Case study 1 – Low frequency absorption in a studio control room

done in order to demonstrate the implementation difficulties in controlling modal sound-fields using a ‘remedial’ technique based on resonant absorption units. The effects on frequency and decay response of the room are discussed.

The room aspect ratios were decided according to an optimisation technique for optimal modal distribution (Cox and D’Antonio 1997), which attempts to avoid modal degeneracy. The inner shell dimensions of the room are 3.98 by 4.95 by 2.7 metres (length, width, height). Even though an optimal modal distribution avoids degenerate modes and their associated problems, it will be demonstrated here that the reverberation time at low frequencies will still be dominated by the decay rates of the existing room modes. Furthermore, the room under test was constructed in double partition brick wall in order to achieve the necessary sound insulation. Due to this sturdy wall construction, the low frequency absorption in the room is very low, leading to long resonant decays.

Following technical recommendations for critical listening rooms (IEC 268-13 1985; ITU-R BS 1116-1 1994; EBU Tech 3276 1998), it was found that the low frequency decay time in the room, below 200 Hz, was too long and thus needed to be controlled. This was identified after appropriate measurements. Two types of measurement, briefly described in Appendix II were carried out and the results are shown in figures 5.2 and 5.3.

Two sets of modules based on the resonant membrane absorber principle, as described in Figure 5.1, were designed with resonant frequencies of 45 Hz and 89 Hz respectively. These modules

Chapter 5: Control of Room Modes

were built using common construction materials, such as medium density fibreboard, a 4 mm thick plywood membrane and bitumen based roof felt, which was used to increase the damping of the membrane. The cavity was filled with fibrous material. The placement of the modules was arranged along each lower vertex of the room where the modal coupling is highest. Thirteen modules for each frequency were installed. This corresponds to an absorption area of nearly 4 m2 _{at each ‘treatment’ frequency.}

Figure 5.2 shows the magnitude frequency response of the room. This was obtained using a dual FFT analyser, and measured as described in Appendix II.i. The response before and after the low frequency control modules have been installed is shown up to 200 Hz. Black dots represent the calculated eigenfrequencies of the room.

Modal Response of Control Room

0 10 20 30 40 50 0 50 100 150 200 Frequency (Hz) Am p li tu d e ( d B)

No modules All modules Eigenfrequency

Figure 5.2 – Modal Response of Control Room. Measured room responses are shown before (- -) and after (--) low frequency treatment with resonant membrane modules. Room eigenfrequencies are indicated in dots.

The dashed line, representing the room response before the low frequency control has been installed, clearly shows the large amplitude variations caused by the presence of resonances. The installation of the absorption modules introduces changes in the modal response, as indicated in Figure 5.2. Changes are shown to occur mainly above 50 Hz. The reduction of the original room modes at 56 Hz and 92 Hz are particularly noticeable. This is not surprising given that at their resonant frequencies (45 Hz and 89 Hz), the two sets of absorbers installed have maximum performance. The remaining frequency range is also altered by the presence of the absorbers, although the potential benefits on the overall room response are difficult to interpret given that the large variations of pressure magnitude still remain after absorption.

Chapter 5: Control of Room Modes

The amplitude is shown to increase at some frequencies. This may be explained by the fact that the introduction of the absorbers changes the source radiation impedance at some frequencies, facilitating more effective energy transfer into the room. The low resolution of the plot does not enable a clear view of the expected widening of bandwidth at each resonance, which is known to correspond to a reduction of their decay rate.

The effects of absorption noticeable on the magnitude frequency response are difficult to interpret, since the amplitude variations have not been removed to a great extent. These effects are also expected to cause a decrease in the reverberation time in the room. This was measured in a number of positions in the room using appropriate measuring techniques based on MLS sequences with the due care necessary to measure reverberation time at such low frequencies (Appendix II). The average results are shown in Figure 5.3, before and after the introduction of the low frequency control modules.

Reverberation Time

Figure 5.3 – Reverberation time measured in the control room before and after the introduction of low frequency control modules.

The resulting reverberation time differences show a more evident control of the resonant decays at the 63 Hz and 125 Hz octave bands. It is important to note that these results correspond to octave band average decay values and not to the decay of individual modes. The changes introduced are indicated in Table 5.1. The frequency range above 500 Hz has remained largely unaffected with only a slight decrease in the reverberation time, caused by the residual effects of the absorbers.

This example demonstrates that the use of membrane absorbers does have an objective effect on the modal sound field. The decay rates at the lower frequencies have been affected and this is shown by a decrease in the measured reverberation after the absorbers have been installed. The changes in the magnitude frequency response measured at a defined position in the room also

Chapter 5: Control of Room Modes

reveal the effects of the absorption, although the potential benefits are difficult to interpret given the remaining large variation of pressure with frequency.

Although a large number of absorption modules were constructed and installed, the final value of measured reverberation time at the two lower octave bands still does not comply with the recommendations (IEC 268-13 1985; ITU-R BS 1116-1 1994; EBU Tech 3276 1998). If this method is to be applied in the treatment of noticeable problems such as those referred in Chapter 4, more modules need to be installed to increase the absorption area and further decrease the decay rates. Defining the necessary reductions is in itself a problem, given that no knowledge of the subjectively acceptable levels for reverberation time at low frequencies has been gathered, and the only guidance available originates from postulates in the published recommendations. The determination of the necessary absorption area remains difficult, and would benefit both from a clear indication of the absorption coefficient of the devices as well as a subjectively valid target level for the noticeability of room modes. The latter topic has been addressed in Chapter 9.

The next section presents the implementation of the same type of absorption device but using commercially available units for which an absorption coefficient measured using standard methods is provided a priory. The practical implementation is thus guided by better information on the absorption characteristics of the devices.