[audioblazer]

Mike ,
Thanks for the recommendation.

Based on yr calculation, to go as low as 20hz, we will need a 55ft length room????? So the professional reviewers would be wrong to state that their reviewed speakers can go as low as 20hz when most of them don't even have a room as big as mine? Interesting

Happy New Year. Let's roar in another exciting audiophile year 2011

[Ethan Winer]

Ethan, what are the real trap products that absorb mostly bass? Do they work on low bass?

Since you asked: All RealTraps "bass traps" have a reflective membrane that increases absorption at bass frequencies while reducing absorption at mid and high frequencies. (We also offer most panels in a High Frequency option that absorbs both bass and highs.) So in order of increasing performance there's MiniTraps, MondoTraps, and MegaTraps:

http://www.realtraps.com/products.htm

--Ethan

[Ethan Winer]

Based on yr calculation, to go as low as 20hz, we will need a 55ft length room?????

No, that's a common misconception but it's totally wrong. As proof, headphones can reproduce down to 20 Hz or lower. So you can have full output at frequencies as low as your speakers or sub can play to, regardless of the room size.

--Ethan

[jtwrace]

No, that's a common misconception but it's totally wrong. As proof, headphones can reproduce down to 20 Hz or lower. So you can have full output at frequencies as low as your speakers or sub can play to, regardless of the room size.

--Ethan

What's your favorite pair of headphones?

[Ethan Winer]

What's your favorite pair of headphones?

I have no idea! The last "good" headphones I owned was 35+ years ago. As I recall they were a top of the line model from Koss and cost about $65 in 1970s dollars. The only phones I use these days are for editing narration to hear soft details clearly, a $10 open-back set made by Sony. So almost anyone else here will have a better opinion on this than me.

--Ethan

[McTwins]

The frequency of your room can be determined (in feet): 1126 (speed of sound) / 24.5 (length of room) = 45.959 (as Hz). That represents the longest (lowest) frequency that will directly image in the room.

That does not mean that I am suggesting that everything below 46 Hz is a waste. However, there are a number of issues, including direct vs. reflective sound. Sure, you can hear freqs lower than 46 Hz in the room, but you are getting them reflectively.

Not surprising, your most recent plots suggest that moving your speakers (as some people suggested) had a significant, positive result. The call for more bass traps, etc. is, I think, unwarranted until at a minimum, you have finalized the placement of your speakers. I would continue to work with speaker placement to see (hear) if you can improve things further.

There is a pretty good sized set of variables to contend with, and since I don't know your background, suggesting references for a better understanding of what you have to deal with may be redundant. The two books that I use the most are:

Sound Reinforcement Handbook, 2nd Edition (written for Yamaha by Gary Davis & Ralph Jones) -- Pretty easy to digest. a lot of basic theory audio theory and loads of practical application. Includes some basic math, and the schematics are pretty easy to follow. It's available through Hal Leonard Publishing, and runs USD $35. I'm assuming it is still in print. A used copy shouldn't be tough to locate.

For more detail, there is Handbook for Sound Engineers (I have a 3rd Ed. I think it's up to a 4th), edited by Glen M. Ballou. A LOT more detail, this has moderate amounts of both schematics and somewhat advanced math. However, there is still much in it useful for people who aren't EEs, etc. It's available through Focal Press, and is a bit more expensive: At 1500+ pages, it will set you back about $100.

You schould divide the 46 Hz by two and get the first fundamental resonance at that leght and the rest is harmonics, so it means, that the 46 Hz is a harmonics. If any can show me any bass panel resonator that can take that first fundamental by construction is very difficult to achive. I haven't seen one yet.

Thanks

[Mike Nomad]

Mike ,
Thanks for the recommendation.

Based on yr calculation, to go as low as 20hz, we will need a 55ft length room????? So the professional reviewers would be wrong to state that their reviewed speakers can go as low as 20hz when most of them don't even have a room as big as mine? Interesting

Happy New Year. Let's roar in another exciting audiophile year 2011

Good observation, and yes your math is correct. As someone else pointed out, this is a common misconception. Let's say you have a room that is 35 feet long, place a set of rear ported speakers half-way into the space, and sit all the way at the back of the room. Anything that comes out of the port travels to the back wall (~17 feet), bounces, then has to move through the space to get to your ears (~33 feet). 50 feet is the length of 22.52 Hz.

This is why I used the word "direct" in my earlier post. Your walls don't absorb everything (and frankly, you don't want them to). This is why I was recommending a couple of books. There really are _a lot_ of things interacting. One thing in particular to look at are the typical response curves with human hearing. We don't hear things flat across the spectrum. To make things sound like they are equal in volume, more (and less) amounts of power are needed at different frequencies. an EQ is an easy way to cheat the power need: An EQ is simply adding (or subtracting) gain at a particular frequency.

A Happy New Year to you too,

Mike

[Ethan Winer]

50 feet is the length of 22.52 Hz.

Just to clarify, the resonances in a room occur at half the wavelengths. So if the lowest mode is at 22.52 Hz then the room is 25 feet long, not 50 feet. The explanation below is from the Help file for my freeware Graphical Mode Calculator utility.

--Ethan

__________________________________

Twice the dimension is used because a room 10 feet long really has a total distance of 20 feet - the wave travels from one end to the other and back to complete one cycle. So for a room 10 feet long the first mode occurs at 56.5 Hz:

   1130 / (10 x 2) = 56.5

[audioblazer]

Ethan ,
Thanks for the clarification . It's sound definitely right. I doubt anybody has a 50ft room to play down to 20 hz

[audioblazer]

Mctwins
I m sure there is more than 1 way to skin a cat. Surely there are some rooms out there using bass trap etc with good freq response . My room problem may be due to construction issue. I m suppose to wrap  my corner bass trap with a pond liner and pull it's a tight as a drum but unfortunately the frame was built into the corner rather than constructing it in 3 pieces to be stack on top of each other. When the workers tried to pull it v tightly, it warped the frames.  This could be the reason why the bass region is not properly tuned.
As for your DiY route using only diffuser it certainly work based on your freq response. However I certainly don't have the expertise or inclination to experiment  with room acoustic . Anyway thks for your feedback. I m considering put rpg skyline at the front resonator to improve imaging

[McTwins]

Hi
Understand your point, it's a tuff job to treat a room. Did you try it with your subwoofers as I mentioned before. You can leave your VR4 in full range and cut off at 80 Hz with your subs and I think you will solve the problem with your suckout at your listening position. Just try it!!
Thanks

[audioblazer]

I have a pair of JL audio F113. Love it for rattling my room for HT duty. In fact bought a tube buffer to try it due to impedance mismatching between my tube CJ preamp and my sub but unfortunately I didn't manage to get good result. Even bassier with the VR4Sr running full range . Maybe when and if I ever buy an external crossover I will try it again. Thanks for the suggestion

[Housteau]

Low frequency dips in a listening room can be very difficult to deal with.  I battled mine for years until I finally took a big chunk out of it.  My dip was around 47Hz and dropped at its max to around 12 db in my untreated dedicated room.  My standard room treatments for bass, all vertical corners filled in as well as several wall to ceiling angles and my cathedral peak, dropped that dip to around 8-9 db.  This is where it remained for years until I finally figured it out.

If you have a dip at a certain frequency more than likely there will also be a strong build-up at that same frequency some other place in the room.  I played a tone at 47Hz and found a very large build-up at the center of my front wall from the floor to my cathedral peak.  This is the place bass trapping had to be to catch that 47Hz and knock it down.  Lowering that peak also diminished that dip by another 4-5 db.  Had I just added more bass trapping in a random manner, I doub't that I would have gotten the same results.

It takes a lot of thickness to do justice to 47Hz and so I used 2.5 complete boxes of 703 stacked from nearly floor to cathedral peak.  I never saw the need to even take the 703 out of the boxes.  I made attachment points and supports for them directly to the wall.  I then covered them with the most decorative, yet acoustically transparent covers I could easily put together.  I ended up buying two orietal room divider screens that I modified to fit.

[steve2701]

Listening to music in that stunningly beautiful room must be a joy. Congratulations on getting the result you wanted.

[audioblazer]

Housteau,
This is certainly an interesting solution. Can it be use for both dip and peak? Any idea where I can get a specific freq test tone? Can stereophile test CD with test tone eg at 31.5hz ( my peak ) 1/3 octave warble tone at -20db be use ? Thks for the excellent suggestion

[audioblazer]

Housteau
How do u find the build up at the centre of the front wall when you play the 47hz test tone?

[Housteau]

This is certainly an interesting solution. Can it be use for both dip and peak? Any idea where I can get a specific freq test tone? Can stereophile test CD with test tone eg at 31.5hz ( my peak ) 1/3 octave warble tone at -20db be use ? Thks for the excellent suggestion

Yes, this works for both dips and peaks located at your listening position.  However, I am a two channel listener and so my target zone (sweet spot) is only for a single listener.  So, for me it is easy to notch out peaks, but I cannot fill in dips and that is why their source needs to be addressed.  I would think that my method of dealing with dips would benefit the multichannel systems with multiple seating locations as well.

You can use any method you like of producing that tone through your system.  I have used the set of tones available from Ethan as a download on the RealTraps site, as well as from my True RTA software on a laptop.  I just dial in the desired frequency and play it through my system.  A test disk will work fine as long as it has your exact frequency on it.

Once the tone is playing I just walk around the room with a simple db meter.  You can actually get a good idea by just using your ears as well.  The meter doesn't have to be very accurate as you are just taking levels of loudness relative to each other.

Remember that getting a flat frequency response using test tones does not automatically create great sound, as music is more complex than solid tones are, but it is the best solution I have found to rough things in.  There are a lot of other quality factors involved that do not show up in a straight frequency graph.

[Housteau]

Listening to music in that stunningly beautiful room must be a joy. Congratulations on getting the result you wanted.

Thank you.  I am thankfull for it each time I sit inside.  My home is actually very small and simple at around 1550 square feet.  About 18 years ago I had the opportunity to build a detached garage in the back and decided to have a listening room built as a part of that free standing structure.  It is nice having it separate from the home as I can listen late into the evening and not disturb anyone.  It is a very nice room, but as I wrote in my recent thread here "A Room Within A Room", I could have done better with the knowledge I have now eliminating a lot of the issues I later had to put band-aids over.

[audioblazer]

Below is a posting by Kye in stereo.net.au. Best DIY perforated bass panel for various bass freq issues .Maybe it can help some of us with bass freq issues.

How to build a perforated panel absorber (bass trap)
Hi All,

In acoustically treating a room, there are a number of ways to overcome issues and achieve good sound. Absorption is one of these techniques, and constructing panels that will absorb from approx 250Hz and up is relatively easy to do, however it is the frequencies below 250Hz which prove to be challenging. This is where the perforated panel absorber can be useful.

Background
A perforated panel absorber is a resonating absorber - ie, it is 'tuned' to a frequency. It will have high absorption at that frequency, but will have lower absorption the further that you go either up or down from this frequency.

Physically, it is a box, where one side of the box has gaps in it where air can move in and out of the box. The other 5 sides of the box are solid and complete, with no gaps. If you build a six sided sealed box and drilled a hole in one side, that would be a perforated panel absorber. The name comes from the fact that one panel (ie, one side of the box) is perforated - ie, it has gaps/holes in it.

The idea of these 'gaps' is an important one, because the shape of these gaps does not matter at all. You can drill holes, you can have thin slots, you can have any shape you like!

The thing that matters is the percentage of the panel that is gaps.
ie, if I have a panel that is 100cm2 and I drill a hole in it that is 1cm2 then 1% of the panel will be a gap, and 99% of it will be solid.

How to work out the size of the box
The box can be any size and shape you like. However, the size of the box does affect the tuning of the box, so (for instance) you won't be able to build a box that is 10cm deep, but will be tuned to 20Hz.

Here's the formulas (taken from the Master Handbook of Acoustics)

for absorbers with holes:

Freq = 200 x square root of (P/(D x T))

where:
P = perforation percentage (eg, 5%)
D = depth of air space (in inches)
T = PT + 0.8 x HD
where:
PT = panel thickness (in inches)
HD = hole diameter (in inches)

for absorbers with slats:

Freq = 216 x square root of (P/(D x PT))

where:
P = perforation percentage (eg, 5%)
D = depth of air space (in inches)
PT = panel thickness (in inches)

notes:
the depth of the air space is the internal depth of the box - ie, the distance from the inside of the panel with the holes/slats to the inside face of the rear panel.
the panels themselves are not meant to move at all, therefore you should make them out of a material sufficiently thick so they don't move. ie, don't use 6mm MDF! If you're going with <16mm MDF then you might like to put in a couple of braces - nothing ridiculous, just to stiffen the panels up a bit.

Bandwidth
Ok, so we know how to tune the box to a particular frequency, but how wide a frequency range will it absorb?
The quick answer is: not very much. However, if we put stuffing inside it, then it will absorb a much wider range of frequencies.

The graphs in the book show that at the tuning frequency the absorption is very close to 100%, but if we take 80% as being our goal, then it will do roughly one octave either side of the tuning frequency.
ie, if we build a trap that is tuned to 50Hz, then we will get about 80% or more absorption between 25Hz and 100Hz.

Stuffing can be anything you like, but fibreglass is the cheapest and most effective performer, so that is what people normally use. Be warned though, fibreglass is nasty stuff, and so you should wrap it in fabric to ensure the fibres don't go everywhere!

Size of trap and absorption
These traps are not magical devices - they don't 'suck' bass from a room. They simply behave in such a way that lots of the bass frequencies that go into the trap (through the gaps in the front panel) don't come out again.
The key concept here is that if you build a bass trap that has a front panel size of 1m x 1m it will absorb a lot more bass than a box 10cm x 10cm with the same tuning frequency.

Guidelines
The trap becomes more effective at absorbing bass the more easily that the sound can get into the box to be absorbed. ie, a 1m2 box with 1% of the front panel open will be less effective than a 1m2 box with 5% of the front panel open, simply because there are more holes to let the sound in!
You will now realise that if you want to tune your trap for low frequencies, you can either make it have less holes, or be deeper. The guideline would be to keep the perforation percentage above 1%.

An example - My system
I own some panels that absorb from approx 250Hz up, so I wanted to complement these with absorption from 250Hz down.
Taking the idea that a trap is >80% effective over a two octave range, I decided to build two different tunings, one at 120Hz and one at 40Hz.
The 120Hz trap would absorb up to 240Hz and down to 60Hz with greater than 80% effectiveness.
The 40Hz trap would absorb up to 80Hz and down to 20Hz with greater than 80% effectiveness.
Combined, they should be effective over a range from 20Hz to 240Hz, fitting my criteria.

I decided that I would build my traps from a commonly available size of wood so I didn't have to do as much cutting, so the front panel of the traps are 900x600.
So, using the above formula, using the tuning frequency as a goal, I calculated a trap that was ~12 inches deep (the internal depth) and had a 3.6% perforation percentage with 1cm (0.4in) diameter holes and a 12mm (0.5in) thick front panel. This gives me the 120Hz I wanted.
So, what does 3.6% perforation actually mean?
Given that each hole has a surface area (using pi*r*r) 0.785cm2, and that the total area for the front panel is approx 5031cm2 (remember the front panel is 576mm x 876mm on the inside of the box) that means we need 230 holes.
These holes should be spread pretty evenly over the front face of the box, although you don't have to be super accurate with this - as long as they're relatively even.
If we had a grid that was 12 rows by 19 rows of holes, that is 228 holes, which is easily close enough to 230, and it gives us a shape that matches the dimensions of the box. Holes would be in a grid with 4.5cms between the centres of the holes.

The same approach was used to determine the 40Hz traps.. which would end up being 0.4% perforation percentage, the same depth, and a grid of 6 x 4 of the 1cm diameter holes with a distance of 12.85cm between hole centres.

You will note that I used a perforation percentage under 1% for this trap, but I did so because I didn't want to make these traps any deeper than the other ones.
I came up with this depth because it is the depth of my bookshelves, and so when I put the traps next to my bookshelves they don't stick out, but actually look rather neat - giving the bookshelves a 'recessed' appearance.
I also build two of each trap (4 in total) so I can put the traps symmetrically on each side of the room. I'm not sure if this matters much, but it fits in with my room layout, and gives twice the absorption of a single trap of each size. They can also be stacked to be 600 wide and 1.8m tall, or 900 wide and 1.2m tall, providing a handy shelf. I am about to build some subs (stereo) and will make these about 600 x 750 x the same depth. This will mean I can slot them under the absorbers and they will take up no additional floorspace, despite being quite large subs. This modular design is fantastic for my needs.
I filled my traps with fibreglass wrapped in the cheapest fabric I could buy, this will stop the fibres coming through the holes into the room.
I made the front face out of 12mm ply and after drilling the holes (LOTS of holes!) varnished them. They look ok, but if I did it again I would use MDF and paint them cream so they blend into the walls. Also, the fabric is a light colour and you can see it through the holes, which makes it look a bit strange with the varnished dark plywood.

Here's a (very old) pic of two of my finished traps:

They're not exactly as I described above as I used approximate numbers for the above examples. In this setup they were positioned in the rear corners of the room, but in subsequent setups they were mounted in the front corners up against the side walls facing the speakers.

Is this the right design for me?
Only you can answer that question. However, in order to help you decide, here are some things to think about:
- this is only one type of bass trap / absorber and the others may work equally well
- these require a fair amount of effort to make, but are cheap and very good performers once installed
- as these are solid, they could be modified to fit with your decor, and have a shelf on the top for plants or other forms of high WAF diffusors
- they are large and heavy, so if you move house a lot this might be a consideration, although you can always put wheels on them

Possibly the largest advantage of these is that they work according to the laws of physics, not the laws of interior decorating or other such 'principles'.. this means you can be creative with them and make them fit in all kinds of interesting places.
Eg, if you have a cupboard that is relatively solid and the door seals ok, you could drill some holes in the door and turn it into a large bass trap. wardrobe? yep - it's even got acoustic padding in it (clothes)  storage under the stairs.. yep.
Got a gap you don't need? Put one in the fireplace going up the chimney. Mount one in the ceiling and put cloth over the holes to hide them. Similarly under the floor. When the boxes aren't 100% square just estimate the depth and use lots of stuffing and you'll get it pretty close.

Why have bass absorbers at all?
Most speakers make bass. Most speakers are used in a space that has walls around it which will reflect the bass and not let it move out of that space. Therefore, most music setups will be in a space with bass echoing around inside it.
Virtually no room will be 100% at containing bass (it leaks out all over the place) but as there is nothing in the room very good at absorbing it and it will take a while for most of it to leak out of the room, it tends to still be bouncing around in the room long after the high frequencies have been absorbed. This means that while your mids and highs will sound nice and crisp, the bass you hear will be echo-ey and reverberating after the bass note is over, so you will not hear clear and clean bass.
Bass traps absorb bass, so when the speakers make bass, it gets absorbed quickly and the subtle details in the sustain and decay of bass notes will be more easily heard. Music may also sound more snappy and nimble as the subtle timings in the bass will be more apparent.

I found that when I installed my traps, bass became tighter, you could hear when notes were allowed to decay and when they were muted, and how the muting was done. Overall, it cleaned up the bass region of the sound.

[audioblazer]

Mctwins, I removed the front passive resonator as suggested & in the process of adding 11 pieces of 2D PRD diffuser 25 x 46" made of mdf board ( really heavy) , 3 to be place on the front wall , 4 on each side wall ie 2 before & 2 after the speakers. My issue is that I have a sub panel located around 24" from the floor . So I can either located all diffusers 30" from the floor for u uniformity or place the diffuser 24" fr the front wall and side wall after speaker & for the side wall before speaker ( nearer front wall) , place it 30" from the floor due to the sub panel. Is there any kind of rules of how height the diffuser must be place from the floor.