Room Acoustics - Understanding the Measurements - Next Steps

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#1
Hello everyone,

i thought id share some of my measurements and gather some Feedback. Also i would like know what Options i can take to lower the 29.2Hz resonance as the "off the shelf" Absorbers dont do anything down there. I use 8 GIK Soffit Traps and some other devices (ceiling, curtain etc...)

Response 1_6_resize.jpg
Smoothing 1/3

Time_resize.jpg
Time (I still have to find a way to fix the reflection)

CSD_resize.jpg
The very deep bass rises cause the most challanges

The room is 5.97m in length, 4.25m width and 2.4m in height.

System_resize.jpg

Cheers

Florian
 
Last edited:

RayDunzl

New Member
Jun 26, 2014
290
0
0
Tampa
#2
Since no one else has responded, I will. I do in-room measurements often.

Figure 1: Frequency response:
Looks reasonable, no big peaks or dips to contend with are obvious at the 1/3 smoothing selected. However, you might want to use no smoothing in the low bass as to not lose the detail in that area.

Figure 2: Impulse Response:
Why is the first millisecond so hashy? It is far from ideal. I use room correction, but even my uncorrected IR looks similar to my "best example" - first two milliseconds:

2016-01-20_0211.png

The apparent levels of the later reflections may be accentuated by the lack of a strong peak in the initial response in the measurement.


Here is a longer IR from a couple of days later which shows the major reflections in my room using dipoles:

1 ms: probably reflections off the top of the couch where the microphone is
7 ms: the backwave off the wall behind the speakers
27 ms: double room-length bounce

2016-01-08_0513.png

If the initial peak on mine were more smeared out (like yours appears) the highest peak would be of lower amplitude and the relative amplitude of the reflections would be higher.

Figure 3: Waterfall Decay time

Your concern about the 20Hz area...

You might want to take a measurement without much input from the speakers, just the ambient noise levels. The lowest frequencies can have quite a high SPL, and while not being particularly audible, are quite measurable.

Here is a Waterfall for my room, with the excitation sweep at -60dB - you can see a little waterfall at, say, 4kHz, but the rest of the sweep would be below the level of ambient noise in the room (or measurement system) so it isn't even seen here:



The point being - it may not be so much a lack of decay (at 20hz), but just mostly ambient noise that you see in the low frequencies of your waterfall. I have a 18hz bump when the AC is on, and plenty of other LF content intruding from the outside world.

So, how much of that 20Hz in yours is always there, and how much of it is lack of decay from your test tone?

I'm not saying my room is noisy - all I hear is a little muted computer fan noise, but the measurement microphones are quite sensitive. Right now, 2am., 35.6dB A-weighted, 44.5dB C-weighted, 50.9dB Z or unweighted


Figure 4: The System
Very nice. I simply must do something better with mine... one of these days...

---

Sorry if these are not the responses you would have liked to have seen, but, at least it is a reply, based on my recent measuring escapades.
 
Last edited:

YashN

New Member
Jun 29, 2015
963
0
0
Canada
#3
I don't think there's a big pb at 29Hz, but there are two troughs higher that could be problematic. Usually, bass traps in room corners help with standing waves.

Otherwise, it looks like a quite smooth response.
 

FrantzM

Member Sponsor & WBF Founding Member
Apr 20, 2010
6,469
0
0
#4
Hi Florian

I side with Ray on the low bass issue. As it is, your room response is extremely good and beg to be left alone :). I know we, audiophiles are constantly looking to improve our systems but ... what you have here is remarkable.
Apogee Grand?
 
Jan 29, 2014
983
0
0
Cape Town South Africa
#5
As it is right now , what is "wrong" with the bass?

You need some DSP eq to remove low bass massive peak issues . no physical treatment will be effective.

Troughs , unless very broad are not anywhere as audible as a peak is . pull down the peaks and if its a bit bass light , apply a house curve. you cannot cure steep troughs with boost..dont even try .. shallow dips can be "fixed"

Im not sure of your path , but if its puter based , download a trial version of J river and use their DSP studio parametric to mess with low bass eq , , you can hear as you tweak , equalizer APO is also a nice freebie option for computer based stuff

If you are not running off a computer, and you use analog sources you will have to use an external unit which will digitise all your delicious analogness and then convert it back to analog .. you have to balance any possible deterioration of the signal with much better reproduction

Alternatively , albeit it sounds counter intuitive , use multiple subs and apply DSP to them ONLY...
Or you could just move the listening chair to get out of nulls and peaks... tho whats best for bass might not be best for the rest
 
May 30, 2010
13,968
42
48
Portugal
#6
IMHO the 1/3 octave smoothing hides the useful information in the bass - 1/12 or 1/24 is needed bellow 200 Hz before debating peaks. Perhaps it is a good time to remember we had an excellent WBF thread written by Amir some years ago, Acoustic-Measurements-Understanding-Time-and-Frequencyhttp://www.whatsbestforum.com/showthread.php?8203-Acoustic-Measurements-Understanding-Time-and-Frequency.

In my longer room (31') the second eigenvalue was around 35 Hz with corresponding very long decays at this frequency and I managed to improve it significantly, both in subjective listening and measurements, building very large bass tuned traps - see http://www.whatsbestforum.com/showthread.php?7710-Diaphragmatic-versus-membrane-bass-traps&p=133862&viewfull=1#post133862. As I had plenty of space behind the speakers, they are hidden by curtains. I am in the process of building a second one covering the whole back wall.
 

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#7
Since no one else has responded, I will. I do in-room measurements often.

Thank you :)

Looks reasonable, no big peaks or dips to contend with are obvious at the 1/3 smoothing selected. However, you might want to use no smoothing in the low bass as to not lose the detail in that area.

I did select 1/6 and higher where it does move out a little more but still within the +-5db realm. I will upload another set once i am home from work. The sound is very nice. If i move in 30cm steps towards the back wall, it gets quite a bit more busy.

Figure 2: Impulse Response:
Why is the first millisecond so hashy? It is far from ideal. I use room correction, but even my uncorrected IR looks similar to my "best example" - first two milliseconds:

Good question, i can influence this in a big way depending on how far i sit back. I assume this is from the reflection points and my listening chair, as i place the microphone on top of my chair. Maybe i should move that out of the way.

Figure 3: Waterfall Decay time

Your concern about the 20Hz area...

You might want to take a measurement without much input from the speakers, just the ambient noise levels. The lowest frequencies can have quite a high SPL, and while not being particularly audible, are quite measurable.

I will take a measurement to see the noise in the room. I never caught that :)

Figure 4:The System
Very nice. I simply must do something better with mine... one of these days...

Thanks, it did take a lot of work.

---

Sorry if these are not the responses you would have liked to have seen, but, at least it is a reply, based on my recent measuring escapades.[/QUOTE]

Thats actually a really good response and i learned something. Thanks!
 

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#8
I don't think there's a big pb at 29Hz, but there are two troughs higher that could be problematic. Usually, bass traps in room corners help with standing waves.

Otherwise, it looks like a quite smooth response.
Its about +-5db, even at 1/6 smoothing. I use 8 full size bass traps from GIK (Soffit) but their value in the very low bass frequencies is close to none.
 

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#9
Hi Florian

I side with Ray on the low bass issue. As it is, your room response is extremely good and beg to be left alone :). I know we, audiophiles are constantly looking to improve our systems but ... what you have here is remarkable.
Apogee Grand?
Thank you, besides the long decay at the deep frequencies i am quite happy with it. Yes, its an Apogee Grand. Its been my speaker since more than 10 years :)
 

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#10
As it is right now , what is "wrong" with the bass?

You need some DSP eq to remove low bass massive peak issues . no physical treatment will be effective.

Troughs , unless very broad are not anywhere as audible as a peak is . pull down the peaks and if its a bit bass light , apply a house curve. you cannot cure steep troughs with boost..dont even try .. shallow dips can be "fixed"

Im not sure of your path , but if its puter based , download a trial version of J river and use their DSP studio parametric to mess with low bass eq , , you can hear as you tweak , equalizer APO is also a nice freebie option for computer based stuff

If you are not running off a computer, and you use analog sources you will have to use an external unit which will digitise all your delicious analogness and then convert it back to analog .. you have to balance any possible deterioration of the signal with much better reproduction

Alternatively , albeit it sounds counter intuitive , use multiple subs and apply DSP to them ONLY...
Or you could just move the listening chair to get out of nulls and peaks... tho whats best for bass might not be best for the rest
Thanks for your post, i do stream from JRIVER as well but am more into Analog. I use a Lyngdorf RP-1 which corrects 7% at the listening position. The chair is placed already at the best measuring location, if i move forward or backward the bass just gets more bumpy :)
 

Florian

New Member
Jun 26, 2016
18
0
0
Switzerland
#11
IMHO the 1/3 octave smoothing hides the useful information in the bass - 1/12 or 1/24 is needed bellow 200 Hz before debating peaks. Perhaps it is a good time to remember we had an excellent WBF thread written by Amir some years ago, Acoustic-Measurements-Understanding-Time-and-Frequencyhttp://www.whatsbestforum.com/showthread.php?8203-Acoustic-Measurements-Understanding-Time-and-Frequency.

In my longer room (31') the second eigenvalue was around 35 Hz with corresponding very long decays at this frequency and I managed to improve it significantly, both in subjective listening and measurements, building very large bass tuned traps - see http://www.whatsbestforum.com/showthread.php?7710-Diaphragmatic-versus-membrane-bass-traps&p=133862&viewfull=1#post133862. As I had plenty of space behind the speakers, they are hidden by curtains. I am in the process of building a second one covering the whole back wall.
Thanks! How did you tune the membrane to said frequency? I would be interested in building a Helmholtz Resonator at 29.2Hz. I will read the other posts, information is always good.
 
May 30, 2010
13,968
42
48
Portugal
#12
Thanks! How did you tune the membrane to said frequency? I would be interested in building a Helmholtz Resonator at 29.2Hz. I will read the other posts, information is always good.
I have used mainly the excellent calculators from mh-audio http://www.mh-audio.nl/ACalculators.asp#showcalc. I choose to use a limp membrane (6,38 kg/ square meter) as I have read that it has a much broadband tuning than solid panels. I calculated it to a frequency about 20% higher as in this site it is stated that
"Filling the cavity with fibreglass or mineral wool tends to lower the resonant frequency by up to 50 per cent as well as doubling the effectiveness of the trap. It also lowers the Q of the trap so that it is effective over a wider frequency range. A typical panel-type trap is effective for frequencies around one octave either side of the centre frequency, which at least has the advantage that you don't have to be absolutely accurate to get results. "
 
Nov 3, 2014
405
0
0
#13
I am exclusively a listener to digital sources. So, Dirac EQ does a wonderful job of controlling room modal issues in the bass for me. Some other EQ tools might be just as effective.

I do no passive treatments. Helmholz resonators might be effective at a single frequency, though difficult to fabricate and calibrate exactly. And, of course, they are forever fixed at that single frequency. So, your room, speaker and furniture configuration must stay forever unchanged.

Have you investigated active bass treatments? These are essentially "negative" subwoofers, with active amps and circuitry to counter modal peaks with out of phase cancellation signal. And, they are not directly connected to your system, other than through the ebb and flow of sound in your room, which they "hear" via a built in mike. They are not dirt cheap, and not outrageous either. But, they seem to be effective at low frequencies, unlike passive bass traps. Bob Katz did a review earlier this year of one brand in Stereophile. There may be a few others, but I do not have the links offhand.

Some subs with mike calibratable internal digital circuitry, JL Audio for example, can also provide EQ for a limited number of modal issues within their range. That, of course, requires the built in a-d and d-a in the sub channel, but it does not require a digital xover and need not affect the rest of the frequency range. However, I am not a fan of using a subwoofer without a high pass and low pass xover, but that could be an analog xover. But, xovers might open their own can of worms, and good ones, especially in the analog domain, are not cheap.

And, multiple subs can also provide smoother bass evening out room modes, but that requires much experimentation and measurement, with final effectiveness unpredictable for your specific situation at specific frequencies.

Personally, I am quite happy with a single sub, a digital xover in my PC via JRiver library/player software and PC Dirac providing full range EQ. Room modes are pretty much gone, and I love the sound.
 
Nov 6, 2014
164
0
0
Um ... SLC?
#14
Hello everyone,

i thought id share some of my measurements and gather some Feedback. Also i would like know what Options i can take to lower the 29.2Hz resonance as the "off the shelf" Absorbers dont do anything down there. I use 8 GIK Soffit Traps and some other devices (ceiling, curtain etc...)

View attachment 29327
Smoothing 1/3

View attachment 29328
Time (I still have to find a way to fix the reflection)

View attachment 29329
The very deep bass rises cause the most challanges

The room is 5.97m in length, 4.25m width and 2.4m in height.

View attachment 29330

Cheers

Florian
god i wish i knew how to use dsp. i recently heard a pair of older monitor audio's with parasound amps and front end with 4 m&k subs and all kinds of tweaking and cool dsp and it was one of the most musical sounding systems i have ever heard. he then switched to a old dynaco amps and images were almost holographic 3d and **** was floating in space and i was shocked. i think the total system was about $15K. i went home and listened to my system (same material) and it sounded lifeless and boring. it is amazing what a well thought out and planned system can do with modern technology. i just dont understand any of this. but i can tell you it works and works well.
 
Nov 6, 2014
164
0
0
Um ... SLC?
#15
i am just barely understanding this dsp stuff. does anyone have a step process or guide on how to learn more about dsp and how to use it in your system. i know there is ton of software out there. is there a like a top 5 list? and what about room acoustics ... does one use dsp in conjunction with room acoustics? thanks in advance for the input.
 

flyer

VIP/Donor
Dec 16, 2012
325
0
16
Belgium
www.ultisone.com
#16
Helmholz resonators might be effective at a single frequency, though difficult to fabricate and calibrate exactly. And, of course, they are forever fixed at that single frequency. So, your room, speaker and furniture configuration must stay forever unchanged.
I am very much in favor of the Helmholtz resonators. The issue posted here is solely due to the measurements of his room. This means that furniture and speakers do not have any (or hardly any) effect on those standing wave. You might tweak the problem a bit but not eliminate it: moving speakers, listening position, speakers that don't go that low, ...
Yes, HH resonators are single frequency, and if you use them without absorber inside then their band width is really narrow, but also really impactful which is what is needed here. And mind you, the impact is not on the resonator frequency alone but also on the harmonics of that frequency!!

On the other hand, the +/- 5 dB bump in the low end is everything but worrisome! I do am surprised to see the decay at that frequency to be that long while this seems to cause only a few dB extra. I would expect the peak to be closer to at least 10 dB, but well acoustics is not an exact science or rather, we do not always have the exact data at hand to be fully informed on effective conditions.

SMT does make and commercialize very effective HH resonators.
 

About us

  • Founded in 2010 What's Best Forum invites intelligent and courteous people of all interests and backgrounds to describe and discuss the best of everything. From beginners to life-long hobbyists to industry professionals we enjoy learning about new things and meeting new people and participating in spirited debates.

Quick Navigation

User Menu

Steve Williams
Site Founder | Site Owner | Administrator
Ron Resnick
Site Co-Owner | Administrator
Julian (The Fixer)
Website Build | Marketing Managersing