Interesting article about the room acoustics by Cees Ruijtenberg: ” Room acoustics is often underestimated. Acoustic research is often limited to spectral behavior but the time domain is just as interesting. The underlying investigation was done with the same loudspeaker under several conditions. The first picture (above left)shows you the waterfall plot outside a building. The first curve shows you the spectral response of the loudspeaker followed by some reflections coming from the ground and nearby buildings. It is also clear that there are no standing waves as there is no room with parallel boundaries.

The second picture (middle one)is made in a normal living room having concrete walls. The high frequency roll off is a bit more if compared to the first measurement as it was measured from the listening position which was approx. 4 meters. As you can see there is more energy in the middle of the spectrum due to multiple reflections coming from several walls. In the lower range we can see a lot of energy around 50 Hz and is affected by parallel walls. This effect is well known as standing waves.

Then we have the third picture (above right) which was made in a famous recording studio. Looking at the first plot we can see a very nice spectrum. In the midrange a small dip is caused by the mixing console. Further we can see a nice reverb behavior due to unparalleled
walls and decent absorption. After all a well-treated environment for monitoring. But have a look in the low ranges. This has nothing to do with loudspeaker or room interaction as it is coming from outside caused by industry or traffic. As a result the entire building is shaking and can be a part of the recording. The picture below will show you the same studio. While energy from the speaker/room combination is already gone, this kind of oscillating is still going on having a frequency of 5.8 Hz. Probably better to do critical recordings by night 🙂
Still the first picture doesn’t tell you the real story . The second picture tells us a lot about the energy in the sub region. In the first plot still the response of the loudspeaker is visible and will have a sound pressure of about 85 dB’s. The 5.8 Hz part is 30dB louder and is present all the time !!!.