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

[Richard Greene]

Parametric equalization does not eliminate standing waves --it has no direct effect on standing waves because standing waves are caused by reflections ... and equalizers

have no direct effect on reflections. Bass traps do.

A room full of bass traps does significantly reduce

standing waves. I have listened to music in a room filled with about 12 DIY bass traps and the sound quality was similar to listening to bass outdoors (smooth bass frequency response and fast decay). But not only are a dozen bass traps expensive, they are also ugly (to me) and completely unacceptable to most women.

So rather than using a room full of ugly bass traps, the second best thing to do is to reduce output at those frequencies that "ring" in your listening room using a parametric equalizer as a notch filter. The Behringer Feedback Destroyer at $129 is also incredibly cheap.

After parametric equalization, the same frequencies will still "ring" in your room, but the peak SPL of room modes will be no louder than other bass frequencies, so they will not be noticed. The room mode frequencies will still decay too slowly, but once again at a lower SPL that is less noticable. So the parametric equalizer has simply reduced the SPL of a room acoustics problem so that we (hopefully) don't notice it at all while listening to music.

The relatively small home listening rooms suffer from

standing wave problems far more often than very large rooms

such as studios and auditoriums. What we should remember is that we usually equalize to reduce standing wave frequency response peaks audible at a specific listening position -- that's not useful in a very large room or auditorium where there are many seating positions and

you can't suboptimize for a single seating position because the sound quality at other seating positions may

deteriorate.

The bass frequency response in my own listening room is so bad (even worse than the +/-9dB measured using 1/6 octave sine waves suggests) that I would not use a subwoofer at

all without equalization.

My primary bass peak is so wide that in the past I was able to reduce deviations to +/-6dB using only the 1/2 octave 45Hz. control of my Audio Control Richter Scale

crossover/equalizer and later to +/-5.5dB with my Alesis

MEQ230 30-band equalizer. My Behringer Feedback Destroyer worked best of all -- just two bands of equalization

reduced the bass frequency response deviations at my listening seat to +/- 4.5dB

All of these equalizers were set using sine waves

(using pink noise there is no room mode problem --

yet my ears tell me there IS a serious room mode

problem that desperately needs equalization. Sine wave

tones correlate with what I hear from my subwoofer

-- pink noise does not).

 

[Ron Shaw]

Well, I guess I'm outnumbered here. If you like sine waves, that's fine. As far as home Hi-Fi vs. pro sound, I should say that I got into the studio/sound reproduction field because of my love for music, interest in DIY, and HI-Fi as a hobby, so I certainly do know where home audio folks are coming from. I knew that disks could be mixed better if the engineers had decent playback equipment. I should say that my consulting work has nothing to do with the recording portion of the studio, but with the music reproduction side. I leave the mics and cables to the mixer to others, and concentrate on the best playback possible for the environment. Most studios are at least as big as the typical home theater room, so the speakers aren't really near field monitors, and a sound stage is very large (it can house an entire 100 piece orchestra). I have experience with almost all types of loudspeakers from sealed bookshelf systems, BR, large horn systems, Magnaplaners, etc. (I have come back to horns for the most realistic reproduction). I know many have low opinions of horn systems, and rightly so if your experience is a poor horn system, but that's another topic.
What I meant by the parametric equalizers not being what most people are used to is that they are not commercial units, nor can they be adjusted. They are fixed modules, tailored to a specific system. We make the modules ourselves, based on system/room measurements, and each band has a set center frequency, Q, and gain (or cut). Similar commercial filters used to be available from Altec Lansing, but they are gone, now. The chassis is rack mounted, and can take up to eight plug-in modules. Our spectrum analyzer is CRT based, not LED, and has an averaging display, so its quite stable, and has very good resolution (seems better than 1/10 octave). We use an EV RE-55 dynamic mic (wide smooth 20-20kHz response), calibrated to the analyzer. We have the system re-calibrated every 6 months, or sooner it its suffered a shock. We pride ourselves on delivering high quality audio, and find that the simplest path is the best path. The best thing about this job, is you get to play with all kinds of equipment at someone else's expense!

 

[Wayne A. Pflughaupt]

Sounds like you do some really interesting work, Ron. I’m happy to see someone with a hi-fi background designing studios. Frankly I wish they were all designed by guys like you. If they were, the recording engineers would (hopefully) be able to hear how nasty all those cheap mics and digital processors sound and stop fostering all those mid-to-low-fi CD recordings on us!
I lived anywhere near I’d certainly be signing up for your crew.
Regards,
Wayne A. Pflughaupt

 

[Sasha_G]

Ron,

This is a facinating thread, and I am trying to understand all the viewpoints. I have a question for Ron. Dr. Hsu thinks that certain pink noise/warble tones are better that sinewaves for measuring bass. I'm no acoustic engineer, but lets see if I undertsand the fundamental reasons for using pink noise:

pink noise/warble tones smooth over small variances in the room's response.

Sinewaves are set for a specific frequency, but the pink noise/warble tones vary in frequency and average at a certain frequency.

Because the room's response can change dramatically with just one Hz, it makes sense to smooth out the measurements a little. Does this make sense, or is your rationale different?

Also, I know this is getting down to the nitty gritty, but I'm curious how many room measurements you take in a typical job using the the pink noise? That is, how many room locations and where are they typically? Do you aim to equalize a sweet spot, or a more general area?

 

[Richard Greene]

YOU WROTE:

"pink noise/warble tones smooth over small variances in the room's response."

********************RG replies:**********************

Actually pink noise and warble tones smooth over LARGE variances in the room's response.

A slow sine wave sweep will typically show frequency response deviations of +/-15dB (or more)!

Audio magazine used to publish unsmoothed in-room frequency reponse "curves" with their speaker reviews -- scary looking charts -- fortunately our ears smooth the most of the many peaks and troughs ... and the music does not sound as bad as an unsmoothed frequency response chart looks.

At low frequencies the peaks and troughs (caused by standing waves) are easier to hear because room modes are not as dense (peaks are too few and too far apart for our ears to smooth them). The room "booms" at certain frequencies related to room dimensions -- sometimes an incredibly loud boom, as in my listening room. If a room booms (aka "rings") at 40-55Hz., as my room does, the problem can be significantly reduced by using a paramtric equalizer as a notch filter so that after equalization the output at 40-55Hz. is no louder than output at other bass frequencies. Steady state sine waves are needed to set up a parametric equalizer for use as a notch filter.

Graphic equalizers need pink noise or warble tones

(warble tones are typically a sine wave sweep that

cycles five times per second).

************************************************** *****

YOU WROTE:

"Also, I know this is getting down to the nitty gritty, but I'm curious how many room measurements you take in a typical job using the the pink noise?"

*********************RG REPLIES:**********************

Two (one for my left ear location and one for my right ear location) -- obviously I'm only equalizing the sweet spot!

Equalizers work best for a single location ... but optimizing frequency response for a single seat can deteriorate sound quality at other seats.

If you had a row of seats in your A/V room you could measure at each seating location and average the data ... or you could measure where YOU sit and ignore every other seat! There is no need to measure locations away from where people will be sitting.

 

[BruceD]

Sasha,

Your comments for using pink noise,

Because the room's response can change dramatically with just one Hz, it makes sense to smooth out the measurements a little. Does this make sense, or is your rationale different?

Myself, I want to measure and identify the frequency of each bad frequency spike so I can correct them with a parametric EQ.

So, pink noise tends to gloss over the problem frequencies and doesn't let you know they are there, while sine waves will clearly identify the specific problem frequencies.

That's the quick synopsis.

 

[Ron Shaw]

Sasha, you are correct in part of your assumptions. Pink noise is wide bandwidth information of equal energy per octave, and being random in nature, does not excite room nodes, and lets you measure the response of the loudspeaker system. Warble tones, which are rapidly changing swept sine waves, do the same by not being constant. Using fixed sine waves allow for room nodes to be excited, and you are now measuring the loudspeaker/room for sustained frequencies. As I mentioned earlier, a lot, if not most, very low frequency information is impulse in nature, not sustained, so using a method of measurement that excites to room nodes will give a false impression of what is happening. Again, let me state that solving problems with standing waves is important. I just think that it is a problem that needs to be addressed as a separate issue. Room nodes are also a function of more than just room dimensions. Many factors influence room nodes, such as dimensions, construction, placement, etc. Typical stud/sheetrock walls are actually a bunch of bass traps, flexing, and therefore canceling/reinforcing certain frequencies (and since stud spacing is more or less uniform, they all act at the same frequency). Floors with a crawl space bellow (as apposed to a slab) will also have its own affect at certain frequencies. Most domestic installations will be hard pressed to solve many of these problems for various reasons, but it may influence your choice of rooms. For instance, I find that basement rooms work better (slab floors/rigid walls).

As far as measurements go, it depends on the installation. For large spaces which involve many listeners (such as an auditorium or theater) we typically take 5 readings. One at center, 2/3 back from the screen, and a reading in each of the 4 quadrants of the room, and average them. If we get readings way off in one location, you cant do much about it, so we usually throw it out! With controlled dispersion loudspeakers (like horns), the readings can be quite close to each other, and we can usually get the response to within +/-6 db throughout. For a recording studio, we aim for the sweet spot, which is all important. We don't care what others in the room hear, its important for the engineer to hear exactly what he needs to hear. When we consentrate on the sweet spot, we can often get to within +/-3 db for the entire spectrum. Let me again give the reasons why I prefer to equalize the entire spectrum. Somewhere, you need to make adjustments to give the smoothest output from a loudspeaker possible. In most commercial systems you by, this is done within the loudspeaker, at the crossover, using resistors, capacitors and inductors various configurations to give the desired result. Some crossover designs I have seen have used over 100 elements! The result on the output can vary from excellent to outrageously poor. I find that the very complex crossovers do not serve the music, and introduce amazing amounts of phase anomalies. I mentioned earlier that we strive for the simplest solution. We prefer to use a simple crossover unit, adjusted to get a fairly even output from the drivers, and do the fine tuning with an equalizer. This not only takes care of the loudspeaker response, but at the same time takes care of the speaker/room interaction, while introducing less phase problems a complex crossover adds. Impulse testing will always show better characteristics using this approach. What at first seems like a more complex system (the addition of an equalizer), actually becomes the simplest (and in my view) a more 'purist' solution.

We sometimes have people bash our use of wideband equalization for the effects of phase response (at least that what they have read), but totally ignore early reflections in their own listening rooms. Spare me, please!

We are advocates of the LEDE approach (live end/dead end), which is the only way we have found to give a phase coherent wavefront (short of being outside). The LEDE approach basically means to treat the front (source) end of the room to eliminate reflections, using sound absorbing material, or diffusers, to eliminate early reflection, which can cause smearing of the wavefront. This can be done in an attractive fashion, even in a domestic environment by using thin panels from floor to ceiling, or even drapes along the walls. The floor is usually taken care of be carpet, and the ceiling may or may not be a problem. Well, Im getting off topic and starting to ramble on, as I can so easily do on this subject, so...

 

[Wayne A. Pflughaupt]

Interesting stuff, Ron. It appears you do a lot of installations with custom –designed speaker systems, even custom electronics (e.g. those parametric modules you mentioned). But again keep in mind this is not pro-audio and most HT enthusiasts use commercially manufactured speakers, which is probably why many of them claim that EQs has a bad effect on the sound (allowing the admittedly generous assumption that they actually know how to use it). It may be aggravating the questionable phase situations the speakers already have.
That said I’m intrigued by your preference to keep crossovers simple and accomplish the tweaking with an equalizer (knowing the often less-than-desirable effects of passive elements). I’ve toyed with the idea of making my own speakers from time to time but quickly dismissed it because of the daunting prospect of passive crossovers. From what I can tell, passive crossovers are the bane of DIY speaker hobbyists.
So I wonder if you have any thoughts on completely loosing the passive crossovers and going solely with electronic crossovers. It seems a no-brainer to me that this is the way to fly, but it doesn’t appear to be a course embraced by the hobbyists.
Also, you keep mentioning “a lot, if not most, very low frequency information is impulse in nature, not sustained.“ I really can’t see where this comes from, and indeed being a bass player myself I totally disagree with this supposition. It is very common for a bass part to have notes that repeat or sustain for a whole measure, even more. Even notes that sustain for only half a measure can easily have a one-second duration, which is plenty of time to excite a standing wave. The same holds true for other bass-capable instruments, like keyboards. Actually, a kick drum is the only instrument I can think of as a quick, impulsed low frequency source.
Not to mention – those low-frequency-intense explosions in action movies can be five to fifteen seconds – even longer - in duration. What’s “impulse in nature” about that?
Thus fully exciting room modes with sine waves and equalizing accordingly makes perfect sense.
Regards,
Wayne A. Pflughaupt

 

[Chris PC]

I can't understand using anything other than a sin-wave. It doesn't matter what you say about techniques. Sine-waves are pure sound waves. If you want to obtain better, or flatter response in your room, you want to find what frequencies are resulting in non-flat response. Sine-waves do this. This is how you find peaks and valleys in repsonse in your room. This is exactly what you want to find. Find the reponse and treat accordingly, be it with placement, crossover or EQ.

 

[Ron Shaw]

I was speaking more in the HT arena when I talk of low freq. info being mostly impulse. Things like explosions, etc., are more like pink noise in their random nature. For music, of course, things change, but not by much. The treatment of room nodes as a separate issue takes care of that, anyway. Even in music, much info (which I find important, so the reason I don't want to sacrifice it) isn't steady state. Hall ambience is very noticeable on classical material. Anyway, we can debate this issue forever, apparently. I prefer to treat response and room nodes as separate problems, and many of you prefer to treat them together. That's fine if it satisfies you. As far as electronic crossovers are concerned, I agree wholeheartedly. We almost always bi-amp. I prefer tube amps for the mid and hi end, and solid state for bass. We use Audio Research electronics almost exclusively. We build our own electronic crossovers. A big improvement can be had from crossing over electronically from the bass to the midrange. We use passive crossover networks for the midrange/tweeter transition. I have tri-amped on occasion, but find the benefits do not outweigh the cost. Tube amps sound very transparent and liquid on the mid and high end, but the high damping factor of a solid state amp sounds better for heavy woofer cones, at least to me. Bi-amping also allows you to use two lower power (and lower cost) amps instead of one large one, and still get the same dynamic range. Once you bi-amp, you will never (want) to use a full range amp again.

No need to fear home built loudspeakers. You will invariably be surprised at how good they can sound for the money invested (the typical cost/MSRP of a commercial loudspeaker is 1:5). If you run into a problem you cant figure out, you can always get info on the web.

 

[Manuel Delaflor]

Ron and Richard,

Must excellent information, very useful.

 

[Richard Greene]

Thanks Manuel

Ron and I are talking about completely different uses

for equalizers.

Parametric equalization to reduce the SPL

of specific room modes heard at a "middle-of-the-room"

sweet spot is optimizing frequency response for a single listening position.

Since bass frequency response varies so much as you move around a ordinary home listening room (often called a "small room"), optimizing one listening position may worsen frequency response at other seating locations, especially those closer to the walls/corners.

I have seen a sound meter change 6dB with only a 12"

change of position while measuring sine wave tones to set up a parametric equalizer. Steady -state sine wave tones have only one very specific use with parametric equalizers -- reducing the SPL of standing wave-related frequency response peaks heard/measured at a specific listening position.

Trying to use a parametric equalizer to tame frequency peaks at a variety of listening positions is a losing battle. At 70Hz. for example, the distance between a room mode peak (loud 70Hz.) and a null (weak 70Hz.) is only about 4 feet -- if you used sine waves, you'd have to select which listening position to optimize ...

and then one listener would be happy and another listener just four feet away might (correctly) believe the sound quality was WORSE after equalization .

 

[Manuel Delaflor]

I know what you mean. I was astonished when I first started to use an SPL Meter with tone tests, it is very difficult and take a lot of work to really get the best bass that your system is able to achieve.