Author Topic: Resonator Length  (Read 8028 times)

Herb

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Resonator Length
« on: May 11, 2013, 06:31:32 AM »
Hi Jim,
Is it possible to use a Resonator with a smaller diameter (say 40mm instead of 50mm). Is there a formula to work out the rough length to get close to the right pitch?  Does it make any difference to the sound?

Jim McCarthy

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Re: Resonator Length
« Reply #1 on: May 13, 2013, 01:33:55 AM »
Hi Herb.
Well ideally we try to match the internal diameter of the resonator as closely as possible to the width of the bar. The reason for this is that wider resonators take more energy away from a bar than smaller ones.... because of this they sound louder... but for less time. Wider bars also produce more energy than narrow ones though... this is why bars get wider as they get lower - to compensate for a human's reduced loudness perception at lower frequencies. So matching the resonator diameter to the bar width tends to keep the sustain and volume even and correct sounding over the instrument range.

That being said.... we certainly don't have to be too fussy about this. There are many reasons why a smaller tube might be used - most commonly because matching the INTERNAL diameter often means with thicker wall pipe, that the EXTERNAL diameter of a matching end cap ends up wider than the bar plus 1/2" gap between bars - meaning they don't fit in the space!

So using a smaller diameter tube is just fine. The difference will be not so noticeable at all with just 20% difference in diameter. A slight decrease in oomph from the sound if you really had to specify a difference... and perhaps a fraction more sustain if you really listened critically.

For this difference - don't even worry about the length change - it will be so small as to not worry. Technically you get the length from: 1/4 of the wavelength of the note minus 0.61R where R is the pipe radius. So you see the difference in diameter makes very little difference to the length in this case. In fact you will get a BIGGER difference because of changes in air temperature.

Really even with this calculation, I have found that you actually need to tune your resonators if you want them accurate - you can't just cut to calculated length. It will be very close, but tuning is better. There are just practical imperfections that affect the results. Try to tune in a room at a constant temperature that is what the instrument will be played at - I usually go for 21 degrees Celsius.

Hope this helps!
Jim

Herb

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Re: Resonator Length
« Reply #2 on: May 17, 2013, 03:38:58 PM »
Thanks that helped, the other thing I would like to know, does the temperature affect the bars and the resonator's equally, in other words do they stay in tune to each other. 

Jim McCarthy

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Re: Resonator Length
« Reply #3 on: May 18, 2013, 12:31:02 AM »
Temperature changes are very much the enemy of bar percussion tuning!
The situation is worse than the bars and resonators not being affected evenly - they are affected in the opposite direction to each other! An increase in temperature will make the bars go DOWN in pitch, but the resonators go UP in pitch.... therefore it is quite important to fine tune both as close to the final playing room temperature as possible. For example when fine tuning bars I tune them to perfect correct pitch, then place the bars in a temp controlled room - just my lounge room with the air con on set to 21 degrees - and leave them for at least 10 minutes there to equalize. Sanding the bar will create friction and warm it up, so after ten minutes in the room I take it back to the sander and quickly touch tune it again as it will have gone up in pitch above the zero mark a little. I repeat this a couple of times to make sure it ends up correct. I make sure I tune resonators in the same room.

The whole point of having that adjustable height on resonator banks, is to compensate for temperature variations. Sometimes if we play the marimba at higher temperatures it will start to sound a little off - the resonators and bars get out of tune with each other so the system becomes less efficient and we hear a sound that is both less harmonious and less fat sounding. This is a common enough scenario as stages sometimes get warm because of lights etc and studios often just heat up! Warmer temps mean that the resonators get too sharp in pitch for the bars.... bars go down and tubes go up.... so by raising the tubes up a little closer to the bottom of the bars we can actually make those tubes flatten in pitch to better match the bars "high temp" pitch. This is because of a kind of proximity effect. You may have noticed that the pitch of a tube will go down if you partially close the mouth of the tube..... having the tube mouth a little too close to the bar has this same effect... and in this case we are using this effect to our advantage.
Jim

Almonte

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Re: Resonator Length
« Reply #4 on: May 30, 2017, 12:25:27 AM »
Temperature changes are very much the https://www.gabeba.com/testo-fuel-review-results testo fuel enemy of bar percussion tuning!

Too right. Can you give any tips on keeping temps stable Jim?
« Last Edit: October 28, 2017, 05:11:36 PM by Almonte »

Jim McCarthy

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Re: Resonator Length
« Reply #5 on: May 31, 2017, 11:23:24 PM »
Hi there Almonte!

SO do you mean whilst tuning the bars/resonators??? Or do you mean on stage - or in your home perhaps?

I've both performed and built now in both extremes of climate - from South Australian Summer to Canadian winter... so I have developed a few strategies for most scenarios now!
Jim