For newbies: common mistakes with REW, how to ask for help on ASR

[klipschsoundsystem]

Looks bright corrected, unless you like it that way of course. Target looks like it could be raised 5db.

im using anechoic data to EQ the mids and treble

The modal zone is estimated by calculating the Schroder frequency. The formula is:

View attachment 408916

looks like its at about 250hz

 

[Keith_W]

looks like its at about 250hz

Yep, I calculated the same with the data you provided. Read the first post about zones, it contains a link - the relevant zones are pressure zone (greatest dimension of the room <= longest wavelength), modal zone (pressure zone to Fs), transition zone (Fs to 4Fs) and diffusion zone (> 4Fs). Also remember the Schroder frequency is an approximation, it is not exact.

 

[haen]

Thanks @Keith_W for taking time to write these guides. A few questions:

(1) Calibration: How important is to have calibrations file(s) for e.g. UMIK-1? In https://www.audiosciencereview.com/...om-measurement-tutorial-for-dummies-part-2.5/ Amir wrote:

Before you hit the buy button, let me talk about calibration. Both the UMM-6 and UMIK-1 come with a serial number that you type online and it gives you a (supposedly) a calibration file to use with REW. My recommendation is to not bother! Yes, you heard me right. You don’t need a calibrated mic and these files themselves can be faulty. The difference between calibrated and non-calibrated mics is small and it is in a region that I suggest you use your ear, not an instrument, to adjust the sound.

I have the calibration files (0/90 degrees) downloaded anyway, just curious.

(2) It seems that your don't think that room treatment is usually appropriate for 'fixing' low freqs? I think I saw many threads even on ASR with people trying to apply various bass traps etc. but I didn't really try yet to put much effort in understanding what are conditions when room treatment for LF makes sense.

(3) You wrote:

When measuring short wavelengths, we want to measure the output from the loudspeakers alone because that determines what we hear.

Would that mean that if there are measurements for my speaker available (e.g. Amir measured my speaker) it would be better to try to apply corrections to HF based on those measures instead my in-room measures?

(4) I've seen some people advocating for measuring both speakers together, and some separately. Also some recommend doing one measurement at the listening position, and then some propose taking similar measurements around the listening position and then averaging them. Is there any 'fast' rule here? I would think that e.g. it makes sense to measure speakers separately - when I put my active monitors on the desk I noticed that the left one in its position is audibly quieter (confirmed with an SPL meter), so intuitively I could aim to have separate EQ settings per a channel.
Probably as long as measurements are taken properly and clearly labelled, then the more data the better. ;-)

 

[Keith_W]

(1) Calibration: How important is to have calibrations file(s) for e.g. UMIK-1? In https://www.audiosciencereview.com/...om-measurement-tutorial-for-dummies-part-2.5/ Amir wrote:

I disagree with Amir on this one. Take a look at this:

These are several dozen Behringer ECM8000 omnidirectional XLR mics tested by a microphone calibration service. Note that the response varies up to +/- 5dB at both frequency extremes. Amir makes the point that at high frequencies, you should be using your ears to adjust the sound and not a microphone. I agree with him that this is what you should be doing, since with high frequencies what you measure isn't really what you hear. If you are very careful, you will at best be measuring the direct response of the loudspeaker. But what you hear is the direct response + early reflections. Correction of the direct response also changes the early reflections since the two are inextricably linked. But I still think it is better to get as accurate a measurement as you can.

(2) It seems that your don't think that room treatment is usually appropriate for 'fixing' low freqs? I think I saw many threads even on ASR with people trying to apply various bass traps etc. but I didn't really try yet to put much effort in understanding what are conditions when room treatment for LF makes sense.

Best to discuss DSP vs. room treatment in another thread to avoid derailing this thread.

(3) You wrote:

Would that mean that if there are measurements for my speaker available (e.g. Amir measured my speaker) it would be better to try to apply corrections to HF based on those measures instead my in-room measures?

No, it is better to have your own measurement. Use Amir's measurement to check that your own measurement is accurate. As for whether you should be correcting HF's - again, better discussed in another thread. In fact, there is a current ongoing thread that is discussing this. Very sorry for continuously redirecting you, but this thread is for making sure that people are taking correct measurements in the first place.

(4) I've seen some people advocating for measuring both speakers together, and some separately. Also some recommend doing one measurement at the listening position, and then some propose taking similar measurements around the listening position and then averaging them. Is there any 'fast' rule here? I would think that e.g. it makes sense to measure speakers separately - when I put my active monitors on the desk I noticed that the left one in its position is audibly quieter (confirmed with an SPL meter), so intuitively I could aim to have separate EQ settings per a channel.
Probably as long as measurements are taken properly and clearly labelled, then the more data the better. ;-)

There is no hard and fast rule. What you measure depends on what question you are asking or what problem you are trying to solve. Re:

- Measuring speakers alone vs. together. I do both. I want to see the individual response of both speakers, and what happens when both speakers play together. This is especially useful for detecting phase issues. Here is an example:

Red/green: L/R measurement of individual speakers. I deliberately inverted the polarity of the right speaker. As you can see, the individual measurements look great. But play both speakers together, and the blue curve is the result.

- Measuring at MLP vs. measuring in various positions and averaging. The former is an artificial representation of reality since you have a pair of ears which point forwards, not an eardrum floating in space. The latter is more realistic but then you can't obtain timing information. Again, there is no hard and fast rule. I do both. I take a single measurement at MLP and use that for all my timing corrections. Then I take an averaged measurement from several positions and use that for amplitude correction.

I might have forgotten to reply to this post so i'll do it now:

I disagree with this one. When you listen to music you are in the room so are part of the sound. I setup microphone where my head is and sit beside it.

This is a compromise but at least my body is included in the process. Bodies are 70% percent water which is an absorbent. A body would be considered part of the furniture and an influence on the sound. Anything that is present in the listening room has to be considered.

Whether or not you choose to include your body in the measurement depends on what you want to measure or what you want to correct. It is absolutely valid to include your body / sofa / listening chair / etc. in the measurement if you want to correct low frequencies. As for high frequencies, that depends on your correction philosophy. Most people, including Toole, say that we do speaker correction and NOT room correction for high frequencies. There is a difference - for speaker correction, you try to replicate anechoic conditions as best as you can. Room correction involves correcting the speaker together with the room.

There are other correction philosophies, e.g. BACCH's ORC which uses a pair of in-ear microphones. I have no idea what they think they will gain from this, but it strikes me as a very bad idea.

 

[Pieter1267]

I have been helping new users interpret REW measurements on ASR and I see many of the same mistakes being made all the time. Rather than repeat myself, I thought that a guide might help.

This is NOT a guide to take measurements or how to use REW. The REW manual is a treasure trove of information, and it is extensive. The only problem is that you may not know where to look if you are new to measurements.


PART 1: HOW TO TAKE MEASUREMENTS

1. Before you start ...
These items are essential before you start taking measurements with REW:

- PC/Mac capable of running REW.
- A USB or XLR microphone. If you choose an XLR microphone, you also need an interface with 48V Phantom Power
- Appropriate cabling for your microphone and interface
- A microphone tripod with a boom.
- Read the getting started with REW guide.

2. What microphone to use
Use an omnidirectional condenser type microphone. Microphones are not the same! Some microphones may be optimised for voice, some may have a cardioid pattern and reject reflections, etc. Omnidirectional condenser microphones work by varying the capacitance in the capsule membrane, and thus need to be powered.

There are two types: XLR and USB. XLR microphones require a microphone preamp that supplies voltage, usually 48V. This is known as "Phantom Power". It is built into interfaces. Common XLR microphones used by ASR members include: Behringer ECM8000, Dayton EMM-6, iSemCon EMX-7150, Earthworks M23/M30. USB microphones have the power supply and ADC built into the mic. Common USB microphones used here include: UMIK-1, UMIK-2, Dayton UMM-6.

3. Calibration
View attachment 400395
Microphones are manufactured to a tolerance and may have large variations in frequency response if not calibrated. The above shows the variance in response of a few dozen Behringer ECM8000 microphones - you can see that the variance is highest at both extremes of the frequency range. Make sure the microphone you buy comes with a calibration file. If not, send it to a calibration service. Also note that mics can go out of calibration after use, especially if the mic has been physically abused (subjected to temperature extremes, dropped, jiggled around, etc).

4. Why you need a mic boom tripod
View attachment 400396
Some microphones come with mini tripods (e.g. UMIK-1 and UMIK-2, Anthem ARC microphone, etc). These are good for landfill only. The first thing you should buy is a mic boom stand. This is because microphones need to be positioned away from reflective surfaces to take a meaningful measurement. Even the design of the mic boom can create reflections that will affect your measurement, but we don't usually worry about those because they are low in amplitude and can be ignored unless extreme accuracy is desired.

5. Microphone orientation
Microphones need to be pointed at the speakers, and not up at the ceiling. The reason is because "omnidirectional" microphones are not truly omnidirectional, they have a polar response and will measure a different frequency response if it is pointed in the wrong direction. Read this. The only time microphones can be pointed upwards is if they include a equalisation file for vertical orientation. Otherwise - (as a presidential candidate said) ... DON'T.

6. Moving furniture
Some guides might direct you to move furniture away so there are no reflective surfaces close to the speaker. Some tell you to leave furniture in place. What is going on?

View attachment 400398

The key is to understand that long and short wavelengths behave differently in listening rooms. The strategy to measure them is different, and the interventions we use to fix these problems are also different. The zones are defined by the Schroder frequency. It is very important to understand this concept. (As an aside: If you have high frequency problems, the best approach is to (a) buy better speakers, (b) consider repositioning your speakers, (c) try to fix anomalies with room treatment, (d) apply gentle DSP. In that order! If you have low frequency problems, the approach is to (a) consider adding more subwoofers, (b) DSP. Room treatment has a minimal role because it is less effective and excessively intrusive.)

When measuring long wavelengths, we want to include all the influence of room furnishings in our measurement because that affects how bass behaves. This includes: sofas, coffee tables, and other large furnishings. When measuring short wavelengths, we want to measure the output from the loudspeakers alone because that determines what we hear. This can be very difficult in a listening room, particularly if it is small. So we may decide to: (a) do the measurement anyway but interpret upper frequencies with caution, (b) choose not to correct the upper frequencies because we know we are not measuring them properly, (c) attempt to get a quasi-anechoic measurement by taking the speakers outside and away from nearby reflective surfaces.

View attachment 400400

If your room is large enough, you can move your speakers and microphone to the middle of the room to measure upper frequencies. You will be able to obtain a meaningful measurement to a lower limit if you window the measurement appropriately. The lower limit is defined by the distance between mic/speaker to the closest reflective surface (e.g. walls, floor, ceiling). See above diagram.

7. Choose the correct SPL for measurement
Measurement sweeps need to be: "not too loud, and not too soft".

"Not too loud": loudspeakers, and to a lesser extent amplifiers, are nonlinear devices. At high volumes they distort, compress, and clip. This WILL affect your measurement, for example high amounts of harmonic distortion will look like a rising frequency response. Loudspeaker compression will look like deviations from the frequency response depending on which driver is compressing.

View attachment 400403

"Not too soft": every listening room has a noise floor. Most of the noise in listening rooms is low frequency, so you may not be aware of the presence of a lot of noise because it is less audible and our brain filters it out. It will look like a rising bass response. The above graph shows a full range (20Hz - 20kHz) measurement of a tweeter (I was able to obtain this measurement without damaging the tweeter by choosing a very short pulse). All that bass you see is the room's noise floor, it is not tweeter output!

The goal of a measurement is to obtain a high signal to noise ratio. Noise can be minimised by choosing to do your measurement when ambient noise is low (evenings, weekends). Signal quality can be improved by increasing the amplitude of the impulse (i.e. turn up the volume) or length (time) of the impulse. A 45 second sweep has about 90dB of noise rejection. You could also take multiple short sweeps, reject those that look flawed or noisy, and average them. How to average graphs in REW.

You will notice I did not include "SPL Meter" in the purchase list, despite REW nagging you to calibrate your mic to an SPL meter every time you boot it up. Why? This is because knowledge of your SPL does not guarantee that your speakers will not behave in a nonlinear fashion. The best way to determine if your speaker is behaving strangely is to measure at different volumes (typically 75dB, 85dB, 95dB - but you can increase the volume in REW by 5-10dB each time). Then overlay the graphs and examine it. They should all look the same, but they won't. At some point the frequency response will start to deviate. You want to measure at a volume lower than where the deviation occurs.

SPL meters are good for determining the noise floor when interpreting spectrograms and waterfalls and for compression measurements. They have a number of other uses, but most of us do not need SPL meters.

8. Before and after measurements
You might want to know the effect of DSP, room treatments, different speakers, etc. The key is to repeat the measurement whilst keeping all variables the same, except the one you are testing. This means: don't move your microphone, or at least take steps to make sure you can re-position your microphone in the exact same position when comparing measurements. Some people hang a plumb line from the ceiling so they know where to place the microphone. Don't make multiple changes at once, e.g. one guy I am trying to help is measuring with one mic in five positions using his HT AVR and verifying with another mic using MMM and a different software program. One variable at a time!

9. Avoid speaker and hearing damage.
Be aware that speaker drivers can overheat and voice coils can melt. Do NOT play loud sine wave tones through your subwoofer for a long time. Do NOT allow your amplifier to clip - this sends high frequency tones to the tweeter and can burn it out. Sometimes, repeated loud measurements can cause drivers to overheat and behave in a nonlinear fashion - i.e. repeated sweeps can cause the frequency response to change!

Wear hearing protection, or exit the room while taking the measurement. You can use REW to delay the start of the measurement to give you enough time to leave.

10. Special measurement techniques
Special procedures are required if we want other types of information. For example:
- if we want to know the bass performance of a subwoofer or speaker independent of the room. Speaker designers and reviewers need to know this. Most hobbyists don't. What you need to know is how your sub or speaker is performing in your room.
- the average frequency response over a listening area. You will need to do an MMM or multi-point averaged measurement. I am not going to discuss that here to keep the length shorter.
- quasi-anechoic response of the loudspeaker and directivity measurements. You need to know this if you want to correct the upper frequencies with DSP. There are several techniques but I won't discuss them here.
- speaker compression.
- loudspeaker port measurement
- measurements of difficult speakers, e.g. dipoles and omnis.



PART 2: HOW TO ASK FOR HELP ON ASR
For us to help you (and we want to help!) we need:

1. Confidence that you have taken a proper measurement. Read the above and make sure you have not made any mistakes. This is very important. We don't know if the anomalies are real or due to an artefact from improper measurement! In your post, state that you have read this guide, and specifically state that you have used the proper microphone, pointed it at the speakers, used a mic boom tripod, and taken noise precautions.
2. The intention of the measurement and the question you are asking. For example, "my left speaker sounds louder than my right, but the measurements look the same! Why?". A question like "please comment on this graph" is not so helpful.
3. Details of your room: height, width, length (people always forget about the height!). Where is the listening position, where are the speakers. A quick drawing is very helpful.
4. Details on your equipment you are measuring: how many speakers, subs, how many DAC channels, etc. and how you have connected them together.
5. Properly labelled graphs. Edit the name of each graph in REW so that the labels CLEARLY tell us what we are looking at, e.g. "L spkr + L sub" or "L sub - no DSP". You can rename graphs by right clicking on the graph panel on the left.

By far the best way to ask for help is to post the .MDAT file on ASR. To do this, you need to zip the .MDAT and use "Attach files" when posting your thread.

If you do want to post the graphs (and a few graphs are always helpful), please try to follow these guidelines:

View attachment 400408

1. Choose a proper zoom. If you are asking a question about bass, zoom in on the bass. REW has a number of options that help you scale the graph for easier viewing:
a) "fit to data" option. (see above)
b) Zoom in to an area of interest. To do this, hold down the CTRL key and right click on the graph to drag a box. Then click on the shaded area to zoom.
c) On the bottom right of your graph, you will see two buttons - "10 .. 200" and "20 .. 20k". One automatically zooms to show the bass, the other to full range.

2. Choose proper smoothing. Normally 1/6 or 1/12 octave smoothing.

View attachment 400409

2. If using a screenshot, make sure we can see the vertical / horizontal scales and labels CLEARLY. This is the best way to post a screenshot: click on the camera (top left) then click on copy to clipboard. Then go to ASR and type "Ctrl-V" to paste.

3. Do not post too many curves in one graph. It makes it hard to see.

Lastly, remember that ASR members are human! If you post a thread without sufficient information, too many mistakes in measurements, a poorly asked question, etc. - many ASR members will not bother to help. I can't blame them. I often help because everyone is ignoring the newbie and I feel pity. These guys are effing grumpy. So please, help us to help you.

This is a great post. Thank you!

 

[Solveit]

I don't know if missed something but I use ONE speaker as reference, not copy Left EQ to right speaker.
Use the ONE speaker and Export-export measurement as txt.
Import it as house curve in EQ .
Then pusch calulate taget level from response.
Then try to meet that response to your other speaker.
When, after hours of measurements from the ONE to the other and copys you have nirvana.
I have it now with my uneven room.
My daughter in law didn't what to leave when she heard it.
But it's an rabbit hole I know but took about 50 measurements....
Just me...

 

[Solveit]

Qf course it's EQ in room, how you choose what is right for ONE speaker.
EQ that ONE, it's better to lower than pusch up and forget those room moodes spikes you see.
You don't hear them.
Try to meet your speakers in the room.