REW Graphic analysis

Tony Teng

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I have just used REW to analysis my audio room. My room size is 5.68meter*3.36meter*2.78meter.
All reference informations such as, speakers & mic location,SPL & waterfalls diagram, room pictures are as attached photos.
During test tones test, I heard my room ringing at low frequency between 30-120Hz
I would like to solve the low frequency ringing problem.
What can I do to improve my space before PEQ solution?
P.S. equipments
CDP burmester 102
pre AMP Burmester 035
Power Amp Burmester 956MK2
Speaker Monitor audio gold X300
Regards
 

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John Mulcahy

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There is a lot of noise and very little signal in that mdat file. Could you hear the sweep when you made the measurement?
 

Matthew J Poes

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As John already stated, something seems amiss in the measurement. Maybe try measuring again at a louder level?

I really love your room. The existing acoustic treatments look great. Once you take new measurements it should be readable to investigate.

Now as for ringing, the only place the current measurements show evidence of ringing is around the 30hz mark where your lonely axial mode lives. Those are exceedingly difficult to treat without eq. The only good option is to treat the source. Damp that length mode reflection. Since it’s such a powerful one I would typically address it on both the front and rear wall using tuned absorbers. It appears your room was already professionally designed so it’s possible they could address it. Helmholtz or panel/membrane traps are likely the only solution that low and you would need a lot of them. If I was treating that space I would build the walls as CLD walls on resilient mounts and I would then treat 100% of both the front and rear wall with bass absorbers. I don’t know what kind of treatment is already there so what I would do would depend on the untreated room. In your case the issue is more about adding absorption targeting such a low mode.

I’ll also just add that Eq is a Kutcher easier and more effective fix at those frequencies.
 

Tony Teng

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I did hear the sweep during my measurement. The SPL meter shows 75dB and sweep once(1). Today I measure again with SPL meter 75 & 80 dB as attached files.
By the way, There is a glass window behind the front wall curtain.
 

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  • 20180813 80 dB.mdat
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John Mulcahy

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Those still do not look like valid measurements. You have not selected an output in REW, how is your mac connected to your system?

Try installing the latest 5.19 beta from the downloads section and measuring with that, if that doesn't product a valid measurement we can try measuring with the acoustic timing reference option and with "wait for timing reference" selected on the Measure dialog.
 

Tony Teng

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I just download REW 5.19 and make measurement as attached.
 

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  • 20180813 rew 5.19.mdat
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John Mulcahy

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That doesn't seem to have helped, unfortunately. Quite a puzzle.

- How is your mac connected to your system?
- If you play a 1 kHz tone through the REW signal generator does the tone sound clean? How does the RTA plot of the tone look (using a Blackman-Harris 7 window)?
 

Tony Teng

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miniDSP UMIK-1 mic was connected to Macbook thought USB as attached photo.
Pink noise file which generated by REW was play thought cambridge 851N USB input
1KHz tone sound clear and RTA (REW 5.19) plot as attached.
 

Tony Teng

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photos
 

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  • REW mic connection.jpg
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Tony Teng

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Do I need to connect Macbook 3.5mm socket output to pre amp RCA input to play pink noise.wav ?
 

Tony Teng

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I have connect my Macbook 3.5mm socket to pre-Amp RCA input. measurement as attached. Does it make sense now?
 

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John Mulcahy

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Looks a little better, still very noisy though and there is no high frequency content. REW does not use pink noise for its measurements, it uses a sine sweep - did you hear the sine sweep this time? You should be hearing the sine sweep REW generates when you click 'Start measuring', you should not be playing any file.
 

Tony Teng

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This time I do not change wiring. I click signal generator and select sine wave 1000Hz.
I press the play button and hear 1000Hz sound then I press measure button (the sound disappear during measurement) and the result as attached file.
I repeat the same measurement of 30Hz and Pink noise for your reference.
I am so surprise they are almost the same curve.
The high frequency disappear may cause by the sound card of Macbook.
 

Tony Teng

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attached files
 

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  • pink noise.mdat
    2.9 MB · Views: 14
  • sine 30Hz.mdat
    2.9 MB · Views: 14
  • sine 1000HZ.mdat
    2.9 MB · Views: 10

Tony Teng

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My question is during measurement the sound disappear. Why the mic still record the diagram ? And why 1000Hz, 30Hz and pink noise have the same curve?
I am so confused.
 

John Mulcahy

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Those measurements look much better. Successive measurements should be virtually identical if you do not move the mic. You do not need to do anything with the signal generator to make a measurement. Just click the Measure button, then click Start Measuring on the dialog. REW controls the signal generator. It doesn't matter what signal was playing before the measurement starts.

Is there a reason you have not selected the mac's built-in output in REW's soundcard settings (output device and output)?

The internal soundcard on any mac I have measured has been very flat, unless it is broken it cannot be the cause of the high frequency roll-off.

You do not need the foam cover on the mic when measuring indoors, it is to help with wind noise for outdoor measurements. It should not cause the high frequency roll-off but best to remove it.

Try raising the mic so it is at ear height, the low position may be partly responsible for the lack of high frequency.
 

Matthew J Poes

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My question is during measurement the sound disappear. Why the mic still record the diagram ? And why 1000Hz, 30Hz and pink noise have the same curve?
I am so confused.
Hi Tony,

The way REW works is that it creates a sweep on its own that goes from some specified low frequency (say 20hz) up to a specified high-frequency limit (say 20khz). The tone generator is for other purposes, not necessarily response measurement. It is highly suggested that you use the sweep method. When you play a tone and then tell it to run a sweep, it will stop the tone generator and run its sweep. You should be hearing this sweep, correct? I know John is trying to help you, I want to let him do that so I don't inadvertently confuse things. It seems like you still are a bit confused in how the program takes a measurement. Be sure to carefully read the manual under downloads which explains how to measure with REW. It's very handy. It's called, "Getting started with REW...".

I'm happy to help address any acoustic questions you have once the measurements are sorted. Until they are sorted I don't want to make any judgments about the acoustics.

What is the name of the firm who handled the original acoustic design of the room? Are those products all RPG or is there a mix of brands? Even once I have measurements, without knowing what is in that room already, it will be hard to know how to address any lingering issues.
 

Adhoc

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It is several years since I used REW together with Umik 1, windows version and it helped me alot with placing treatment in my room to cure SBIR problems. But, I also think your meaurements look ”odd”. That mode around 30 Hz from the waterfall looks to linger on ”forever”. If you have a large window area behind those curtains it feels likely a length mode 30 Hz would just pass right through the curtain and window area, not bounce back and forth in the room and build up such a mode.

Is it is possible to borrow another laptop, perhaps a windows based one, and make new measurements?

Otherwise, a personal reflection; you have good gear and a very nice looking room. So if there’s any truth in the measurements you have made so far, this shows good gear and a nice looking room doesn’t guarantee good sound. The interaction speaker positions <=> surrounding reflection areas <=> listening position often influences bass sound a lot more than good versus less good gear. Speaker directivity and toe in are other variables to remember, impact can be seen in REW's ETC diagram and has to do with SBIR.

I understand you don’t want to change anything in your room, so I would start with simple things which requires no money and little of efforts. That would be to move the microphone forward and sideways at ear height at least 1 m and not place it on anyhting that can resonate at 30 Hz. Any change? If yes, a move of speakers and/or listening position can mean improvement. (A shot in the dark, -if your couch under the leather cushions is constructed around a sealed empty plywood box, it could be a resonating box, porly damped at 30 Hz.) A simple ”real” floor standing tripod, instead of the little one you got together with the mic, can be a good investment. Always mark out your measuring positions with a tape mark on the floor and number it so you know which measurement it goes along with.

My basement room has a concrete shell 20-50 cm (8”-20”) thick. A change of 1 m shows a substantial change in mode strength. I realised before I started out with treatment that the 3 lowest length modes would be next to impossible to treat with any common purchaseable and visually acceptable ”acoustical fixes”. So, careful positioning of speakers and listening place was treatment # 1 at no cost. # 2 for 4 th length mode at around 86 Hz and 2nd width mode at 82 Hz were limp membrane traps + DIY helmholtzes of substantial areas (but not very deep). # 3 (later on) were arrays of subs at front and back wall. So, I don’t fully agree with what Matt wrote ”The only good option is to treat the source. Damp the length mode reflection”. Check first how it sounds with a repositioning of speakers and / or listening position. Important to remember is that the best position for mid and treble element is seldom the best for low bass element. To lessen bass problems 2 subs, 1 at the front and 1 at back wall can improve a lot at listening position versus length modes. 2 separated at front and 2 separated at back wall and you can reach an even bass across the width of the room. The subs would take up less volume than any useful acoustical treatment, probably less money (and no / little rebuilding …) An EQ instead is cheap, could improve on 1 more or less fixed position but not overall in the room.

First things first though, check out if the measurements show true or false results.
 

Tony Teng

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First of all, I did appreciate what you gentlemen have taught me in the past few day. Today I follow the REW help step by step. First I setup the input and output device as attached photo. check level,disconnect wi-Fi, remove the foam on the mic and higher the mic position. So, here is my measurement again. Hopefully it will be correct. please advise.
 

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John Mulcahy

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The measurement looks good, but there is still the dramatic high frequency roll-off. It almost looks as if the tweeter is not being driven, If you change the output selection to the left channel do you see the same roll-off?
 

Adhoc

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I extented the waterfall time range in your mdat file and John may have a point. It looks peculiar from about 140 Hz and up. -What happened within the midrange / higher frequencies and why?

Tony Teng WF 2000 ms.JPG


I also googled Monitor Audio Gold 300. Very good and positive reviews overall. Regretably no speaker measurements were found on the Gold 300. There were some on their Silver 300 though which has similar bass elements, = 2 x 6,5”. The Silver 300 starts to roll off around 70 Hz with its ports blocked, around 40 Hz with open ports as it seems. I expect if not identical, at least similar response from the Gold X300. See ”Measurements” in: https://www.stereophile.com/content/monitor-audio-silver-300-loudspeaker-measurements . -So I wonder why your measurement show that high 30 Hz peak if the speakers has probable roll off above that frequency. Also as your speakers are placed quite far away from walls, I wouldn’t expect much room gain in the low range.
 

Tony Teng

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It make no difference while switch output to left channel. my ears still don't hear any sound at the last 1/3 of sweap
 

Tony Teng

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When I play 30~39Hz test tone and walk around my room. I did hear serious ringing here and there. I will try re-position my speakers first after my business trip to Japan. I will keep you guys update my test. Thank you.
 

Tony Teng

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Do I need to install my minisdp 2*4HD to help REW measrement?
 

John Mulcahy

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No, 2x4 HD is an equalizer/filter product, it is not needed for measurements. If you want to apply filters later though you could use a 2x4 HD for that.

For your high frequencies, make sure the links are in place to connect the LF and HF terminals on your speakers. It is probably a good idea to try making a measurement with the mic 1m from your speakers at the height of the tweeter, to see if the speaker is behaving normally.
 
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