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massive 120hz dip. what to do? Dynamics Plugins
Old 4th August 2010
  #1
Gear Head
 

Thread Starter
massive 120hz dip. what to do?

Hi guys, I've read around a bit here but don't really understand a lot when it comes to acoustics so I can't take anything from it.

I borrowed an SPL mic today at the studio and took it into my suite to check out the response at the listening position. I fired out some pink noise and what returned was rather good and within 3db of 0 for the most part. The problem was that there was a -7db dip at 120hz.

How could I address this issue so that my low end is flatter and I can make better judgements.

My monitors are Genelec 1031a's.

Thanks in advance
Old 4th August 2010
  #2
Gear Guru
 
Glenn Kuras's Avatar
Can you please explain how you fired pink noise and got a reading? I have a feeling you did not really test the room right. I would recommend using the following program to test. Needless to say also it would be good to see a layout of the room if possible to give you some better advise.
Room EQ Wizard - REW Home Page
Old 4th August 2010
  #3
Lives for gear
 
frans's Avatar
Describe your room, make a sketch where everything is located, break out your measuring tape and then people like Glenn have a chance to help you. heh
Old 4th August 2010
  #4
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DanDan's Avatar
RTA

Must have used a Real Time Analyser, hardware or soft.
These are useful for very quick and nasty broad stroke fixing.
We use them all the time in PA. It can be difficult to get people to give up their concert seats in favour of bags of fibreglass :-)

If you decide to do proper measurement, brace yourself, it will look awful.
It is the only sure path to sonic heaven though.

Regarding your question. More info would be useful. Room dimensions can cause such a dip. So, room dimensions and materials which the surfaces are made of plse, e.g. Stud/Plasterboard, Brick/Concrete, Wood, etc.

Be sure that you have not engaged that 160Hz desk reflection filter in those speakers!

Try playing a sine wave around 120 Hz. Sweep slowly and carefully until you hear resonance, it will be obvious. Then move about the room with the SLM. Write down the peaks and dips. This should give great clues as to locations for traps, or better locations for speakers or listening. e.g. if you are sitting in a null, move!
DD
Old 4th August 2010
  #5
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Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by MusicTart View Post
How could I address this issue so that my low end is flatter and I can make better judgements.
I'm surprised everyone is telling you to measure. No matter what you measure you still need to address the problem, and that's done with bass traps. The more traps you can manage, the better. It's really that simple. Of course measuring is not a bad idea! But it's not actually needed to know how to treat the room. More here:

Acoustic Basics

--Ethan

________________
The Acoustic Treatment Experts
Old 4th August 2010
  #6
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Glenn Kuras's Avatar
Quote:
Originally Posted by Ethan Winer View Post
I'm surprised everyone is telling you to measure. s[/url]
I think measuring is a good thing if for no other reason you can see the improvements your hard earned money got yea!thumbsup The nice part these days is it is free with programs I listed above.
Old 4th August 2010
  #7
Gear Head
 

Thread Starter
Oh My,

by the replies I've received so far I'm scared to try anything else. This is going to be harder than I thought. Maybe I'll just look up some people in the profession around the Sydney area and get them to come and do it for me. I don't think I have the patience or the expertise for any of this trial and error.

I took a lot at Room EQ wizard a few weeks ago and it scared the crap out of me. What a complex program with calibration files and all those fields to ender DB levels etc etc. I just want to make music lol.

Frans, I placed the mini SPL microphone right about where my ears would be and aimed it at my computer screen right in between my 2 speakers. I sit with my head between the speakers so that each tweeter is aimed at my ears. This eliminates most of the room I find. I then recorded a looped bit of pink noise at 80db and viewed the result through various spectrum analyser plug-ins on the master bus in my DAW. They all returned that there was a steep dip at 120hz. The rest of the spectrum looked pretty good and I can say that it sounds quite good when I run commercial mixes through it. I have always noticed a bit of a hole where the punch from most mixes should come from and this has been explained by the 120hz dip I can see on the analyser.

I don't know the exact measurements of the room but I can measure it tomorrow at work. It's about 5 metres by 2.5 metres. The walls are all slightly irregular lengths. The speakers are positioned in the middle of one of the 5m long walls. The ceiling is sloped from the 5m wall behind me and comes down to the same height as the speakers on the wall in front of me. So the speakers are directly in front of the wall + ceiling corner. Weird room that was originally designed for video editing.

Thanks for the tip about the null davedave, I moved the mic back half a meter from my listening position just to see if it would change things and the results came back with the same stubborn dip at 120hz and by the same amount.
Old 4th August 2010
  #8
Gear Guru
 
Glenn Kuras's Avatar
Well I guess you could hire someone or just do what Ethan says. Load up the room with bass traps, treat the early reflection points and call it the day.
See the following for a layout you can use.
GIK Acoustics: Room Setup
Old 4th August 2010
  #9
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DanDan's Avatar
Not everyone

Musictart, the fact that the null at 120Hz didn't change in one direction tells you something. Try it again moving your head up and down. The simple modes run along the three axes L W H. If you are lucky your speakers may be in a vertical null. If you can change the height, by adjustable stands, or inverting the speakers so that the woofers get away from the null, you may be able to solve this. Positional changes are very powerful. The beauty of the simple Sine test over complex measurement is that it points to the solution as much as the problem.
DD
Old 4th August 2010
  #10
Gear Guru
 
Glenn Kuras's Avatar
Quote:
Originally Posted by DanDan View Post
Musictart, the fact that the null at 120Hz didn't change in one direction tells you something. Try it again moving your head up and down. The simple modes run along the three axes L W H. If you are lucky your speakers may be in a vertical null. If you can change the height, by adjustable stands, or inverting the speakers so that the woofers get away from the null, you may be able to solve this. Positional changes are very powerful. The beauty of the simple Sine test over complex measurement is that it points to the solution as much as the problem.
DD
thumbsupthumbsup
Old 4th August 2010
  #11
Lives for gear
 
avare's Avatar
 

Quote:
Originally Posted by DanDan View Post
Try it again moving your head up and down. The simple modes run along the three axes L W H. If you are lucky your speakers may be in a vertical null. If you can change the height, by adjustable stands, or inverting the speakers so that the woofers get away from the null, you may be able to solve this. Positional changes are very powerful. The beauty of the simple Sine test over complex measurement is that it points to the solution as much as the problem.
+1 , again DanDan.

Andre
Old 5th August 2010
  #12
Gear Head
 

Thread Starter
DanDan and others, thanks for all your help.

DanDan I tried the test again moving the speakers around drastically and the position of the microphone just to see what would happen. The drop at 120hz was actually 140hz and it's still there no matter what I do. What exactly do you mean by inverting the speakers? Swap them around? I haven't tried that yet.

It's looking like I will have to get someone to come and fix this. Saving my pennies now.

Best
Old 5th August 2010
  #13
Gear Guru
 
DanDan's Avatar
The Scientific Method

Tart, if I may call you that heh
confusion will reign until you get with the scientific method.
120 is now 140?
Moving everything around is well, just fishing.
Measurement by software is superb, but you need even tighter focus with all that sophisticated information.
Modes are usually the strongest influence in our small rooms.
Axial Modes are the strongest and run along a room axis. DOH. If you play a sinewave at exactly a modal frequency, that mode will get strongly triggered. If you walk the room, including height, you should be able to find a pattern of peaks and nulls running along that axis.
Life can get complicated when there are several modes combining, either the same frequency or close. They pull together.
This is why I say 'exactly'. You need a very fine sine sweep to be exact. You can also position the source or receiver at the nulls of modes you wish to ignore/ filter out.
Signalsuite and the facility in REW are excellent, but anything can be used if you are meticulous.
It is useful to calculate the modes first using a room mode calculator. This will forewarn you of close modes.
If you don't know what that is, tell me your room dimensions (exactly) and I may be able to shine a light on this for you.
DD

Last edited by DanDan; 23rd February 2012 at 06:41 PM.. Reason: EXTRA BITS
Old 19th February 2012
  #14
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Cuban Science's Avatar
 

Use REW to run test on your room.. REW is very handy, and once you understand the graphs, can tell you virtually all you need to know.. i can give you a formula to use that can give you a reasonably tight ball park figure, and i did it on my studio pre REW testing and the result s match near perfect to what i was expecting from measurements.
first you need to measure the length, width, and height of your room and also the temperature, this is critical.
next we need to do some physics. work out the speed of sound for your room, speed of sound varies with temperature. sound travels at 332.5 Mtrs/sec at 0 degrees C. use this calculation
V= 331.5 =0.6T {V=Velocity(speed) T=temp in C} so.
room temp at 20c will give you (0.6x20)+331.5=343.5m/s
using the next calculations we can work out the fundamental (loudest) frequency for your room. {F=frequency}
F=343.5/(length of room x2) = ??
i.e F=343.5/(4.56x2)= 37.6 0r 38Hz
do the same measurement for the 3 axis of your room then you can create a chart with the frequencies of your room, my room looks like this, L - 4.56m W - 4.56m H - 2.37m
this gives me 38Hz, 38Hz, 73Hz as my fundamental frequencies.
Mode Mode 1 Mode 2 Mode 3 Mode 4
Height 73Hz 146Hz 219Hz 292Hz
Length 38Hz 76Hz 114Hz 152Hz
Width 38Hz 76Hz 114Hz 152Hz
this gives me a rough idea of where i need to treat my room as this is where the standing waves are going to build up, I'll have especial trouble at 38HZ.
room modes are the harmonics of the fundamental. room modes are of less importance as they get higher, (multiples of the harmonics become less important)
its a bit in depth but that's acoustics for you.. hope it helps, if you can follow what I'm saying.
Old 19th February 2012
  #15
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
If you play a sinewave at exactly a modal frequency, only one mode will get triggered.
This is not always the case.

Assuming the source is located in a pressure zone for a given mode, it will excite all modes (more or less) that are close enough to the driving frequency and/or with a low Q so it covers the driving frequency. This is why you need to take into account adjacent modes when doing modal calculations:

http://downloads.bbc.co.uk/rd/pubs/reports/1992-09.pdf


EDIT: Re-worded.
Old 22nd February 2012
  #16
Gear Guru
 
DanDan's Avatar
I may have generalised a little, however 'All modes within' is overstated IMO.

Indeed Modes can be very close, commonly only a handful of Hertz wide.
However if a secondary or even more modes are stimulated, the simple pattern associated with the one under investigation will not be detectable.
i.e. PNP or PNPNP for Axials 1 and 2.

Should that occur we can nuance the Sine Method by using the fact that we know where particular modes live and where they can be detected. Their pattern of peaks and nulls.

The source can be placed in modal nulls to minimise stimulus of those modes thus filtering out the desired one(s).

e.g. Source at front wall, mid width and mid height, will not strongly stimulate W1 and H1, but will strongly stimulate all the axial Length modes.
Moving the Listener (Mic) position can further single out the mode of interest.
e.g. L1 will show very little energy at the centre of the room's length.
L2 at quarter and so on.

I call this type of enhanced Measuring, "Interrogating the room"

Recently I found it possible to identify two modes, both L1 using Sines alone. The second one was caused I believe by a layer of plasterboard/insulation on the concrete wall. i.e. Two boundaries. The frequencies were 34Hz and 37Hz.

It would be very easy to underestimate the power and finesse of the simple Sine Method.


DD
Old 23rd February 2012
  #17
Gear Guru
 
DanDan's Avatar
Noise

MusicTart, are you still about?
Another test or two and we could nail the cause of this.
e.g. Using one speaker, pink noise, RTA, move the speaker towards the front wall. Does the frequency of the null change?

As Jens has pointed to, modal density increases it becomes increasingly difficult to tell one mode from another.
I wouldn't recommend using Sines to identify modes above the third axial, and not at all for oblique or tangential. Not so sure the Waterfall is any better at it either.
Particularly when we see frequency shifts later in the decay. Which I assume is modes influencing each other.

However, irrespective of single or clusters of modes, I am advocating a treatment advisory role here.

So, lets say we play a Sine at 174.965Hz and hear, or with an SLM measure, an obvious resonance....
Then check the practically trappable locations for Hot spots.
The Hottest would be the best places to trap.

I reckon this would apply to the 120/140Hz irrespective of what is causing the dip, even if it is a Boundary Interference Null.

Have you tried playing a Sine Wave at 120Hz?



DD

Last edited by DanDan; 23rd February 2012 at 06:47 PM.. Reason: Rewrite
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