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I finally got around to taking some room measurements. These are with Joseph Audio RM25si speakers in my new room, which is approximately 18'8" x 15. I used the Stereophile CD 2 test frequencies and a Radio Shack 33-2050. I then plotted the responses on paper and transferred them to Excel (tell me this is not a labor of love). I started with the speakers at 35 inches from the back wall, then progressively moved them backwards to 31, 28, and 24 inches.
The bass response seems difficult to pin down, and I see evidence of some comb filtering, but I'm not completely sure. However, I can definitely say that I don't like the 8dB suckout at 80Hz!
So all this by way of saying: would the room responses posted lead me to believe I need bass traps? I know they can smooth out the peaks and assist with articulation, but I really don't know how they'll help with the suckouts. Perhaps I should read up more...
Anyway, just wanted to post and try to keep moving (however slowly) forward on my room.
Many thanks!
(also: I ram into a discussion about Room EQ Wizard and will being trying to use it. Looks complicated and cool!)
Base on personal experience, one of the reasons for the suckout is due to 2nd and 3rd Harmonic of 1L mode.
http://img.photobucket.com/albums/v241/joamonte/Small%20room%20bass/velodyne40hz.jpg
This is a 40Hz signal measured in a big and asmall room, notice the 2nd and 3rd harmonic at 80Hz ad 120Hz?
The root solution of this to reduce the 1st room mode at 31.5 Hz ,it will also reduce the 2nd Harmonic 63Hz and 3rd Harmonic 126Hz (I think the resolution of your graph is too coarse).....check out the other measurement I made before and after I make 2 huge Frequency-Adjustable Helmholtz resonators into a customer room, notice how the 31Hz 1L room mode reduced along with the 2nd 62Hz.
http://img.photobucket.com/albums/v241/joamonte/Small%20room%20bass/Koopanelbasstrap.jpg
Very interesting.
> > The root solution of this to reduce the 1st room mode at 31.5 Hz
The funny thing about it is this: that is (or is very close to)the tuning frequency of the port on my speakers! Oh no! Well, some serious room tuning is ahead!
I'll check out the graphs you posted tonight. Thanks!
here is another room..
Before.
http://img.photobucket.com/albums/v241/joamonte/Small%20room%20bass/1605090fr01.jpg
After reduce the 35Hz room mode.
http://img.photobucket.com/albums/v241/joamonte/Small%20room%20bass/1605090fr02.jpg
> Read the Asylum FAQ on speaker placement methods.
Hi David. Will do. I've seen enough evidence with my rudimentary tests to know that their reaction within a space can change what I hear.
> Speaker placement is important too, but not..
Hi Ethan. Yes, this is a different context than David's above! I see your point, and the point that bass traps will be a good investment.
> Your room is surely much worse than what's shown in those plots. The third-octave frequencies on that CD are way too coarse to see the true detail. Most rooms the size of yours have peak/null spans of 30 dB or even more, and the peaks and nulls are often very close together. All that is hidden at 1/3 octave.
OK, so I fired up REW. All I can say is wow. After about an hour I had things calibrated, and the initial tests seemed close to what I had plotted a few days back...that is, until I zoomed in. Ouch! The magnitude of some of my problems was bigger than I thought! You were right on. Ah well, lots to do.
> Software like REW or ETF is much better than static tones at third octave intervals.
Proven to me very quickly!
Many thanks to you both.
You already got good advice from David and Klaus. I'll add a little:
Yes, you definitely need bass traps. All rooms do. Not only to flatten the response, but to reduce ringing at bass frequencies which makes bass sound muddy and indistinct.
Speaker placement is important too, but not as important as bass traps. The more traps you have, the less it matters where your speakers are and where you sit while listening.
Your room is surely much worse than what's shown in those plots. Bass response needs to be measured at high resolution. The third-octave frequencies on that CD are way too coarse to see the true detail. Most rooms the size of yours have peak/null spans of 30 dB or even more, and the peaks and nulls are often very close together. All that is hidden at 1/3 octave.
Software like REW or ETF is much better than static tones at third octave intervals. Not only because of the increased resolution, but because they also show time-based properties. And as I'm sure you'll appreciate, software takes less than 5 seconds to measure the room and generate a graph!
This article explains how I use ETF, but the principles apply to all such programs.
--Ethan
> The basic room performance isn't going to change with speaker movement except that changes in the speaker distance to the front wall will affect the frequency of the quarter wave cancellation to some degree
So maybe my best bet in this case, then, would be to get the speakers to where:
1) the trough is the smallest
2) the frequency can be dealt with more easily by use of traps
So the frequency (of the problem) does change based on distance from the front wall. Reading this earlier this morning (minus coffee and focus) made me think this was not the case.
But again, though, would bass traps really be a major part of the solution?
Thanks!
Read the Asylum FAQ on speaker placement methods. There are a number of methods, all of which yield slightly to largely different layouts but some of them are really only practicable in a dedicated room and some can be used in a living room where there are constraints on where you can put things. Experiment a bit with where the speakers and listening position go because this can make a big difference to your results and it's free, apart from your time and effort in moving things around. Good placement of speakers and listening position will minimise the effect of the room upon the sound but it can't eliminate all room effects on its own.
Traps will definitely help but, as I said, it depends on the effectiveness of the traps (not all traps are equally effective) and how many you use. Frequency isn't a big issue unless you use something like Helmholtz resonaters which have a narrow bandwidth and have to be tuned to your specific problem frequencies. Traps using fibreglass are broad band and work over a range of octaves so you don't have to worry about frequency but their effectiveness is falling off as frequencies drop below 100 Hz. Greater thicknesses and placement with air gaps behind them extends their range of operation to lower frequencies but takes up space and increases cost. If you're going DIY make sure you get the density right because if you don't use dense enough materials you won't get the performance you want at low frequencies but provided density is in the range of 2 to 6 lbs per cu. ft. thickness is much more critical than density.
David Aiken
Room dimensions: I did have them in my original post, but they are 18'8" L, 15'1"W at the ends.
At 4' into the room from both ends, there is a 1' addition on one side only, so for 9' in the middle the room width is 16'1".
Also, at that same 4' into the room, the ceiling goes from a standard 8' to 10'.
Wonky description. I have a Google SketchUp pic if that would help. Let me know and I can post it or a jpg of it at least.
What I'm hearing here, then, is that bass is bass in the room. That makes sense. I'll dial in the best of those plots (or somewhere in between) and continue to consider adding bass traps.
Thank you for your help.
Your room response at bass frequencies is determined by the room's dimensions and the stiffness of the walls, floor and ceiling. The basic room performance isn't going to change with speaker movement except that changes in the speaker distance to the front wall will affect the frequency of the quarter wave cancellation to some degree and placement across the width of the room will cause some degree of cancellation or reinforcement of some of the modes associated with the room width. You didn't mention the room dimensions but I've watched a friend measuring his room while moving speakers and his plots at different locations had the same kind of broad similarity to each other as yours have.
You would probably get bigger differences if you left the speakers in the one location and took a series of plots moving the mic to different positions along the listening axis. You'll see more change then because at some locations you will be measuring a peak and at others a null in the various modes. I've been in a room where you could hear a bass note strongly in one location and you could move a pace or two forward and be totally unable to hear the same note. The locations of the peaks and troughs are fixed by the room dimensions and don't change when you move the speakers so if the mic stays in the one place the measurements won't change anywhere near as much as you would expect when you move the speakers, but they will change a lot more if you try moving the mic.
Every room has room modes which will cause peaks and troughs in the frequency response. You can't avoid that and bass traps will help smooth them out. Floyd Toole, an acoustics researcher and speaker designer, comments in his recent book "Sound Reproduction" that bass traps will be almost universally beneficial so you can basically take it as read that they will produce some benefits in your room. How much benefit depends on your room and how bad its problems are, and on the effectiveness of the trap design and how many you install.
It's worth reading up on acoustics. Toole's book is good but if you want a better explanation of how room modes work, I think Everest's "Master Handbook of Acoustics" may be a better choice. Both are very good books with different technical emphases but their basic treatment recommendations are actually very similar. Everest provides a lot of useful info on the construction of treatments so if you're planning on going the DIY route I'd incline more to his book than Toole's. Toole has a lot more on psychoacoustics and how we process/interpret what we hear plus a fair bit on surround sound matters while Everest devotes little time to surround sound. I'm glad I have both.
David Aiken
> I see evidence of some comb filtering>
Human hearing has a mechanism called "binaural decoloration" which means that effects such as comb filters can be measured but not necessarily be perceived, provided that they are below established audibility thresholds:
Salomons (1995), “Coloration and binaural decoloration of sound due to reflections”, Thesis, Delft University
http://www.darenet.nl/en/page/repository.item/show?saharaIdentifier=tuddare:oai:tudelft.nl:200755
> I can definitely say that I don't like the 8dB suckout at 80Hz <
The same as above applies: it's not because you measure something that you also will perceive it. Considering the equal loudness curves an 8 dB suckout may not be audible at all.
Bücklein (1981), „The audibility of frequency response irregularities“, J. of Audio Eng. Soc., S.126
Robertson et al., "Threshold of Hearing and Equal-Loudness Relations for Pure Tones, and the Loudness Function", J. Acoust. Soc. Am. Volume 29, Issue 12, pp. 1284-1288 (December 1957)
I would use measurements as diagnostic tool AFTER having noticed a problem when listening to music. As long as you don't have a solid correlation between what you hear and what you measure measurements as such are of little use, IMHO.
Klaus
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