In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: In the normal speech range of 500 to 3 , 000 Hz , an estimated difference of 0 . 99 dB ( - 0 . 17 to 2 . 14 dB ; P = 0 . 09 ) was detected .
### Output Text:
In the normal speech range of 500 to 3 , 000 Hz , an estimated difference of 0 . 99 dB ( - 0 . 17 to 2 . 14 dB ; P = 0 . 09 ) was detected . 