Broadband absorption behind monitors?

Some background. I have a room which I use for mixing and tracking. Overall the room sounds good for tracking, and have just bought a pair of Eve SC208 monitors. It is compromised in terms of geometry and the requirement to be a tracking room as well as used for mixing. I have placed the monitors up against the front wall. I have read various articles which indicated to: 1. Place the monitors as close to the front wall. 2. Place 100mm absorbers on the front wall, behind the Monitors and place the monitors up against the absorbers. My questions are: 1. Is there a way of understanding whether placing the absorbers will be beneficial without building them and trying it? 2. If so, how would I go about this analysis? Many thanks in advance. This is a great forum.. I have also measured my room dimensions, and I have a picture attached to the file below.. Hopefully this works.. Regards Rob

greenlounge wrote:

Some background. I have a room which I use for mixing and tracking. Overall the room sounds good for tracking, and have just bought a pair of Eve SC208 monitors. It is compromised in terms of geometry and the requirement to be a tracking room as well as used for mixing. I have placed the monitors up against the front wall. I have read various articles which indicated to: 1. Place the monitors as close to the front wall. 2. Place 100mm absorbers on the front wall, behind the Monitors and place the monitors up against the absorbers. My questions are: 1. Is there a way of understanding whether placing the absorbers will be beneficial without building them and trying it? 2. If so, how would I go about this analysis? Many thanks in advance. This is a great forum.. I have also measured my room dimensions, and I have a picture attached to the file below.. Hopefully this works.. Regards Rob

The reason for placing the monitors as close to the front wall as possible is to reduce the effects of speaker boundary interference, it is the next best solution to actually flush mounting them in the wall itself. Another solution is flush mounting the speakers in soft and deep treatment wall, with the intention of absorbing some of the low frequencies before they have a chance to reflect back out into the room, delayed by however many ms to the direct signal. Of course, this requires very deep treatment in order to be effective all the way down the spectrum, if the loudspeakers go down to 40hz then the treatment would need to be around 2' thick in order to absorb down that low at 60% efficiency. Thicker than that and you quickly reach a point of diminishing returns with velocity based absorbers and membrane absorbers need to be deployed. My point is, 100mm absorbers behind the speakers aren't going to help much. low frequency radiates spherically from the driver, creeping behind the speakers, whereas mids and highs are more directional and do not radiate behind the speakers (apart from potential reflections off of room surfaces/rear of desk). If your speakers are as close as you can get to the front wall already then any LF that goes behind will reflect off of the front wall with minimal delay, causing minimal audible problems. Ideally the front wall would be angled each side to follow the toe-in angle of your speakers, but even without that the issues will be minimised. You may find that your speakers sound too "bassy" now, but that's because they probably have a built in baffle step compensation, if you can disable that then great, or if you just have a Low frequency control then turn it down until you get the flattest response. Paul

Paulus87. Thank you very much for the prompt reply. Based on this I might leave the front wall as is. I do have another question, as to the speaker and listening position. I have enclosed the room layout. 1. Is there a better place to put the speakers and the listening position? If I need to have a seperate post, please let me know and I will do so. Again, thanks for the information. Regards Rob

greenlounge wrote:

Paulus87. Thank you very much for the prompt reply. Based on this I might leave the front wall as is. I do have another question, as to the speaker and listening position. I have enclosed the room layout. 1. Is there a better place to put the speakers and the listening position? If I need to have a seperate post, please let me know and I will do so. Again, thanks for the information. Regards Rob

Unfortunately it is a bit of an awkward room...but even the most ideal rooms are compromised, the trick is finding the best compromise. a few accepted ideals for a critical listening room are: - at least 1200 cubic feet in volume - no dimensions the same or multiples within 5% of each other. - a ceiling higher than 8' - Left/Right Symmetry at least from the listening position forward (ideally the whole room) - a listening position of 37.5% of the length between the front and rear walls - the speakers fire down the longest dimension of the room - appropriate treatment to deal with first order reflections back to the listening position and to attenuate modal resonances As it is, your left/right symmetry is off due to the fireplace in the left corner, this could affect stereo imaging... measuring would confirm. Ideally, in your room you would want to face one of the short walls, but I'm not sure I would advise it due to you having a fireplace in one corner and a door in the other, which prevents you being able to put decent bass trapping there. On the plus side, with your room the way it is, it looks like you have an opening directly behind the mix position, which is good since it will extend the amount of time the sound needs to travel before bouncing back at your ears, which ideally would be more than 20ms after the direct sound and attenuated by at least 20db. Also, some of the low end frequencies will diffract around the opening and disperse into the open space behind. Do you have any kind of acoustic treatment in your space at present? To be honest, it's quite an unusual room and the "best" listening position may be achieved through experimenting and lots of measuring. Paul

Paulus87. Yes it is a bit of an unusual room.. The alcove part of room (the one with the external door), actually has a lot of stuff in it, all a bit random.. No I do not have any acoustic treatment at all, unless you consider a Hammond M series spinet in one corner and a Leslie 147 in the other.. There is a couch on the opposite wall from the window, next to the fireplace. I have taken some measurements using REW but I am not 100% sure how to interpret them. Again.. Thanks heaps..

Acoustics of spaces not designed for sound purposes, are highly complex. Theories derived from more ideal spaces are not too far from guessing. Acoustic testing is very affordable now, and it has certain results. Absorption behind monitors vs not, finding optimum locations for speakers and listener (which is interactive)...... REW and an Omni Mic.

Thanks DanDan. I do have an Omni mic and REW. Is there a post which gives some insights on how to interpret the data? I have followed the post on how to calibrate the setup, but haven’t found anything which talks about analysing the data itself. Regards Rob

greenlounge wrote:

Thanks DanDan. I do have an Omni mic and REW. Is there a post which gives some insights on how to interpret the data? I have followed the post on how to calibrate the setup, but haven’t found anything which talks about analysing the data itself. Regards Rob

You can upload the .mdat file online somewhere and provide us with the link, then we can take a look at it for you. But, I think we're all learning more and more about what the data is telling us all the time, at least I am. The main things to learn how to read (in my opinion) is the ETC graph and the waterfall or spectrogram plots. Paul

+1 The REW manual is extraordinarily good. IMO Measurement is superbly useful for telling if A is better than B. Not so great at doing a full diagnostic on C..... What are we looking for? Well no reflections for a little while after a hypothetical impulse. -20dBFS for the first 20mS, or better. Then Room Decay third octaves varying no more than 10% from their neighbours either side. 200mS is good, but we would be happy with a rise to 400mS at LF.

Thanks for all the responses.. I suppose the 1st things I would like to understand from the measurements are: Are my measurements valid, and have I set everything up in REW correctly? I followed the sticky topic. - I am using a DBX measurement mic, and do not have a calibration file for it. Model DBXRTA-M. - I have calibrated SPL to 80 db as suggested but used a mobile phone app, weighted C slow. How symmetrical is my stereo image across the frequency spectrum, considering I don't have a symmetrical room? Looking at the different listening positions, how do I understand these, in view of the room geometry ? Should I be looking for other listening positions, or even re-orient the speakers ? - If so, is this trial and error ? Please find enclosed a link to the REW file. The speakers are positioned as per the drawing in the original post for all measurements. https://www.dropbox.com/s/p3p4r4m021338 ... .mdat?dl=0 Once again, thanks for the great help here.. Regards Rob..

Are my measurements valid, and have I set everything up in REW correctly? I followed the sticky topic. They do seem to be. How symmetrical is my stereo image across the frequency spectrum, considering I don't have a symmetrical room? Compare the SPL L vs R with the mic central. Looking at the different listening positions, how do I understand these, in view of the room geometry ? Eveness of FR and Decay, as in my post above. Should I be looking for other listening positions, or even re-orient the speakers ? - If so, is this trial and error ? Yes, including varying the listening position. The RTA with Pink Noise will help you eliminate No Fly zones in real time. I note that your speakers are between 1 and 2 metres from the Front Wall. That is a generally regarded as a No Fly Zone. http://arqen.com/acoustics-101/speaker- ... erference/

Thanks DanDan.. Maybe a bit of confusion here, but in the measurements, my speakers are approx 40mm from the front wall, and for the various measurements, I have been moving the listening position only, the speakers have remained in this position. In terms analysis.. If I look at the 1.2m listening position, which I am currently using: The low frequency analysis shows substantial differences between the left and right speakers (enclosed file). The overall frequency analysis does show differences, which I assume are due to the non-symmetrical nature of the room (I haven't enclosed this file, since I can only seem to do one at a time), but its in the data in the link. How would I use the Time Domain analysis to get a better understanding of the characteristics of the room? It would help me greatly if you can explain what I am looking for Once I understand how the time domain analysis works, I will move the speakers and listening position around the room, and measuring. Thanks again..

REW is brilliant at comparing A vs B. Not so great at giving C a health check, i.e. an 'analysis'. Many treatments are an absolute given e.g. Cloud, Side Reflection Absorbers, Back Wall Absorbers and Corner absorbers. They work in every room. There is virtually no chance of doing too much of them.

DanDan wrote:

REW is brilliant at comparing A vs B. Not so great at giving C a health check, i.e. an 'analysis'. Many treatments are an absolute given e.g. Cloud, Side Reflection Absorbers, Back Wall Absorbers and Corner absorbers. They work in every room. There is virtually no chance of doing too much of them.

Thank you DanDan. I do have one further question that isn’t really clear to me. How do I interpret the REW data, specifically the time domain data to understand how much smearing I am getting from early reflections, at which frequency they are at? I am thinking if I can deduce this from the data, I can then have a strategy to deal with these early reflections in an appropriate manner, considering the constraints of my room. For example I might need moveable absorbers placed at specific positions while mixing, which might block windows/doors or walkways. Or are you saying I need to just take before measurements and try a treatment, measure and see what happens? Thanks again.

Well I made the broadband absorbers today. 600x1200x142 with 100mm of FI48. Will hang them them correctly and take some measurements to see the impact. Regards Rob.

You are welcome Rob. The REW ETC will display reflections. By their timing you could work out what distance they are etc. But if you play snooker, you can easily see them. Or use a mirror. You can also confirm which boundary is a particular reflection by moving the mic or speaker and observing what the reflection does in time. When you have a good suspect in your sights, install treatment, or have a friend hold an absorber blocking a suspected location...... REW will show the reflection's absence now. At low frequencies, it is pretty easy to connect a reflection with say a null. Typically the Floor Reflection...... Higher up, it gets trickier and trickier. But the simple approach of moving and blocking remains totally valid. Better is better. The minimal goal here is eliminating early reflections to the extent of -20dB for the first 20mS. You will probably do better than that.

DanDan. I have now positioned the panels and will redo the measurements. I have 2 more panels under construction to try and take the 1st reflections off one side wall. Here is a picture of the front wall now.

Wow that room is narrow. That places the traps almost in the corners. Where they would be remarkably effective. Much more so if you trap floor to ceiling.

DanDan. The above photo probably gave you an unrealistic perspective of the room width. Here is another photo further back in the room. On the left is a fireplace, which I think is actually doing good things to the room. It has a cast iron insert. Sorry about the mess in the room. I have 2 more panels to be completed for 1st reflections on right hand side. 1 have 3 panels of 100mm FI48 left after this, each sheet being 2400x1200 and have a plan for this. From this photo you can see the underlying asymmetry of the room. I am thinking of having 2 absorbers one in front of the fireplace and the other that can be put across the front right hand side. These would be moveable, since the right one would block the door. When not mixing the 2 absorber panels would be moved into a position for tracking, either for vocals or behind the drum kit. So I don’t know whether this would make them moveable absorber panels or Gobos. I feel that this potentially requires another thread? Thanks for all the assistance so far.

So I have completed 4 x 600 x 1200 mm panels, with FI48 100 mm panels, set in a 150mm deep frame.. These were placed in the following: 1. 1st reflection point on RHS - one. Obtained using the mirror trick. 2. Cloud over the mixing area. - one. 3. First reflection point over the fireplace. - two panels. Covers the 1st reflection point but wider. The speakers are 80 mm from the front wall (back edge of the speaker to front wall) with NO treatment behind speakers. Enclosed is the mdat file for this measurement, which contains only the L and R speaker measurements. The waterfall shows very strange things at low frequencies, but my speaker specs (Eve SC208) say they go down to 36Hz (-3db), so I am not too sure whether what I see in the waterfall is actually valid or relevant? Is it possible to get some feedback on these measurements to see what my next steps in treating this rooms should be? Many thanks in advance. dropbox link to mdat file is below: https://www.dropbox.com/s/9dleie7jsr4nc ... .mdat?dl=0

3D Chess. We are trying to optimise FR and Decay via locations of two speakers and a listener zone. Not easy. Quite often moving the speakers to the Front Wall will increase modal activity and overall LF level. It can be helpful to simply turn it down. Or block the ports if your speakers are reflex.

Yes.. it’s a bit like that DanDan. I have 3 sheets of F48 (2400x1200) left. It is in the plastic packaging it came in.. I straddled one across the front right hand side (temporarily) and did the measurement. The Right speaker measurement was better. This room actually is very complex. I am not done yet. One question. What is the best REW reports to use for IR for low frequencies? Waterfall shows it but not really accurate. Spectrograph is the same. I am now focusiing on RT60 using Topt as a measure. Thoughts? Thanks again for your help.

Hi Rob. To be honest I would often apply the certain treatments and then take the first baseline measurements. Judge by clear improvements, rather than some big and unreadable differences or the unanswerable 'what is this response like, any good? ' There are different ways of assessing. BBC used to look a the Room Decay. Topt or T30 1/3 Octave in modern REW language. Their wish was that no band should differ from it's next neighbours by more than 10% You can also view the Waterfalls or Spectogram and so on. A smooth spectrum of FR and Decay, gracefully sloping down at HF........ You could try different speaker locations to drive the modes harder. On the floor, in a corner, has three boundaries boosting LF and all modes gather there. Mic in the opposite ceiling corner.

The speakers are 80 mm from the front wall (back edge of the speaker to front wall) with NO treatment behind speakers

. Not clear, but can you try them 1mm from the Front Wall? I skip around, different fora, threads, so you may have answered this already. Do you have a Cloud?

Thanks for the help DanDan. I will focus on the Topt and T30 reports. Yes there is currently 1 1200x600 100mm thick cloud, suspended off the ceiling (ceiling height is 3 meters.. Regards Rob

Paulus87 wrote:

Another solution is flush mounting the speakers in soft and deep treatment wall, with the intention of absorbing some of the low frequencies before they have a chance to reflect back out into the room, delayed by however many ms to the direct signal. Of course, this requires very deep treatment in order to be effective all the way down the spectrum, if the loudspeakers go down to 40hz then the treatment would need to be around 2' thick in order to absorb down that low at 60% efficiency. Thicker than that and you quickly reach a point of diminishing returns with velocity based absorbers and membrane absorbers need to be deployed. If your speakers are as close as you can get to the front wall already then any LF that goes behind will reflect off of the front wall with minimal delay, causing minimal audible problems. Ideally the front wall would be angled each side to follow the toe-in angle of your speakers, but even without that the issues will be minimised. Paul

I was thinking about building a tapered wedge bass trap so the the slim taper just begins to fill the the gap edge of the monitor against the wall due to splaying. In reading this post, I'm wondering if there would be any benefit had to essentially build the treatment around the monitor, so that it is flush with the front baffle and still leaving the back of the monitor against the wall? It would be easy enough to use some 703 to frame out the pink fluffy around the monitor, even encasing the stand into the treatment. Essentially a faux soffit. I am using passive monitors so cooling isn't an issue.