davewantsmoore Posted November 13, 2022 Posted November 13, 2022 1 hour ago, acg said: I do. Left channel is looped back as timing reference and right channel feeds the preamp, or the other way round, I can't remember. So, then how is there: "delay via the electronics and then the sound waves leaving the speaker and finding the microphone." This should be nullified by your loopback channel (if working correctly).
davewantsmoore Posted November 13, 2022 Posted November 13, 2022 56 minutes ago, acg said: I think it could do better at really loud volumes which I sometimes/ often do partake What is the reason to think that absorption is the problem/solution..... I honestly think it is rarely an issue even with low DI speakers.... let alone high and sitting close. "Uneven" OTOH.... this is usually a big issue (and sometimes only noted as "bad" when the SPL goes up) Anyways.... measurements can show you how even the decay times are.
acg Posted November 13, 2022 Author Posted November 13, 2022 53 minutes ago, davewantsmoore said: Anyways.... measurements can show you how even the decay times are. It does not take much volume to clip my microphone, a Dayton EMM-6, at least I think it clips as it gives some rubbish results sometimes, so I can't really test at what I would call high volumes. My experience is that different room effects become audible at different volumes. For example, one room I was familiar with would excite a resonance in an air-conditioning duct behind the listening chair which first became audible as a developing "tinny reverb" following the music then as something stronger and more obnoxious as the SPL rose. Keep the volume reasonable and everything sounded fine. In my room it may be simply some feedback from a vibrating floor through a vacuum tube or it might be some resonance somewhere else, even a reflection from the midrange horn from the top of the Upperbass Horn, for instance, or something similar. The pneumatics on the DSET amplifiers are still deflated because the amps are still open (and I don't want them wobbling helter skelter while trying to solder) so it may be a feedback happening there, and it predicitably comes on when I set the volume very high. It may also be none of those things. Most rooms sound shabby when things get proper loud
acg Posted November 13, 2022 Author Posted November 13, 2022 1 hour ago, davewantsmoore said: So, then how is there: "delay via the electronics and then the sound waves leaving the speaker and finding the microphone." This should be nullified by your loopback channel (if working correctly). I do not know where REW sets zero for the excess phase plot, perhaps it is higher in the frequency spectrum than that shown. Perhaps it is at the "quickest" frequencies to hit the microphone. If my channels are not time aligned (they are not) then it will show as different offsets along the excess phase graph according to the bandwidth of the channels and the deviations from being time aligned. So, looking at the black line in the graph below, the UB Horn (140Hz+) is 10mS behind some other channels higher in the frequency range and the Bass Cannons is 5mS behind the UB Horn...at least that is how I read that graph. This all sounds like it is not correct, because 5mS is about 1.7 metres, which is most of the distance between the Cannons and the listening chair where the measurement was taken...it could not be that far out.
davewantsmoore Posted November 13, 2022 Posted November 13, 2022 2 hours ago, acg said: It does not take much volume to clip my microphone, a Dayton EMM-6, at least I think it clips as it gives some rubbish results sometimes, so I can't really test at what I would call high volumes. <shrug> it can supposedly do ~ 127dB without clipping..... but I wouldn't be looking at room issues at high SPL.... the underlying room issues are not that SPL dependant. OTOH, if you mean room rattles, etc.... then fair enough. 2 hours ago, acg said: My experience is that different room effects become audible at different volumes. That's true... but the decay times, etc. will measure quite similar vs SPL.... so you do not need to test them at extreme SPL. 2 hours ago, acg said: For example, one room I was familiar with would excite a resonance in an air-conditioning duct behind the listening chair which first became audible as a developing "tinny reverb" following the music then as something stronger and more obnoxious as the SPL rose. Keep the volume reasonable and everything sounded fine. Yes, in that case, fair enough.... but if you are testing for things which would (potentially) need absorption in the room to fix (like what we were discussing) then you don't need extreme SPL for this. 1
davewantsmoore Posted November 13, 2022 Posted November 13, 2022 1 hour ago, acg said: If my channels are not time aligned (they are not) then Then you should not test them playing together (for anything to do with timing). 1 hour ago, acg said: according to the bandwidth of the channels and the deviations from being time aligned. So, looking at the black line in the graph below, the UB Horn (140Hz+) is 10mS behind some other channels higher in the frequency range and the Bass Cannons is 5mS behind the UB Horn...at least that is how I read that graph. This all sounds like it is not correct, because 5mS is about 1.7 metres, which is most of the distance between the Cannons and the listening chair where the measurement was taken...it could not be that far out. You also need to account for the effect of your filters (?!) Either way.... if they're not time aligned, don't play them together and expect correct timing information (or even correct amplitude information, for the most part).
acg Posted November 19, 2022 Author Posted November 19, 2022 (edited) Brought 2 VPR's into the listening room today. They are heavy and awkward to move so I rigged up the winch to the beam running at the peak of the ceiling and lifted them into the room before closing the trapdoor beneath them and dragging them into place. The 1.8m x 1.2m traps are in the room. Tomorrow I'll bring in the 2.2m x 1.2m traps. That winch is soooo handy not only to lift the amplifiers off their mounts for maintenance but now to bring the room treatments into the room. The VPR's are giving what I think are spectacular results. On another forum I read about consultant using these to straddle corners which is not really their use-case. They were designed to be unobtrusive in the room, to sit on the walls, and to give performance equivalent to a LOT of soft and fluffy foam in that position. I can confirm that they do just what they are supposed to do as two of these behind the speakers standing against the wall do pretty much the same job as making a triangle of soft foam (a Superchunk) of equivalent dimensions. In my case two 1.2m wide VPR's against the wall gave a little better performance than a single Superchunk 1m x 1m x 1.4m and 2.2m high. But, similar to the aforementioned consultant posting on another forum, a single VPR straddling the corner does something special... Look at that ^^^...about 10dB SPL change circa 65Hz and up to 15dB at about 90Hz. That is huge! It has also milked a dB or three from the 39Hz and 56Hz modes even though they are not really the proper target of a trap in the front corners. When I straddled the other front corner with a second 1.8m x 1.2m VPR that 48Hz tangential trough lifted another 3dB, which is very promising. Tomorrow I will put some VPR's on the front wall to see what can be achieved with the 39Hz and 79Hz modes and also on the side walls to attack the 84Hz axial mode. With any luck these positions will also show promise with the tangential modes. Exciting! Edited November 19, 2022 by acg 7 1
acg Posted November 20, 2022 Author Posted November 20, 2022 Today, the remaining two VPR's were winched into the room. The pair already up are 1.8m x 1.2m and these additions are 2.2m x 1.2m (because the south wall is 2.2m high to the ceiling). Ideally, these 2.2m tall VPR's would be 1.5m wide to maintain the 1:1.5 side-length ratio, but I wanted to see just how they may work with dimensions that fit the south wall. Turns out they are ok and I cannot discern any difference in performance to the 1.8m tall panels so I cannot see any reason to cut them down to 1.8m tall. So, four VPR's all up, the two 1.8m tall fellows straddling the corners of the front wall and the 2.2m tall panels shifted from midway along the front wall to all parts of the east wall. Unfortunately, no more off-the-charts performance from them as was gianed yesterday by straddling corners, but nonetheless some solid gains that are much appreciated. See below for the final measurement of the day with two 1.8m panels straddling the front corner and two 2.2m panels on the left side-wall. Note that only the right speaker is making noise so these taller panels are effectively on the far side-wall. A couple of dB have been grazed from the top of the 40Hz reinforcement and 5dB of the 48Hz trough has been filled...I think these are pretty big numbers given the frequencies involved...48Hz is now twice as loud. So, what have I learned? Wherever I put these two new panels in the room they each knocked a dB or two off 40Hz and added a dB or two to 48Hz...it did not really matter where on which wall I put them the result was similar...so to get an even better result I am probably going to need brute force i.e. more panels. These have only been trialled with the 2mm steel panel as resonator because theoretically it will be more effective for the lower frequencies and given these results I have no compulsion to reset them such that the 1mm panel is the resonator as it is too much work which I think I will have to do twice (to set them back as they are now). I can build another five VPR's for the walls, and already three are underway for the ceiling: that totals a neat dozen in the room...potentially. With any luck the SPL delta from 39Hz - 48Hz - 50Hz wil be greatly reduced, likewise that dip in the 90's. Should things pan out that way I will need to call the Bass Cannons my midbass channel and implement a couple of true subwoofers to fill up 20Hz-40Hz. 7
tripitaka Posted November 20, 2022 Posted November 20, 2022 I've followed this amazing build from early on and I'm starting to feel exhausted by all the work involved, how are you holding up?!
acg Posted November 20, 2022 Author Posted November 20, 2022 @tripitakabit knackered right now...been a big couple of days litfting and shifting heavy, awkward things. Plus the humidity has kicked in yesterday to make things particularly sweaty. The bigger VPR's weigh about 60kg each. Five days to recover then back into it next weekend. Orders will be made this week to get the steel cut and folded for the remaining VPR's, some more glue and screws plus design some brackets to hold them to the walls and ceilings. Assuming a good run at the Christmas break the room may nearly be more or less done sometime about the end of December. Then I have to run my attention to diffusers and making it all look great. Thankfully I'm at the pointy end of things now, not far to go and the sound is already fantastic and all the changes I make lately seems to incrementally improve the sound and not in small ways. 3
The Mad Scientist Posted December 7, 2022 Posted December 7, 2022 On 23/12/2014 at 1:57 AM, acg said: All excitedly I laid these out and explained to my darling wife what they meant and all she could say is "that's too big". I smiled, gave her a kiss and retorted "that's too late". Hehehe. This is golden! Not sure it would play well with Mrs. Mad Scientist, though. My current wide-baffle 3-way construction is on the edge of WAF...... They might fit well in a batchelor bedsit though 1
acg Posted December 7, 2022 Author Posted December 7, 2022 @The Mad ScientistGraeme, yes, not WAF at all, but she does bring all her friends up here to gawk and listen, as do my kids. Not really a lounge room system for sure... 2
davewantsmoore Posted December 10, 2022 Posted December 10, 2022 On 8/12/2022 at 10:39 AM, acg said: too late Heh. That's the same tactic I used. It was the only approach that would have ever possibly worked. 1
acg Posted February 26, 2023 Author Posted February 26, 2023 So, subwoofers. (long post...sorry) I've managed to put 10 VPR panels up into the room and affix them to the wall. Will take some measurements at some stage but the sound is not great as...surprise surprise...when you line the walls with steel panels a lot of top end energy stays in the room. Bass sounds magnificent with so much resolution and texture but the mids and highs are quite forward now. This was to be expected and the idea is to skin the VPR's with whatever foam etc. is necessary to bring in an even reverb time for the room. Like I said earlier, no measurements yet, but I have been thinking subwoofers and even dusted off Hornresp to model some scenarios with various drivers. Current thinking is for a stereo pair that go behind me underneath the vinyl racks. There are five racks currently full of soft n fluffy batts acting as bass traps each circa 700mm tall x 1200mm long x 400mm deep. The Hornresp modelling was interesting in that it has really brought home a few things to look for in subwoofer drivers namely I don't need sensitivity really at all but I do need lots and lots of cone area if I want to use a SET amp for the channel. High sensitivity is actually a hindrance for something that I expect to sign off at about 40Hz because no driver I've found goes flat to 20Hz without some EQ or room gain and the high sensitivity is generally relevant at 100Hz+ which I have to filter out anyway. A low sensitivity driver actually works better here because it is much lower SPL at 100Hz than the high sensitivity driver, but they are similar output at 20Hz...within reason of course. There is no replacement for displacement. My entire system is based on miniscule excursion and the new subs will be no different. Basically I need massive cone area, prefer low sensitivity, it needs to work in 250-300 litres sealed box which gives as high as possible electrical load to the amplifier (because I do not want to have to commission another giant bass OPT that I no longer have room for in the amplifier chassis). I've got 16 watts to play with and as the plan currently sits the sub will sit in 1 pi space (on the floor against the wall). The contenders thus far: Stereo Integrity HS-24: A 24" driver from the US. I think I can make this fit where it needs to fit. Available cabinet size is perfect, is low sensitivity so has low output in the filter band so may get away with a lower order electrical filter than the other options. Box Fs = 35Hz, Ze 6r at 20Hz, 36r at 35Hz, 6.5r at 40Hz At 1 watt/20Hz: 92.5dB with 0.9mm excursion At 7w/20Hz: 108dB with 6mm excursion At 16 watts/20Hz: 116dB with 14mm excursion Dayton Kraken 21": Two of these wired in parallel. The pair can certainly fit in the available cabinet size. Box Fs = 35Hz, Ze 4.1r at 20Hz, 44r at 38Hz, 32rr at 40Hz At 1 watt/20Hz: 92dB with 0.6mm excursion At 7w/20Hz: 108dB with 4.2mm excursion At 16 watts/20Hz: 116dB with 9.5mm excursion Vitavox K15/40: Three of these wired in parallel. Can fit in the available cabinet size...just. I already have these drivers. Not really sub drivers though. Box Fs = 54Hz, Ze 4.5r at 20Hz, 9.1r at 40Hz, 25r at 54Hz At 1 watt/20Hz: 95dB with 0.6mm excursion At 4w/20Hz: 108dB with 2.6mm excursion At 16 watts/20Hz: 119dB with 10.6mm excursion On the face of the numbers above the Vitavox trio of 15"ers seems to be a winner due to the relatively even impedance in the passband, the extra 3dB of output and very low excursion. However, at 80Hz they are 4dB louder than the Kraken pair and 11 dB louder than the HS-24 which means the electrical low pass filter has more work to do for the same frequency response and may need to be one or two orders higher which could be an issue. Unfiltered, both the HS-24 and the Kraken pair are 20dB below their 20Hz SPL at about 1kHz, whereas the Vitavox trio is at 5kHz-6KHz. Also, the Vitavox trio don't have 10mm of excursion to give so will likely distort quite quickly as the SPL increases. But how loud do they need to go? To put 105dB at 20Hz at the listening chair they will need (according to Hornresp and my DATs measured driver parameters) about 4 watts of amplifier power which puts the modelled excursion at 2.6mm...which is probably quite ok. The other drivers have much larger linear excursion (the Kraken pair is 21mm xmax and needs 7w at 4.2mm excursion for 105dB at the chair while the HS-24 is 39mm one-way and also needs about 7w at 6mm excursion for the same output) so are possibly going to distort less than the Vitavox trio at that 105dB SPL, and certainly much less than the Vitavox trio at higher volumes. An older version of the HS-24 was measured in a larger sealed box with remarkably low distortion at 20Hz at high output (see here)...<3% distortion at 20Hz 110dB which is super competitive even with the best bandpass and tapped horns. Sensitivity is lower than the other two contenders which means out of band SPL is much lower and a lower order electrical filter can be used (i.e. fewer caps in the signal path). The HS-24 as modelled will also present a slightly higher load to the amplifier which is an advantage when designing an output transformer for the SET amplifer that runs the channel. 1
jamiebosco Posted February 26, 2023 Posted February 26, 2023 2 hours ago, acg said: So, subwoofers. (long post...sorry) The contenders thus far: Stereo Integrity HS-24: A 24" driver from the US. I think I can make this fit where it needs to fit. Available cabinet size is perfect, is low sensitivity so has low output in the filter band so may get away with a lower order electrical filter than the other options. Box Fs = 35Hz, Ze 6r at 20Hz, 36r at 35Hz, 6.5r at 40Hz At 1 watt/20Hz: 92.5dB with 0.9mm excursion At 7w/20Hz: 108dB with 6mm excursion At 16 watts/20Hz: 116dB with 14mm excursion Dayton Kraken 21": Two of these wired in parallel. The pair can certainly fit in the available cabinet size. Box Fs = 35Hz, Ze 4.1r at 20Hz, 44r at 38Hz, 32rr at 40Hz At 1 watt/20Hz: 92dB with 0.6mm excursion At 7w/20Hz: 108dB with 4.2mm excursion At 16 watts/20Hz: 116dB with 9.5mm excursion Vitavox K15/40: Three of these wired in parallel. Can fit in the available cabinet size...just. I already have these drivers. Not really sub drivers though. Box Fs = 54Hz, Ze 4.5r at 20Hz, 9.1r at 40Hz, 25r at 54Hz At 1 watt/20Hz: 95dB with 0.6mm excursion At 4w/20Hz: 108dB with 2.6mm excursion At 16 watts/20Hz: 119dB with 10.6mm excursion Hi ACG Awesome post, looking forward to seeing your results with the VPRs! I'm not the smartest tool in the shed,but can you explain how you are getting the calculated SPL numbers? I thought a doubling of power only netted +3dB?
davewantsmoore Posted February 26, 2023 Posted February 26, 2023 4 hours ago, acg said: On the face of the numbers above the Vitavox trio of 15"ers seems to be a winner due to the relatively even impedance in the passband But this is becuase you are operating it below it's Fc (system resonance). I would expect that the compliance of this driver is pretty nonlinear.
acg Posted February 26, 2023 Author Posted February 26, 2023 3 hours ago, jamiebosco said: I'm not the smartest tool in the shed,but can you explain how you are getting the calculated SPL numbers? I thought a doubling of power only netted +3dB? No claims here to be a rocket surgeon either...ha! Now that you point it out I have probably muffed the amplifier voltage calculations in Hornresp...will go have a look 1
acg Posted February 26, 2023 Author Posted February 26, 2023 (edited) So, I've got 10 VPR's in the room and up on the wall. Crappy phone photos...should have turned on the light...mind the mess... They look hideous up on the wall but will be skinned soon to remediate. The SPL graph just goes to show that you cannot easily beat the room until you pretty much suck all of the energy from it. What you start with is going to be similar to what you end with. The bass SPL response looks worse now than when I started... ...but I am yet to put up the VPR's that straddle the corners behind the speakers. In early testing it was the straddled trap that really Kong'd that 40-100Hz region. I would prefer not to have the straddled traps but by the look of this I am going to have to at least try them. Bass sounds better now because the reverb times have dropped markedly...so much clarity and texture...but I do think there is more to achieve when the front wall corners are straddled and I trial some ceiling VPR's. Been some major change in the spectrograms...look at that change in the bass much of which now has shorter reverb time than the mids...overall it does not sound great to be honest, but that could also be due in part to the rampant specular reflections of the steel sheets on the front of the VPR's and to the unusual decay times (i.e. bass shorter than midrange). That upward curve in the RT60 from 300Hz up makes the sound very forward and "bright"... As an experiment so that I can get some VPR skins designed and made I brought up seven Basotect G+ "skins" (a different kind of skin...these are throw aways when they cut into a brand new block of G+ so they are literally the G+ skin) and leant them again the far wall as shown below... ...and as you can see above they did so nice work to flatten the RT60/reverb time in the frequency range of interest. I also played around with the Basotect G+ skins in varying thicknesses on the ceiling and behind the listening position and have identified a few of those modes seen on the spectrogram in the 200Hz-600Hz range so I reckon 'll be able to cancel most of them when the time comes. I was hoping for -60dB to happen at 250ms or maybe even a little earlier down to 60Hz. At the moment I'm confident that I can get -60dB at 200ms down to 100Hz and remain hopeful that the corner straddled VPR's will further tidy up into that 60Hz territory. Then it will be on to ceiling absorption. For side-wall VPR skins I would like to add some diffusion. The skin cannot be thick or I'll never get up the stairs to am really limited to BAD or MLS diffusion. The easiest would be to forego the diffusion and use some architectural acoustic wall panelling such as this... Absorption is in the correct range for my room... ...but I am worried about specular reflections. If only the boards could be adjusted for a MLS sequence. Should have samples here this week. Edited February 26, 2023 by acg 1
cdave Posted February 27, 2023 Posted February 27, 2023 20 hours ago, acg said: The easiest would be to forego the diffusion and use some architectural acoustic wall panelling My 2 cents. Looks fantastic, and if this was your lounge room, absolutely! However, given this is a dedicated listening space and your herculean efforts so far.... With the dramatic changes diffusion can make, It feels like putting road tyres on a race car. 1
almikel Posted February 27, 2023 Posted February 27, 2023 On 26/02/2023 at 6:39 PM, acg said: The easiest would be to forego the diffusion and use some architectural acoustic wall panelling such as this... In a DIY context, why wouldn't you just add the slats in a 1D BAD pattern so some diffusion is achieved? Mike
cdave Posted February 28, 2023 Posted February 28, 2023 Darko's latest video on treating his room. https://youtu.be/lVfxxUEhmnQ Note 3 different types of diffusers flanked by absorbers. 1
almikel Posted March 2, 2023 Posted March 2, 2023 On 26/02/2023 at 11:47 AM, acg said: So, subwoofers. one of my favourite topics! On 26/02/2023 at 11:47 AM, acg said: no driver I've found goes flat to 20Hz without some EQ or room gain Have you looked at the Acoustic Elegance IB drivers? Being designed for Infinite Baffle they end up with a Qtc >0.707 with a sealed box of ~300 litres. My 1st goto simple tool for designing boxes for drivers doesn't include driver excursion or amp power modelling, but: AE IB15HT in a 300 litre box, F3 is 19Hz, Qtc 1.10 (ie some peaking prior to rolloff) Being designed for IB the box size to achieve a Qtc of 0.707 is ridiculous (22K litres) But a 600 litre box would get you an F3 of 17.24Hz and a Qtc of 0.92 (still some peaking prior to roll off) When I first discovered Acoustic Elegance (AE) through Stereonet over a decade ago (thanks @Paul Spencer), they only had the IB15 - it was the 1st driver I'd found that could hit 20Hz in a 240 litre box without EQ. Acoustic Elegance now have a bunch of drivers in their IB line-up (and the specs have changed a bit) - 12", 15", 18" in both Automotive and Home Theatre options. A summary of their automotive (AU) and home theatre (HT) IB models in 15" and 18" below: I ended up heading down a different path and using AE TD18s as my mid bass below PSE144 horns, with a tapped horn sub underneath the TD18s - using DEQX crossovers and digital EQ... ...so I've never listened to the Acoustic Elegance IB drivers...but my AE TD18s are truly sensational at mid bass - Acoustic Elegance make fabulous drivers! You're welcome to listen to my setup anytime you're in Brisbane! cheers, Mike 1
almikel Posted March 2, 2023 Posted March 2, 2023 (edited) On 01/03/2023 at 9:10 AM, cdave said: Darko's latest video on treating his room. https://youtu.be/lVfxxUEhmnQ Note 3 different types of diffusers flanked by absorbers. nice if you can afford it!!! $5K Euros - nearly AUD$8K of treatment! From a DIY perspective: Cinema Round Premium >300Hz it has an absorption co-efficient of 1 - the curve would provide a minor amount of diffusion - likely less than slats in a 1D pattern over 150mm of fluffy absorption Vic Strip >300Hz has an absorption co-efficient of 1 - no proper diffusion, only scattering - why didn't they use a 1D BAD pattern? because they'd have to pay RPG a license fee. For DIY replace the regular pattern with a 1D BAD pattern to add some diffusion Super Bass Extreme Ultra VMT in the corners - no results under 100Hz - just straddle corners with DIY Polymax XHD as wide as you can get away with (min 600mm wide and 100mm deep - wider and deeper is better) Wavewood ultralight on the back wall - different take on a 1D BAD - just DIY a 1D BAD over absorption. I love the clap test - don't get me wrong - the clap test is relevant for higher frequencies such as slap echo. Here's another youtube for the installation of Primacoustic traps: What I was looking for but couldn't find was an old video on the difference a Primacoustic London 12 kit could make in a room on the "clap test", which was way less expensive than $8K... ...but managing slap echo is trivial compared to managing <150Hz "in room" bass bouncing around, which is @acg's goal. I totally agree with Darko that the $ spent on room treatment has a way bigger impact than the same $ spent on any other upgrades such as electronics/cables etc.... ...but IMHO room treatment should focus on getting the room's bass under control, whilst trying to not make the room too dead in the top end. Once the room's bass is under control, further treatment is icing on the cake... @acg is deploying VPRs, which conveniently reflect treble back into the room whilst absorbing room bass. Mike Edited March 2, 2023 by almikel clarification 3
acg Posted March 23, 2023 Author Posted March 23, 2023 On 02/03/2023 at 11:55 PM, almikel said: nice if you can afford it!!! $5K Euros - nearly AUD$8K of treatment! Well, the VPR's have cost more than that so far, but I have built 15 of them, not that all of them are yet in the room. Mike, thanks for your earlier posts. I've been crazy busy, got the big job out at the end of last week and then promptly caught the flu (not the spicy flu) on Monday and it has knocked me about a bit this week. Just getting active again now but I did find and model those drivers. Talking about VPR's, I put two more up into the room last Sunday straddling the front corners. I had hoped to not have to use the corner VPR's but there is more bass reverb that needs killing. Managed to measure the room just now and got a shock at how bad the frequency response has become...look at this... ...that is a MASSIVE suckout from say 40Hz to 120Hz. At first I did not believe the measurement, but I set REW to output 72Hz and moved the mic about the room. 90dB at the speaker, 54dB at the listening chair, much, much closer to 90dB pretty much everywhere else in the room. Moved the mic from the sweet spot to the floor and it gradually increased to the high 80's and same at the ceiling above the listening chair. I'm sitting right in a massive floor to ceiling modal null. This made me realise how much work the wall VPR's are doing in this frequency range given that this null is now so prevalent...like the other 72Hz reverberations that were interfering with this floor-ceiling mode have been sufficiently dissipated for this last one to take centre stage and ruin the performance. Of course I've planned for this and have three more VPR's downstairs that I have designed especially for ceiling duties and it seems I need to get them up as soon as possible. While I had everything set up and the mic with a long cord so I could roam the room, I went to the new VPR's that straddle the front corners and with the 72Hz playing 90dB at the speakers the mic read 84dB at the VPR face and with the back of my finger I could feel the steel panel resonating soaking up so much of that energy. Tried the same with other panels in the room and they could also be felt but to a lesser degree, so corner straddling is still "where it's at".. I've always had a problem with 30Hz in my room. The room resonance would build up so much pressure at the left and right sidewalls that the energy would transfer directly to the cartridge on the turntable. Not through the turntable, or through the wall/floor/turntable but directly via air from the speaker to the cart...rare...and it would set up a nice feedback loop with the cart being plucked at 30Hz then playing that 30Hz again through the speakers and so on and so forth until it got VERY loud. The plan has been to put the TT in a sealed box so the direct air interference was sufficiently attenuated to not cause the issue. Anyway, with that long leadup I set the REW generator right on that 30Hz mode and put 90dB at the speakers and instead of measuring about 110dB at the side walls like it used to it now measured 84dB. That is a miracle as it is proof that my VPR's are effective at 30Hz and yes I can feel them vibrating with the back of my finger...absolutely sterling! The TT is not in the room while I'm doing the room treatments so I don't actually know that the 30Hz feedback problem is solved, but I am 99% sure the problems is gone. I might still put the TT in a box though as it is a nice way to keep dust at bay.
acg Posted March 23, 2023 Author Posted March 23, 2023 On 27/02/2023 at 9:41 PM, almikel said: In a DIY context, why wouldn't you just add the slats in a 1D BAD pattern so some diffusion is achieved? Mike Mike, Ive done a bit more research into the BAD panels and they only start diffusing from about 2kHz. Research indicates better sound quality results when diffusion is circa 500Hz to 5kHz with high frequency diffusion not necessarily sounding great nor diffusion into the bass frequencies. Even got a quote for RPG BAD wall panels but that was definitely out of reach. My speakers are very narrow dispersion and thus don't engage the room nearly as much as conventional speakers (high ratio of direct to reflected sound) so I think the first thing to sort out after bass are the early reflections within the room which I can decide individually to absorb or diffuse. There will be diffusion in my room, but I'm not yet sure just where and how much. Also read some more recent research on traditional QRD diffusion that used the equivalent of the "spin-o-rama" to measure scattering and diffusion which really mapped out how non-neutral that form of diffusion can be...hence how it is often described to have a "sound". This has made me to not want to build my own diffusors instead relying on newer, more neutral QRD variants and/or the scattering/reflector types. 1
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