Big Challenge ( Picture now attached)

invisibl · Post by **invisibl** » Wed Jun 29, 2005 12:09 pm

Hi Steve and all...

Quick question...

Should I be measuring my room from the front of the wardrobe where the doors are? 4.4 mtrs

Or the Wall that is at its back? 3.85 mtrs

I am getting the hang of this but Until I understand which dimension is to be adhered to...All my calculations are for nada.

Thank you

Jeremy

knightfly · Post by **knightfly** » Thu Jun 30, 2005 12:50 am

I think you got dimensions reversed; anyway, you would use the ENTIRE room dimension for calculation, keeping in mind that if the doors of the wardrobe are closed that they will ALSO cause fairly pronounced modes - IOW, some of each

As to location for fiberglass, a membrane/panel absorber should NOT have the fiberglass touching the front panel. A Slat/Slot/Helmholtz absorber SHOULD have the insulation touching the front panel; otherwise the front panel may ring and actually ADD sounds back into the room... Steve

knightfly · Post by **knightfly** » Thu Jun 30, 2005 2:05 am

Sorry, missed a point or two -

Bass traps do NOT have to be cylindrical, in fact they're more effective when NOT - there are so many kinds of traps, and people tend to call them ALL "bass traps" - most cylindrical traps other than Helmholtz types don't do much for bass anyway; more for low mids.

Better if you think of traps as three distinct types - absorptive (soft outside) panel (hard outside) and perforated (Helmholtz, also hard outside) -

Absorptive are generally quite broadband, and don't do a lot for bass until the air space between them and boundaries are fairly large (couple feet or so) - they work on the 1/4 wavelength principle, meaning that they peak at the frequency where the distance from their front surface to the wall = 1/4 wavelength of the sound to be absorbed. They have useful absorption at LOWER frequencies than that, maybe an octave or so; and will absorb all frequencies HIGHER than the 1/4 wavelength pretty completely.

Panel traps are tuned resonant traps; they work by absorbing(turning into low grade heat) the energy that they are resonant to; this causes flexing of the panel, which converts the sound energy into heat; then, INSIDE the trap, the moving air caused by panel flex gets restricted in movement thru the insulation, causing MORE sound-into-heat losses. If the front panel is allowed to touch the insulation, it dampens the vibrations and lowers absorption; it also widens the "Q" of the trap, making it more broadband.

Perforated panel traps is just another name for a perforated Helmholtz absorber - all these should have the insulation up against the surface where the holes or slats are located; this minimises ringing of the slats or front panel, broadens the absorption frequency, and acts as part of the "acoustic resistance" by increasing the inertia of the air within the holes or slots - the Helmholtz formula in the Acoustics forum here (NOT on the REST of the net, INCLUDING the original SAE site) takes this into account.

Resonant traps (panel, Helmholtz) need to be located at points of high sound PRESSURE to work well, but Absorbent traps (rigid fiberglass, rockwool, whether cloth covered or not) need to be at places of higher sound VELOCITY to work best.

When in doubt, using broadband absorption is nearly ALWAYS the first thing to do when treating a room - then, if there is still uneven response, and you've done everything possible to locate things away from peaks/nulls, it may be time for tuned traps (generic name for panel traps, Helmholtz, slat/slot absorbers...

HTH... Steve

invisibl · Post by **invisibl** » Thu Jun 30, 2005 6:20 pm

Cheers for the reply Steve.I know that your time must be very scarce. So your answer is most appreciated by me ( and no doubt by anyone who reads this that is in a similar predicament.

Another coupla questions for you....

Based on my room dimensions ( which are for now and for ever based on the length of 4.4 metres, again cheers!) I drew up a series of to scale boxes eg 4.4 x 2.71 then made up a same sized box with division lines representing halves quarters thirds fifths etc and overlayed them. I did this for length width and height. Basically creating what I imagineis a map of the standing waves and their positions in my space.

Essentially this was a variation of the info in the post "are my speakers in a null"

I was thinking of using this as a starting point to find optimum positioning for both myself and my monitors.

A is this a mathematically correct thing to put my faith in. and

B are the above fractional divisions sufficient??

Coz if so then this seems to be a real nice way to find a starting position using fractions of the room size rather than frequency based divisions.

And finally ( for tonight) Is there any sort of way i can exploit the wardrobe at the rear of the room to my advantage.

Again cheers

Jeremy

knightfly · Post by **knightfly** » Fri Jul 01, 2005 12:36 am

First part; divide your room into 2,4,6, and 8ths in all three axes; don't put anything there. This will cover both peaks and nulls of the first 4 harmonics for each axis, axial modes ONLY. (at each of those divisions, there will be either a peak or a null or both.) The smaller the division, the less you'll need to move the speaker (or your head) to effect a change. (Note that 3/8 = 37.5%, very close to the "38% rule" - hence my warnings about being careful on this one)

Second part - Sure; just leave the stuff in it, leave the doors open, and voila - nice, deep broadband trap your wife can't object to (other than the doors being open, but that's only necessary when you're using the room)

(Just make sure nothing in there rattles; you can use any sort of sweep tone thru your speakers at mix levels to check this) Steve

invisibl · Post by **invisibl** » Tue Jul 05, 2005 5:57 pm

Biggest thanks Steve.

I have done as you instructed/suggested.

Now I have only one issue ( I believe)

Where/How I sit means my ears are at 125cm. Which corresponds to a Mode of 259Hz on the vertical axis.

How much higher do i need to raise the tweeters/myself to avoid this??

Just above this point? Halfway tween this and the next modal point??

BTW amidst all of this...I have discovered my mixes beginning to translate a little better.

Again. Cheers.

knightfly · Post by **knightfly** » Wed Jul 06, 2005 4:25 am

Your original post mentioned 9 foot ceiling; if so, your ear height is approx. 49 inches, which is exactly halfway BETWEEN null points by my calculations; doesn't seem to be a problem??!?

BTW, it's the WOOFERS that need to be kept out of modal points (either peak or null) for flatter response - usually 6-8 inches (15-20 cm) off center, measured from the center of the woofer cone.

Tweets are so far above modal considerations that they may as well not even exist for this purpose. They should, however, be aimed directly at (or slightly behind) your head.

Keep in mind that these are only AXIAL calculations, and that Tangential modes can be nearly as troublesome; so these are only guidelines, which will almost ALWAYS need some "real world" tweaking for best results... Steve

invisibl · Post by **invisibl** » Tue Jul 19, 2005 5:06 pm

Steve..

Bro first up. I have been uber busy the last few weeks and as I have no internet at home I havent had the chance to thank you for your help in helping me understand the benefits and the reality of the benefits in treating my room.

So Thanks.

HUUUUUUUUUUGE thanks.

I havent finished just yet but when I do I will post a wee chronicle of what i did right and wrong so That others may laugh and maybe even benefit from your help.

Again Thanks

jeremy

knightfly · Post by **knightfly** » Tue Jul 19, 2005 6:10 pm

No prob, Jeremy; been a bit busy lately myself -

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=3967

Glad you're getting there, keep us posted... Steve