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Old 2nd June 2012, 02:31 PM   #71
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Quote:
Originally Posted by puppet View Post
How do SPL and reflection intensity interact?

I've notice that low to medium SPL/reflections don't impart a confused sound whereas very high SPL/reflections do.
There is a reason for this sensation. Yes, there is a relation to SPL, but its easier to ignore this for a moment to make a point - lets say there is a 'reference' listening level where muddiness happens, and we are not going to lower the listening level to solve the issue.

There are two different reasons - one for small rooms the other for large concert halls. First there is a image forming time span, Haas interval. Reflections don't change the apparent source position that is established by the direct signal - law of first reflection. This doesn't mean that high gain reflections are ignored! They can cause image smear, tonality errors, loss of intelligibility etc. Apart from flutter, usually this is the main reason a small room can sound muddy and .. small. (aren't generalizations nice! Lets ignore modal activity; a 150Hz highpassed signal can also sound muddy)
This is one reason why some control rooms (critical listening rooms) are designed to have an effectively anechoic response to ~20ms. This gives the brain enough time to form a 'unconfused' image while a strong reflection ending this period gives a nice spacious sensation to the presentation without giving the 'confused sound' effect.
Muddiness in a large space is found to come from reflections arriving around 50-80ms. Thats why some of the best halls have dual slope decay - strong lateral reflections beginning ~20ms, fast(er) decay until 120ms, then a slowly decaying reverb giving good envelopment.

In small rooms and large rooms alike when a 'ff' moment comes in music there are 2 possibilities - either confusion/muddiness/fatigue or the sound 'magically' envelopes the listener. This doesn't explain why there is a level dependent threshold, but it is good to know the effect exists so that it can be 'engineered' around.
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Old 2nd June 2012, 03:24 PM   #72
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Originally Posted by markus76 View Post
Well, physics stays the same regardless of SPL. What changes is our perception. There are little muscles in our ear that might have a bigger impact on sound perception than commonly thought.
It's definitely a perceptual change with SPL, not something else.

The better you become as a "trained" listener, the more you start to notice the changes in your perception of sound which occur for everyone, but most people don't notice.

I often notice that my system sounds different from day to day, (since I'm always a critical listener, even when I just want to relax and enjoy the music...) and it used to drive me around the bend trying to figure out what was changing, until after lots of critical measuring and testing of equipment, checking connections and so on, I realised nothing in the system was changing, only my perception was changing

I notice changes in apparent tonal balance between too much bass and not enough bass from one day to another, changes in imaging between immersive and engrossing one day to somewhat flat and disappointing a few days later, changes in apparent room liveness from moderately well damped one day to overly reverberant another day, even changes in perceived volume - where the volume setting I'm using on the same music will be quite a lot higher or lower to achieve the same perceived "optimal" listening level...

Some factors that I've been able to correlate include time of day, (things seem louder to me early in the morning or late at night, making me set the volume level lower subconsciously) whether I'm tired or alert, (it always sounds worse when I'm tired) whether I've exercised / worked out in the last few days, (sounds much better if I have) mental state - happy and invigorated, or depressed - sounds worse if I'm feeling depressed, whether I've spent most of the day in a quiet serene place like staying at home indoors, or whether I've been subject to loud traffic/city/work noises all day.

All these (and probably more) affect my perception, so I've learnt that when I sit down and switch on some familiar music and it sounds a bit flat and un-involving not to go looking for whats wrong with the stereo and just accept that its not a good time for me to listen. Likewise I'll sometimes switch it on and listen and it can be an amazing experience where I'll listen for hours and can't stop because it sounds so good...

This is why as much as I place importance on how a speaker sounds, I never go by listening alone, I'll always back it up by measurements, and any changes to the response that I'm testing I'll typically evaluate over several weeks, to even out the daily "fluctuations" in my perception. Otherwise you just end up going around in circles tweaking never making any progress.

In regards to the original question about the effects of reverberation at different SPL - as I mentioned above I perceive not only differences in room reverberation on different days, but also different required volume levels for "normal" playback, and I think the different playback levels that seem "normal" on different days directly result in the difference in perceived room reverberation, and probably some of the perceived difference in tonal balance in the bass too.

Ears are very acute measuring devices in some ways but they are certainly not to be trusted on their own if you want any sort of consistency...they constantly go in and out of "calibration"
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Last edited by DBMandrake; 2nd June 2012 at 03:30 PM.
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Old 2nd June 2012, 04:24 PM   #73
puppet is offline puppet  United States
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^ .. thanks Simon. Now I know what to call each of the three presets I have on the DSP.

:P
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Old 2nd June 2012, 04:46 PM   #74
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Originally Posted by DBMandrake View Post
It's definitely a perceptual change with SPL, not something else.
Yep, it always amazes me how inaudible the acoustic reflex really is.

Last edited by markus76; 2nd June 2012 at 05:03 PM.
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Old 4th September 2012, 03:49 AM   #75
boris81 is offline boris81  United States
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Default OB amplification trouble

I am still working on this project and have some promising updates. I got into big trouble in the beginning of the summer when I realized that I need a huge amount of power to compensate the dipole loss in the woofer section. I had to reconsider my requirements or abandon the project.

After some thought I decided that a single channel of amplification for the 4 woofers is unrealistic. I added another amplifier channel to the woofer section and now each pair of woofers are driven in parallel by their own amp.

speaker-channels.JPG

The speaker requires 8 channels of amplification total (+ an optional subwoofer). I am planning to use a high-end class D 7.1 receiver which has 7x110W channels. Additionally I plan to use a pair of miniDSP modules which will have 8 channels as well and that setup won't allow for an additional subwoofer.

I decided it will be best to convert the Mid-Tweeter crossover into a passive one. The speaker will be a hybrid passive-active 3-amped one.

I ran some simulations and I don't think that a phase-accurate system can accomplished with more than 1 le'Cleach crossovers. My initial idea of a phase-accurate system with IIR filters was probably unrealistic. At this point I'm looking into a passive acoustic 2nd order Linkwitz-Riley on the Midwoofer and Tweeter at around 2kHz, then an active 2nd order LR on the Mid and the Woofer array somewhere into the 200-400Hz region. As I mentioned earlier the woofer array will be driven by 2 amplifier channels.

The overall sensitivity should be 85-88dB @ 1m, not bad for OB.

I've been listening to the prototype and getting very excited. The bass remains very even throughout the room, it's quite a contrast from my boxed speaker.
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Old 6th September 2012, 10:21 AM   #76
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Very interesting project Boris - keep us posted!
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Old 7th September 2012, 05:34 AM   #77
boris81 is offline boris81  United States
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Default bass measurements

I have some measurements of the speaker outside.
speaker.jpg

Here are the unequalized drivers on the baffle.
woofer array = blue
midwoofer = red
tweeter = green
raw-w-m-t.png


I want to equalize the speaker along the line denoted by -30dB in this graph. The woofer array is at that level around 400Hz and everything below is reduced at rate of 6dB/octave that is characteristic of open baffles. In order to extend down to 50Hz I would need to amplify by 18dB.

While that's doable, I tried it and I did not like the dynamics. I decided to dedicate 2 amplifier channels to the woofer array which will boost their output by 6dB. I'm also experimenting with small side wings which can boost the useful signal by another 3dB.
woofer array naked = red
woofer array with small wings = green
out3-rNaked-gSmallWings.png


Wings introduce a problem however that is visible in the phase plot above. The top woofer pair reaches it's dipole peak as the bottom pair goes into a dipole dip.
woofer array with small wings combined = blue
woofer array with small wings top pair = red
woofer array with small wings bottom pair = green
out3-bBoth- rTop-gBottom.png


I believe to have solved the problem by equalizing the peak on the bottom pair and thus improving the phase tracking. I took measurements of the equalized response at different distances.
equalized woofer array with small wings @1.5m = red
equalized woofer array with small wings @2m = blue
out3-Weq-b20-r15.png


I think it looks very useful. I'm not sure yet what the woofers-to-mid crossover will be yet. I can probably pull off a 2nd order @ 200Hz or a 4th order at 400Hz. Below is a 180 degrees frequency response of the woofer array.
unequalized woofer array on a naked baffle
out-bass-naked.jpg

equalized woofer array with small wings
out-bass-smWings.png

Last edited by boris81; 7th September 2012 at 05:38 AM.
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Old 7th September 2012, 09:21 AM   #78
Rudolf is online now Rudolf  Germany
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Did you equalise the woofer array by applying some bandpass filters? Your last diagram looks like that.
I've learned from SL and MJK that dipole woofers should be EQed by applying a 6 dB low pass at the point where the 6 dB dipole roll-off starts:
boris81.gif
For your woofer array on a naked baffle this point would be 60 Hz.

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Old 7th September 2012, 01:17 PM   #79
puppet is offline puppet  United States
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boris81 ... could you explain what's going on with the two different baffle arrangements you have depicted? I noticed the first photo shows the baffle as a solid 1-1/2" ... second photo (with the shortened wings) seems to have a space between the 1-1/2" sandwich ... with an insulation? wrap around the drivers?

I'd listen to the speaker playing music (in room) before settling on a xo point ..(your 200 vs 400hz proposal). While the response might look good on paper ... it might not sound right. This area, to me, seems to be pretty critical for getting vocals to sound natural. ... and there is the room resonance that might show up in the 300hz range. How severe that is will determine how much of a notch a guy needs to implement.

Because of the way my system is developing and the rooms 310hz resonance (pretty strong) , I chose to deal with that notch in the mid/woofer. ... but my bass woofers are separate from the mains ... so the xo is low to begin with. Anyway ... might be something to consider.
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Old 7th September 2012, 06:20 PM   #80
boris81 is offline boris81  United States
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Default Woofer EQ

Quote:
Originally Posted by Rudolf View Post
Did you equalise the woofer array by applying some bandpass filters? Your last diagram looks like that.
I've learned from SL and MJK that dipole woofers should be EQed by applying a 6 dB low pass at the point where the 6 dB dipole roll-off starts:
Attachment 300111
For your woofer array on a naked baffle this point would be 60 Hz.

Rudolf
I should not have put all those different graphs in one post, I'm getting confused looking at it myself. The 2 directivity plots don't belong together because the top one is of a Naked baffle and the bottom one is of an Equalized Baffle with Small Wings. It's a different baffle.

You are correct, the equalization of the Naked baffle should be done with a 6dB/oct low pass.

On the Baffle with the Small Wings the trouble is that the top woofer pair has it's dipole peak at a different frequency than the bottom pair. I equalized the signal by attenuating the dipole peak on each woofer pair individually.
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