This study has been on the to-do list ever since I saw the first Avalon clones with the middriver stuck into a very narrow and deep rectangular hole. I always thought the midrange would suffer congestion but I never had the chance to hear any of the clones. As often as possible I stress the importance of chamfering the driver holes to allow free ventilation for the rear energy coming from the driver. Obviously the energy coming from the rear of the driver is exactly the same as what comes from the front and as most drivers have very thin and acoustically transparent membranes we need to do all we can to get rid of the rear energy in order not to have that energy being reflected back towards the membrane. The reflected rear energy will be delayed and hitting the membrane it will cause smearing of detail and increased (subjective) distortion. Set-up: I was sure the differences would show up on CSD (cumulative spectral decay) and step response, but it didn't. Much to my surprise! But what surprised me even more was that it had a significant impact on frequency response. What I already knew from mounting a small middriver into a narrow tube (A) was the impact on sound (TJL3W and Classic 3-Way). Comparing A and C on pink noise leave no doubt of the importance of chamfering driver holes. On C we hear a full-bodied sound with deep and wide low-end response where A will make the sound thin with an unpleasant presence. It sounds congested.
Worst case
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