June 15, 2024
The most important physical parameter for characterizing the acoustic properties of a room is its reverberation time. It is a measure of a room’s reverberation. Reverberation time is frequency-dependent, since materials such as stone, wood, carpet, and textiles absorb sound to varying degrees at different frequencies.
Reverberation Time is the time interval during which the sound pressure in a room drops to a specified fraction of its initial value when the sound source suddenly stops (reverberation). In most cases, this fraction is set at one-thousandth, which corresponds to a decrease in sound pressure level of 60 dB. The corresponding reverberation time is then designated as T60 or simply T; in English, it is usually referred to as reverberation time (RT). It is one of the most well-known parameters in room acoustics.
German Wikipedia (translated)
Music is produced in the recording studio for a “standardized” acoustic environment. This process is based on a reverberation time in the listening room as specified by DIN standards. The closer the reverberation time in the listening room matches that specified by DIN, the more accurately the music you hear will correspond to the version mixed by the sound engineer in the studio. The reverberation time specified by DIN varies depending on the size or volume of the listening room.
Acourate Wiki (translated)
We measure the RT60 reverberation time using Acourate. The reverberation time is determined from the measurement of the room’s impulse response.
An Example of Improving the Reverberation Time
August 31, 2021
During one of the countless room acoustics measurements I’ve performed, we discovered that the door to my listening room has a significant impact on the room’s reverberation time. I’d like to describe here how we minimized that impact. As always, the first step is to take a measurement.
The image above shows the initial setup. In the upper right corner, “DIN 18041 Music” is selected as the measurement tolerance. Below that are the dimensions of my listening room. These specifications are used to generate the two dashed lines. Ideally, the reverberation time curves for both channels should now fall within these two boundaries.
You can clearly see how the reverberation time increases below approximately 100Hz and exceeds the interval at around 80Hz, meaning it becomes too large. The measurements also clearly show that I am using a mono subwoofer. Below 125Hz, the crossover frequency, the curves for the left and right audio channels overlap.
As a first step, I applied a sealing strip with a rubber profile — the kind used in window construction — to the top and side of the lock, tucking it into the door’s rebate. This made the door close so tightly that it could no longer move within the lock.
The results are impressive. The reverberation time is now quite acceptable. Apparently, the door moved in its frame due to the bass vibrations in the room. This measure effectively prevents that from happening. Incidentally, it’s a change that goes unnoticed and can therefore be made in a living room without any issues.
However, I didn’t stop there. I have my own listening room, so I don’t have to worry about the famous WAF (Wife Acceptance Factor). I also covered the door on the listening room side with heavy-duty automotive foil.
The heavy foil does indeed offer another improvement, though not as noticeable as the first one. However, we’re talking about high-end audio here, so I’m grateful for even the smallest positive change.
Up to this point, I took all the measurements without the active absorber; I wanted to assess the door’s behavior on its own. In the next measurement, we’ll see how it behaves with the absorber turned on.
It’s very clear to see how the situation has deteriorated. Apparently, the absorber is now operating at an incorrect setting, which is having a negative effect on the reverberation time.
But this is easy to explain: The heavy foil significantly lowers the door’s resonance frequency, and this must, of course, be taken into account in the absorber’s settings.
A minor adjustment to the frequency response in the absorber’s DSP resolved the issue. The reverberation time in my room now falls within the specified range across the entire frequency spectrum. Furthermore, the situation improved even more with the use of the active absorber; from around 50Hz onward, I have a fairly constant reverberation time of around 0.4 seconds all the way up to the cutoff frequency. A result that is satisfying not only in terms of measurements. As I’ve already mentioned elsewhere, I now have bass reproduction in my listening room that I never would have thought possible.
Addendum
In August 2021, I modified my measurement setup and now take measurements exclusively using the Lynx Aurora(n). This was made possible by adding an additional card with microphone inputs to the converter. The measurement signals naturally also pass through the Dante network, so I took measurements at 96 kHz/24-bit. Otherwise, there have been no changes.
Measurement using the Lynx Aurora(n)
The measurement results above clearly show the impact that a suboptimal measurement chain has on the measurement results.
If you compare the two graphs above, you can see that the RT60 curves for both measurements are around 0.3 seconds starting at approximately 250Hz. It can therefore be said that the measured values follow roughly the same trend here.
Below this frequency, however, there are significant differences in the results. This becomes most apparent below approximately 50 Hz. With the old measurement setup, the reverberation time increases sharply; in the measurement using the Lynx, however, the value remains below 0.4 seconds.
I attribute the differences to the rather basic microphone preamp I was using before. The Lynx’s microphone input clearly performs much better here.