Most young retrievers have sensitive hearing. There is no need to use the same loud volume at 20 yards as at 200 yards. I like to use a soft,chirp for whistle-sit up close, and a loud, loooooong whistle for long distance, difficult to hear conditions. Conditions like running through lunging water, running through tall grass,etc. Under these conditions, the retriever may hear the whistle only at the top of his stride.. so a looooong whistle helps ensure he has a better chance of hearing the whistle at the top of his stride. Some times a whistle refusal is the handler's fault because the dog can not hear the whistle. Retrievers may also have difficulty hearing the whistle on windy days, especially long windy water blinds. Here is a quick example on a dog walk..soft, quick whistle for up close, long whistle for difficult hearing conditions...I blow until the instant the retriever turns to sit.