Active Silencers and Absorbers

In comparison with the well known principle of noise cancellation by destructive interference of out-of-phase pressure waves (anti-noise) actively absorbing silencers represent a new advantageous technique of active noise control in HVAC-ducts. This refers not only to the robust structure of the system but also to the underlying principle of operation which goes without sophisticated and expensive digital signal processing.

Our active silencers are built up of single cassettes which contain the active components (loudspeaker, microphone, analogue signal processing) covered by perforated sheet metal and glass fiber fabric (Figure 1).

If deactivated the membrane of the loudspeaker and the volume of the back cavity form a kind of a serial resonator. Near the resonance frequency of this lining the increased compliance of the passive system leads to a insertion loss typical for reactive silencers. Now, the microphone voltage proportional to the pressure in front of the membrane is fed back linearly with a high gain forcing the membrane to withdraw even further. Thereby, on one hand the pressure in the back cavity is increased but on the other the pressure at the interface to the duct is decreased and the insertion loss rises significantly. As a result more acoustic energy is absorbed into the ASC.

Figure 2 shows the insertion loss of such an active silencer (1 m long consisting of 4 ASC) measured in a test duct with a cross-section of 0,25m x 0,25m. The peak insertion loss of about 40 dB around 100 Hz is achieved by equal tuning of all ASC. However, with different designs for each ASC a more broadband attenuation is possible.

Active quarter-wave resonator

© Fraunhofer IBP

For a low-frequency noise control in ducts, active sound absorbers represent a possible alternative to porous sound absorbers that work particularly efficiently at medium and high frequencies. Industrial applications in the field of heating and air conditioning technology are considered to be especially promising. However, the potential of this technology is limited by unfavorable ambient conditions. High temperatures, extreme sound levels, static and dynamic pressure loads and humid and aggressive media, for example, can impede the use of active components. In order to ensure the efficiency of active sound absorbers also in these cases, either resistant yet expensive components have to be used, or the sensitive parts of the active sound absorber have to be protected adequately against unfavourable ambient conditions. One possible solution is to spatially separate loudspeaker, microphone and electronics from the sound channel in order to avoid direct contact with the medium. The practical solution is based on a λ/4 resonator with known transmission properties, which is mounted at the side of the sound channel and connected to it by an opening. The insertion loss of such a branch with acoustically rigid termination can be changed and increased considerably by influencing the sound field in the resonator chamber with an active sound absorber cassette (ASDK) installed at its end.

Construction of the Active Exhaust Silencer (AES)
The active silencer, shown in figure 1, forms itself around the circular main duct and consists of two parts. At the bottom a porous absorber layer surrounds the main duct half. At the top side of the main duct there is a tube attached via a small opening. To protect the side branch mechanically and to avoid an exchange of gas from the main duct with that of the side branch the opening is covered by perforated sheet metal and fibre fabric. Additionally, the opening is sealed hermetically by heat-resistant foil.

In the side, there is attached over the whole length a porous absorber layer which serves at one hand as heat insulation and at the other hand as an acoustic absorber for the interior of the resonator. The end of the side branch is terminated by a small box containing a standard loudspeaker, an electret microphone and an analogue controller which forms an active silencer cassette. With these active components an electro-acoustic feedback loop is established. The feedback gain can be manually or automatically adjusted at the controller and determines within the stability limits the acoustic wall impedance of the ASC.

Experimental Results
Figure 2 shows the results of the insertion loss measurements of the silencer in figure 1. The black curve depicts the case where a plate of sheet metal rigidly terminates the end of the silencer branch as in conventional passive side resonators. This configuration forms a combination of Helmholtz- and Quarter-Wavelength-Resonator with a first IL-maximum of 17 dB at about 160 Hz. Additionally, the IL increases with rising frequency due to the porous layer at the bottom half-pipe.

The measured IL changes little if an active cassette without feedback is assembled at the end of the side branch instead of the rigid termination (red curve). However, if the feedback loop is closed with the highest possible feedback gain the IL-maximum becomes broader and is shifted to about 50 Hz (green curve). With a feedback gain between the maximum and zero the low-frequency-maximum in the IL will be located somewhere between 160 Hz and 50 Hz. Thus, it is possible to tune the IL-maximum from about 160 Hz to about 50 Hz simply by adjusting the feedback gain. This opens up the opportunity for a manual or an automatic control of the IL depending on SPL-related operational states of a noise emitting device as e.g. the rotational speed of an internal combustion engine or the temperature of a burners exhaust gas.


Active Absorbers in Small Rooms

Messung der Übertragungsfunktion in diagonal gegenüberliegenden Ecken mit und ohne Absorber in den Ecken

Bild 1

Übertragungsfunktion im schallharten Raum ohne Absorber (grün), mit aktivem Absorber (rot) und Rechnung mit A1 (schwarz)

Bild 2

Übertragungsfunktion im bedämpften Raum mit (rot) und ohne Absorber (grün)

Bild 3

The variety of small enclosures where the acoustic environment decides their usefulness and quality includes professional sound studios as well as conference rooms and speaker cabinets. For economic reasons laboratories for certain acoustical measurements become smaller and smaller. The requirements concerning acoustical quality, however remain on the same high level. Therefore an effective acoustical conditioning of small rooms becomes a current topic in practical applications. Compared to, e.g. larger concert halls the speciality of small rooms becomes evident at lower frequencies where the room size has the same dimensions as the wavelength. The sound field in an untreated room is influenced by standing waves, the so-called eigenmodes of the room. These eigenmodes result in a distorted sound field which strongly depends on the positions of the receiver. Therefore, not only the sound absorbing material but the position of the sound source and the receiver must necessarily be taken into account when designing small rooms for acoustical purposes.


Active Absorbers in rooms with rigid walls

Apart from commonly used resonating absorbers, active absorbers are a very small and effective solution to influence the sound field in a room. These absorbers act like actively absorbing silencers where the local reaction of a mass-spring-system is fed back and amplified with a microphone in front of a loudspeaker membrane. To characterize their effectiveness the transfer functions of a room (Fig. 1) with and without an active absorber are compared. These transfer functions are obtained by driving a source in one corner of the room and measuring the sound pressure level in the opposite corner. In a small untreated room with rigid walls the transfer function shows large maxima and minima as seen in Fig. 2. The peaks in the transfer function are smoothed significantly with one single active absorber only. This also implies a shorter reverberation time at the resonant frequencies of the room.

A calculation method has been developed for the exact placement of the active absorber, whose effect is strongly dependent on the position. The example solution (Fig. 2) shows the accuracy of the implemented theory valid for rectangular rooms.


Active absorbers in damped rooms

The design of acoustic rooms certainly extends to the middle and higher frequency range, e.g. with porous materials as wall panels. Depending on the thickness and absorption of such panels there are changes in the lower frequency range as well. A layer of 10 cm porous foam on the walls and on the ceiling of the room in Fig. 1 leads to a smoother transfer function with increasing frequency (Fig. 3).

Apart from the eigenmodes between 30 and 50 Hz the minima at 77 Hz and 90 Hz impair the listening conditions. Such minima in the audible frequency range cannot be compensated, e.g. in a music studio by simply equalizing the sound signal. In this specific case the frequency dip at 77 Hz can be eliminated with one active absorber placed in the room (Fig. 3). The use of additional active absorbers leads to a further acoustical improvement of the room.