Grooves and needles

Until 1925 acoustic recording was the only technique in use. Recording company engineers had their own individual preferences for the shape of their groove cutting tool and there was no standardisation for the profile of the record groove. With the introduction of electrical recording in 1925 a more systematic approach was needed, and within about 10 years it had been agreed that a V-shaped groove with an angle of about 80 degrees and a bottom radius of about 0.0025 inches ("2.5 mil" - 0.06 mm) was best. The difference in groove profiles before standardisation, such as the HMV and the UK Columbia records shown in the diagram, was not considered to be important until the introduction of hard sapphire and diamond styli.

Most needles were made of either of steel or thorn. They were designed so that they would wear down to fit the groove during the first few revolutions of the record. The tip of the needle then came into contact with the bottom of the groove. To decrease the wear time so that the needle fitted the groove more quickly, special fillers of abrasive substances were added to the shellac used for record material. These included rottenstone (a silicious limestone which can be used for polishing metal), emery powder, slate powder and carbon black, augmented with tougheners such as cotton flock.

Not surprisingly, abraded granular material from the record surface became embedded into the grooves. This gave rise to surface noise whose component frequencies tended to lie well within the audible region. Characteristic qualities of surface noise were likened in The Gramophone during the 1920s to "coarse sand-paper" for a peak was in the 2500 Hz region; "tearing of brown paper" for noise peaking at 3500 Hz; and "fish frying" noise for a noise peak around 5000 Hz. Some of the cheaper British records were relatively coarse grained and tended to produce more low frequency "coarse sand-paper" noise than the "fish frying" noise of the best HMV and Columbia record composition. Many other record manufacturing countries used finer composition that British companies with the result that such records sound better after restoration than some of those made in the UK.

After playing a record a few times using steel needles, it was found by microscopic examination that small particles of steel become embedded in the groove walls. A heavily "steeled" record has much more groove damage than one played many times using a thorn needle regularly re-sharpened with a device such as that pictured below. The drawback to fibre needles was that they left a layer of organic debris in the bottom of the groove, which can prove difficult to clean out.