AudioRolloffMode.Logarithmic  

// Sets the rolloff of the audio source attached to
// this script to Logarithmic.

@script RequireComponent(AudioSource)

audio.rolloffMode = AudioRolloffMode.Logarithmic;

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(AudioSource))]
public class example : MonoBehaviour {
    void Example() {
        audio.rolloffMode = AudioRolloffMode.Logarithmic;
    }
}

import UnityEngine
import System.Collections

[RequireComponent(AudioSource)]
class example(MonoBehaviour):

    def Example():
        audio.rolloffMode = AudioRolloffMode.Logarithmic