Home Theater Forum and Systems banner
1 - 20 of 25 Posts

·
Registered
Joined
·
32 Posts
Discussion Starter · #1 ·
Hi,

I am a real newbie at this so please bear with me.

What I need is some help understanding the graph I am posting here. Just what does this show to you experts. Is this a poor result?

I should explain what I am doing...

I want to see how the monitors I have on my PC are performing in a rather small room 2x3m. I am not going to change any EQ or use filters in anyway. All I want to do is alter the room acoustics by adding absorbers / reflectors etc to make any changes. The position of the Monitors are not symmetrical in the room (not easy do do).

I am using the PC for mixing original recordings, not what most of you use the software for to tune your systems and room to improve listening of existing CD’s movies etc.



On the graph Left is Red, and green is right. I turned off the right speaker /monitor when doing the measurements for the left etc. Is this correct? The right monitor is nearer to a wall than the left monitor. When I first did this I assumed the software would only use one speaker at a time, but it does both, have I set it up wrong?

I used the Radio Shack SPL Meter with the cal file I downloaded. The monitors are Alesis M1 Active Mk2 with that have a flat output between 50Hz and 20KHz with cutoffs at 40Hz and 23.5KHz and is driven by an M-Audio Fast Track Pro USB box. (I don’t use a PC sound card quality is not good enough)

Any help appreciated.

Mike
 

·
Registered
Joined
·
7,514 Posts
I would increase the level of the test from the ~60dB you're at to something like 80dB. (edit)

I would change the vertical limits of the graph from the (10dB-96dB) to something like (45dB-105dB).

I would turn on smoothing of about 1/3 octave to remove the spikes and make it a bit easier to read.

All in all the response is about as expected. The lower end is fairly flat down to about 45Hz and begins to drop off. The upper end cannot be verified above 10KHz, because the RS meter basically sucks above that range. But your response is quite flat.

brucek
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #3 ·
Brucek thanks for your advise.

Here is another graph with the changes you suggested, except that the level is at 70dB any higher and it clipped.



So basically you say this is ok, but it does have a couple of dips in the ranges 240~550Hz and 1KHz~3.5KHz. Is the best way to flatten this out by removing the peaks in other areas using absorbers tuned to certain frequencies thereby reducing the peak levels to fall in line with the lower parts of the curve. Or is it the other way round and try to increase the dips with reflectors? The latter sounds more difficult, but as a novice I would be grateful of any advise.

Or would you be happy with the graph as it is?

Would I be correct in saying that the right speaker (green) is higher at the bottom end because it is closer to a wall, in fact it is almost in a corner of the room. And to improve the bottom end I would either have to box in the left speaker or add a bass trap behind the right speaker? (By improve, I mean even out the left and right so they are the same levels.)

Thank you,

Mike
 

·
Registered
Joined
·
7,514 Posts
Yeah, the small lower end boost you're getting on the green speaker is likely due to corner placement. As is the case with a sub, corner placement accoustically couples the speaker better to the room and excites all resonances to give a boost in gain.

I'm afraid my experience is limited to low frequency equalization and not to room treamant. Hopefully others can chime in and help out with that.

Overall though I would say you need to lower somewhat the peak between ~120-150Hz and try and lower the upper end which must be a bit bright? A slow declining 'house curve' I've shown usually sounds best.





brucek
 
G

·
We have some very knowledgeable room treatment guys hangin' around and I would think those areas that brucek point out would definitely benefit from room treatments. I would probably suggest this be your first avenue to tame those areas too, if you can afford it or you could use DIY acoustical panels.

You might list your equipment in your profile too... so we know what you have to work with.
 
G

·
The response shape above 2k is primarily due to the meter, if you look at the graphs in the leaflet that comes with with the meter you'll see the same shape. To do any EQ above a few hundred Hz you really need a calibrated microphone, else you are likely to do more harm than good.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #7 ·
Great help so far, many thanks, getting there slowly :T

I have got some Acoustic tiles 40mm thick see ebay item: 7381039576. Works well for high frequency absorption, can also be fitted multiple thickness to lower the frequency. I can also do some DIY. But can't find a supplier of the rigid Rockwell wool (Manchester, UK) only the softer type used for loft insulation.

I will compile an equipment list and post in the next few days.

Re John's message about a calibrated microphone, any suggestions? I am prepared to go down this route to get it done right. Would prefer a not too expensive one though. But can a good microphone with a published profile do instead (by this I mean a small response graph that comes with the mics and on the website, no figures just a graph). I have a number mainly by Rode:
NT1a
NT2000
NT5
NTG-2
An example of a Frequency Response graph is http://www.rodemic.com/downloads/NT5_Brochure.pdf

Thanks,

Mike
 
G

·

·
Registered
Joined
·
32 Posts
Discussion Starter · #10 ·
Fantastic, I am going to get the Behringer ECM8000 it's only £38.78 inc VAT from
Boomerangsounds

Thanks for the help, I will get the mic, do some more measurements and perhaps post another graph and take it from there. May take a couple of days.

Incidentally the Rode NT5 is not an omni and costs about 3 times the price of the ECM8000. The Rode NT2000 has variable polar pattern including omni, which I will also try alongside the ECM8000. It would be interesting to see how they compare.

Your a great bunch of guy's :hail:

Mike
 

·
Super Moderator
Joined
·
9,310 Posts
I am not going to change any EQ or use filters in anyway. All I want to do is alter the room acoustics by adding absorbers / reflectors etc to make any changes.
If the objective is to tame the "hot" upper frequencies your graph is showing, I would imagine this tact would have less of an effect with PC speakers than with regular speakers set up 10-15 ft away from you. PC speakers are essentially near-field monitors, so you’re hearing more direct sound and less reflective sound than you would normally have. Maybe our acoustics experts can comment.

Regards,
Wayne
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #12 ·
In light of Wayne's message, I thought I should include a photo of the room so you know what I am up against...



The centre of the speakers are 0.75m (2ft 6inch) apart and my head forms an equilateral triangle with them. The room is 2.8m wide and 2m deep (across the window wall). It is quite packed with cupboards and bookcases.

So yes, these are near-field in every sense of the word! Perhaps not what you guy's are used to with home theatres.

You can see the wall has some acoustic foam panels, they were really to reduce the PC noise, which it does quite well. Behind the monitor is a greater thickness of foam up to about 150mm thick.

The speakers are standing on Auralex MoPADs.

All the PC's have extensive sound insulation inside, ultra quiet fans and the main PC (on the right) has a fan-less power supply.

The audio interface is the M-Audio Fast Track Pro (on the left), that feeds the speakers and is also the audio input to the PC.

Despite the cramped space, there is still the ceiling which is acoustically untreated, and I could box in the left speaker a bit to help balance the base response. The ceiling, which is quite low, if treated with 40mm thick foam may help with taming the higher end of the graph. But I will wait till I measure again with the Behringer ECM8000.

Mike
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #13 ·
OK, I have now re measured with a Behringer ECM8000. I also realised that I had not done the soundcard response properly (miss-read the help and did not do a loopback with cables!! just used the speakers :duh: ) Anyway, new graph below...


[Red= Left, Green=Right, Grey=Soundcard]

Now this shows a different picture. More high end, about the same as before at the bottom end and also a few troughs.

I thought I would see what would happen putting in some low/midrange absorbers that I had made a while ago, 200mm thick rockwool 0.75m x 0.4m area (three of them). These were placed behind the listening position. The result is...



For an absorber to remove troughs, I am correct in saying that they are absorbing standing waves? This was a bit of a surprise, I thought the base end would benefit by getting reduced.

If I was to suspend/fix to the ceiling such absorbers just above the speakers/listening position would I loose this benefit? Could I gain a taming of the lower end?

And what can be done to tame that rather high top end?

How would you acoustic experts out there attempt to flatten this graph out?

Remember, I can’t use any EQ or filters, just moving things around a bit and adding absorbers/reflectors.

Boy, you could spend a great deal of time experimenting getting this just right can’t you, but then you have been there haven’t you. :)

I really could do with some guidance on what to do next. Thank you,

Mike
 
G

·
To adjust the top end toe the speakers out, the off-axis response will be progressively more rolled off at the top so the further off axis you turn them the more roll-off you should see.

The dips come about from cancellations due to reflections, modal or otherwise, so any absorption will reduce the reflections and so reduce the dips.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #16 ·
I have added some acoustic foam tiles to the ceiling. Kept the two temporary acoustic absorber panels behind me (which are 1.25m x 0.45m x 180mm deep each not what I said earlier) and done some more measurements with the ECM8000 and a Rode NT2000 microphone which is set to omni to compare.


Used Behringer ECM8000 microphone


Used Rode NT2000 microphone

The graphs look quite different and to be honest I think it more likely that the NT2000 has produced a more accurate curve. I say this because the slight hump I am getting at 10KHz corresponds with the graph shown in the microphone manual. This can be seen here.

Also Sonnie showed in the thread Professional Mic Calibration where there is hump at 15KHz for the Behringer ECM8000 that was calibrated. So I suppose it looks a bit like that as well. I can see why getting a mic calibrated can help, just need to find someone in the UK.

So after all that, apart from the peaks around 50-60Hz it looks reasonably flat, would you agree or not.

I have not toed out the speakers very much as they would end up being too far off axis with my listening position which is very close.

Not sure how to remove those low peaks, any pointers appreciated.

Thanks,

Mike
 

·
Super Moderator
Joined
·
9,310 Posts
In light of Wayne's message, I thought I should include a photo of the room so you know what I am up against...
Nice set up, Mike! :T

So after all that, apart from the peaks around 50-60Hz it looks reasonably flat, would you agree or not.
Yup, I’d say it’s really good! What kind of speakers are those?

Not sure how to remove those low peaks, any pointers appreciated.
Bass traps would probably do the trick, if you have the physical space for them. There really aren’t that many non-electronic compensations (read treatments) available for low frequencies.

Regards,
Wayne
 
G

·
I've already purchased some material for some bass traps. I've got 100mm thick RWA45 which you can get from Sheffield Insulations at a reasonable price. It's important I believe to have an airgap behind rather than having it hard up against a wall so where you have your 200mm thick insulation it would be better to have 75mm with 125mm cavity behind. I believe all absorbers work better with an airgap. If you made some panels 75-100mm thick and covered them in fabric (I'm using some speaker fabric) you could move them around until you found the best position and sound :T

Cheers,

Bob.
 
G

·
MikeOates said:
The graphs look quite different and to be honest I think it more likely that the NT2000 has produced a more accurate curve. I say this because the slight hump I am getting at 10KHz corresponds with the graph shown in the microphone manual. This can be seen here.

Also Sonnie showed in the thread Professional Mic Calibration where there is hump at 15KHz for the Behringer ECM8000 that was calibrated. So I suppose it looks a bit like that as well. I can see why getting a mic calibrated can help, just need to find someone in the UK.

So after all that, apart from the peaks around 50-60Hz it looks reasonably flat, would you agree or not.

I agree... if you haven't had your ECM calibrated you could probably take my .cal file and load it in REW to get closer. They are pretty uniform out of the box according to Kim, the calibrator. Not always exact but close. But definitely that looks to be the differnece between the two.

It's gonna be tough to get rid of those peaks below 80hz with bass traps.
 

·
Registered
Joined
·
32 Posts
Discussion Starter · #20 ·
This is an update of my measurements now that (thanks to Sonnie) I have got a calibration file for my Behringer ECM8000.

Just to remind you: I am setting up my room for mixing music therefore no EQ can be used, just changes to the room acoustics using absorbers.

Here is a starting graph before any room treatment...


[Right = Green, Left = Red]

Now after adding acoustic ceiling tiles, three home made movable acoustic panels each measuring 1.2m x 0.45m x 200mm thick (look like wooden draws filled with rockwool and fabric covered) plus two triangular corner base traps (1ft x 1ft x 3ft) I have now made major improvements to the way the room sounds. This is certainly noticeable and the resulting graph is proof.



One other change was made after reading the manual :blush: for the Alesis M1 Active Mk2 monitors, due to the position of the monitors, one near a corner, I plugged some of the ports on the front panels firstly with socks! (black ones of course, so they don’t colour the sound ;) ), then changed that for some acoustic foam which looks a bit neater. This is the method used to fine-tune the base output from the monitors. This has reduced the obvious difference between left and right. (Right is in the corner). I plugged both ports on the right one and one on the left.

The speakers cut off at 40Hz that’s why it’s lacking in base. Perhaps I may look for a woofer and crossover unit. What crossover would you recommend that I can fit between the PC’s USB audio interface box and the woofer/monitors that has adjustable crossover.

Thanks for all the input and help so far. :T

Mike
 
1 - 20 of 25 Posts
Top