Quantcast
Nulls and peaks - Gearspace.com
The No.1 Website for Pro Audio
Nulls and peaks
Old 14th December 2009
  #1
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
Nulls and peaks

We put absorbers on wall and in corners to reduce the peaks caused
by modes because those are places where those peaks are present
(and they're conveniently out of the way). But what about the nulls
caused by the modes ? Am I right in thinking that reducing the peaks
will have no influence on the nulls ?

For example, for the null in the middle of the room at the mode frequency,
no amount of absorption anywhere will have any effect ?

So how do you get rid of nulls ?

Paul P
Old 14th December 2009
  #2
Lives for gear
 
🎧 10 years
Absorption (assuming its of the proper thickness/materials/placement/etc.) will help with both the peaks and the nulls.

Both problems are caused by sound reflecting and combining both in (peaks) and out (nulls) of phase.
Old 14th December 2009 | Show parent
  #3
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
Quote:
Originally Posted by Smithcok ➡️
Absorption (assuming its of the proper thickness/materials/placement/etc.) will help with both the peaks and the nulls.

Both problems are caused by sound reflecting and combining both in (peaks) and out (nulls) of phase.
I agree for most frequencies, but for the mode and its harmonics I don't
think phase enters the picture. I could have been clearer by asking how
does one get rid of nulls at modal frequencies and I believe the answer is
you can't.

Paul P
Old 14th December 2009 | Show parent
  #4
Lives for gear
 
Nordenstam's Avatar
 
🎧 15 years
Hi Paul!

There's no room null in free space. If the absorbers did the job, there would be no null in the room either. Less than ideal absorbers gives more of the peaks and nulls.

As Smithcok wrote, both the nulls and peaks are caused by superposition of waves.

+ and + is double plus, + and - is nought.

With standing waves, it's + and +, a pair of slightly dampened + and + from the next reflection, and so on with repeats to the decay limit. That's why it's so loud, there's so many plusses in one place! (replace with + and - for the null point counterpart)

Have a feeling you knew this already...
Old 14th December 2009 | Show parent
  #5
Lives for gear
 
🎧 10 years
The peaks cause the nulls. Well...the walls cause the nulls. Yin and yang. Peaks and nulls can't exist without one another. Perfect absorption removes the walls, removing the nulls (no reflection interference).
Old 14th December 2009 | Show parent
  #6
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
Thanks for the replies, I guess I'd forgotten all about the "-" .

What's confusing me are the spl vs room dimension graphs in the MHofA.
There's no negative in the graph, just two peaks at the walls on either side
of zero in the middle. It this just because only half the picture is shown ?
Shouldn't there be a mirror negative spl under the bottom of the graph ?

I think I need someone to explain the graph to me

Paul P
Old 14th December 2009 | Show parent
  #7
Lives for gear
 
Nordenstam's Avatar
 
🎧 15 years
I'm at fifth edition of the MHoA. If you have the same, which page do you refer to?
Old 15th December 2009 | Show parent
  #8
Lives for gear
 
ciro's Avatar
 
🎧 10 years
Paul, see what happens in my small (treated) room;it´s the #4 post of thread:
https://gearspace.com/board/bass-traps-acoustic-panels-foam-etc/376986-how-much-buildup-wall-corners-2x-3x.html#post4039999

First (left) pic, a comparison between my mix position (black, 40% front wall) and the center of lenght (50%, green).As you can see, there´s not a "huge" dip in the first harmonic lenght mode (50hz) even in the center of room.
And room sounds basically "flat" in any position you are due to the treatment.

Ciro
Old 15th December 2009 | Show parent
  #9
Gear Head
 
FastlanePro's Avatar
 
🎧 10 years
This always seems to get a little confusing and I hope I'm not making it worse.

Room Modes are dependant on dimensions and wavelengths, not surfaces/absorption. Technically, the modes exist regardless of how apparent they are. In the context of modes, the absorption we put on the wall is a fairly crude attempt to stop *all* of the reflection in the room - which will definitely "flatten" the response. The term "broadband absorber" is accurate, in that when we passively absorb LF, we basically absorb everything above the target frequency if we aren't careful.

The trick is - unless you are going for an (semi-)anechoic chamber - there has to be a balance of absorptive & reflective surfaces or the room is typically going to sound like poop. Anechoia isn't a state the human ear tends to find pleasing, or natural. So - if we have too much absorption (i.e. to remove all reflection), we also muck with our RT60 times. Basically - throwing out the baby with the bathwater.

Sometimes, the "all-absorption" solution is better than the options, but it still is a compromise in the sound of a room. If the room is a cube or a closet - it's likely better to absorb everything and deal with the consequences. If the room has relatively appropriate dimensions, there are usually better ways.

Just know that there's a difference between using absorption to tame early reflections/ITD gap issues, and to deal with mode or modal distribution problems. Truly dealing with modal frequencies often requires changing dimensions and/or "active" absorption principles.

Hope that helps.
Lane
Old 15th December 2009 | Show parent
  #10
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
Quote:
Originally Posted by Lupo ➡️
I'm at fifth edition of the MHoA. If you have the same, which page do you refer to?
Pages 228 and 427.
Old 15th December 2009 | Show parent
  #11
Lives for gear
 
🎧 10 years
Quote:
Originally Posted by PaulP ➡️
I agree for most frequencies, but for the mode and its harmonics I don't
think phase enters the picture. I could have been clearer by asking how
does one get rid of nulls at modal frequencies and I believe the answer is
you can't.

Paul P
(I guess everyone else has covered this, but) in fact you can.

I guess its a little weird to wrap around the concept that by absorbing things, you can make nulls go away (or at least become less dramatic). The pressure waves are there, they are just being defeated through interaction with other waves - an epic battle!
Old 15th December 2009 | Show parent
  #12
Lives for gear
 
Nordenstam's Avatar
 
🎧 15 years
Quote:
Originally Posted by PaulP ➡️
What's confusing me are the spl vs room dimension graphs in the MHofA.
There's no negative in the graph, just two peaks at the walls on either side
of zero in the middle. It this just because only half the picture is shown ?
Shouldn't there be a mirror negative spl under the bottom of the graph ?
There is no such thing as a negative SPL. It can only go from zero and up (if zero is no pressure change at all). The condensations and rarefactions in the air are the driving force behind the fact that there's high SPL at some places and lower SPL's at others. Both the negative and positive swings counts as pressure.

PS: the picture at page 427 shows a line across the room, it's 1D. The Y axis is for level - not physical position. Not that you insinuated it, but I remember having an aha moment myself when discovering that quite obvious fact. :D


Quote:
Originally Posted by FastlanePro ➡️
Room Modes are dependant on dimensions and wavelengths, not surfaces/absorption.
Modes are dependent on all those aspects. No room, no modes. Totally absorptive surfaces, no modes.
Old 15th December 2009 | Show parent
  #13
Lives for gear
 
🎧 10 years
Quote:
There is no such thing as a negative SPL. It can only go from zero and up (if zero is no pressure change at all). The condensations and rarefactions in the air are the driving force behind the fact that there's high SPL at some places and lower SPL's at others. Both the negative and positive swings counts as pressure.
Of course, there is negative SPL relative to the nominal air pressure in the room. The negative pressure relative to the nominal air pressure is naturally still positive relative to a vacuum. As Nelson Pass put it, "air is Class A."
Old 15th December 2009 | Show parent
  #14
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
Quote:
Originally Posted by Lupo ➡️
There is no such thing as a negative SPL. It can only go from zero and up (if zero is no pressure change at all). The condensations and rarefactions in the air are the driving force behind the fact that there's high SPL at some places and lower SPL's at others. Both the negative and positive swings counts as pressure.
I think I may have finally got it. At the fundamental mode frequency if you
could see the pressure at the wall it would cycle through from negative to
positive. The center of the room would remain at zero (variation from
normal air pressure). And the opposite wall would be at opposite pressure
to the first wall. (You're right, I knew this, I just have trouble hanging on
to it).

So SPL is a continuous variation from plus to minus and back.
Ergo, the graph is not a wave.

Trying to grasp this I stumbled across another simulation applet, the
Box Modes Simulation (expand the window for better presentation).
For the (1,0,0) mode it clearly shows the pressure cycling from positive
to negative while the center stays black (at zero). I haven't fully explored
this applet but you can select different modes and see resulting pressure
distribution. You can combine modes as well. Could be useful to understand
absorber placement.

Paul P
Old 15th December 2009 | Show parent
  #15
Gear Nut
 
🎧 10 years
sorry to hijack, guys but: should I get the Master Handbook of Acoustics or the other book (whose title I forgot) that everyone seems to recommend? MUCH APPRECIATED!
Old 15th December 2009 | Show parent
  #16
Lives for gear
 
PaulP's Avatar
 
🎧 10 years
I just discovered something in the Box Modes Simulator. If you select the
"mouse = Adjust Angle" you can see the box (room) in 3-D and rotate it into
whatever position you want (by pressing and holding the mouse button while
sliding the mouse over the box).

Oh, and set the image resolution as high as possible, the image will be much nicer.

Paul P
Old 15th December 2009 | Show parent
  #17
Lives for gear
 
Nordenstam's Avatar
 
🎧 15 years
Quote:
Originally Posted by Brainchild ➡️
Of course, there is negative SPL relative to the nominal air pressure in the room. The negative pressure relative to the nominal air pressure is naturally still positive relative to a vacuum. As Nelson Pass put it, "air is Class A."
We're both right. There's instanteous SPL and there's effective SPL. The instantaneous value does have a negative indeed. Normally, SPL is used to imply the effective pressure and that's a RMS calculation. By the time the squaring part of the equation is done, negative numbers have turned positive.


Quote:
Originally Posted by PaulP ➡️
(You're right, I knew this, I just have trouble hanging on
to it).
Glad you saw it! We're all getting mind blocks at times.

Quote:
Originally Posted by PaulP ➡️
Ergo, the graph is not a wave.
Bingo! Remember that I took me some while to understand such pictures exactly because I tried to see them as waveforms.

Quote:
Originally Posted by PaulP ➡️
Trying to grasp this I stumbled across another simulation applet
The Falstad site is immense. There's an applet for just about anything physics and engineering related.


Cheers,

Andreas
📝 Reply

Similar Threads

Thread / Thread Starter Replies / Views Last Post
replies: 202 views: 63709
Avatar for Old Goat
Old Goat 27th August 2017
replies: 427 views: 14820
Avatar for dluther
dluther 14th January 2018
Post Reply

Welcome to the Gearspace Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…

Forum Jump
Forum Jump