Monday, August 26th, 2019

SP Acoustics : The continuing validity of the Infinite Baffle design

By editorApril 30, 2012


Steve Phillips, owner and founder of SP Acoustics has been involved with the audio industry for over 25 years and more specifically has spent the past 15 years in the design and development of high end loudspeakers for both domestic and professional markets. He has been responsible for development of numerous award winning loudspeakers and AV systems which have gained many ‘product of the year’ awards.

Is it a studio monitor or isn’t it?
“What is the difference between a studio monitor and other speakers, or indeed between a good studio monitor and a bad one?  Can’t I use my Hi-Fi speakers or the small PA we use for our pub gigs?…. there are so many questions on such a contentious and, in some ways, subjective subject.

“There are some parameters we all expect from a studio monitor; a relatively flat frequency response within a sensible frequency range (this usually eliminates PA speakers), distortion figures within usable limits, and power handling that will allow a reasonable output before we have to re-cone the woofers or extinguish the flames.  Assuming all these criteria are met, how does an engineer begin to choose from the plethora of products available today and how does a company, looking to make the best studio monitors, set out design criteria?

“At SP Acoustics, the first stage of defining an internal “term of reference” is understanding exactly what an engineer wants and needs from his system.  There is no “one size fits all” and many criteria need to be taken into account.  As well as the aforementioned parameters, we believe that the low frequency (LF) performance, especially in terms of timing, is imperative to good monitor design, and to us, one of the most important ones.

“Our brains are very proficient at compensating for irregular frequency response, phase shifts and uneven output levels.  The thing we are most sensitive to, and often overlooked, is timing.  As an evolutionary defence mechanism, our senses can detect slight variances in the arrival times of sound impulses and we use this to detect where a sound is coming from and, if it’s moving, which way.  It is inherent as part of our fight or flight reactions, so we are naturally susceptible to it.

SP Acoustics SP1MLA

“The LF timing in a monitor or any other speaker is generally a result of the chosen monitor bass alignment.  At SP Acoustics we use Infinite Baffle (Sealed box) designs, one of the oldest ways of designing speakers that, in modern times, has been pushed aside in favour of Bass reflex (Ported) alignments looking for more bass from a smaller box.  Both have their uses and both have advantages and disadvantages.  There are others, but we will stick to these two main variants for now.

“The reflex method takes the output from the rear of the woofer and sums it with the front radiating output from the same woofer, usually just below the natural roll-off frequency of the woofer.  This has the advantage of increasing the LF output of the monitor so you can use smaller cabinet designs, but has the disadvantage of adding a group delay on the port output component, effectively smearing the LF signal when the two components are combined together.  It also has a very fast low frequency roll off, so what you may gain at 45Hz drops off very quickly below that.”“Our chosen method, the Infinite Baffle design, does not use ports; the woofers are housed in sealed enclosures so you only hear what is coming from the front.  This also has some potential drawbacks – Lower power handling, higher LF distortion and compromised LF response, but our use of modern materials and manufacturing processes, allow us to bring this classic design in to the modern studio, on a par with the reflex designs in terms of power handling, distortion and LF response.  This alignment also provides superior LF timing performance and slower roll off, giving increased LF extension well beyond that of the ported designs.  The only compromise we have had to make is a bigger box.  The advantage of this cleaner LF output is a clearer distinction between LF signals, useful when balancing a kick drum and a bass guitar and very handy for highlighting artists’ …… can I put this….. “Musical timing issues”.

“Continuing our look at what makes a good monitor, we can turn our attention to frequency response, or more specifically off-axis frequency response.  It should be obvious to all that a monitor needs to be relatively consistent in this field.  You can usually get accurate response curves from most manufacturers, but which is more important, the on-axis or off-axis output?  Well consider this; what you are actually listening to when working with your monitors – may be 40% the monitor and 60% the room?  The ratio changes with frequency and will depend on how well treated your room is, but unless you are in an anechoic chamber, the room is going to play a big part in your listening experience.  The on-axis response of your monitors is going to be the bit that creates the “sweet spot”, great if you have a well controlled room, never move, constantly listen at a set level and you are the only one in the room.  In reality you need that sweet spot to be as big as possible so that you can move around and listen with clients or co-workers and all hear the same thing.  To achieve this you need a good off-axis frequency response.  One that has a gentle High Frequency (HF) roll-off as the angle is increased but doesn’t have dips and peaks appearing all over the response curve.  This off-axis response is what will drive most of the energy in your listening environment so it is important that it is smooth.

SP Acoustics SP1MA Amplifier

“At SP Acoustics, we design all our monitors so that the optimum measuring position is horizontally 5°off-axis.  This gives us 2 sweet spots per monitor, one 5° to the left and one 5° to the right. In the case of out SP1M, this is a vertically arrayed three way monitor, so the horizontal off-axis response is faultless.  The vertical off-axis response can vary a little as, when you change your head height in relation to the monitor, you are changing the distance between your ears and each individual driver.  At frequencies where these drivers overlap (due to the 2nd order crossovers) there is a certain amount of cancellation.  This happens on any monitor that uses more than one driver but the effect can be kept to a minimum with a well designed crossover.  We use 2nd order crossovers on our passive systems to reduce the vertical off axis effect and minimise distortion, a constraint that isn’t relevant in the DSP filter systems used in our active SP1MA and SP1MLA monitors.  The SP1MA is an active version of the SP1M that uses higher order filters therefore reducing the number of frequencies shared by different drivers so they interfere with each other less, this improves the vertical off-axis response.

“As we have moved on to active monitors, a word about DSP (Digital Signal Processing).  It is a very powerful tool.  To be able to create seemingly endless chains of filters and group delays within your monitoring system can make the impossible seem possible … It isn’t!  It is a great way of making a monitor work well within its own parameters and allows an experienced designer to time align a system at a specific point.  It will not fix bad acoustic spaces!  When used properly with experienced acousticians that know how to measure a room, it can be used to fine tune a complete acoustic system (the electro-acoustic equipment and the acoustic space it’s operating in) but it will not compensate for room anomalies.  These are dynamic issues that need to be corrected physically.

Although all our active monitors have full DSP capabilities, we only allow our engineers to access the settings.  When SP1MA or SP1MLA monitors are sold we will deliver them, set them up and measure the installation.  If required we can set or alter the DSP settings at that stage to fine tune the installation and it’s all included in the purchase price!”“I cannot stress enough how important it is to get the room acoustics as close to perfect as possible.  There is no way around this.  As a manufacturer we can not design monitors that specifically work well in bad acoustic space.  The one monitor that came close to achieving this was the infamous NS10.  Yes it sounded mid-range heavy when compared to today’s offerings but that was its party piece.  It had minimal bandwidth (very little bass & lots of mid range) so it didn’t excite the LF energy in the room too much.  It was also a sealed infinite baffle design so there was no port to upset to room modes and what bass it did produce was very tight with good timing.  It had lovers and haters but because it didn’t energise the room with lots of bass it sounded the same in every room.  If nothing else, it was a constant for engineers moving around the world from studio to studio.

“Choosing a monitoring system is a big challenge.  As we have already said there isn’t a one stop shop for all, and everyone wants something just different enough to make things complicated.  First recommendations have to be to get the acoustics sorted before any major listening or comparisons are done.  What works in a reverberant room may sound dull in a properly treated room.  It may take a while to get it treated but it will be worth it and will make the choice of monitors a lot easier.  You may find your existing system works just fine in a good room.

“In life, most people probably wouldn’t buy a pair of shoes without trying them on – Just because they are in your size, look great and your mate has a pair doesn’t mean they will fit!  Whenever possible, try monitors in the room that they are to be installed in and make sure they fit acoustically.  The room or acoustic space is part of the monitoring system; it’s the air in that room that the monitors will need to drive and it will be different to that in a dealers’ showroom.  At SP Acoustics we try to sell our products to customers only after they have had a pair setup in their listening room and have been left alone to put them through their paces for a few hours maybe longer in a commercial studio.”

Steve Phillips
SP Acoustics

Our thanks to Steve for providing such a thorough yet understandable review of monitor loudspeaker design principles.  You can check out the full SP Acoustics product range at


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