Concepts 2: Acoustics

AEROnet Audio & Electronic Research

14-Nov-2006

Room Acoustics: a brief introduction

	room acoustics An understanding of room acoustics is vital to getting the most from your sound system. As noted on the concepts page, the speakers are the most important part of a reproduction system. The speakers act upon the air in the room causing pressure and velocity changes. The room is thus a direct extension of the speakers. So much so, that any discussion about the perceived sound of speakers without reference to room acoustics is largely meaningless! So let's see what we can discover...
	reflections The human ear can discriminate between the original sound source and single reflections that are heard more than about 30ms later. At times less than this, any reflections from objects near the speakers are integrated by the ear as belonging to the sound source. These reflections are called 'early reflections'. Unfortunately, because the ear cannot distinguish between early reflections and direct sound, the early reflections appear to be part of the sound of the speakers. Reflected sound waves will interferre the direct sound from the speakers and cause an uneven frequency response. In a typical listening room, any solid surface within about 10m of the speakers is liable to cause early reflections. Thus most rooms aren't large enough to avoid the effects of early reflections. The effects can be mitigated to some degree by providing strategically placed acoustic absorbers and diffusors. By imagining sound as emanating from the speakers as might a light ray, absorbers and diffusors can be positioned along walls adjacent the speakers to reduce the deleterious effects of early reflections. Speakers with controlled dispersion are also of benefit as they tend to illuminate adjacent reflected surfaces to a lesser degree. After multiple reflections, the sound will have exceeded the 30ms threshold to become part of the reverberant sound field of the room.
	reverberation

The most important metric of a room is the reverberation time or RT60. This is the time it takes for the reverberant sound field to decay away by 60dB after the sound is suddenly switched off. The more reflective the room, the longer the reverberation time.

The situation is complicated by the frequency dependant nature of absorbent materials. Typical listening rooms have more absorption at high frequencies than low ones. Thus the reverberation time is almost always longer at lower frequencies.

In order to illustrate the behaviour of a real room, I have measured the RT60 in my current listening room.

RT60 Measurements 19-3-06

Octave bands: Time (s)

40-80Hz: 0.9

80-160Hz: 1

160-320Hz: 0.7

320-640Hz: 0.7

640-1.25kHz: 0.5

1.25-2.5kHz: 0.4

2.5-5kHz: 0.3

5-10kHz: 0.3

The trend of increasing RT60 at lower frequencies is clear. Another way of looking at this information is that the size of the room increases at lower frequencies from an acoustic point of view!

The room is storing more energy at the low frequencies. This is a characteristic of almost all listening rooms, and is therefore part of our normal everyday experience of listening to sound indoors. In this case, ideally the reverberation time would be a bit lower at low frequencies and perhaps slightly longer at high frequencies, but the critical midrange RT60 is about right.

Various references suggest that an RT60 of about 0.5s is optimum for a general listening room, with a shorter RT60 of 0.3 for home theatre and with less than 20% change between adjacent octave bands.

My current listening room is constructed within a concrete shell and it is therefore difficult to reduce the reverberation time at the low end where the RT60 is increasing a bit more than recommended. Bass traps can help here, but their effectiveness is limited by their size. Given the aesthetic, structural and size limitations, the room sounds surprising good.

I also calculated the expected RT60 using a Java applet written by Dr. Jörg Huneckeby and this gave results in close agreement with the measured values, both proving the validity of the experiment and the usefulness of the calculations.

The directivity of the speaker also plays a part and interacts with the RT60 of the room. See the following section about Critical Distance.

room modes

Because of the smaller amount of absorption at lower frequencies the sound tends to bounce around between walls more than it does at higher frequencies. The wavelengths at low frequencies are also much longer; at the very lowest frequencies, often longer than the largest dimension of the room. These factors give rise to room modes. Room modes always exist in an enclosed space. They cause an uneven bass response, making rooms boomy and bass loudness inconsistent at different locations within the room.

Floyd Toole has done a huge amount of research into room modes. His white papers are essential reading for anyone designing or installing sound systems. As Toole shows there's no magic in acoustics or room modes. Some simple rules can be derived to overcome the problems.

Just to summarise a couple of the main points of his work: Speaker and listener placement within a room are far more important the actual dimensions of a listening room. He suggests that perhaps the best compromise placement for subwoofers are as a pair along opposite side walls.

critical distance

The distance from the speakers where the measured sound field is 3dB greater than the reverberant sound field alone is called the critical distance. It's a yardstick that can be used to give an idea of how useful the on axis frequency response graph of a speaker is. Unfortunately the critical distance is frequency dependant because of the differing amounts of absorptive material in a room making the determination more complicated .

To give some indication of what the situation is like in a medium sized listening room, here's a graph showing how critical distance varies with frequency. The room under test was my listening room which has an effective size about 5x4x2.5m. The test speaker was a small 2-way with 8" woofer and 1' tweeter loaded by a 150deg waveguide.

In the lowest octave, the speaker is omni-directional and the critical distance is a very low 0.5m from the speaker. At higher frequencies up to around 500Hz the critical distance is about 1.5m. Above 1kHz the controlled directivity of the waveguide plus the reasonably constant RT60 of the room gives a consistent critical distance of around 2.5m from the speaker. The 640-1.25kHz band is a bit strange, the data suggests that dispersion is wider at this frequency than others, but it isn't clear why that might be.

So at a typical listening distance of 3m, the reverberant sound field would seem just as important to the overall perceived sound of the speaker as the direct on axis response. Only at the higher frequencies does the direct sound begin to overpower the reverberant sound field. At the lowest frequencies the reverberant soundfield completely overwhelms the the direct sound from the speaker. The low frequency amplitude response published by the speaker manufacturer is largely meaningless unless the characteristics of the room are known.

Of course each room and speaker set is different. Rooms with less absorption and speakers with wider dispersion will have shorter critical distances.

This information is significant when assessing the suitability a particular speaker system for a particular room. Speakers with wider dispersion will suit rooms with shorter RT60's. Rooms with longer RT60's will benefit from a more controlled speaker dispersion. Conversely, the RT60 of the room can be controlled to best match the directivity characteristics of the speakers.

Speaker dispersion can be controlled through manipulating driver size, radiation pattern eg dipole, or by adding directivity controlling devices such as horns or waveguides.

As noted earlier, RT60 is a function of room size and the amount of sound absorbent material in the room.

near field monitoring

Sound professionals often listen at distances very close to the loudspeakers. This is referred to as 'near field monitoring'. The near field is basically the region well inside the critical distance. Conversely, the far field is the region where the reverberant sound field exists well beyond the critical distance.

Near field monitoring can give the listener a very clear understanding of what is actually part of the recording. The room becomes much less important in shaping the perceived sound. Of course the accuracy of the perceived sound is directly determined by the axial frequency response of the speakers. The manufacturer can promote flat axial response over the off-axis response to achieve this end. (more about the off-axis or power response can be found in Speaker Design: An overview of we're trying to achieve:) The actual listening experience is quite similar to listening on headphones.

room response time

The reverberation characteristics of a room are normally thought of a steady state phenomenon. The sound field builds up to a point where the amount of energy entering the room is balanced by the energy leaving it. Music however is transient in nature. The reverberant sound field is similar to background noise that follows the musical signal in a harmonically related fashion.

With this in mind, the direct sound has a greater significance upon the perceived sound of a speaker than the critical distance would suggest. This seems clear from everyday experience. Even in an extremely reverberant space such a cathedral, speech is still audible to a reasonable extent.

I suggest that listening at a distance in accordance with the critical distance of the room at midrange frequencies is a good compromise between the normally quite accurate frequency response of the direct sound of the speaker and the sense of space that the reverberant sound field adds to the listening experience. There will also still be considerable transient information to give good directional clues to the ear.

For home theatre where the created reverberant sound field is a greater feature of the original direct sound, the listening distance should be further within the direct sound field at a distance less than the critical distance.

envelopment

Envelopment is a term used to describe the effect of sound surrounding the listener. A peculiar effect of reproduced sound in small rooms is that the low frequencies aren't easily localised outside of the head. Bass sounds can appear as though they are inside your head. In a large concert hall there are enough reflections to provide a sense of space for high frequency sounds as well as low frequency ones. There is a sense of envelopment which is often missing at low frequencies in small rooms where there are static structures of standing waves.

David Griesinger spends a great deal of time and effort developing an understanding of envelopment and how it can be encouraged to occur in small listening rooms by thoughtful speaker and listener placement. In summary, he concludes that multiple subwoofers are required are often best placed to the sides of the listening space. He also suggests that while control of standing waves is worthwhile to a degree, that some room modes are in fact necessary to establishing envelopment at low frequencies.

the next step

Now it's time to have a look at the various attributes of speakers and how they affect performance and sound perception. Speaker Design: An overview of we're trying to achieve:

references

Floyd Toole at Harman has reseached and written extensively about acoustics for sound reproduction rooms. His papers are essential reading.

A lot of valuable ideas on room acoustics are also to be found at Siegfried Linkwitz's site.

David Griesinger has also progressed the state of the art of acoustics in rooms for sound reproduction amongst his other work on recording techniques and the application of electronic reverberation. Technical but very interesting work.

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