Theoretical plot of driver response on a circular baffle.
But when you talk to panel loudspeaker adherents they will be the first to describe box speakers as sounding plummy and coloured. That might partially explain why panel speakers are often described as ‘thin’ or ‘lightweight’ by some listeners on first hearing an electrostatic. In fact I formulated a theory many years ago that we are so used to hearing box resonances that the sound seems ‘wrong’ when you take them away. Now most of us grow up hearing these box resonances from every loudspeaker we listen to. So, considering that most people like small speakers in their living rooms, what is the point of pursuing the open baffle? I’ll tell you – it is that the box is missing!ĭespite the best intentions of the loudspeaker designer in providing bracing, damping, internal absorption, adding a port, a horn or a quarter wave pipe, there is no getting away from the fact that putting a box behind a speaker just encourages resonance. But you do need large speakers to really make it happen. That ability to really move the air, so that it has visceral as well as audible impact, is something lacking in most hi-fi systems, box speakers or not. As my wife pointed out when hearing the reproduction of a Bach organ work on the radio “organ music sounds better in a church where you can feel the power of the low notes”. I’m not going to claim that you’ll achieve the same bass power in the room from an open baffle speaker as you can from, say, a transmission line speaker of similar overall dimensions. Now we are getting somewhere, and it starts to all make sense when we look at panel loudspeakers like the Quad Electrostatic where the baffle width clearly isn’t over 2 metres! By increasing just one dimension, say height, and allowing the speaker to couple to the floor and, possibly, the side wall, strong bass output can be achieved to satisfy even the organ enthusiast! Peaks at multiples of Fp which is why, in general, circular baffles Radiation become coincident, shown here as Fp. For a circular baffle there will also be a peak where front and rear Until the driver resonance is reached when the slope changes to 18dB per Below this frequency output will fall by 6dB per octave This table shows the relationship between baffle diameter (in metres) and theĬut off frequency Fc. This might be as much as half that calculated as, at frequencies below 100Hz, room gain becomes considerable.įor example, if we take room gain at 40Hz to be 6dB above that at 80Hz – a not unreasonable premise for speakers placed on the floor and near a side wall – then our 215cm round baffle can generate substantial output down to 40Hz (where it would normally be -6dB measured under anechoic conditions). Obviously you will start to get cancellation at the smallest dimension, say the width, but this will be blurred by the reinforcement of radiation occurring at the larger dimensions.Īs a result of the differing dimensions, complete cancellation will not occur until a much lower frequency. Who wants total cancellation at the baffle cut-off frequency? Change the baffle to a rectangular one and you can see that there are three dimensions that determine the half-wavelength cancellation, namely height, width and corner to corner. This sounds frighteningly large to the home constructor dreaming of attractive wood enclosures in the living room, but all is not always as it seems in acoustics.įor a start only an idiot would make a circular baffle. For example a circular baffle of diameter 215cm will allow strong radiation down to 80Hz and below. When the drive unit is mounted on a baffle this distance is markedly increased. Yet bass did come out of this radio, perhaps not very deep bass but bass was present nevertheless! The answer, as I came to find out when I studied speakers rather more intently, is that frequencies are only cancelled when their half-wavelength exceeds the distance between the front and back of the drive unit. Table radios of the 1930 – 1950 era, such as this EKCO 75, used the deep,īakelite cabinet housing to act as an extended baffle, normally fitted with a How could an ‘open box’ possibly produce any bass? Surely the front and back waves would cancel each other out? Having been brought up in the hi-fi tradition of speakers in boxes I remember being horrified when I first discovered an old radio with a perforated back plate. If we look at the early history of loudspeakers, however, it is littered with, what we now call, open baffle speakers. With the exception of panel speakers like electrostatics, an enclosure and a drive unit seem inseparable.
Peter Comeau explores the alternatives.į or as long as many of us can remember, loudspeakers have come in boxes, but that wasn’t always the case. When you think of a speaker, you think of a speaker in a box. Designing Loudspeakers - Part 15 Open Baffles and Bass