Solving Low Frequency Issues in Constructions

Hi, I'm talking from Brazil I have a doubt... I'm currently working for a night club in order to solve insulation problems The night club is having problem with the neighbors due to the loud music... But the problem is essentially in the low frequency region. The place has also big room modes problems in the low frequency region ... Here's my question... Should I first solve the room modes issue or work on the structure of the house first? Knowing that maybe by solving the room modes it could reduce the total level of the low frequencies It would be nice to hear from you guys, perhaps you have been through a similar situation Sorry for my bad english

Hi Fernando, Low frequencies are the hardest to isolate, since they are very long wavelengths, and they also carry a lot of energy. Isolating your club for that will not be easy, but it can be done: However, in order to design the isolation that you will need, you will first have to take some measurements with a sound level meter to find out how loud you are, and how quiet you need to be. So if you don't have a meter, you will need to buy one, or borrow one. They are not expensive: you can get a good one for around US$ 100. You will need to measure the level inside the club during a typical event, then measure the levels outside at various points, especially where the complaining neighbors are. You should also draw an accurate diagram showing the locations and distances of the club and the surroundings, and also provide some photos, so we can get a better idea of the situation. To answer your questions: You should first deal with the isolation problem, because that will involve making changes to the walls, ceiling, windows, doors, and maybe also the floor of the club, as well as the ventilation system. When you are done with that, and have the isolation properly controlled, then you can proceed to do the treatment for the modal issues. It makes no sense to treat the modal problems first, as you'd have to do it again once the isolation is completed.

Knowing that maybe by solving the room modes it could reduce the total level of the low frequencies

Not necessarily. They might be happening at different frequencies: Modal issues are due to the dimensions inside the room, whereas isolation issues are due to the construction of the walls. They are probably not related to each other. Even if they are related (for example, if you have a mode at 70 Hz and also an isolation resonance at 70 Hz), reducing the modal resonance would not really solve the problem; sound would still get out at 70 Hz due to the isolation resonance. So first fix the isolation, then treat the modal problems. - Stuart -

Thanks Stuart for your answer I've already taken some measurements at various points, especially where the complaining neighbors are to find out how quiet we need to be. It turns out that we need to isolate only 5 dB, we have already replace some weak points (replace some doors, take care of ventilations systems...) My point is that, by treating the room modes we can have a flatter frequency response and by doing that we can lower the overall level of the sound inside the club Because the "DJ / FoH" is bosting the low frequencies to compensate the "nulls" caused by room modes I think that it could be a solution to solve the problem at the source, and be able to lower the total level by 7/9 dB Does it seems totally wrong? Fernando Guilhen

It turns out that we need to isolate only 5 dB,

Well, that information is no use at all to me! I need the actual readings from your sound level meter, "C" weighted, slow response. The actual levels. both inside and also at several points outside. Saying you need "5 dB more" is like saying your car needs to go "20% faster".... 20% faster THAN WHAT? 20% faster than 1km/h is only 0.2 km/h, but 20% faster than 300 km/h is 60 km/h. On the one hand it is a tiny difference, on the other hand it is a huge difference. In the first case, I could make you go that much faster just by pushing your car with one hand, but in the second case, I'd have to add a turbo-charger, put in bigger engine, new gearbox, different wheels, new suspension.... I think you see the point! The decibel scale is logarithmic, so saying you need "5 dB more", is meaningless, unless you specify the actual values. If you need to decrease from 15 dBC to 10 dBC, I'd give you a thin sheet of cardboard to do that, but if you need to decrease from 135 dB to 130 dB, I'd give you a thick piece of concrete! And also, I'm assuming that you measured correctly. There's a huge difference in measuring on the "A" scale and the "C" scale for bass-heavy dance music.... If you measured on the "A" scale, you might well think that you only need 5 dB more, but if you measure on the "C" scale you might find out that you really need 30 dB more.... Those are two very, very different scales. The lower the frequency, the more different they are! I find it hard to believe that your neighbors said you only need 5 dB more isolation: to most people, 5 dB is not much at all. Most people can only just notice a difference of 3 dB, so going from "the neighbors are complaining all the time" to "no problem at all" is not going to be a 5 dB difference. In most cases, that would be a difference of 20 dB or more. Something does not make sense here.

My point is that, by treating the room modes we can have a flatter frequency response

No. Treating room modes will not flatten your frequency response curves significantly. It will greatly reduce the RT-60 times for those modes, yes, but it won't necessarily flatten your response by much. You might reduce the intensity of three or four modes by maybe 5 to 10 dB each, but that would have hardly any effect on the total sound level inside the room. You would probably not even notice the difference on your meter> It would be maybe 1 or 2 dB reduction in the total level.

... and by doing that we can lower the overall level of the sound inside the club

Very unlikely! Unless you have major modal issues, you won't be changing the overall level just by treating a few modes. As I said above: the difference would be barely noticeable. That's not the way modal problems work. Modes are not even your problem here, anyway! Your problem is isolation, not modes. Fixing your modes wont fix your isolation. That would be like expecting that pumping up your car tires with more air will make your engine run smoothly... There is no relation. I'd also like to know how you are so sure that you have modal problems at all! It would have to be a very small night club to show drastic modal issues. Night clubs are usually large rooms, sufficient to accommodate dozens or even hundred of people> way too big for modes to be a big issue. So I'd be surprised if you have serious modal issues. You might have resonance issues, but its not probable that you would have modal issues. 1. How did you determine that modes are the problem? How did you measure that? 2. What are the dimensions of your room?

Because the "DJ / FoH" is bosting the low frequencies to compensate the "nulls" caused by room modes

:shock: :roll: He is trying to accomplish the impossible! That shows a major misunderstanding of how modes work. You CANNOT "fill up" a mode by making that specific frequency louder. In reality, what you will do is make it QUIETER! That sounds crazy, and non-intuitive, but it is the truth. It is physically impossible to fill in a modal null by adding more energy to it. That's like trying to slow down your car by pressing harder on the accelerator... A mode is a standing wave. If you are listening at a location in the room where there is a null for that specific mode, then that is because the velocity component of the wave is at its peak value there, while the pressure component is at its minimum value, at that location. Our ears do not respond to velocity, only to pressure. If you add more energy to that mode, then the velocity component peak will go up even higher (and you wont notice it), while the pressure component null will go down even lower: you will make the null even deeper at that location. (On the other hand, in the room corners, you will make the pressure peak much higher, and the velocity dip much lower....)

I think that it could be a solution to solve the problem at the source, and be able to lower the total level by 7/9 dB

Sorry, but that won't work. Assuming you have ten modal problems below 200 Hz (very unlikely, but let's assume a "worst case"), and each of them is creating a 10 dB peak in your frequency response, and assuming you manage to treat them all perfectly, getting perfectly flat response below 200 Hz (extremely unlikely!), your actual level inside the room would decrease by less than 2 dB. The only way to decrease the overall level by 7 dB is to take hold of the master fader on the DJ's console and pull it down until your sound level meter shows a decrease of 7 dB (assuming the sound level meter is set to "C" and "slow"), measured at several points in the room, and averaged. And that change would not be noticed very much by your neighbors anyway...

Does it seems totally wrong?

Yes. :) Because sound does not work the way you are hoping it works. Sound is not intuitive: we cannot see it, but we assume it must work the same way as things that we can see. However, it doesn't. Resonance is a very hard concept to grasp, and it can do some very strange things. We expect sound to behave in a linear fashion, like the things we can see, but it actually behaves exponentially. There is no visual reference in nature that can help us fully understand things like diffraction, diffusion, or the interaction of particle velocity and wave-front pressure in a 3D sound wave that is passing a specific point in a standing wave.... I know you'd like to treat your problems cheaply and simply, but it just doesn't work the way you are wanting it to work. Unless you supply the information that will help us identify your problems properly, we won't be able to help you fix them. - Stuart -

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