Bass trap

[Milan237]

Hi, I have a question for you, what are the bass traps, I have read the description of many companies, but I want to know the opinion of an expert.

 

[Inner Space]

Bass traps are physical constructions, designed to be placed at or close to walls and corners, with the aim of killing acoustic energy in long and powerful low frequency wavelengths, thereby preventing detrimental reflections back into the room.

First principles would demand tremendous depth to achieve that goal, so bass traps are designed with various ingenious features to get more performance while limiting depth to something domestically acceptable. They aim to be pressure absorbers, rather than the kind of velocity absorbers used at higher frequencies.

Generally, at domestic depths up to a foot or so (30cm), they are modestly effective and can often solve minor problems. But you can't beat math, and for real results you need much more depth than people are willing to contemplate.

 

[Longshan]

Bass traps are physical constructions, designed to be placed at or close to walls and corners, with the aim of killing acoustic energy in long and powerful low frequency wavelengths, thereby preventing detrimental reflections back into the room.

First principles would demand tremendous depth to achieve that goal, so bass traps are designed with various ingenious features to get more performance while limiting depth to something domestically acceptable. They aim to be pressure absorbers, rather than the kind of velocity absorbers used at higher frequencies.

Generally, at domestic depths up to a foot or so (30cm), they are modestly effective and can often solve minor problems. But you can't beat math, and for real results you need much more depth than people are willing to contemplate.

Most bass traps are velocity absorbers, not pressure absorbers.

 

[Inner Space]

Most bass traps are velocity absorbers, not pressure absorbers.

If they are, they shouldn't be. Don't buy those.

 

[pozz]

Not an expert.

There are a few kinds of bass trap, if we are really talking about absorbing subwoofer kind of bass:

• Diaphragm: A flexible panel that resonates at low frequencies. Typically tuned for maximum absorption at one frequency, but things like where and how it is clamped, attached springs or weights and perforations can help it be more broadband.
  • A variation of this trap uses a limp mass absorber. Basically a rubber sheet that hangs loose. The lowest resonant frequency can be tuned through size and weight, although the absorption is generally more broadband than a panel. Has modest effectiveness down to about 50Hz if using heavy sheets.
• Helmholtz: A tuned container that resonates at low frequencies. Even more precise tuning compared to panel. The only passive absorption that works at any low frequency.
• Electronic: Active absorbers. Bag End ETrap and PSI Audio AVAA are examples. They have microphones built in that detect incoming sound, which is sent to a speaker driver that plays out of phase, effectively cancelling it. The ETrap can do two frequencies, the AVAA is more broadband.
  • The AVAA was favourably reviewed by Sound & Recording with good measured performance. However, it is very expensive, and there no reliable in room measurements that show its effectiveness. It seems that you'd need a minimum of two, and that four would be more sensible.
• Metamaterial (experimental; no commercial products exist): Material designed at the molecular level to absorb certain frequencies.

Almost every other kind of product out there is broadband with some sort of dense filling (fibreglass, foam, recycled denim) and has declining effectiveness below 200Hz, and very little below 100Hz, based on the typical thicknesses offered.

One boutique manufacturer to avoid is Acoustic Fields. They advertise a combo diaphragmatic and activated carbon absorber. I have found exactly one independent published paper on using activated carbon for low frequency absorption, the conclusion of which was a "probably effective". The testing data Acoustic Fields posts on their website is questionable and they've never posted comprehensive in room measurements. Their Youtube content is semiaccurate at best, and the prices very high. The founder does not have a background in acoustics.

 

[Bjorn]

A simplistic way of putting is that bass traps are used to remove low frequencies that build up in corners and cause audible resonances and uneven frequency response. Take note that a corner is also where two surfaces meet each other. The result is cleaner and tighter bass and also with the experience of a quicker start and stop of low frequencies which is often descrobed by many as "more dynamic". If the treatment is sufficiently effective, the frequency response will even out as well.

Contrary to EQ and also normally compared to the multiple subwoofer approach, it works to improve the low frequency everywhere in the room since you're dealing with the root, it can cover all the bass frequencies, and generally doesn't have any negative audible effects like to others often will have to some degree.

As others have mentioned, there are two categories of bass traps. Velocity and pressure based. Velocity bass traps are built out of porous materiale and need to be very thick for effectiveness in sub frequencies. Below is a graph of a 20 cm (8") optimal porous material and you can see the absorption coefficient drops significantly already below 200 Hz.

Several companies call porous products that are only 15 cm (6") thick bass traps, but that's quite misleading. I wouldn't call a velocity type a bass trap before it's minimum 30 cm (11.8") thick.

Pressure based traps are effective close to the surface where the pressure is highest and don't need to build out that much. Some of these are very narrow tuned (to a small frequency area) like Helmoholtz resonators or membrane traps, while others can be quite broadband like VPRs.

With velocity based traps one looses considerable more SPL (gain from surfaces) vs pressure based traps.

FIY: I'm a RPG retailer.

 

[caught gesture]

Hi, I have a question for you, what are the bass traps, I have read the description of many companies, but I want to know the opinion of an expert.

https://en.wikipedia.org/wiki/Bass_trap

 

[sweetchaos]

 

[Longshan]

If they are, they shouldn't be. Don't buy those.

Ridiculous.

 

[MRC01]

...
There are a few kinds of bass trap, if we are really talking about absorbing subwoofer kind of bass:
Diaphragm ... Helmholtz ... Almost every other kind of product out there is broadband with some sort of dense filling (fibreglass, foam, recycled denim) and has declining effectiveness below 200Hz, and very little below 100Hz, based on the typical thicknesses offered.
...

Where would you put tube traps in this category? The ones I built measured a difference of 6 dB from 25 - 50 Hz. IME they are more narrow band than porous absorbers, but broader band than tuned resonators.

 

[pozz]

Where would you put tube traps in this category? The ones I built measured a difference of 6 dB from 25 - 50 Hz. IME they are more narrow band than porous absorbers, but broader band than tuned resonators.

Definitely a lower Q version of a Helmholtz resonator.

I saw you mentioned ASC. Their Subtraps were interesting, but they cost so much relative to what they are, and there's no data to speak of.

 

[Inner Space]

Ridiculous.

Whatever. You'll figure it out. Or not. Either way is fine with me.

 

[Frgirard]

Hi, I have a question for you, what are the bass traps, I have read the description of many companies, but I want to know the opinion of an expert.

You have numerous web sites on this subject no linked with a brand.

You can found in downloading the Bible : Acoustic absorbers and diffusers by Trevor Cox and Peter D'Antonio

 

[DVDdoug]

I'm also not an "expert"...

Hi, I have a question for you, what are the bass traps,

The important thing is they "trap" REFLECTED bass to reduce standing waves. With standing waves you get nodes (where the sound waves cancel) and antinodes where the waves combine in-phase and build-up, all at different frequencies and a different places in the room.

The biggest issue is the nodes where cancelation causes a dip in frequency response. You can fix a bump in frequency response with EQ but it takes huge amounts of power and huge woofers/subwoofers to fix a dip. And, you can only EQ one place in the room. Bass traps (and other acoustic treatment can help with all of these things.

Before you go randomly buying or building bass traps it's a good idea to measure your room... Diagnosis before treatment.

 

[Snoochers]

I have a fair bit of room behind my theatre chairs and I might be able to get in a large triangular bass trap in the corner. I could maybe built-it out to 24-30" deep at the thickest point of the triangle. The hypothenuse of the triangle might be as long as 4'.

I don't know anything about tubes, Helmholtz, etc. I just know insulation. This stuff would be pretty dense, maybe comparable to OC 703.

Is this a good idea? Would this help get to some of the lower frequencies while avoiding the need for a really fancy bass trap?

 

[MRC01]

What frequenc(ies) do you want to tame/absorb? Bass traps can have a tuned membrane in addition to the fiberglass insulation in order to increase their Q. For example, here's some info on the bass traps that I use: https://realtraps.com/p_megatraps.htm

 

[Snoochers]

What frequenc(ies) do you want to tame/absorb? Bass traps can have a tuned membrane in addition to the fiberglass insulation in order to increase their Q. For example, here's some info on the bass traps that I use: https://realtraps.com/p_megatraps.htm

I am starting from 0 absorption so I need to catch everything. I know the high end is super easy so I'm more worried about the deepest deep. I was thinking throwing a chonker in the corners would help.

 

[MRC01]

I am starting from 0 absorption so I need to catch everything. I know the high end is super easy so I'm more worried about the deepest deep. I was thinking throwing a chonker in the corners would help.

In that case I recommend tube traps, as they are easy to build yourself and quite effective. In my case, they made a big improvement not only subjectively but in before-after measurements. Sounds like you could go with about 2' diameter, and floor to ceiling in the corners. In my case this affected frequencies from 20 to 100 Hz. The site I linked above has more info and a DIY "how-to" guide.

An alternative would be something like the Megatraps I mentioned above, stacked in each corner. They are also highly effective but that would cost a lot more than the DIY tube trap option.

What's a chonker?

 

[Snoochers]

In that case I recommend tube traps, as they are easy to build yourself and quite effective. In my case, they made a big improvement not only subjectively but in before-after measurements. Sounds like you could go with about 2' diameter, and floor to ceiling in the corners. In my case this affected frequencies from 20 to 100 Hz. The site I linked above has more info and a DIY "how-to" guide.

An alternative would be something like the Megatraps I mentioned above, stacked in each corner. They are also highly effective but that would cost a lot more than the DIY tube trap option.

What's a chonker?

A chonker is just a big thing, sorry. I have tons of pretty dense insulation that works very well for absorbing sound, and as I mentioned I can go pretty thick. Is this not a good approach?

 

[Snoochers]

In that case I recommend tube traps, as they are easy to build yourself and quite effective. In my case, they made a big improvement not only subjectively but in before-after measurements. Sounds like you could go with about 2' diameter, and floor to ceiling in the corners. In my case this affected frequencies from 20 to 100 Hz. The site I linked above has more info and a DIY "how-to" guide.

An alternative would be something like the Megatraps I mentioned above, stacked in each corner. They are also highly effective but that would cost a lot more than the DIY tube trap option.

What's a chonker?

Do you have any photos of these traps? I see some steps but no photos so hard to grasp.