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south-east european

INDUSTRIAL MARKET 3

PAID ARTICLE

Acoustics - The Phenomenon of

"acoustic short circuit"

When using dynamic loudspeaker systems, which transform

electrical signals into audible acoustical signals called sound

waves, their radiation characteristics, and therefore finally the per-

ceived sound quality will be strongly influenced by the mounting

situation in the application device, known as "the acoustic environ-

ment". For taking this into account, the size of the device and also

as its design should be carefully designed. The specific design

should take care of the method of mounting, if it is stand-alone

(like for example a kitchen radio), free flowing (like for example a

telephone) or fixed (like a communication column or door phone

device). In this paper we are going to describe the problems occur

in free-cone speaker systems, explain the background of the back-

to-front cancellation and suggest preventive solutions.

The effect

In real life, it can be noticed, that the sound quality of a loud-

speaker system perceived by a listener can sometimes vary a

lot, depending on different impacts like for example location

(position), or whether additional mechanical pressure is put

onto the loudspeaker unit.

Electro-dynamic loudspeakers are constructed and designed

for an as-good-as-can-be reproduction of sound signals in a

fairly wide frequency range, so that not only single tones but

also tone sequences, melodies and more complex signals (like

voice or music) can be played in an appealing quality.

Considering any specific loudspeaker unit, the usable fre-

quency range will normally represent only a certain part of the

total spectrum that is detectable by human ear. This usable

range will reach from approximately the (own) basic resonant

frequency of the oscillating system up to a constructively pre-

described upper limit, which is usually several Kilohertz (kHz).

Although below and above these limits, a loudspeaker will gen-

erate some sound, too. However, the sound intensity outside

the frequency limits will be significantly lower, so as a conse-

quence, there is no reasonable benefit for a practical use of

that extended range.

Now experience shows that the possible-in-theory (and spec-

ified) frequency range is practically significantly lower (narrow-

er), if -for example- such speaker specimen is placed and op-

erated freely flowing on a table or desk, i. e. without being

mounted into any final device.

Now if this speaker is driven by say a music signal, then it

will sound "less rich", which can be subjectively described

as "thin", "fade" or "tinny".

This behavior will dramatically improve after the speaker is

mounted into a housing environment, device or box. Then sud-

denly, the same specimen will sound more "full" or "voluminous".

Background /Explanation

Why does this happen? For correct understanding, it is help-

ful to remember that sound - by its nature - is nothing else than

air that is moved back and forth frequently. Like in meteorolo-

gy, considering the high- and low- pressure areas, there are

high and low pressure zones created with every movement of

the membrane (diaphragm) in a specific direction: high pres-

sure on the front side of membrane, and at the same time a

corresponding low pressure at the backside of membrane.

In case of a real loudspeaker unit, the movement direction of

the membrane is changing frequently; it is helpful to consider

the whole situation dynamically.

The entire system is striving for pressure compensation at

any point in time. Compressed air (enhanced pressure) uses

to compensate the lower Pressure ("thinner" air) at the other

side of membrane. In weather science this effect is known as

"wind", at a far greater scale. Despite of the scale, we can con-

sider this as a very similar thing, however much smaller.

• The sound (the moved air) takes the shortest possible way for

compensation, and this shortest way is "straight around the

membrane edge" for a free (non-mounted) speaker unit. To

enlarge the path most of the units have a gasket formed by

foam or rubber ring on the edge of the membrane. This cone

surround can be rubber or polyester foam, or a ring of corru-

gated cardboard, resin coated fabric to close the edge of the

diaphragm and the basket. The shape and the material of the

Dipl.Ing. Zoltan Kiss Sales manager East Europe Endrich Bauelemente Vertriebs GmbH

dipl Ing. Fred Kubert - Product Manager/ Acoustics

Zoltan Kiss

Sales manager - Eastern-Europe

e-mail:

z.kiss@endrich.com

surround can dramatically affect the acoustic output of the sys-

tem. Each material has advantages and disadvantages. Poly-

ester foam is lightweight and economical, but is sensitive to

ozone, UV light, humidity and higher temperatures.

• The effect is depending on the specific wavelength, and

therefore on the frequency. That's why for lower frequen-

cies, it is more significant than for higher frequencies.

• For wideband signals (like music or human voice) it will be-

comemost noticeable for the lower frequencies (bass range) for

which the generated pressure is suppressed most. Whereas

themiddle range is affected less, and the higher frequencies are

almost not affected, practically not noticeably. In fact, the higher

frequencies will be reproduced in a satisfactory way.

In practice, this means a displaced balance in the sound im-

pression and for any listener the perceived sound impression

will become "thinner".

It is important to point out that, despite of this effect; every-

thing is fine with the speaker unit. In this case, the speaker unit

is neither faulty nor defective.

For an exact technical measurement and judgment, the man-

ufacturers specify in their datasheets specific and reproducible

conditions, for example the exact and defined mounting posi-

tion in a sound wall of certain size.

Summarizing all above facts, the sound wave from back of

the cone (or even behind the speaker) tends to cancel the wave

from the front, because it is opposite in phase (180 degree) to

sound wave emitted by the membrane in front. This effect is

significant on lower frequencies.

Possible countermeasures

• One way out is extending the "bypass" for the sound, to sim-

ply make the shortest possible way a longer one, before it

comes to pressure compensation. This can be done for exam-

ple as a long tube like shown on the photos. The loudspeaker

is mounted into a tube that has holes on the surface. When

applying music on the speaker, we can hear thinner sound.

The high pressure in front of the membrane and the low pres-

sure behind it will generate acoustic wind; the waves will find

the shortest path through the holes. In order to make its way

longer, we can cover the holes by a paper roll, and the per-

ceived sound becomes immediately much richer.

• It is to point out that any "successful" pressure compensa-

tion is finally to understand as „lost sound energy", which is

no longer available to create sound as wanted.

• A second way is the tight closure of the loudspeaker's

edge while mounting it. To reach this, there are different

practical measures possible and commonly used, like for

example „gentle" pressing from inside direction towards the

housing front by using a backward foam piece, pressing/

clamping, elastic frontal edge sealing ring from suitable

material (like for example rubber, foam rubber, silicone

etc.). Also gluing or potting (total sealing) can be consid-

ered as possible ways where applicable.

Practical help

By the measures as described above, the unwanted direct pres-

sure compensation around the membrane edge can be effectively

prevented. Depending on the specific mounting situation and the

design of the device now the sound will try to compensate via the

shortest possible bypass, i. e. the closest opening. And depending

on the distance of this opening (hole) from the speaker (it's mostly

several centimeters), this results in a gain for the usable frequency

range towards the lower frequency band.

Designers should really take care of mounting the loudspeak-

er into the final place in the application, most commonly into

cabinets as shown on the figure. Closing the edges of the

speaker the length of the sound wave paths increase, causing

better sound quality.

The easiest way to avoid

the acoustic short circuit is

to mount the speaker into

a large plate (open wall),

the sound waves of the

front and end side of the

membrane cannot elimi-

nate each other (A).

Even better ( longer

wave path) solution is the

folded wall solution (B),

which is used for those

electric equipments where

the backside is open,

mainly for thermal cooling

considerations, like e.g.

guitar amplifiers.

The completely closed boxed systems, with appropriate sealing

is many times the best and easiest solution (C and E), but with

subwoofers the losses are too high, the efficiency goes down.

As an inherited solution of the closed housing, the bass reflex-

housing (D) has an additional hole and a channel on the front

panel next to the speaker. The system is designed in a way, that

the air cushion in the rest of the housing works as resonator.

Near the resonance frequency, a higher sound pressure level

can be achieved. Below this frequency, due to the acoustic short-

circuit sound is damped while above this frequency no positive

or negative effect can be noted. In this way, the bandwidth of the

loudspeaker can be extended by approximately one octave.

Besides the best possible tightening of the speaker's edge in an

appropriate manner, there is another, ready-made and quite practi-

cable solution, buying loudspeaker systems in „boxed" version, that

means pre-mounted in a completely closed environment (minibox).

Here, the sound wall is so to say „folded backwards" and

then closed, so that an unwanted pressure compensation

(=the „acoustic short circuit") is practically not possible any

more. By doing this, it is possible to exploit the full (=maxi-

mum possible by the specific design) frequency spectrum for

any specific loudspeaker system given.

Such „boxed" solutions can be found in our portfolio, for ex-

ample from the manufacturer Vansonic („VECO").