The reason why the duff is permitted and other instruments are not
Question

                I know that music is haraam all together and musical instruments.
However, my question is, WHY is the duff instrument allowed on certain events (like marriage etc. for women of course)? I mean, it makes rhytmic sound too, isnt that an instrument also?! There is not much difference between some drum sounds and duff beating sound. Shouldnt duff be haraam as well if that is the case? Why is it allowed, what is the wisdom behind its allowance?.
Praise be to Allah.
    The Muslim is obliged to submit to the ruling of Allah, may 
    He be exalted, even if he does not understand the wisdom behind it, because 
    Allah has perfect wisdom, and He has not prescribed anything except for a 
    reason. Allah, may He be exalted, says (interpretation of the meaning):

    “But no, by your Lord, they can 
    have no Faith, until they make you (O Muhammad SAW) judge in all disputes 
    between them, and find in themselves no resistance against your decisions, 
    and accept (them) with full submission”

    [al-Nisa’ 4:65]. 

    Part of the wisdom of sharee‘ah that it does not 
    differentiate between things that are similar and it does not regard as 
    equal things that are different. If Islam differentiates between two things, 
    such as the duff and the tablah (another kind of drum), then they are 
    definitely not the same and there is a difference between them which 
    dictates that they should come under different rulings. 

    But… There are some people whom Allah has enabled to 
    understand this difference and others who do not understand it, but in 
    either case it is obligatory to submit to the ruling of Allah, may He be 
    exalted. 

    The difference between the duff and the tablah is that the 
    sound of the duff is less stirring and it has less of an effect on the soul 
    than the tablah. The hollow space inside of the tablah causes vibration and 
    a more stirring sound, which does not happen with the duff, because it has 
    no hollow space inside, and its sound is also less powerful than that of the 
    tablah. 

    For this reason sharee‘ah forbids the kind of duff that has 
    jangling pieces of metal around it (i.e., tambourine), because the jangling 
    pieces make more of a stirring sound than the duff that does not have them. 

    The daff is the least stirring of all musical instruments, 
    hence sharee‘ah has permitted it in some cases, because of the interests 
    served by that, such as announcing a wedding and making it well known among 
    the people. 

    For more information please see the answer to question no.
    20406. 

    And Allah knows best.