Acoustic Panels

[edwin]

Yeah, I spent probably $500 on Auralex about 10 years ago for my basement jam room/studio. I ended up giving it all away with an apology that no matter how excited they were about putting all that stuff up, not much was going to happen. I then spent $50 on a huge bale of the acoustic cotton (which was a lot cheaper then for some reason. It was also local), went to Home Depot and had them cut some 1x6 to length and got a bunch of muslin. An afternoon later I had 2 2x6' panes and 4 2x4' panels. I also discovered I could hear my monitor speakers for the first time, which I had wasted tons of money trying different speakers in the room.  I later bought two of the GIK 2x4' panels and put them right over my head to help kill floor to ceiling reflections. Very helpful and the construction they use allows them to be hung safely overhead. Total cost: way less than $500.

[edwin]

Oh yeah, Ethan Winer is also considered an expert in the field. His products, RealTraps, are excellent, but a bit pricier.
 
His website has a lot of good info and he's got a pretty good general audio book out.
http://www.realtraps.com

[keith_h]

Here's a do it yourself site.  
http://acousticsfreq.com/blog/?p=62
 
Keith

[ies_2000]

[no commercial postings on the board., please]
 
(Message edited by adriaan on November 19, 2012)

[5sicks]

I use 2X3' 3 open cell packing panels at odd angles on the walls and ceiling. Styrofoam packing in various shapes kill the bass in corners. The goal is to diffuse and scatter the sound waves much like stealth technology on military aircraft.

[edwin]

I've never heard of styrofoam being used as a bass trap before. Interesting. I've always heard that you need absorbent and compressible material for that kind of function. Styrofoam strikes me as pretty reflective at low frequencies, in fact at most frequencies.
 
I think Keith's link is pretty right on. It's similar to what I did. I will say that controlling reflections was very useful, but the bass traps were even more effective. His stuff can be adapted to bass trap duty by layering the rigid fiberglass to 6 or more inches and then placing it across wall or floor/wall or wall/ceiling boundaries. Unless you have 4 inches or more of absorption, your device is going to be limited at the lower end to the very upper midrange at best.
 
Overall, though, I'd recommend not wasting time with foam, styro and otherwise, egg crates, etc.  If you look over the acoustical forum at Gearslutz, you'll find that pretty much everything you can imagine has been tried and 99.9% of it doesn't measure up to good old rigid fiberglas or acoustical cotton. It's a fun place to read the threads: http://www.gearslutz.com/board/studio-building-acoustics/

[hankster]

Think about tube traps:
http://www.tubetrap.com/
 
R.

[mario_farufyno]

You have to distinguish what problems you're having (or which frequencies are exceding). Lows in excess is usually related to a problem known as Standing Waves. This can only be really eliminated by changing room's proportions in order to spread any reinforcements in some low frequencies the best possible, which is not in question (you will not rebuild your room, I supose). But you can use sound absorbers to tame them a little.
 
Even in appropriate proportioned rooms is known that Lows will sound exaggerated near walls and even more in the corners (where walls join). So it is common to use sound absorbers exactly on these spots. The problem is that porous absorbers (such as foams and fibers) are not as effective in decreasing bass resonances.

[mario_farufyno]

A simple rule teaches us that a porous material can reduce sound energy at frequencies that have wavelengths greater than 4 times its thickness. That is, if we are talking about excess at 100Hz (which has a length of approximately 11.3 feet) we'll need something an absorber close to 33.85 in thickness. Somewhat too thick to lose in a room and it will not absorb frequencies below 100Hz, just above (if the problematic frequency is low as an open E, wich rings in 41Hz, it would require an absorber more than 70 thick!). So, any porous absorber isn't really helping.
 
* For calculating the wavelengths, we have the following formula:
 
f = v / L
- where f is the frequency
- V is the speed of sound (about 1130 feet per second, since the velocity varies with environmental conditions, specially temperature)
- and L stands for waveLengh
 
** a simple formula to porous absorber is:
 
f = 1/4 d
- where f is the low frequency above which our absorber will be effective
- and dis de thikness of it

[sonicus]

If you really want to know what going on in a room ; first of all I would do a few Pink Noise Shots  with an RTA  (real time analyzer) to get an idea of what you have in front of you. Other wise it's almost like trying to grab a fish in  a 12 foot deep  barrel  with muddy water with no visibility.  Don't fall in the water  ___ LOL !

[edwin]

If we are trying tame very specific room modes, then broadband absorption is the wrong tool. Tuned diaphragmatic bass traps work much better for this. However, they are much more involved to make well.  
 
I'm curious about the simple rule of absorption of wavelengths 4 times greater than the thickness. I wonder if this is a rule across all types of absorptive materials or a generalization or an average. My anecdotal evidence is that even 6 is very effective at tightening up the low end in a room that is roughly 12x20x7 (low ceiling-grrrrr). Counterintuitively, bass traps actually improve the perception of bass response rather than attenuate it. I went through a bunch of different monitors trying to find some with good bass response before I added the bass traps. As soon as I did, it became obvious that any of the monitors I had chosen would have been fine.

[mario_farufyno]

However, to our joy, we can create membrane absorbers that use the mass of the material to lower its frequency of operation without requiring an increase in thickness as large as would be required with porous materials. Thus, with a wooden plate (or even fiberwood) just a few inches away from the wall, can act in these severe. And it's good to know that this type of absorber acts only on Lows.
 
While a porous aborvedor acts as a hi-cut (or Low Pass Filter), attenuating sounds above its cutoff frequency, a membrane absorber acts as a lo-cut (High Pass Filter). That's why they are called Bass Traps.
 
The operating principle is that bass can easily pass through a plate due diffraction. Whenever the wavelength of a sound is greater than an obstacle, he can overtake him as if it didn't exist. Thus, the bass pass through the plate and flap against the wall. Upon returning to the room, they meet with the board by behind. At this time the board will tend to vibrate, consuming the sound energy from air converting it into plate motion (transforming acoustic energy into mechanical energy).
 
If we associate some kind of cushioning to damp this vibration, we can dissipate excessive energy in a bass resonating room. It's like what happens when you put your fingertips on a vibrating string. Bass Traps doesn't act over Highs because their smaller wavelenght makes them bounce at the plate and they reflect back to the room.
 
That's why we combine different absorbers to treat a room. In fact, we have to calculate Standing Waves and Reverberation Time to know exactly how much area of each we'll need on any specific room (because we don't want it sounding too dead, too). But you can spread Bass Traps to improve how your room sounds and make further adjustments ahead, anyway.
 
The advantage is that you can tune your bass trap just puting it closer or far way from a fixed wall, because cutoff frequency depends on its mass and distance to the wall.
 
*This is the formula:
 
f = 1900 / √(m x d)
 
- where f is the cut off frequency, below what our bass trap will be effective
- 1900 is simply a constant related to metric measures (sorry, being brazilian I don't know the constant to imperial measurements)
- and this constant is divided by the square root of m (surface mass in Kilograms per square meter - Kg/m2 - weigh divided by area) multiplied by d the distance to wall (in milimeters, mm)

[mario_farufyno]

Sonicus is right, an RTA can show you wich frequency is offendant  
 
Edwin, that 1/4 rule is probably an generalization, for sure. And it is clearly better to control just the specific problematic frequency than start to kill all animals in a forest just to catch a single rabbit. Hope that membrane Bass Trap formula helps somehow  
 
That √ in the formula was an attempt to write down Mac's square root symbol that went wrong...

[mario_farufyno]

ofensive or problematic frequency would be better (sorry, I suffer to express myself in english)

[mario_farufyno]

Important is that membrane is meant to vibrate freely, so hold it by its corners. You can use any material other than plywood, as glass sheets, fabric or even some foam sheet.
 
It can act as broadband absorber if you put it on a corner, since distance to wall will vary.  
 
Don't forget the damping behind the plate, it can increase its strenght.
 
You can weigh plywoods in a scale and divide by its dimensions to know its surface mass.
 
If you wll use Imperial measurements, you must change the constant.
 
If you measure in meters, you must change constant to 19. If using centimeters the constant will be 190 (1900 works just with milimeters). Metric measurements are based on decimal divisions...