Tuning Helmholtz (how accurate)

[Paulkent]

Hi there all,

Just a quickie.. When tuning my Helmholtz Resonators I just have a few reservations.. Here they are :

1) - I've done the Harman Mode calculator (check the sticky) for my live room. I've then used the Helmholtz Resonator calculator and can get within 1hz of a problematic frequency. Do i need to get it spot on to achieve the desired effect? How much +/- should I aim for..

The reason for the above question is because I would prefer to work within the parameters of standard cuts (here in the UK) so I have Slot width and Distance from wall to play with.

2) - Whats the ratio of performance for having a single unit with a repetitive pattern for a single helmholtz frequency vs a unit with several problematic frequencies tuned in...

3) - I was looking at having a tuned Corner unit, but the actual nature of a corner means that the distance from the wall will make the unit broadband. Is this desirable..

In my mind I'm thinking .. I'm looking to suck out some of the problem frequencies without touching the frequencies around them.. But the more I think about it, the more that seems contrary to most of the advice I've been given previously. Perhaps (I theorise) the problem frequencies because they are more pronounced will be impacted more. And therefore a broadband resonator or absorber will ,by the nature of physics, have a bigger resultant effect..

-

Apologies for the long winded rant. Any help will be greatfully recieved. I should have some pics of the units I'm making.

And by the way, I'd encourage anyone to start making your own absorbers.. very therapeutic! much easier than you'd think.

Paul

P.S - i will not start sentences with 'and', i will not start sentences with 'and', i will not sentences with 'and', i will not start sentences with 'and'

[AVare]

You are doing all sorts of calculations, which is good. Bear in mind the relationship between calculations and practice. Have a look at Table 1 (pdf page 13) in BBC RD 1992-09 as an example of the differences between calculated and actual. It is an eye (ear?) opener!

Andre

[Paulkent]

oh... now I'm a little confused.. would I get a better idea of modes and effect of traps if I were to go in there and test for myself rather than rely on a formula?

Paul

[AVare]

would I get a better idea of modes and effect of traps if I were to go in there and test for myself rather than rely on a formula?

Without quite knowing all the acoustic parameters and your goals with the room, yes, actual measurements are the best.

What are hoping to achieve? How are handling the LF decay rate? Also known as the reverb time.


Soundly:
Andre

[Paulkent]

the room is a live room.

I am attempting to get as even a reverb time as possible across the spectrum. how long a reverb time? I'm not looking to deaden the room too much as ill just use gobos if I want a really dead sound. to be honest I don't have a specific reverb time in my head.. which is probably a downfall.

the room is roughly 5.5m by 5m by 2.4m which is less than ideal, but I have put some angled broadband helmholtz.... johns side wall slot resonators across one wall which covers just over half the wall with bass traps underneath.

a non opposite wall is treated with 4 deep bass and mid bass absorbers as per ethan winers designs. the wall opposite has an external window and some absorbers. no two absorbers are ever facing each other.

I am now looking at the corners and realise there is a potential big problem around 32 hz and so I'm looking at treating that and some other problem frquencies...

so there it is.. floor is carpeted ceiling is flat.

I appreciate any input but would refer anyone to my questions in the original post.

cheers

paul

[Eric_Desart]

I've then used the Helmholtz Resonator calculator and can get within 1hz of a problematic frequency. Do i need to get it spot on to achieve the desired effect? How much +/- should I aim for.

In addition to Andre's rightful comment and ref to the BBC doc.
And related to the recent MSM calculations in function of panel traps and the effect of the mass of the backpanel on those calculations:

An Helmholz is as well an MSM (mass-spring) system as a paneltrap.
Which means that the boundaries (mass) of the enclosed space of a Helmholtz resonator as well as valid for a panel trap will influence the resulting tuned MSM frequency.
Also the tradition Helmholtz formulas assume the air to be enclosed by infinite stiff or heavy boundaries, only taking the dynamic stiffness of the air into account.
But this is more complicated to calculate already since a Helmholtz can show a million shapes.

Therefore, and Andre's referred pdf, it's always a good idea to allow tuning to be adjustable somehow.


Eric

[Paulkent]

Ok, thanks a lot for the advice.

any ideas for, or designs for changeable Helmholtz resonators? obviously I can just screw the slats on, which would mean they are moveable and the distance from the wall is somewhat changeable...

Also, do I want to try and hit the frequencies that I've got on my reading, or can I hit a 50hz wide area around the trouble frequency?

cheers,

Paul

[knightfly]

Been tryin' to figure out a "dial-a-slat" mod for the last year or so, may be getting close but no time to build it this year... Steve