Recording - June 2014 - 8

Note to faithful writers: When sending your letters by email,
please remember to include your name and where you hail from.
And the resonance goes on and on...
Our March 2014 issue's article by Bruce Black on building a
Helmholtz resonator is still bringing in a ton of mail. Last month
Bruce answered a reader's questions about room analysis and
placement; this month the subject is dimension choices and how
to "cover" multiple resonances...
Hi Bruce. After searching the web, I found a page with a
'Room Mode/Standing Wave Calculator'. After entering the
dimensions of my room, I got all the frequencies for Axial,
Tangential, and Oblique modes for my room laid out in a chart.
Cool! So I wrote down the frequencies that appeared to be the
most troublesome, and it looks like I need Helmholtz resonators
for 110 Hz and 250 Hz. With a Q of 1, these two resonators
should cover most of the bass buildup problems in my room.
I can figure out by the hole measurements given in your article that 110 Hz would be about an 8" hole. Is that right? But for
frequencies above 120 Hz, you say to reduce the size of the
box but offer no further instructions. I realize there's no way to
list everything in the article; where can I find info about the box
dimensions and hole sizes for the frequencies between 120 Hz
and 300 Hz?
Craig Johnson
Abilene, TX

Bruce Black replies:
Hi, Craig! A room mode calculator is a good start, but it is
only a start. There are a number of things that influence what the
actual modes are and how they act in the room. For example:
~ If your walls aren't consistently at the dimension you measure
~ If your walls aren't exactly parallel or are wavy
~ The vibration of wall panels (like drywall) can absorb certain
frequencies
~ How well the wall panels are attached to the frame work
yields more or less vibration
~ Absorption/diffusion from furniture, etc.
The first two issues above can shift the frequency of the resonances. This will show up in a waterfall plot as an extended
decay that curves away from the initial frequency as time progresses. A room mode calculation won't reveal this.
The other thing is I've found some mathematical formulae have
"fudge factors" added in to reduce, but not eliminate, errors. This
is true especially with resonator port calculations. I designed a
spreadsheet that does the calculation quickly, and in the end it
gives me three numbers, not one. This is because in my research
of formulae, I found a number of different ones from quite reputable sources that ended up not giving the same frequency for a
given set of dimensions. So I then look at all three numbers, pick
the one in the middle, and then make adjustments to make the
port an easy hole to cut by rounding to the nearest 1/8".

And in all this, you have to remember that the upgrades and
rooms I design will most likely be built by framing carpenters,
not cabinet makers. They usually work to within ±1/8" anyway.
And if they have a hole saw blade that is "close", they'll likely
just use that! Now add in the blade wobble because the carpenter may not have a steady hand (if they did, they'd be cabinet makers instead), the kerf of the cut, and you have a lot of
inaccuracies affecting the size of the port. And add in the different atmospheric pressure and density, different densities of
lumber, and on and on. Everything affects it.
So you have many places where inaccuracies can creep in
and the actual frequency can vary from the calculated value. But
in the end, with a Q of 1 (roughly a one-octave bandwidth), a
resonator will have significant influence where you want, even
with all these variables.
This all indicates that there is no substitute for real-world measurements. This can be done easily with your laptop or computer. Just do a search for acoustical measurement software, and
you'll find free or inexpensive applications. Add an audio interface and a good measurement mic and you're on the way, as
described in last month's Talkback. A waterfall plot will tell you
a great deal.
Volumetric resonators like the one in the article are good up
to around 150 Hz. Above that, juggling the cabinet size and the
port size gets pretty unwieldy. So for 250 Hz, I'd recommend
using a perforated panel absorber. You can build a box like the
resonator in the article, but put a thin layer of fluffy insulation in
it, and replace the port panel with pegboard with half the holes
plugged. This should get you pretty close to 250 Hz.
I recommend you build the 110 Hz resonator first and live
with it for a while before building the 250 Hz one. This is so any
improvement you hear can be definitely attributed to whichever
resonator created it. You should hear a greater sonic improvement with the 110 Hz resonator because the lower the frequency, the greater the sonic issues.
By the way, the port size for 110 Hz is 8" for the box size in
the article. Best-BB

Correction
Just after we went to press with our May 2014 review of the
Z-Mod Z-67 from the ADK Custom Shop (www.adkmic.com), we
got a brief letter from Larry Villella of ADK that we'd like to share
with you. "I did find one error! Just to set the record straight,
while some of the lower-cost Custom Shop mics might have metalwork made in Asia, this is not correct for the Z-MODs. We do
some of the Chassis at Latch Lake in Minnesota. By the time you
go to press, all of the Chassis should be sourced in USA, part of
the Z-MOD quest to have 90% Labor / 90% Components from
US and European Union sources." Thanks for updating us,
Larry.-MM

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RECORDING June 2014

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Recording - June 2014

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