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ROOM MODES AND SBIR

Room Modes

Room modes cause peaks and dips in frequency response and associated boomy frequencies. We hear this as areas in a room where bass is very loud at certain frequencies, and in areas where bass seems to ‘drop out’ or disappear.

These are known as bass hot spots and dead spots. But what causes these hot spots and dead spots? Answer.. Standing Waves. Standing Waves are formed when the distance between 2 hard surfaces (ie walls, & floor / ceiling) is the same as half the wavelength of a certain frequency.

For example a 55hz sound wave (A1 musical note) has a wavelength of 6.18m (20ft 3”). In room acoustics we halve that figure, which means that a room 3.1m in length will produce standing waves and room modes at :

55hz fundamental (A1)

110hz harmonic (A2)

165hz harmonic (very close to E3)

220hz harmonic (A3)

275hz harmonic (between C4 and C#4)

330hz harmonic (E4)

In practice this means that a room with a dimension of 3.1m will have peaks and dips in frequency response and associated boomyness throughout the room at the above frequencies, however the most severe of these issues will be at the fundamental and lowest harmonics. Because in this example these frequencies are directly related to the frequencies of musical notes (A and E) particular problems would be expected when these notes are played back in this room.

The above gives the room modes for only 1 dimension of a room, but rooms have 3 dimensions - so you will have a separate set of room modes for :

• Room length
• Room Width
• Room Height

Types of Room Mode :

The equation shown above gives predicted room modes for Axial Modes, which are usually the most dominant / problematic, but are also the easiest to establish where to treat in a room to deal with.

However there are 3 types of Room Mode :

Axial - formed by 2 surfaces

Tangential - formed by 4 surfaces, about half the intensity of Axial

Oblique - formed by 6 surfaces - about half the intensity of Tangential

Figure 1 shows how these different types of modes are formed

Axial

Tangential

Oblique

Figure 1

Calculate the Full list of Room Modes :

The equation for finding axial modes is : speed of sound / 2L = f

Speed of sound = 1130ft / second or 340 meters per second, L = distance between parallel surfaces, f = frequency

Tangential and Oblique modes can also be calculated. The equation for the calculation of all room modes is

There are several online mode calculators that will do the calculations for you..here are some good ones :

• http://www.bobgolds.com/Mode/RoomModes.htm - Gives full list of modes including tangential and oblique
• http://www.hunecke.de/en/calculators/room-eigenmodes.html - Gives full list of modes, and a handy graphical representation of where the bass energy will be highest and lowest for a given frequency.

How does this information help in deciding where to treat and with which products?

If you try inputting your room dimensions in the hunecke calc this will show you where bass energy is high at a modal frequency and where it is low. High energy / pressure areas are where you need to treat to control that mode.

Concentrate on the lowest frequency modes as these will be the most problematic, and look for frequency areas where there is a close build up of modes. Check where in the room this energy is highest using the graphical interface, and put trapping there.

We are more than happy to do this for you and advise on which of our products to use where.

As an example if you have axial modes down to 50hz use our BF-612 Corner Traps in vertical corners floor to ceiling.

If you have axial modes down to 40hz use our BF-850 Corner Traps or BF-4040s

If you have axial modes down to 30hz use our BF-1200 Corner Traps

Ideally use BF-175 traps for back wall in most rooms.

This normally deals with most low frequency modal problems - although targeted absorption may also be needed if you have tangential and oblique modes closely spaced together. This is best determined via room testing.

Speaker Boundary Interface Response

Speakers fire high frequencies in a ray out of the front of speaker, which makes predicting where reflections are going to occur fairly straight forward.

However, below 400hz 99% of speakers radiate energy in 360 degrees out of the speaker. Certainly below 300hz you can expect as much bass energy to be coming out of the sides and rear of the speaker as is coming from the front of the speaker.

The bass energy radiating from the front of the speaker arrives at the listener’s ears as a nice clean direct signal. However the bass radiating from the sides and rear will bounce off any nearby surfaces. This reflected signal then arrives at the listener’s ears slightly after (in time) the direct signal. The difference in the time of arrival between direct and reflected signals causes phasing. If the reflected signal arrives in phase with the direct signal it will cause a peak in frequency response, and if it arrives out of phase it’ll cause a dip / reduction In level at a specific frequency - meaning bass will seem to ‘drop out’.

The peaks and dips caused by SBIR can be very significant - and sometimes just as severe as the room modes.

The solution to this is to treat side walls adjacent to speakers and front wall / directly behind speakers.

Speaker positioning plays a massive role here too - in fact you can tune in or out peaks and dips in frequency response using speaker positioning relative to walls / ceiling / floor, and the listening position.

This tweaking of speaker positioning is best done once a room has been treated, so that room modes are properly controlled and not interfering with the search for the best speaker position. However even in untreated rooms playing with speaker locations can bring big improvements.

To really benefit from this process room testing will give the best results - even moving speakers an inch or 2 can vastly change the low frequency response heard at the listening position.