This is a quote from Trumpet Secrets Volume 1 by Jerome Callet and Bahb Civiletti. Sadly this book is now out of print but luckily I have it!

If the player tries to play with the clearest, most compact and brilliant sound, the player will produce overtones that will project the tone in the largest concert halls. Most trumpeters and trombonists are trying to develop a tone that is not truly characteristic of these instruments. The incorrect tonal abuse causes it to spread the tone too wide. When you play this way, with a “spread” tone, you cannot produce purity of sound for which your instrument was built, and you will always force your tone and over-blow.

Open your ears to the true trumpet sound.
~iii<0