Building a bass trap - advice please?

[AngelEyes]

Hi,

A few years ago I bought some high density rockwool from Wicks with the intention of building a bass trap but I never got round to it...

What I am hoping for is some advice before I finally pull my finger out. Firstly, can I even achieve what I want with what I have (rock wool), if so how? If not, erm... how?

My room dimensions are:

Height: 239cm
Length: 477cm
Width: 486cm

...so not particularly great unfortunately. I also have a very large Helmholzt resonator in the form of a large Inglenook fireplace, but based on my calculations it is unlikely to affect frequencies above a few fractions of 1 hertz.

REW's room mode calculator suggests my worst modes are 35.5, 71.8 and 106.1 and the measurements seem to agree:

This graph shows the same sub positioned either side of the inglenook. I currently have slightly different designs (same driver) in each position but that is another story. Either way I would love to tame some of the room modes with (discrete) treatment if I can.

Due to SWMBO (who hits harder than me) I would prefer something I can place in the gaps either side of the Inglenooks, probably diagonally across a corner but am open to options.

There are some photos of my room on my blog if that helps at all: Live Digital – Home Cinema Blog

Thanks very much for looking

Adam

 

[AngelEyes]

Just to clarify, I don't have a problem building them, there is plenty of info available for that, I just want to know how to target specific frequencies.

Thanks,

Adam

 

[Amioa]

how deep are the inglenooks?

 

[AngelEyes]

how deep are the inglenooks?

Approx 90cm deep by 68cm wide.

Adam

 

[AngelEyes]

how deep are the inglenooks?

To clarify my plural on inglenook was a typo, there is only one inglenook that covers most of the front wall. The measurements above are the gaps to the Walls either side. So subtract those from the width measurement for the inglenook width. Depth is the same, 90cm.

Adam

 

[Avi]

Hi,

A few years ago I bought some high density rockwool from Wicks with the intention of building a bass trap but I never got round to it...

What I am hoping for is some advice before I finally pull my finger out. Firstly, can I even achieve what I want with what I have (rock wool), if so how? If not, erm... how?

My room dimensions are:

Height: 239cm
Length: 477cm
Width: 486cm

...so not particularly great unfortunately. I also have a very large Helmholtz resonator in the form of a large Inglenook fireplace, but based on my calculations it is unlikely to affect frequencies above a few fractions of 1 hertz.

REW's room mode calculator suggests my worst modes are 35.5, 71.8 and 106.1 and the measurements seem to agree:

image






This graph shows the same sub positioned either side of the inglenook. I currently have slightly different designs (same driver) in each position but that is another story. Either way I would love to tame some of the room modes with (discrete) treatment if I can.

Due to SWMBO (who hits harder than me) I would prefer something I can place in the gaps either side of the Inglenooks, probably diagonally across a corner but am open to options.

There are some photos of my room on my blog if that helps at all: Live Digital – Home Cinema Blog

Thanks very much for looking

Adam

Have you taken measurements in the areas you are considering placing the treatment ? Also could you post a waterfall plot of the measure you posted ?

I've found Helmholtz can be effective and "relatively" compact but address too narrow frequency range when used in isolation resulting in a less than ideal result. I've used low frequency resonant panel absorbers with some success with full range speakers but they are large and to gain maximum benefit need to be positioned in a pressure maximum area for the target absorption frequency. This can be impractical or aesthetically unacceptable especially at very lower frequencies.

In addition to physical treatment do you have any electronic correction tools available and have you tried alternative room location for the sub ?

Avi

 

[AngelEyes]

Have you taken measurements in the areas you are considering placing the treatment ? Also could you post a waterfall plot of the measure you posted ?

I've found Helmholtz can be effective and "relatively" compact but address too narrow frequency range when used in isolation resulting in a less than ideal result. I've used low frequency resonant panel absorbers with some success with full range speakers but they are large and to gain maximum benefit need to be positioned in a pressure maximum area for the target absorption frequency. This can be impractical or aesthetically unacceptable especially at very lower frequencies.

In addition to physical treatment do you have any electronic correction tools available and have you tried alternative room location for the sub ?

Avi

Hi Avi,

I have owned most EQ tools out there BFD, SMS-1, AS-EQ1 etc and my Processor has a very good onboard PEQ too but my room is weird, it should have amazing bass given the subwoofers, it sure measures that way but for some reason it just doesn't feel like it looks. EQ doesn't help, a flat graph down to 5Hz, or even a room curve, nothing gives the bass I should be getting so I am hoping addressing the room first might help.

I'll go measure the places I think I could place the traps. I assume I am looking for peaks in the frequencies that are most bothersome?

Does this mean the trap itself is not tuned to the F you want to cull but positioning is more important?

Thanks,

Adam

 

[Avi]

EQ doesn't help, a flat graph down to 5Hz, or even a room curve, nothing gives the bass I should be getting so I am hoping addressing the room first might help.

I'll go measure the places I think I could place the traps. I assume I am looking for peaks in the frequencies that are most bothersome?

Does this mean the trap itself is not tuned to the F you want to cull but positioning is more important?

Thanks,

Adam

Adam

I'm not suggesting chasing a flat frequency graph using filters as this may produce a nice graph but not a sound you like. How effective these tools can be may depend on what you're targeting and thus how you affect a solution.

The design of resonant panel absorber in terms of depth, the surface density of the resonating material and a mix of air space/absorption material in the cavity establishes a general target frequency. The greater the pressure at the target frequency where the unit is positioned the greater the effectiveness. As pressure reduces at the target frequency so does the effectiveness of the panel as it's acting a diaphragm that requires appropriate pressure at a given frequency to resonate. Also surface area is a factor in how effective the panel can be.

Avi

 

[AngelEyes]

Firstly here is a FR of the listening position with no EQ (followed by AS-EQ1) with both subs running, this is the best FR i have been able to achieve with the 2 subs and positioning alone. Given the size of them, the alcoves are the only real option but a bit of jiggling around does make a difference.

Listening Position No EQ

Listening Position AS-EQ1

Here are a few measurements in the left and right alcoves and within the hidden upper area of the inglenook.

Left

Right

Inglenook

The question that springs to mind is 'do absorbers help with dips as well as peaks?' In which case the inglenook looks worth addressing too.

Cheers,

Adam

 

[Avi]

Do you have a waterfall plot ?

Avi

 

[AngelEyes]

Do you have a waterfall plot ?

Avi

Sure, all of them or...?

 

[Avi]

Sure, all of them or...?

Just listening position with and without EQ out to around 2kHz.

Avi

 

[AngelEyes]

Listening position no EQ

Listening Position AS-EQ1

 

[Avi]

Listening position no EQ
image


image




Listening Position AS-EQ1
image




image

Looking at the 500-2kHz the room mean decay time looks to be just under 200ms.

It may be worth experimenting with REW to identify and design filters in the under 200hz range with a goal to reducing the decay time to the room mean for problem frequencies. The filters will have an impact on FR but not in terms of attempting to flat peaks/dips. Don't attempt build filters to address dips. This is similar to some room correction solutions that focus on time and not creating a flat FR. There is an AES papers from Bob Stuart on this subject as the calculation in terms of decay reduces based on frequency bands. Physical treatment is still very beneficial but you may find the time approach more preferable to some room correction solutions.

Also what is it about the existing sound you don't like ?

Avi

 

[AngelEyes]

Hi Avi,

Thanks for all your input but at present I am not too worried about the mid range and want to start with the biggest problem first, which is the bass.

Any suggestions on how to design and build bass traps to address the 35.5Hz peak?

The other problem is identifying whether the inglenook is sucking the bass out of the room. Even though the FR plot in the listening position looks fine there is no real bass in the LP.

Thanks,

Adam

 

[Avi]

Hi Avi,

Thanks for all your input but at present I am not too worried about the mid range and want to start with the biggest problem first, which is the bass.

Any suggestions on how to design and build bass traps to address the 35.5Hz peak?

The other problem is identifying whether the inglenook is sucking the bass out of the room. Even though the FR plot in the listening position looks fine there is no real bass in the LP.

Thanks,

Adam

The link below outlines a basic panel resonator absorber design. The Master Handbook or Acoustics also provides some general guidance with regard to surface density based on different thickness plywood, chamber depth and frequency. There's also some older background research by the BBC below that may be of interest.

Build a Better Bass Trap

https://docs.google.com/viewer?url=http://www.bbc.co.uk/rd/pubs/reports/1992-10.pdf&embedded=true

Avi

 

[AndyST]

You're going to struggle to have any affect at all on a 35Hz wave with standard porous absorption traps. They'd have to be enormous. So it's unlikely you'll be able to do much to tame that peak with Rockwool, even adding additional membranes to increase it's effectiveness.

Room dimensions are unfortunately pretty bad for modal issues, being almost exactly square and the height again being about half the wall lengths. So where you have issues they're going to compound.

The EQ'd plots don't look that bad, though as you've found they're hardly the full story as to how good it sounds. Un-EQ'd plot shows dips at roughly 45Hz and 90Hz (which is probably a harmonic of the former), so this may be where your problem lies. I suspect whilst the AS-EQ1 may be applying boosts and/or phase shifts which appears to correct this on the plots, the underlying issue will still be there and is audible.

By lack of bass is this upper bass slam you're missing or deep pressurising sub-bass?

 

[AngelEyes]

Hi Andy,

Thanks for chipping in, I knocked up a trap very quickly the other day and you are correct in that it did nothing to the areas I was most concerned with.

To be honest all aspects of the bass are 'disappointing' but I suppose I am more blasé about bass than some... The deep room pressurising stuff is probably the least impressive if I had to choose between the two.

Cheers,

Adam