# What does a good waterfall look like?



## Kenneth R.

I've perused quite a few of the threads created by users submitting their first graphs. I will no doubt seek opinions on mine at some point as well.

My assumption on theory is that an ideal waterfall (barring constraints of the real world) would look like a smooth hill. It would increase fairly uniformly across the frequency band, tapering toward the extreme low and high ends, and decay fairly reasonably back down without any frequency ringing. Am I far off on this? I want to have a grip on what I am looking for to compare against what I see in the real world measurements, and I'm fairly sure I'm not the only one.


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## Wayne A. Pflughaupt

Welcome to the Forum, Kenneth!

A waterfall merely shows signal decay time for a frequency response plot. In the “real world” you won’t get the “smooth hill” you’re talking about. Any peaks in response will display a longer ring time than a depression in response. Also, in most rooms decay times increase at the lower end of the frequency spectrum. The primary use of waterfalls is to gauge the effectiveness of low frequency treatments such as bass traps – i.e, “before and after” measurements.

Regards,
Wayne


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## Kenneth R.

Right, but in a non-real-world situation, where nulls and hot spots are not a factor? I should just see the target curve, right?


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## Wayne A. Pflughaupt

Not sure I get what you mean. If all you see is the Target, that means there is no ringing. :scratch:

Regards,
Wayne


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## Kenneth R.

Ah, okay. so then the 'target' is a zero point? and level above that is the ringing of the room at specific frequencies...? I am trying to wrap my head around what a nonreal, ideal case would be. It is definitely an odd question!


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## JohnM

Here is an ideal waterfall:







In a room response the contributions of your room and its surfaces affect the level of the overall response (which roughly speaking appears in the first slice of the waterfall) and the rate at which the response decays (which you can see from the difference between levels of successive slices). Resonances in particular have slow decays and hence form the characteristic outcrops we see in real waterfalls. An alternative view of the behaviour is obtained from the spectrogram, which is in effect a top-down view of the waterfall in which colour is used to show level.


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## Wayne A. Pflughaupt

Kenneth R. said:


> Ah, okay. so then the 'target' is a zero point? and level above that is the ringing of the room at specific frequencies...?


”Below” that, actually. I’m sure you’ve seen waterfall graphs on other threads (or John’s above). The horizontal lines that show the signal decay are “slices.” In the REW program, you can reduce the number of slices. When you’ve reduced it to a single slice (i.e. showing no decay), that trace is identical to the frequency response graph. Make sense?

Regards,
Wayne


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## gsmollin

If you don't mind my jumping in on this thread, there seem to be two waterfalls being discussed, the good one in the subject line, and an ideal one that popped up later. The ideal waterfall that John provided has both flat frequency response, and flat decay time. Is this also "good"? Would we like room acoustics that produced this waterfall? I'm not sure, but I think we would like our delay time to decrease monotonically as the frequency increases. At least I think that is what we are used to hearing, and what's "good" may simply be be what is natural in a room without strong standing waves.

I just got a thought, where would one find waterfall plots, or other similar data on the room response of known venues that musicians consider as having "good" acoustics.


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## Wayne A. Pflughaupt

gsmollin said:


> I just got a thought, where would one find waterfall plots, or other similar data on the room response of known venues that musicians consider as having "good" acoustics.


 Waterfalls are mainly useful for the low frequencies in small rooms. As I understand, in auditoriums RT-60 measurements are used, not waterfalls, because they’re concerned with decay times for the entire audible frequency spectrum, not just the low frequencies. 

Regards,
Wayne


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## Kenneth R.

excellent, thanks for answering my question guys. I'm sure my room (and no room) will ever approach such a flat line, but I know visually where the ideal case is now.


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## gsmollin

Wayne A. Pflughaupt said:


> Waterfalls are mainly useful for the low frequencies in small rooms. As I understand, in auditoriums RT-60 measurements are used, not waterfalls, because they’re concerned with decay times for the entire audible frequency spectrum, not just the low frequencies.
> 
> Regards,
> Wayne


It's true that RT-60 times have been used in auditoriums, with or without frequency-bands imposed. I see no reason why a waterfall can't be used instead. Besides, I'm mainly interested in smaller venues anyway. I know my basement will never be a concert hall, but it would be interesting to see how it compares to famous small-venue theaters.


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## Wayne A. Pflughaupt

The point was that you most likely will not be able to find a waterfall for comparison, because AFAIK the professionals who take acoustic measurements in venues don’t use them.

Regards,
Wayne


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## GuitarCry

Thank you for the waterfall explanation. I'm starting to work my way on the fantastic REW software, and this info is priceless. Seems that my room is highly lively... A lot of work to do.


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## Kenneth R.

Right, I brought this topic up to get a sense of the never-reachable, ideal case just to visualize it. We'll always have bumps and wavering in the response, but it helps imo to have something to compare against.


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## robbo266317

So, are you going to show us your waterfall...


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## Kenneth R.

As soon as I can get the expensive mic from my coworker haha.


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## anwaypasible

i'm a little curious why the 'perfect example' has so many slices.. i thought that 'perfect' would have no slices at all (or at least, only the length of time because the actual frequency takes that much time to complete a sinewave)

and in that case.. the lowest frequencies would have the most slices and the highest frequencies would have the least amount of slices.

i am no audio technician.. therefore i am certain that my 'exact lengths in time' are not accurate.. but i can make an example that makes sense (then just replace the values with appropriate ones if you ever come across the exact values)

so..
if it takes a 10hz sinewave 1ms to complete.. it would show 1ms decay on the waterfall.
and if 10hz takes 1ms.. then it would take 20hz 0.5ms to complete.. and it would show 0.5ms decay on the waterfall.

to complete a note ONE time.. you have to make the positive and the negative of the sinewave.
remember sinewaves look like -^v^v^v-
and a SINGLE note is a single rise and a single fall.. like this -^v-

the amount of time it takes 10hz to complete is probably in the nanoseconds and not the milliseconds (again, i am not an audio technician)

also.. it appears as though REW clearly displays large amounts of milliseconds, which might make such small increments in time hard to see without zooming in.

but yeah, a perfect waterfall would be ruler flat along the top and slightly diagonal along the bottom.

since REW displays decay data in 50ms increments.. there should be only ONE slice of data in a perfect waterfall.

(feel free to make edits wherever necessary if you are indeed an engineer/technician)

personally.. i think that you would need to allow some milliseconds so that the audio actually makes its way to your ear drum... which is probably why the perfect example on page one of this thread shows more than one slice.
because in all-due reality.. who can hear a sound that lasts for less than 1ms ??

there must be some threshold that dictates what speed can be heard.. and therefore all tones must meet this threshold before being taken out of the air.


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## JohnM

anwaypasible said:


> i'm a little curious why the 'perfect example' has so many slices.


The waterfall plot is produced by taking the system impulse response and applying a window to it, outside which the content is ignored. The window starts at the peak of the response and is tapered away to zero either side. An FFT is then used to generates a plot of the frequency content of that windowed portion, which produces a slice of the waterfall. The window is then moved forward in time and the process is repeated. A "perfect" system has an impulse response that consists of a single peak, just one sample wide, and zero everywhere else. As the window moves forward the level of that peak is attenuated by the shape of the window, gradually reducing, until the peak falls entirely out of the window after which all slices are zero. 

The period of a single cycle of some frequency "f" is 1/f seconds, so a 10Hz wave completes a cycle every 1/10 of a second or 100ms. A 500Hz wave completes a cycle ever 1/500s or 2ms. The speed of sound in air is approximately 343m/s, it travels about 1 foot every ms, but that only affects the time it takes sound to reach us, not whether we can hear it or not. Our hearing is limited by the intensity of the sound, eventually it gets too low for us to detect, and by the frequency content, we can hear between roughly 20Hz and 20kHz.


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## anwaypasible

thanks for that bit of clarification.

to edit what i said with more accurate information.. the 20hz frequency MUST have at least 50ms so that the soundwave can complete a full cycle.
and 40hz MUST have at least 25ms
80hz must have 12.5ms
160hz must have 6.25ms

now if you wanted to stand 1ft away from the speakers.. the treble frequencies will have to play one SINGLE soundwave with high velocity and we wait for it to reach our ears.
because 1 divided by 20khz = 0.00005 seconds or 0.05ms

see 0.05ms / 1ft = 0.05ft or 0.6 inches

12 inches / 0.6 inches = 20 increments

you can either play the treble early or play it 20x so that the ear can hear it from 1ft away at 1ms of duration (but playing a 20khz sinewave 20x just so the ear can hear the sound at 1ft away for a duration of 1ms is only my guess-timate)

if you played a SINGLE 20khz sinewave cycle.. the result is obviously inaudible based on the math that john has given us.

being perfectly honest.. some people have a harder time hearing than others.. therefore some people can hear different decay times.
you basically shave the decay times down to as low as you are capable of hearing comfortably.

this might mean a ruler flat frequency response at the top of the waterfall and a ruler flat response down at the bottom of the waterfall with whatever decay time you are capable of hearing (but must AT LEAST be 50ms so that the 20hz sinewave cycle can complete.

therefore it might be completely normal to see a flat line of decay at 200ms or 300ms

either way.. flat one way and flat the other way is 'perfect' as can be seen in the pic that john posted.


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## JohnM

You are misunderstanding how sound propagates. You do not need to keep playing a sound until it reaches the listener, and the decay times in a waterfall plot have nothing to do with the period of any individual frequency. The Master Handbook of Acoustics has a good introductory chapter on the fundamentals of sound, I recommend you get hold of a copy and read through it.


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## GuitarCry

If a rock hits the water, the waves will propagate and reach some distance. You do not have to keep the rocks falling into the water to make the waves reach a reasonable distance. Cheers


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## anwaypasible

i know that the audio doesnt have to be constant directly in front of the speaker cone all the way to the ear.

i am surpised that i came off as if that is my method of thinking.
although i was speaking from the edge of my knowledge, so i probably worded something poorly.

anyways.. i got my point across that the frequency response should be flat and the bottom decay should be flat and the amount of decay depends on how well your hearing ability is.


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## Kenneth R.

Alright, I've finally snagged the nice mic.

I've run into two issues so far:

1. I need to calibrate the sound card (actually not a sound card, it is a firewire i/o interface) through a preamp channel as that is the way the mic connects, with phantom power. I don't want to damage my unit doing this.

In the mean time, I calibrated just fine using the line in and line out connections, however... that won't give an accurate result since when I actually measure, I won't be using those jacks.

2. There is no way the levels in this room will ever reach 75dB SPL. 50 was really pushing it, and 90% of the time the level will not exceed 30dB SPL. Is there a way to calibrate and measure at a lower level? This is to be a studio mixing room, not a home theater. I need an accurate picture of my room at the levels I'm going to use it at.

Suggestions...?


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## JohnM

It is possible to calibrate via a mic input, but you need an attenuator between the line out and the mic in to counter the preamp gain and of course you need to switch off the phantom power.

Are you sure about those SPL's? 75dB is not at all loud, 30 is virtually silent. You can calibrate and measure at any level that gives you enough signal above the noise floor of the room, but I think you are off on those figures. If you are reading them from the REW meter, you have to calibrate that against an external meter before using it.


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## Kenneth R.

Ah, I suspected something was up. Looks like I need to grab the external meter anyway. 

Supposing I put an attenuator in the signal chain, but remove it when I am running my test. Wouldn't this affect the calibration?


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## JohnM

Kenneth R. said:


> Supposing I put an attenuator in the signal chain, but remove it when I am running my test. Wouldn't this affect the calibration?


No, shouldn't do.


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## Kenneth R.

Finally have found time to take some sweeps:

Here's a first attempt.








First sweep, 1/48 smoothing, no treatments.







Same again, with 1/3 smoothing.







Waterfall data - default settings.







Waterfall data - extended time shows room noise after decay of sound. Frequencies point to computer noise, AC noise. 

Room is finished basement with double carpeting, fabric on most walls but nothing substantially acoustic-minded. 
Speakers are KRK Rokit 8" G2, pair. 
Audio I/O is PreSonus Firebox
Measurements calibrated and taken with Earthworks M30 mic, phantom power from Firebox.

Listening position faces open wooden stairs, side-incident. First reflection wall is about 8' behind listening position. Room is approximately 17'x15'. I've been attempting to recreate with AutoCAD as an exercise in teaching myself CAD, but that is still a work in progress.

*Thanks to JohnM and Wayne for the top-notch support!*


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## Wayne A. Pflughaupt

Wow, that’s some outstanding frequency response, Kenneth. :T Waterfalls are only useful up to about 300 Hz, so you might want to re-scale those graphs. It’s also best to lower the floor of a water fall graph to about 35 dB.


Regards,
Wayne


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## Kenneth R.

Will post a fixed graph then! Though I'm primarily concerned with the main speakers... the low end is still an interest. 

Now, to decide how to further improve the room with treatments.


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## GuitarCry

Wow, that is some good measurements for an untreated room. Wish my starting point would be like yours :clap:


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## Kenneth R.

As promised,

corrected scaling of waterfall to show default window:








Now to find some time to treat that 200Hz bump and the 60Hz ringing...


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## eljf

JohnM said:


> Here is an ideal waterfall:
> View attachment 25229
> 
> In a room response the contributions of your room and its surfaces affect the level of the overall response (which roughly speaking appears in the first slice of the waterfall) and the rate at which the response decays (which you can see from the difference between levels of successive slices). Resonances in particular have slow decays and hence form the characteristic outcrops we see in real waterfalls. An alternative view of the behaviour is obtained from the spectrogram, which is in effect a top-down view of the waterfall in which colour is used to show level.


hi John,

Is this decay time for a home movie room setup? 

Some people say, 2channel music decay times should be longer e.g. 300-400ms (low freq) and 200-300ms high freq.?

Or does it depend on the room, with a smaller volume needing a shorter ideal Rt60 e.g. 27ft cube small room has ideal RT60 of 160ms?

kind regards,
ian


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## JohnM

That plot is not a room measurement, and the waterfall is not an RT60 plot. How a waterfall is generated is explained in the help, as is the RT60 plot. The waterfall plot is useful in the low frequency region where RT60 plots are no longer meaningful. 

ISO 3382 has some recommendations for RT60 times, smaller spaces have lower recommended RT60 times but the lower volume limit for the ISO3382 recommendations is 1800 cubic feet. You'll find other recommendations from a search. It is sometimes suggested that multichannel listening environments have RT60 times below 0.3 s so that the room's reverberation does not obscure the surround cues and reverberation that have been put in the recording, but such a dry/dead environment may be less pleasant for music reproduction.


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