B&O Tech: But what if my room is Scandinavian?

#13 in a series of articles about the technology behind Bang & Olufsen loudspeakers

 

The following question recently arrived in my inbox via our customer service department.

“I am an admirer of B&O Hifi products as of over 20 years, but a great mystery for me is how you achieve great sound reproduction in the typically minimalist Scandinavian interior design environmment with polished floors, bare walls and bare glass windows. Effectively such environments are acoustical disasters !?!”

I thought that this was a great question – so it’s the topic of this week’s article.
Of course, you are correct. A room comprised of large flat reflective surfaces with little acoustical absorption has a very different acoustical behaviour from a recording or mastering studio where the final decisions about various aspects of a recording are made. And, consequently, this must have an effect on a listener’s perception of a recording played through a pair (assuming stereo reproduction) of loudspeakers in that room. The initial question to be asked is “what, exactly, are the expected effects of the room’s acoustical behaviour in such a case?” The second is “if the room has too much of an effect, how can I improve the situation (i.e. by adding absorption or changing the physical configuration of the system in the room)?” The third, and possibly final question is “how can we, as a loudspeaker manufacturer compensate (or at least account) for these effects?”
The effect a room’s acoustical behaviour has on a loudspeaker’s sound can, at a simple level, be considered under three general headings:
  1. early reflections
  2. room modes
  3. reverberation

Early Reflections

Early reflections, from sidewalls and the floor and ceiling, have an influence on both the timbre (tone colour) and the spatial characteristics of a stereo reproduction system. Let’s only think about the timbral effects for this article.
Fig 1. The sound arriving at a listener from a loudspeaker in a room with only one wall. Note that the sound arrives from two directions - the first is directly from the loudspeaker. The second is a "first reflection" off the wall.
Fig 1. The sound arriving at a listener from a loudspeaker in a room with only one wall. Note that the sound arrives from two directions – the first is directly from the loudspeaker. The second is a “first reflection” off the wall.
Let’s start by assuming that you have a loudspeaker that has a magnitude response that is perfectly flat – at least from 20 Hz to 20 kHz. We will also assume that it has that response regardless of which direction you measure it in – in other words, it’s a perfectly omnidirectional loudspeaker. The question is, “what effect does the wall reflection have on the measured response of the loudspeaker?”
Very generally speaking, the answer is that you will get a higher level at some frequencies (because the direct sound and the reflection add constructively and reinforce each other) and you will get a lower level at other frequencies (because the direct sound and the reflection work against each other and “cancel each other out”). What is potentially interesting is that the frequencies that add and the frequencies that cancel alternate as you go up the frequency range. So the total result looks like a comb (as in a comb that you use to comb your hair, if, unlike me, you have hair to comb). For example, take a look at Figure 2.
Fig 2. Distance to loudspeaker = 2 m. Distance to wall = 1 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
Fig 2. Distance to loudspeaker = 2 m. Distance to wall = 1 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
So, you can see in Figure 2 that, at the very low end, the reflection boosts the level of the loudspeaker by a little less than 6 dB (that’s two times the level!) at the listening position. However, as you go up in frequency, the total level drops to about 15 dB less before it starts rising again. As you go up in frequency, the level goes up and down. This alternation actually happens at a regular frequency spacing (i.e. a notch at every 200 Hz) but it doesn’t look regular because the X-axis of my plot is logarithmic (which better represents how we hear differences in frequency).
What happens if we move the wall further away? Well, two things will happen. The first is that the reflection will be quieter, so the peaks and notches won’t be as pronounced. The second is that the spacing of the peaks and notches in frequency will get closer together. For example, take a look at Figure 3.
Fig 3. Distance to loudspeaker = 2 m. Distance to wall = 3 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
Fig 3. Distance to loudspeaker = 2 m. Distance to wall = 3 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
Similarly, if we move the wall closer, we do the opposite, as in Figure 4.
Fig 2. Distance to loudspeaker = 2 m. Distance to wall = 0.25 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
Fig 4. Distance to loudspeaker = 2 m. Distance to wall = 0.25 m. Wall is perfectly reflective and the loudspeaker is perfectly omnidirectional.
So, if you have a room with only one wall which is perfectly reflective, and you have a perfectly omnidirectional loudspeaker, then you can see that your best option is to either put the loudspeaker (and yourself) very far or very close to the wall. That way the artefacts caused by the reflection are either too quiet to do any damage, or have an effect that starts at too high a frequency for you to care. Then again, most room have more than one wall, the walls are not perfectly reflective, and the loudspeaker is not perfectly omnidirectional.
So, what happens in the case where the loudspeaker is more directional or you have some fuzzy stuff on your walls? Well, either of these cases will have basically the same effect in most cases since loudspeakers are typically more directional at high frequencies – so you get less high end directed towards the wall. Alternatively, fuzzy stuff tends to soak up high frequencies. So, in either of these two cases, you’ll get less high end in the reflection. Let’s simulate this by putting a low pass filter on the reflection, as shown in Figure 5, 6 and 7 which have identical distances as the simulations in Figures 2, 3, and 4 – for comparison.
Fig 5. Distance to loudspeaker = 2 m. Distance to wall = 1 m. Wall is absorptive at high frequencies and/or the loudspeaker is directional.
Fig 5. Distance to loudspeaker = 2 m. Distance to wall = 1 m. Wall is absorptive and/or the loudspeaker is directional at high frequencies .
Fig 2. Distance to loudspeaker = 2 m. Distance to wall = 3 m. Wall is absorptive at high frequencies and/or the loudspeaker is directional.
Fig 6. Distance to loudspeaker = 2 m. Distance to wall = 3 m. Wall is absorptive and/or the loudspeakers is directional at high frequencies.
Fig 2. Distance to loudspeaker = 2 m. Distance to wall = 0.25 m. Wall is absorptive at high frequencies and/or the loudspeaker is directional.
Fig 7. Distance to loudspeaker = 2 m. Distance to wall = 0.25 m. Wall is absorptive and/or the loudspeaker is directional at high frequencies.
What you can see in all three of the previous plots is that, as the high frequency content of the reflection disappears, there is less and less effect on the total. The bottom plot is basically a proof of the age-old rule of thumb that says that, if you put a loudspeaker next to a wall, you’ll get more bass than if it’s farther from the wall. Since there is not much high frequency energy radiated from the rear of most loudspeakers, Figure 7 is a pretty good general representation of what happens when a loudspeaker is placed close to a wall. Of course, the exact behaviour of the directivity of the loudspeaker will be different – but the general shape of the total curve will be pretty similar to what you see there.
So, the end conclusion of all of this is that, in order to reduce undesirable artefacts caused by a wall reflection, you can do any combination of the following:
  • move the loudspeaker very close to the wall
  • move the loudspeaker farther front the wall
  • sit very close to the wall
  • sit farther away from the wall
  • put absorption on the wall

However, there is one interesting effect that sits on top of all of this – that is the fact that what you’ll see in a measurement with a microphone is not necessarily representative of what you’ll hear. This is because a microphone does not have two ears. Also, the direction the reflection comes from will change how you perceive it. A sidewall reflection sounds different from a floor reflection. This is because you have two ears – one on each side of your head. Your brain uses the sidewall reflections (or, more precisely, how they relate to the direct sound) to determine, in part, how far away a sound source is. Also, since, in the case of sidewall reflections, your two ears get two different delay times on the reflection (usually), you get two different comb-filter patterns, where the peaks in one ear can be used to fill in the notches in the other ear and vice versa. When the reflection comes from the floor or ceiling, your two ears get the same artefacts (since your two ears are the same distance to the floor, probably). Consequently, it’s easily noticeable (and it’s been proven using science!) that a floor or ceiling reflection has a bigger timbral effect on a loudspeaker than a lateral (or sideways) reflection.

Room modes

 Room modes are a completely different beast – although they exist because of reflections. If you pluck a guitar string, you make a deflection in the string that moves outwards until it hits the ends of the string. It then bounces back down the string, bounces again, etc. etc. See the diagrams and animations on this page – they might help. As the wave bounces back and forth, it settles in to a total result where it looks like the string is just bouncing up and down like a skipping rope. The longer the string, the lower the note, because it takes longer for the wave to bounce back and forth on the string. You can also lower the note by lowering the tension of the string, since this will slow down the speed of the wave moving back and forth on it. The last way to lower the note is to make the string heavier (i.e. by making it thicker) – since a heavier string is harder to move, the wave moves slower on it.
The air in a pipe behaves exactly the same way. If you “pluck” the air in the middle of a pipe (say, by clapping our hands, or coughing, or making any noise at all) then the sound wave travels along the pipe until it hits the end. Whether the end of the pipe is capped or not, the wave will bounce back and travel back through the pipe in the opposite direction from whence it came. (Whether the pipe is closed (capped) or open only determines the characteristic of the reflection – there will be a reflection either way.) It might help to look at the animations linked on this page to get an idea of how the air molecules behave in a pipe. As the wave bounces back and forth off he two ends of the pipe, it also settles down (just like the guitar string) into something called a “standing wave”. This is the pipe’s equivalent of the skipping rope behaviour in the guitar string. The result is that the pipe will resonate or ring at a note. The longer the pipe, the lower the note because the speed of the sound wave moving in air in the pipe stays the same, but the longer the pipe, the longer it takes for the wave to bounce back and forth. This is basically how all woodwind instruments work.
What’s interesting is that, when it comes to resonating, a room is basically a pipe. If you “pluck” the air in the room (say, by putting sound out of a loudspeaker) the sound wave will move down the room, bounce off the wall, go back through the room, bounce of the opposite wall, etc. etc. etc. (other things are happening, but we’ll ignore those). This effect is most obvious on a graph by putting some sound in a room and stopping suddenly. Instead of actually stopping, you can see the room “ringing” at a frequency that gradually decays as time goes by. However, it’s important to remember that this ringing is always happening – even while the sound is playing. So, for example, a kick drum “thump” comes out of the speaker which “plucks” the room mode and it rings, while the music continues on. You can see this in Figure 8, below.
Fig 8. The concept of the effect of a room mode. The sound coming out of the loudspeaker is shown on the top plot, in black. The response in the room is shown in blue. You can see there that the room keeps "ringing" at a frequency after the sound from the loudspeaker stops. The red plot on the bottom is the difference between the two plots - in other words, the "sound" of the room mode in isolation.
Fig 8. The concept of the effect of a room mode. See the text below for an explanation.
Figure 8 shows the concept of the effect of a room mode. The sound coming out of the loudspeaker is shown on the top plot, in black. The response in the room is shown in the middle plot in blue. You can see there that the room keeps “ringing” at a frequency after the sound from the loudspeaker stops. The red plot on the bottom is the difference between the two plots – in other words, the “sound” of the room mode in isolation (note that it’s at a different scale than the top two plots to make things easier to see).

There are two audible effects of this. The first is that, if your music contains the frequency that the room wants to resonate at, then that note will sound louder. When you hear people talk of “uneven bass” or a “one-note-bass” effect, one of the first suspects to blame is a room mode.

The second is that, since the mode is ringing along with the music, the overall effect will be muddiness. This is particularly true when one bass note causes the room mode to start ringing, and it keeps ringing when the next bass note is playing.  For example, if your room rings on a C#, and the bass plays a C# followed by a D – then the room will be ringing at C#, conflicting with the D and resulting in mud. This is also true if the kick drum triggers the room mode, so you have a kick drum “plucking” the room ringing on a C# all through the track. If the tune is in the key of F, then this will not be pretty.

 

If you would like to calculate a prediction of where you’ll have a problem with a room mode, you can just do the following math:

metric version: room mode frequency in Hz = 172 / (room length in metres)

imperial version: room mode frequency in Hz = 558 / (room length in feet)

Your worst modes will be the frequencies calculated using either of the equations above, and multiples of them (i.e. 2 times the result, 3 times the result, and so on).

So, for example, if your room is 5 m wide, your worst-case modes will be at 172 / 5 = 34.4 Hz, as well as 68.8 Hz, 103.2 Hz and so on. Remember that these are just predictions – but they’ll come pretty close. You should also remember that this assumes that you have completely immovable walls and no absorption – if this is not true, then the mode might not be a problem at all. (If you would like to do a more thorough modal analysis of your listening room, check out this page as a good start.)

Sadly, there is not much you can do about room modes. There are ways to manage them, including, but not exclusive to the following strategies:

  • make sure that the three dimensions of your listening room are not related to each other with simple ratios
  • put up membrane absorbers or slot absorbers that are tuned to the modal frequencies
  • place your loudspeaker in a node – a location in a room where it does not couple to a problematic mode (however, note that one mode’s node is another mode’s antinode)
  • sit in a node – a location in a room where you do not couple to a problematic mode (see warning above…)
  • use room correction DSP software such as ABC in the BeoLab 5

 

Reverberation

Reverberation is what you hear when you clap your hands in a big cathedral. It’s the  collection of a lot of reflections bouncing from everywhere as you go through time. When you first clap your hands, you get a couple of reflections that come in separated enough in time that they get their own label – “early reflections”. After that, there are so many reflections coming from so many directions, and so densely packed together in time, that we can’t separate them, so we just call them “reverberation” or “reverb” (although you’ll often hear people call it “echo” which is the wrong word to use for this.

Reverb is what you get when you have a lot of reflective surfaces in your room – but since it’s so irregular in time and space, it just makes a wash of sound rather than a weird comb-filter effect like we saw with a single reflection. So, although it makes things “cloudy” – it’s more like having a fog on your glasses instead of a scratch. Think of it like the soft focus effect that was applied to all attractive alien women on the original Star Trek – you lose the details, but it’s not necessarily a bad thing.

 

So what are you gonna do about it?

Fine, this is a short-form version of what a room’s acoustics does to the sound of a loudspeaker, but how do we, as a manufacturer of loudspeakers, ensure that our products can withstand the abuse that your listening room will apply to the sound? Well, there are a number of strategies that we use to do what we can…

1. ABC. The BeoLab 5 has a proprietary system built-in called Adaptive Bass Control or ABC. Pressing a button at the top of the loudspeaker starts a measurement procedure that is performed using a built-in microphone that measures the loudspeaker’s behaviour in two locations. Actually, what it’s doing is looking at the difference in the loudspeaker’s response in those two positions of the microphone to determine the radiation resistance that the loudspeaker “sees” as a result of reflective surfaces in the room. The ABC algorithm then creates a filter that is used to “undo” the effects of some of the low-frequency effects of the room’s acoustics. For example, if the radiation resistance indicates that the loudspeaker is close to a wall (which, as we saw above, will boost the bass) then the filter will reduce the bass symmetrically. That way, the loss in the filter and the gain due to the wall will cancel each other.

2. Position switches. ABC in the BeoLab 5 is a very customised filter that, in part, will adjust the loudspeaker’s response for placement near a wall or in a corner. Almost all of the other BeoLab loudspeakers (and other sound systems such as the BeoPlay A8 and A9, for example), include a manual-adjusted “position switch”. This allows you to use one of three filters that we have customised in the development of the loudspeaker to account for its behaviour according to whether you have placed it away from a reflective surface (“Free”), near one surface (“Wall”) or in a Corner. This is not just a filter that adjusts the bass level. The three filters for “free”, “wall”, and “corner” have been calculated using three dimensional measurements of the acoustical behaviour of the loudspeaker. So, the filters for the BeoLab 3 are completely different from those for the A9, for example, because they have very different directivity characteristics.

3. Sound design in multiple rooms. As I talked about in a previous posting, when we do the sound design of all of our loudspeakers, we tune each of them in at least 4 or 5 rooms with very different acoustical behaviours ranging from a very “dead” living room with lots of absorptive and diffusive surfaces to a larger and very “live” space with a minimalistic decorating, and large flat surfaces (just like the description in the original question). Once we have a single sound design that is based on the common elements those rooms, we test the loudspeakers in more rooms to ensure that they’ll behave well under all conditions.

 

Wrap-up

Of course, I haven’t covered everything there is to know about room acoustics here. And, of course, you can’t expect a loudspeaker to sound exactly the same in every room. If that were true, there would be no such thing as a “good”concert hall. A room’s acoustical behaviour affects the sound of all sound sources in the room. On the other hand, humans also have an amazing ability to adapt – in other words you “get used to” the characteristics of your listening room. Back when I was working as a part-time recording engineer in Montreal, I did a lot of recordings in churches. Typically, we (the producer and I) would set up a control room with loudspeakers in a back room, and the musicians would sit out in the church. When we arrived to set up the gear, the first thing was to set up the monitor loudspeakers and a CD player, and we would play CD’s that we knew well while we set up everything else. That way, we would “learn” the characteristics of the control room (since we already knew what was on the discs and the characteristics of the monitor loudspeakers). So, if all of our CD’s sounded like that had too much bass, then we should do a recording with too much bass – it was the fault of the control room.

However,  there is no debate that, due to lots of issues (the first two that come to mind are frequency range and directivity) two different loudspeakers will behave differently from each other in two different rooms. In other words, if you listen to loudspeaker “A” and loudspeaker “B” in a showroom of a shop, you might prefer loudspeaker “A” – but if you took them home, you might prefer loudspeaker “B”. This would not be surprising, since what you hear is not only the loudspeaker but the loudspeaker “filtered” by the listening room. This is exactly why, when you are buying a loudspeaker, you should audition it in your home in order to ensure that you will be happy with your purchase. And THIS is why you can arrange a home demonstration of Bang & Olufsen loudspeakers through your dealer.

10 Responses on “B&O Tech: But what if my room is Scandinavian?”

  1. David Keener says:

    I have two questions with regard to this article.

    Some listeners have commented on the “weak” bass of the BeoLab 5 compared to other BeoLab speakers, especially the BeoLab 9. I theorized that the BL 5’s bass may seem less prominent but more accurate than the BL 9 because of the ability of the BL 5 to account for room effects on the bass response vs. other speakers. What is your opinion of this? Independent reviews have provided very flat bass response graphs of the BL 5.

    Second, you provided equations showing approximate room mode frequencies. The equation makes reference to “length” which I’m assuming is the front to back dimension. Below that, you appear to reference the room width (side to side) dimension. Would it perhaps be more accurate to not to refer “length” but “room dimension” as I’m guess the room length, width, and height all contribute to the room mode frequencies?

    David

  2. geoff says:

    Hi David,
    It is certainly possible that a DSP-based room compensation algorithm like ABC can result in some people saying “not enough bass” if they’re expecting things like room modes or boundary effects to provide some assistance. It is possible to play BeoLab 5’s with the ABC filter disabled, so a customer can A/B the loudspeakers with and without room compensation to determine which they prefer. Personally, I never tell people that they should prefer something just because it’s more “correct”. If you like ketchup on your sushi, then you should put ketchup on your sushi. :-) However, whether this is the one and only explanation for people’s comments with respect to bass level comparisons of BeoLAb 9’s and 5’s – I would be, cautious. When comparing two loudspeakers, unless you are absolutely sure that nothing but the loudspeakers has changed – then any conclusions are dangerous. So, unless the loudspeakers are in the same place, in the same room, playing the same tune at the same level while you sit in the same place, then I would not trust anything. And even then… I would be careful. In particular the question of how you match the listening level of two loudspeakers with different responses is a significant one. And, since a small change in level can result in a large change in perception, my advice is “be careful”.

    As for the question of “length” – actually, to quote Dr. Suess, “I said what I meant and I meant what I said”. However, to be fair (and more precise), I was assuming that the loudspeakers are set up in a rectangular room, symmetrically, near one of the shorter walls. So the “length” of the room is the distance from the front wall, between the loudspeakers, through you, to the back wall. The reason my personal opinion is that this mode is worse than the width one is because, in this particular case, the correlation of a stereo signal will have an influence on the “width” room mode. As Floyd Toole has been pointing out for years, when your loudspeakers are set up in that way, any correlated signals (i.e. voice, bass, kick drum, etc…) in the stereo signal will cause the loudspeakers to act oppositely to the width modes that have a null at the centre of the room. This is because the loudspeakers, for that signal, are “in phase” but the mode is “out of phase” (more accurately, “opposite in polarity”) – so, in this case there is no mode. So, for the width dimension, in this configuration, half of the modes typically don’t exist. However, for the “length” dimension, they all do. As for the “height” modes – there’s not much you can do about them, but they’re typically higher in frequency than the others (because most rooms are shorter than they are long or wide), so, going up in frequency, you’ve already run into trouble by the time you hit the fundamental frequency of the height dimension. One additional “advantage” of the height mode is that you typically don’t move too much vertically (you’re either sitting or standing) so you don’t get the variations in level at the modal frequencies as you move around the room.

    Cheers
    -geoff

  3. Peter Schaefer says:

    In the chapter “Room modes are …” you wrote in the second passage:
    “Whether the pipe is called or open”
    Could it be a typo? (Whether the pipe is closed or open).

    Very nice article!

  4. geoff says:

    Oops. Nice catch. Thanks!

    I actually meant to say “capped” – but now I’ve edited it to both “capped” and “closed” – just to cover all the bases.

    Cheers
    -g

  5. Jan Sand says:

    Can playing through four speakers improve room accoustics? I have never seen this discussed, but maybe putting speakers in the back can override/improve the accoustics of a room?

    I have BL9 as fronts and and can play BL4000 in the back connected as “fronts” when playing music through Beoplay V1.

    Best regards, Jan

  6. geoff says:

    Hi Jan,

    The fastest answer is “yes, but it really depends on the details”. There has been a lot of great work by Todd Welti published at the AES (see, for example, this paper as a start).

    Generally, if you have matched loudspeakers all over the room, all playing the same low frequency content, then they will have better “control” of the room modes. However, I’m painting with a VERY wide paintbrush here. There are specific cases (i.e. loudspeaker positions, or frequencies) where you will actually make things worse.

    Since you have a BeoPlay V1, you could certainly try it to see if, with your specific loudspeakers in their specific placements in your specific room, you get an advantage. Instead of setting all of the Speaker Roles to be the same in a Speaker Group, you could do it this way:

    1. use a Sound Mode with Bass Management set to ON (i.e. Movie mode)
    2. go to MENU -> SETUP -> SOUND -> SPEAKER GROUPS -> your speaker group -> ADVANCED SETTINGS -> BASS MANAGEMENT -> REDIRECTION LEVELS
    3. Set all values to 0 dB
    4. go back one menu step and then into REDIRECTION BALANCE
    5. Set all values to “0” (the centre position)

    This will result in the bass management signal to be a mono signal (step 5) and sent to all loudspeakers (step 3). It might mean that you have too much bass (if so, you could drop the levels in Step 3 by a couple of dB – but keep them all matched).

    Have a listen to see if this gives you a better “punch” in the low end of your system. As I said, it may be that it helps – but it might make things worse at some frequencies – but this will be highly dependent on the details of your configuration.

    Cheers
    -geoff

  7. claudio says:

    Very enlightening article indeed!
    My (small 3 x4 meters 2.6 height) listening room made of pure concrete was softened with some absorbing foam in certain points of the walls anc ceiling, carpet on the floor and 3d diffuser int the back wall. BL3 sounded fantastic.
    Just bought a pair of BL9 but not sure if puttin the BL3 in the back or in the attic. Made some tests and found the BL3 do a good job if pointed ‘backwards’ but ot in front of the BL9. That way they improve the ambience sensation without compromising holographic stereo.
    What would you suggest? How can I send you a diagram of my setup?
    Tnx in advance!
    Claudio

  8. geoff says:

    Hi Claudio,
    Without being there to hear how it sounds, I’m afraid that I don’t know what to suggest – apart from saying that, if you fid a position for your loudspeaker that you like, then you should definitely go with that!
    Cheers
    -geoff

  9. Svein Haakon Lia says:

    Hi Geoff,
    thank you for a very interesting article!
    You mention that “It is possible to play BeoLab 5’s with the ABC filter disabled”, how would you go about doing that?

    Thanks in advance,
    Svein

  10. geoff says:

    Hi Svein,

    Press and hold the button at the top of the bl5 to start the ABC measurement. The microphone will move out and the measurement will start. Keep holding the button down until the procedure stops. The LED will blink, indicating that no ABC filter has been created – but you can still use the loudspeaker.

    Note that this erases the filter – so you’ll have to redo the measurement to create it again.

    cheers
    – geoff