Improved damping of acoustic room modes by tunable Helmholtz resonators

In certain frequency ranges, disturbing sound waves cannot be damped with conventional absorbers. An optimization procedure and the use of finely tuned resonators, which are correspondingly dimensioned and positioned in the room, allow different damping effects to be achieved.

Problem

Room modes determine the sound propagation behaviour in small rooms, in the case of which the wavelength of the airborne sound lies in the order of the wall clearances. This manifests itself in strong variations of the frequency response of the sound propagation between a sound source (e.g. a loudspeaker) and a receiving transducer such as, for example, the human ear. In addition, this disturbance depends on both the place of the sound source and the place of reception and can manifest itself in an increase or decrease in the sound pressure at resonance frequencies.

Solution

At frequencies below 100 Hz, sound cannot be damped with conventional sound absorbers. Investigations were carried out on which possibilities arise when Helmholtz resonators are used. When tuned to one of the room resonances and placed in a suitable position in the room, these resonators can be coupled to the respective room mode and lead, when optimally dimensioned, to a clearly improved frequency response of the sound propagation. As statements on a corresponding optimization procedure have so far not been made in the technical literature, corresponding calculations were performed within the scope of a diploma thesis and verified in real rooms.

Applications

Applications can be found in many fields, for example in the acoustic irradiation of loudspeakers in sound control rooms, in room and building acoustic measurement technology or in sound recordings in small studios. In particular when hard parallel walls are irradiated with one of the resonance frequencies, so-called standing waves are formed, where integral multiples of half the wavelength λ/2 exactly fit between the two parallel walls. Between the local sound pressure maxima, places arise where the sound pressure of this frequency becomes minimal: the so-called sound pressure nodes. In addition, these places are characterized by the fact that here a sound source at the respective resonance frequency can hardly irradiate any sound.

Effect of the resonators

Depending on the optimization criterion, the use of tunable resonators allows three different effects to be achieved:

1. An enhancement of the frequency response of the sound propagation can be reduced to a minimum.
2. If its frequency agrees exactly with a resonance of the room, a single disturbing tonal noise (examples: transformer hum, fan noise) in the room can become evident in an unpleasantly amplified way. A tuned resonator of high quality can damp this sound strongly and selectively, without affecting the acoustic properties of the room at other frequencies. Proof of level reductions of more than 30 dB could be furnished.
3. The transient behaviour of a sound at the room resonance frequency can be clearly reduced. The effect of this measure increases with the quality factor of the undamped room mode.

In practical applications, optimal tuning can be realized by a special mechanism, in the case of which the resonance frequency and the damping of the Helmholtz resonator can be adjusted largely independent of each other.

Measurements in a test facility of building acoustics at the lowest resonance frequency of 38 Hz allowed the theoretical values of all three criteria to be verified.

(Audio)example: Damping of a resonance in the staircase of PTB's new Guest House.

When the lower door of the staircase is closed, a room resonance of 38.5 Hz is excited which lingers on clearly audibly for up to 7 s. Through a tuned and optimally damped resonator, the decay process at this frequency could be reduced to an inaudible minimum. This effect can be made audible in a sound example on the basis of a comparison of the door noises without and with a resonator. For this purpose it is recommended to use headphones with good bass reproduction.

Figure 1: Calculated sound pressure distribution at 38.5 Hz in the staircase without a resonator at a certain moment, red: positive sound pressure, blue: negative.

Contact person:

Ingolf Bork, Dept 1.6, WG 1.63, e-mail: ingolf.bork@ptb.de