Acoustic Panels

[manis]

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.

[drewh1]

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.

[rlyach]

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.

[Paul Birkeland]

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.

[4krow]

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.

[rlyach]

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.

[Paul Joppa]

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.

[bainjs]

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

[rlyach]

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.

[rlyach]

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.

[rlyach]

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.

[Paul Joppa]

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.

[rlyach]

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.

[rlyach]

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.

[4krow]

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.