# Help with interpreting REW graphs and general understanding of audio characteristics



## djryanash (Nov 5, 2015)

Hi all,

I'm new to all this, so please bear with me.

(Have attached 2 x REW settings files for your reference.)

I got a Mac with REW, Behringer ECM8000 and an Apogee One audio interface.

I have done a measurement at my home studio and I did one for a restaurant where I recently did a sound system installation.

I have also manually programmed the audio processor in the restaurant with the suggested EQ settings from REW.

I'm having trouble understanding the graphs and what they mean and I haven't found anything satisfactory online.

The main thing that I want to use REW for is checking the quality of audio in restaurants and bars which I do sound system installations in. I know that's it's main use is balancing a system for sound studios (correct me if I am wrong) but I figured that if you can find the problem areas in bears, restaurants and clubs, that would be a boon.

From what I understand, for my purposes, the SPL, Waterfall and Impulse Response graphs are the most important. Is that correct?

If ou had to list out the order of importance of all the graphs, what would be the order i.e. which graphs must I consider most important when making decisions about acoustics in the room measured?

I kinda get the SPL graph. From what I understand, by sending a logarithmic sine sweep, it picks up at what frequencies the room (speaker, etc) is resonating at i.e. resonant frequencies. Theoretically this should give one the opportunity to select acoustic treatment to eliminate or reduce said resonances. So the question arises: How do I go about selecting acoustic materials based on that analysis? For instance, if I have a resonant frequency at 386 Hz (see the graph below) how do I choose the specific material that will counteract that specific frequency? And then how do I know where to place that material in relation to my listening position?

So then comes Phase graph. Mine are generally all over the place i.e. seem to be rotating those every 1/24th of an octave (or so it seems) especially in the upper frequencies(see same graph below). What does this mean? Should I be concerned? And if so, what can I do about it?

The Waterfall is the decay of the FFT over time. I get that. But again, how do I use this information to improve the acoustics of my room - with regards to treatment or other methods?

Impulse Response: What does a good graph look like (compared to mine)? What can I do to improve my graph with treatment? How do I know which treatment to select?

How do I read the Distortion Graph? I understand about fundamentals and harmonics, but I still don't understand how to read the graph.

RT60: U understand that it is the time that a given frequency takes to get to -60dBFS. But how does that help me? What does a bad measurement look like vs a good measurement?

Decay Graph? Spectrogram Graph? How do I read them? What do they mean? What can I do to improve them? Should I spend my time improving them i.e. how important are these facts and figures?


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## Niick (Jan 2, 2015)

djryanash said:


> Hi all, I'm new to all this, so please bear with me. (Have attached 2 x REW settings files for your reference.) I got a Mac with REW, Behringer ECM8000 and an Apogee One audio interface. I have done a measurement at my home studio and I did one for a restaurant where I recently did a sound system installation. I have also manually programmed the audio processor in the restaurant with the suggested EQ settings from REW. I'm having trouble understanding the graphs and what they mean and I haven't found anything satisfactory online. The main thing that I want to use REW for is checking the quality of audio in restaurants and bars which I do sound system installations in. I know that's it's main use is balancing a system for sound studios (correct me if I am wrong) but I figured that if you can find the problem areas in bears, restaurants and clubs, that would be a boon. From what I understand, for my purposes, the SPL, Waterfall and Impulse Response graphs are the most important. Is that correct? If ou had to list out the order of importance of all the graphs, what would be the order i.e. which graphs must I consider most important when making decisions about acoustics in the room measured? I kinda get the SPL graph. From what I understand, by sending a logarithmic sine sweep, it picks up at what frequencies the room (speaker, etc) is resonating at i.e. resonant frequencies. Theoretically this should give one the opportunity to select acoustic treatment to eliminate or reduce said resonances. So the question arises: How do I go about selecting acoustic materials based on that analysis? For instance, if I have a resonant frequency at 386 Hz (see the graph below) how do I choose the specific material that will counteract that specific frequency? And then how do I know where to place that material in relation to my listening position? So then comes Phase graph. Mine are generally all over the place i.e. seem to be rotating those every 1/24th of an octave (or so it seems) especially in the upper frequencies(see same graph below). What does this mean? Should I be concerned? And if so, what can I do about it? The Waterfall is the decay of the FFT over time. I get that. But again, how do I use this information to improve the acoustics of my room - with regards to treatment or other methods? Impulse Response: What does a good graph look like (compared to mine)? What can I do to improve my graph with treatment? How do I know which treatment to select? How do I read the Distortion Graph? I understand about fundamentals and harmonics, but I still don't understand how to read the graph. RT60: U understand that it is the time that a given frequency takes to get to -60dBFS. But how does that help me? What does a bad measurement look like vs a good measurement? Decay Graph? Spectrogram Graph? How do I read them? What do they mean? What can I do to improve them? Should I spend my time improving them i.e. how important are these facts and figures?


 the answers to the questions you just asked many people spend a lifetime and an entire career trying to answer! You're gonna have to take one thing at a time. I HIGHLY recommend reading the REW help files. They are immensely informative. There are many softwares out there like REW, but none for free which are SO powerful, SO in depth. I guess the first thing that I would recommend is to understand what it is you are wanting to measure and why. I know that sounds a lot like your question, but you must know why you're wanting to take measurements, what it is you're trying to answer. 

I suppose the most common question one would try to answer is "what is the frequency response of the sound system at this point in space?" 

The answer to this question is found in the SPL graph. The SPL graph doesn't exactly show you what frequencies are RESONATING, but, you can use the waterfall graph for this. Often, two (or more) different points of view are necessary to get an idea of what it is you want to know. 

Also remember (with REW) that a single point in space (the microphone capsule) is all you're ever measuring at any one time. This is where "spatial averaging" comes into play. 

Time domain data (IR, ETC, step response, etc.) is simply another way of looking at the same data as the SPL graph. It's another "point of view". 

Time domain data is often used to compare arrival times between speakers. 

Time domain data can also be used to look at the ratio of direct to reflected energy (ETC) and how quickly that energy decays, such as the RT60 metric you previously mentioned. 

The first thing to do is to really get a good understanding of how to acquire the data. Then you can worry about interpreting it. Acquiring valid data is half the battle. REW uses a logarithmic sine sweep as its excitation signal. It then measures the transfer function, or basically divides the measurement channel by the reference channel. 

The measurement channel in REW is whichever channel of your interface you have your mic plugged into. The reference is one of two things. It's either an internal "loopback" of the signal being sent out to the SUT (system under test), or, it's a physical "loopback" that you actually hard wire your two (or more) channel audio interface. You then select in "preferences" "use loopback as timing reference". This is the best way to go. This way, you have valid time domain data. 

As far as I am aware, everything I've said so far is accurate. If not, I'm sure someone will help clarify things. That's about as far as I can go so far, until you get a little more specific with your questions/goals. Good luck, have fun!


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## djryanash (Nov 5, 2015)

Hi Nick,

Thanks for your reply.

My main objective is to eliminate boominess in the bass and excessive toppiness in the highs. I want to get a more even response from the system as a lot of bars/restaurants, etc sound absolutely terrible. I want to achieve a more balanced sound overall.

Does that answer your question?

Now perhaps you could help me out further with my previous questions about the various graphs, etc?

I know its taken many men's lifetimes to compile this data about audio and acoustics, but I think my questions are specific enough to invite some answers.

Thanks in advance,

Ryan


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## DrDyna (Jul 21, 2011)

djryanash said:


> So then comes Phase graph. Mine are generally all over the place i.e. seem to be rotating those every 1/24th of an octave (or so it seems) especially in the upper frequencies(see same graph below). What does this mean? Should I be concerned? And if so, what can I do about it?


That phase response looks about like what you'd expect from a measurement at some distance in a reverberate space that's not acoustically treated. Sound tends to bounce around, and all the waves going to and fro across surfaces and interacting with one another makes those graphs a little nonsensical in practical-land. I wouldn't worry about it too much. (Weapon: Room treatment / better loudspeakers)



djryanash said:


> The Waterfall is the decay of the FFT over time. I get that. But again, how do I use this information to improve the acoustics of my room - with regards to treatment or other methods?


 It gives you "areas of interest" for bass trapping. Most bass trap type devices are tuned to have greater effect at certain frequencies, I.E using thicker or thinner material, with more or less space between them and walls, or using devices like helmholz resonators that are specifically tuned to a narrow band of frequencies. If you notice one band of excessive ringing, sometimes something as simple as relocating the subwoofer can help. (Weapon: Bass trapping, multiple subwoofers, subwoofer DSP)



djryanash said:


> Impulse Response: What does a good graph look like (compared to mine)? What can I do to improve my graph with treatment? How do I know which treatment to select?


The graph essentially tells you how quick the speaker reacts to a signal, and then how quickly (and gracefully) it returns to base after the signal is gone. A perfect impulse response would be nothing but a sharp, square box that hits the top instantly, and returns to zero instantly with no ringing...think...the long tetris piece standing upright. It helps with time alignment, especially for subwoofers, because when you send a little *blip* at the speakers, you want them to all respond to it at the same time, or at least appear that they all respond at the same time from whatever distance you are from them, because drivers are typically separated by some distance, putting them at different distances from the ear. A bad graph would be two (or more, yikes) blips that are far enough apart to cause phase issues, or cancellation. (Weapon: Room treatment / DSP for time alignment, better loudspeakers.)



djryanash said:


> How do I read the Distortion Graph? I understand about fundamentals and harmonics, but I still don't understand how to read the graph.


That's pretty easy, the distortion curve is just telling you how loud each of the distortion products are relative to the fundamental frequency. On your graph, it looks like you have a slight increase in distortion from 1.5 khz and up, indicating that whatever tweeter is in use, has slightly higher distortion than the other drivers. It's not great, but it's not atypical of a lot of tweeters, especially if driven hard. (Weapon: Better loudspeakers)



djryanash said:


> RT60: U understand that it is the time that a given frequency takes to get to -60dBFS. But how does that help me? What does a bad measurement look like vs a good measurement?


Really all it does is tell you how long sound hangs out before it's eaten by the langoliers. This is one of the areas that room treatment will tighten up. RT60 is just a chosen time frame so that people have an agreed upon metric when discussing decay time, at least as far as I know. (Weapon: All of the things.)



djryanash said:


> Decay Graph? Spectrogram Graph? How do I read them? What do they mean? What can I do to improve them? Should I spend my time improving them i.e. how important are these facts and figures?


Waterfalls and decay are just fancier ways of showing resonances. A perfect one would probably look like a round tube laying on it's side, but in practice, modal response dominates here. You'd need an acoustically large space to get a really, really good one, but looking at it gives a clue as to why the phase response is everywhere. (Weapon: Room treatment, extreme weapon: be outdoors and 50 feet in the air.)

Pretty much every measurement can be improved one way or the other with room treatment, better loudspeakers with more predictable polar response, more careful time alignment between all the speakers (and individual drivers in the speakers themselves) or introduction of some gentle EQ in problem areas. A lot of this information "overlaps" in what it tells you, but is presented in different ways.

Disclaimer: I'm not an expert, and some of this may very well be slightly off or incomplete. We'll see what others have to say.


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## Wayne A. Pflughaupt (Apr 13, 2006)

djryanash said:


> The main thing that I want to use REW for is checking the quality of audio in restaurants and bars which I do sound system installations in. I know that's it's main use is balancing a system for sound studios (correct me if I am wrong) but I figured that if you can find the problem areas in bears, restaurants and clubs, that would be a boon.


REW can be used for any situation where an audio system is used – home, car, auditorium, studio, etc.



> From what I understand, for my purposes, the SPL, Waterfall and Impulse Response graphs are the most important. Is that correct?





djryanash said:


> My main objective is to eliminate boominess in the bass and excessive toppiness in the highs. I want to get a more even response from the system as a lot of bars/restaurants, etc sound absolutely terrible. I want to achieve a more balanced sound overall.


The frequency response (SPL) graph is best for that. You can use it to help equalize the system’s frequency response to achieve a smoother sound (e.g. eliminate the boomy bass and excessive highs you mentioned). 

Waterfall, IR, RT60 and all the other graphs you’re concerned about are of minimal use for achieving smooth response. They give acoustical information about the room that can be used for treatments, in order to reduce reverberation, echo, etc. I don’t know how things are in Hong Kong, but I’ve never seen anyone install acoustical treatments in restaurants and bars. Studios yes, but not restaurants and bars.

Still, if you’re interested in the acoustics measurements simply for your own edification, I’d second Nick’s suggestion on reading the REW Help Files. There is a link with details on each type of graph.
http://www.roomeqwizard.com/help/help_en-GB/html/

You can find further information at the Acoustics Frontiers blog. I’d suggest the starting with the topics on Early Reflections and Room Acoustics 101. You’ll find enough reading here to keep you busy for days, including “case studies” on applications of acoustical treatments.
http://www.acousticfrontiers.com/category/blog/

At the Tape Op Message Board and Gearslutz Forums you’ll find people knowledgeable on acoustical treatments for studios.

Regards, 
Wayne


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## djryanash (Nov 5, 2015)

Hey everyone,

Thanks for all your help. Your answers were fantastic - very detailed. 

I really appreciate your time. 

I'll do some research and experiments, etc and tell you how I get along. 

Regards,

Ryan


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