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Room Modes 101

by ct_acoustic_frontier July 06, 2022

What are room modes and how do they impact sound quality?

Some unavoidable facts:
  • ALL rooms have naturally occurring resonance frequencies called room modes or standing waves. If you read around the web you'll find some more esoteric terms like eigentones or eigenmodes. I quite like calling them modal resonances.
  • Room modes are the main cause of acoustic distortion below the transition frequency, causing peaks and dips in the frequency response of 20dB or more.
  • Modal resonances store energy and decay slowly compared to nearby frequencies causing audible problems such as 'one note bass' and 'boominess'.
  • NEARLY ALL systems, even those with extremely high end equipment, suffer from the negative impact of modal resonances.
If are unhappy with the performance of your home theater, listening room or recording studio in the bass and lower midrange frequencies then it is highly likely that the problem is related to the negative impact of modal resonances. You will need to learn a little about the science behind room modes to understand how to reduce or remove their impact on sound quality.  

What causes Room Modes?

Room modes are created when a sound wave travels between two opposite boundaries, for example the left and right side walls or the floor and ceiling. The first modal resonance occurs at the frequency where the distance between the two boundaries is equal to half a wavelength. If a room is 18ft long then the first modal resonance will be at 31Hz. There are further resonances at multiples of this frequency. There is a huge amount of good information available on the web about room modes so I won't try and replicate it here. Read some of the following links:
  • Mark Wieczorek's 'crash course on acoustics' offers a good overview of the topic.
  • The Institute of Sound and Vibration in the UK has some excellent animations on standing waves and room modes. It's good to realize that the same physical phenomenon that causes room modes is responsible for generating the resonances that create sound when a stringed instrument is plucked.
[caption id="attachment_2634" align="aligncenter" width="509"]Standing Waves, ISVR A screen capture of the awesome animation over at the ISVR page[/caption]

Room mode calculators - what are they good for?

Whilst learning about room modes, you will almost certainly run into 'Room Mode Calculators'. These predict the frequencies at which modal resonances will occur. For rectangular spaces room modes can be easily predicted from the length, wide and height dimensions. If you have a non-rectangular or odd shaped room (e.g. L-shaped or open plan) then I would recommend skipping calculators completely and going straight to measurement, or hiring a professional with the capability to do the right modeling. Predicting the room modes in such a space is a non-trivial exercise - we use boundary element modeling (BEM). Using the results produced by a calculator it is possible to see which peaks on a frequency response measurement are room mode related and which are caused by other mechanisms such as speaker boundary interference. Check out a couple of these links: [caption id="attachment_2633" align="aligncenter" width="545"]Room Mode Calculator The Harman Room Mode Calculator[/caption] It is important not to get too hung up on the results of the room mode calculator since it does not tell you the impact of modal resonances IN YOUR ROOM. The first reason is that a room mode calculator does not show you the effect of modal resonances on the frequency response or time domain performance of your system. Two rooms with the same dimensions can have very different performances due to the materials used in the construction of the room and the furnishings used. The second reason is that in real life the actual room modes can be greatly affected by walls with doors, windows and other departures from the infinitely stiff surface used in the math that the calculators use. Therefore it is important to measure your room to identify what modal resonances issues there are and have a reference point to compare with once you have implemented some improvements.  

How can I identify modal resonances using acoustic measurements?

Evidence of resonances can be seen in three areas in our measurements:
  • A narrow peak in the frequency response.
  • Ringing in the time domain.
  • Changes in frequency response and time domain behavior at different points within a room.
Clear evidence of room modal activity can be seen in this high resolution frequency response plot taken with XTZ Room Analyzer II Pro. You can get similar plots from software like Room EQ Wizard and Omnimic as well as the old school graph paper, test tones and SPL meter. For more on acoustic measurement options see our blog article: four approaches to room acoustic measurement. [caption id="attachment_2631" align="aligncenter" width="500"]Frequency response Frequency response, as taken with XTZ Room Analyzer II Pro[/caption]   This 2D waterfall or cumulative spectral decay plot (CSD), taken with Room EQ Wizard, clearly shows evidence of modal resonances, identifiable by their slow decay in the time dimension. [caption id="attachment_2568" align="aligncenter" width="700"]Room EQ Wizard Spectrogram Room EQ Wizard Spectrogram[/caption]  

How can I check if my room has modal resonance problems?

Room modes can be identified through peaks in the frequency response and slow decay in the time domain. We made a video showing how to interpret the acoustic measurements and spot room modes. This video shows XTZ Room Analyzer II Pro but you can use a similar technique for other measurement solutions.

How do I reduce the impact of modal resonances?

There are a number of different solutions to room mode issues.
  1. Acoustic treatment - see our blog article on bass trap placement
  2. Positional equalization - changing the location of the listening positions and speakers / subs
  3. Electronic equalization - see our blog articles Room Correction: A Primer, Hard Proof that EQ kills room modes and Audyssey XT32 vs. Parametric EQ
  4. Room mode cancellation - see our links to articles on multiple low frequency sources
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ct_acoustic_frontier
ct_acoustic_frontier

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