task6_low

Table 1: first 10 modes of the room

The eigenfrequencies correspond to the real room, thus we have to take into account the scaling of 1:10 for our model. That means we are supposed to have modes at 286 Hz, 451 Hz, 543 Hz etc. in our measurement results of the test box.

We make a first measurement of the empty box, the microphone placed in one corner of the "room" (attached to the amplifier with a cable getting through a hole in one of the side walls at the rear end of the room) and the loudspeaker being represented by a headphone plugged into a hole at the front side of the room (at the same side wall). We used a microphone position in one of the other corners to obtain a clear modal pattern.

We use Room-Capture and chose following settings: pink noise and a frequency range of 200 Hz - 1 kHz which corresponds to 20 Hz - 100 Hz for the real room. As a first result for the frequency response we get:

Figure 1: Frequency response of the empty room

The first peaks that we can see, correspond nearly exactly to the room modes we calculated analytically before. The goal of this lab is to give a more neutral character to the room. The first step to take is to reduce the amplitude of the first resonance frequencies in order to flatten out the very low frequencies. One easy way to absorb specific frequencies is to use Helmholtz resonators.

We checked also the "software-calculated" frequency for each mode by the "Room Modes" icon in Room Capture. Those calculated frequencies are really close to the measured ones and the hand-calculated ones. We are thus quite sure about those values.

Helmholtz resonator

In this case, as the model is quite small and we need rather high frequencies (286 Hz, 451 Hz ...) we use beer cans as resonators. The specific frequencies are achieved by filling more or less the cans with water, so that the air volume inside the can changes. The usual mode pattern of a rectangular room shows that the highest pressure levels appear near the walls. That's why we always put the absorbers (porous and Helmholtz) next to walls.

For the first try we put two cans in two corners of the room (at the opposite side wall to the loudspeaker). The microphone is placed rather in the center where the mixing table is supposed to stand. The two cans are filled in the way that they absorb the two first frequencies at around 280 Hz and 450 Hz. The exact frequencies can't, of course, be achieved as we deduce the necessary amount of water just by blowing over the cans and controlling the emitted frequency with the Spectrum analyser of Room-Capture.

Figure 2: set up with 2 cans in the corners

We do the same measurement again, for the same frequencies, but for different positions of the cans. This time they are placed at the centers of the side walls.

Figure 3: set up with 2 cans in the centers

When we compare the modification brought by the Helmholtz resonators and compare them to the frequency response of the empty room, we get:

Figure 4: frequency response of set-up with Helmholtz resonators (280 Hz, 450 Hz)

As we can see on the figure above, the first two peaks decrease by 7 dB in amplitude (with frequency shifts). There is a visible difference between the configurations where the cans are in the middle or in the corners. The SPL reduction is approximately the same. However the response is much flatter for the second try (cans at side walls) up to 450 Hz. Above this frequency the second try shows big antiresonances which don't appear neither at the first try nor in the empty room.

What we can observe is that even if the cans are configurated to reduce the first two modes, the other modes are a bit attenuated too.

The resonance frequencies differ a bit using the can. This could be explained in different ways. The cans in the room will modifiy its geometry (since the cans use the volume of the room) and then the modes of the room will be changed. One can also witness different degree of coupling between the room and the cans; this coupling can be positive but also negative and creates two peaks at a mode instead of one.

Instead of only comparing the different positions of the resonators we can also vary the frequencies we want to reduce. First we configurated two cans for the 2 first modes. Now we use three cans and choose the frequencies (286, 534, 572) and (534, 572, 728). Again, as the way of getting these frequencies is not exact, we finally have 280, 530, 590 and 790 Hz.

Figure 5: Set-up with three cans

Figure 6: Frequency response of set-up with 3 Helmholtz resonators (280 Hz, 530 Hz, 590 Hz) / (530 Hz, 590 Hz, 790 Hz)

Let's first have a look at the pink curve (280, 530, 590 Hz): there is a little reduction at the first mode and, as expected, none at 450 Hz. At 530 Hz the attenuation is huge, nearly 15 dB. At 590 Hz the SPL is reduced of around 8 dB with a small frequency shift.

For the brown curve (530, 590, 790 Hz) there is an unexplainable reduction at 280 Hz. It has exactly the same shape as if there had been a can configurated for this frequency. At 530 Hz the reduction is nearly non-existant while at 590 Hz it's exactly the same as for the other configuration. As we didn't manage to adjust the third can at exactly the modal frequency, but only at 790 Hz, the attenuation at this frequency range is rather modest.

One way to improve the absorption of the resonance requencies of the room would be to add a certain flow resistance into the neck of the cans. Due to the fact that we could get problems trying to remove them, we prefered not to test this possibility.

The usual advantage of a Helmholtz resonator is that you can focus on very special disturbing frequencies and try to remove them. When the modal density of the room is still very low, as it is in the very low frequency range, the resonators can be of big help. The big drawback however is, that there is absolutely no general flattening of the frequency response. As we can see, for instance on figures 4 and 6, the response gets even more "peaky" and irregular.

Porous absorber

In order to flatten out the whole low frequency range it might be useful to use porous absorbers. We have two devices: small patches and corner wedges, both made with absorbing material.

Figure 7: set-ups (absobers, absorbers + corner wedges, corner wedges)

When using the corner wedges and the absorbers separately and then both mixed we get the following frequency responses:

Figure 8: Frequency response of the set up with absorbers + corner wedges

What can be observed at first sight is that the set-ups with corner wedges and with/without absorbers are nearly identical. That shows that the main device that brings modifications is the corner wedge (this can be expected since the corner wedges are in the corners...), while the absorbers nearly don't change anything. When we have a look at the green curve (only absorbers) we have again another proof for the fact that the absorbers don't bring much change. The curve still follows the one obtained for the empty room. It's a bit attenuated but not flattened at all. The two other curves, with the corner wedges included, however, show a sensible flattening, especially in the frequency range between 400 Hz and 1 kHz.

Until here one could notice that often the resonance frequencies (especially in low frequencies) were a bit shifted down in frequency. This can be explained by the fact that the room looks apparently larger because of the change of the sound speed inside the absorbing material of the room (non adiabatic).

At last we compare the efficiency of the small absorbing patches to the Helmholtz resonators.

Figure 9: Set-up with absorber panels and Helmholtz resonators

Figure 10: Frequency response for the set-ups absorbers and absorbers + Helmholtz resonators

For the resonators we took the cans of the last configuration. What we can see in general, is that the frequency response is not really flattened out. In some frequency ranges it is even more uneven. Furthermore there's nearly no difference with or without Helmholtz resonators. So we can conclude that the resonators have no real interest for our studio.

As a conclusion we can say that the best method to use is the absorption by corner wedges. The modifications bring significant improvements in the frequency response of our studio and are really easy to use!

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