(many)room acoustic questions

[Bevan]

i got myself a radioshack spl meter last week and have been getting some surprising results, which leave me with some questions i'm hoping someone can answer.

i have a some dynaudo 52s firing down the lenth of a 5.5m(18ft) room,(4.4m/14.6' wide). they are 112cm from the front wall, and 130cm from the side walls.

walking back and forth in the room with the spl meter and playing a 31hz sweep tone at 70db, i have a 12db dip(58db) just about at the half way point in the room(15cm behind the half way point actually). 60cm front and back of this it reads flat at 70db. another 60 or so centimeters back, which is 1.3m from the back wall behind me, i have a peak of 76db.

but when i play a 59hz tone which is also a major bass node, that near-middle point in the room that had a 12db dip at 31hz is now 9 db above flat at 79db!

firstly, is a 18db range between peaks and troughs excessive??????

isnt the half way point or any even division of the room meant to maximise bass??? why the 31hs dip then?

another surprising find was that there is very little fluctuation of the needle when moving laterally across the room. i would have though the width nodes would have been as severe as the lenth nodes

also strange was that another big node at 156hz seemed to occur at different points in the room to the 31hz node. i would have thought that base nodes of any frequency would have occured at the same point.

now i'm thinking of trying some new speaker placements. rives audio room simulator diagram suggests 120 from front wall and 105 fron side wall. but elsewhere in their site they say all formlas work on putting the speakers 1/3 or 1/5 of the way into the room. but 120cm is not either of these two ratios? and another website says "A speakers' distance from the front wall should not be within 33% of he distance from the side walls." 120 and 105 seem a bit close?

putting speakers and listning possition at odd divisions of the room is supposed to minimise bass nodes i've read. does this mean that the frequency response curve is flattened(good), or is bass reduced across the board(bad?), because i've read somewhere that one can chose to sit at even divisions to maximise bass(surely not bass nodes)

last question, what is the difference, and which is preferable, between moving speakers back and forth, and moving listning position.

sorry for being so long winded. if anyone can understand what i'm trying to say and has any answers i'd be most gratefull.

cheers

bevan

 

[jaxvon]

While I can't answer your questions, I can explain why nodes and peaks occur at different points. Depending on the frequency of the sound, the wavelength of the sound wave varies. The lower the frequency, the longer the wavelength.

Also, when doing measurements, it's best to put your SPL meter on a tripod so it isn't affected by your movement and presence near the microphone. This will help you achieve more accurate readings.

 

[Bevan]

thanks.

i had known that the lower frequencies had longer physical lenths, i had just assumed that because of the lenth of my room, i would have a primary node at 31hz, then again at 62, 124 etc. and if the width node was at say 40hz, i would have them at lesser degrees of severity at 80, 120 etc. and because 120hz has nodes from both lenth and width it would be twice as bad. but i had thought that all these nodes would have occured at the same phsical locations within the room, which doesnt seem to be the case.


...just moved my speakers to 1.98m from the front and 1.22 from the side, as per a formula i found(caradas possibly?)

now i have a massive node of 14db above flat at 90db!? and a massive dip of 10 bellow at 104hz. now i have 24db between the highest peak and lowerst valley, even worse than before!

and now the bass also rolls of at least 10hz above where it did when i had the speakers only 112cm from the front wall but the imaging is phenominal

guess i need a sub with the speakers this far out. thinking of the new velodyne with room correction. hopefully it will be able to iron out these mountains in my frequency respose curve.

b

 

[bpape]

Room modes vary in position depending on which harmonic of the fundamental they are. The primary mode will have a null at the half way point. The 2nd harmonic of that length will have a peak there.

What it measures anywhere in the room but at your seating position doesn't matter.

If you want to try to optimize things here is a basic methodology.

- Put the seating in a good place in the room first. No matter where the speakers are, this will help. 38% from the back wall is one rule of thumb. Others are to put the seat at some position in the length that is divisible by 3, 5, or 7. When getting into the higher numbers, you're sitting in modes that are less intense (2nd is less than primary, 3rd is less than 2nd, etc.)

- Start with the speakers the same distance apart as the distance from your head (seated) to the speaker baffle in a straight line. Tweak as desired for your imaging preference, smoothing of bass response (maybe deliberately causing a hump where you're seated in a dip), etc.

- Toe the speakers in so they point at a position just slightly behind your seated head position.

- Sub - get it out of the corner if at all possible. Use the same criteria as you did for the seating. Maybe 1/7th from the front wall and 1/7th from the side wall as a starting point. Move to tweak response curve as desired. When you do this, don't forget that there are 3 axis in a room. Try getting the sub up off the floor too. Some people use what's called the 'crawl around' method. Put your sub where your head would normally be. Then crawl around and find the position where the bass response is the smoothest - not necessarily the loudest. Then put the sub in that position.

 

[Bevan]

thanks for that bpape.

one question, does sitting at 1/3 or 1/5 or 1/7 of the room lenth put one at a point where all nodes(peaks and dips) are attenuated, or does it just put one on a dip?(i've read somewhere thats dips can be more severe than peaks....but maybe as they seem to say in hifi, sins of omission are worse than sins of addition?)

b

 

[bpape]

You're better to sit in a peak than a null. A peak can be tamed with EQ. A null can't. When you treat the room, you smooth the peaks and curves.

The best place to sit in terms of being as far as possible from any severe peaks and nulls is 38% from the front or rear wall and 38% from the side walls and 38% from the floor. In practice, that's not always feasible.

There is no place in a room where you are totally out of all peaks and nulls. All you can try to do is minimize their impact on the seating position as the best starting point before tweaking speaker location, treating the room, or EQing.

 

[Bevan]

cheers for that.

i'm thinking of getting the dynaudio sub 250 rather than the volodyne with eq(portability and cost reason, not performance)

leaves one suitable option: room treatment

going to look into building a helmholtz resonator to attack the 31 and 156hz peaks

b

 

[bpape]

I'd start with broadband first so you not only smooth things out, but you also begin to control the decay times in the bottom end appropriately. THEN if you still have very specific issues that you haven't resolved AND you still need more decay time shortening, then you should look at targeted absorbtion like a Helmholz.

If you do the broadband and have the decay time where it should be and still have the issues, then consider a parametric EQ instead of a Helmholz solution.

 

[Bevan]

nice to be talking with someone so knowlegable.

what parametric eq's are there on the market? i know of rives parc but that is above my means. there are the new high end av receivers like the new denon that have eq i belive, but i'd like to keep my multichannel analogue pre-amp.

how effective is using two subs to fight nodes?

and lastly, what level of success can one hope to achieve with room treatment or eq, i.e should one be happy with 3?, 6?, 10? db peaks and nulls.

thanks again

b

 

[bpape]

If you can get your seating position to be smooth within +/- 4-5db you're doing VERY well.

A parametric EQ on the sub channel only is many times sufficient. Something like the Behringer BFD is a perfect low cost solution. I wouldn't think about putting in on my mains but for the sub it is fine. If you want somthing nicer, there are other options for more money. Personally, if your're really serious about getting the room as good as you can, just plan up front to both treat the room and add a parametric EQ on the sub. If you can or will do only one, treat the room.

2 subs (IF you're willing to put the subs where they need to be) can help smooth things out considerably. It's also 2-3x the work to get right. If you're willing to do it, I'd certainly consider it. If you're going to restrict the location to just the front of the room, just the corners, just right against a wall, don't bother. You'll likely end up with just as many issues but still more enveloping bass.

 

[Bevan]

it'll be treatment then, and when funds allow, a second sub that possibly has eq in it. that should do the trick

b

 

[bpape]

EQ in the sub is great - for one band. Many times you need more than one and need more flexibility over the Q of the band. Seriously, for $100, the BFD is almost a no brainer.

I personally run a Symmetrix 551E just on my sub channel. I run all 5 bands between 15 and 60Hz at different center points and Q's. Works like a champ.

 

[Bevan]

reading up on the BFD i'm surprised i havent come upon it before. seems just the trick. very glad you mentioned it. seems like it will provide/neccesitate many hours of tweaking, which is fine with me!

cheers

b

 

[Sonnie Parker]

Hmmm... I thought I heard somebody say BFD. I'd say it works wonders.

 

[Bevan]

impressive!

sonnie, what program you use to make those graphs?

b.

 

[Sonnie Parker]

This is from the Microsoft Excel Workbook available for download on the GUIDE.

 

[Bevan]

Hi Sonnie, or anyone else,

was wondering, with a 12db dip like you had at 40hz, is it safe to boost that by 12db with a sub having only a 230W amp?

if one were to lower a 12db peak and raise a 12db dip to flat, would the load the amp sees be the same in net term? (though i'm guessing raising a dip at 40hz is not the same power wise as raising or lowering a kink at 80hz?)

cheers

b.

 

[bpape]

Not really. That's how you clip an amp and fry a sub.

You're better off to use room treatments and careful placement to minimize hugh nulls. For what is left, bring the overall volume up a little and use the EQ to bring the peaks down.

Be realistic in your expectations. If you can get overall response to within +/-4db you're doing VERY well.

 

[Sonnie Parker]

bpape is right... although room treatments are typically going to absorb more frequencies vs. boosting any... they might help eliminate some standing waves which might be causing a room null. Unfortunately you won't find many room treatments effective below ~100-125hz. A 40hz room treatment would usually be extremely large and it would most likely be something you would build (quite complicated to say the least).

You can try boosting anything you want but listen carefully at your amp and sub and make sure you do not hear signs of clipping, which can go undetected very easily. Sometimes it happens before you can do anything about it. My amp just happens to have plenty of headroom (3000watts) which enables me to add that 13db boost, and it sounds perfectly fine and has been working for me for about a year and a half now. Generally speaking... the lower the frequency the more headroom an amp will need for boosting, but I always encourage people to use caution and as bpape suggested +/- 4db is reasonable to accept. Sometimes you have to accept what you can get.

Keep in mind too that there are 12 notes in an octave, so in my case there are 24 notes between the 2 octaves of 20hz to 40hz and 40hz to 80hz. If I divide those octaves up into 1/6 octaves and take measurements and see a narrow null at say 40hz, chances are that I may not even notice that null, especially if I get it to within +/- 4db. Many people only measure 1/3 octaves because they don't feel like they can notice any fluctuations between, which would include every other 1/6 octave measurement. Then there are some who go to the extreme and measure 1/12 octave and apply correction. The bottom line is listen to what your ears are telling you on your favorite demo material.

 

[Bevan]

hi.

i got a question for anyone familiar with the bfd or similar typr eqalizers.

if i have the spb crossed over at 80hz, how far above that point will the bfd be effective in taming peaks?

i have a peak of 8db at 120hz. i know the sub is outputting something at that frequency, so will it be fixable.

i'm also guessing that if i fix my peaks at 80-90hz it might automaticly change the 120hz peak?

thanks

b.