Panel Absorber  -
Helmholtz Panel Absorber  -
Slot Absorber

Placing the Traps  -
Skyline Diffusor

Quadratic Residue Diffusor (Size)  -
Quadratic Residue Diffusor (Design
frequency)  -
Links

There are two commonly used traps, both of which are
easy to build. They are the panel absorber and the Helmholtz resonator.
However, to be successful you first have to know precisely where your problem
frequencies are and then you have to build these traps accurately to ensure
that they work at the right frequency.

Both types of absorber take up a large area, but have the advantage of being
only a few cm deep. Even so, you must bear in mind that these are tuned traps
and so are normally used to reduce specific resonances. They are not suitable
for use as broad-band absorbers, with the exception of a panel trap
constructed with a highly damped, limp membrane.

Helmholtz Absorber

A Helmholtz absorber is simply a box with a port on
its front side to couple the enclosed volume of the airspace in the box to
the air in the room. The depth of the enclosed airspace in the box behind the
port and the width and depth of the port control the resonant frequency of
the bass trap.

Another form of helmholtz
resonator is created using perforated plywood - i.e. plywood with hundreds of
holes in it. You see it in hardware stores holding up tools etc. If you place
a panel of this over an air cavity like in a panel absorber not only do the
little holes act like bottle necks the whole panel acts as a low frequency
panel absorber!

Standing waves occur at harmonics of the fundamental frequency - that is 2,
3, and 4 times the fundamental. Thus a room with an 2.45 meter ceiling has
standing waves forming at 70 Hz (the fundamental frequency or first
harmonic), 140 Hz (the second harmonic), 210 Hz (the third harmonic) and 280
Hz (the fourth harmonic).

Rooms with smaller dimensions often have standing waves or resonance build
ups that are very noticeable causing coloration at around 200 Hz.

Calculate
standing waves for one dimension

Room dimension (length, width or
height)

m

fundamental frequency (fo):

Hz

first harmonic:

Hz

second harmonic:

Hz

third harmonic:

Hz

The formula for determining the fundamental frequency of a standing wave for
a particular room dimension is:

fo = V / 2d

where:

fo = Fundamental frequency of the standing wave

V = Velocity of sound (345
meter per second)

d = Room dimension being considered in meter (length, width, or height)

The formula for determining fres:

fres
= c/2 * sqrt( (l/lx)^2 + (m/mx)^2
+ (n/nx)^2 ), with l,m,n
= 0,1,2...

c = 344 m

Helmholtz-Resonator

Calculate
resonant frequency, Bandwidth and Q from given Volume, Port- length and
width.

Hight:

Width:

Depth:

Volume

cm

cm

cm

Liter

Port Length:

Port Diameter:

Calculated Port Area:

cm

cm

cm2

calculated resonant frequency: Hz

Q

Bandwidth

flow (f1)

fhigh (f2)

Hz

Hz

Hz

Die gr??te Schallabsorptionsfl?che A bei gleichzeitiger
Breitbandigkeit erreicht man, wenn das Resonatorvolumen V m?glichst gro? gew?hlt
wird. Das bedeutet aber, da? gro?e Resonatorhalsquerschnitte bzw. geringe
Resonatorhalsl?ngen zu realisieren sind.
Hinsichtlich der Bemessung des
Kanals gibt es allerdings Grenzwerte, die nicht ?ber- bzw. unterschritten werden
sollten: der Radius mu? kleiner sein als eine Achtel Wellenl?nge der
Resonanzfrequenz und die Kanall?nge mu? gr??er Null sein.

Durch st?rkere
Bed?mpfung des Volumens und/oder Kanals wird die G?te des Resonators verringert
und die Bandbreite vergr??ert.
Allerdings h?ngt die G?te in gr??erem Ma?e vom
Verh?ltnis der Abmessungen ab.
Die G?te des Helmholtz-Resonators ergibt sich
wie folgt:

Q?=?2??pi??sqrt(V???(l/A)?)?=?f/(f2?-f1)

mit
Q?=?G?te
V?=?Volumen des Resonators
[m??=?cm?/1000000]
l?=?L?nge des Kanals in
[m?=?cm/100]
A?=?Fl?che der Kanal?ffnung
[m??=?cm?/10000]
sqrt()?=?Quadratwurzel
pi?=?3.141592654
f?=?Frequenz
[Hz]
f1?=?Frequenz [Hz] unterhalb von f (-3dB)
f2?=?Frequenz
[Hz] oberhalb von f (-3dB)

Nochmal obenstehendes Beispiel:

Wir
haben einen rechteckigen Holzkasten (Wandst?rke 19?mm?=?0,019?m) mit einem
Volumen von 100?Litern (=?0,1 m?), der eine rechteckige ?ffnung von 0.07?m? hat.
Die Helmholtz-Resonator-Frequenz betr?gt gerundet 91?Hz.

Eingesetzt in
obige Formel erhalten wir eine G?te des Resonators von 0.28.

Damit
erhalten wir auch einen Wert, mit dem wir die Bandbreite der Resonanzkurve
bestimmen k?nnen. Die Bandbreite ist die Differenz der beiden Frequenzen, bei
denen die Amplitude gegen?ber der Resonanzfrequenz auf 70% des H?chstwertes
(-3dB) abgefallen ist.
In unserem Beispiel betr?gt die Bandbreite
91?Hz/0.28?=?325?Hz, so da? wir bei einem symmetrischem Verlauf der
Resonanzkurve einen oberen Frequenzpunkt f2 von 325/2?+?91?=?254?Hz
erhalten.
Mit diesem Helmholtz-Resonator w?rden wir also eine breitbandige
Bed?mpfung des gesamten Grundtonbereiches erhalten.
Es ist also auch bei der
Dimensionierung das Verh?ltnis der Faktoren untereinander zu
ber?cksichtigen.

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Panel Absorbers

A panel absorber is created when you place a sheet of plywood or fibreboard, with insulation glued to the back of it, over
an air cavity. The panel will have a resonate
frequency of its own, tap it and you will hear it. When it is placed over a
sealed cavity, and insulation is attached to the back, everytime
it hears its own note it resonates and the air in the cavity resonates and
the insulation absorbs the resonance, hence absorbing the frequency! It is
important to note that here we have an absorber that reflects the high
frequencies and attenuates the low. With the hangers all that exposed
insulation absorbs the high frequencies as well so the panel absorber has a
place in the studio. The two factors determining the frequency of absorption are:

The mass or density of the panel.

The depth of the air
cavity, i.e. depth of the sealed timber frame.

Filling the cavity with fibreglass or mineral wool
tends to lower the resonant frequency by up to 50 per cent as well as
doubling the effectiveness of the trap. It also lowers the Q of the trap so
that it is effective over a wider frequency range. A typical panel-type trap
is effective for frequencies around one octave either side of the centre
frequency, which at least has the advantage that you don't have to be absolutely
accurate to get results.

Calculate frequency of absorption of the Panel Absorber

Panel mass

[kg/m2]

Depth

[cm]

Frequency of absorption

Hz

recommended width

cm

recommended length

cm

Area

m2

Calculate
depth of the Panel Absorber

Panel mass

[kg/m2]

Frequency of absorption

[Hz]

Depth

cm

recommended width

cm

recommended length

cm

Area

m2

Calculate mass of the Panel

Frequency of absorption

[Hz]

Depth

cm

Panel mass

[kg/m2]

recommended width

cm

recommended length

cm

Area

m2

You can discover the resonant frequency by sticking a cheap contact mic on to
the panel's surface, then plugging the mic into a preamp or mixer with a VU
meter. Play a test tone from an oscillator or test tone CD using
loudspeakers, and vary this around the frequency the trap is designed for
until you get a maximum meter reading. This will be at the trap's resonant
frequency.

How to build a contact microphone

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Helmholtz
Panel Absorber

By fixing a perforated wooden panel over a
frame and putting an absorbent material inside the space created, a resonant
bass trap is formed with each perforation acting as a single 'bottle' in our
virtual bottle array.

By varying the percentage of perforation, the design can be applied to both
the bass and mid range.

However, predicting the performance of these traps is difficult because the Q
or bandwidth varies depending on the amount of internal damping.

The other problem is getting the right perforation percentage. Common
hardboard is usually used in mid traps rather than bass traps. For example:

hardboard (1 x 1 m)

6 mm thick

with 500 holes

and a hole diameter of 5 mm

has a perforation percentage of 0.98 per
cent.

Fixed over a 10 cm
air gap, this gives a resonant frequency of a little onder
160 Hz.

Calculate Resonant frequency of
Helmholtz Panel Absorber

Panel area
[m2]

Panel thickness
[cm]

Depth of the air space [cm]

Hole diameter [mm]

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Amount of holes

Percentage of perforation
[%]

Resonant frequency [Hz]

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Slot Helmholtz
Resonator

Slot width

mm

Slat width

mm

Depth from wall

mm

Slat depth

mm

effective depth of Slat

mm

Absorption Frequency

If the gaps vary say 5mm, 10mm, 15mm, 20mm and the
wall is angled as shown below, a broad band low mid absorber is created that
still keeps the high frequencies alive.

Remember the cavity behind must be airtight!

By
working out the different slat widths and slot gaps you can create a
broadband low mid absorber at specific frequencies.

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Placing the Traps

The
first pair of traps should be inserted in the front corners of the listening
room behind the loudspeakers. Care must be taken to use the trap only for its
intended purpose: pressure absorption.

The next pair of traps should be placed in the rear corners behind the
listening position. It is common to place larger traps in the rear corners
and smaller traps in the front corners. However, if your listening position
is at or forward of the room's midpoint, place the larger set of traps in the
front corners.

The next, and usually final, step is to put one trap at the midpoint of the
front wall. This trap, often referred to as the imaging trap, helps tame
cross-correlated reflections and takes care of the room resonance which
occurs at the midpoint of any of a room's walls.

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Links

Fundamentals
of Sound (very good!)

Acoustics Basics

Trap Setups

Optimizing Traps

JON RISCH
.versus. DECWARE Room treatments - A Comparative Review

Long Awaited Diffusor
Recipe

Calculate HELMHOLTZ-RESONATOREN (german)

Acoustic Treatment and Design for
Recording Studios and Listening Rooms

Technical
Articles

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Disclaimer

Please keep in mind that I am not an expert
or an authority on acoustics, and the information presented here is just my
opinion. I do however have many years of practical experience and my opinion
is based on that. Please take this information for what it's worth and
hopefully you will find it useful.

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