Successful loudspeaker design with Scott Krix
The key questions are – what positive effect is the product expected to have on the users’ daily life to induce them to acquire it? What price range will it be in? Where will the product be used and what application is it most suitable for? For example, is the product intended for home hi-fi or home cinema use, a discrete audio application, a commercial environment such a cinema or shopping centre, or possibly professional audio involving a live performance?
Without knowing this, it is nigh impossible to build a speaker that people want.
Aesthetics and application
All of this often goes hand in hand with the suitability for purpose. Loudspeakers that are to be used in a home environment are obviously expected to have aesthetic appeal to fit in with or at the very least not clash with interior design preferences.
These considerations normally set some upper limit on the size of the product, irrespective of other considerations. The trend toward simpler, less cluttered interior spaces has changed people’s expectation towards home entertainment equipment. This has changed the design paradigm with the conventional ‘box’ style loudspeaker being one possible approach, ‘concealed’ audio being another.
Size and Sensitivity
The most important constraint in designing any loudspeaker starts with estimating the size of the environment that the product will be used in and the required Sound Pressure Level (SPL) that is expected to be delivered to the listener in this environment.
Commercial and professional audio loudspeakers are often justifiably large, as the required SPL needed for a live performance or the accurate rendition of a movie at a cinema requires products with sufficient levels of performance and obviously in this case, size does matter. As well as being larger, commercial and professional audio loudspeakers are generally more efficient with the advantage that less power is required to generate a given SPL at a particular distance.
Loudspeakers exhibiting higher efficiency and sensitivity almost always have less distortion for a given SPL and sounds with a high peak to average ratio (such as drum hits, string plucks or explosions at the movies) can be rendered accurately. This is perceived as a more dynamic, exciting and effortless listening experience.
This is especially the case for horn loaded loudspeakers where the efficiency and sensitivity are many times greater than conventional direct radiating loudspeaker units, and they are perceived to be even more dynamic and involving as long as other important criteria are satisfied such as frequency response and directivity control.
The home is not as large as the environments mentioned previously, so by definition the loudspeakers required for the job don’t have to be large as their commercial cousins. However in this day and age of miniaturised handheld electronic devices (phones and tablets), there is the misplaced expectation that very small loudspeakers can offer a satisfying listening experience. Unfortunately wavelengths of sound are independent of the size of the listening space and especially at low frequencies the air that needs to be moved to generate sound still remains considerable. This brings increased pressure on the design of products that offer the same level of performance in ever smaller packages. So in the relative scale of the home environment, size still matters. However, with the exciting innovative development of an increasingly wide range of so called “concealed” audio products, offering a very satisfying performance with a very much reduced visual design footprint is now more widely available.
The frequency response of the loudspeaker has the largest bearing on the overall sound presentation and specifically that which falls within the vocal range. The midrange is where we live, and we are very sensitive to small variations of timbre, distortion, and so on. Our ears are most sensitive to the speech range (refer Fletcher-Munson equal loudness contours) with average hearing sensitivity falling off at very high and very low frequencies.
It follows therefore, that a consistent response within the speech intelligence band is of paramount importance, such that vocal and genres of music with sounds mirroring the vocal range (for example a cello) can be naturally and accurately reproduced. As simple as this objective may be, it is often difficult to achieve. The crossover point between loudspeaker driver units often falls within the vocal range due to the performance constraints of the individual driver units selected for use. When crossing over from one driver unit to the next, the frequency response of those respective units tends to be quite variable as they are working in regions subject to their diaphragm breakup modes and resonances.
Achieving a combined frequency response free from adverse dips and peaks involves careful driver selection and skilful crossover design to blend the acoustic output from all drive units together. It should also be noted that to gain the full advantages of a three-way loudspeaker design, where a dedicated low distortion high sensitivity midrange driver normally handles the important vocal range, the complexity of the filters required makes the design much more difficult.
Power handling and maximum power rating
This is the quantity of electrical power the loudspeaker can absorb from the amplifier without damage, and combined with nominal sensitivity figure, allows calculation of maximum possible SPL. The acoustic power delivered by the loudspeaker is the product (multiplicand) of the electrical current and voltage and conversion efficiency. The size and number of loudspeaker drivers used in a product has the greatest effect on its power handling, more so than any specific technological innovation.
As soon as the cone or dome or diaphragm vibrates to make sound waves, distortion is generated. The more it vibrates or moves, the more distortion it makes, but the larger the loudspeaker, the less movement required. Thus in all cases distortion will rise as output SPL rises and it is really only a question of how much for so much. Distortion is generated mostly due to non linearities in all the subsystems of the loudspeaker driver and enclosure. Odd order harmonics e.g 3rd 5th etc are unpleasant whereas the even ones e.g. 2nd, 4th are more benign. Good design ensures that cone movements are as small as possible and the driver motor system is designed to minimise any distortion generated. This is achieved by meticulous attention to the detailed design of the loudspeaker drivers, e.g. optimised magnet and pole piece layout to achieve the largest, most linear cone excursion and a copper capped magnet pole to minimise eddy current and hysteresis effects.
Correctly designed horn loudspeaker systems have a natural and considerable advantage. The horn loading increases the acoustic pressure on the driving cone/diaphragm thus dramatically reducing the movement required to generate a given SPL and this in turn reduces the distortion. Horns can exhibit distortions due to the nonlinearities of air itself but this is only an issue for extreme sound levels (in the vicinity of 140dB).
In summary, all loudspeakers generate distortion when operating, but good loudspeakers have lesser amounts of, and more benign, distortion.
Directivity and Dispersion
These factors relate to the distribution of the sound that is radiated into the room. If the frequency response is designed to be flat on axis or at the listening position but the off axis responses vary considerably with frequency then the energy in the room also varies considerably, which can result in a blurring sound stage presentation. This is always an issue of varying degrees in systems using only direct radiating loudspeaker drivers. If the directivity could be well controlled at say 90 degrees horizontally and 40 degrees vertically the amount of energy that reflects off the side walls and ceilings is substantially less than that of the spherical pattern of 120 to 180 degrees that comes from a direct radiating loudspeaker system. Horn loudspeakers that successfully control the directionality of the sound offer a more direct and visceral acoustic presentation for this reason.
This is an important characteristic as it relates to how hard the amplifier has to work in driving the loudspeaker to a given SPL. As the loudspeaker’s output is purely a response to the applied voltage, the lower the impedance (in Ohms), the greater the current (in amps) demanded by the loudspeaker load on the amplifier (Ohm’s law). Depending on the loudspeaker in question, nominal load impedances usually vary from 4 Ohms to 8 Ohms. Loudspeakers with a lower impedance rating draw more current from the driving amplifier, but also cause it to dissipate extra power and heat in order to deliver this extra current. Commercial loudspeaker systems for professional audio and cinema often use lower load impedances of 4 Ohms and high power commercial amplifiers with continuous fan cooling are used to drive them. For home Hi Fi loudspeakers smaller amplifiers are used and higher impedances from 6 Ohms and 8 Ohms are more common.
Manufacturing considerations – Finish and Quality Control
It is important in the manufacturing process that any given unit produced is a faithful replica of the original reference loudspeaker design in every respect. For stereo loudspeakers the pair matching is critical for a balanced sound presentation, otherwise stereo imaging may be degraded. It is vital that comprehensive testing be carried out on all units produced, with respect to frequency response and distortion.
As with any product, the sum of its parts can be much less than the whole and without all of those parts working together to deliver the best possible result, the product may fail to meet expectations. Consider a modern motor car, it may have an engine which allows outstanding fuel economy, but if the seats fitted to the vehicle were grossly uncomfortable surely no one would want use it to travel any distance?
Loudspeakers are very much similar in that all parts must be expertly combined to deliver a satisfying listening experience for the user in the intended environment, whilst also being easy to install and most importantly having pleasing aesthetic aspects in order to blend in with the surroundings.