cavity. A larger cavity means lower bass (because the cavity appears in the denominator).

A is the area of the neck. Because that appears as the numerator, a smaller hole means lower bass. The length L of the foot also plays a role: a longer foot means lower bass.
a djembe with smaller 'port' will have a lower bass with less attack...

It will have a lower bass, but I'm still not sure that a smaller hole will result in less attack, both from my own experience and theoretically. Increasing the hole size increases the inertia of the air in the resonator. More inertia means lower ability to quickly propagate a pressure wave, which translates to less attack. A larger hole also decreases the velocity of the air moving in and out, which also translates to less attack.

My personal experience with drums with a large hole is that they tend to go "boooiiiing". That sort of makes sense intuitively too: the resonant frequency of shell with a large hole is higher and therefore closer to the resonant frequency of the skin, which allows the skin to swing longer before things dampen out.

For drums with a small hole, I find that the sustain is short and the attack sharper. The sharper attack would be due to the higher air velocity; the shorter sustain would be due to increased difference between the resonant frequency of the shell and the skin, causing the system to dampen out more quickly.

If you want a sub to have a lot of attack, you dont port it but have it facing out, if you want it to have a deep rumble, you put it in a bandpass box (fully enclosed) and put in a small port.

I honestly don't know whether the comparison is correct because an unported enclosure is no longer a Helmholtz resonator but here, we want to compare Helmholtz resonators with different port sizes.

Not really so... if you tighten the head you will bring the bass note up in pitch. So they are not at all independent... a tighter head will vibrate with less movement, causing smaller sound waves = higher pitch.

That does not align with research by Albert Prak:

As Ben already pointed out, the Helmholtz resonator produces the bass, not the membrane. I still had difficulties to believe that the rope tension doesn't have an effect on the bass frequency. So I made up for the decisive experiment. I measured the bass frequency (mic at the end of the pipe of course): 77.6 Hz. I untied 5 knots. Slaps sounded awful. Same experiment. Guess what? 76.5 Hz. The rope tension did indeed have little effect on the the bass frequency. (PS I just read that Kent Multer has the same experience).

Cheers,

Michi.
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Re: Bass Djembe

Postby Rhythm House Drums » Fri Mar 18, 2011 3:21 am

... but then you have to take into account the relative humidity, the moisture content on the shell, the density of the wood, the thickness of the shell, the elevation (air pressure) and the size of you're ear hole, the acoustics of the room you're playing in, you're proximity to the drum relative to the room, the phase shift of reflection...

This math stuff can really get in the way of just enjoying a drum!! so a note to anyone other than sadistic analysts of sound ... enjoy your drum and find out what works for you! Too much science takes away the spirit and joy of drumming. Someone with a huge spirit and good rhythms can make a crap drum sound inspiring despite what lack of golden ratio was used in it's making.

But for me...
The length L of the foot also plays a role: a longer foot means lower bass.

True, this is expressed in ported subs also.


The hole size thing... it's more about the size of the hole relative to the size of the head.. but given all other factors are the same, a smaller hole has less attack, and more sustain.

From Wiki on the Helmothden thing :)
This effect is akin to that of a bungee-jumper bouncing on the end of a bungee rope, or a mass attached to a spring. Air trapped in the chamber acts as a spring. Changes in the dimensions of the chamber adjust the properties of the spring: a larger chamber would make for a weaker spring, and vice-versa.


To me this varifies what I said. especially the "Air trapped in the chamber acts as a spring" I read that as meaning, the air can not escape as quickly with a small hole... so it bounces back and forth creating lower frequency and.... longer sustain. As opposed to a large hole where there is less back and forth, less pressure, more attack and less sustain.

If you want a sub to have a lot of attack, you dont port it but have it facing out, if you want it to have a deep rumble, you put it in a bandpass box (fully enclosed) and put in a small port.


I honestly don't know whether the comparison is correct because an unported enclosure is no longer a Helmholtz resonator but here, we want to compare Helmholtz resonators with different port sizes.


Me either, it just sounded good :)

With a djembe the attack comes off the head, not really through the drum, which disqualifies some of the higher pitched sounds from the Helmholts thingy, right?? Seems like only the sound generated from air being pushed through the drum would apply to this, like the bass... because there is SPL built up in the shell as the head is struck, the volume of the attack is on top (mostly). This is why we mic the top for tones and slaps, we want the attack. Less resistance in the shell (larger hole) means quicker escape which means more attack... for the bass note.

I tell my students that the slap comes off skin, the tone comes through the shell, and the bass comes out the bottom.... It's how I visualize it...

It's late and I'm not sure what I'm typing anymore... :) :uglynerd:
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Re: Bass Djembe

Postby Rhythm House Drums » Fri Mar 18, 2011 3:33 am

That does not align with research by Albert Prak:

As Ben already pointed out, the Helmholtz resonator produces the bass, not the membrane. I still had difficulties to believe that the rope tension doesn't have an effect on the bass frequency. So I made up for the decisive experiment. I measured the bass frequency (mic at the end of the pipe of course): 77.6 Hz. I untied 5 knots. Slaps sounded awful. Same experiment. Guess what? 76.5 Hz. The rope tension did indeed have little effect on the the bass frequency. (PS I just read that Kent Multer has the same experience).


Hmm.. this works to an extent, you'd have to mic the top too.. of course there is less change coming out the bottom of the drum, because that is determined more by the shape of the shell, but the majority of the attack comes off the top... but even so, his experiment shows lower frequency at a looser drum.. which means, :dance: I'm right :) haha. but really, it can't be said that head tension and bass frequency are independent of each other. And on a practical basis... next time you head a drum, hit the bass as you tune it tighter, that'll give you your answer.
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Re: Bass Djembe

Postby e2c » Fri Mar 18, 2011 7:29 am

Interesting tor realize that most members here live in very different kinds of climates.

In other words, the norm in my particular part of the US's Eastern Seaboard is likely not the same as the norm where michi is, or RHD, or d-n, or... And likely most of us live at differing heights above sea level, which also plays a part in this.

That (imo) makes a difference in what we all hear on a day to day basis, in terms of tuned/not tuned, etc. (Humidity and its effects on the shell and skin - or lack of humidity, etc. etc. etc.)

Must admit that I'm not really into the scientific analysis side of this, although I can see why others might be.
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Re: Bass Djembe

Postby michi » Sun Mar 20, 2011 12:43 am

Rhythm House Drums wrote:... but then you have to take into account the relative humidity, the moisture content on the shell, the density of the wood, the thickness of the shell, the elevation (air pressure) and the size of you're ear hole, the acoustics of the room you're playing in, you're proximity to the drum relative to the room, the phase shift of reflection...

Moisture content, density, thickness and so on wouldn't affect the bass very much, I would think. That's determined largely by the proportions of the drum. The room will have some effect though.

This math stuff can really get in the way of just enjoying a drum!!

Yes, I agree with that. Maths are strictly for nerds :)

especially the "Air trapped in the chamber acts as a spring" I read that as meaning, the air can not escape as quickly with a small hole... so it bounces back and forth creating lower frequency and.... longer sustain. As opposed to a large hole where there is less back and forth, less pressure, more attack and less sustain.

I don't think that's how it works. The pressure wave that is started when you hit a bass has to make its way out of the bowl. With a smaller hole, the pressure wave has less area to get through, so its velocity increases. With a larger hole, the velocity is lower. I think that this would translate to sharper attack and less sustain with a smaller hole.

With a djembe the attack comes off the head, not really through the drum, which disqualifies some of the higher pitched sounds from the Helmholts thingy, right??

The bass comes pretty much exclusively from the foot. You can feel the pressure if you hold your hand in front of the opening while hitting a bass.

This is why we mic the top for tones and slaps, we want the attack.

Yes, but a bass doesn't have that much attack on the skin. Most of the sound is the low rumble that comes out of the foot. To me, sharper attack means a steeper slope in the rise of volume at the beginning of a bass. The higher air velocity caused by a smaller hole should result in a sharper attack.

Less resistance in the shell (larger hole) means quicker escape which means more attack... for the bass note.

I don't think so, because the larger hole means that the air actually moves slower.

I tell my students that the slap comes off skin, the tone comes through the shell, and the bass comes out the bottom.... It's how I visualize it...

I agree with that :)

but even so, his experiment shows lower frequency at a looser drum.. which means, :dance: I'm right :) haha.

The change in frequency is miniscule—it's essentially the same frequency. Pretty much no-one can tell the difference in pitch between a 77.6Hz and 76.5Hz bass.

And on a practical basis... next time you head a drum, hit the bass as you tune it tighter, that'll give you your answer.

That's a very sensible suggestion :)

I currently have a student drum sitting here waiting to be re-skinned. I'll record the bass before and during the dry pull when I do the drum. A few Fourier transforms later, we should have an answer. Stay tuned :)

Cheers,

Michi.
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Re: Bass Djembe

Postby bubudi » Sun Mar 20, 2011 12:55 am

lol michi & rhd :uglynerd: :uglynerd:

i love you guys!
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Re: Bass Djembe

Postby Djembe-nerd » Sun Mar 20, 2011 1:13 pm

This math stuff can really get in the way of just enjoying a drum!!
Yes, I agree with that. Maths are strictly for nerds :-)


Even the Nerd is confused now :giggle:
If you want to see me kick some butt, just tell me about all the things you think I won't be able to do
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Re: Bass Djembe

Postby Rhythm House Drums » Mon Mar 21, 2011 10:32 am

I'll get some recordings too :) I've got one drying right now.
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Re: Bass Djembe

Postby michi » Mon Mar 21, 2011 10:58 am

Nerds rule! :-)

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Re: Bass Djembe

Postby michi » Wed Mar 23, 2011 5:15 am

For those of you who don't want to read through the entire analysis, for the record: the bass stays put as the drum is tuned and only tones and slaps go up in pitch as the skin tension increases. So there, I was right :)

I re-headed that student drum and took recordings. I used my Sony PCM M-10 for the recordings, set to 96kHz sampling rate at 24-bit resolution. I left the level setting and microphone position identical for the three recordings, and I made sure that me and the djembe were in the same position each time. I also did my best to strike consistently every time, not harder or softer. The microphone was positioned 60cm from the drum, at the same height as the waist of the drum.

Each recording contains bass-tone-slap followed by a few basses. (These are MP3s to save space; of course, I did the spectrum analysis on the uncompressed WAV recordings.)

Here is the sound after the skin dried, without having done any dry pull:

Djembe 1.mp3
Sound before 1st dry pull.
(324.9 KiB) Downloaded 234 times

Here is the sound after the first dry pull:

Djembe 2.mp3
Sound after first dry pull.
(280.43 KiB) Downloaded 199 times

And after doing a second round of pulling verticals:

Djembe 3.mp3
Sound after second dry pull.
(195.88 KiB) Downloaded 193 times

Here is a spectrum analysis of the basses, in the same order as the recordings:

Bass.jpg
Spectrum analysis of basses in increasing order of tension.
Bass.jpg (354.56 KiB) Viewed 937 times

In all three recordings, there are spikes at 85/86Hz and 124/125Hz. What's happening here is that the bass fundamental (1st harmonic) is at around 42Hz. But that frequency is so low that it can't be sustained because both the drum and skin are too small for so low a frequency. So what we are seeing in these spikes is the 2nd and 3rd bass harmonic.

In recording one, there is another fundamental at 52Hz. We see the 3rd harmonic of that fundamental again at 156Hz. The next spike to the right (unlabelled) is at 205Hz, which is the 4th harmonic of that fundamental.

In the 2nd and 3rd recording, the spectrum no longer shows this fundamental. The 52Hz, 104Hz, and 156Hz bands are empty. However, both recordings still show the 4th harmonic of that same fundamental at 205Hz.

If you listen to the recordings, you will hear that the tone has increased by about a semi-tone, but the bass stays exactly the same. In the first recording, the fundamental at 52Hz is most likely a vibrational mode that is in a odd-numbered relationship with the frequency of the shell, which explains why it (and its 2nd and 3rd harmonic) disappear as soon as there is any real tension on the skin. The 4th harmonic at 205Hz finds a resonance with some part of the shell and so hangs around in all the recordings. (That same resonance is also present in the spectrum of the tones at different amounts of tension.)

Here is the spectrum analysis of the tones:

Tones.jpg
Spectrum analysis of tones in increasing order of tension.
Tones.jpg (410.57 KiB) Viewed 937 times

You can see the same components in the bass at 51Hz at low tension, and at 84Hz, 86Hz and 87Hz. The spike at 205/207Hz is still there, but much less dominant. The 3rd harmonic of the bass fundamental is also still visible at 123Hz, but is down -70dB.

The tones are visible in the spikes further to the right. For the before the dry pull, the tone fundamental is at 309Hz. After the first pull, it has moved to 342Hz, and after the second pull, it is at 352Hz. The additional spikes further to the right are various harmonics. At that point, the skin no longer vibrates in a clean nodal pattern, and you get additional harmonics from parasitic vibrations as well as beat effects between the different fundamentals. Still, it's pretty easy to see that the series of harmonics shifts to the right while the bass stays put.

Now, strictly speaking, the bass doesn't quite stay put. Strictly speaking, the shell and the skin are coupled resonators. That means that the overall resonant frequency is influenced by both. So, as skin tension goes up, the bass does move up a tiny fraction. However, the system is almost completely dominated by the shell, so the change in tension in the skin has a negligible effect on the bass frequency.

Cheers,

Michi.
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Re: Bass Djembe

Postby michi » Wed Mar 23, 2011 5:32 am

After completing the first row of weave, it sounds like this:

Djembe 4.mp3
Sound after completing first row of weave.
(240.65 KiB) Downloaded 208 times

Here is the spectrum of a bass:

Bass final.jpg
Spectrum of bass after first row of weave.
Bass final.jpg (123.82 KiB) Viewed 937 times

As you can see, the bass is still firmly stuck at 68Hz.

And here is the spectrum for a tone:

Tone final.jpg
Spectrum of tone after first row of weave.
Tone final.jpg (156.07 KiB) Viewed 937 times

As you can see, the tones have moved up to a 402Hz fundamental.

So, overall, we've gone from 309Hz to 402Hz in the frequency of tones, and the bass has stayed at 68Hz all along. That's a third of an octave change in tones with no change in bass, which is pretty convincing, I'd say :)

Cheers,

Michi.
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Re: Bass Djembe

Postby bubudi » Wed Mar 23, 2011 8:37 am

nice work, michi. that concurs with my experience, however i have found that the bass is 'lost' somewhat at a certain point, and that where this point is, varies from drum to drum, and possibly also on the skin. with some drums there is a very minor discernible difference in the frequency, but it's the volume of the bass that is most affected. these things are harder to measure accurately. there are variables in the drum shape and interior, and no skin is the same. also, when measuring volume, how do you know you're hitting the bass equally hard during each recording? our usual yardstick for that is how loud it sounds to our ears, but when it's the effect on volume that you're measuring, you can't do that!
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Re: Bass Djembe

Postby Rhythm House Drums » Wed Mar 23, 2011 7:14 pm

Nice work Michi! Guess I was wrong. Experience over theory everytime... :)
Little embarrassing as a drum builder, but hey, I'm never too proud to learn.
I love this forum!

On another note, your sound after the wet pull but before dry pull was lower than I expected., the drum was dry? My skins are much tighter once they dry out but before I tune. I wonder if this has much effect on the sound or longevity of the head?
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Re: Bass Djembe

Postby michi » Wed Mar 23, 2011 9:31 pm

bubudi wrote:that concurs with my experience, however i have found that the bass is 'lost' somewhat at a certain point, and that where this point is, varies from drum to drum, and possibly also on the skin. with some drums there is a very minor discernible difference in the frequency, but it's the volume of the bass that is most affected.

I've noticed this too. If you keep tightening the skin more and more, there is a point where the bass disappears. The first time I saw this was at a workshop where the teacher had brought his drum press along, and a student used it to tune his drum. We were standing there while he was flipping diamonds on the second row one after the other. He kept pulling more diamonds without checking the sound and, at some point, I noticed that he had almost completed the second row without hitting the drum once.

I walked over and pressed a little onto the skin. I swear that this is the tightest skin I've ever seen. It's a miracle it didn't break. When he took the drum off the press and hit it, it was insanely high, but there was no more bass. When you hit a bass, all you got was this dry "tok" sound with no body at all. (Imagine a bass where you still have the sound that comes from the hand making contact, but nothing comes out of the foot.)

When you think of the drum as coupled resonators, that makes sense. As the skin is tightened, its resonant frequency moves higher and further away from the resonance of the shell. At some point, they are so far apart that the skin simply can longer swing at the lower frequency demanded by the shell. What you get is a bass that tries to be too high and dampens out almost instantly. Needless to say, realising that his drum was within seconds of breaking, the guy let out five or six diamonds in short order :) The bass came back after that, of course.

these things are harder to measure accurately. there are variables in the drum shape and interior, and no skin is the same.

The shape of the drum is what determines the bass frequency. That's not a variable (for a given shell, that is). I don't think inside texture makes any difference at all. The low bass frequencies won't be affected by the texture. (The texturing affects the high-frequency overtones and reflections inside the shell, not the bass.) The skin will have an effect, but not on frequency. The frequency of the skin depends on its tension, not on its thickness. But sustain is obviously affected by the skin. A thick skin has more internal friction than a thin one and, in general, will have shorter sustain.

also, when measuring volume, how do you know you're hitting the bass equally hard during each recording? our usual yardstick for that is how loud it sounds to our ears, but when it's the effect on volume that you're measuring, you can't do that!

For this experiment, I wasn't interested in volume really, but in frequency, which is independent of how hard I hit (at least for the bass; we can have a separate argument about whether how hard I hit affects the pitch of tones and slaps ;) ) So, I just focussed on hitting equally hard each time. Not that difficult to do: the tactile sensation in my hand tells me a lot about impact force.

I know that this isn't terribly scientific and that I can't completely control variation across the different recordings, but I'd say I would have been pretty close to getting the same impact force each time. BTW, the recordings show that, for consecutive strikes, volume is very much the same. The following shows bass, tone, slap followed by six basses. The amplitudes of the basses are very nearly equal.

Amplitude.jpg
Bass, tone, slap, followed by six basses.
Amplitude.jpg (108.88 KiB) Viewed 908 times

Cheers,

Michi.
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