Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: rlyach on March 16, 2013, 10:47:09 AM
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Well, I finally gave up trying to compensate for the fact that my system is on a dresser, and less than 1 foot from the wall. When I play the music a little louder, I get distortion from the wall. I can't move the speakers any further out and I like to listen a little louder sometimes. I finally placed a 1" piece of foam behind each speaker and that helped tremendously. I finally broke out my miter box and router table and built a pair of acoustic panels to put behind the system. I am using 2" stone wool and acoustically transparent cloth from ATS acoustics. Here are the boxes. I am waiting for the insulation and cloth so I can finish the project.
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Hi, I personally made good experiences with stone and glasswool with a wooden bracing for stopping the sound to grow...!
chris
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I will suggest that everyone interested in acoustic treatment check out the cotton material manufactured by Bonded Logic. It is made from shredded t-shirts and blue jeans. The "acoustical" version is monopolized by one company and commands what I feel are inordinately high prices - because I used to buy the stuff wholesale. But the thermal insulation Bonded Logic makes is essentially the same stuff and can be had at green building supply stores. The material doesn't fill your skin with itchy glass fibers and it actually works better at low frequencies.
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Doc,
I wish I had done a little more research before I ordered my insulation. Here is the table of acoustic properties of different materials. The recycled cotton is a little better than the Roxul AFB that I ordered. Oh well. The next time I need to build panels I will rethink the insulation choice.
Randy
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One of the biggest payoffs for me has been with acoustic treatment/management. Glad that you will benefit too. Opens up the soundstage and depth, etc etc. as you already know.
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i second that - I have had great luck with absorption panels. I now use them on front and back walls. I would put them at an angle to speakers on side walls too. I just don't have the room for it in my small living room. Room treatment is definitely the best bang for the buck in this racket - unless you invest in shiny pebbles :)
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Can the back panel be tuned for a bass absorber?
Is there are way to fire proof the cotton batting?
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i second that - I have had great luck with absorption panels. I now use them on front and back walls. I would put them at an angle to speakers on side walls too. I just don't have the room for it in my small living room. Room treatment is definitely the best bang for the buck in this racket - unless you invest in shiny pebbles :)
You have shiny peebles? Wow, I only have Tibetian singing bowls arranged in a heart shape on the floor of my living room. It pleases the gods. But seriously, panels are great because they can easily be moved around to determine best placement. I once used a large, heavy quilt behind my speakers and the effect was dramatic. Then I tried it behind my listening position,,,equally dramatic. In the end I preferred it behind my speakers.
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VoltSecond,
First, the cotton batts are class A fire retardant. Other insulation is also fire retardant. Using panels to control bass requires different panels and insulation. From what I have read, bass traps are physically larger and many work best in corners. The best bass absorbers are at least 4" thick. If you look at the table in the previous post, 4" Corning 703 gives the best bass absorption. The 2" panels I am building only offer about half the low frequency absorption, but I also need to reduce some mid frequencies. I plan on putting these panels directly behind and on the wall beside the speakers. I experimented with 1" foam and it made such an improvement that I decided to build the panels.
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Randy,
You are probably familiar with the side wall reflection waves being able to be visually detected by placing a mirror at a point along the side wall where you can see your speakers from your listening position. This indicates the 'bouncing point' if you will, of the sound waves coming from the speakers and arriving as an unwanted wave at your listening position. Absorbers are most effective at those reflective points. And as I recall bass control devices work best, as you stated, in corners where the bass is concentrated. I look forward to your reported progress in this effort.
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4krow,
My situation is very non-standard. I do not have a dedicated listening room. I have placed my system in my bedroom in a corner on a dresser. I know this is not optimal but that is where I have it. The side panel is actually in the corner to get any reflected waves off the back that then are reflected off the side. I don't have room for a full corner trap so this is the best I could do. I tested the setup with 1 inch foam and the sound was much better. Additionally, there are window treatments that help to deaden the reflections off the wall, and the ceiling is vaulted. As you can see, I am limited in what I can do but I think you can see from the picture where I plan on putting the panels. If there is something else I could do I would appreciate any feedback.
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Randy,
Your ability to recreate the bedroom as a diagram is very helpful. Let me just say this, experimentation is an incredible tool. What seems odd may well work. I have had bedroom systems too, and they are a challenge. As I recall, lower volume helps disassociate some of the acoustic problems encountered in typical rooms. And if possible, near field listening is helpful. No doubt, you are a thinker, so try brainstorming ideas. You may well be surprised with the results
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I finally finished the panels. The sound improvement was very noticeable. The sound-stage is more defined. It is also more consistent over all listen levels. This is a very inexpensive improvement and very easy to build. Notice the pilot light under the 12AT7 :).
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Kudos, that looks very very nice!
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Yep room treatment is awesome. I have always wanted to build a diffuser.
http://www.flickr.com/photos/39812136@N08/3711143217/
Maybe my next project - You can make them out of styrofoam and so they can be pretty light. Maybe my next project.
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Yep room treatment is awesome. I have always wanted to build a diffuser.
http://www.flickr.com/photos/39812136@N08/3711143217/
Maybe my next project - You can make them out of styrofoam and so they can be pretty light. Maybe my next project.
Those definitely have a lower WAF, though.
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well, I am single and so do whatever I want - there are tradeoffs to that though!
Actually some of the diffusers look pretty artistic IMO, they can be done in wood and look quite nice.
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I have some data to report using the panels. The first thing I noticed was that the sound was much tighter. The second thing I discovered was that the system seemed louder. This was very subjective so I got out my R/S sound pressure meter. I tried two different songs, both had a large dynamic range. I tried both slow and fast readings. Without the panels I barely got 86db peaks. With the panels hung and nothing else changed I got 91db. I am not an expert in acoustics but is it possible that the sound coming out the back of the ported speakers, being 180 degrees out of phase with the front, is reflecting off the wall and canceling some of the sound in the room? This explanation sounds sketchy to me, but the wavelengths in question range from 267 inches at 50Hz to 0.7 inches at 20KHz. The speakers are 8 inches from the wall, which is 1673Hz. I can almost convince myself that frequencies up to about 5KHz or 6KHz will experience some destructive interference from the wall. In any even, I can't argue with the measurements. Plus, the sound is much, much better. I love this amp and the Orcas.
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You could gather some evidence to support or refute your hypothesis by tightly plugging the ports with a cork made of rubber or cork, then remeasuring.
I like your approach to this: listen, measure, postulate. Far too often we have lots of postulation, debatable listening, and no measurements, lol.
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Randy,
I am glad that you use your head in these matters. Sometimes, I find myself stabbing in the dark. Strangely, it works more often than it should, but not very scientific.
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PB,
The only problem with plugging the ports is that it will also change the sensitivity of the speakers. A sealed cabinet will not allow the speakers to move as freely. I just downloaded a spectrum analyzer app and will do a frequency spectrum of the room with and without and panels at the same volume level and the same microphone placement. Since I will be comparing two curves, I don't need the microphone to be calibrated exactly. A quick calibration will be sufficient. This is starting to get interesting.
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My guess would be that the reflection from the wall without the absorbers caused a cancellation in a frequency band where the music had a high level, and the reinforcements happened in bands where the level was lower.
Above the port resonance (probably below 100Hz) the signal from the port is small and inconsequential.
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Randy,
I also have the Orcas and would like to experiment with some similar panels.
Do you think leaving the wood frame exposed instead of covered in fabric would affect the effectiveness of the panels? I would just cover the insulation area and finish the frame.
Thank you
Joel in TN
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Joel,
Since the material is acoustically transparent, it should not make much difference if you leave the frame exposed or cover it with speaker cloth. the only trick is how to attach the insulation to the frame. With the frames covered, all you have to do is spray contact cement on the backing board and stick the insulation to it. The cloth will then hold the insulation in. If you use rigid insulation instead of flexible insulation it will hold it's shape better without the frame. You still have to think about how you want to attached it. Good Luck.
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Paul,
I think you hit the nail on the head. I spent some time and calibrated my microphone with my RS sound pressure meter. Then I obtained a 1/f frequency sweep audio file. I played the file at full volume with the FFT set on peak detect for 1000 samples. This gave the sweep time to complete. It was neat watching the graph progress as the frequency increased. The surprising thing for me is the bass response of the Orca's without a sub. The increased sound pressure with the panels was probably due to the 1KHz spike in the room at my listening position. The other thing I noticed is that there is less troughs at mid-range frequencies in the data with the panels in place. This is what improved the sound and also gave the added impression of volume. I now know why people build listening rooms. The sharp drop at 20KHz is probably due to the microphone. If I add a bass trap in the room I might flatten out the response more but I like the way it sounds now, especially since the Orcas only have 3 inch drivers.
Update: I think the negative slope of the graph is due to the fact that I used a 1/f power frequency sweep instead of a constant power frequency sweep. That little fact makes the data much better. I was warned not to do a constant power sweep because it might damage my speakers. I hope someone can set me straight here and make sense of the data.
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One last post. This is the FFT at normal listening levels. I do not listen at full volume. this looks much better :)
Update: The "normal" data was taken while listening to a song instead of using a 1/f power frequency sweep. I had the sampling set to peak detect at 1000 samples. Thus it shows the peak output over all frequencies for this particular song. I have also added a white noise response for consideration.
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A 1/f power spectrum (called "pink noise") gives equal power in each fractional-octave band - 1/3 octaves are widely used. White noise is equal power per Hz, such as what you get with an FFT. Because half the energy is in the 10kHz-20kHz band, it is rough on tweeters - which is not really a problem with single-driver systems!
Your white noise data is quite similar to what we measured last week in setting up the Stereomour/Orca system at Bottleheadquarters. The main difference is as you suggested in the highest octave; we measured a smoother and more gradual rolloff, but I can't vouch for our microphone either. Someday I'll get out my serious mic and give it a try.
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Paul,
I was suspicious of the white noise data so I imported the white noise file I had into audacity and did a spectral analysis on the source. It shows hard cliffs at 42 Hz and 10KHz. Additionally, the level is -36 dB. If I correct my white noise data for this file's limitation I get 96 db output at full power. The cliff at 10KHz is due to the source sound file and the cliff at 20KHz is due to the microphone filter (as shown by the frequency response curve for the internal microphone on my iPhone). The data between 10KHz and 20KHz is the background noise in my room. The low-end cut-off at 60 to 70 Hz is probably due to the Orcas. I will look around for a better white noise file to use.
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I finally found a high quality white noise file. Here is the response from the system with Orcas in my bedroom at my listening position. The source file had a -27.2dB level +/- .2dB. You can see there are some frequency traps in my room. Also, the lower frequency 3dB point is 75Hz. The Orcas have an amazing base response considering that they only have 3" drivers. Also, the drop above 20KHz is due to the filter on the microphone.
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As far as frequency response goes, I would think that this is more than acceptable. The bigger problem may be with getting a good soundstage. It is uncanny what a 3" driver can do, and they have an immediacy that I prefer due to their speed.
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Yes, I am very happy with the Orcas. I just realized that the white noise source file has no data below 86 Hz, so the bass response is probably not accurate, although the frequency sweep data seems to also point to a bass limit of around 75Hz. The Orcas have an amazing frequency range, and I am very glad I decided to go with the them.
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Have you tried a program like REW? It sounds like you have a mike. The program is free and had tis one built in test signals...John
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John,
I am using SignalScope by faberacoustical. It also has built in test tones. I am using this on my iPhone using the built-in microphone. You can also use an external mic as well. The iPhone mic is actually pretty good, although they have a 20KHz 3rd order filter on it so it drops off pretty quickly. You can also export either a graphical output file or a comma separated data file that can be directly imported into excel. Thanks for the reference to REW. I will take a look...
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Randy,
Now that you have a good source, how about adding a before to the above post so we can see side by side comparison of what the sound panel did?
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Grainger,
Here is the final comparison. I put the microphone on a stand at my ear position, and took data with and without the panels. The difference is definitely measurable, and in general the panels made the sound tighter and more uniform. The standard deviation is lower with the panels. The bass response is also lower with the panels. The mid-range frequencies up to 2k have more peaks without the panels. Above 2K the response is almost the same. Here are three graphs. One full range over the audio spectrum. One from 100 to 1K and one from 1K to 10k.
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It's confirmed. I definitely need to get/make some panels!