How does one clalculate room nulls?

klh · « **on:** 5 Apr 2005, 11:59 pm »

I've used data sheets to calculate the resonate frequencies for my room (21'6" x 16'3" x 9'). The only spot where two walls double up are at multiples of 105 Hz; there are no frequencies where they triple up. So... of concern for peaks are really only 105 Hz, 210 Hz and possibly 315 Hz.

Now that I know the peaks, how do I calculate and anticipate the nulls? Besides broadband absorption, is there a way to deal with them? The only null I know how to calculate is the one having to do with distance from the rear wall (If memory serves me correct, an example is the dip at 100Hz if seated 38" from the rear wall).

I'm still going to put a ring of 705/equivalent around the room at the wall to ceiling corners as well as 702/equivalent at all first reflections so I have a RFZ. I also want to put 705/equivalent about 1' from the rear wall behind all listening positions on the couch to somewhat attenuate the null from the first reflection. Even though the seating will only be 31% into the room, the sound should be pretty good.

Please comment on calculating nulls or my design. Thanks in advance.

ted_b · « **Reply #1 on:** 6 Apr 2005, 12:07 am »

I have a great room mode calcualtor that shows seated peaks and nulls, etc. It's nice. PM me with your email address and I'll send it to you. I'd forward on a site address but don't remember where I got it. It's an Excel spredsheet.

Edit: nevermind, you have an email already sent.

Ted

BradJudy · « **Reply #2 on:** 6 Apr 2005, 01:08 am »

Googling 'room mode calculator' turns up several. I like to use this one: http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html

ted_b · « **Reply #3 on:** 6 Apr 2005, 01:10 am »

Quote from: BradJudy

Googling 'room mode calculator' turns up several. I like to use this one: http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html

It's a decent one, but doesn't ask for seated position; ie. doesn't calculate nulls at the listening position.

Ted

ted_b · « **Reply #4 on:** 6 Apr 2005, 01:12 am »

Found the one I like.

http://www.ultimateavmag.com/news/10388/. Includes some directions/notes. You'll find it on the link that says "click here to download the file".

Ted

Ethan Winer · « **Reply #5 on:** 6 Apr 2005, 04:19 pm »

klh,

> Now that I know the peaks, how do I calculate and anticipate the nulls? <

Great question, and the answer is simple too. All rooms suffer peaks and nulls at all frequencies, not just those that align with the room dimensions. At any location in the room you'll probably be able to find a frequency that peaks and another that nulls. However, modal peaks have a resonance that causes those musical notes to sustain, where peaks created by simple boundary interference do not resonate.

> Besides broadband absorption, is there a way to deal with them? <

Broadband absorption is best in most cases because it flattens the response at all room locations, and at all frequencies, regardless of whether the cause is modal or non-modal. Tuned absorption could be appropriate in an extreme case, like a room that's 8 feet cubed. But from what you described about your room, broadband makes the most sense.

--Ethan

klh · « **Reply #6 on:** 6 Apr 2005, 08:57 pm »

Thank you all

I knew that there were peaks and nulls as one moves about in any room, but wasn't sure how they came about or if there was a way to calculate and deal with the more prominant ones. I'm very lucky to have such good proportions in the room... in fact, it was the main reason why I wanted that house (of course my wife doesn't know that, nor would she understand!).

klh · « **Reply #7 on:** 7 Apr 2005, 12:03 am »

Thanks to all. Ted's seems to be the most informative. I have a question, though, what is Delta?