danieletarab
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Sound holes!!
Hello everybody!
I am thinking to get a Moussa oud and I am quite confused about which model I have to choose! One of my main concerns are about the soundholes..
Specifically, I wonder how the sound hole's shape, number and size affect the sound. Thank you in advance for answering!
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ultragroove
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Hi Daniel!
This question has been partially discussed here:
http://www.mikeouds.com/messageboard/viewthread.php?tid=13629
http://www.mikeouds.com/messageboard/viewthread.php?tid=14292
Greetings and happy playing with your new Oud!
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Edward Powell
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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.
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Luttgutt
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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.
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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!
The wood might be dead, but the oud is alive.
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jdowning
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There are so many variables involved in the creation of an oud (or lute, guitar and other related acoustic instruments) that it is surely impossible
to accurately calculate and so predict the potential acoustic performance by considering some aspects of design such as soundhole shape, area or
diameter. Indeed no two 'identical' instruments built even by a master luthier using woods from the same trees will have the same acoustic performance
- even if that difference could be objectively measured rather than being a subjective assessment in the 'ear of the beholder'.
Having said that there are certain basic acoustic concepts such as the Helmholz resonance phenomenon that hold true generally. However, as an oud is
far from being a perfect Helmholz resonator (a rigid glass sphere with a long neck open to the atmosphere), predicting the required soundhole diameter
to bowl air volume by calculation is not a practical proposition.
A luthier will determine the best proportions (in his or her assessment) by trial and error requiring the construction of many instruments for (again,
most likely, subjective) comparison.
Four years ago, reported on this forum, I ran some brief acoustic trials - out of curiosity - to establish if the Helmholz effect is a reality and to
try to find out a bit more about how sound hole size might affect the fundamental resonance frequency of the air mass of an instrument. (see here and
part of page 9 of the thread)
http://www.mikeouds.com/messageboard/viewthread.php?tid=8488&pa...
Part of the trial was to investigate the effect of a rigid rosette inserted into an open sound hole. Interestingly - although the open area of the
rosette was only about 29% of the open sound hole area, the Helmholz resonant frequency and its amplitude was not significantly altered, however, the
sustain of the sound was doubled!
As a rosette is equivalent to a multitude of tiny sound holes I concluded that this experimental observation was due to an increase in equivalent
length of each of the rosette sound holes compared to the equivalent length of the open sound hole - the net result being about the same as far as the
Helmholz resonance frequency is concerned. Note that the equivalent length of a sound hole is not just its measured thickness (i.e. soundboard
thickness - a couple of millimeters say) but significantly greater due to the length of the air column (on each side of the sound hole) oscillating at
resonant frequency through the sound hole. The length of this column of air is related to the sound hole diameter - increasing in length as diameter
is reduced.
From these limited, preliminary trials I concluded that the number or shape of sound holes has no significant effect on the Helmholz resonance
frequency (it is their total area relative to the air volume of the bowl that matters) and neither does the presence of a rigid rosette. Indeed the
rosette might improve the sustain of the resonant frequency - if that is considered to be desirable. As far as a rosette made from a thin flexible
material such as paper or parchment is concerned I would also expect this to have no significant influence on acoustic performance.
Note that oud rosettes are made rigid by use of thick material whereas in a lute the rosette - cut as it is into soundboard material that is only
about a mm in thickness - is reinforced with small braces glued underneath.
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.
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Edward Powell
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| 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! |
yes "where the tones are" means "what are the pitches".
if you cover the small holes you 'sort of ' get more bass. What you do is lower the AIR TONE. The notes nearby the AIR TONE note are very strong and
punchy with bass... so if you lower the AIR TONE then that means that the punchy effect will affect lower notes, but keep in mind that after lowering
the AIR TONE, the notes where the AIR TONE used to be will not have as much punch and bass.
You can cover the holes with anything as long as you block the flow of air.
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Luttgutt
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Thanks Jdowning and Edward!
that was very informative indeed!!
I'll try it later and report back..
The wood might be dead, but the oud is alive.
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Edward Powell
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| 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.
|
covering one of the soundhole as I just said, in fact radically changed to sound of the oud RIGHT IN THE AREA WHERE THOSE NOTES ARE THE SAME AS THE
HEL.RES.... as I described but other than that the rest of the oud will basically sound the same I would say.
John this is very interesting what you say about the rose... I have not experimented with this. I only remember asking Faruk about this and he said
something to the effect of having no rose give a more "naked, 'in your face', raw sound" whereas WITH the rose the sound is more "refined, gentle,
warm, rich". . . . . . . . To me this might imply that you get more highs and more metalic sounds WITHOUT the rose... which would fit in with the
hel.res. going up somewhat.
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ultragroove
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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. 
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Luttgutt
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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...
The wood might be dead, but the oud is alive.
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danieletarab
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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!
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Edward Powell
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| 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... |
yes this is what happens when you cover the holes. I would only recommend doing this if the oud sounds weak in the lowest 2 strings... otherwise you
will end up with TOO MUCH punch in those lowest strings. This is an unbalanced oud.
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Edward Powell
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| 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.
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jdowning
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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.
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jdowning
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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.
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jdowning
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My old Egyptian oud has three sound holes - one large, measuring 5.7cm in radius and two small each of 2.35 cm radius. Adding together the three radii
gives a total of 10.4cm so the square root value is 3.22 which is proportional to the resonant frequency f1 (which in this case is an unknown value,
the oud currently being dismantled under repair/restoration).
If the two small sound holes are blocked, so leaving only the large sound hole as functional, the square root of the large sound hole radius is 2.39
which again is proportional to the new resonant frequency f2.
So, f2/f1 = 2.39/3.22 hence f2 = 0.74 f1 or about 75% of the original Helmholz resonant frequency.
At the present time the Helmholz resonant frequency of my oud is unknown (although could be estimated by measuring the volume of the oud bowl and
calculating the resonance frequency). However, assuming Arabic tuning C E A d g c' and assuming that the design resonance value happens to fall midway
between E and A string pitches (82 and 110 Hz at A440 standard) for a value of 96 Hz, then blocking the two small sound holes would theoretically
shift the Helmholz resonance to a lower value of 0.74x96 = 71 Hz (i.e. between C and E string pitches).
In the case of Luttgutt's trials, the design Helmholz resonant pitch (determined by sound hole radii and bowl volume) would appear to to be somewhere
between A and d (110 and 147 Hz) so that blocking the two small sound holes lowered the resonance to somewhere between E and A (82 to 110hz) judging
by the subjectively assessed loss of 'brightness' of the d string and improved sound volume of the A string? The quantitative amount of the resonant
frequency might not be the same as for my oud (0.74xf1) but might be readily calculated by measuring sound hole diameters and following the above
sample calculation substituting the new values of radii as appropriate.
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jdowning
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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.
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jdowning
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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.
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