Pensados Place #21 - Bob Hodas

[boggy]

Originally Posted by johndykstra

It's always been my understanding that while a room of say a 10' depth has the ability, as a room of any size, to reproduce 20hz,

You are not the only one who think so. I meet people which still think the same, even if they hear this low frequencies in the small room where it "not should be heard", following this "theory". This looks like well spread misunderstanding or even myth... but I don't know who is the real author of it.

Originally Posted by johndykstra

it's dimensions are not large enough to reinforce that frequency with resonance. The loudest this frequency can be heard is that of the intensity emitted from the speaker.... I.e. no ringing or modal peaks reinforced due to the room's size.
.....

No, room resonance is not needed to "boost" low frequency to be audible. Room resonances are still the worst problem in the small room, so there is no benefit from it at all, from 300Hz down to lowest room mode.

Room gain IS the benefit (if any at all) in the small room. Sealed box woofer with F(-3dB)=50Hz and 12dB/oct slope may sum to nearly flat response to the very low frequencies, in the room where is lowest room mode at about 50Hz. DSP crossover/EQ may be usable for tweaking, but the main point is reached relatively easily with acoustics only.

[boggy]

Originally Posted by BriHar

So if a smaller room eliminates lower frequency reinforcement, helping eliminate modal peaks and ringing, why are small rooms considered bad for mixing in esp. in the LF spectrum. I realize we are focusing here on a single principle and it is probably some other that is the root of the problem. This is a grey area. I always thought that because the (small) room cannot support the lower frequencies, this is why they (small rooms) are a problem, but what is said here seems to show that this is good. I'm confused...

Small rooms are problem because there are too little space (volume) for enough absorption to dampen (very distinct) room modes, below 300Hz. Because (small) dimensions of small rooms, room modes are more pronounced, because in larger rooms sound waves decrease intensity only because they travelling more to boundary (and back to listener), and we need a couple of decibels even more of absorption at boundaries, than in larger room, to get nearly equal results at listening position. So... these are two problems because dimensions, smaller space and shorter path... both of it make problem of acoustical treatment of small room tremendously difficult, if we like to have, at least, acceptable frequency response (+/-3dB, 1/3 oct. smoothing) at listening position.

[johndykstra]

Boggy,

I think you are still misunderstanding what I am saying... even if what I am saying is my own misunderstanding, I'd like us to be on the same page in my interpretation, so you can then effectively shoot it down if need be.

I'm not asserting that frequencies below the dimensions of the room are inaudible, rather that they are not enhanced or louder due to modal supported dimensions.

Say your axial length mode of the room is 90hz. You should expect wild variations in the response of this frequency. Intensities above playback volume and well below depending on your location.

I submit that frequencies lower than 90hz will still vary with intensity again depending on your location, but will never peak louder than the decibel level of the playback. Certainly we should expect intensity to fall below...

Being that there are a multitude of frequencies above 90hz gaining from room resonances gaining intensity due to all the modes supported above this lowest mode, it's often that we see graphs with hyped "high bass" (in this case above 90hz) and a stiff and sudden drop off in the "really low", sub modal region of the room (in this case below 90hz).


Am I wrong here?

[boggy]

Originally Posted by johndykstra

......
Am I wrong here?

Yes. On the contrary, below lowest room mode (defined by room dimensions), room gain can be fairly strong, at lowest frequencies it may be stronger than single room mode "gain" (+6dB). There is a rising slope of something between 6 and 12dB/oct when we go below lowest room mode.

We don't have anything similar to this in the modal frequency region in the small room. This is a reason why is relatively easy to have low frequencies inside car.

[Ethan Winer]

Originally Posted by kasmira

I suppose its a matter of what you consider the term "develop" to mean

This could mean one of two things, both incorrect.

1) You need to be at least 30 feet away for the sound to reach its full potential.

2) The room needs to be at least 30 feet long for the wave to fit, and in turn to be heard.

--Ethan

[Salty James]

Originally Posted by Ethan Winer

That's simply not true. If it were, headphones wouldn't work, and you'd never hear anything below 400 Hz in your car.

--Ethan

The Acoustic Treatment Experts

uh.. wow,
I never considered this obvious fact, Ethan.

So how does a 150hz tone get generated from headphones. Headphones produce down to that far. So does the wavelenght change if very light volume is used -that doesn't make sense..

[Ethan Winer]

^^^ See my Post #18 above.

Also, acoustics is linear, so whatever happens at soft volumes happens the same when loud.

--Ethan

[BriHar]

Does this mean that pressure-based acoustic treatments are more effective than velocity based treatments in a small room?

[boggy]

Originally Posted by Ethan Winer

.......
Also, acoustics is linear, so whatever happens at soft volumes happens the same when loud.
....

I agree, acoustics is really linear, but human hearing isn't, so people is more sensitive to low frequency room modes at higher listening levels.

Originally Posted by BriHar

Does this mean that pressure-based acoustic treatments are more effective than velocity based treatments in a small room?

Not generally, because in small rooms, room modes are more denser and you usually have all of it in audible range. In bigger room you have a chance that lowest ones may be below 40Hz e.g.

So only very efficient, low physical volume AND wide range pressure based absorbers may be more effective than velocity based in the small roms for lowest room modes.

You spend available volume in small room very fast with treatment, so if you occupy a couple of cubic meters of room physical volume with high Q, single freq. pressure based absorber to kill one single room mode, you will not have more space for others...

[Jens Eklund]

Originally Posted by BriHar

Does this mean that pressure-based acoustic treatments are more effective than velocity based treatments in a small room?

A pressure based devise is most efficient close to the boundary where the pressure variation is greatest. A velocity based devise is most efficient where the particle velocity is greatest; away from the boundary (depending on wavelength).

[BriHar]

I do understand that concept Jens, I was just thinking in regard to Ethan's post#18

Originally Posted by Ethan Winer

The term you're looking for is called pressure mode. This means the waves are too long to fit into the enclosed space, so instead they just increase (and decrease) the ambient atmospheric pressure. But they're still waves, and they still impinge on your ear drums to create sound. As John explained, all that really happens when a room is too small is you don't get a ringing peak. Which IMO is a Good Thing.
--Ethan

So Boggy, you are reaffirming for me the general guideline that small rooms benefit most from broadband treatments.

Peaks and nulls (nodes and antinodes) will still manifest even in small rooms. Am I correct in assuming these to be caused generally by standing waves? Could these not still be handled by tuned trapping or geometric deflection, specifically in relation to the sweetspot (i.e. best position discovered by measurement, as they say, no position is perfect but there will be a best)?

[boggy]

Originally Posted by BriHar

...
So Boggy, you are reaffirming for me the general guideline that small rooms benefit most from broadband treatments.

Yes, from my experience at least.

Originally Posted by BriHar

Peaks and nulls (nodes and antinodes) will still manifest even in small rooms. Am I correct in assuming these to be caused generally by standing waves?

These aren't caused only by standing waves, there are a non resonant interferences too...

Originally Posted by BriHar

Could these not still be handled by tuned trapping or geometric deflection, specifically in relation to the sweetspot (i.e. best position discovered by measurement, as they say, no position is perfect but there will be a best)?

You can find best position, of course, but you still need absorption at each boundary but not only at single frequency to get best results.

e.g. if we look at Pressed Lizard studio (described in MyRoom Design white paper), which have dimensions 3.67x3.56x2.55m you have pretty strong (double and resonant) null at listening position at ~50Hz, virtually wherever you place your speakers and listener. So if you choose to efficiently dampen this null with only velocity based absorbers, following Jens note above, about their functioning, we need to put absorbers at place where we already have a listener, because velocity is highest there. This is a problem in all rooms where we have the lowest room mode in audible (or usable) frequency range. So we MUST place absorber in place where pressure is higher, because there is no listener
Also ... next axial modes in this room are at 67Hz, 93Hz, 97Hz,... which aren't easy to dampen with only velocity based absorbers, also frequencies are too spread for single frequency (non wide band) pressure based abosrbers (we have modes at ~50Hz AND ~95Hz which need to be dampen...)

So only combined and complex construction of wideband pressure based absorbers below 100Hz and wideband velocity based absorbers for frequencies above 100Hz may works efficiently in rooms where lowest room modes are in audible frequency range or, at least, in working range of our loudspeakers.

[BriHar]

OK, your saying we can't place an absorber at the listening position (obviously) but that is where the velocity is highest so we MUST place the absorber where the pressure is highest (i.e. the wall involved). But an absorber I understand functions on a velocity principle so would it function in a high pressure zone, or will it simply be less efficient, or do you mean we would need a pressure treatment such as a panel trap as opposed to an absorber.

[boggy]

Originally Posted by BriHar

......But an absorber I understand functions on a velocity principle so would it function in a high pressure zone, or will it simply be less efficient, or do you mean we would need a pressure treatment such as a panel trap as opposed to an absorber.

We can't do things ideally. We can use pressure based wideband absorbers in place where they are more efficient than velocity based (in this same place) for some frequency range. So it is not that important to place it at theoretically best place, or place on highest pressure... we must trying to find the best compromise, because ideal solutions isn't possible.
Complex construction of VPR inspired design (modex plate) combined with wideband absorber in front (and air transparent diffuser in front of all), seems to be nearest practical example of compromise of which I speak.

[Jens Eklund]

Or in general; use velocity based absorbers only where needed (at early reflection points if not redirection of early energy can be applied) and accept that they will not be very efficient below about 100 Hz (assuming reasonably deep panels). Use pressure based absorbers (perforated or membrane) for the modal region at other surfaces, possibly in conjunction with diffusers on top forming a membrane absorbers at areas where diffusers are applicable (depending on design concept used).

[Glenn Kuras]

OR start with broad band straddling corners which will reach way below 100hz and then follow up with something tuned for the last of the problems (if needed or wanted). It really comes down to budget at the end and to use only pressure based is not a easy trick to pull off.
Need advice on waterfall!

[pitchfork]

I'm confused...

Welcome to the world of small room acoustics and the...

latestflavor

[Ethan Winer]

Originally Posted by BriHar

Does this mean that pressure-based acoustic treatments are more effective than velocity based treatments in a small room?

Probably not. Small rooms especially have peaks and nulls at all bass frequencies, not just those related to the room dimensions. Pressure absorbers are tuned, so broadband (velocity) traps are usually a better choice.

--Ethan

[Ethan Winer]

Originally Posted by boggy

I agree, acoustics is really linear, but human hearing isn't, so people is more sensitive to low frequency room modes at higher listening levels.

I could argue the opposite.

In most rooms, nulls are more of a problem than peaks. So if you're listening at a low volume, a null can make something that would have been audible to now be too soft to hear. In that case the problem is worse at low levels. Of course, your point is valid too. Which is why bass traps are so darn important in all rooms.

--Ethan

[Ethan Winer]

Originally Posted by boggy

These aren't caused only by standing waves, there are a non resonant interferences too.

Yes, the key word here is interference. Peaks and nulls are both caused by acoustic interference. Room modes and standing waves are a subset of interference.

--Ethan

[Jens Eklund]

There’s a widespread misconception that pressure based absorbers always are high Q devises (and perhaps this is the reason a lot of people are afraid of using them unless costly builds where there’s an acoustician involved). This can be true if very low fc and/or shallow construction depth but if not, you seldom need more that 3 different designs to cover the entire modal region:


As a bonus, if using pressure based absorbers (at other areas than early reflection points unless splayed walls are employed to redirect the early energy), you effectively avoid creating a “dead” room with uneven decay times (very short in the highs and normal too long in the lowest octave) since these reflect the mids/highs (depending on design naturally). Having said that, it is important to decide on type of treatment and how to use it, AFTER deciding on what kind of acoustic response is requested. If aiming for an NE-room or other designs resulting in a "dead" acoustic (speaker) response, you might not need to use a lot of pressure based absorbers (assuming you have enough depth available for very deep velocity based absorbers). If LEDE/RFZ or other designs that relies on a marked termination of the ISD-gap and a (semi)-diffuse field following it, you probably want to avoid velocity based absorbers as much as possible and use a lot of (if not solely) pressure based absorbers.

[boggy]

Originally Posted by Jens Eklund

.....but if not, you seldom need more that 3 different designs to cover the entire modal region:

..........

Which method do you recommend to build a single perforated resonant absorber that can resonate (absorb) at both 45 and 90Hz with absorption coefficient higher than (at least) 0.8 at this frequencies?
Let's say that absorption above 100Hz is not needed for this absorber.

[Glenn Kuras]

The fact of the matter is that most people do not have the budget nor the room to do those kind of designs totally correctly, BUT hey keep preaching as I would retire early if it truly worked in most rooms.
Note: I have nothing against pressure based traps if it works for the given customer.

[Jens Eklund]

Originally Posted by boggy

Which method do you recommend to build a single perforated resonant absorber that can resonate (absorb) at both 45 and 90Hz with absorption coefficient higher than (at least) 0.8 at this frequencies?
Let's say that absorption above 100Hz is not needed for this absorber.

Depends on some factors like available build depth etc, but if one can spare 250 mm, I would do a slotted panel like this for example:

[gullfo]

i would keep it simple and build several controls into a single wall unit

[boggy]

Originally Posted by gullfo

i would keep it simple and build several controls into a single wall unit

Nice idea gullfo, combining membrane and perforated absorber may possibly give a better chance to make an absorber which efficiently absorb at two frequencies below 100Hz, also it will be good that when we control one frequency, second become twice as higher or lower (like primary and secondary room modes from one room dimension)...

Primary and first next axial room mode in audible range, and below about 100Hz, is worse problems that we may have in the small rooms, and there are nearly always two of it in regards to each dimension... ('two there shall be... a master and an apprentice...' Sith "rule of two" )

[SörenHjalmarsson]

Originally Posted by boggy

Nice idea gullfo, combining membrane and perforated absorber......

.....('two there shall be... a master and an apprentice...' Sith "rule of two" )

Jedi Master Brandt often utilize that kind of device/method in his designs....

[garryrobson]

High praise indeed from Bob Hodas, for Focal Solo and their CMS range in episode 21 of Pensado's Place! Play from about 1hr in: The secrets behind tuning your room!