[drphoto]

Ok, so are bass traps supposed to give 'better' (by which I mean 'punchier and more articulate) bass? Or do they simply remove 'boominess'?

If the former is true, do they do this by eliminating standing waves that both reenforce and cancel parts of the low frequency response?

Speaking of which, what is the best way to determine what kind of response you have? Rent an RTA from the local pro music shop?

Does the homemade type trap (a wire cage wrapped w/ fiberglass....ala Jon Risch) work?

And finally.....where do you put the thing? Behind speakers...in a corner...?

[JackStraw]

Ok, so are bass traps supposed to give 'better' (by which I mean 'punchier and more articulate) bass? Or do they simply remove 'boominess'?

If the former is true, do they do this by eliminating standing waves that both reenforce and cancel parts of the low frequency response?

Speaking of which, what is the best way to determine what kind of response you have? Rent an RTA from the local pro music shop?

Does the homemade type trap (a wire cage wrapped w/ fiberglass....ala Jon Risch) work?

And finally.....where do you put the thing? Behind speakers...in a corner...?

My comments address some of your questions in order...

1) It's foolish to think that the bass that get trapped might be 'better'. I think that the fact that they have been caught in trap indicates they they are probably dumber, and therefore worse. Although I don't know if anyone has studied the relationship between bass intelligence and taste.

2) While breaching is dramatic and somewhat common among whales, when bass jump out of the water they generally just make a little splash. So, I don't think that you need to worry much about standing waves.

3) How many bass has Jon Risch caught? Have you actually seen his bass trap in action, or just read about it on the internet? From your description of the design, it sounds like the bass would just swim right out the other end.

4) If you're trying to catch a fish you're going to have to leave your listening room. Clearly the best place for your bass trap is in the water.

 

Ok, maybe I'm a little punchy tonight.

[drphoto]

Hmmm, something seems fishy about this explanation.  

[Sedona Sky Sound]

Hello JackStraw,
I must say that was one of the funniest post I have seen in a really long time  . I was rolling on the floor with laughter   .

Drphoto:
There is no way I can top JackStraw's post but here are some "non-fishy" thoughts:
 
The purpose of a bass trap (or practicly any absorber for that matter) is to reduce the RT60 time of a particular frequency range. This time is the amount of time in seconds that a signal takes to reduce in intensity by 60 dB. A general rule of thumb for Home Audio is that you want this to be somewhere in the range of .5 to .7 seconds for all frequecies. Anything less than about .4 seconds and the room will seem too dead and more than 1 second will produce echo/smearing/blanketing of the sound.

Absorption devices are rated by the amount of energy (sabins) that they take out of the air and convert to heat for a standard set of frequencies.

One thing to note is that term "bass trap" seems to have several different definitions within the audio community. A tube trap (ie, John Risch or ASC design) uses a combination absorber/panel diffuser technique to turn waves into heat as they try to pass through the trap. The frequecies being absorbed is directly related to tube trap diameter. They are typically placed at the frequency maximum (which is typically in a corner).  

The other main type of bass trap is a a true Helmholtz resonator. Basically think of a coke bottle. A Helmholtz resonator works by the low pressure of the wave pressing down on the air in the neck of the "bottle". This increased pressure within the "bottle" is converted to heat and thus results in the wave energy being removed from the air. Helmholtz resonators are tuned to very specific frequencies.

Generally speaking, the two are not interchangable. Tube traps are generally only good down to 80 Hz or so (they would be prohibitively big to provide significant absorption below that) while Helmholtz resonators work quite well in the 20Hz to 100 Hz range. If you have a room with no bass absorption (ie. the walls are made of concrete) then a tube trap will work very well for bringing down the RT60 times for all the bass frequencies. The bass modes will also be "smoothed out" somewhat since the louder/higher energy modes will be absorbed more than the rest.

However, for most homes in the US that have sheetrock walls and wood floors, the RT60 times for most of the bass frequencies can already be pretty low (< .5 seconds). By installing a bass trap you would be "fixing" some of the mid-bass modes but would also be further reducing the RT60 times for the rest of your bass frequencies (making a really bass-shy room). In this case a Helmholtz resonator is a much better bet (especially if the nusiance bass mode is in the 30Hz to 60 Hz range).

John Risch does not recomend Helmholtz resontators because 1) they are hard to make correctly 2) You need to know exactly what frequency you are trying to fix 3) if not placed correctly they can interact and cause a resonant hum. I personally believe they are necessary to get the best possible sound but I fully understand John's points. Unless you are extremely patient, good with math, and are handy in a woodshop then it may be best to leave Helmholtz resonators to the professionals.

That being said, there are several simple/cheap things you can do to help find problem areas (and better understand if the use of a tube trap is correct for your room). First do a back-of-the-envelope calculation of your RT60 times. You can do your own numbers or use a simple calculator like this one: http://www.saecollege.de/reference_material/pages/Reverberation%20Time%20Calculator.htm

Next, make or buy a CD with 1/6th octive frequency steps that last about 10 seconds each starting at 20Hz and going up to about 500 Hz. Use a SPL meter to measure each frequency at your primary listening position and several points through-out your room. This will give you a good idea where your most problem frequencies and areas are.

There are a lot more complex/costly/better ways to measure your room but this should get you started and allow you to quickly fix a majority of your problems.

I hope that helps.
 
Julian
www.sedonaskysound.com

[Jay S]

Julian,

This post was wonderfully helpful!  the RTA calculator was especially helpful.  

A quick calculation of my room indicated that RTAs are between 0.56-0.69 for 500 Hz - 4000 kHz.     But, RTA rises to 0.97 and 1.03 for 125 Hz and 250 Hz, respectively.     Substituting an audience in seats for an upholstered seat reduces RTA in those 2 lower frequencies to 0.77 and 0.81, respectively.  No wonder my system sounds better with more people sitting on the couch listening!  

This is a good rough cut, but its too bad the calculator can't model having multiple types of material along a wall, open doorways, the effect of having multiple pieces of furniture, etc.  For example, I have wood speaker stands as well as a wood coffee table in my listening area, which I think reduce the RTA of lower frequencies (based on the coefficients listed for plywood over an airspace).  

In general, there is great info on this site on acoustics and absorbers and I have printed out several pages to read later on.  

Thank you again!

[drphoto]

Julian,

Very nice....thanks.

BTW: by my use of RTA I meant a real time spectral analyzer in conjunction w/ a pink noise generator.