Tools of the trade – the anechoic chamber

Tools-of-the-Trade---The-Anechoic-Chamber

In the first of a new series of posts, Senior Product Manager, Mike Gough, looks at some of the tools used by the professional speaker designer including their limitations and pitfalls.

The anechoic chamber

Anechoic means “no echoes” and, if you have never seen an anechoic chamber, this is one of the best. It’s at the Building Research Establishment, just north of London, UK and is the largest in Europe. It’s size is indicated by the Bowers & Wilkins engineer in the photo and the speaker is an early Model 801, which puts the date somewhere in the 1970s. We used to hire the chamber in those days to make really accurate bass response measurements. The open air might have been an alternative, but there you have to cope with noise from wind and passing traffic and worry whether the sun is shining or there’s snow on the ground.

Anechoic chamber at the Building Research Establishment in North London

Anechoic chamber at the Building Research Establishment in North London

The longer the wedges the lower the bass

The walls, ceiling and floor are lined with wedges. They’re over a metre long and absorb sound, partly because they’re made from fibreglass, a very good sound absorbing material, and partly because the tapered shape blurs the effect of having a sudden change of material. Having the taper alternately vertical and horizontal also helps. As a general rule, the larger the chamber and the longer the wedges, the lower into the bass you can measure accurately. Because of its size, this chamber needs an open metal grid above the floor wedges to support objects under test and allow people to walk around.

As you may guess, it costs a fair amount to build a chamber like this and you need the space for it. Nowadays, its use is reserved for when you want to test much larger objects than a loudspeaker. For those familiar with London’s Albert Hall, the discs in the roof were tested here before being installed to reduce that hall’s famous echo.

The bass response of a speaker is now more predictable using modern computer modelling techniques, which allows us to use a much smaller chamber. This is the one at our Steyning R&D facility. The outer dimensions are a much more manageable 5.5m x 4.2m x 4.2m high. It’s accurate down to around 200Hz and here it is shown during a simple tweeter measurement. The wedges are still a metre long, but are made from open cell foam. Having once accidentally fallen into fibreglass wedges and suffered a good week of itching as a result, I can testify that foam is a much more acceptable material.

It’s as important to stop sound and vibration getting into the chamber from outside as it is to stop internal reflections. Of course, the wedges absorb sound in both directions, but our chamber is also isolated from vibrations in the building, hence the gap between it and the floor.

Why we need to measure

At this point, it’s worth taking a step back to ask why the speaker engineer needs measurements at all; surely the final arbiter of what is good and bad should be the ear. That is certainly true, especially as we can’t yet measure every property of our equipment that goes to make up the total listening experience. But, however good our ears are at telling us whether what we have is good or bad, they are not very useful at quantifying how good, or pointing us in the right direction to make any improvements. Only when you can put a figure a property can you tell if you are making an improvement. You have to measure.

Now, nobody in their right mind would want to listen to music in an anechoic environment. Listening in a room or auditorium of some type is much more satisfying and that’s what we do, so why not measure under the same conditions that we listen in? Well, let’s look at some basic frequency response measurements. This graph shows three measurements of the same speaker. In each case the microphone is placed directly in front of the speaker.

Frequency response

 

The green trace is an in-room measurement of the speaker close to a rear wall. The grey trace is with it closer to a corner. Both these measurements are very ragged and it’s difficult to decipher what is going on. They show the effects of reflections and resonance modes in the room and, while they are similar at higher frequencies, they’re very different in the bass. If you were to move to a different room, the results would be different again. Fortunately, the ear is able to cope with some of these differences, but even then, the professional engineer cannot optimise for just one room and even one position in that room. He needs to know what the speaker itself is doing and assign measurement targets that will make it sound good in the majority of practical listening rooms.

The red trace is an anechoic measurement of the speaker. It’s much smoother and, more importantly, it is consistent and repeatable. In the bad old days, it was considered the right thing to make the speaker’s response extend as far as possible into the bass and measure ruler flat as it did so. If nothing else, it was good for published specifications. Actually, that’s about all it was good for. Nowadays, most designers have more sense and make some allowance for the bass boost that virtually all rooms will provide.

DM70 in Anechoic Chamber

DM70 in Anechoic Chamber

What comes next…

We’ve briefly touched on frequency response, but the anechoic chamber is used for a much wider range of measurements that benefit from this type of environment. However, a discussion of distortion, dispersion and so forth belongs in a future article. And before fans of impulse response measurements point out (justifiably) that you can make ‘anechoic’ measurements in a live room, we’ll compare that approach as well.

For a larger selection of anechoic chamber images, please visit our Facebook gallery.

Mike Gough, Senior Product Manager.

12 Comments

  • Jake Purches says:

    Remember the anechoic chamber well – I shot the nautilus video in it. So silent, and very strange to hear no echoes. Regards to all at Steyning Research.

  • Matthew Simpson says:

    Wow it looks like something out of a 1920′s German Expressionist film. It’s fun and informative to see just how much science goes into the sounds we hear from our headphones.

    http://www.bestheadphonesforclassicalmusic.com

  • Robin Perry says:

    On the thread “Nowadays, most designers have more sense and make some allowance for the bass boost that virtually all rooms will provide.” I think sucks.
    My new house has a room that cancels all the bass out of my speakers. Unless you stand on the chair then at height you can hear it again. What do I do to get my Bass back…Move again? I’ve tried the speakers in corners out of coners close to the wall away from the wall with no improvement. Not a happy bunny. Can’t people just make speakers work as they should and not make allowance for boosts?

  • Hal Owen says:

    Greetings from sunny southern California.
    Interesting article regarding a non reflective room. Is it true some people loose their sense of balance when entering?
    At present I listen to three 800s, (left center, and right channel,) along with a pair of 801s as my rear surrounds. The five channel timber match is pretty close but volume levels are about 2.5 db louder with the 801s using the same interconnects and cable runs. Are the 800s simply less efficient? Is this more a matter of room acoustics or something else? The surround audio experience is truely enveloping as is two channel stereo. Many thanks for such glorious sounding and looking speakers.
    Best regards,
    Hal Owen

  • Jim Barry says:

    Robin, it sounds like you are suffering from the dreaded “bass null” problem. Towards the centre of a room, bass frequencies tend to cancel out. The effect relates to the position of the listener rather than the speakers. Square rooms are the most problematic, as the modes are similar in both axes.

    I have this exact problem in one of my rooms. In my case, the room is squarish with a low ceiling, making it almost a perfect cube. This is a complete nightmare acoustically, as the room modes combine in all three axes. The only way to hear the bass is to move away from the centre of the room – either horizontally or, as you say, by standing on a chair. In the end, I had no choice but to rearrange the furniture so that my listening position is close to the rear wall.

    In addition to the “bass null” problem, room modes can play havoc with the low frequencies, causing large peaks and troughs in the bass response. If you have a sub/sat setup it’s well worth considering a DSP equalizer such as the excellent Anti-Mode 8033. It corrects for lumpy/boomy bass caused by the room itself, resulting in tighter, more even bass response.

    Finally, I’m sure you know what you’re doing, but it’s always worth double checking that the speaker polarity is correct!

  • Robin Perry says:

    Greetings from OZ
    Thanks Jim for the input.
    My room is carpeted and about 6m long 3.5m Wide 2.7m Tall with the speakers at on end pointing down the 6m length. The seating is well away from the opposite wall which has timber blinds and curtains. I added curtains to see if that would help reduce reflections that might be cancelling out sound coming from the speakers as they bounce back towards them, all to no avail. This was the driving force in reading this article. I use a Quad 909 Pre Power with a pair of Monitor Audio RS800. Aspiring for B&W 800′s. Do you think more room treatments to reduce reflections will help or is there more to the problem than reflections? Have I just the worst acoustically shape room ever? Anyones input would be good. I used to love the sound from my system in the last two houses. ;-(

  • Lafonte says:

    Good Morning. I’m very happy that i checked my e-mail today. Seeing this very intesting article about The Anechoic Chamber, has helped me understand why it is used and how it works. This article was very educational for me. Thank you and have a great day.

  • Jim Barry says:

    Greetings from the UK. Hey Robin, strangely enough I’m getting on a flight to Sydney later today for a 3 week holiday in Oz. As for the room acoustics – bass frequencies have wavelengths measured in metres, so carpets and curtains will have no effect whatsoever. However, you may be able to improve the situation with acoustic room treatment, specifically bass traps. If you search for “bass null” and “bass trap” you should find some good info out there.

  • Robin Perry says:

    Greetings Jim. Enjoy your three weeks in Sydney albeit in the middle of a crappy winter.
    I am in Adelaide South Oz in the middle of a crappy winter too, moved here 4 years ago from Stockport near Manchester.
    Thanks for the tip on bass traps, makes for an interesting read but a no go with the missus I reckon. The video was really good on the RealTraps web site illustrating the full effects of traps and diffusers. I reckon a node may be being generated by the ceiling due to the height change making the most striking difference audibly. Front to rear adjustments, even large exagerated ones of both speakers and listening positions don’t really make a significant difference. Not sure what to do about it really, that’s wife friendly. The wife thinks it sounds fine of course and would suggest nothing. May sneak a sub in one day in a hidden position even though I think that’s Sacrilege.

  • Hal Owen says:

    Robin, your listening room/waf situation sounds all too familiar. Any chance of reversing your speaker/sweet spot placement to the short side of the room? Certainly you can go the EQ route and/or add a sub or two but you might also consider the decorative approach to room acoustic management – by using say fabric wall hangings placed at first side wall reflections for example or the addition of indoor plants in lieu of corner bass traps – silk trees are especially useful here as wife friendly diffusers. If possible try temporarily arranging the listening room in thirds or fifths to try and better determine where your low end null points are and observe as much as possible the golden triangle or equidistant approach to speaker/sweet spot placement. If more adjustments seem useful experiment by closing the gap between speakers ever so slightly. Securing a pair of 800s will definitely make things more interesting.
    Best wishes, Hal Owen

  • Robin Perry says:

    Thanks for the input again guys…..
    OMG I used my iphone as a basic SPL meter and found, thanks to some sine wave waves tracks from RealTraps, a 20db, yes 20db drop in the 60 to 90hz area exactly in the seating area. I convinced the wife to let me move the furniture slightly… Bringing the speakers forward almost 1 metre away from the wall moved the null closer towards them, and then I moved the listening sofa further back towards the opposite wall.
    Results are it sounds much much better now with it’s guts back albeit not as strong as it used to sound due to the speakers being so far away from the rear wall.
    However there is a lift in 150hz area which sucks. I must admit till moving house so much recently I never really appreciated how much room acoustics influnence the sound. People should spend more money on acoustics treatments for the perfect room and then less on the equipment as this would probably yield far better results as I am begining to learn……;-)
    Rob….

  • Stephen j says:

    Hey Robin Perry. Head room for bass boost is very essential & is something a good speaker engineer would do! For the fact, a human ear acts like a microphone itself. And this human microphone(ear drum) has its own frequency response strengths or sensitivity levels. So if we take the sensitivity of human ears, they are more dominant over the lower frequencies than the mids or highs. Thus designing a speaker having flat response from the bass frequencies to high freqs will actually make the low frequencies very dominant than the highs.

    But the problem your suffering with no bass in your room, probably has nothing to do with the speakers. Your room is a dead room. If you want to improve the bass in your room, you could make your room filled up with drapes, carpets,tables , chairs, etc. Make sure the room looks as much as asymmetric in its geometry. And another quick fix would be, along with all the above said- Why dont you add a bean bag to the room. Sitting on the bean bag is just going to improve the bass as a quick fix. Because frequencies below 80hz is not only heard but also felt because they are mechanical vibrations (sound) by the end of the day.

    If you require any detailed help in acoustics . reach me through stephen.j2412@gmail.com or
    stephen.j@soundofmusic.cc

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