Useful Tips and Advice

The speed of sound is.......

Well actually, it isn't.

The speed of sound can vary with the media it travels through, the temperature, and the humidity of air. It is an important thing to know as you could otherwise be looking for problems that don't exist.

In normal atmospheric conditions where one could expect to find a human listener there can be significant variations in the "reference" speed of sound.

The speed of sound is an immensely complex variable to pin down, a bit like trying to catch a fly. The atmosphere we are in is in a state of constant change, in many ways, including temperature, humidity, density, and other movement currents. some of these will affect the propagation of the sound-wave, others the speed. They are all changing as we work or listen to something. Air, in itself, as a medium is non-linear in many ways.

The speed of sound increases with increased humidity and this effect increases with an increase in temperature.

Large amounts of Carbon Dioxide in the air such as in an unventilated room can lower the speed of sound.

There are also minor effects that may cause up to a 0.1ms drop in the speed of sound below 100Hz down to 10Hz.

So.... Using the speed of sound for accurate fine measurements is rather like using a rubber tape measure. Take care, get a reference before you make any fine adjustment, you can be as far as 10% away from what you may expect, or have previously set, and in many circumstances 10% is a significant variation that can change a perfect summation into a total cancellation.

But this means that "modal effects" in a room which are dependent upon the speed of sound through air can also vary as much as 10% (in extremes) from one day to another. (although such environmental differences my be a little uncomfortable a 5% shift is possible in more benign situations.) - That's rather inconvenient isn't it, especially for modelling and "tuning" such issues.

Phase is not polarity.

Or should we say, Polarity is not phase. (Stop saying "phase reverse" it's wrong)

Yeah, but it's out of phase ins't it? Well, No it isn't, not in that context.

Phase is a relative difference in time from one signal to another, a phase shift is a time shift. We use phase (measured in degrees) as a convenient way of describing a time difference of one amplitude / time signal to another as a measure of the relative cycle of a waveform, but in a linear time delayed signal that measure is only relative to one infinitely small frequency point.

If we delay a signal by 0.5ms we introduce a 180 degree phase shift at 1,000Hz, but at 500Hz we have introduced a 90 degree shift, and at 2,000Hz we have a 360 degree shift. Conversely, if we introduce a 180 degree phase shift to a full bandwidth signal at 1000Hz, we introduce a 90 degree phase shift at 500Hz, (which is twice as long, therefore half the rotational difference) and so on. There are complex phase relationships in non-linear circuits which we shan’t go into here, but suffice to say none of this relates to polarity.

Polarity on the other hand does nothing to time, there is no time domain effect at all from a polarity inversion.

Look at it this way, if you wave your arm from left to right a video camera with a video delay can invert this movement in a delayed replayed image, it has lag, it is slow to respond by one half cycle. When you look at the monitor the image will behave like a mirrored response of your movement. This is a phase shift. If you move your arm faster the delay will alter the relative position to your real arm, and the mirrored movement will be lost, just as a phase shift delay changes relative to frequency.

A mirror on the other hand has no delay, it will seem to equally laterally invert the movement of your arm relative to what you would see if you were at the mirror's perspective, however, no matter how fast you move your arm you will always see the inverse of what others would see, and this relates to a polarity inversion.

Take care, applying a 180 degree phase shift to correct a polarity problem is wrong, just as applying a polarity inversion to correct a 180 degree phase shift is wrong.

The correct place for a critical motoring loudspeaker is either in a wall, or in totally free space.

But wait, I hear the cry, "users don't listen like that". True, very true, but two different and not inverse wrongs never make a right.

If a monitoring loudspeaker were to be totally directional, throwing all of its radiated energy forwards within a tight nominal coverage area it would be fine to place this loudspeaker "free standing" close to boundaries outside the directivity pattern. As it happens we have not seen any monitoring loudspeaker with such a directivity response, and are not likely to see for some time.

In all circumstances when free-standing a vast quantity of energy in the lower frequency region, depending on the geometry of the source loudspeaker, is propagated in all directions at the same time. Some of that energy, a minority, goes towards the listener, the rest of it, the majority, goes off in other directions until it hits something, at which point it reflects and heads back at various angles. Some of this eventually reaches the listener, now late, and out of time with the initial first "true" arrival, it reacts with the pressure of the true arrival and modifies that pressure to create a "false" response. In higher frequencies we are able to easily differentiate between a source and a reflection, not so in lower frequencies where wavelengths are long and reflections are often well within the cycle of the direct arrival. What happens is a modification of the direct arrival in the time domain, and we get a series of peaks and dips in the bass response frequency-amplitude domain that can totally colour the direct sound in a way that means the loudspeaker can never reproduce the input signal truthfully. The loudspeaker-room combination is then said to have a very strong character that will impose itself on all program played.

Mounting a loudspeaker in a wall very simply and totally effectively removes all the problems related with rearward propagation. There is no acoustic compromise to this, it is wholly effective. All energy is radiated forwards so long as the integrity of the wall is adequate.

Mounting a loudspeaker in a wholly absorbent space is also as effective, but exactly what constitutes a wholly absorbent space at 25Hz is very debatable, and very expensive to do. In addition, a wholly absorbent space is a horrid environment for human listeners, especially when we can see the space but not hear that space. This is not a viable option for anything except a laboratory environment.

A loudspeaker wall cannot be "seen" by the sound propagating from the loudspeaker, it cannot reflect from that wall, and therefore the wall can be bright and reflective to give a comfortable space for the user. The only walls requiring acoustic treatment being the ones the sound will be directed towards.

A loudspeaker mounted in a wall will exhibit the flattest response it can, far flatter than a free standing loudspeaker which is always subject to a bass roll-off as the bass propagation begins to expand into a greater omnidirectional space.

Bass from a flush mounted loudspeaker will exhibit more impact and detail, and be a far truer representation of the electronic signal.

Well, what about the listener at home?

Well as the bass response of any free standing loudspeaker is totally dependent upon the precise geometric position in the room, virtually every listener will have their own different problems in their bass response. It is impossible to pre-empt this situation in the production environment. If their problems are not too bad they may even be able to hear through them slightly. Whatever the case the only professional solution is to make the music correct in the first place and leave the problem solving to the user in their unique surroundings.

Where should I place my loudspeakers for best stereo image?

Hmm. Maybe we could say "at the end of the rainbow".

No, seriously? where?

Well, that is a serious answer to this question.

That and most definitely, sometimes, occasionally the universal answer "42"!

The audio world, and the internet is awash with theories and equations for such questions, but really devoid of any single "correct" answer.

"It depends" is probably the best professional expert answer we can give in this circumstance.

Again, the internet is awash with pictures of professional recording studios, many with vastly varying Left-Right loudspeaker geometry. Some of those pictures show absolutely ludicrous positioning that really wouldn't work well at all, but the fact that they exist in the first place, and the fact that the studios are not laughed out of business is testament to the vagueness of the answer to our question.

So what is the long answer?

Well. The biggest issue is the room, if you have a good room and a good set-up then you are probably not even asking this question, maybe 42 was the answer, but, of course, you knew the question.

The room can greatly affect where you would place a loudspeaker for least compromise, where you would listen from, and what will reflect and create phantom images and a collapse of stereo imaging. Go too wide and we get a "hole in the middle" or "sound in our head" effect, too narrow and the stereo sound field collapses to almost mono. In a more "live" listening environment we sometimes need to spread the speakers a little to create a wide enough soundfield as reflections can be masking the image. In a very controlled environment we can close in the speakers a little without losing the image and get a better centre phantom image, if pushing the loudspeaker out to a great stereo position ends up pushing them into a reflective corner, then often the stereo advantage is outweighed by the reflective disadvantage.

If the loudspeaker is a poor point source with poor time response you will find that the stereo image is poor and the speaker has to be pushed out wider to exaggerate the soundfield.

Strongly reflective surfaces behind the head can make almost any geometry useless.

Stereo is a psychoacoustic effect, we create virtual sources in our brain based on what we hear binaurally, the only way to verify any psychoacoustic effect is to run it for real and use the human ears and brain for verification. When the listening environment includes distracting influences we have to do test runs to see if we can make things work.

At a design stage we can't go around waving our speakers in the air to find a sweet-spot, we have to nail down our positions, especially when they are wall-mounted flush, in our case it is based on an intimate knowledge of our rooms, years of adjustments, and great control over the loudspeakers we use. Thee is no substitute here for experience and decades of customer feedback on which geometry was deemed to be best.

So what about "42"?

Well, actually, 42 degrees off the centre line is actually not too bad a starting point for a beginner.

Nothing is exactly what it seems in this game.

One of the most common totally correct, absolute, answer we find ourselves giving is "it all depends on.....(insert infinite parameters here)"

This doesn't mean we don't know. The more we know the more we know that we don't know, and the more we know that other factors beyond our immediate control are huge modifiers of our work causing our answers to be vague.

We have already ascertained that even the speed of sound "depends", that where we put our speakers "depends", and how our speakers sound "depends". Even how to build a recording studio always "depends", just as how to cable a studio "depends".

In a world of people who want absolutes, black and white answers, this or that solutions, this vague incalculable tendency to come back and say "maybe" is deeply dissatisfying to many.

It can be hard to teach a subject to eager students when all we can say is "maybe that is ------- but it depends on so much you don't know yet". That is deeply dissatisfying to the student, especially when they are being sold knowledge.

In the absence of black-and-white answers we see endless opportunist merchants preying upon those seeking certainty, folks purveying "the absolute answer" to "the ultimate question", folks saying, if you do this, or buy that then you will have that answer you seek.

Snake-oil sometimes has a very bitter after taste that burns, despite being so sweet and soothing initially.

In many things we can, as yet, only ever "ball-park" our predictions, and results. We can get things there, or thereabouts with a reasonable degree of certainty within the variation, but that often leaves room for unscrupulous operators to move in and be critical of our lack of precision. Even where they have no better (often worse) results they find benefit to their position in claiming certainty that they do not have, and cannot prove (but do not need to) to those who do not know.

The problem here lies with the human trait that is a desire for absolute answers, black-and-white binary logic, something that I can do that is simple and absolute to resolve my problem. Something I can just buy with my money and not take any responsibility for if it doesn't work.

Audio and acoustics often don't work like that. We're incredibly grateful that they don't too. This is an industry of almost infinite probabilities, a space where we know precious little in the bigger picture and where we have great scope to improve upon things. It should not be a disappointment that we are not in full control, but a great point of excitement for what is left to do, how we can embark upon great journeys of enlightenment, how there is so much to master. Maybe we should stop seeking to have control, and better learn to navigate this vast complex electro-acoustic universe in a more imprecise enlightened way to better achieve the essence of what we strive to achieve.

I am struck by the number of lifelong experts in our industry who in face-to-face conversations reply with "We don't fully know that yet" "Ah, yes, in that situation it is all different" or "It all depends on so may unknown factors" to seemingly simple questions that lesser knowledgeable folk claim to have the answer to, and how their eyes light up with any suggestions of new results, new approaches, or new methods.

Many lifelong experts rely equally on a lifetime of experience of empirical results, educated intuition, putting together of different experiences in different situations, and a knowledge of what we yet do not know, just as much as they rely on a library of calculations and learned texts (which we should always hold a shred of doubt over) . When working in such a vague subject it can be as useful to know and observe what is actually happening and search for an understanding of why it is happening, as it is to know what should have theoretically happened and figure out what to blame for why it did not.

In may ways, professionally, at a high level, to say "it may probably do that if all is as expected" is an incredibly precise answer, and to say "Oh, that's easy, it always does this" is a very dangerous assumption to make.

It is a point of great amusement that as an industry, we are an industry of so-called experts, who often insist on micro-measuring immensely vague and variable circumstances to fractions of units with immensely expensive finely calibrated equipment, only to finally report that the immensely precise fractional number to the nth degree they come up with is actually only able to be qualified as a vague approximation to 5% or 10% certainty due to uncontrollable environmental factors and inadequacies in the measurement concept itself.

There is something almost dishonest in the very process presented to the layperson that implies immense precision in the measurement of imprecise often moving targets to achieve approximate answers.

If there was only one piece of advice I could give to any student in this subject it would be to be suspicious of certainty and to embrace and understand the concept of uncertainty in all we refer to.

Beware those who pedal certainty, and trust those who express their qualified uncertainty openly.