Using wool felt for baffles?

[Jason]

I was wondering if wool felt for baffles would be a good choice to mellow the sound of a concertina? It would look pretty too. Would it be breathable enough or would it need a standoff like leather? Thoughts? 

Edited March 2 by Jason

[d.elliott]

sounds like a lot of free fibres and a potential choke on airflow to me. 

[ttonon]

Hi Jason,

 

What do you mean by "mellow" the sound?  My guess is that you mean less harsh, which often translates to a reduction in higher overtones.  If that's the case, I don't think it would do that because felt would likely attenuate most all the overtones, resulting in only a reduction in volume.  

 

In some full size accordions, makers invented what they call a "cassotto," which is a 5-sided box, or slot, or chamber, in which the pallets are mounted.  This acoustic feature is fairly effective at mellowing the tone for much of the pitch range of the instrument.  It's most effective on the lower notes, because of their frequency relation to the dimensions of the chamber, and I don't think such a device can be effective the entire pitch range.  It's used by most jazz players in the States, who describe the sound as "ballsy." 

 

I suppose it's possible to build such a thing for a concertina.  Good luck!

 

Regards,

Tom

www.bluesbox.biz

[March Hare]

Hello Jason,

 

I think the felt is worth trying.  I have glued a layer of thin felt (ordinary craft felt, probably polyester rather than wool) to the inside of the fretwork on lots of concertinas.  It works for me, reducing the high overtones and taking the edge off the sound.  I just brush a few small smears of weak pva on the inside, no stand-offs, to cover the holes.  In my experience it doesn’t shed fibres or choke the airflow, and it pulls off easily leaving negligible residue.

 

When I sing with concertina accompaniment the felt helps my words to come across, and I feel that the instrument is supporting my voice not getting in the way.

 

I think that this is the result of a lucky coincidence: thin felt absorbs sound noticeably above about 1000Hz but not much below that frequency.  Human hearing is much more sensitive in the range from roughly 1000Hz to around 3000Hz which is (naturally) the range needed to hear the consonants in our speech, particularly T, S, K, D etc which enable us to understand what is said.  So the felt baffle helps to prevent the high frequencies from the reeds from covering up the words of the song.  As the listeners’ hearing is sensitive in that range the accompaniment sounds less intrusive (although the loudness measured by a meter would probably not show much difference, as the thin felt has little effect on the fundamental notes of the music).

 

This is my rough and ready theory - the only way to find out if it works for you and your instrument is to try it.

 

Some years back on the forum there was a post from Wim Wakker, saying that Edeophones had mahogany innards which reduced the high overtones to mellow the sound.  My favourite concertina to sing with is an Edeophone and I have never wanted to add any sort of baffle.  Also a New Model tenor with mahogany structure - same applies.

 

Hope this helps.

 

[ttonon]

March, I agree with what you say.  My own response didn't consider the frequency response of absorption, and you got it better.  In fact, I attach a figure showing how the sound absorption of felt increases with increasing frequency.  The three curves shown are for different thicknesses (4, 8, 10 mm) and densities (76, 116, 148 kg/m^3).  Here's the source:

https://www.researchgate.net/publication/321858922_Improvement_of_the_sound_absorption_performance_of_jute_felt-based_sound_absorbers_using_micro-perforated_panels/figures?lo=1

 

Best regards,

Tom

[David Lay]

I have designed theaters and auditoriums (auditoria?).  The acoustitians always had us apply dense but soft material to walls from which undesirable reflections could bounce.  The soft material (usually fiberglass board) cut reflections both at the surface and by attenuating any pass-through reflections that made it to the back-up wall.  Lower frequencies always caused problems for adjacent spaces rather than for an audience.  A heavy separating wall (mass) was the only way to mitigate that.  Soft materials will definitely change the sound of a concertina wherever they are placed inside.  Wood choices will also change the sound with their different densities.  That mahogany might have resulted in a different sound in-part because mahogany typically has open pores.  The pores possibly provide small chambers into which higher frequency waves enter and die rather than reflect.  I suspect that the effect is subtle, however.  (Based upon this principle, concrete masonry blocks are available with openings and internal chambers to be used for sound attenuation.)

[ttonon]

2 hours ago, David Lay said:

Wood choices will also change the sound with their different densities.  That mahogany might have resulted in a different sound in-part because mahogany typically has open pores.  The pores possibly provide small chambers into which higher frequency waves enter and die rather than reflect.  I suspect that the effect is subtle, however.  (Based upon this principle, concrete masonry blocks are available with openings and internal chambers to be used for sound attenuation.)

Hi David, the effect I believe would be too subtle to notice.  For sound attenuation within pores or cavities, the sound must cause air vibrations so that frictional (viscous) work is done in the boundary layers, and for that to happen, the dimensions of the cavity must be not too far different from the wavelength of the sound.  So with wood, the depth of the pores is so small only extremely high frequencies would be affected.  The table below shows that wood offers very little attenuation at the higher frequencies. 

 

I also have doubts that the effect of wood density would have much effect on impinging sound waves.  It could have an effect on the vibration of the tongue, but not much effect on impinging sound waves for the reasons I explain above.  If the wood structure vibrates from the pressure oscillations in the sound, that could cause some attenuation, because that vibration will cause viscous dissipation (friction) that absorbs the sound energy, though this is likely to occur only for the lowest frequencies that couple with a resonant frequency of the wood structure as a whole. And there, wood density could have an effect, but only to help determine the particular low frequencies the structure vibrates at.

 

The table below shows this effect, wherein the absorption coefficient for wood is much higher at the lowest frequencies, presumably because a region of the wood is set to vibrate. 

[Devils' Dream]

IMO experience felt cotton and such fibers with lint are a BAD idea. It will end up in your reeds and valves. Maybe maybe a very hard felt.  Better and what is most often used to mellow melodeon tone is fairly dense linen. Or some synthetics. 

[ttonon]

Hi Devil, if that's the case, maybe a fibrous plastic that doesn't shed would be useful, such as the one side of Velcro strips, which seems like a dense cluster of fibers that offer much surface area on which viscous forces can act, as with felt.

 

Best regards,

Tom

[David Lay]

20 hours ago, ttonon said:

Hi David, the effect I believe would be too subtle to notice.  For sound attenuation within pores or cavities, the sound must cause air vibrations so that frictional (viscous) work is done in the boundary layers, and for that to happen, the dimensions of the cavity must be not too far different from the wavelength of the sound.  So with wood, the depth of the pores is so small only extremely high frequencies would be affected.  The table below shows that wood offers very little attenuation at the higher frequencies. 

 

I also have doubts that the effect of wood density would have much effect on impinging sound waves.  It could have an effect on the vibration of the tongue, but not much effect on impinging sound waves for the reasons I explain above.  If the wood structure vibrates from the pressure oscillations in the sound, that could cause some attenuation, because that vibration will cause viscous dissipation (friction) that absorbs the sound energy, though this is likely to occur only for the lowest frequencies that couple with a resonant frequency of the wood structure as a whole. And there, wood density could have an effect, but only to help determine the particular low frequencies the structure vibrates at.

 

The table below shows this effect, wherein the absorption coefficient for wood is much higher at the lowest frequencies, presumably because a region of the wood is set to vibrate. 

I agree that the pores are too small.  This chart shows that even a B6 has a wavelength of about 6".

[ttonon]

4 hours ago, David Lay said:

I agree that the pores are too small.  This chart shows that even a B6 has a wavelength of about 6".

 

Well, how big is a pore?  If around 0.1 mm, that would give a frequency of the order of a couple million Hertz!  I don't think even dogs can hear those.

Tom