Author: Georg Merkert
The idea of an in-house acoustic measurement room had been around for quite some time. There were several advantages for us. Our aim with the room was to:
- check the serial variation of speakers and microphones
- validate the suitability of speakers and microphones for developments and modifications
- provide a consistent measurement environment for comparative measurement
- carry out customer acceptance tests, such as FAT (Factory Acceptance Test)
At the beginning of 2025, we began to view and qualify available rooms.
In order to take the correct insulation measures, the room had to be measured accordingly. One of the most important parameter is the reverberation time of the room, in this case the RT60 (it shows how long it takes for a sound to decrease by 60 dB in a room after the sound source is turned off).
To measure the RT60, you need an appropriate measuring device and an impulse generator, e.g. a blank-firing pistol, an omnidirectional speaker (dodecahedron) or similar.
Image of a dodecahedral speaker
The image shows a visualization of the basic principle of reverberation time measurement.
We repeated the RT60 measurements several times in order to calculate an average value and thus increase the measurement accuracy. The measurements taken in the unmodified room show relatively high reverberation times in octave bands.
Before:
| Band [Hz] | RT60(T20) [s] | MeasUncertainty [%] |
| 63 | 2,48 | -,– |
| 125 | 1,43 | -,– |
| 250 | 0,88 | 7,95 |
| 500 | 0,79 | 5,93 |
| 1000 | 0,77 | 4,24 |
| 2000 | 0,78 | 2,97 |
| 4000 | 0,73 | 2,17 |
| 8000 | 0,60 | 1,70 |
Table values for the unmodified room
However, the low frequencies could not be measured with sufficient accuracy due to the wavelength of the signal and the room geometry. They were ignored due to measurement uncertainty during measurement and subsequent evaluation.
From 250 Hz onwards, the values drop to < 1 sec, which would be sufficient for a room for speech reinforcement, but is still too much for acoustic measurements.
These RT60 measurements were sent to manufacturers specializing in room acoustics. This data provided them with the parameters they need to determine and select the insulation materials.
The modification:
The walls and ceiling of the room were covered with appropriate panels by a specialist company.
Mobile acoustic partitions and bass absorbers (basetraps) provide further improvements in insulation values.
The measured values show a significant improvement in all octave frequencies.
Afterwards:
| Band [Hz] | RT60(T20) [s] | MeasUncertainty [%] | Reduction [%] |
| 63 | 1,91 | -,– | -22.98 |
| 125 | 0,94 | -,– | -34.27 |
| 250 | 0,45 | 5,77 | -48.86 |
| 500 | 0,31 | 3,73 | -60.76 |
| 1000 | 0,28 | 1,55 | -63.64 |
| 2000 | 0,27 | 1,26 | -65.38 |
| 4000 | 0,21 | 1,03 | -71.23 |
| 8000 | 0,20 | 1,03 | -66.67 |
These values create a “low-noise” room, and the reverberation times are so low that audio measurements can be carried out with more than sufficient (repeat) accuracy.
What do we use to measure? – The measuring equipment
The audio measuring equipment is installed in an adjacent room and consists mainly of the following:
- XL2 audio analyser, Minirator generator, M4261 microphone
- True RMS measuring device
- 4-channel DSP measuring amplifier 4 x 50 watts / 100 V
Image shows our measuring equipment
Where do we measure? – The measuring room:
The image shows the measurement room during near field measurement of one ASH 125 siren horn. The mobile acoustic walls (gray) and the rectangular bass traps (white) are visible in the background.
What are the results? Was it worth the effort?
Previously: In the past, we mostly took measurements in free fields and were always interrupted or disturbed by:
- Weather (wind, rain, etc.)
- Other external influences (road noise, etc.)
- Setups were not identical and took a lot of time
- Comparative measurements were often not identical and therefore not comparable
Today: We are able to measure our products in a professional acoustic environment and to improve the sound quality.
a) The measured values are identical to the free field measurements to such an extent that they are within the measurement tolerance.
b) Set-up times are short, saving us a huge amount of effort.
c) Customers are also impressed by this further professional step in the field of acoustic measurements.
Our acoustic measurement room has proven to be an extremely profitable investment.
We hope you enjoyed this overview on our in-house acoustic measurement room.
If you want to have more information, please feel free to contact us.
