Tequila Slammer wrote:......A lot of the research I've done so far says a good initial set-up is to put treatment above, in front of, behind and to either side of... The monitoring position!
That always niggled at me. Why would you ever get standing waves there? Any sound crossing that point has to be going at an angle to the wall other than 90 degrees, unless you wear your monitors like headphones. Wide angles won't exhibit standing wave properties at all, and very narrow angles will lose energy bouncing around before they creep over to you.
The mirror trick, which I picked up on the studiotips website after Paul mentioned it on the Mick-Traps thread, sounds like a much more scientifically valid method of treatment placement.
Acoustic treatment ain't just about standing waves.
Treatment for standing waves at Room Mode frequencies is best treated by broadband absorption in corners. Tri-corners being the optimal placement if you can't do ALL corners. Room mode standing waves NEED two or more boundaries, and in the real world they can affect the freqeuncy response of a room from the bass upto around 500Hz.
The mirror trick is used to determine placement of broadband absorption for EARLY REFLECTIONS ( i.e those reflections whose path length is less than around 20 feet ( about 20ms ) greater than the direct sound path length. These reflections are heard by the ear as part of the direct sound and cause reduced clarity and detail, innacurate stereo image/source location. And a 'comb filter' type response. All across the audio bandwidth where the sound is directional.
So... "A lot of the research I've done so far says a good initial set-up is to put treatment above, in front of, behind and to either side of... The monitoring position!"
is great advice. As it helps to create an RFZ ( Reflection Free Zone ) at the listening position.
And in particular, with free standing non-soffited speakers, placed close to one or more boundaries, there will be a rise in bass response. This is due to the omnidirectional response of a speaker cabinet at lower frequencies, and the wavelengths of the lower frequencies being large compared to the reflection path length resulting from the boundary. As the wavelengths are so long in comparison the reflected sound will pretty much be in phase with the direct sound, causing an additive increase in level. IIRC you get theoretical 6dB increase in level for every boundary. Which means peeps who place their speakers in a reflective tricorner will get a theoretical increase of 18dB. In practise it will less than this due to real world losses, etc.
You can also compensate for this increased bass response by EQ. In fact many active monitors have tilt and shelving controls to do just that.
As you increase the frequency, so the wavelength and path length become a similar size, the reflections cause interference patterns, which results in a 'comb filtered' response.
So with free standing speakers that are close to walls, floors and ceilings ( unavoidable in small rooms ), LOTS of absorption behind, below, above and at the sides of the speakers is well recommended.
The other option is to soffit mount the speakers, which places the sound source at the SAME PLANE as the wall, thus their are NO reflections from behind the speaker to cause any increase in bass levels or comb filtering at higher frequencies. The other advantage of soffit mounting is to stop sound diffracting at the edges of the speaker cabinet.
Another advantage of soffit mounting speakers is the increase in efficiency of lower frequencies ( Sound energy that otherwise would radiate in directions other than forwards, now CANNOT, due to the soffit wall. EQ is used to compensate the response, and thus your monitors have more bass head room. Meaning less speaker distortion.
Genelec have the best FAQs on soffit mounting here..
All this though , in regard to real world treatment, is a seperate issue to Room Mode treatment ( trapping ).
What's everyone's thoughts on having high monitors angled down? It makes complete sense to me, but as I siad, I have all of an A level in physics.
What are the sonic advanages of having your monitors' tweeters level with your ears? Is it worth it to suffer the hideous mangling of audio that comes with standing waves?
The sonic advantages of having the tweeters at ear level is to help the clients sitting on the couch at the back of the studio hear a balanced HF. Angle them down and it's likely the client will hear a dull version ;) Important considerations in the real world.
I've actually designed my monitors to angle downwards in my new studio, as I don't have clients and I angled the speakers ( and front soffit wall ) downwards by 10 degrees to optimise the sound at the listening position in regard for other pieces of equipment like computer monitors obsuring the Audio Monitors.
Sorry if my reply has been too much or too long :) I got carried away.
But it is important, IMO, to show that when treating rooms, the innacuracies we wish to treat, are caused by various situations, and different treatments are used for those different situations.
In short(er).... ;)
Room mode Standing waves need TWO or more Boundaries to exist, and have their maximum pressure at those boundaries. They are pretty much independent of speaker placement, although certain speaker placements can excite them more easily ( such as in the case of soffit mounting )
. Corners are where maximum pressure of modes from more than one room dimension add together. So corners are where to maximise the efficiency of room mode absorption treatment.
Early Reflection problems are treated by absorbing ay reflections that will end up at the listening position. The Mirror trick is the easiest way to find the spots, although it can be done pretty well by visualisation for those without mirrors ;)
IMO those are the two types of treatment that shoudl be considered first off. And then once the early reflections and room mode standing waves have been tamed you can then start thinking about diffusion to smooth out the LATER reflections in the case of large rooms, or smoothing out any ambience left over in a small room.
Right! that's enough for now.
back to designing my studio build acoustic treatment.