Bass/Room probelm

I have theil 2.2s that are place four feet out from the back wall (all glass) and 3 feet in from each side wall in an L shaped room that is roughly 24 ft by 22 ft with a 11 ft by 11 ft inset in one corner. Using a radio shack spl and stereophile test cd (vol 1)my bass response measures (and sounds)10db higher than normal at 31 hz and 10 db lower than normal at 64 hz and is otherwise pretty flat from 20 hz to 150 hz. What is going on? Is there anything I can do to address this with speaker placement/ room treatment etc?


One more thing. The thiels don't extend to 20hz I have an ACI Titan II subwoofer crossed at 35 hz. but the response problem occurs with or without the sub in place.
Quite honestly, your measurements are not that out of the ordinary. Most speakers do not take into account room acoustics and are not designed for linear "in room" response. This problem is further compounded by improper placement or selecting speakers that are not really suitable for one's listening environment.

You have standing waves or nodes that are being excited with specific frequencies. These are resonances due to room dimensions and speaker placement. The resonances that are "in phase" at 32 hz ( bass reinforcement ) and out of phase ( causing cancellation ) at 64 Hz can somewhat be juggled and manipulated. Since these are harmonics of each other, you need to somehow stagger the nodes and break them up. Since you can't really just change room dimensions at the drop of a hat, the simplest solution is to move the speakers and / or your seated listening position.

While you can try all different locations for your speakers and not come up with anything any better, i would simply start by adjusting them so that they are NOT an equal distance from each side wall. While this will change the point of the first reflection from each speaker, that is more easily dealt with since those frequencies are more easily absorbed / diffused.

Once you can find a spot for each speaker that smooths out the response, you can further alter the response by moving the speakers closer or further away from the back wall. This can also be manipulated by changing the distance between your "sweet spot" and the speakers. If you doubt this, simply move the spl meter or your head forward a foot or two and listen / measure the difference.

By taking this approach, you will produce several smaller peaks and dips. Your measurements will probably look a lot more like a very wiggly line rather than the major hill and valley that you are currently experiencing. The end result should be something that is generally smoother in response and a little easier to deal with.

The use of good sized heavily padded cloth covered furniture that is strategically placed along with acoustic treatments geared towards low frequencies ( read this as large and very thick ) can also be put to work to your benefit.

Keep in mind that most "acoustic panels" are pretty worthless below appr 400 Hz. As such, don't go crazy thinking that you're going to solve your problem by buying some RPG Pro-Foam or something like that. All that type of stuff does is to create even more erratic response up higher in the audio spectrum. One look at the absorption co-efficient graph will tell you that they would not work too well for you ( nor in most other situations ).

If you really want to strive for a better "in room" response, try picking up a book by F. Alton Everest entitled "the master handbook of acoustics". It can walk you through why this is occuring, how to deal with, how to build your own acoustic treatments, etc... and help you to better understand how to achieve "killer sound" on a budget. Sean

Well said, Sean!

There is a way to make an end run around the bass problems in a typical room. And that is to start out with speakers that adequately address the situation.

One such design is the Gradient Revolution, whose dipole bass loading results in exceptionally smooth in-room response. Okay I'm not normally one to get excited about numbers, but sometimes something you can hear shows up in measurements, and it strikes a chord. In 1997 Stereophile's John Atkinson measured the smoothest in-room response they ever recorded, and it was from a pair of Revolutions. Plus or minus 1.3 dB from 32 Hz to 10 kHz in an actual room, using 1/3 octave pink noise. Yup, re-read that. Very few speakers can pull that off in an anechoic chamber, let alone a real room! The point is, there are unorthodox techniques that offer significantly better real-world performance by seriously addressing the speaker/room interface. I tip my hat to Jorma Salmi for his brilliant design.

The secret to the Revolutions is dipole bass loading, which very significantly reduces the amount of reverberant bass energy put out into the room's resonant modes, along with cardioid radiation patterns for the mid/tweet modules. These unorthodox radiation patterns minimize the room's influence on the sound. The net result is a speaker that not only sounds very good, but does so in a normal (i.e. crappy) room.
Hi Greg,

I own a pair of 2.3 s, and was experiencing bass problems with my 12 x 15 listening room. I have had good luck with floor to ceiling Jon Risch’s DIY Bass Traps. I have also been corresponding with ASC and Garfish(Agon) on their new Planar Trap for sidewall reflections.

I currently have my Thiels 46” off the rear wall and 31” from the side 5’ 6” between the speakers. Listening position is 3ft behind the nearfield listing position. For my room this is the most natural sounding location.

Hope this helps.

Sean's advice is excellent. You could try the CARA software as well. It's very helpful at speaker and listener positions. As Sean said, it is doubtful you will solve the problem, but you should be able to reduce the problem to some degree. Everest's book describes some more elaborate bass attenuating devices (such as Helmholtz resonators). They can be difficult to build, but can be very effective, when you have a very specific peak as you do.
Great advice on this thread. I've had good luck with ASC quarter round bass traps in the two corners on the speaker wall. Unfortunately, you don't see them used very often on Agon.
You can make an end run around all room interface problems with the Tact RCS 2.0. It will correct time as well as frequency anomalies It's a great pre-amp as well!
I agree, excellent post Sean. An exception to most speakers that don't take in-room response into account are the Vand. 5s-- which have in room adjustable bass from 20 HZ to 125 HZ. The V5 system is very effective. However, I still use ASC tube traps and Panel traps to provide a damping affect in the soundfield. Good Luck. Craig