Sines vs Warbles and what's really happening in the room (1 Viewer)

[Steve_Ma]

What's the difference between sine and warble tones and what's it mean with regard to my room response?

I've been experimenting with the Stryke disk, and various x-overs, connections, speaker settings, and etc. If I use sine waves vs warble tones, I seem to get different responses above 120hz or so. The good news is that my bass response is reasonably flat from 25 to 80hz or so. From 80 and up....not so great, depending on which tone I use.

Any input appreciated.

--Steve

 

[Ron Shaw]

The problem with using sine waves for response testing is that they set up 'standing waves' that re-enforce or cancel each other at certain places throughout the room, and give a misleading response curve. This is the reason that 'pink' noise is the noise source of choice for measuring room response. The random nature of pink noise will not set up standing waves. Warble tones are used for the same reason. The continuous variation of a sine wave makes standing waves less problematic. I find warble tones still not quite as good as pink noise, but acceptable, anyway.

 

[Richard Greene]

Use steady-state sine waves or a slow sine wave frequency sweep for subwoofers (under 100Hz.) to fully excite standing waves because standing waves are audible and should be equalized. Use of pink noise or warble tones for bass equalization is a serious mistake because bass sound quality is determined mainly by listening room acoustics, particularly standing waves.

Use pink noise or warble tone sine waves to equalize main speakers (above 100Hz.) because at those frequencies standing waves are close enough together that they are rarely audible as individual frequency peaks (our ears "smooth" the frequency response we hear --

roughly 1/6 octave smoothing)

 

[Steve_Ma]

Thanks for the responses. So Richard, Does this mean I'd be better off using Sine tones up to the crossover (fixed at 100) and then engage the Warble tones to measure through and beyond that?

If I read your post correctly, a flat response in the upper freqs is not really that big of a deal or is certainly not going to affect the quality of the reproduction as much as having a good bass response or dealing with room treatments and etc.

Am I on the right page here?

--Steve

 

[Ron Shaw]

I disagree. Sine waves shouldn't be used for low frequencies either. Standing wave problems should be handled as a separate issue, not by equalization.

 

[Richard Greene]

To Ron:
You can deal with one specific room mode with clever
subwoofer placement, or by using two subwoofers on opposite sides of a room. To deal with room modes in general, you need parametric equalization -- there are no acoustical solutions other than a room full of bass traps that are not pleasant to look at - a very low SAF (Spouse Approval Factor). There are no alternate feasible solutions to
sharply reduce bass frequency response variations heard at the listening position.
If you don't want to believe me you may want to read an article by Floyd Toole of Harman.
See this link.
http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt3.pdf
In my own listening room, a subwoofer is not tolerable without equalization. The bass frequency response deviations are +/-9dB before EQ measured using 1/6 octave sine wave tones (and an unequalized subwoofer sounds even worse than +/- 9dB suggests) compared with +/- 4.5dB after applying just two bands of parametric equalization. Reducing frequency response deviations by 50% or more is a typical result from the use of a parametric equalizer with a subwoofer.
Based on 20 years of experience with bass equalization,
it is my opinion that in 90% of all listening rooms, an inexpensive subwoofer such as the Adire Audio Rava for $400, when used with an inexpensive parametric equalizer such as the Behringer Feedback Destroyer, will
outperform any expensive subwoofer used without equalization. "Outperform" refers to bass quality --
not peak bass SPL.
Bass sound quality is determined primarily by room acoustics -- the brand and model of subwoofer used is far less important to the sound quality that reaches your ears.
There is no logical reason to use pink noise or warble tones for subwoofer equalization. Use of these test tones is equaivalent to pretending that standing waves don't exist, or are not audible.

 

[Ron Shaw]

I agree that equalization is needed (and not just for the low frequencies, but for the entire bandwidth), but I find I get more accurate results using pink noise, which lets you measure speaker response, and not room nodes. Placement and room treatment are needed for standing waves.

 

[Michael R Price]

Well, think of it this way. Standing waves usually take a little while to fully 'excite'. Play a low tone, chances are the SPL will start out at some level and then increase or decrease over time. The warble tone, or a sine sweep doesn't last long enough to excite the resonance.

So I honestly don't know which you should focus on: the 'instantaneous' response or the 'over time' response. But this should be a good discussion.

BTW, I measured my frequency response with the 'long sine wave' method and the result was terrible. 15db dip at 60Hz and decreasing in general under that (only a 1/3-octave measurement so I don't know exactly what happened). But playing a quicker sweep, I heard a much smoother response which was strong to 20Hz! And music sounds fine. So I don't know what the deal is.

 

[Wayne A. Pflughaupt]

Actually, a room node is fully excited in less than a second. Sound travels pretty fast, y’know.
I’m not especially fond of sweeps. I prefer fixed-tones so I know exactly where the problems are, so I know where and how much to equalize.
I’ve used both pink noise and sine waves, and I find I prefer sine waves – at least for low frequencies.
The randomness of pink noise makes it difficult to take accurate readings because the meter or RTA is all over the map. Broadband pink noise especially is a very difficult signal to negotiate below 100Hz.
I had much better results with filtered pink noise, but again the meter was all over the place, and it gets worse the lower you go. Fortunately, I have the digital RS SPL meter, and I was able to use the time-lapse averaging feature to successfully equalize my subs.
Then I got the Stryke disc with sine waves, and it was like going from an old Volkswagen van to a Cadillac. Sine waves are a very stable test tone, so the meter pretty much stays put. You can get an accurate reading within 4-5 seconds, vs. 15-20 seconds per frequency with filtered pink noise (and about that much time again to reset the meter for each reading and start the tone again). You can accomplish complete 1/6-octave sub-range readings with sine waves in the time it takes to do about 3-4 accurate filtered pink noise readings.
There has always been debate about whether or not an equalizer can deal with standing waves of room nodes. Regardless of that, an equalizer can smooth response and deliver excellent performance to an otherwise unlistenable room/sub combination. Are the room nodes still there after equalization? Perhaps, but ask anyone who has successfully EQ’d their sub if they care.
I’ve seen dozens of people attempt sub equalization on this Forum in the past two-and-a-half years I’ve been participating; I’ve yet to see anyone ditch their EQ and conclude, “This sucks – I had better luck experimenting with treatments and different locations.”
Regards,
Wayne A. Pflughaupt

 

[BruceD]

Ron,

The only truly effective methodology for using shaped pink noise to get an in-room frequency response is to use a system based on an MLS (Maximum Length Sequence) signal and then perform FFT (Fast Fourier Transform) analysis on the captured impulse response for a frequency response.

Otherwise, I agree with Richard, especially for in-room bass response. You need to identify the room's modal peaks and the only way to do that effectively with test tones and an SPL meter is to use at least 1/6th octave sine waves as a source signal.

Warble tones are good for tuning the crossver between a sub and mains.

 

[BruceD]

I agree that equalization is needed (and not just for the low frequencies, but for the entire bandwidth),

I tend to disagree with this statement. I would never equalize any of the frequency spectrum above the bass xover.
IMHO I find a speaker's character above the bass frequencies (how it sounds) is why I bought the speaker in the first place. As long as first reflection points don't cause an imaging problem, I feel equalization won't improve the sound of my speakers, in fact it is more likely to degrade it with the phase shifting introduced by EQ.

 

[Wayne A. Pflughaupt]

Wow, Bruce, you surprise me here. I’m used to seeing such well-tempered responses from you on these subjects!
I usually “high five” every post I see of yours, Bruce, but I have to part ways with you on this one. Room effects start to influence speaker response at 500Hz and can be considerable below that point, so IMO it makes no sense to dismiss full-range equalization out-of-hand.
For instance, if your mains are in an asymmetrical room, say one near a corner and one near an open doorway, the speakers can exhibit audibly (or at least measurably) different in-room response, especially at frequencies approaching the crossover point. Thus 1/3-octave or parametric equalization can help restore spectral consistency between the L/R speakers.
If you’re concerned about transparency you could biamp the mains and equalize only the mids/woofs – typically that is where any equalization would be needed anyway (if indeed it was needed at all). I recently noticed another participant here on the HT Forum who successfully did this with his rather high-end speakers; he was very pleased with the results; I was surprised - shocked, actually - to see that he was using a very cheap 1/3-octave EQ (I wouldn’t even use the one he was using – for anything!).
I tend not to take claims of “phase shift” seriously because no one seems to have any idea exactly what it sounds like. Besides, a speaker’s passive crossover elements also introduce phase issues.
Like all things subjective, I think it’s best to form an opinion based on experience, not theories. Can’t tell you how many times I’ve made impassioned arguments on this forum based on what seemed to be sound and logical theories I had studied, (theories complete with impressive mathematical formulas ), only to come away blushing sheepishly once I took the time to put them to real-world tests.
Regards,
Wayne A. Pflughaupt

 

[BruceD]

Wayne,

Yea, I'm probably a bit off the mark.

I never liked how graphic EQs seemed to distort the sound. I can't biamp my 2-way main speakers anyway, so I can't actually use a parametric on them.

Even though I have an asymetrical room with openings and a cathedral ceiling my mains maintain audible consistancy and imaging. Measurements are slightly different, but not by much.

But, you're right others may benefit from EQing their mains.

 

[Richard Greene]

The test tones you should use depends on what you want to

do with them. Sine wave sweeps and sine wave tones that are used for setting parametric subwoofer equalizers are

not useful at all for graphic equalizers.

A very slow frequency sweep tells you the total bass frequency response deviations including all standing waves.

You could also do this with steady state sine waves for every frequency from 20Hz. through 100Hz. = 80 SPL measurements!

Sine waves spaced 1/6 octave apart allow quick measurements (only 15 SPL measurements required from 20Hz. to 100Hz. rather than 80) and I use them for that reason (setting up a parametric equalizer is an iterative process).

But 1/6 octave-spaced sine waves do smooth the frequency response somewhat compared with a frequency sweep.

For example, using a frequency sweep may tell you the loudest bass peak is 90dB ... but using 1/6 octave sine waves, the loudest bass peak might measure 87dB.

This can happen when the center frequency of a standing wave peak happens to be located at a frequency between the frequencies of two 1/6 octave sine wave tones... and means you really need about 3dB more attenuation of that frequency peak than measurements using 1/6 octave sine waves suggested.

3dB doesn't sound like much but it is audible for bass frequencies (for subwoofer bass frequencies, +5 to +6dB sounds like doubling the SPL)

 

[Steve_Ma]

Great thread. Thanks for taking the time. Very informative and it's giving me some solid points to consider.

--Steve

 

[brucek]

Yeah, I have to agree with Richard on his last point.
I use 1/6th octave sine wave tones for setting up my parametric equalizer and recommend this to anyone that asks.
But, when I find a peak using 1/6th octaves and want to enter a filter value, I will use 1 hertz tones around that peak to establish the optimum value for the filters frequency. Sometimes, the real peak is hiding between the 1/6th octaves.... Probably being a little crazy doing this, but it makes me feel good..
brucek

 

[Ron Shaw]

I still stand by pink noise (or warble tones) for low frequency measurements and equalization. I agree that readings can be more difficult. That's the way it is. It helps to use the slow response of your meter, if you have one. We actually use a spectrum analyzer, which makes it easier. The reason you don't want to use sine waves is because a lot (if not most) information in the very low part of the spectrum isnt sustained information. Its things like explosions, room reverberance, kick drums, and a whole multitude of other misc. sounds that are impulse in nature. If you use sine waves, and equalize based on standing wave patterns, you are sacrificing the accuracy of this type of information, and just moving a foot will uncover a different set of problems. I am not saying that dealing with standing waves isnt important. It is indeed, but not at the detriment of the audio signal. It needs to be delt with in other means. Again, not necessarily an easy task, but taking shortcuts isnt the ideal solution. As far as a persons opinion on wide range system equalization, I guess it's a personal choice. I found decades ago that for most systems, applying eq to the whole system results in a far more satisfying system than no eq. If you are lucky enough to have a system and room that work well together, than that's great. When we design and build a studio or sound stage, we always voice (eq) the system for the installation. The amount of phase shift is negligible when you consider the entire system. I don't think I have ever ran across a client who preferred the sound unequalized. We use both 1/3 octave units, and parametric types. The parametric eq. we use isnt what most consumers are used to. We measure the room, and custom build individual filters for each point we need to adjust. It may only take two or three filters, or it can take as many as eight. If it needs more than that, we use a 1/3 octave unit.

 

[brucek]

Ron,
Interesting stuff. I'm always trying to better understand acoustics.
I'm having difficulty getting a handle on what you're saying here. Although this is obviously your field of expertise, I'll have to agree to disagree with you and go with the accepted idea, (around this forum anyway) that pure sine wave tones are the preferred signal to carry out a low frequency response test...
I have found in my system, that when I use MLS signals and a spectrum analyzer as BruceD mentioned above, to carry out a response of my subwoofer at my listening position, that the resultant graph very closely matches the frequency response curve that I get when I play my sixth octave sine wave tones and take readings with a corrected Radio Shack meter.
I simply have a couple of room modes (that are also easily calculated given the room dimensions) around 35Hz and 55Hz. These two peaks exist and cause uneven response of my low frequency spectrum. A simple sine wave exciting these modes shows this to be true. The two peaks cause a bloated sounding bass at those frequencies when I play music and when I play movies.
When I use my parametric equalizer in the subwoofer chain that has 12 possible filters that I can set any cut, bandwidth and frequency that I can bring to bare on these peaks, I end up with a nice flat response. The bass then sounds very even and smooth for both music and HT.
It takes less than a second to excite a room mode. Music and HT have copious quantities of low frequency energy at single frequencies for that long, and when it does, the room mode is excited and I get a peak.
My feeling is, that I want to equally excite that mode when I do my tests, so I can tame it with my EQ. Pink noise just doesn't seem to cut it at these low frequencies. I'm not really interested in subwoofer speaker response - I know it's pretty good by doing near field teats. Its response is a small issue in relation to the havoc that the room causes. My interest is in exciting and then taming that room with EQ. I need sine waves to do that.
I don't understand the "shortcuts" you are referring to in your post, since in the last few sentences you talk about using a parametric EQ to make a room sound better? I can design any filter I desire with a simple BFD EQ. Why is your parametric any different?
I wonder, is a home HT/music system, where we sit in a defined spot, not different from the EQ situations you are talking about in a large club or sound stage?
brucek

 

[Michael R Price]

Hmm, maybe the change in sound level was from me moving the sound meter slightly. Yeah, probably. Sorry about that, it makes perfect sense that standing waves get excited very quickly.

I read that above 140Hz or so, while the room does affect frequency response your ear can tell the difference between the initial sound from the speaker and the reflected sound from the room. If you have a piano playing in your room, it will sound like a piano in your room. If you have a speaker playing the sound of the piano, the reflected sound will be different but it will still sound like a piano. If you equalize the room effects, then you will have a very strange sounding piano. Just a thought.

 

[Wayne A. Pflughaupt]

Ron,

When we design and build a studio or sound stage, we always voice (eq) the system for the installation.

I think this is the basis for the conflict we’re having here, Ron. I used to be a pro audio installer, so I’ve been where you are; but I was into hi-fi long before that. So as one who comes from both sides of the fence I know that pro audio and home theater are worlds apart.
Of course there are speakers and amps, etc. but that’s about where the similarities end.

• There are divergent acoustics involved: A living room or dedicated space over a garage bears little acoustical resemblance to a 2000-seat auditorium (where reverberations are a chief concern) or an acoustically “dead” studio with Sonex-covered walls.
• The types of speakers used are radically different: Home theater or hi-fi speakers are not designed for near-field use, are not designed to play all day at 110dB and seldom use horn-loaded compression high frequency drivers, minimal-excursion woofers, or horn-loaded subs.
• Purpose of use and application also have nothing in common: The home theater is used solely for personal entertainment, not for an audio-related work product or sound reinforcement for a live band.

Even beyond all that, pro audio has its own “culture” apart from home audio. Just imagine, Ron, if some of our more esteemed colleagues at this forum were running one of your installations: “Okay, we’re going to loose the Meyers. Martin-Logans image a lot better, so we’re going be flying those instead. QSC? Hafler? Grainy. We’re going with Krell for the mains and Mark Levinson for the subs. Loose this Mogami crap and rewire the whole place with Kimber. That will give the Martins better detail. BSS? Sorry, no equalizers, no balanced lines either. We want to eliminate all that extra clutter from the signal chain, people. Has our delivery from BPT come in? We're using balanced power for everything.”
Or, at the other extreme, how ’bout some input from our brethren from the “Basics” Forum: “EV? That’s nice, but I’d really prefer a Sony sub. Crown? What the heck is that? Let’s get some Kenwood in here.”
Let’s be honest, Ron: Both the audiophile and the “newbie” would be laughed right off a pro job, wouldn’t they? And rightly so – they have no idea what they’re doing there.
Sad but true – the CAD guy at the company I used to work for once described for me his “dream” home stereo system, which included Turbosound speakers. Or the audiophile guy I worked with who thought he was a good installer - he liked to 1/3-octave EQ systems with nothing but his favorite reference music CDs. Mixing a live band he wouldn’t use the EQ to cut that hot, resonating frequency that made certain notes on the sax pierce your eardrums – didn’t want to sacrifice any “resolution.”
The point is, Ron, that a lot of people here have spent a lot of time developing various methods of EQing – subs or otherwise – borrowing from home audio, pro audio and even computer technology. The result is methods that work very well in the home environment. As I have already noted, there are dozens of “happy customers” here who are grinning from ear to ear after utilizing the capable advice of Richard Greene, Patrick Sun, Sonnie Parker, BruceD, brucek and others here.
But I don’t think any of those people would presume to show up at a professional installation and say “Guys, you’re doing this all wrong. I’m telling you, it’s going to suck.”
Despite my endorsement of using 1/6-octave sine waves for EQing subs, when I set up a pro system I don’t use any of that stuff. I pack my RTA and shoot the place with pink noise, just like you do. I also pack a few reference CDs to make sure things sound a good as they measure. Why the seeming contradiction? Because I know that very few pro audio systems are what an audiophile or even a hi-fi buff would call a “reference” system. I know I’ve seldom seen one. Set the crossovers and gains for an initial spectral blend, EQ out the feedback (if applicable), bumps and ripples as best as you can: boom, we’re outta here to the next job. I promise you no one in pro-audio spends 20 hours tweaking the subs.
I did a search on your previous posts, Ron, and found that you really have a lot of good things to say, really helpful and useful information. So there is definitely a place for a pro guy here. Just try to keep in mind the audience you’re dealing with here is not the same as the one at liveaudio.com.
Regards,
Wayne A. Pflughaupt