Paul's Posts — 16 August 2012
By Paul McGowan
Yesterday’s post got us connected and playing our system for the first time. Through the use of Stereophile’s test disc we’ve determined that the channels are correct, the phasing is right and the balance between the two speakers the same.
Then we played our first music: a simple naturally recorded voice that will help us get 80% of the setup right and achieved proper level so we can make sure the size of the voice is correct for the room and the loudspeakers.
Next we need to get the voice in the right place from front to back. In an earlier post Where the music’s supposed to be, which I encourage you to read, we learned that our soundstage should be behind the loudspeakers, not on the same plane and certainly not in front. I know this is going to rub some of you the wrong way as you’ve been enjoying your soundstage in front of your loudspeakers for years. I hate to be the one to break it to you, but that’s wrong and you’re going to have to get over this idea to move forward. I fully realize it is counter intuitive because intellectually we all understand the sound is projecting forward and into the room – so what’s up with insisting it appears from behind the loudspeakers? Please, read the link and absorb what it is telling you.
So now that we’re listening for depth, where is the singer? If you’re using the same track from Jane Monheit I am using she should be a little forward of halfway between the front of the loudspeaker and the rear wall. Now here’s where my mention of staying away from formulaic approaches is going to bite either me or you – because only some of you will have the proper space of 1/3 the total length of the room behind the loudspeaker to play with. I am going to guess only a very few of you will enjoy this much real estate in your homes. The rest of us won’t have that luxury (including me in my system at home) and will have perhaps only a few feet. Should you have less than the ideal space to play with, don’t use the imaginary reference of halfway between the speaker and the rear wall. Your task is to imagine where that place would be and achieve it anyway. Close your eyes if necessary and don’t let the physical reality of where your speakers are convince you there’s not enough room behind them for the soundstage. It’s all an illusion anyway.
Making sure your blue tape marks are in place so you have a reference to return to, start playing around to see what happens in your room. I am going to give you some general guidelines and then you need to simply play and take note of what works to get this depth – and equally important what causes the depth to get worse – it’s all valuable. Keep at it until you have gotten as close as you can to what I asked you to get and then mark that position with a second set of blue tape – and keep the old tape as well. The changes you make should be in inches at this point – no giant changes, please as Millimeters matter.
- Moving the pair closer together with the fronts remaining parallel to the wall behind you will increase the focus of the voice, decrease soundstage width in the middle and push the image farther towards the back. It will also increase the lower ranges of the voice because you’ll get better midbass coupling of your loudspeakers – more midbass (below 500Hz) gives the illusion of greater depth.
- Moving the pair away from the rear wall increases depth and decreases focus of the voice but also can add to its roundness and space as an individual performer. Moving closer towards the rear wall can flatten out the voice and compress the space of the soundstage.
- Toe in can do a lot and go either way when it comes to depth. This is because depth is partly a function of tonal balance (which is part of what we’re working with here). If we have a loss of energy in the 500Hz to 1.5kHz region we’ll have greater depth and when you toe in a pair of speakers not only do the tweeters get “hotter” and more direct at your listening position but this can also have the affect of changing the overall tonal balance to that of less lower energy by virtue of more higher frequency energy. It isn’t the amount of total energy but the balance between the frequencies that counts.
You will have to experiment and take notes! Work with it until you maximize the singer’s depth as best you can making sure she is always firmly behind the loudspeaker pair. This is a big step and one that will be different in execution for each room and loudspeaker pair.
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Soundminded
In this type of sound system, the perception of distance to the performer is strongly influenced by the factors you say. Avoiding interaction with the room boundaries, hence the speaker placement away from the walls and fairly strong high frequency directivity matter. Precise phase relationships between your ears and speakers. Avoiding interactions with the room boundaries especially from the direction of the speakers is critical because phase displaced reflections degrade the effect of perceived distance. Further improvements to this effect can be obtained by using sound absorbing material at the end of the room where the speakers are, and using geometric diffusers to reduce the predominance of reflections from any one direction from arriving at the listener. The location of the listener is also critical, hence there is a sweet spot where all of these factors are optimized. Altering the geometric relationships change the effect just as you say.
The effect also depends on the recording containing early reflections. Dead recordings made with little or no early reflections will not have this effect to the same degree if at all. Boomy midbass will also degrade it by imposing many reflections that give strong clues to the small dimensions of the room itself. Weak midbass will enhance it but at the cost of spectral inaccuracies. Nevertheless many audiophiles prefer it that way.
Why does it matter? Because the farther away the perceived source of the sound is, all other things being equal, the more powerful it is perceived to be at any given loudness. In fact the perceived power varies with the square of the perceived distance making even small improvements very noticable. How do you know the correct loudness? The answer is through experience. Since all of us have experience with the human voice, we should usually be able to tell rather accurately if for a perceived distance to the source, a human voice is too loud or too soft. It partly depends on the voice itself and how it is used. At a given distance, a shout will be much louder than a whisper. This is a factor also. Experience for many with other musical instruments may not be as great and so it may be more difficult to get the loudness for other recordings to be accurate.
It is an equally legitimate goal for a sound system to reproduce the sound of musical instruments as they would be heard in your room. This only works for soloists and small ensembles playing acoustic instruments because anything larger and more powerful would have to be played at unacceptably loud levels to be accurate. A concert grand piano in the average living room even in a large house can be overwhelming. Toward this end an entirely different set of criteria must be met by an entirely different kind of sound system. Now imaging the opposite extreme. One where the musicians aren’t a foot or two behind the speaker but dozens of feet beyond the front wall. This alone would increase their perceived power enormously. Add to that the sense of space of a room hundreds of times larger than your listening room and reverberation that last 10 times or longer than what you hear and you’re in a different world entirely.
LarryB
Paul is certainly not the first to discuss the concept that the image should be behind the speakers. Putting aside for a moment his argument as to why this is “correct,” I think far too much emphasis is placed on this concept. In large part this is beause focus is placed on the speakers when in fact, the ideal situation is when the speakers seemingly disappear as sources. If this is achieved, it should not matter if the speaker plane represents the front of the perceived (i.e, imaginary) soundstage, the rear of the soundstage, or anything in between. Either way there is depth to the soundfield, which hopefully adds to the illusion of the music being live, rather than recorded.
This of course is simply my opinion, which may not be shared by all.
Regards,
Larry Borden
Mark Malboeuf
I agree with Larry. I also feel the “thought experiment” outlined in “Where’s the Music Supposed to Be?” gets too convoluted for me. I guess I have always thought of the mics as ears. Where the mics are placed is where the instruments are imaged and recorded from, not from the position of a hypothetical listener sitting behind. What row is the listener in? ; )
Let’s suppose we’re in a jazz club, and we put up a coincident pair of mics to capture the band. We choose a spot that captures a balance between the instruments, and also between the overall band and the room. That spot is where we are listening from when we reproduce the music, and it includes the room sound as imaged by the mics from that position. If we are sitting out in the room behind these mics during the show, we will hear a different balance and a different set of room reflections. You can noodle with the recording with electronics, but the straight two channel feed can’t image it from the audience unless that was where the mics were.
Paul McGowan
All that being said if you imagine yourself as the microphones, as you suggest, the sound will always be in front of you. Right? Or, put another way, the sound will always be in front of the microphones. If you step back a few feet and observe the microphones recording the stage then the sound that is being recorded (which is in front of the microphones) is now behind the microphones from your new vantage point.
Now replace the microphones with loudspeakers playing back whatever was on the recording. The sound shouldn’t reverse its position relative to the transducers.
What’s difficult to comprehend is that because the microphones are pointing towards the music and the speakers are pointing towards the listener, you’d think the sound position should switch – but it really won’t and really shouldn’t. For the sound to image in front of the loudspeakers you’d have to imagine yourself in the music not listening to it out in front.
Soundminded
The sound that reaches each microphone is a vector field. The microphone may be more sensitive to sounds arriving from some directions than from others or it may be nearly as sensitive to sounds arriving from all directions. It doesn’t matter. The microphone output is a scalar. It has only the dimensons of amplitude and time. It stays that way until the speaker converts it back into a vector field. And its the wrong vector field. It’s nothing like the field incident on the microphones let alone what arrives at the audience’s ears no matter what seat it’s heard at.
The theory of a microphone being an analog of an ear reaches its full glorious absurdity in the abject failure of binaural sound played through headphones. I was reading more of that Audiophile editor’s prattle the other day and he said something to suggest to me that he’d nearly got it. But not quite. He’d nearly stumbled on a critical piece but didn’t understand the importance of it and he didn’t know how to put the pieces together to figure out the picture in the puzzle. But he actually knew something I hadn’t given him credit for. Wonders!
Mark Malboeuf
Soundminded – just to be clear, I’m not suggesting that mics can be like ears, I’m talking position in the room. The mics are where we’re hearing the performance from.
What do you think about a Blumlein pair recording played back on speakers set at 45 degree toe-in? Or Ken Christiansen’s True Stereo recordings for Naim? Those can make some neat sounding and coherent images. I understand how both of these types of recordings are made (have done a fair amount of recording myself), so no technical description is necessary.
Soundminded
IMO current recording techniques whether two or three microphones or multimiking can fairly well capture the sound coming from the performing stage at the audience. With suitable tweaking this aspect can be fairly well reproduced. What can’t be reproduced is the sound that is the result of the concert hall’s acoustics. Since that constitutes 90% or more of what we hear, it’s a real problem. No techniques for recording or reproducing that sound has been shown effective in any commercial product. Binaural sound has failed, quadraphonic sound failed. Last year I heard what may be the ultimate ambiophonic sound system and IMO that failed too. My analysis is that it is probably impossible to record and reproduce that sound but if it isn’t, it’s not by any method demonstrated so far. By comparison to “the real thing” recordings of music sound to me dull, flat, and two dimensional. Even with the best systems they lack concert hall reverberation, sense of space, sense of power, and are tonally and dynamically inaccurate. I feel like this industry has been largely spinning its wheels for the last 30+ years endlessly tweaking the 10% it knows how to deal with. (I’m not impressed by any sound system where you have to sit in one particular spot either. What kind of accuracy is that?) The one bright spot was the introduction of the digital compact disc, a far better method to store and retrieve electrical signals that are analogs of music IMO than vinyl phonograph records or analog magnetic tape.
Mark Malboeuf
Don’t get me wrong, Paul – I don’t actually disagree – I’m trying to work through the logic of the idea of imaging behind the speakers conceptually because I think it’s an interesting idea. I’m neither against it nor fully sold on it. My speakers image from between and behind depending on source and placement, etc. When they are doing that, the sound seems more likely to be floating free of the speakers, or I am less aware of the speakers than when the image is coming ‘out at me’ – perhaps because in the case of forward imaging I notice changes due to my head position more.
What I guess is bugging me is that there seems to be an extra element in this construct. How about if we limit it to sound source and listener. When we listen to the musicians onstage and no one is recording it, should the sound come from behind them? Most of the sound we hear first is what is going toward us. When we listen to speakers, they are now the sound source – playing a reproduction of the sound as recorded from the position of the mics, not the sound as it actually emanated from the acoustic instruments onstage. So it’s this analog of the sound that the speakers are projecting toward us.
Is it really possible for it to be a sort of ‘relay’ as you have outlined, where the sound goes out from the performers, to the mics, then (skipping that whole troublesome middle bit) from there in a straight line out of our speakers to our ears? Can we really say, despite the undeniable visual fact that the mics were between us and the performers during the show, that the audio chain is recreating this setup the way we saw it?
If I plug a mic and preamp into my speakers you’re going to get the same stuff out of my speakers as came out of the PA onstage (at least the same signal went in – let’s not get into what happens after that). If I stand in front of, or behind the speakers (or in another room) it doesn’t matter – the sound should go toward me, just as the sound of my voice would in a room with no recording going on. It’s when we get into reproducing acoustic performances in a given space that this gets complicated. Certainly the phase information of the original sound gets run through a blender. Is ‘imaging from behind the speakers’ a way of visualizing setup that leads to tweaking the phase to a place that creates a more pleasing and convincing illusion of the room acoustic?
No doubt, something really cool and special can happen when all the elements converge in a great live performance and recording, well reproduced. I’m still not sure what the mechanics of that is!
KipnisStudios
I am reminded of THIS article by John Atkinson, back in 1986, in Stereophile:
”…by recording amplitude information only in a two-channel system, we can create a virtual soundstage between and behind the loudspeakers….” “…the brain is able to work out which signal is intended for which ear. If a wavefront reach
es the left ear from the left speaker, the brain knows that that wavefront will reach the right ear around 0.7ms later, the time taken for the wave to travel around the head, and therefore can ignore it….” “…When it comes to recording music, there are two mutually incompatible philosophies. One is to capture as faithfully as possible the acoustic sound produced by a bunch of musicians, in effect treating a performance as an event to be preserved in a documentary manner. The second, which is far more widespread, is to treat the recording itself as the event, the performance, using live sounds purely as ingredients to be mixed and cooked. This, of course, is how all nonclassical recordings are made. The sound of an instrument or singer is picked up with one microphone, and the resultant mono signal, either immediately or at a later mixdown session, is assigned a lateral position in the stereo image with a panpot. As this is a device which by definition produces a ratio of amplitudes between the two channels, it would seem that every recording made this way is a true amplitude-stereo recording, capable of producing a well-defined stereo image….” “… When producing such a recording, the producer decides how much and what type of reverberation should be associated with each of the mono sound sources, and also decides where in space that reverberation should be positioned. There is no reason at all why the ambience surrounding, say, a centrally placed lead vocalist, should have any relationship with that around the drums. Or the guitar. Or the synthesizer. And if it doesn’t, then the listener doesn’t hear a soundstage. Rather, he hears a collage of individual musical events, bearing no spatial relationship to one another….” “…Early stereo rock recordings, such as the Airplane’s After Bathing at Baxter’s, illustrate this graphically: while such a recording can undoubtedly be satisfying musically, a soundstage it just doesn’t have. Since the late ’60s, producers nearly always take care to coordinate the artificial ambience on rock recordings to result in the production of a convincing soundstage. Recordings from Paul Simon, Andreas Vollenweider, and Clannad, for example, create a wholly artificial, but nevertheless effective, soundstage hanging between and behind the speakers, which bears no relation to anything that might have existed in real life….” “…Remember that a true stereo image is produced by a two-channel recording consisting of amplitude information only. What if the microphones are separated in space, not by a small distance, but by a distance larger than the wavelength of most of the musical sounds—10′, say? Unless an instrument or voice is exactly halfway between the two microphones, there will be, in addition to the amplitude information, a time delay introduced between the electrical signal it produces in one channel and the signal it produces in the other. Such time information pulls the image of the source further toward the nearest speaker, resulting in an instability of central imaging and a tendency for sources to “clump” around the speakers. Add to that the fact that the inter-channel amplitude differences produced by spaced microphones do not have a linear relationship with angular direction of the sound sources, and it is hard to see how a pair of spaced microphones can produce any image at all….” “…To make a recording capable of producing a true soundstage, we have to go back to Blumlein’s 1931 paper, in which he outlined two microphone techniques producing outputs containing amplitude information in the correct linear relationship with the direction of the sound sources.
First is the M-S technique, where a sideways-facing velocity (figure-eight) microphone is spatially coincident with a forward-facing mike. Rediscovered in the ’50s, this presents the two outputs in matrix form, as sum and difference signals (footnote 2), and is the basic philosophy behind the Soundfield microphone.
Second is to arrange two figure-eight microphones horizontally coincident at 90°, each positioned at 45° to the forward direction. This is the classic “coincident” technique as used for early EMI stereo recordings, by James Boyk for his Performance Recordings piano records, and by Sheffield for their recent Firebird….” “…I am certainly not saying that only recordings made in this manner are worth listening to. This discussion of stereo imaging has ignored such equally important aspects of miking as frequency response and balance; microphone coloration, distortion, and noise; capturing the true dynamics of the music; the quality of the concert-hall acoustic; getting the most musically desirable ratio between direct and reverberant sound; and even the time available to find the best places in which to position the mikes. When all these are taken into consideration, any good recording engineer will tell you that he often has to sacrifice the potential for true stereo imaging, in Blumlein’s amplitude sense, to gain benefits elsewhere. And if the results are still musically justifiable, why not?…”
Stereo & the Soundstage By John Atkinson : http://www.stereophile.com/asweseeit/1286awsi/index.html
~~~
Food for thought!
Cheers -
Jeremy
Kipnis Studios
http://www.Kipnis-Studios.com