When a speaker loads the room, what is it doing?

[Wrinkles]

I have heard the term "loading the room" or some variation ot that, but never really heard a definition.

It is one of those sayings that I can kinda visualize a meaining, but when I have asked a few people, they cannot explain it very well.

Thanks,

Wrinkles

[Quiet_Hollow]

they cannot explain it very well.

That's because it's a [bs] idea to begin with.

[mustang guy]

Loading might be BS, but pressurizing is not. That is what takes place with low frequencies with wavelengths longer than the size of the room. Since the full effect of the wave doesn't fit, the pressure change lets you feel that note anyway. A 20hz tone has a 56' wave. Unless the room you are in is huge, pressurization needs to take place in order to feel the force of that note. Bigger room=more subs...

[Chris A]

I have heard the term "loading the room" or some variation ot that, but never really heard a definition...It is one of those sayings that I can kinda visualize a meaning, but when I have asked a few people, they cannot explain it very well.

I once asked Roy Delgado this question in an email. He mentioned three "regimes" or areas of loading:

1) far-field effects

2) near-field effects

3) room pressurization effects

I'll attempt to discuss these three below..

Chris

Edited April 15, 2014 by Chris A

[Chris A]

1) Far-Field Effects -

My understanding of this effect is like antenna theory or directivity due to reduced radiation space into an infinite volume. This is where the "pi" space discussions usually fall, IMHO. It is also where a lot of what PWK talked about corner loading reside. You get "gain" due to the fact that your space into which you radiate the energy is reduced. Think of a flashlight ("torch") bulb illuminating a room without the reflector, then add the focusing reflector behind the bulb, and the effects on the illuminated area of a room.

What is the far field? Generally, this is defined as more than 2 wavelengths at the frequencies that you are worried about. Clearly, for frequencies below ~100 Hz, anything in most listening rooms is in the "near field". I'd say that the Schroeder Frequency defines the near-field/far-field transition frequency. So now we are talking about mid-bass frequencies and above, in terms of "room loading". I think that you probably are thinking what I'm thinking - the math involved with "room loading" in the near field is going to be nasty since we cannot assume far-field effects when talking about low frequency room loading.

Chris

Edited April 15, 2014 by Chris A

[Chris A]

2) Near-Field Effects

Okay, if you are still with me, the next topic is the near field. The Wikipedia article on near field divides this into two regions: reactive near-field and radiative near-field. But this is only for electromagnetic fields with separate electric and magnetic fields that are not in phase. So we'll confine our discussions to "radiative near field" or the "Fresnel region" for acoustics.

So what we are talking about is the "re-radiation field" around the speaker. This is important for any objects that are within about one wavelength of your speakers. If you are using corner-loaded speakers, guess what...this is the effect that PWK talks about "mirror images" of the floor and side walls. The walls of the room must be smooth in order for the near-field reflections to be well in-phase with the direct acoustic energy exiting your speakers. This is what I talked about in the Corner Horn Imaging FAQ in terms of midrange bounce issues off the side walls from corner horns or corner-located speakers. PWK was a strong advocate of placing his speakers in the corners of the room.

Now we have a peek into what is happening - these are near-field effects, and many pundits shy away from this topic - even big names like Beranek, Olson, Toole, Geddes, etc. I think that there needs to be more papers explaining these effects. I advocate using absorption around the midrange horn mouths (and upper mid-bass) if using corner horns - since this significantly improves the stereo imaging performance of the speakers. But I don't advocate using absorption for low bass or high frequencies. Perhaps more on this subject later.

Also note that directionality is still a major factor in near field discussions - so we are talking about horn effects in the near field, and the Khorn actually makes use of the corners of the room to complete the last fold of the horn, albeit at a greatly increased effective flare rate of the horn which directly affects impedance bounces of acoustic energy back into the horn, and then back to the driver. When you test a Khorn in a non-reflective environment (such as testing it outside in a flat field) its electrical impedance curve is actually different than testing it in a corner of a room. Why?

Now we are getting to the meat of the discussion of "loading the room": since horns are excellent at transforming the mechanical motion of the cone driver into acoustic waves traveling in your listening room, it also works just as well in reverse, too. Horn-loaded loudspeakers also work like microphones. If you look at the Corner Horn Imaging FAQ section on SET amplifiers, you will see a discussion on why SETs and other high output impedance amplifiers are poor choices when using horn-loaded loudspeakers: all kinds of non-linear effects begin to color the high fidelity reproduction if used in small rooms with highly efficient horn-loaded speakers.

"Loading the room" in this case is talking about the near-field effects of the horn coupling into the room and presenting an added impedance or load to the horn's driver that it "feels". If the coupling is poor, the room isn't "loaded" and the horn's drivers don't react very much to the room's reflections.

Chris

Edited April 15, 2014 by Chris A

[Chris A]

3) Pressurization field effects

Pressurization (cabin gain) occurs when the closed acoustical listening room space is smaller than one wavelength of the reproduced signal. Acoustic wave propagation is in the form of pressure waves and is no longer wavefront propagation. This overlaps the definition of "near field", but we are now talking about how the waves propagate. Rooms with open doors and/or windows are not typically called "pressurization zones", but frequencies below the lowest room mode are considered to be pressurization zone frequencies.

Think of being inside a field drum with both top and bottom drum heads. If someone hits the drum head, the volume inside the drum is "pressurized" (actually it's alternately pressurized and rarefacted). This is the area that I believe a lot of people get hung up. Note that most rooms are not "pressure chambers" or even "acoustic chambers" in the sense of pressurization.

It occurs in automobile interiors quite well, however. And also in "acoustic suspension" or air suspension loudspeaker cabinets. The SPL levels inside these cabinets are way above the non-linear thermodynamic effects of moisture condensation in diffraction slots of horn-loaded speakers, so note that you are getting non-linear spring effects from closed cabinets. Additionally, the same effects occur in ported or slotted speaker cabinets, albeit at lower levels.

I don't believe that the term "loading the room" is synonymous with "room pressurization".

Chris

Edited April 15, 2014 by Chris A

[Don Richard]

http://audioroundtable.com/forum/index.php?t=msg&th=16684

[Chris A]

http://audioroundtable.com/forum/index.php?t=msg&th=16684

By the definitions used above, I'd call Mr. Parham's examples "near field" cases. The visualizations that he produced are good at highlighting some of the issues with placing speakers in corners, especially corner horns.

If you use K-402 horns, by the time that you get to 1Khz and above (up to the point where reflections are heavily attenuated: usually above 10Khz), you are looking at side wall interference patterns that he produced. Hence the suggestion to place 1-to-2 absorption pads on the side walls at the exit to the K-402 horn's mouth.

If you are using Khorns, then the frequencies at which interference occurs can be higher or lower, depending on the exact corner wall geometries and room-furnishing acoustic reflectors within about 3 feet of the top-hat. My calculations indicate that 4-5 KHz, you start to have a problem with side-wall reflections. Unfortunately, this is also a crossover region between the tweeter and midrange, and in the pre-2002 models, is governed by the natural roll-off of the K-400 midrange horn/K-55 driver combination. I would highly recommend placing absorption pads on the side walls next to the mouth of the midrange horns and some absorption material on the top of the speaker, sticking out a little in front of the top-hat front surface in order to absorb some of the midrange "ceiling bounce" energy due to its loss of directional control in the vertical axis at ~1.5-2Khz.

This also explains why PWK recommended placing his Khorns on the long dimension corners of the listening room - it further mitigates close-field reflection effects in the near field, by turning these reflections into far field issues.

Chris

Edited April 15, 2014 by Chris A

[mustang guy]

Great response Chris!

I had never considered the pressurization the same as cabin gain. This is interesting, as there is a 12db cabin gain in a car with a sub, so if you load up a room with enough sub power to pressurize the room, you will be building that same cabin gain. This means that the normal 3db gain when stacking subs, would be compounded by the cabin gain of 12db more once the room is pressurized.

Am I talking BS, or is this right?

[Chris A]

This means that the normal 3db gain when stacking subs, would be compounded by the cabin gain of 12db more once the room is pressurized.

 

If you are talking about a relatively small room, with the doors and windows closed - then the effect is like cabin gain in an auto interior.

 

Note that drywall flex is significant in my room, starting at ~20-22 Hz and below. Even though the SPUDs put out a great deal of energy (with flat FR) down to ~14 Hz, the drywall flex in my room nullifies this up to about 17-to-18 Hz.

 

Also note that my room is 15.5'w x 10'h x 39.5'd, (4.7 x 3.1 x 12 meters), and the Schroeder frequency is ~100 Hz: most other listening rooms are more like 200 Hz or above, so the "cabin gain" effect will probably extend quite high in frequency in these rooms if their doorways and windows are shut.

 

Chris

Edited September 3, 2015 by Chris A

[mustang guy]

My assumption was that even if you have a huge room, you could achieve max cabin gain if you added enough subs. I wasn't considering the surface area flexibility.

I am still thinking of cabin gain as the magical 12db increase, which is likely my problem. If you have enough subs relative to the size of the volume of the room, does the equation change (notwithstanding the construction materials and flex)?

As for the high frequency cabin gains in small rooms compared to low frequency gains with larger rooms, that is surprising. I didn't even consider that.

[Chris A]

My assumption was that even if you have a huge room, you could achieve max cabin gain if you added enough subs. I wasn't considering the surface area flexibility.

What you are advocating is basically the same thing that Toole recommends.

Note a word of caution: you need to use the exact same model-brand of sub all the way around in order to match the group delay of all the subs to achieve smooth response. You can think of this as the same kind of problem as matching the drivers/speakers in a stereo array.

Note that Toole really doesn't talk about near-field and far-field effects, above, and particularly avoids the discussion on AM and FM distortion in speakers (i.e., the reason why horn-loaded speakers sound so good). I believe that he is brand-name-constrained to not talk about these issues, since they will begin to chip away at some of his pronouncements on acoustics and speakers in his otherwise excellent book. Toole, I think, just doesn't "get it" when it comes to horn-loaded, corner-placed speakers. No matter: there are other JAES authors that recommend corner-located subs exclusively:

https://community.klipsch.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=69304

If you are willing to forgo room gain, there is a much more interesting approach to placing subs in a room:

http://www.avsforum.com/avs-vb/showthread.php?t=837744

Here is the original article, translated from German by yours truly, the double bass array. All the longitudinal modes are simply swept clean by this intriguing approach.

Chris

Edited April 15, 2014 by Chris A

[Quiet_Hollow]

It smells in here. [*-)]

[Quiet_Hollow]

The idea gets the button from me because I feel there's simply no way to establish a metric for "loading". Ranks right up there with musicians talking about "tone".

Model "such and such" really loads the room one might say? Okay, prove it.... and simply saying so isn't enough because there's no repeatability in that.

From an acoustics perspective, I find the descriptor terribly short on functional merit. Good sound is more complex than that. Hence the button.

In most instances, I wager on the room being responsible for letting the speaker do what it's supposed to do.

Just imagine if you saw it on a spec sheet:

"- Loads the room well."

Cool, now what does a tech do with that information? Does that mean it'll load itself? Maybe they should drop a load in advance? [:^)]

[Chris A]

The idea gets the button from me because I feel there's simply no way to establish a metric for "loading".

Actually, I've heard a metric that could be considered somewhat quantitative: how low the speaker loads (FR) below its expected anechoic or calculated -3 dB (Webster formula) cutoff point.

Roy Delgado (Klipsch professional product chief engineer and the co-inventor of the Jubilee bass bin) talks about horn/driver combinations in terms of how low they "load" the drivers before in-room horn unloading occurs. Note that this effect can be a relatively good one since horn unloading can sometimes be quite sharp vs. frequency. So this can be a design criterion or a in-room placement criterion and still have fairly good quantitative value.

Chris

Edited April 15, 2014 by Chris A

[Wrinkles]

Thanks Chris for the detailed reply.

And I thought it would be a simple explanation.

[Chris A]

And I thought it would be a simple explanation.

It's like almost everything in life...

"The simplest problems which come up from day to day seem to me quite unanswerable as soon as I try to get below the surface" Justice Learned Hand

Edited April 15, 2014 by Chris A

[dtel]

Now, explain women...........

[Quiet_Hollow]

Chris took the formal route over the course of four posts...I took the short, albeit curt route in my first post. []