Sound holes!!

Quote: Originally posted by Edward Powell

Very generally speaking the bigger the holes and the more of them, the more bright and metalic the oud will sound. I have heard that theoretically oval shaped holes are supposed to be acoustically advantagous for some reason (?). If the hole has a decorative rose built in remember that this will effectively make the hole size smaller. However all of these assessment criteria aside the best way to judge the effect of the holes is to determine what is the helmholtz resonance, and knowing this note will tell you really a lot about how the oud will sound. What I do as a minimum test for an ouds tone is 1) find the AIR TONE (helm.res.) by singing into the hole. 2) find the main soundboard tap tone by tapping the middle of the soundboard - this tap tone generally and usually is about 1 semitone higher than the AIR TONE... so depending on where these 2 pitches are will tell you A LOT about the character of this oud.

Quote: Originally posted by Luttgutt

Quote: Originally posted by Edward Powell   Very generally speaking the bigger the holes and the more of them, the more bright and metalic the oud will sound. I have heard that theoretically oval shaped holes are supposed to be acoustically advantagous for some reason (?). If the hole has a decorative rose built in remember that this will effectively make the hole size smaller. However all of these assessment criteria aside the best way to judge the effect of the holes is to determine what is the helmholtz resonance, and knowing this note will tell you really a lot about how the oud will sound. What I do as a minimum test for an ouds tone is 1) find the AIR TONE (helm.res.) by singing into the hole. 2) find the main soundboard tap tone by tapping the middle of the soundboard - this tap tone generally and usually is about 1 semitone higher than the AIR TONE... so depending on where these 2 pitches are will tell you A LOT about the character of this oud. Hi Edward! That was very well written! Thanks... I have two questions: 1- What do you mean by "where", when you write: "depending on where these 2 pitches are"? Do you mean which Tones? 2- does that mean that if you have a Oud with 3 holes, and you "cover" the small ones, you kan expect MORE BASE? In in that case, does it depend om WHAT you cover them with? i.e. do you have to cover them with wood, or could you cover them with paper, or nylon...? thanks again!

Quote: Originally posted by jdowning

Covering the small sound holes of a 3 soundhole oud might have some effect by shifting the Helmholz resonance (due to reducing the total sound hole area relative to the air volume of the bowl) - although this might not be significant enough to alter the overall acoustic performance of a particular oud. As Edward suggests you would have to try it yourself to find out.

I think the rosette affects the sound mainly by reinforcing the edges of the sound hole. It stops the soundboard from indifferent vibrating around the hole. That's what makes the sound "refined, gentle, warm, rich".
I removed the shams on my Gamil George oud once and noticed a significant change in sound - a little bit louder and less mellow but somewhat chaotic... so i glued the rosette back...
On my Shukar oud, which sounds great anyway but have no rosette as you may know, I hear the same effect - the sound is a little uneven compared my ouds with a shamsa.
Anyway. Complete covering the holes don't make the resonance tone deeper but higher by decreasing the virtual air volume. A proper working Helmholtz resonator virtually increases the air volume of a closed sound chamber. For instance a closed/sealed loudspeaker box with a given bottom resonance frequency will become smaller if one build that box as Helmholtz resonator aka bass reflex.

I hope my bad English makes the thread not to confusing.

Dear members, thanks to all you! As usual you keep on surprising me with your knoweldge of the oud I will try to cover the small soundholes in my mounir banshir model and I'll let you know! Unfortunately, I won't have the chanche to try any Moussa oud but I will have to order one among his models. I noticed that several of his models have just a big round soundhole and I wondered how the lack of the 2 little ones may affect the sound. Thank you again!

Quote: Originally posted by Luttgutt

OK! Now I have done it. I covered the two small holes.. And the differance is much more then I have expected.. Defficult to explane With Words, but I'll try (I'll try tomorrow to post sound of before and after): 1-The A string became deeper, a litle bit more powerful (maybe) and got a better "character" (feels more mature). I love it. 2-The D string losts its brightness.. So that was a negative thing. 3- I did not feel difference in sustain. 4- The oud is as loud, but it losts its overtones!! So it sounds somewhat different. Neither better or Worth to my ears, just different. It Depends on what you like! The Nice thing is that, in a way, it is like getting a New oud :-) Keep playing...

Quote: Originally posted by danieletarab

Dear members, thanks to all you! As usual you keep on surprising me with your knoweldge of the oud I will try to cover the small soundholes in my mounir banshir model and I'll let you know! Unfortunately, I won't have the chanche to try any Moussa oud but I will have to order one among his models. I noticed that several of his models have just a big round soundhole and I wondered how the lack of the 2 little ones may affect the sound. Thank you again!

So what you need to keep in mind is that as John said, the TOTAL amount of AIR from the combined size of all the holes will determine the HELM.RES.... BUT, the difference between one big hole and 3 smaller holes is that with just one big hole the soundboard main res. pitch will be LOWER than if you have 3 holes simply because those two small holes remove resonance area of the soundboard, which of course makes the pitch of the soundboard go UP. So with just one hole you will have a deeper, darker sounding oud then if you have 3 holes.

In my opinion THIS is the reason why those 2 small holes are used instead of 1 big hole. I think that the attempt here is to create more distance between the AIR TONE (which should be the LOWEST RES. MODE on the instrument) and the SOUNDBOARD RES. TONE. The danger with only one hole is that the soundboard res.tone will be too low and therefore create imbalance (too much res. in one area in the bottom 2 strings, and not enough punch in the mids)... so by adding the 2 small holes the soundboard provides the punch in the mids, and the AIR provides the punch in the deep lows - BINGO, balanced oud

It seems to me that this is the trick we notice in so many instruments... the attempt is to get a DEEP AIR TONE, but keep the soundboard tone relatively high. This is why in my opinion instruments like the TAR, and the RABAB have a double chamber and actually 2 resonating area for the soundboard. The effect of this of course is a very large AIR space, but actually quite a small resonating space for the soundboard.

It seems to me that the sound likes 1) a lot of AIR, 2) quite small resonating area on soundboard.

I have made instruments with huge soundboards, and relatively small air space and they sound terrible. When I started to reverse this relation -- making BIG AIR + SMALL SOUNDBOARD, my instruments improved enormously.

It may be helpful in trying to visualise the effect of sound hole changes as it affects the Helmholz resonance by going back to the basic formula that defines the resonance of a Helmholz resonator (a rigid sphere with a short neck vented to atmosphere). Derivation of the formula requires a knowledge differential calculus and gas thermodynamics but this exercise has already been undertaken by those familiar with the science and is shown in the attached image.

f = the Helmholz resonance frequency.
c = the speed of sound in air (343 m/sec at 20°C)
A = cross section area of the neck.
V = volume of the resonator.
L = equivalent length of the neck.

So it can be seen that if the volume and length of neck remains constant, increasing the diameter of the neck (and so increasing the cross section area) will increase the Helmholz frequency and vice versa if the neck diameter is reduced.
Alternatively if the neck area and length remain constant and volume is increased the Helmholz frequency will be lowered and vice versa if volume is reduced.

The basic concept of a resonator is that the trapped air volume acts as a spring that drives the cylindrical 'slug' of air in the neck to oscillate back and forth in simple harmonic motion once disturbed. Apart from this dynamic disturbance there is no net loss of air from the resonator.

More to follow.

Ouds , lutes, guitars and other related instruments are not perfect Helmholz resonators - not only because their geometry is different but because their bodies are not perfectly rigid. This lack of rigidity (dynamic swelling or contracting) effectively reduces the 'springiness' of the contained air mass resulting in a lower resonance frequency than predicted by the Helmholz resonance equation previously posted. The quantitative effect will depend on the structure of an individual instrument so likely defies precise calculation.

The other obvious difference is that the length of the 'neck' (i.e soundhole length) is physically very short - about 2mm (i.e. the soundboard thickness). However, if end corrections are included (equivalent to those applicable to a short pipe open at both ends) the theoretical end correction is 0.85 times the radius of the sound hole - which applied to each side of a sound hole gives a total length of the oscillating 'slug' of air within the sound hole equal to 2 mm plus 1.7 times the sound hole radius which is significant compared to the sound board thickness. As the length of the 'slug' L relates to the radius of the sound hole as does the area A of the sound hole, the Helmholz equation may be simplified - as shown in the attached image - where the resonant frequency is proportional to the square root of the product of sound hole radius divided by the volume.

For the purpose of investigating what happens to the resonant frequency if some sound holes are blocked (i.e. the volume remains constant before and after), the equation further reduces to the frequency being proportional to the square root of the product of the sound hole radii added together.

Next a 'real life' illustration of what happens to the calculated Helmholz resonant frequency if the two small sound holes of an oud are blocked.

A recent topic on the forum on the subject of the geometry of an old Abdo Nahat oud includes restoration of the oud by luthier Michael Cone.

http://www.mikeouds.com/messageboard/viewthread.php?tid=10882

In the condition as received for restoration it appeared that someone had added additional material to both the large sound hole and the small sound holes to reduce their diameter (thus blocking part of the original rosette pattern). The restoration work included removal of this added material.

I wondered at the time why anyone would want to tamper with reducing the original diameter of the rosettes. Now I think I know why, given that this would have lowered the Helmholz resonance frequency of the oud. Whoever made this change must have been knowledgeable about the effect this would have and felt that lowering the Helmholz resonance frequency improved the overall acoustic balance of the instrument?

If so, this suggests the possibility of the small sound holes being used to fine tune the acoustic balance of an oud. Here the oud would first be set up without the rosettes in place and with the small sound holes being first made undersized in diameter. With the instrument strung and brought up to tension the small sound holes would be gradually increased in diameter by uniformly trimming away the sound board material around the circumferences until the required acoustic balance is achieved. The rosettes could then be finally glued in place to complete the job.

Tuning of the resonant frequency could also be achieved by adjusting the diameter of the large sound hole but finer adjustments might be possible by working only on the small sound holes.

The attached images show the Nahat oud before and after restoration.

The Helmholz resonance is not the full story and I tend to agree with Edward that cutting holes in a sound board will also affects sound board response - although I am not sure if this would necessarily result in raising instrument pitch.

The attached images show sound holes placed in 'unconventional' positions on lute and oud sound boards The lutes are large bass instruments with correspondingly large sound boards and have in some cases a single small sound hole positioned low down on the bass side. I suggested on an earlier thread that this may have been not only to enhance the bass response by lowering the Helmholz resonance but also by locally weakening the sound board to favour lower sound board frequencies. However, I do not know how to predict the latter possibility - if true.