Room Modes & STC ratings

[whitelion88]

Do room modes (room shape) have any effect the STC rating of a room or do room modes only have an effect the acoustics within the room itself?

In other words, all other things being equal, if I build a room with non-parallel walls and then build the same size (in terms of square feet) room with a rectangular shape, will one have a different STC rating than the other?

[lilith_envy]

If a tree falls in the woods.....

I would say no difference.
But maybe if in one senario the speakers were firing directly at a wall from 2m(6.6ft)
compared to senario 2 where the speakers were firing from 5 m???
Are standing waves the enemy of mass?

I still say no.
Great question though, keen to hear everyones elses thoughts


Mass= STC
treatment = room modes

[AVare]

STC is a barrier classification system. It has nothing to do with room modes.

Andre

[xSpace]

... if I build a room with non-parallel walls and then build the same size (in terms of square feet) room with a rectangular shape, will one have a different STC rating than the other?

No.

The suggestion that parallel walls or non parallel walls are the deciding factor in how well an assembly will isolate against sound are uneducated guesses.

The square footage alone is irrelevant while the actual climate temperature and sea level might be better barometers:)

[lilith_envy]

temperature does affect speed of sound

[Eric_Desart]

Do room modes (room shape) have any effect the STC rating of a room or do room modes only have an effect the acoustics within the room itself?

In other words, all other things being equal, if I build a room with non-parallel walls and then build the same size (in terms of square feet) room with a rectangular shape, will one have a different STC rating than the other?

While I know this can look as bean counting versus the other contributors, there is some logic in your question.
Since one compares the diffuse field level (several methods can be applied) in one room with the same in the other room (for normal STC lab tests), modal behavior can certainly influence the real live STC.
Even the volume of the receiving room can influence STC (mainly in the low frequencies).
What's wrong in your assumption is that non-parrallel walls should not show modal behavior in the room. Hence it all depends on circumstances. if you have modes in your room which coincide with the mass-spring of your walls then you can be in trouble.
At studiotips long time ago there was a typical situation where a room mode DID coincide with this mass-spring resulting in a wall which acted as a filter.

ASTM E 413 – 04
Classification for Rating Sound Insulation

4. Significance and Use
4.1 These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling allies, power transformers, musical instruments, many music systems, and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.
Note here that the standard itself refers to its limitations in function of types of noise sources.

7. Keywords
7.1 architectural acoustics; building design; ceiling attenuation class (CAC); field sound transmission class (FSTC); noise isolation class (NIC); normalized noise isolation class (NNIC); partitions (buildings); sound insulation rating; sound transmission class (STC)

ASTM E 90 – 04
Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements

1. Scope
1.1 This test method covers the laboratory measurement of airborne sound transmission loss of building partitions such as walls of all kinds, operable partitions, floor-ceiling assemblies, doors, windows, roofs, panels, and other space-dividing elements.
1.2 Laboratory Accreditation—A procedure for accrediting a laboratory for performing this test method is given in Annex A3.

6. Test Rooms
A lot of conditions must be fulfilled (too much to copy here & copyright issues) but a few short (incomplete) things:
Source and receiving room >= 50 m3 (1765 cft)
Rooms must be diffuse (diffusers)
Rooms must be below a max allowed absorption.
Average Temperature in rooms for walls 22 +/- 5° C.
6.2.3 During the sound pressure level and sound absorption measurements, variations in temperature and humidity in the
receiving room shall not exceed 3°C and 3% relative humidity respectively. Temperature and humidity shall be measured and
recorded at the beginning and end of each test to ensure compliance.

This isn't complete. It just gives an idea that STC is a value within certain boundary conditions. And more than atmospheric conditions, which don't differ that much in common applications, modal behavior and receiving side volume will influence practical STC values.

While not obliged, certain labs therefore will or can measure in both directions and average these values, since THEORETICALLY (not taking flanking effects into account) transmission loss is reversible. And take into account that such rooms and the measurement methods are designed to exclude at a maximum such disturbing phenomena.

The question of the OP isn't that illogical, with this exception that he starts from the wrong assumption that non-parallel walls should prevent or avoid room modes.

[valentin]

with this exception that he starts from the wrong assumption that non-parallel walls should prevent or avoid room modes.

this assumption is all over the web and as you state its wrong non-parallel walls just make prediction a lot more difficult too predict but modal behavior will exist in small rooms with non parallel rooms

rectangular rooms are a lot simpler for modal analysis

[Juan.Nightmare]

temperature does affect speed of sound

i almost forgot about this..

[whitelion88]

The question of the OP isn't that illogical, with this exception that he starts from the wrong assumption that non-parallel walls should prevent or avoid room modes.

Actually, the assumption I make is that non-parallels do not "prevent or avoid" room modes, but rather that in comparison to, say, a perfectly cube room (10'x10'x10'), a room with non-parallel walls will have substantially less modal problems.

[Eric_Desart]

The question of the OP isn't that illogical, with this exception that he starts from the wrong assumption that non-parallel walls should prevent or avoid room modes.

Actually, the assumption I make is that non-parallels do not "prevent or avoid" room modes, but rather that in comparison to, say, a perfectly cube room (10'x10'x10'), a room with non-parallel walls will have substantially less modal problems.

Nice explenation. And a room with non-parallel wall is also better than a room build as a sphere from 10 m3.
What a comparison ...... Everything is better than a cube room (that's stamping an open door). You spoke about a rectangular room, but if you like to play with words that's OK for me too (without me).

[whitelion88]

Nice explenation. And a room with non-parallel wall is also better than a room build as a sphere from 10 m3.
What a comparison ...... Everything is better than a cube room (that's stamping an open door). You spoke about a rectangular room, but if you like to play with words that's OK for me too (without me).

Yes, I understand that in terms of modes, a non-parallel wall room is better than either a rectangular or cube-shaped room. I was wondering if a cube/rectangle shaped room would have a worse STC than a wall with non-parallel walls. IF you have any data or studies on this that you can point me to, it'd be appreciated, though I myself am not aware of any. My guess (based on what I know and have read about), such as explained here:

http://www.audioholics.com/education/ac ... coustics-1

is that since a cube/rectangular room creates more sound pressure INSIDE the room from increased modal activity, it therefore will sound louder outside of the room than an equally well-constructed room with non-parallel walls under then same test circumsances, all other things being equal. This, in turn, would make the room with non-parallel walls have a much STC. Whether or not it is significant is to be determined. However, this is just conjecture and I have yet to find any hard data to back this up yet and was wondering if anyone had any.

[valentin]

i think you are not getting the point read again

Hence it all depends on circumstances. if you have modes in your room which coincide with the mass-spring of your walls then you can be in trouble.

where is this assumption coming from

is that since a cube/rectangular room creates more sound pressure INSIDE the room from increased modal activity

some simple words

All rooms, of all shapes and sizes, have resonant modes. Those in rectangular
spaces are well understood and easy to predict. Modes in nonrectangular rooms
are diffi cult to predict.

A recurring fantasy about rooms is that if one avoids parallel surfaces, room
modes cannot exist. Sadly, it is incorrect. Among the few studies of this topic,
Geddes (1982) provides some of the most useful insights. He found that
“room shape has no signifi cant effect on the spatial variations of the pressure response. . . . The spatial standard deviations of the p2 response is very nearly uniform for all the data
cases ”

chapter 13.2 Sound Reproduction, Floyde Toole

there you have a easy and serious book to read

[Soundman2020]

This is a really old thread, but there's a lot of misinformed commentary in it that needs correcting. I came across it while searching for something else, and felt the need to update it with correct info:

Do room modes (room shape) have any effect the STC rating of a room or do room modes only have an effect the acoustics within the room itself?

As Eric clearly pointed out, if a room mode happens to be at the same frequency as the resonant frequency of the wall, then you would have a large reduction in isolation at that frequency. However, since room modes and MSM resonance are normally problematic in the lowest part of the spectrum, and STC does not take into account the lowest part of the spectrum, there would probably not be much difference in the STC rating, even though there might be a large difference in isolation. STC is not a good measure of studio isolation anyway.

In other words, all other things being equal, if I build a room with non-parallel walls and then build the same size (in terms of square feet) room with a rectangular shape, will one have a different STC rating than the other?

Thre would probably be no difference, since having some walls non-parallel won't change the overall modal response much.

Actually, the assumption I make is that non-parallels do not "prevent or avoid" room modes, but rather that in comparison to, say, a perfectly cube room (10'x10'x10'), a room with non-parallel walls will have substantially less modal problems.

You changed the goal posts! You never mentioned cubed rooms at all in your original question. You mentioned two similar rooms, both rectangular, one with parallel walls and one with non-parallel walls. This comment on cubed rooms is irrelevant. And in reality, it doesn't change the outcome very much anyway...

Yes, I understand that in terms of modes, a non-parallel wall room is better than ... a rectangular ... room.

Then you understand wrong! Angling walls does not necessarily improve the modal response of a room. It could, in fact, make it much worse. It is a myth that angling your walls to make them non parallel somehow magically makes room modes go away.... it does not. All it does is to move the modes to a slightly different frequency, which makes it harder to predict, and broaden the Q of the mode, which makes it harder to treat.

Your basic assumptions are flawed.

since a cube/rectangular room creates more sound pressure INSIDE the room from increased modal activity,

Huh? Nope. Just nope. That is NOT true at all. The axial modes at the specific resonant frequencies will be higher in intensity, and will potentially decay more slowly, but that has no effect at all on the "ound pressure inside the room".

Your basic assumption here is also flawed.

it therefore will sound louder outside of the room than an equally well-constructed room with non-parallel walls under then same test circumsances, all other things being equal.

The conclusion does not follow, since the premise is incorrect.

This, in turn, would make the room with non-parallel walls have a much [better] STC.

Same as above: The conclusion does not follow, since the premise is incorrect, and while there might be a difference in isolation in low frequencies due to a modal resonance coinciding with an MSM resonance, that would very probably not be apparent in STC testing, since STC does not deal with low frequencies.

----

Conclusion: Having non-parallel walls will have little to no effect on either modal response or STC rating.


- Stuart -