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Question about dip at 1KHz
Old 28th April 2020
  #1
Question about dip at 1KHz

Hi everyone!

I just built a new studio setup and installed all the acoustic panels I had as leftovers from my other studio and I measured the room response yesterday with IK Multimedias ARC2 (just a habit to use this software instead of RoomEQ Wizard). See attached chart link:

https://ibb.co/cx6f7z2 (imgbb image upload)

I can see that I need to control the bass some more (I know this since no bass traps have been installed yet) but what I wonder is above the Schroeder freq where there's a dip at 1KHz and then at 3KHz?

I have studied some acoustics myself and learned how to tame peaks but for some reason newer learned how to raise dips?

Any ideas why this might happen? I've described my room below.

Any ideas would be wonderful!! :D

Cheers
Toni

ROOM DESCRIPTION

The room dimensions are: 3,7 (D) X 3,5 (W) 2,5 (H) meters.

I use 100 X 65 X 5 cm custom made panels with wooden frames filled with hard/heavy Rockwool with similar density as mid-absorbers and they are spaced different distances from the walls (some up till 10 cm from the walls).

Also I have the same panels in the ceiling suspended some 7 cm.

My back wall is filled with 6 of these panels: https://www.mega-acoustic.pl/sklep/d...pm-8klu-60x60/

My desk is the Zaor Onda Angled - rather small as to not have too many direct reflections from the monitors.

The monitors/desk are not in the middle of the room but a bit asymmetrical to the side (to fit a vocal booth corner) and some 20-30 cm from the walls and with mid-absorbers between the back of the monitors and the walls.
Old 28th April 2020
  #2
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Jens Eklund's Avatar
 

Most likely desk related.

Make sure you measure one speaker at the time if mic is not exactly (millimetres count) the same distance from both speakers (tweeters). If not; dips in upper midrange/highs can be a result of this.

Cover your desk with thick blankets, folded towels or similar and check if the dips disappear.
Old 28th April 2020
  #3
Thanks for the info Jens (formoder du er dansk)

As for measurement the ARC software measures 14 different positions around the listening position and with each speaker separately. So this should be right.

I'll try to cover the desk tomorrow and see. But can you explain mathematically why the desk should create a dip? Doesn't the desk reflect frequencies that are this high? It should only absorb low end frequencies... I'm confused.

Thanks!
Old 28th April 2020
  #4
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Jens Eklund's Avatar
 

Jag är svensk

It is often said that an object needs to be about ½ wavelength in size to reflect. This is obviously not a hard limit, but a gradually increasing thing. At 1 kHz, a surface larger than about 0,17 meters (1/2 wavelength) will start to noticeably reflect. An 0,34 m surface (1 lambda) will definitely reflect.

If the path length difference between direct sound and reflection is ½ lambda (or n + ½ lambda multiple); destructive interference will occur and you´ll experience a dip.

If reflection and direct sound are in phase (one or more whole wavelengths difference in path length); there will be constructive interference and you’ll get a boost (close to 6 dB assuming perfect reflection and minimal difference in travel length between direct and reflection (often the case if desk reflection).
Old 29th April 2020
  #5
Hej Jens! Cool. Det är faktisk i Stockholm jag har byggt mit extra studie. Sickla området Tack för dina svar och information!!

Perfect! Now it makes sense. The wavelength for 1KHz is approx the 34 cm you mention. I will try covering the desk later but need to deliver a project first (deadline today).

So what do people do to avoid these dips? I mean I can't remove the desk and I won't buy a new one since this is made for the studio. Any ideas or thoughts?

----------------------------

Also I've checked the room response at another place in the studio by the acoustic piano I have standing by the wall:

https://ibb.co/g6p4Ppt

There was also a dip in the high freq. as well - more around 1,5KHz. Do you think this is related to waves coming directly from the speakers (behind me) and then creating destructive interference with the reflections from the wall/piano itself (in front of me)? The distance from the wall to the chair is around 60-70 cm which equals 3 X wavelength. Correct? I do have an acoustic panel on the wall there. Would it help to add another panel that is suited to dampen 1-2KHz?
Old 30th April 2020
  #6
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Jens Eklund's Avatar
 

Quote:
Originally Posted by ToniMartin View Post
Hej Jens! Cool. Det är faktisk i Stockholm jag har byggt mit extra studie. Sickla området Tack för dina svar och information!!

Perfect! Now it makes sense. The wavelength for 1KHz is approx the 34 cm you mention. I will try covering the desk later but need to deliver a project first (deadline today).

So what do people do to avoid these dips? I mean I can't remove the desk and I won't buy a new one since this is made for the studio. Any ideas or thoughts?

----------------------------

Also I've checked the room response at another place in the studio by the acoustic piano I have standing by the wall:

https://ibb.co/g6p4Ppt

There was also a dip in the high freq. as well - more around 1,5KHz. Do you think this is related to waves coming directly from the speakers (behind me) and then creating destructive interference with the reflections from the wall/piano itself (in front of me)? The distance from the wall to the chair is around 60-70 cm which equals 3 X wavelength. Correct? I do have an acoustic panel on the wall there. Would it help to add another panel that is suited to dampen 1-2KHz?

This is unfortunately a very (VERY …) common issue in most studios, including the expensive ones.

The solution consists of a combination of two things:


1. An acoustically “small” desk (as seen by the speakers):

Question about dip at 1KHz-holonor-desk-1.jpg
Question about dip at 1KHz-holonor-desk-2.jpg

This normally means shallow and/or perforated horizontal surfaces, angled rack sections (enough angle to actually do something useful) and keeping the screens down low (and/or smaller screens).


2. Using speakers that offer controlled directivity in the vertical plane:

Cardioid speakers


In general, the total response is the vector sum of all phasors. If one or more of the vectors apart from direct sound is relatively strong, and out of phase (as discussed above) compared to the direct sound; you will see a dip at the frequency of interest.

I made a simple excel sheet to quickly calculate nulls due to floor (same for desk) related SBIR. I´ll add it here if anyone wants to check their floor/desk related issues and see if it adds up.
Attached Thumbnails
Question about dip at 1KHz-holonor-desk-1.jpg   Question about dip at 1KHz-holonor-desk-2.jpg  
Attached Files
Old 7th June 2020
  #7
Dear Jens!

This was the most thorough and well explained answer ever! Thanks

I will try different things and move around and use your beautiful excel sheet.

Cheers
Toni
Old 7th June 2020
  #8
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Jens Eklund's Avatar
 

Quote:
Originally Posted by ToniMartin View Post
Dear Jens!

This was the most thorough and well explained answer ever! Thanks

I will try different things and move around and use your beautiful excel sheet.

Cheers
Toni
You’re welcome
Old 18th September 2020
  #9
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Quote:
Originally Posted by Jens Eklund View Post
This normally means shallow and/or perforated horizontal surfaces, angled rack sections (enough angle to actually do something useful) and keeping the screens down low (and/or smaller screens).
How much is "enough angle"?

If it's flat down, it will cause reflections from the ceiling and if it's straight up it will block the sound from the speakers. So the best would be angle directly towards the speakers when the speakers and desk are in the final position?

I'm asking as i'm planning new space that will need a new desk.

Desk is a compromise of one or all of these:
1. Ergonomics
2. Acoustics/sound
3. Space

Many so called "studio desks" compromise on ergonomics and acoustics, but have plenty of space.

For me the ergonomics is the most important as bad ergonomics can do permanent physical damage in a long run. Then comes sound/acoustics and then space.

My space, like many modern control rooms have no need for easy access 19" racks. I think 18" depth for the desk would be enough for me. Display would be on a stand where it can be freely to moved on any direction and angle. That allows good ergonomics for most of the work and can be moved away when mixing.

Does it make difference is the desk deep and narrow or shallow and wide if m2 is the same?
Old 18th September 2020
  #10
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Jens Eklund's Avatar
 

I think most questions will be answered by checking out these threads:

Desk Replacement. Design ideas?

Cardioid speakers
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