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Does open doors count as infinite traps?
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ohmtom
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#1
25th February 2012
Old 25th February 2012
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Does open doors count as infinite traps?

Hi!

Sorry if my english is not perfect, but I think it is readable...

My control room is quite small, it is 12.7´x 11.7 x 8.2´. (The tracking room is in the basement below).

I have built absobers at the front half of the room, see pictures below.
At the front wall I have 6" mineral wool with a 2" air gap to the wall.
To the sides I have 3" mineral wool with a 1" gap to the wall.
The front corners are filled with mineral wool, so the largest depth in the bass traps is 24".

So, the front wall is quite "dead" I guess, and have bass traps in the corners.
The front ceiling has a "cloud" consisting of a 2" rigid glass wool panel.

I thought that building a front end that "dead" shoud avoid me having problems with standing waves in the bass frequency region, but not so. Also, since almost half of the back wall is missing, (it is a passage into the living room) I thought no bass would be able to build up. But I was wrong.

Right at the sweet spot there is a cancellation at about 80 Hz. And at other spots there are cancellations from 120 Hz and down, in different places.

Would not the passage and the open door count as bass traps? Infinite bass traps?
With no door into the hallway, and a big hole into the living room, and two huge bass traps at the front, I am surprised that the room still has bass buildup and standing waves.

When I tried to lean a stack of absorbers (about 10" total) to whats left of the rear wall, it definitely got better. See picture below.

Should I build a deep absorber to the back wall? Maybe even buy those GIK soffit absorbers for the top, on the back wall and all the way to the middle of the side walls? Is there any point in buying diffusors? I do not have much wall space left for those? Maybe between the speakers?

I would like to swap those old Tannoy Reveals for Focal Solos, but it feels like throwing money away when the room still is so bad.

I would be very happy to get any suggestions what to do next. I have run out of ideas...

/Thomas

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28th February 2012
Old 28th February 2012
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I'll bump this for you, as the question in the title interests me as well.
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28th February 2012
Old 28th February 2012
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Did you do anything to the ceiling besides the cloud?

Is that center "box" a closet. If so you could turn it into a huge bass trap. Hanging clothes make good absorbers.

Then there is always the wall/ceiling intersection you could treat.

Great job by the way on what you have done.
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29th February 2012
Old 29th February 2012
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Good

Nice look to your traps and room. The open door is bound to let LF escape but will also allow a secondary later decay bloom. Maybe nice.

There is a lot more to life than modes.
RealTraps - Room Modes

Measurement will make everything you do more certain.

The ceiling treatment won't affect the height mode which has a null around listener and woofer height at around 70Hz. You could try moving your speakers up or down to get out of the null. Up higher but upside down springs to mind. Speakers can work fine upside down. http://mission.co.uk/
Back Wall treatment is bound to help.
Try moving your speakers as close as possible to the Front Wall.

I hope I don't sound like a broken record but I do think this is the future for DIY enhanced LF trapping.

My Experiment with a Metal Panel Absorber

DD
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29th February 2012
Old 29th February 2012
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In small rooms of this nature, some varying peaks and dips in the response at different listening positions, is common... nothing to stress out too much about.

If the cancellations you are getting are at specific places only, we are dealing with SBIR/LBIR issues - how the wall, speakers and listener are positioned relative to each other. You might be able to resolve some issues by moving your speakers and listening positions, to find the optimal spot. From your pictures (nice room, btw), it appears that you have the space to move stuff back and forth, and sideways, and up and down. A few inches can make a huge difference.

Try to optimize the response at the sweet spot (wherever you finally find it to be) and don't worry too much about peaks and dips elsewhere.

In response to your title - yes. the bass that leaves the room and doesn't come back is the cheapest type of absorption. but it will not solve all problems
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29th February 2012
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When I took off a big 3x5 desk that was serving only to support a screen , a (PC) keyboard and a mouse , it improved my response in 4dbs (my personal experience, of course). But I don´t know why to expect a so better response ("building a front end that "dead" shoud avoid me having problems with standing waves in the bass frequency region")if you have only 2" above your head, it does very little to anything below 200hz or so.Consider a much thicker cloud and it should improve something.

Ciro
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29th February 2012
Old 29th February 2012
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Thanks for your replies and nice words!

Quote:
Originally Posted by sandersd View Post
Did you do anything to the ceiling besides the cloud?

Is that center "box" a closet. If so you could turn it into a huge bass trap. Hanging clothes make good absorbers.

Then there is always the wall/ceiling intersection you could treat.

Great job by the way on what you have done.
No, it is just the cloud above the desk, angled to steer away early reflections. Maybe I should build a thicker cloud for the back half of the room? I guess the more damping the better in a small room like this?

Unfortunately, the center "box" has the stairs down to the basement. I have to keep it (or move the stairs to another place in the house, but that would be just too much... I rather move to another house with a better room )

Yes, I think I will try the wall ceiling intersection next.

Quote:
Originally Posted by DanDan
Nice look to your traps and room. The open door is bound to let LF escape but will also allow a secondary later decay bloom. Maybe nice.

There is a lot more to life than modes.
RealTraps - Room Modes

Measurement will make everything you do more certain.

The ceiling treatment won't affect the height mode which has a null around listener and woofer height at around 70Hz. You could try moving your speakers up or down to get out of the null. Up higher but upside down springs to mind. Speakers can work fine upside down. Mission
Back Wall treatment is bound to help.
Try moving your speakers as close as possible to the Front Wall.

I hope I don't sound like a broken record but I do think this is the future for DIY enhanced LF trapping.

http://www.gearslutz.com/board/studi...-absorber.html

DD
OK, it´s a relief to hear that the open doors is not just a big big no no. Thanks!

Yes, I will measure the room soon, just waiting for the microphone to arrive.

OK. I will experiment more with the speaker placement. I bought a couple of PMC TB2SA yesterday, and put them on Auralex on the desk, closer to me, lower, and further away from the front wall, but I still has some lack of bass in the sweet spot. If I move my listening position 4 feet closer to the back wall, the bass feels more "true". Well, again, measurement. Amazing speakers btw!

Maybe a VPR on the rear wall would solve much of the problems?

Quote:
Originally Posted by audiothings
In small rooms of this nature, some varying peaks and dips in the response at different listening positions, is common... nothing to stress out too much about.

If the cancellations you are getting are at specific places only, we are dealing with SBIR/LBIR issues - how the wall, speakers and listener are positioned relative to each other. You might be able to resolve some issues by moving your speakers and listening positions, to find the optimal spot. From your pictures (nice room, btw), it appears that you have the space to move stuff back and forth, and sideways, and up and down. A few inches can make a huge difference.

Try to optimize the response at the sweet spot (wherever you finally find it to be) and don't worry too much about peaks and dips elsewhere.

In response to your title - yes. the bass that leaves the room and doesn't come back is the cheapest type of absorption. but it will not solve all problems
OK, I´ll focus on finding the "sweetest" spot in the room. Thanks for the information!

Quote:
Originally Posted by ciro
When I took off a big 3x5 desk that was serving only to support a screen , a (PC) keyboard and a mouse , it improved my response in 4dbs (my personal experience, of course). But I don´t know why to expect a so better response ("building a front end that "dead" shoud avoid me having problems with standing waves in the bass frequency region")if you have only 2" above your head, it does very little to anything below 200hz or so.Consider a much thicker cloud and it should improve something.

Ciro
Thanks. Well, I will try more absorbers in the back half of the room first. If that´s not enough, I´ll change the cloud to a thicker one. I was so happy about how it turned out, so that will be the last resort
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29th February 2012
Old 29th February 2012
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Bass

Quote:
I bought a couple of PMC TB2SA yesterday, and put them on Auralex on the desk, closer to me, lower, and further away from the front wall, but I still has some lack of bass in the sweet spot.
They will deliver more bass if you place them close to the front wall. I am working with a friend who has the same ones. He ended up removing part of a 12inch deep wall trap in order to get the speakers as close as possible to the wall. We finally have an acceptable LF level.

DD
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2nd March 2012
Old 2nd March 2012
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I´ve tried John Slayers way to treat low frequency with his corner traps with very good results in small rooms
Wall Units

I think this will work great to put in the corners of your front wall, just make them as deep as possible. You will have a good low-freq broadband pressure trap that work really well ,from my experience and it´s easy and cheap to build.
Put the speakers as close to the front absorbers as possible to avoid SBIR.

Skål !!
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2nd March 2012
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there´s a detail It´s not clear no me:
"The box created must be completely sealed to be airtight. The only access to the outer air is via the gaps between the slats"

How it´s possible having gaps and be sealed at same time?

Ciro

*Btw my room has some traps this way - slats in front - (7 for while) and it makes a big difference.
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3rd March 2012
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I have often wondered what would happen in a situation like this (control room linked to another room by a door less opening) if the opening was plugged with a nice thick LF broadband absorber as a door. My instinct is that it would be very effective at taming some of the low end... I would be interested to hear the opinions of our experienced members on this.
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3rd March 2012
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Quote:
Originally Posted by ciro View Post
there´s a detail It´s not clear no me:
"The box created must be completely sealed to be airtight. The only access to the outer air is via the gaps between the slats"

How it´s possible having gaps and be sealed at same time?

Ciro

*Btw my room has some traps this way - slats in front - (7 for while) and it makes a big difference.
Like John says, the box(the backsides and top/bottom) needs to be sealed(just glue it , nail it, Latex it) so no pressure will escape from the joints .

Balluda
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3rd March 2012
Old 3rd March 2012
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My knowledge of room acoustics is quite limited compared to many posters in the forum, so forgive me if this is a stupid question... but....

Doesn't the open doorway turn the room into a helmholtz resonator?
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3rd March 2012
Old 3rd March 2012
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Quote:
Originally Posted by DanDan View Post
They will deliver more bass if you place them close to the front wall. I am working with a friend who has the same ones. He ended up removing part of a 12inch deep wall trap in order to get the speakers as close as possible to the wall. We finally have an acceptable LF level.

DD
This was so stupid, I almost hesitate to write this:

I moved the desk forward to the front wall, put the PMC speakers on the desk and raised a couple of desk boards (desktops?) against the front wall behind the speakers, to get the most bass reflections possible from the speakers.

With the signal generator plugin in Pro Tools, I swept from 200 Hz down, and the speakers almost went quiet below 100 Hz. They say the PMC have a good bass response. I was so disappointed. I soldered a long speaker cable, and moved one speaker to the living room. A little better there.

Well, after some experimentation with speaker placement and moving absorbents around, I finally realized that I had an EQ on the 2bus in the old project I opened where I put the signal generator on a channel. Low shelf at 100 Hz. For some unknown reason. Haha. That evening had its ups and downs I tell you

I now have the speakers on stands in front of the bass traps in the front corners, and the desk a little closer to the front wall. The bass volume is a little higher than before, but not as good as I would want it. Placement is also a little to wide relative to my position, so the image is not as good as I want it.

I am still waiting for the ECM8000 to arrive from Germany. I would like to have a waterfall to see how the room reacts as it is at the moment, but I thought about closing the door to the hallway and put a trap there instead, and soffit traps at the wall/ceiling intersection, as in the picture below. Results in a few days.

/Thomas

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3rd March 2012
Old 3rd March 2012
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Bass

Thomas, my client also believed the PMCs would have great response.
The simple fact is they don't. We had to use the help of the front wall to get acceptable bass.

Quote:
Doesn't the open doorway turn the room into a helmholtz resonator?
Yes of course but I expect the resonant frequency would be extremely low, and the quality, Q, of resonance would be weak, due to the very large door area.
As someone suggested a door sized fibre trap would be interesting.

The possible downside to this doorway may be unequal LF response.
Measurements will show this. e.g. Nulls from the Back Wall BIR.
Or not. Maybe bass waves have spread enough to consider the BW as just one singularity.
Pretty much any mic will show this, if you want to find out sooner than the ECM.

DD
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4th March 2012
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a bit off topic ...

I also own a pair of PMC TBS2A

When I went to buy them at a selling shop, I took a selection of my own CDs, mostly classical music and small jazz bands. I found the PMC to be as good as some more expensive well renowned monitors on demo when playing upright and double bass music.
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5th March 2012
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If the door/passageway opened into an infinite space, with no boundaries, in other words opening to the outside, then I could see how you could consider it a bass trap, but if it opens into another enclosed space, then you just create a more complicated acoustically coupled space that is capable of creating any number of anomalies in low frequencies.

I found this out in a practical way by measuring my 1500 cu' home mixing room with the door both open and closed. The back, off-center door leads to a large atrium-like area several times larger than my mix room. What I found is that when the door was closed, the acoustic response was predictable. When the door is open, the bass issues don't go away, they just change character, becoming more complex and less predictable. When you think about it, this seems like a logical outcome.
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5th March 2012
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Quote:
Originally Posted by ggegan View Post
If the door/passageway opened into an infinite space, with no boundaries, in other words opening to the outside, then I could see how you could consider it a bass trap, but if it opens into another enclosed space, then you just create a more complicated acoustically coupled space that is capable of creating any number of anomalies in low frequencies.

I found this out in a practical way by measuring my 1500 cu' home mixing room with the door both open and closed. The back, off-center door leads to a large atrium-like area several times larger than my mix room. What I found is that when the door was closed, the acoustic response was predictable. When the door is open, the bass issues don't go away, they just change character, becoming more complex and less predictable. When you think about it, this seems like a logical outcome.

+1

Ciro
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5th March 2012
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Door

Good point. The after bloom or boom from the other room (poetry) may or may not be intrusive or nice. Listening and Measurement would easily determine this. If there was a door....
I am pondering, would I recommend installing this enormous door or treat the other room...?
ohmtom do you have a Impulse Response measurement that we could look into?
You can post them directly here if you Zip them first.
DD
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5th March 2012
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Quote:
Originally Posted by mhch View Post
I also own a pair of PMC TBS2A

When I went to buy them at a selling shop, I took a selection of my own CDs, mostly classical music and small jazz bands. I found the PMC to be as good as some more expensive well renowned monitors on demo when playing upright and double bass music.
Yes, that is what I´ve been told too, so I am a bit confused at the moment. Maybe the measurements will give me a clue.
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5th March 2012
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Quote:
Originally Posted by DanDan View Post
Good point. The after bloom or boom from the other room (poetry) may or may not be intrusive or nice. Listening and Measurement would easily determine this. If there was a door....
I am pondering, would I recommend installing this enormous door or treat the other room...?
ohmtom do you have a Impulse Response measurement that we could look into?
You can post them directly here if you Zip them first.
DD
Hi.

No, I have not done any measurements yet. What I have done the last couple of evenings is to put a door in, at the opening into the hallway, in the back left corner. I found out that I had ditched the door some years ago, and a new door did not fit to the old frame, so I had to change the frame also. Phew! I just finished that off.

I thought it would be good to see what happens with the readouts with the door open and then closed.

I just downloaded REW, now I just have to find out how it works

Hopefully, the ECM8000 arrives tomorrow so I can start to measure...
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5th March 2012
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This is my example, having a door in the rear wall. Measurements with the door closed and opened. Hope it helps.

Opening the door and there´s a big and non symetric room "behind".It improves the decay at lowest lenght mode (50hz), but creates a null (10dB) and ringing at same frequency (105hz).

Ciro


*measured with MXL 2003. My ECM8000 broke last month ...
Attached Thumbnails
Does open doors count as infinite traps?-closed-door.jpg   Does open doors count as infinite traps?-open-door.jpg  
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5th March 2012
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Measurements

That's pretty convincing in favour of the door Ciro.
Nice looking waterfall too.
DD
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5th March 2012
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Yes, I prefer to work having a little "boom" (door closed) that I can eq with a narrow filter than having a null that nothing will "compensate".

Tks DD !


Ciro
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5th March 2012
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I tried to set up REW with my Rode K2, just to understand how REW works. But I don´t understand the input preferences for the soundcard. I have a Profire 2626 connected to a Mac. And all I can choose from is "default input", and input channel "Left" or "Right".

But the 2626 has 8 channels...? I tried a program called FuzzMeasure earlier, and there I could choose from input channel 1-8. But not here.

In the Help they say something about using the soundcards mixer (I have everything off there, as I monitor through Pro Tools normally):

"The Output and Input used will be those which have been selected in your soundcard's mixer. Make sure that the input is not feeding into the output signal in your soundcard's mixer and that monitoring is not enabled (if available)."

I feel confused, maybe because it is bedtime here in Sweden. Any suggestions? Goodnight everybody.

/Thomas
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6th March 2012
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REW/Mac

Apple don't do Java correctly so REW won't work with multichannel interfaces.
Use the on Apple on board I/O. They are good enough for the job.
You could do half and half. Playback through your current rig, but select Inbuilt Line Input for REW. System Prefs/Sound or Audio Midi Setup.
Fuzz has none of these problems because it doesn't use Java.
DD
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6th March 2012
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Quote:
Originally Posted by DanDan View Post
Apple don't do Java correctly so REW won't work with multichannel interfaces.
Use the on Apple on board I/O. They are good enough for the job.
You could do half and half. Playback through your current rig, but select Inbuilt Line Input for REW. System Prefs/Sound or Audio Midi Setup.
Fuzz has none of these problems because it doesn't use Java.
DD
Aha, I could have spent hours on the net trying to find that out. Thanks.

Yes, and I have a small Behringer mixer with a mic input with 48V I could use. I will try it out this evening.

/Thomas
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6th March 2012
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So now I have done some measurements...

I have measured with the left rear door open and closed, after adding absorbents to the rear wall, and with left, right and both speakers.
All measurements with the ECM at listening position, except one with the ECM close to the rear wall.

I am no good at interpreting the graphs, but I guess that there is a small difference between left and right speakers due to the open wall-thing at the rear right. There seems to be no wave coming back from the living room, at least not within the 500 msek sweep length...

It seems that the room is quite dead, don´t you think?

The dip at 185 Hz may be because the PMC has this "Transmission Line"-thing. I read somewhere that that is the drawback with the technology, but that the Q is so narrow that it doesn´t matter.

I am mostly worried about the low numbers below 100 Hz. No bass?

What´s your verdict?

(The zipfile has no measurement with the ECM at the rear wall, the file was too big.)

/Tomas
Attached Thumbnails
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Does open doors count as infinite traps?-rew-6-mar-2012_6.jpg   Does open doors count as infinite traps?-rew-6-mar-2012_6_all.jpg   Does open doors count as infinite traps?-closed-door-absorbents.jpg   Does open doors count as infinite traps?-closed-door-no-absorbents.jpg   Does open doors count as infinite traps?-open-door-absorbents.jpg  

Does open doors count as infinite traps?-open-door-no-absorbents.jpg   Does open doors count as infinite traps?-front.jpg  
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File Type: zip ohmtom 6 mar 2012_3.mdat.zip (8.68 MB, 4 views)
#29
6th March 2012
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Start

Good start with Measurement but let's not trust them yet.
I suspect your mic preamp has a High Pass Filter.
The dip at 185 is worth checking out. It could be BIR. Try moving the mic and/or speaker small increments say 5cm, and let's see if the frequency changes.

DD
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7th March 2012
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Quote:
Originally Posted by DanDan View Post
Good start with Measurement but let's not trust them yet.
I suspect your mic preamp has a High Pass Filter.
The dip at 185 is worth checking out. It could be BIR. Try moving the mic and/or speaker small increments say 5cm, and let's see if the frequency changes.

DD
I did a loopback calibration from out of the 2626 to the CME Matrix preamp that is connected to the Mac audio input, and it was a flat curve, so it seems that there is no EQ going on with the preamp...

I measured with the old Tannoys too, and got a similar curve, with lack of bass under 100 Hz, see picture. Maybe it is the room? In that case, what should I do? More bass traps at the rear wall? I measured the SPL under 100 Hz with the signal generator at the rear wall/ceiling area, and it was about 10 dB higher there than at the speaker position.

I did a lot of measurements with my PMC speakers today, and found that the dip at 185-190 decreased when I moved the speakers a bit closer to the middle of the room. Having done that, I measured from my monitor and back to the rear wall. It seems that the 185 dip is smallest now around the listening position, so the next step would be to measure with small speaker and mic movements around that area.

It also seems that I have comb filtering from 300 Hz all the way up. Maybe that is normal in a small room?

/Thomas
Attached Thumbnails
Does open doors count as infinite traps?-tannoys-desk-7-mar.jpg   Does open doors count as infinite traps?-5-feet-front-wall-7-mar.jpg   Does open doors count as infinite traps?-listeing-pos-6-feet-7-mar.jpg   Does open doors count as infinite traps?-5-feet-rear-7-mar.jpg   Does open doors count as infinite traps?-2-feet-rear-7-mar.jpg  

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File Type: zip ohmtom 7 mar 2012_5.mdat.zip (7.60 MB, 3 views)
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