[DanMan]

I am in the process of sealing my trunk baffle for 2 AE IB15 subs in my 98 Civic sedan. I will then concentrate on additional Raamat, Ensolite..etc. There is already 1 layer of Raamat in the trunk. I am also planning to pick up some Luxury Liner Pro. I hear it's great stuff, although it expensive.

I understand that LLP acts as a noise barrier. For some time I mistakenly believed that Butyl mat(Raamat,Damplifier,Dynamat..etc)acted as a barrier to "outside noise" like tire hum, exhaust drone..etc. Reading posts by noise control experts like "FoxPro5" and "Mooble" I now see that a dedicated noise barrier is needed in addition to resonance dampening materials.

My concern now is in regards to the IB sub system I am working on. After sealing the baffle and isolating the front and rear waves, is there a point where too much noise barrier in the trunk would be too much? For arguments sake let's say I completely enclosed the trunk in LLP. Would a point be reached where it would be detrimental to sq? Should I cover only the floor with LLP?

P.S. the cones of the subs fire into the trunk.

 

[FoxPro5]

I understand that LLP acts as a noise barrier. For some time I mistakenly believed that Butyl mat(Raamat,Damplifier,Dynamat..etc)acted as a barrier to "outside noise" like tire hum, exhaust drone..etc. Reading posts by noise control experts like "FoxPro5" and "Mooble" I now see that a dedicated noise barrier is needed in addition to resonance dampening materials.

Not an expert, I just try to understand then apply.

Here's the deal, you can get barrier-like effects from pretty much anything. Imagine a room in your house that you want to keep quiet. If you took actual before and after SPL measurements to calculate transmission loss (TL) of a wall using a steady sound source (maybe a speaker playing a 100hz tone) after applying ANYTHING to the wall, you will probably see some TL.

If you add a 3 mm constraining layer (butyl) and then another layer of sheet rock you will get pretty good TL. If you open the wall up and add a decoupled barrier (ie LLP) you'll probably get even more. Combine both - get even more. Add an absorber to the wall (like a recording studio) and you might get to 60 STC which is the industry standard for "complete TL" meaning the wall is "dead." Add some diffusers and bass traps in the room and now you're in a "dead" room and starting to control the acoustics within it. It's endless!

Question is: how can you maximize TL with the best choice in material(s)? Truth is; it's not so cut and try and there's some overlap across dampers, barriers, absorbers, diffusers, etc. Noise is pesky and difficult to control and a lot of things will affect it.

What we know is that vibration damper is best used as vibration damper. It can block b/c it has mass. Is 3 layers of CLD mat (Damp Pro, for ex) the same as LLP because it has the same mass? Close, but not exactly. How much do you have to spend in DP to get LLP performance, is the question in my mind. There are a lot of quiet vehicles with grossly over-used dampers (look at Buzz Thompson's truck, for ex) that could have gotten better results with less $$$.

We can get into some pretty hefty duty physics and science with regards to TL and sound barriers like frequency-specifics, critical frequency, Mass Law, coincidence, and more. As a general rule, just follow Mass Law when you need a barrier. It's the easiest way to predict IMO. Combine this with the known/tested STC or TL figures of the product in question (LLP) and you have a very good idea of what to expect.

For arguments sake let's say I completely enclosed the trunk in LLP. Would a point be reached where it would be detrimental to sq? Should I cover only the floor with LLP?

Knowing what we just talked about, then we can answer this question. First, what is the sub's passband? How high will it play frequency wise? For ex, if it's 80 hz then look at the STC figures for LLP at 80 hz and you'll see how much it will block.

 

[Rudeboy]

What possible effect can treatments done on the back wave side of the baffle have on what you hear in the passenger compartment? If you have sealed the trunk off all you need to worry about is sound that travels around the baffle and what can be heard outside the car.

Below 800 Hz it takes 3 layers of Dynamat Xtreme on 22 gauge steel to block as well as a single layer of LLP on 22 gauge steel. Above 800 Hz, the LLP beats the 3 layers of DX by between 5 and 10 dB. Above 5kHz, the LLP's foam comes into play, but the lower frequency results suggest that density and not just mass plays a role. 3 layers of DX outweighs a layer of LLP by almost 50%.

 

[Rudeboy]

Some numbers to support the statement above. Measured using white noise at 100 dB at the sheet metal. Bold numbers are the difference between treated and untreated in dB, other numbers are dB read via RTA.

 

[markland556]

Seems like some very interesting info. If you were to just use one layer of butyl material then LLP overtop that it seems like you would get close to maximum dappening without taking away from the overall sound inside the cabin.

 

[FoxPro5]

A damped surface is always better to put a barrier over than an "alive" one. That should come as no surprise. Question is: how dead does it need to be?

Here's the deal in kindergarten terms (how i've tried to understand it, anyway)....

If you have a metal panel where it's natural RF (resonant frequency) is GREATER than the excitation frequency (music sound waves) = stiffness dominates.

If you have a metal panel where it's natural RF is LESS than the excitation frequency = mass dominates.

If you have a metal panel where it's natural RF is the SAME as the excitation frequency = damping dominates.

If Wn is the natural frequency of a panel and W is the frequency of excitation;

(a) when W << Wn, stiffness dominates,
(b) when W == Wn, damping dominates and
(c) when W >> Wn mass dominates.

Also note the following:

If a panel is mechanically excited, most of the energy is produced by resonant panel modes irrespective of W.

Roughly translated, the vibration caused by your moving car is just as important than the vibration caused by your sound system when it comes to damping.

If a panel is acoustically excited by incidence, its vibrational response comprises both a forced vibrational response at W and a resonant response at all relevant natural frequencies which are excited by the interaction of the forced bending waves with the panel boundaries.

Rough translation, by adding damping materials you're telling the substrate to not only change it's RF but you're throwing harmonics of it's RF at different places along it. So, you shift the RF and you "tell" the vibration to go to a different spot as well.

Obviously, you're always playing with changing RF no mater what. The solution depends on identifying the problem and using the best approach within your own budget.

Reference link: Noise Transmission

 

[Rudeboy]

A damped surface is always better to put a barrier over than an "alive" one. That should come as no surprise. Question is: how dead does it need to be?

Here's the deal in kindergarten terms (how i've tried to understand it, anyway)....

If you have a metal panel where it's natural RF (resonant frequency) is GREATER than the excitation frequency (music sound waves) = stiffness dominates.

If you have a metal panel where it's natural RF is LESS than the excitation frequency = mass dominates.

If you have a metal panel where it's natural RF is the SAME as the excitation frequency = damping dominates.

Also note the following:

Roughly translated, the vibration caused by your moving car is just as important than the vibration caused by your sound system when it comes to damping.

Rough translation, by adding damping materials you're telling the substrate to not only change it's RF but you're throwing harmonics of it's RF at different places along it. So, you shift the RF and you "tell" the vibration to go to a different spot as well.

Obviously, you're always playing with changing RF no mater what. The solution depends on identifying the problem and using the best approach within your own budget.

Reference link: Noise Transmission

That's a very good summary in that link. I might have some slightly different interpretations or I might just be misunderstanding what you're saying.

If a panel is mechanically excited, most of the energy is produced by resonant panel modes irrespective of W.

Roughly translated, the vibration caused by your moving car is just as important than the vibration caused by your sound system when it comes to damping.

This is true but it also applies to incident broad spectrum noise - most of the resonance, by a very large margin is going to occur at the panel's natural resonant frequency and its harmonics, with each higher harmonic producing less energy than the one below it, no matter how the panel is excited. This is hugely helpful to us since most of the action is going to be in the 50Hz-250Hz range, given us a relatively narrow target to worry about. It's also useful to know this if you want to find a panel's resonant frequency - tap on it and that's what you will hear (or measure).

If a panel is acoustically excited by incidence, its vibrational response comprises both a forced vibrational response at W and a resonant response at all relevant natural frequencies which are excited by the interaction of the forced bending waves with the panel boundaries.

Rough translation, by adding damping materials you're telling the substrate to not only change it's RF but you're throwing harmonics of it's RF at different places along it. So, you shift the RF and you "tell" the vibration to go to a different spot as well.

Obviously, you're always playing with changing RF no mater what. The solution depends on identifying the problem and using the best approach within your own budget.

The linked page says:

Increasing the amount of damping applied to the panel will not alter the frequencies of resonance and coincidence but will act to reduce their effect.

and that definitely matches up with the results I've been getting. You really have to load the stuff up to shift the RF in any meaningful way, but relatively small amounts will attenuate resonance significantly.

I think what he is saying is that with forced acoustical excitation, the panel resonates somewhat at the frequency of the incident sound which excites the panel's natural resonant frequencies which will actually have higher amplitudes than the resonances at the forced frequencies. In any case, a resonating panel presents a reinforced noise source in close proximity to any barrier we are using, so damping is always going to be an important piece of the puzzle.

 

[FoxPro5]

Well on my third read through I guess I didn't really draw the total gist of all that thoroughly. Now going back and trying to put two and two together from what you said makes more sense of it. I've went through that course a few times but never fine combed it. Everytime I read it again I get it a little more. I suppose it would help reading it when not so damn over-hung from the night before. [note to self: when random bar chick want's to buy you another 'Bazooka Joe' shot, pretend you have to pee and bail instead].

 

[Rudeboy]

Well on my third read through I guess I didn't really draw the total gist of all that thoroughly. Now going back and trying to put two and two together from what you said makes more sense of it. I've went through that course a few times but never fine combed it. Everytime I read it again I get it a little more. I suppose it would help reading it when not so damn over-hung from the night before. [note to self: when random bar chick want's to buy you another 'Bazooka Joe' shot, pretend you have to pee and bail instead].

I think you have your priorities skewed if you are willing to sacrifice interaction with random bar chicks for vehicle acoustics