RMA – ROOM MODE ANALYZER
The Room Mode Analyzer (RMA) offers an easy check of the most important acoustic listening room properties without the need for an expensive measurement microphone.
Each listening room is a confined space where sound waves with a frequency below the so called Schroeder Transition Frequency (STF) become standing waves creating room modes. For small rooms that STF can be as high as 500Hz, whereas in regular listening rooms the transition happens below 250Hz.
The main timbral changes are caused by wave acoustic effects (room resonances) below the STF. Above the STF sound waves propagate in an optical manner, just reflecting of the walls, similar to light beams reflecting in a mirror, causing comb filter effects which can be relatively easily controlled through acoustic measures like sound absorbing furniture.
As a first conclusion we can say that the room response is most critical for frequencies below 500Hz.
If you start the RMA then you’re able to hear the sinus sweep (1Hz – 500Hz) which should have an equal sound level, getting louder in perception with increasing frequency because of the hearing curve. In reality you will hear a sound volume that fluctuates, sometimes by more than 20dB, caused by standing waves resonating between walls where the listening position is either in a maximum or minimum of a room mode.
Room modes are defined by the standing wave resonances possible within the dimensions of the room. There are several different modes but the most critical are standing waves between two opposite walls, creating clearly separated peaks in the RMA measurement.
Bass reproduction in rooms with distinctive room resonances is quite boomy. It is impossible to separate different bass notes.
The Room Mode Analyzer (RMA) is an easy to use free of charge application to measure the relative amplitudes and frequencies of the different room modes by applying a slow linear sinus sweep to assure that all resonances are excited.
There is no need to use a special measurement microphone because even the laptop internal microphone helps you to identify the frequencies of the different room resonances caused by standing waves.
As discussed before it is not of importance to look at frequencies above the Schroeder Transition Frequency, therefore the RMA measures the Room Response up to 500Hz covering the most critical frequency range.
Select the Audio Input Device
This can be either the laptop internal microphone or a dedicated measurement microphone like the UMIK-1.
Select the Audio Output Device
This must be the sound card output connected to your HiFi-System you want to measure.
Press start to initiate the measurement
The RMA needs 60 seconds to execute a full measurement, starting with the left speaker and after 30 seconds switching over to the right speaker.
Preconditions for a flawless measurement:
Make sure that the background noise level is as low as possible.
Adjust the volume of your sound card or amplifier to assure that the resulting measurement curve is above -60 dB but below -6dB.
RMA measurement showing left and right channel.
The mouse pointer indicates a room mode at 139Hz.
RMA measurement of the left channel. The display of the right channel has been deactivated by a click on the “right” label.
The mouse pointer shows a room mode at 52Hz.
RMA measurement of the right channel.
A room mode at 56Hz is marked by the mouse pointer.
It is quite difficult to apply acoustic measures to damp standing waves because the necessary bass traps are quite large which renders them nearly unusable to be integrated into an average listening room.
Luckily, moving the loudspeaker and the listening position within the room can make a huge difference. It should be possible to find a location where the listening position isn’t in a standing wave maximum avoiding the typical booming bass reproduction.
Just use the RMA to measure the room response for different positions to find the best loudspeaker/listening combination.
Another method is the classical room equalization by minimum phase full parametric equalizers.
It is known that room modes exhibit a minimum phase (causal) response, therefore it is common practice to use dedicated equalizers to reduce the amount of energy put into the room mode frequencies.
Don’t try to fill gaps in the room response by amplification of those frequencies! Only acoustic measures can compensate minima within the room transfer function.
There are several solutions in the market trying to correct the full room response (amplitude & phase) either already in audio players or as additional plugins.
A room response correction above the Schroeder Transition Frequency tries to mitigate the effects of early wall reflections and the not perfect frequency response of the loudspeaker.
It is our experience that a correction of the loudspeaker response is useful but to achieve good results it is necessary to do the measurement under free field conditions (e.g. within an anechoic chamber).
Correcting the room response beyond the STF is extremely difficult and only perfect for a very small sweet spot. This is due to the fact that the wavelength gets shorter with increasing frequencies. A 5 kHz sound wave has a wavelength of c/f (c = 340m/s at 20°C) which is around 6.8cm (2.68 inches).
For those frequencies the smallest listener head movement would render the correction obsolete. To mitigate that effect some solutions try to combine several measurements to broaden the listening spot.
We recommend to make sure that the listening room shows a good frequency response below the STF (around 250Hz) by avoiding the excitation of the strongest room modes and to handle all frequencies beyond the STF by having regular furniture absorbing the most critical sound wave reflections to keep the RT60 reverberation time within acceptable limits.