Do resonances/modes ever change with room contents?

[nickdahl]

From my reading, I've been able to figure out that room contents affect sabins. But, do room contents, especially heavy, dense stuff, ever affect a room's modes?

Thanks,

Nick

[Ethan Winer]

Nick,

> do room contents, especially heavy, dense stuff, ever affect a room's modes? <

You bet they do. When absorption is added to a room the mode frequencies go down. The change is not usually much, maybe a few percent.

The first graph below shows an empty room, and the second graph shows the same room with bass traps added. Look carefully at each peak and null frequency and you'll see that adding traps lowered them a small amount.

Besides shifting the frequency, adding absorption also lowers the Q of a mode. In fact, the change in Q is much larger than the shift in frequency.

--Ethan

[z60611]

But, do room contents, especially heavy, dense stuff, ever affect a room's modes?

Sure.
There's three effects that a pair of large flat solid armoirs could do:
a) reflections (flat)
b) constriction (change in volume of some part of room)
c) membrane (LF resonant absorber)

from: Acoustics and Psychoacoustics by David M. Howard pg 188

A constriction near a pressure node (displacement antinode) lowers that mode's frequency.
A constriction near a pressure antinode (displacement node) raises that mode's frequency


A constriction at a pressure node (displacment antinode) has the effect of reducing the flow at the constriction since the local pressure difference across the constriction has not changed. Benade (1976) notes that this is equivalent to raising the local air density, and the discussion in Chapter 1 indicates that this will result in a lowering of the velocity of sound (see Equation 1.1) and therefore a lowering in the mode frequency (see Equations 4.7 and 4.9). A constriction at a pressure antinode (displacment node), on the other hand, provides a local rise in acoustic pressure which produces a greater opposition to local air flow of the sound waves that combine to produce the standing wave modes. This is equivilent to raising the local springiness in the medium (air), which has been shown in Chapter 1 to be equivalent for air of Young's modulus (Egas) which raises the velocity of sound (see Equation 1.5) and therefore raises the mode frequency (see Equations 4.7 and 4.9). by the same token, the effect of locally enlarging will be exactly the opposite to that of constricting it.

There's lots of examples of constrictions affecting the modes of musical instruments. For fun, try adding smoothed wax inside of a pipe at the nodes and antinodes.

[nickdahl]

Thanks for the help, guys. I'm trying to get the most out of my efforts, and before I re-measure my room's frequencies, it seemed important for me to figure out what the furnishings and equipment might do to the numbers. That is, I'm trying to sketch out a plan of action, such as:

Calculate modes using room dimensions.
Measure the room frequencies when the room is empty.
Re-measure when room is full.
Design treatments to deal with problem frequencies.
Pray that I deal with the problem.

Given what the room furnishings and equipment might do to measurements, should I skip the "empty" room frequency measurements?

Nick

[z60611]

nickdahl:

Calculate modes using room dimensions.
Measure the room frequencies when the room is empty.
Re-measure when room is full.

If you've got the time, experiment (apparatus, method, observations) -- and post results.

I'm sure I'd like to see the field results of
- "Calculate modes using room dimensions."
vs
- "Measure the room frequencies when the room is empty. "
vs
- "Re-measure when room is full."

But I'm one of those idiots who wrote his own calculator.
http://www.bobgolds.com/Mode/RoomModes.htm

(Speaking of which, I've been meaning to add a few more calculations to that ... Forgot about that.)


Measurements for
a) speaker in one tri-corner, microphone in opposite tri-corner (one test)
b) speakers in normal positions, microphone in three different positions (four tests)

In the case of both (a) and (b), use positions that work in both empty and full room.


Ultimately, all you're going to need is the

measure when room is full.
Design treatments to deal with problem frequencies.

Shampoo, rinse, repeat as nessessary.

[Ethan Winer]

Nick,

> Calculate modes using room dimensions. <

That's not as useful as you might think, assuming you designed the room with a favorable ratio in mind. And if not, what could you do about it now anyway?

> Measure the room frequencies when the room is empty. <

Yes, measuring trumps predicting every day of the week. I've seen measured modes vary quite a lot (>20 percent) from what was predicted.

> Design treatments to deal with problem frequencies. <

Most rooms do best with broadband absorption, as opposed to absorption that targets specific frequencies. You name a frequency, and I guarantee it's a problem somewhere in your room!

--Ethan

[AVare]

The world of reality vs theory is amazing in acousitcs. In BBC RD 1992-09 table 1 on pdf page 13 shows teh calcualted vs measured frequencies for the 14 modes in a test room. Only mode agreed, and in 5 cases reality followed "gut theory" with the ameasured frequency higher thatn calculated!

In case someone thinks this is an anomally to modea analysis, BBC RD 1991-07 describes reverb testing where the reverb time INCREASED wioth added abosprption! Fig 7, pdf page 10 is start into that.

Enjoy!
Andre

[Ethan Winer]

Andre,

> table 1 on pdf page 13 <

Wow, only 6 percent difference? I measured 20 percent difference in the lab room at our factory which is standard sheet rock construction. I'm sure you recall the huge fight that ensued when I reported that at recording.org!

--Ethan

[z60611]

AVare:

table 1 on pdf page 13 shows teh calcualted vs measured frequencies for the 14 modes in a test room. Only mode agreed,

Only one mode agreed to 4 decimal places. Where else in acoustics would one demand greater than 4 decimal places of accuracy?
The others were close enough that you could use the calculator to give an indication of the mode number (i.e. which walls) that was causing the mode. All but 2 were within 2% (seems to me that calulators are useful), the worst being 6.77% off.

from pdf pg 13, document page 7

There is a slight tendency for the measured frequency to be lower than calculated, corresponding to an acoustic size slightly greater than the physical size. The largest errors occur at frequencies ... the effect of panel resonances in the low-frequency acoustic treatment on the walls.

The calculators should be best at frequencies where walls are mostly/totally reflective, and in rooms that are empty (no furniture, no added absorption, no diffusion, just reflection) and rectangular (six right angle surfaces). The further you get from that, the greater the error.

I like the phrase they use "Practical Rooms".

[AVare]

Only one mode agreed to 4 decimal places. Where else in acoustics would one demand greater than 4 decimal places of accuracy?
The others were close enough that you could use the calculator to give an indication of the mode number (i.e. which walls) that was causing the mode. All but 2 were within 2% (seems to me that calulators are useful), the worst being 6.77% off..

Agreed. It provides some perspective for tuing resonant absorbers etc.

Andre