AVS makes it easy to create new threads and, as a result, we often have relevant information about specific topics spread over many threads over years. I'm going to try and aggregate a few earlier riser threads here.
The Riser Article
A lot of riser info comes from the seminal article in Home Theater Builder magazine in 2003 written by Dennis Erskine. That magazine is out of print and the article very difficult to find, so it's transcribed in the initial post there: https://www.avsforum.com/forum/19-ded...ne-2003-a.html
The thread continues with a few changes
to the design since 2003, along with links to riser builds
that followed Erskine's designs over the years. BIGmouthinDC likely has the most experience building these risers other than the Erskine Group itself and he contributes some practical wisdom in the thread, including why you might want to use a rubber pad
(like Serenity Mat) instead of felt. SierraMikeBravo, from the Erskine Group, contributes
Erskine did an interview on Home Theater Geeks that touched on a number of topics, including using a riser as a pressure absorber. That portion of the audio interview is transcribed here
Broadband Absorber vs Helmholz Resonator
Back in 2011, user Felgar asked a question about designing a riser as a broadband absorber when the riser is not touching any walls, in the middle of the room: https://www.avsforum.com/forum/19-ded...nst-walls.html
The thread quickly took off and the next 71 posts contribute quite a bit to defining how the risers work, with interesting info from (at least) Felgar, Bigus, and Erskine. The question is raised if the Erskine design is a broadband absorber or a Helmholz Resonator. Felgar points out
the Helmholz design
by BasementBob is notably more complex. Bigus opines that if the Erskine riser is a broadband absorber, then the vents may be in an inefficient location
Erskine steps in and clarifies that his design is not a Helmholz resonator but is rather a broadband pressure absorber that targets modal frequencies
. He mentions that the riser must be against the back wall and at least one side wall. In his offered example, there are two vents in the back and one on each side. The vents are adjustable to allow for some level of tuning.
Erskine also provides a link
to where he gets his linear bar grilles.
Bigus (and others) describe ways to create a more generic broadband absorber
while Erskine goes into more details
on how his design is specifically targetting modal frequencies.
More on Broadband for Targeted
Last year, user fulminis asked for a review of his rather uniquely designed riser: https://www.avsforum.com/forum/19-ded...ap-review.html
A vigorous discussion followed with input by such people as Erskine, SierraMikeBravo, Bigus, HopefulFred, TMcG, BIGmouthinDC, Mfusick, and others.
TMcG points to a calculator
that would allow you to design a riser to capture a specific frequency. On clarification that a broadband absorber is desired, Bigus describes what makes a good broadband absorber
BIGmouthinDC gave the many reasons
why he likes putting internal supports in risers as well as what makes a quiet riser
fulminis described a design
that might be a hybrid between a targetted absorber and a broadband one. TMcG is dubious that it's possible
to have both in one device, which Erskine confirms
. Even as just a broadband absorber, SierraMikeBravo cautions
that it might not as effective as hoped and explains why
The question comes up on how to build a tall riser and Erskine steps in with plans
on doing that.
Converting a Riser into a Broadband Absorber
In 2009, user mtbdudex decided to convert his riser into a bigger variant and to make it be a broadband absorber at the same time: https://www.avsforum.com/forum/19-ded...7-ft-deep.html
This thread is somewhat unique in that there are a number of "before and after" tests that show dramatic differences between an open vent and closed vent situation.
mtbdudex starts by noting that he will create multiple zones
(three of them) in his riser rather than have one large cavity. The idea is to capture different peaks.
On advice from Ethan Winer, a total of fourteen 4'x14"vents were installed
Multiple tests were done using REW both with the vents close and with them open, to see what kind of impact the riser was having as a broadband absorber and graphs of each were posted
As an aside, the question of using standard fiberglass insulation vs Roxul Safe n Sound came up, and a GIK Acoustics rep answered that it wouldn't make much of a difference
in this case, so use whatever is on hand.