An Introduction to Automobile Audio System Design, Stereo Imaging and Staging

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Though many automotive audio fanatics have already got a very good understanding of how sound is produced, no technical handbook ought to begin with out protecting the fundamentals of its subject material. Writers typically wrongly assume that the reader will already know quite a bit concerning the topic and they also skip the fundamentals. Due to this, some could discover it obscure the explanations for putting in audio system specifically positions within the automotive, for instance, or why a woofer operates finest in a specific measurement and kind of enclosure. Most individuals discover it embarrassing to confess that they don’t perceive one thing which they assume everyone else is aware of. The reality is that there are most likely a number of different folks in the identical group who wish to ask the identical query.

Right here I’ll define the fundamentals of the topic and briefly focus on how these fundamentals apply to the world of automotive audio. It’s not purported to be an entire information to sound as a result of we do not need to ship you to sleep! If you want to know extra about sound and the rules of hi-fi sound replica, there are numerous good books on the topic.

So what’s sound?

Sound is attributable to the motion of air. If a big picket panel is vibrating, the air subsequent to it’s pushed away. If the vibration is gradual then the air simply strikes out of the way in which, however as the speed of vibration will increase to between a couple of tens and some tens of hundreds of excursions per second, the air can’t transfer out of the way in which quick sufficient and subsequently compresses because the panel pushes it away. The air’s pure elasticity comes into play. The particles of air in opposition to the panel are pushed in opposition to the subsequent particles, which push in opposition to the subsequent ones and so forth, making a ‘stress wave’. Because the panel returns to its beginning place, the air turns into much less dense because it fills the void in entrance of the panel.

The speed at which ‘stress waves’ are produced is known as the ‘frequency’ and is measured in ‘cycles per second’. As a substitute of ‘cycles per second’ we often describe frequency when it comes to Hertz (1 cps = 1 Hz). Think about a shower filled with water. For those who put your hand within the water and transfer it slowly backwards and forwards, not a lot occurs – the water merely slips across the sides of your hand. However if you happen to transfer your hand very quick, the water would not have time to maneuver out of the way in which and also you create a wave within the bathtub. Quickly the water is flowing over the perimeters of the bathtub and soaking the carpet. For those who used one thing bigger than your hand to maneuver the water, corresponding to a dinner plate, you could possibly trigger the water to overflow with out shifting your hand as quick as earlier than. Air reacts in a lot the identical approach. Massive objects and small objects can generate stress waves and subsequently sound, however a small object should transfer faster to trigger the air in entrance of it (and behind it) to be compressed.

You might be most likely one step forward of me by now, however because of this we discover tiny tweeters and really massive woofers. All sizes of audio system can produce sound, and the the explanation why tweeters are small and woofers are large has a lot to do with the scale of the stress wave they’re attempting to breed and the load and traits of the shifting ‘panel’ – which within the case of a speaker is often a cone, a dome or sometimes a flat diaphragm. To breed excessive frequencies, the cone or dome wants to maneuver very quick. The smaller and lighter it’s, the better it’s for the amplifier to regulate it. We’ll come again to this later. First we have to perceive a bit of extra about sound itself.

Strain Waves

Strain waves of a sound journey at a set pace of round 1100 toes per second (truly, air temperature impacts the precise pace however we needn’t fear an excessive amount of about that). If we all know the frequency (the variety of waves per second), we are able to calculate the gap between corresponding factors on successive waves – in different phrases, we are able to measure the size of a wave, or the ‘wavelength’. A sound that has a frequency of 1100Hz has a wavelength of 1 foot. A sound of 2200Hz could have a wavelength of six inches, and a sound with a frequency of 550Hz could have a wavelength of two toes.

The notes of the musical scale merely signify sounds at sure frequencies. Center A on a piano is 440Hz for instance (a wavelength of 2ft 6in). Within the case of a church pipe organ or digital synthesiser, it could be attainable to play an A which is 4 octaves beneath center A. This has a really low most important or ‘elementary’ frequency (further frequencies or ‘harmonics’ will probably be added naturally, which supplies every instrument its personal particular person tonal character) of 27.5Hz, a wavelength of round 40 toes! At actually the opposite finish of the dimensions, an A performed three octaves above center A has a frequency of round 3500Hz and a wavelength of simply 3.75 inches.

The scale of the musical instrument (and loudspeaker) tends to recommend the scale of the wavelength it is designed to supply. The soundboard of a piano and the big pipes of a church organ are able to producing massive wavelengths, which implies decrease frequencies. Equally, a 12-inch diameter subwoofer is designed to present low bass. If we ignore for a second the mechanical limitations, there may be nothing to cease a very good 6 inch mid-woofer from reproducing very low frequencies. It might probably transfer on the required cycles per second, however its measurement implies that it will probably solely displace a certain quantity of air throughout every cycle. It might produce very low frequencies, however solely at very low output ranges. This takes us again to the analogy of the hand and the dinner plate within the bathtub filled with water. The floor space of a 12-inch subwoofer is sort of a dinner plate, in a position to transfer sufficient air in a single cycle to supply a stress wave able to rattling the home windows in the home throughout the highway!

Imaging and Section… creating the phantasm of actuality

After we set up a hi-fi system within the dwelling or within the automotive, we are trying to breed a really complicated sample of sound waves inside an atmosphere that’s fairly completely different to the one through which the devices and vocals had been recorded. We are attempting to recreate not solely the sounds of the assorted devices and singers precisely but additionally their positions on the ‘sound stage’. That is usually referred to as ‘imaging’ or ‘stage picture’. It might even be described as ‘staging’, however in automotive audio this phrase is mostly used to explain the place of the stage itself (whether or not it seems to be in entrance of the listeners or behind them), quite than the place of the performers on the stage.

If we listened to and recorded the sound of a single flute in our front room, for instance, after which replayed it through a single full-range speaker positioned in precisely the identical place because the flautist, there’s a good probability (if the recording gear and hi-fi system is of a adequate high quality) that it’ll sound roughly an identical. The reverberation traits of the room would be the similar and, since we’re utilizing just one speaker as a substitute of attempting to artificially recreate the place of the flautist within the room by utilizing stereo strategies, the instrument ought to seem like in precisely the correct place.

If we had an entire orchestra in our front room and wished to repeat what we did with the only flute, we would wish to make use of a multi-channel recorder, a number of channels of amplification and nevertheless many audio system it took, every organized in precisely the correct place for every instrument. Having witnessed this performed at a hi-fi present in Paris some years in the past, I can let you know that it will probably sound very reasonable. The issue is that it isn’t at the moment possible to do that in most properties, and definitely not in a automotive, which is why we’ve… yep, good outdated stereo.

Stereo goals to recreate the precise positions of the assorted devices by utilizing solely a single pair of loudspeakers, and it achieves this principally by growing the extent of a specific instrument in one of many channels relative to the opposite channel. Solely the degrees are completely different – the wavelengths from each the left and proper channels will probably be an identical in each different approach. They’re mentioned to be ‘in part’. Which means if you happen to might by some means freeze the sound and see the wavelengths emanating from the 2 audio system, each wavelengths could be on the similar level of their cycle.

When sound is recorded for stereo replica, it’s assumed that the listener will probably be positioned equidistant between the 2 loudspeakers. In fact that is not often the case in a automotive, except you occur to drive a McLaren F1. We compensate for not being seated central to the loudspeakers by adjusting the steadiness management on the CD participant, which will increase the quantity degree on one aspect relative to the opposite. This solely corrects the stereo picture up to a degree, as a result of we’re nonetheless bodily nearer to 1 speaker than the opposite, and adjusting the steadiness management has no impact over the part and timing of the sign reaching our ears.

If we’re sitting within the entrance of the automotive, near the left-side speaker, then we’ll hear the sound from that speaker very barely sooner than the sound from the speaker on the correct, and from these audio system behind us. Additionally it is possible that within the higher frequencies, the place the wavelengths are very quick, the sound we hear by our left and proper ears could also be barely ‘out of part’. These timing and part distortions confuse the mind and may destroy the stereo impact. When this occurs, you hear a lot of the sound is coming from the speaker closest to you, as a substitute of from an imaginary stage throughout the windscreen – the stereo trick fails to work.

Our earlier analogy with water might also assist us to grasp what is supposed by sound being ‘in part’ and ‘out of part’. Think about that you just begin a delicate wave from one finish of the bathtub. For those who use each palms to start out two waves at precisely the identical time, the peaks of the waves will happen at precisely the identical level and the waves could possibly be mentioned to be ‘in part’. Now begin a delicate wave from the opposite finish of the bathtub. What occurs when the 2 waves meet? Each waves conflict and successfully cancel one another out. Now think about audio system on the entrance and rear of a automobile, every producing sound stress waves. These combine and create some very complicated alterations to the sound. Because the stress waves meet, the air could also be pushed stronger in the identical path through which it was already travelling (so we get additions which may result in peaks within the frequency response), or one stress wave could compress in opposition to one other travelling in the wrong way. Within the latter case, if the 2 waves are an identical when it comes to frequency and stress and are precisely half a wavelength out of part, they are going to cancel themselves out utterly, leaving silence.

Normally the 2 sound patterns are so complicated that they are going to solely partially cancel, however comparatively pure tones of lengthy wavelengths and excessive stress usually tend to noticeably cancel. The decrease the frequencies being produced and the extra loudspeakers there are reproducing that vary of frequencies, the extra possible it’s that part cancellation will happen. If audio system are wired with the + and – terminals reversed on one speaker, the audio system are positioned 180 levels (half a wavelength) ‘out of part’. In concept they need to cancel themselves out, however in observe the bass area suffers most cancellation and the rest of the sound turns into confused, with little or no central picture.

Speaker placement… for these with out a McLaren F1

By putting in many audio system inside a automobile, we create a really complicated mixture of stress waves which may trigger issues with the general sound. This doesn’t essentially imply that it’s a unhealthy factor to put in numerous audio system. We could need to cut up the frequency vary into small parts – sub-bass, bass, mid-bass, midrange, higher midrange, excessive frequencies and ultra-high frequencies – so that every vary has a pair of audio system devoted to it. Due to the constraints of loudspeakers, this could be a good suggestion if we’re on the lookout for absolute sonic constancy. In fact area in a automobile is restricted and so we have a tendency to put in fewer audio system, often protecting the sub-bass, mid-bass, midrange and higher frequencies. This can provide good replica of the tonal qualities of the assorted devices and voices however there are nonetheless the issues of part, ‘time alignment’ and ‘off-axis’ efficiency to contemplate.

We should always typically attempt to keep away from reproducing the identical frequencies from audio system positioned at completely different distances from the listeners. For instance, if we’ve a pair of 6-inch audio system producing mid-bass on the entrance of the automobile, we must always keep away from having one other pair of audio system reproducing precisely the identical vary of frequencies from the rear shelf. The colliding stress waves will trigger additions and subtractions, in keeping with the part of every wave, on the level the place they meet. Frequency peaks and troughs could possibly be adjusted with a third-octave equaliser, and the timing of the sound from every speaker could possibly be adjusted utilizing a ‘time alignment’ digital sign processor, however there isn’t any sensible approach of compensating for the part distortions. It’s rather more wise to keep away from the issues from the beginning, as a lot as attainable.

When deciding the place to place audio system, particularly midrange and tweeters, attempt to set up them in order that the left and proper audio system of every matched pair are equidistant from the listeners. That is usually very tough to realize in fact, however do the very best you possibly can. The footwell positions usually work effectively for midrange audio system and typically tweeters too. For those who determine to mount the tweeters larger, attempt if attainable to mount them ready which is half the gap to the midrange speaker – there may be some proof to recommend that putting the mids and tweeters precisely 180 levels aside can enhance stereo imaging.

Bigger bass audio system can usually be put in within the doorways with out an excessive amount of have an effect on on imaging, if you happen to make it possible for the crossover level is about in order that their output doesn’t overlap the frequency vary of the midrange audio system an excessive amount of. The subwoofer, or subwoofers, often should be put in within the rear of the automobile due to their measurement. Take care when selecting the slope and setting the crossover level on the channel(s) feeding these audio system. With a 6dB per octave ‘low-pass’ filter (a filter which solely passes frequencies beneath a sure ‘crossover level’) set to 100Hz, the output at 200Hz will solely be 6dB decrease, and since subwoofers are typically pushed loud, the stress waves created within the center frequency area will most likely be sturdy sufficient to work together with the audio system on the entrance of the automobile. It’s typically smart to decide on a 12dB or 18dB slope for subwoofers.

Putting in full-range audio system within the rear shelf, or within the rear doorways, usually confuses the stereo picture on the entrance of the automotive as a result of the identical sounds are coming from a couple of supply and that is unnatural. If you must set up audio system right here to offer ‘rear atmosphere’ or a ‘double entrance stage’ since you usually carry passengers within the rear seats, regulate the entrance/rear fader management on the CD participant in order that the entrance audio system are significantly louder than the rear audio system when listening from the driving force’s seat.

You should still discover that the stereo imaging on the entrance of the automotive is confused. If that is so, it’s value experimenting by reversing the +/- connections to all of the full-range audio system on the rear, placing them 180 levels ‘out of part’ with these within the entrance. It might sound loopy however typically works effectively. At all times be sure that you reverse the connections on each audio system in a pair.

In a perfect scenario, all of the sound in any respect the assorted frequencies ought to begin out from precisely the identical place. A speaker system which makes an attempt to offer that is usually described as a ‘single level supply’ as a result of the assorted drive items are bodily aligned in order that, in concept no less than, the sound will probably be ‘in part’ and ‘time aligned’ throughout your entire frequency spectrum. What often happens in a automobile, nevertheless, is that audio system are positioned a way aside and at completely different angles relative to the driving force and passengers. A speaker that’s pointed straight on the listener is alleged to be ‘on axis’, whereas a speaker put in low down within the door subsequent to the driving force is described as being ‘off-axis’. A speaker’s output traits alter when it’s listened to ‘off axis’. Its frequency response is affected, and part adjustments might also happen. It is very important know this and experiment, the place attainable, by altering the mounting angle of the speaker to realize the very best outcomes.

Shake, Rattle and… Why supplies resonate

One factor we need to keep away from in a automotive audio system is any panels (such because the steel panels of the automobile itself) that resonate, that’s, vibrate due to the motion of air inside the enclosed area of a automotive. It is a unhealthy factor as a result of these panels will produce a sound of their very own or, within the case of the partitions of a bass enclosure, if they don’t seem to be inflexible then we’ll lose among the energy and definition from the low frequencies.

Musical devices which produce sound naturally (not electronically) accomplish that by both inflicting one thing to resonate – corresponding to guitar and piano strings or the skins and metallic surfaces of a drum package and so on. – or by straight shifting the air, as within the case of wind and brass devices. Each materials has a specific ‘resonant frequency’ – that is the frequency at which the fabric will vibrate or resonate most freely – and this, along with the scale of the resonating object and lots of different components, contributes to the sound which an object will produce whether it is sufficiently excited.

In fact one thing very dense and inflexible like a brick is much less prone to be made to resonate than a guitar string or a skinny piece of wooden. Brick is a a lot much less environment friendly resonator than wooden. Typically this may be helpful, particularly when constructing a bass enclosure the place we want the edges to be very inflexible and acoustically ‘useless’ in order that it would not resonate, since this is able to add a few of its personal sonic character (or ‘coloration’) to the bass sound we are attempting to breed.

Sadly, brick-built bass cupboards in automobiles have not actually caught on, and extra sensible options needed to be developed for making picket enclosures and the steel elements of the automotive much less resonant. Good high quality particle board (MDF) is particularly dense and supplies a very good place to begin, and this may be handled, if vital, with sound deadening sheet or a sprig remedy. Within the case of sound deadening sheet, this bonds an acoustically ‘useless’ materials to the extra simply excited panel, such because the automobile’s steel inside aspect panel, door panel or inside boot (trunk). The sheet helps to soak up panel resonance as a result of its personal resonant frequency may be very low, so the resonance happens at a a lot decrease frequency and turns into much less noticeable. Within the case of NoiseKiller, a sprig remedy developed by Swedish firm Audioform, the resonance absorbed by the fabric just isn’t merely frequency shifted. As a substitute, it’s reworked right into a tiny quantity of warmth. The producer claims that the fabric produces completely no sound of its personal – in different phrases, the panel resonance is totally eradicated.

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Source by Kevin Obyrne