This term generally refers to the percentage of available inventory that an advertiser will achieve through a proposed ad placement. If an advertiser bought all available impressions on a site (i.e., had an exclusive sponsorship), they would achieve 100% share of voice. The formula for calculating is:
Ad impressions included on proposal / Total ad impressions available for unit during relevant period
Share of voice is often requested by an advertiser / agency when completing proposals for an upcoming display ad campaign. Typically, there is a column on a proposal template where the publisher can indicate the share of voice associated with each line item that comprises that campaign.
Below is a sample display ad proposal (SOV column highlighted for emphasis):
SOV is usually calculated on an ad unit basis, and can be referenced as a percentage of a single ad unit on a page or group of pages. For example, assume that every page on a site contains a 728×90 leaderboard and the site generated about 10 million pageviews monthly. A proposal for 1 million leaderboard impressions during a one month period would equate to a 10% share of voice.
If an advertiser is running a homepage takeover or other roadblock, these would generally be considered to be a 100% share of voice placement. Even though the advertiser doesn’t necessarily have the entire site taken over, they would own all of the impressions associated with the specified pages
Many advertisers like to achieve at least 10% share of voice on their placements. There is no real benefit to this threshold, and the share of voice realized ultimately has no impact on the performance of a campaign. This has more to do with the preferences of advertisers; they like to feel as if their ads have a meaningful presence on a site instead of appearing to only one out of 100 visitors or so.
This is one of those aspects of direct ad sales that will have to be managed by the representative. If a client is really concerned about realizing a high share of voice, the publisher may need to consolidate multiple line items into one.