Senior development engineer, Dr John Dibb explains how AM-1 overcomes the acoustic limitations of traditional all-weather speakers.
We know that different speakers can produce different results depending on the acoustics of the space they’re used within. How do you approach that particular challenge when designing an all-weather loudspeaker?
Speakers designed for use outdoors are generally subject to less performance variation caused by their surroundings, as these tend to be larger, more open and less room-like. The results of our own outdoor tests were very positive in this respect. The sound was better than expected in every way, particularly at the low frequencies. We were especially pleased with the attack and the even nature of the bass.
Of course, AM-1 is also suitable for use indoors, so we also needed to take into account the changes in the acoustic environment when the speaker is used in this way, particularly at the low-frequency end.
What we have done is engineer a carefully judged balance that delivers optimum results in all possible applications.
Do loudspeakers intended for outdoor use require any technologies not employed in conventional designs? If so, how do these technologies impact on sound quality?
In general outdoor speakers don’t require any special technologies compared with ‘normal’ systems. However, you do need UV-resistant structural materials and paints. And of course, any parts of the speaker that are exposed to the weather need to be waterproof and corrosion-proof. Hence the use of aluminium in our grille design.
The enclosure has to be completely waterproof, but it should also have a slight, long-term, air leak. This is because outdoor speakers can undergo much greater temperature changes (leading to internal air expansion or contraction) than indoor speakers. This is one of the key reasons for our decision to give the AM-1 an auxiliary bass radiator (ABR), which offers most of the advantages of a reflex system whilst delivering the other design criteria we need.
Unlike other Bowers & Wilkins loudspeakers, AM-1’s tweeter sits below the bass/midrange driver. Why the change in orientation?
When used outdoors, speakers tend to be mounted above the listener, and so we wanted to angle the AM-1’s optimum listening axis (i.e. the point where the units operate exactly in phase) down by around 10 degrees. With limited front-to-back driver positioning on this baffle, it was easier to achieve the downward tilt with the tweeter-below configuration.
The design of the AM-1’s enclosure, which integrates the ABR, is very unusual. What are the acoustic advantages of this engineering approach?
Every aspect of AM-1 is optimised for the best possible bass performance. The ABR approach allows us to achieve close to the performance of a ported system while maintaining all the structural advantages of a sealed box design. Combine this with high-quality Bowers & Wilkins drive units plus a simple crossover, and we think the AM-1 sounds better than any weatherproof speaker we’ve yet produced.